Composition
11471860 · 2022-10-18
Assignee
Inventors
- Brian Elliott Hayden (Southampton, GB)
- Christopher Vian (Southampton, GB)
- Kieren Bradley (Southampton, GB)
- Hugo Jungius (Southampton, GB)
- Kyriakos Giagloglou (Southampton, GB)
Cpc classification
H01M4/9033
ELECTRICITY
C01P2002/76
CHEMISTRY; METALLURGY
B82Y40/00
PERFORMING OPERATIONS; TRANSPORTING
B01J2523/00
PERFORMING OPERATIONS; TRANSPORTING
C01G45/1264
CHEMISTRY; METALLURGY
Y02E60/50
GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
B01J23/002
PERFORMING OPERATIONS; TRANSPORTING
B01J2523/00
PERFORMING OPERATIONS; TRANSPORTING
C01P2002/72
CHEMISTRY; METALLURGY
C01P2004/64
CHEMISTRY; METALLURGY
International classification
B01J23/00
PERFORMING OPERATIONS; TRANSPORTING
B01J35/00
PERFORMING OPERATIONS; TRANSPORTING
H01M4/86
ELECTRICITY
C01G45/12
CHEMISTRY; METALLURGY
Abstract
A composition consisting essentially of a perovskite crystalline structure includes ions of a first metal M1 which occupies an A-site of the perovskite crystalline structure and ions of a second metal M2 which occupies a B-site of the perovskite crystalline structure. M2 has two oxidation states capable of forming a redox couple suitable for reversibly catalyzing an oxygen reduction reaction (ORR) and an oxygen evolution reaction (OER). The composition also includes ions of a third metal M3 at least a portion of which substitutes for M1 in the A-site of the perovskite crystalline structure, and at least a portion of which optionally also substitutes for M2 in the B-site of the perovskite crystalline structure. At least some of the ions of M3 have a different oxidation state to the ions of M1. The composition also includes atoms of an element X, which is a chalcogen.
Claims
1. A composition consisting essentially of a perovskite crystalline structure, the composition comprising: ions of a first metal M.sup.1 which occupies an A-site of the perovskite crystalline structure; ions of a second metal M.sup.2 which occupies a B-site of the perovskite crystalline structure, M.sup.2 having two oxidation states capable of forming a redox couple suitable for reversibly catalyzing an oxygen reduction reaction (ORR) and an oxygen evolution reaction (OER); ions of a third metal M.sup.3, at least a portion of which substitutes for M.sup.1 in the A-site of the perovskite crystalline structure, and at least a portion of which optionally also substitutes for M.sup.2 in the B-site of the perovskite crystalline structure, at least some of the ions of M.sup.3 having a different oxidation state to the ions of M.sup.1; and atoms of an element X, which is a chalcogen; wherein the ions of the metals M.sup.1, M.sup.2 and M.sup.3 are present in atomic ratios within the ranges (a) or (b): (a) 25 to 49.9 atomic % M.sup.1, 30 to 60 atomic % M.sup.2, and 5 to 45 atomic % M.sup.3; (b) 10 to 30 atomic % M.sup.1, 50.1 to 60 atomic % M.sup.2, and 25 to 45 atomic % M.sup.3; each expressed as a percentage of the total metal ions in the composition excluding oxygen; wherein presence of the ions of M.sup.3 causes a change in an oxidation state of some of the ions of M.sup.2 in the perovskite crystal structure, thereby creating the redox couple suitable for reversibly catalyzing the ORR and OER; and wherein the ions of the metals M.sup.1, M.sup.2 and M.sup.3 are present in atomic ratios (a1): (a1) 35 to 45 atomic % M.sup.1, 30 to 45 atomic % M.sup.2, and 15 to 25 atomic % M.sup.3; all expressed as a percentage of the total metal ions in the composition excluding oxygen.
2. The composition according to claim 1, wherein at least some of the ions of M.sup.3 are in an oxidation state which is capable of oxidising some of the ions of M.sup.2 in the perovskite crystal structure, thereby creating the redox couple suitable for reversibly catalyzing the ORR and OER.
3. The composition according to claim 1, wherein M.sup.1 is a trivalent metal.
4. The composition according to claim 3, wherein M.sup.1 is selected from the group consisting of Y, La, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sb and Bi.
5. The composition according to claim 3, wherein M.sup.1 is La.
6. The composition according to claim 1, wherein M.sup.2 is a trivalent metal capable of oxidation to a tetravalent oxidation state to form the redox couple.
7. The composition according to claim 6, wherein M.sup.2 is Mn.
8. The composition according to claim 1, wherein M.sup.2 is selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Nb, Mo, Tc, Ru, Rh, Pd, and Ag.
9. The composition according to claim 1, wherein M.sup.3 is a divalent or trivalent metal.
10. The composition according to claim 1, wherein M.sup.3 is selected from the group consisting of Mg, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Y, Zr, Nb, Mo, Tc, Ru, Rh or Pd.
11. The composition according to claim 10, wherein M.sup.3 is Ni.
12. The composition according to claim 10, wherein M.sup.3 is Ca.
13. The composition according to claim 1, wherein M.sup.1 is La, M.sup.2 is Mn and M.sup.3 is Ca.
14. The composition according to claim 13, wherein the composition comprises 15 to 25 atomic % La, 51 to 58 atomic % Mn, and 25 to 35 atomic % Ca, all expressed as a percentage of the total metal atoms in the composition excluding oxygen.
15. The composition according to claim 1, which is formed as a nanoparticle.
16. The composition according to claim 15, wherein the nanoparticle is supported on a support.
17. The composition according to claim 1, wherein neither M.sup.1 nor M.sup.3 is Sr.
18. The composition according to claim 1, wherein the composition does not have the formula La.sub.1−ySr.sub.yZO.sub.3, wherein y is greater than 0 and less than 1, and Z is a transition metal.
19. The composition according to claim 1, wherein the composition does not have the formula La.sub.1−ySr.sub.yZO.sub.3, wherein y is greater than 0 and less than 1, and Z is selected from the group consisting of Co, Cu, Fe, Mn, and Ni.
20. A core-shell composition comprising (a) a core particle comprising a first composition; and (b) a second, different composition applied as a shell on the core particle, wherein either the first or the second composition is the composition according to claim 1.
21. A method of preparing the composition according to claim 1, the method comprises providing a source of each component element of the composition, wherein the sources comprise at least a source of the ions of M.sup.1, a source of the ions of M.sup.2, a source of the ions of M.sup.3, and a source of the atoms of X; and depositing the sources of each element onto a substrate, wherein the component elements from the sources react on the substrate to form the composition consisting essentially of the perovskite crystalline structure.
22. The method according to claim 21, the method comprising forming the crystalline composition in the form of a film on the substrate.
23. A method of using a composition, comprising: providing the composition according to claim 1; using the composition as a catalyst.
24. The method according to claim 23, wherein the catalyst catalyses the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER).
25. An electrode comprising the composition according to claim 1.
26. A cell comprising: an anode; a cathode; an electrolyte; wherein the anode and/or the cathode includes a catalyst comprising the composition according to claim 1.
27. The cell according to claim 26, which is a fuel cell.
28. The cell according to claim 27, which is a regenerative fuel cell.
29. The cell according to claim 27, which is an alkaline fuel cell.
30. The cell according to claim 26, which is a metal-air cell.
31. A composition consisting essentially of a perovskite crystalline structure, the composition comprising: ions of a first metal M.sup.1 which occupies an A-site of the perovskite crystalline structure; ions of a second metal M.sup.2 which occupies a B-site of the perovskite crystalline structure, M.sup.2 having two oxidation states capable of forming a redox couple suitable for reversibly catalyzing an oxygen reduction reaction (ORR) and an oxygen evolution reaction (OER); ions of a third metal M.sup.3, at least a portion of which substitutes for M.sup.1 in the A-site of the perovskite crystalline structure, and at least a portion of which optionally also substitutes for M.sup.2 in the B-site of the perovskite crystalline structure, at least some of the ions of M.sup.3 having a different oxidation state to the ions of M.sup.1; and atoms of an element X, which is a chalcogen; wherein the ions of the metals M.sup.1, M.sup.2 and M.sup.3 are present in atomic ratios within the ranges (a) or (b): (a) 25 to 49.9 atomic % M.sup.1, 30 to 60 atomic % M.sup.2, and 5 to 45 atomic % M.sup.3; (b) 10 to 30 atomic % M.sup.1, 50.1 to 60 atomic % M.sup.2, and 25 to 45 atomic % M.sup.3; each expressed as a percentage of the total metal ions in the composition excluding oxygen; wherein presence of the ions of M.sup.3 causes a change in an oxidation state of some of the ions of M.sup.2 in the perovskite crystal structure, thereby creating the redox couple suitable for reversibly catalyzing the ORR and OER; and wherein the ions of the metals M.sup.1, M.sup.2 and M.sup.3 are present in atomic ratios (b1): (b1) 15 to 25 atomic % M.sup.1, 51 to 58 atomic % M.sup.2, and 25 to 35 atomic % M.sup.3; all expressed as a percentage of the total metal ions in the composition excluding oxygen.
32. The composition of claim 31, wherein M.sup.1 is selected from the group consisting of Y, La, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Ky, Ho, Er, Tm, Yb, Lu, Sb and Bi.
33. The composition of claim 31, wherein M.sup.2 is selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Nb, Mo, Tc, Ru, Rh, Pd, and Ag.
34. The composition of claim 31, wherein M.sup.3 is selected from the group consisting of Mg, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Y, Zr, Nb, Mo, Tc, Ru, Rh or Pd.
35. The composition of claim 31, wherein the composition does not have the formula La.sub.1−ySr.sub.yZO.sub.3, wherein y is greater than 0 and less than 1, and Z is selected from the group consisting of Co, Cu, Fe, Mn, and Ni.
36. A core-shell composition comprising (a) a core particle comprising a first composition; and (b) a second, different composition applied as a shell on the core particle, wherein either the first or the second composition is the composition according to claim 31.
37. An electrode comprising the composition according to claim 31.
38. A cell comprising: an anode; a cathode; and an electrolyte; wherein the anode and/or the cathode includes a catalyst comprising the composition according to claim 31.
39. The cell according to claim 38, which is a regenerative fuel cell.
40. A composition consisting essentially of a perovskite crystalline structure, the composition comprising: ions of a first metal M.sup.1 which occupies an A-site of the perovskite crystalline structure; ions of a second metal M.sup.2 which occupies a B-site of the perovskite crystalline structure, M.sup.2 having two oxidation states capable of forming a redox couple suitable for reversibly catalyzing an oxygen reduction reaction (ORR) and an oxygen evolution reaction (OER); ions of a third metal M.sup.3, at least a portion of which substitutes for M.sup.1 in the A-site of the perovskite crystalline structure, and at least a portion of which optionally also substitutes for M.sup.2 in the B-site of the perovskite crystalline structure, at least some of the ions of M.sup.3 having a different oxidation state to the ions of M.sup.1; and atoms of an element X, which is a chalcogen; wherein the ions of the metals M.sup.1, M.sup.2 and M.sup.3 are present in atomic ratios within the ranges (a) or (b): (a) 25 to 49.9 atomic % M.sup.1, 30 to 60 atomic % M.sup.2, and 5 to 45 atomic % M.sup.3; (b) 10 to 30 atomic % M.sup.1, 50.1 to 60 atomic % M.sup.2, and 25 to 45 atomic % M.sup.3; each expressed as a percentage of the total metal ions in the composition excluding oxygen; wherein presence of the ions of M.sup.3 causes a change in an oxidation state of some of the ions of M.sup.2 in the perovskite crystal structure, thereby creating the redox couple suitable for reversibly catalyzing the ORR and OER; wherein M.sup.1 is La, M.sup.2 is Mn and M.sup.3 is Ni; and wherein the composition comprises 35 to 45 atomic % La, 40 to 45 atomic % Mn, and 15 to 20 atomic % Ni, all expressed as a percentage of the total metal atoms in the composition excluding oxygen.
41. A core-shell composition comprising (a) a core particle comprising a first composition; and (b) a second, different composition applied as a shell on the core particle, wherein either the first or the second composition is the composition according to claim 40.
42. An electrode comprising the composition according to claim 40.
43. A cell comprising: an anode; a cathode; and an electrolyte; wherein the anode and/or the cathode includes a catalyst comprising the composition according to claim 40.
44. The cell according to claim 43, which is a regenerative fuel cell.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
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DETAILED DESCRIPTION
Definitions
(20) In this specification the term ‘oxygen reduction reaction’ or ‘ORR’ means the half-reaction which results in the reduction of oxygen gas. In an alkaline fuel cell, this takes place according to the following half-reaction:
O.sub.2+2H.sub.2O+4e.sup.−.fwdarw.4OH.sup.−
(21) In this specification the term ‘oxygen evolution reaction’ or ‘OER’ means the reverse of the above half-reaction which results in the evolution of oxygen gas.
(22) In this specification the term ‘element’ means an element of the Periodic Table.
(23) In this specification the term ‘atom’ means an atom of an element of the Periodic Table. The term therefore generally includes the atom when in a charged (i.e. ionic) form and includes forms where the interaction with other atoms is totally ionic and forms where the interaction is partially covalent.
(24) In this specification the term ‘ion’ means an atom (as defined above) or a molecule comprising a plurality of atoms, when in a charged (ionic) form, and includes forms where the interaction with other atoms in the structure is totally ionic and forms where the interaction is partially covalent. The terms ‘monovalent’, ‘divalent’, ‘trivalent’ and ‘tetravalent’ when referring to metal ions, mean ions having an oxidation state of +1, +2, +3 and +4 respectively: the term ‘oxidation state’ taking its normal meaning in the art of the charge assigned to the atom after ionic approximation of its heteronuclear bonds (i.e. on the assumption that all heteronuclear bonding, irrespective of the actual nature of the bonding, is totally ionic).
(25) In this specification the term ‘metal’ includes both metallic elements and metalloid elements, preferably metallic elements.
(26) In this specification the term ‘redox couple’ takes its usual meaning in the art of a reducing species and its corresponding oxidizing form. Examples of redox couples include Mn.sup.3+/Mn.sup.4+ and Fe.sup.2+/Fe.sup.3+.
(27) In this specification the terms ‘cell’ means an apparatus capable of generating electricity through an electrochemical reaction. For the purposes of this specification it is equivalent to the term ‘battery’.
Advantages and Surprising Findings
(28) The composition typically consists essentially of a crystalline material, having the perovskite structure M.sup.1M.sup.2X.sub.3 described herein wherein the M.sup.1 ions occupy A-sites of the perovskite structure and the M.sup.2 ions occupy B-sites of the perovskite structure. Surprisingly, it has been found by the present inventors that when ions of a third metal M.sup.3, having a different oxidation state to the atoms of the metal M.sup.1, are introduced into the perovskite crystalline structure such that at least a portion of the metal atoms M.sup.3 substitute for the metal atoms M.sup.1 in the A-sites of the perovskite structure, and optionally at least a portion of the metal atoms M.sup.3 substitute for the metal atoms M.sup.2 in the B-sites of the perovskite structure, this causes the oxidation state of the M.sup.2 atoms to change to thereby give rise to a redox couple having a low overpotential for both the oxygen reduction reaction (ORR) and its reverse reaction, the oxygen evolution reaction (OER). This makes the material particularly suitable as a catalyst for use in fuel cells, particularly alkaline fuel cells. This would not have been predicted from the art, as there is generally known to be an anti-correlation between OER and ORR catalytic activity: as the compositional ranges of a particular system change, OER catalytic activity normally decreases as ORR catalytic activity increases and vice versa.
(29) In particular, it has been surprisingly found that, when the metal ions M.sup.1, M.sup.2 and M.sup.3 are present in the atomic ratios defined herein, the redox couple formed by the M.sup.2 ions is particularly favourable for both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). While this may be known in the art for perovskite crystalline structures having compositions along the pseudo-binary line (i.e. where all of the A-sites are fully occupied and all of the B-sites are fully occupied, such that the structure has 50% A-site ions and 50% B-site ions, giving an effective 1:1 ratio of A-site ions to B-site ions), it would not have been expected by the skilled person that structures having compositions in the atomic ratios specified herein, away from the pseudo-binary line, would exhibit favourable catalytic properties for both the ORR and the OER.
Composition
(30) The composition comprises, consists essentially of or consists of a perovskite crystalline structure having ions of at least three different metals M.sup.1, M.sup.2 and M.sup.3, together with ions of a chalcogen X, as described in more detail below.
(31) In this specification the term ‘perovskite structure’ generally means a crystal structure of general chemical formula M.sup.1M.sup.2X.sub.3, where M.sup.1 is a first metal ion, M.sup.2 is a second metal ion of different size to M.sup.1, and X is a chalcogen that bonds to both. In the perovskite structure, the M.sup.1 ions are larger than the M.sup.2 ions. In the idealized cubic unit cell of such a compound, the M.sup.1 ions sit at cube corner positions (0, 0, 0) (referred to herein as ‘A-sites’), the M.sup.2 ions sit at body-center positions (½, ½, ½) (referred to herein as ‘B-sites’) and chalcogen atoms sit at face centred positions (½, ½, 0). The ideal cubic structure has the M.sup.2 cation in 6-fold coordination, surrounded by an octahedron of anions, and the M.sup.1 cation in 12-fold cuboctahedral coordination.
(32) An example of an idealised perovskite structure is shown in
(33) In one embodiment, at least 90% by weight of the total weight of the composition has a perovskite structure. In one embodiment, at least 95% by weight of the total weight of the composition has a perovskite structure. In one embodiment, at least 96% by weight of the total weight of the composition has a perovskite structure. In one embodiment, at least 97% by weight of the total weight of the composition has a perovskite structure. In one embodiment, at least 98% by weight of the total weight of the composition has a perovskite structure. In one embodiment, at least 99% by weight of the total weight of the composition has a perovskite structure. In one embodiment, at least 99.5% by weight of the total weight of the composition has a perovskite structure. In one embodiment, at least 99.7% by weight of the total weight of the composition has a perovskite structure. In one embodiment, at least 99.9% by weight of the total weight of the composition has a perovskite structure.
(34) In this specification, when used to describe atomic sites in a crystalline structure generally, the term ‘A-site’ means the sites normally occupied by the larger M.sup.1 ions in the crystalline structure (and which are typically fully occupied by those atoms in the absence of metal ions M.sup.3). The ions M.sup.1 may be the same or different. Preferably, all ions M.sup.1 are the same.
(35) The ions M.sup.1 are larger than the ions M.sup.2. The preferred ratio of ionic sizes M.sup.1 to M.sup.2 is defined by the Goldschmidt tolerance factor t (Goldschmidt, V. M. Skr. Nor. Viedenk.-Akad., KI. I: Mater.-Naturvidensk. KI. 1926, No. 8), defined by the equation:
t=(r.sub.M1+r.sub.X)/√2(r.sub.M2+r.sub.X)
wherein r.sub.M1 is the empirical ionic radius of atom M.sup.1 in the oxidation state in which it is present in the structure; r.sub.M2 is the empirical ionic radius of atom M.sup.2 in the oxidation state in which it is present in the structure; and r.sub.x is the empirical ionic radius of atom X in the oxidation state in which it is present in the structure, all of the above being applicable at room temperature. In one embodiment, 0.75<t<1.0. In one embodiment, 0.85<t<1.0. In one embodiment, 0.9<t<1.0. In one embodiment, 0.95<t<1.0. In one embodiment, 0.97<t<1.0. In one embodiment, 0.99<t<1.0. For an ideal perovskite t is unity.
(36) Typical elements which may form the metal ion M.sup.1 include alkaline earth metal elements (such as Ca, Sr or Ba), rare earth elements (Sc, Y or a lanthanide, such as La, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb or Lu) or p-block elements (such as Ga, In, TI, Ge, Sn, Pb, As, Sb or Bi).
(37) In one embodiment, the metal ion M.sup.1, when present at the A-site of a perovskite structure, does not present relevant electrochemical behaviour at voltages within the range of the OER/ORR (typically between +1.0 V and +1.5 V, preferably between +1.1 V and +1.4 V, more preferably between +1.2 V and +1.3 V, still more preferably between +1.21 V and +1.25 V and most preferably between +1.22 V and +1.24 V).
(38) In one embodiment, M.sup.1 is a trivalent metal ion. In one embodiment, M.sup.1 is selected from the group consisting of Y, La, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sb and Bi. In one embodiment, M.sup.1 is a lanthanide metal ion (i.e. selected from the group consisting of La, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu). Preferably, M.sup.1 is La or Gd, more preferably La.
(39) In one embodiment, M.sup.1 is a divalent metal ion. In one embodiment, M.sup.1 is an alkaline earth metal ion. In one embodiment, M.sup.1 is selected from the group consisting of Sr and Ba. In one embodiment, M.sup.1 is Pb.
(40) The metal which forms the ion M.sup.2 is a different metal from the metal which forms the ions M.sup.1. The ions M.sup.2, in the oxidation state in which they are present in the crystalline structure have a smaller ionic radius than the ions M.sup.1 in the oxidation state in which they are present in the crystalline structure. In this specification, when used to describe ionic sites in a crystalline structure generally the term ‘B-site’ means the sites occupied by the smaller M.sup.2 ions in the crystalline structure. The ions M.sup.2 may be the same or different. Preferably, all ions M.sup.2 are the same.
(41) In the composition of the present invention, the atom which forms the ions M.sup.2 has two oxidation states capable of forming a redox couple suitable for reversibly catalyzing an oxygen reduction reaction (ORR) and an oxygen evolution reaction (OER). The change in oxidation state is caused by the presence in the A-sites of the ions M.sup.3 as described below.
(42) In one embodiment, the potential of the redox couple between the two oxidation states of the atom M.sup.2 is between +1.0 V and +1.5 V. In one embodiment, the potential of the redox couple between the two oxidation states of the atom M.sup.2 is between +1.1 V and +1.4 V. In one embodiment, the potential of the redox couple between the two oxidation states of the atom M.sup.2 is between +1.2 V and +1.3 V. In one embodiment, the potential of the redox couple between the two oxidation states of the atom M.sup.2 is between +1.21 V and +1.25 V. In one embodiment, the potential of the redox couple between the two oxidation states of the atom M.sup.2 is between +1.22 V and +1.24 V. These potentials are measured relative to that of the standard hydrogen electrode (at which the half-reaction follows the equation 2H.sup.++2e.sup.−.fwdarw.H.sub.2) which is assigned the potential of 0 V.
(43) Typically, the potential of this redox couple is measured at a pH of 7 to 14, preferably 10 to 14, more preferably 12 to 14, even more preferably 12.5 to 13.5, and most preferably 12.8 to 13.4.
(44) Typical elements which form the ions M.sup.2 include transition metal elements, such as first transition group elements (Ti, V, Cr, Mn, Fe, Co, Ni and Cu) and second transition metal group elements (Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag).
(45) In one embodiment, M.sup.2 is a trivalent metal ion capable of oxidation to a tetravalent oxidation state to form the redox couple. In one embodiment, M.sup.2 is Mn.
(46) In one embodiment, M.sup.2 is a divalent metal ion capable of oxidation to a trivalent oxidation state to form the redox couple. In one embodiment, M.sup.2 is Fe, Co, or Ni.
(47) In one embodiment, M.sup.2 is a tetravalent metal ion capable of reduction to a trivalent oxidation state to form the redox couple. In one embodiment, M.sup.2 is Ti.
(48) In one embodiment, M.sup.2 is a trivalent metal ion capable of reduction to a divalent oxidation state to form the redox couple. In one embodiment, M.sup.2 is Fe or Co.
(49) In the present invention the redox couple formed by the two oxidation states of M.sup.2 is typically suitable for reversibly catalyzing an oxygen reduction reaction (ORR) and an oxygen evolution reaction (OER). Preferably, such a redox couple will have the potentials (relative to the standard hydrogen electrode) within the ranges specified above, and in particular at the pH ranges listed above. As is well known to the person skilled in the art, a large number of possible redox couples have potentials within this range and are capable of performing this function. Examples of typical redox couples formed by atoms M.sup.2 include Ni.sup.2+/Ni.sup.3+, Mn.sup.3+/Mn.sup.4+, Fe.sup.2+/Fe.sup.3+ and Co.sup.2+/Co.sup.3+.
(50) In the compositions of the present invention, at least a portion of ions of the third metal M.sup.3 substitutes for M.sup.1 in the A-site of the perovskite crystalline structure. In one embodiment, at least 0.1% (as a proportion of the total metal atoms M.sup.1 and M.sup.3 in the A-sites of the composition) of M.sup.3 substitutes for M.sup.1 in the A-site of the perovskite crystalline structure. In one embodiment, at least 0.5% (as a proportion of the total metal atoms M.sup.1 and M.sup.3 in the A-sites of the composition) of M.sup.3 substitutes for M.sup.1 in the A-site of the perovskite crystalline structure. In one embodiment, at least 1% (as a proportion of the total metal atoms M.sup.1 and M.sup.3 in the A-sites of the composition) of M.sup.3 substitutes for M.sup.1 in the A-site of the perovskite crystalline structure. In one embodiment, at least 2% (as a proportion of the total metal atoms M.sup.1 and M.sup.3 in the A-sites of the composition) of M.sup.3 substitutes for M.sup.1 in the A-site of the perovskite crystalline structure. In one embodiment, at least 3% (as a proportion of the total metal atoms M.sup.1 and M.sup.3 in the A-sites of the composition) of M.sup.3 substitutes for M.sup.1 in the A-site of the perovskite crystalline structure. In one embodiment, at least 5% (as a proportion of the total metal atoms M.sup.1 and M.sup.3 in the A-sites of the composition) of M.sup.3 substitutes for M.sup.1 in the A-site of the perovskite crystalline structure. In one embodiment, at least 10% (as a proportion of the total metal atoms M.sup.1 and M.sup.3 in the A-sites of the composition) of M.sup.3 substitutes for M.sup.1 in the A-site of the perovskite crystalline structure. In one embodiment, at least 20% (as a proportion of the total metal atoms M.sup.1 and M.sup.3 in the A-sites of the composition) of M.sup.3 substitutes for M.sup.1 in the A-site of the perovskite crystalline structure. In one embodiment, at least 30% (as a proportion of the total metal atoms M.sup.1 and M.sup.3 in the A-sites of the composition) of M.sup.3 substitutes for M.sup.1 in the A-site of the perovskite crystalline structure. In one embodiment, at least 40% (as a proportion of the total metal atoms M.sup.1 and M.sup.3 in the A-sites of the composition) of M.sup.3 substitutes for M.sup.1 in the A-site of the perovskite crystalline structure. In one embodiment, at least 50% (as a proportion of the total metal atoms M.sup.1 and M.sup.3 in the A-sites of the composition) of M.sup.3 substitutes for M.sup.1 in the A-site of the perovskite crystalline structure. In one embodiment, at least 60% (as a proportion of the total metal atoms M.sup.1 and M.sup.3 in the A-sites of the composition) of M.sup.3 substitutes for M.sup.1 in the A-site of the perovskite crystalline structure. In one embodiment, at least 70% (as a proportion of the total metal atoms M.sup.1 and M.sup.3 in the A-sites of the composition) of M.sup.3 substitutes for M.sup.1 in the A-site of the perovskite crystalline structure. In one embodiment, at least 80% (as a proportion of the total metal atoms M.sup.1 and M.sup.3 in the A-sites of the composition) of M.sup.3 substitutes for M.sup.1 in the A-site of the perovskite crystalline structure. In one embodiment, at least 90% (as a proportion of the total metal atoms M.sup.1 and M.sup.3 in the A-sites of the composition) of M.sup.3 substitutes for M.sup.1 in the A-site of the perovskite crystalline structure. In one embodiment, at least 95% (as a proportion of the total metal atoms M.sup.1 and M.sup.3 in the a-sites of the composition) of M.sup.3 substitutes for M.sup.1 in the A-site of the perovskite crystalline structure. In one embodiment, at least 97% (as a proportion of the total metal atoms M.sup.1 and M.sup.3 in the A-sites of the composition) of M.sup.3 substitutes for M.sup.1 in the A-site of the perovskite crystalline structure. In one embodiment, at least 99% (as a proportion of the total metal atoms M.sup.1 and M.sup.3 in the A-sites of the composition) of M.sup.3 substitutes for M.sup.1 in the A-site of the perovskite crystalline structure.
(51) In one embodiment, from 0.1% to 99.9% (as a proportion of the total metal atoms M.sup.1 and M.sup.3 in the A-sites of the composition) of M.sup.3 substitutes for M.sup.1 in the A-site of the perovskite crystalline structure. In one embodiment, from 1% to 99% (as a proportion of the total metal atoms M.sup.1 and M.sup.3 in the A-sites of the composition) of M.sup.3 substitutes for M.sup.1 in the A-site of the perovskite crystalline structure. In one embodiment, from 2% to 98% (as a proportion of the total metal atoms M.sup.1 and M.sup.3 in the A-sites of the composition) of M.sup.3 substitutes for M.sup.1 in the A-site of the perovskite crystalline structure. In one embodiment, from 3% to 97% (as a proportion of the total metal atoms M.sup.1 and M.sup.3 in the A-sites of the composition) of M.sup.3 substitutes for M.sup.1 in the A-site of the perovskite crystalline structure. In one embodiment, from 5% to 95% (as a proportion of the total metal atoms M.sup.1 and M.sup.3 in the A-sites of the composition) of M.sup.3 substitutes for M.sup.1 in the A-site of the perovskite crystalline structure. In one embodiment, from 10% to 90% (as a proportion of the total metal atoms M.sup.1 and M.sup.3 in the A-sites of the composition) of M.sup.3 substitutes for M.sup.1 in the A-site of the perovskite crystalline structure. In one embodiment, from 15% to 85% (as a proportion of the total metal atoms M.sup.1 and M.sup.3 in the A-sites of the composition) of M.sup.3 substitutes for M.sup.1 in the A-site of the perovskite crystalline structure. In one embodiment, from 20% to 80% (as a proportion of the total metal atoms M.sup.1 and M.sup.3 in the A-sites of the composition) of M.sup.3 substitutes for M.sup.1 in the A-site of the perovskite crystalline structure. In one embodiment, from 30% to 70% (as a proportion of the total metal atoms M.sup.1 and M.sup.3 in the A-sites of the composition) of M.sup.3 substitutes for M.sup.1 in the A-site of the perovskite crystalline structure. In one embodiment, from 40% to 60% (as a proportion of the total metal atoms M.sup.1 and M.sup.3 in the A-sites of the composition) of M.sup.3 substitutes for M.sup.1 in the A-site of the perovskite crystalline structure.
(52) In addition to the ions of the third metal M.sup.3 which substitute for M.sup.1 in the A-site of the perovskite crystalline structure, optionally at least a portion of the ions M.sup.3 also substitutes for M.sup.2 in the B-site of the perovskite crystalline structure. In one embodiment, at least 0.1% (as a proportion of the total metal atoms M.sup.2 and M.sup.3 in the B-sites of the composition) of M.sup.3 substitutes for M.sup.2 in the B-site of the perovskite crystalline structure. In one embodiment, at least 0.5% (as a proportion of the total metal atoms M.sup.2 and M.sup.3 in the B-sites of the composition) of M.sup.3 substitutes for M.sup.2 in the B-site of the perovskite crystalline structure. In one embodiment, at least 1% (as a proportion of the total metal atoms M.sup.2 and M.sup.3 in the B-sites of the composition) of M.sup.3 substitutes for M.sup.2 in the B-site of the perovskite crystalline structure. In one embodiment, at least 2% (as a proportion of the total metal atoms M.sup.2 and M.sup.3 in the B-sites of the composition) of M.sup.3 substitutes for M.sup.2 in the B-site of the perovskite crystalline structure. In one embodiment, at least 3% (as a proportion of the total metal atoms M.sup.2 and M.sup.3 in the B-sites of the composition) of M.sup.3 substitutes for M.sup.2 in the B-site of the perovskite crystalline structure. In one embodiment, at least 5% (as a proportion of the total metal atoms M.sup.2 and M.sup.3 in the B-sites of the composition) of M.sup.3 substitutes for M.sup.2 in the B-site of the perovskite crystalline structure. In one embodiment, at least 10% (as a proportion of the total metal atoms M.sup.2 and M.sup.3 in the B-sites of the composition) of M.sup.3 substitutes for M.sup.2 in the B-site of the perovskite crystalline structure. In one embodiment, at least 20% (as a proportion of the total metal atoms M.sup.2 and M.sup.3 in the B-sites of the composition) of M.sup.3 substitutes for M.sup.2 in the B-site of the perovskite crystalline structure. In one embodiment, at least 30% (as a proportion of the total metal atoms M.sup.2 and M.sup.3 in the B-sites of the composition) of M.sup.3 substitutes for M.sup.2 in the B-site of the perovskite crystalline structure. In one embodiment, at least 40% (as a proportion of the total metal atoms M.sup.2 and M.sup.3 in the B-sites of the composition) of M.sup.3 substitutes for M.sup.2 in the B-site of the perovskite crystalline structure. In one embodiment, at least 50% (as a proportion of the total metal atoms M.sup.2 and M.sup.3 in the B-sites of the composition) of M.sup.3 substitutes for M.sup.2 in the B-site of the perovskite crystalline structure. In one embodiment, at least 60% (as a proportion of the total metal atoms M.sup.2 and M.sup.3 in the B-sites of the composition) of M.sup.3 substitutes for M.sup.2 in the B-site of the perovskite crystalline structure. In one embodiment, at least 70% (as a proportion of the total metal atoms M.sup.2 and M.sup.3 in the B-sites of the composition) of M.sup.3 substitutes for M.sup.2 in the B-site of the perovskite crystalline structure. In one embodiment, at least 80% (as a proportion of the total metal atoms M.sup.2 and M.sup.3 in the B-sites of the composition) of M.sup.3 substitutes for M.sup.2 in the B-site of the perovskite crystalline structure. In one embodiment, at least 90% (as a proportion of the total metal atoms M.sup.2 and M.sup.3 in the B-sites of the composition) of M.sup.3 substitutes for M.sup.2 in the B-site of the perovskite crystalline structure. In one embodiment, at least 95% (as a proportion of the total metal atoms M.sup.2 and M.sup.3 in the B-sites of the composition) of M.sup.3 substitutes for M.sup.2 in the B-site of the perovskite crystalline structure. In one embodiment, at least 97% (as a proportion of the total metal atoms M.sup.2 and M.sup.3 in the B-sites of the composition) of M.sup.3 substitutes for M.sup.2 in the B-site of the perovskite crystalline structure. In one embodiment, at least 99% (as a proportion of the total metal atoms M.sup.2 and M.sup.3 in the B-sites of the composition) of M.sup.3 substitutes for M.sup.2 in the B-site of the perovskite crystalline structure.
(53) In one embodiment, from 0.1% to 99.9% (as a proportion of the total metal atoms M.sup.2 and M.sup.3 in the B-sites of the composition) of M.sup.3 substitutes for M.sup.2 in the B-site of the perovskite crystalline structure. In one embodiment, from 1% to 99% (as a proportion of the total metal atoms M.sup.2 and M.sup.3 in the B-sites of the composition) of M.sup.3 substitutes for M.sup.2 in the B-site of the perovskite crystalline structure. In one embodiment, from 2% to 98% (as a proportion of the total metal atoms M.sup.2 and M.sup.3 in the B-sites of the composition) of M.sup.3 substitutes for M.sup.2 in the B-site of the perovskite crystalline structure. In one embodiment, from 3% to 97% (as a proportion of the total metal atoms M.sup.2 and M.sup.3 in the B-sites of the composition) of M.sup.3 substitutes for M.sup.2 in the B-site of the perovskite crystalline structure. In one embodiment, from 5% to 95% (as a proportion of the total metal atoms M.sup.2 and M.sup.3 in the B-sites of the composition) of M.sup.3 substitutes for M.sup.2 in the B-site of the perovskite crystalline structure. In one embodiment, from 10% to 90% (as a proportion of the total metal atoms M.sup.2 and M.sup.3 in the B-sites of the composition) of M.sup.3 substitutes for M.sup.2 in the B-site of the perovskite crystalline structure. In one embodiment, from 15% to 85% (as a proportion of the total metal atoms M.sup.2 and M.sup.3 in the B-sites of the composition) of M.sup.3 substitutes for M.sup.2 in the B-site of the perovskite crystalline structure. In one embodiment, from 20% to 80% (as a proportion of the total metal atoms M.sup.2 and M.sup.3 in the B-sites of the composition) of M.sup.3 substitutes for M.sup.2 in the B-site of the perovskite crystalline structure. In one embodiment, from 30% to 70% (as a proportion of the total metal atoms M.sup.2 and M.sup.3 in the B-sites of the composition) of M.sup.3 substitutes for M.sup.2 in the B-site of the perovskite crystalline structure. In one embodiment, from 40% to 60% (as a proportion of the total metal atoms M.sup.2 and M.sup.3 in the B-sites of the composition) of M.sup.3 substitutes for M.sup.2 in the B-site of the perovskite crystalline structure.
(54) The third metal which forms the ions M.sup.3 is a different metal from the metals which form the ions M.sup.1 and M.sup.2. The ions M.sup.3 may be the same or different. Preferably, all ions M.sup.3 are the same. Typically, the ions M.sup.3 are in a different oxidation state to the atoms M.sup.1. As detailed herein, in the compositions of the present invention, ions of the third atom M.sup.3 substitute for ions M.sup.1 in at least a portion of the A-sites of the perovskite crystalline structure, and optionally in at least a portion of the B-sites of the perovskite crystalline structure, and cause a change in the oxidation state of the atoms M.sup.2, thereby creating a redox couple in the atoms M.sup.2 suitable for catalyzing the ORR and OER reactions.
(55) Typical elements which form the ions M.sup.3 include alkaline earth metal elements (such as Mg or Ca), transition metal elements, such as first transition group elements (such as Ti, V, Cr, Mn, Fe, Co, Ni and Cu) and second transition metal group elements (such as Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd or Ag). In one embodiment, M.sup.3 is selected from the group consisting of Mg, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Y, Zr, Nb, Mo, Tc, Ru, Rh or Pd.
(56) In one embodiment, M.sup.3 is a divalent metal ion. In one embodiment, M.sup.3 is a trivalent metal ion. In one embodiment, M.sup.3 is a metal capable of exhibiting both divalent and trivalent oxidation states.
(57) In one embodiment, M.sup.3 is selected from the group consisting of Mg, Ca, Fe, Co and Ni. In one embodiment, M.sup.3 is selected from the group consisting of Fe, Co and Ni. In one embodiment, M.sup.3 is selected from the group consisting of Ca and Ni. In one embodiment, M.sup.3 is Ni. In one embodiment, M.sup.3 is Ca.
(58) In one embodiment, M.sup.1 is a trivalent metal and M.sup.2 is a trivalent metal capable of oxidation to a tetravalent oxidation state to form the redox couple. In one embodiment, M.sup.1 is a trivalent metal, M.sup.2 is a trivalent metal capable of oxidation to a tetravalent oxidation state to form the redox couple, and M.sup.3 is a divalent metal.
(59) In one embodiment, M.sup.1 is La and M.sup.2 is Mn. In one embodiment, M.sup.1 is La, M.sup.2 is Mn and M.sup.3 is selected from the group consisting of Mg, Ca, Fe, Co and Ni. In one embodiment, M.sup.1 is La, M.sup.2 is Mn and M.sup.3 is selected from the group consisting of Ca and Ni. In one embodiment, M.sup.1 is La, M.sup.2 is Mn and M.sup.3 is Ca. In one embodiment, M.sup.1 is La, M.sup.2 is Mn and M.sup.3 is Ni.
(60) The composition also comprises anions of an element X, which is a chalcogen atom (i.e. an atom of an element selected from O, S, Se and Te). Typically, X is O or S, preferably O.
(61) Typically, the element X is present in an amount such that the crystalline structure is electroneutral.
(62) In one embodiment the metal ions M.sup.1, M.sup.2 and M.sup.3 are present in an atomic ratio within the ranges (a): 25 to 49.9 atomic % M.sup.1, 30 to 60 atomic % M.sup.2, and 5 to 45 atomic % M.sup.3, each expressed as a percentage of the total metal ions in the composition excluding oxygen.
(63) In this embodiment, preferably M.sup.1 is present in an atomic ratio within the range 27 to 49 atomic %, expressed as a percentage of the total metal ions in the composition excluding oxygen. More preferably, M.sup.1 is present in an atomic ratio within the range 30 to 47 atomic %, expressed as a percentage of the total metal ions in the composition excluding oxygen. Even more preferably M.sup.1 is present in an atomic ratio within the range 35 to 45 atomic %, expressed as a percentage of the total metal ions in the composition excluding oxygen.
(64) In this embodiment, preferably M.sup.2 is present in an atomic ratio within the range 30 to 47 atomic %, expressed as a percentage of the total metal ions in the composition excluding oxygen. More preferably M.sup.2 is present in an atomic ratio within the range 35 to 45 atomic %, expressed as a percentage of the total metal ions in the composition excluding oxygen.
(65) In this embodiment, preferably M.sup.3 is present in an atomic ratio within the range 10 to 30 atomic %, expressed as a percentage of the total metal ions in the composition excluding oxygen. More preferably M.sup.3 is present in an atomic ratio within the range 15 to 25 atomic %, expressed as a percentage of the total metal ions in the composition excluding oxygen.
(66) In one embodiment, the metal ions M.sup.1, M.sup.2 and M.sup.3 are present in atomic ratios within the ranges (a1): 35 to 45 atomic % M.sup.1, 30 to 45 atomic % M.sup.2, and 15 to 25 atomic % M.sup.3; all expressed as a percentage of the total metal ions in the composition excluding oxygen.
(67) In one embodiment the metal ions M.sup.1, M.sup.2 and M.sup.3 are present in an atomic ratio within the ranges (b): 10 to 30 atomic % M.sup.1, 50.1 to 60 atomic % M.sup.2, and 25 to 45 atomic % M.sup.3; each expressed as a percentage of the total metal ions in the composition excluding oxygen.
(68) In this embodiment, preferably M.sup.1 is present in an atomic ratio within the range 12 to 27 atomic %, expressed as a percentage of the total metal ions in the composition excluding oxygen. More preferably, M.sup.1 is present in an atomic ratio within the range 15 to 25 atomic %, expressed as a percentage of the total metal ions in the composition excluding oxygen.
(69) In this embodiment, preferably M.sup.2 is present in an atomic ratio within the range 50.5 to 59 atomic %, expressed as a percentage of the total metal ions in the composition excluding oxygen. More preferably M.sup.2 is present in an atomic ratio within the range 51 to 58 atomic %, expressed as a percentage of the total metal ions in the composition excluding oxygen.
(70) In this embodiment, preferably M.sup.3 is present in an atomic ratio within the range 27 to 42 atomic %, expressed as a percentage of the total metal ions in the composition excluding oxygen. More preferably M.sup.3 is present in an atomic ratio within the range 25 to 35 atomic %, expressed as a percentage of the total metal ions in the composition excluding oxygen.
(71) In one embodiment, the metal ions M.sup.1, M.sup.2 and M.sup.3 are present in atomic ratios within the ranges (b1): 15 to 25 atomic % M.sup.1, 51 to 58 atomic % M.sup.2, and 25 to 35 atomic % M.sup.3; all expressed as a percentage of the total metal ions in the composition excluding oxygen.
(72) In another embodiment, there is provided a composition consisting essentially of a perovskite crystalline structure, having the formula:
M.sup.1.sub.xM.sup.2.sub.yM.sup.3.sub.zX.sub.w
(73) wherein:
(74) M.sup.1 is an ion which occupies an A-site of the perovskite crystalline structure;
(75) M.sup.2 is an ion which occupies a B-site of the perovskite crystalline structure, M.sup.2 having two oxidation states capable of forming a redox couple suitable for reversibly catalyzing an oxygen reduction reaction (ORR) and an oxygen evolution reaction (OER);
(76) M.sup.3 is an ion at least a portion of which substitutes for M.sup.1 in an A-site of the perovskite crystalline structure, and optionally at least a portion of which substitutes for M.sup.2 in a B site of the perovskite crystalline structure, at least some of the atoms M.sup.3 having a different oxidation state to the atoms M.sup.1;
(77) X is O or S;
(78) x is greater than 0 and not more than 2;
(79) y is greater than 0 and not more than 2;
(80) z is greater than 0 and not more than 2; and
(81) w is greater than 2, but not more than 4.
(82) wherein the ions M.sup.1, M.sup.2 and M.sup.3 are present in the atomic ratios specified herein.
(83) As will be readily understood by the person skilled in the art, the formulae above define the atomic (molar) ratios of each of the constituent metals M.sup.1, M.sup.2 and M.sup.3 relative to each other and to the chalcogen atom X. Thus a formula of M.sup.1.sub.xM.sup.2.sub.yM.sup.3.sub.zX.sub.w is the same as a formula of M.sup.1.sub.2xM.sup.2.sub.2yM.sup.3.sub.2zX.sub.2w, and so on.
(84) In one embodiment, x ranges from 0.5 to 1.3. In one embodiment, x ranges from 0.7 to 1.1. In one embodiment, x ranges from 0.8 to 0.9. In one embodiment, x ranges from 0.82 to 0.88. In one embodiment, x ranges from 0.84 to 0.86.
(85) In one embodiment, y ranges from 0.5 to 0.9. In one embodiment, y ranges from 0.6 to 0.8. In one embodiment, y ranges from 0.65 to 0.85. In one embodiment, y ranges from 0.68 to 0.72. In one embodiment; y ranges from 0.69 to 0.71. In one embodiment; z ranges from 0.2 to 0.7. In one embodiment, z ranges from 0.3 to 0.6. In one embodiment; z ranges from 0.4 to 0.5. In one embodiment, z ranges from 0.43 to 0.57.
(86) In one embodiment, x ranges from 0.25 to 0.45. In one embodiment, x ranges from 0.28 to 0.32. In one embodiment, x ranges from 0.29 to 0.31.
(87) In one embodiment, y ranges from 1.05 to 1.15. In one embodiment, y ranges from 1.08 to 1.12. In one embodiment, y ranges from 1.09 to 1.11.
(88) In one embodiment, z ranges from 0.55 to 0.65. In one embodiment, z ranges from 0.58 to 0.62. In one embodiment; z ranges from 0.59 to 0.61.
(89) In one embodiment, w ranges from 2.5 to 3.5. In one embodiment, w ranges from 2.7 to 3.3. In one embodiment, w ranges from 2.8 to 3.2. In one embodiment, w ranges from 2.9 to 3.
(90) In one embodiment, x ranges from 0.25 to 0.45; y ranges from 1.05 to 1.15; z ranges from 0.55 to 0.65; and w ranges from 2.8 to 3.2. In one embodiment, x ranges from 0.28 to 0.32; y ranges from 1.08 to 1.12; z ranges from 0.58 to 0.62; and w ranges from 2.8 to 3.2. In one embodiment, x ranges from 0.29 to 0.31; y ranges from 1.09 to 1.11; z ranges from 0.59 to 0.61; and w ranges from 2.8 to 3.2.
(91) In one embodiment, x ranges from 0.8 to 0.9; y ranges from 0.65 to 0.85; z ranges from 0.4 to 0.5; and w ranges from 2.8 to 3.2. In one embodiment, x ranges from 0.82 to 0.88; y ranges from 0.68 to 0.72; z ranges from 0.43 to 0.47; and w ranges from 2.8 to 3.2. In one embodiment, x ranges from 0.84 to 0.86; y ranges from 0.69 to 0.71; z ranges from 0.44 to 0.46; and w ranges from 2.8 to 3.2.
(92) In one embodiment, M.sup.1 is La, M.sup.2 is Mn, and X is O. In one embodiment, M.sup.1 is Sr, M.sup.2 is Ti, and X is O. In one embodiment, M.sup.1 is Ca, M.sup.2 is Ti, and X is O. In one embodiment, M.sup.1 is Pb, M.sup.2 is Ti, and X is O. In one embodiment, M.sup.1 is Bi, M.sup.2 is Fe, and X is O. In one embodiment, M.sup.1 is La, M.sup.2 is Yb, and X is O.
(93) In one embodiment, M.sup.1 is La, M.sup.2 is Mn, and X is O. In one embodiment, M.sup.1 is La, M.sup.2 is Mn, M.sup.3 is selected from the group consisting of Mg, Ca, Fe, Co and Ni and X is O. In one embodiment, M.sup.1 is La, M.sup.2 is Mn, M.sup.3 is selected from the group consisting of Ca and Ni, and X is O. In one embodiment, M.sup.1 is La, M.sup.2 is Mn, M.sup.3 is Ca and X is O. In one embodiment, M.sup.1 is La, M.sup.2 is Mn, M.sup.3 is Ni and X is O.
(94) In one embodiment, the composition comprises, consists essentially of or consists of a single phase. In contrast to the biphasic materials reported in Demina et al., the compositions comprise all of the ions M.sup.1, M.sup.2, M.sup.3 and X in a single phase. In contrast to these biphasic materials, for which Demina et al. does not provide any evidence of any electrochemical behaviour, the single phase materials of the present invention exhibit favourable electrochemical properties as described herein, in particular efficient catalysis of the OER and ORR reactions. This makes these materials particularly useful for use in cells, especially alkaline fuel cells.
(95) In another embodiment, there is provided a composition of formula:
M.sup.1.sub.xM.sup.2.sub.yM.sup.3.sub.zO.sub.w
(96) wherein:
(97) the composition consists essentially of a perovskite crystalline structure containing ions of M.sup.1, M.sup.2 and M.sup.3;
(98) M.sup.1 is an ion which occupies an A-site of the perovskite crystalline structure;
(99) M.sup.2 is an ion which occupies a B-site of the perovskite crystalline structure, M.sup.2 having two oxidation states capable of forming a redox couple suitable for reversibly catalyzing an oxygen reduction reaction (ORR) and an oxygen evolution reaction (OER);
(100) M.sup.3 is an ion at least a portion of which substitutes for M.sup.1 in an A-site of the perovskite crystal structure, and optionally at least a portion of which substitutes for M.sup.2 in a B-site of the perovskite crystal structure, at least some of the atoms M.sup.3 having a
(101) different oxidation state to the atoms M.sup.1;
(102) x is greater than 0 and not more than 2;
(103) y is greater than 0 and not more than 2;
(104) z is greater than 0 and not more than 2; and
(105) w is greater than 2, but not more than 4;
(106) wherein the presence of the M.sup.3 ions causes a change in the oxidation state of some of the M.sup.2 ions in the structure, thereby creating the redox couple suitable for reversibly catalyzing the ORR and OER; wherein the ions M.sup.1, M.sup.2 and M.sup.3 are present in the atomic ratios specified herein.
(107) In one embodiment, M.sup.1 is La, M.sup.2 is Mn, M.sup.3 is Ni and the composition comprises 30 to 50 atomic % La, expressed as a percentage of the total metal atoms in the composition excluding oxygen. In one embodiment, M.sup.1 is La, M.sup.2 is Mn, M.sup.3 is Ni and the composition comprises 35 to 45 atomic % La, expressed as a percentage of the total metal atoms in the composition excluding oxygen.
(108) In one embodiment, M.sup.1 is La, M.sup.2 is Mn, M.sup.3 is Ni and the composition comprises 30 to 60 atomic % Mn, expressed as a percentage of the total metal atoms in the composition excluding oxygen. In one embodiment M.sup.1 is La, M.sup.2 is Mn, M.sup.3 is Ni and the composition comprises 40 to 45 atomic % Mn, expressed as a percentage of the total metal atoms in the composition excluding oxygen.
(109) In one embodiment, M.sup.1 is La, M.sup.2 is Mn, M.sup.3 is Ni and the composition comprises 5 to 30 atomic % Ni, expressed as a percentage of the total metal atoms in the composition excluding oxygen. In one embodiment, M.sup.1 is La, M.sup.2 is Mn, M.sup.3 is Ni and the composition comprises 15 to 20 atomic % Ni, expressed as a percentage of the total metal atoms in the composition excluding oxygen.
(110) In one embodiment, M.sup.1 is La, M.sup.2 is Mn, M.sup.3 is Ni and the composition comprises 30 to 50 atomic % La, 30 to 60 atomic % Mn, and 5 to 30 atomic % Ni, all expressed as a percentage of the total metal atoms in the composition excluding oxygen.
(111) In one embodiment, M.sup.1 is La, M.sup.2 is Mn, M.sup.3 is Ni and the composition comprises 35 to 45 atomic % La, 40 to 45 atomic % Mn, and 15 to 20 atomic % Ni, all expressed as a percentage of the total metal atoms in the composition excluding oxygen. It has been surprisingly found that compositions having the above elements in these proportions exhibit a low overpotential in the oxygen reduction reaction (ORR) and simultaneously exhibit a low overpotential in the reverse reaction, the oxygen evolution reaction (OER). Reversible OER/ORR behaviour means that the material is particularly suitable as a catalyst for use in alkaline fuel cells.
(112) In one embodiment, M.sup.1 is La, M.sup.2 is Mn, M.sup.3 is Ni, x ranges from 0.8 to 0.9; y ranges from 0.65 to 0.85; z ranges from 0.4 to 0.5; and w ranges from 2.8 to 3.2. In one embodiment, M.sup.1 is La, M.sup.2 is Mn, M.sup.3 is Ni, x ranges from 0.83 to 0.87; y ranges from 0.68 to 0.72; z ranges from 0.43 to 0.47; and w ranges from 2.8 to 3.2. In one embodiment, M.sup.1 is La, M.sup.2 is Mn, M.sup.3 is Ni, x ranges from 0.84 to 0.86; y ranges from 0.69 to 0.71; z ranges from 0.44 to 0.46; and w ranges from 2.8 to 3.2. It has been surprisingly found that compositions having the above elements in these proportions exhibit a low overpotential in the oxygen reduction reaction (ORR) and simultaneously exhibit a low overpotential in the reverse reaction, the oxygen evolution reaction (OER). This excellent reversible OER/ORR behaviour means that the material is particularly suitable as a catalyst for use in alkaline fuel cells.
(113) In one embodiment, M.sup.1 is La, M.sup.2 is Mn, M.sup.3 is Ca and the composition comprises 10 to 30 atomic % La, expressed as a percentage of the total metal atoms in the composition excluding oxygen. In one embodiment, M.sup.1 is La, M.sup.2 is Mn, M.sup.3 is Ni and the composition comprises 15 to 20 atomic % La, expressed as a percentage of the total metal atoms in the composition excluding oxygen.
(114) In one embodiment, M.sup.1 is La, M.sup.2 is Mn, M.sup.3 is Ca and the composition comprises 40 to 60 atomic % Mn, expressed as a percentage of the total metal atoms in the composition excluding oxygen. In one embodiment, M.sup.1 is La, M.sup.2 is Mn, M.sup.3 is Ca and the composition comprises 50 to 55 atomic % Mn, expressed as a percentage of the total metal atoms in the composition excluding oxygen.
(115) In one embodiment, M.sup.1 is La, M.sup.2 is Mn, M.sup.3 is Ca and the composition comprises 20 to 45 atomic % Ca, expressed as a percentage of the total metal atoms in the composition excluding oxygen. In one embodiment, M.sup.1 is La, M.sup.2 is Mn, M.sup.3 is Ca and the composition comprises 25 to 30 atomic % Ca, expressed as a percentage of the total metal atoms in the composition excluding oxygen.
(116) In one embodiment, M.sup.1 is La, M.sup.2 is Mn, M.sup.3 is Ca and the composition comprises 10 to 30 atomic % La, 40 to 60 atomic % Mn, and 20 to 45 atomic % Ca, all expressed as a percentage of the total metal atoms in the composition excluding oxygen.
(117) In one embodiment, M.sup.1 is La, M.sup.2 is Mn, M.sup.3 is Ca and the composition comprises 15 to 20 atomic % La, 50 to 55 atomic % Mn, and 25 to 30 atomic % Ca, all expressed as a percentage of the total metal atoms in the composition excluding oxygen. It has been surprisingly found that compositions having the above elements in these proportions simultaneously exhibit a low overpotential in the oxygen reduction reaction (ORR) and a low overpotential in the reverse reaction, the oxygen evolution reaction (OER). This means that the material is particularly suitable as a catalyst for use in alkaline fuel cells.
(118) In one embodiment, M.sup.1 is La, M.sup.2 is Mn, M.sup.3 is Ca, x ranges from 0.25 to 0.45; y ranges from 1.05 to 1.15; z ranges from 0.55 to 0.65; and w ranges from 2.8 to 3.2. In one embodiment, M.sup.1 is La, M.sup.2 is Mn, M.sup.3 is Ca, x ranges from 0.28 to 0.32; y ranges from 1.08 to 1.12; z ranges from 0.58 to 0.62; and w ranges from 2.8 to 3.2. In one embodiment, M.sup.1 is La, M.sup.2 is Mn, M.sup.3 is Ca, x ranges from 0.29 to 0.31; y ranges from 1.09 to 1.11; z ranges from 0.59 to 0.61; and w ranges from 2.8 to 3.2. It has been surprisingly found that compositions having the above elements in these proportions exhibit a low overpotential in the oxygen reduction reaction (ORR) and simultaneously exhibit a low overpotential in the reverse reaction, the oxygen evolution reaction (OER). This excellent reversible OER/ORR behaviour means that the material is particularly suitable as a catalyst for use in alkaline fuel cells.
Methods
(119) The compositions of the invention can be prepared by a number of methods well known to those skilled in the art.
(120) Typically, the method comprises providing a source of each component element of the compound, wherein the sources comprise at least a source of atoms which form ions M.sup.1, a source of atoms which form ions M.sup.2, a source of atoms which form ions M.sup.3, and a source of atoms which form ions X; and depositing these onto a substrate, particularly although not exclusively a substrate heated to between about 50° C. and about 800° C. The component elements from the sources react on the substrate to form the composition consisting essentially of the perovskite crystalline structure containing at least M.sup.1, M.sup.2, M.sup.3 and X.
(121) Suitable substrates are well known to the person skilled in the art, and include metals (such as platinum, aluminium, titanium, chromium, iron, zinc, gold, silver, nickel, molybdenum, including alloys thereof, which may include non-metals such as carbon, examples of which include steels such as stainless steel), metal oxides, such as aluminium oxide, particularly conducting metal oxides such as indium tin oxide), silicon, silica, silicon oxide (including doped silicon oxide), aluminosilicate materials, glasses, and ceramic material.
(122) One general method used in accordance with one embodiment of the invention is a physical vapour deposition (PVD) method. According to this method, the crystalline composition is formed from the component elements M.sup.1, M.sup.2, M.sup.3 and X, by providing a vapour source of each component element of the compound and co-depositing the component elements from the vapour sources onto a substrate, typically a heated substrate.
(123) Accordingly, in this embodiment, there is provided a vapour deposition method comprising: providing a vapour source of each component element M.sup.1, M.sup.2, M.sup.3 and X, wherein the vapour sources comprise a source of M.sup.1, a source of M.sup.2, a source of M.sup.3 and a source of X, to deliver a flux of M.sup.1, a flux of M.sup.2, a flux of M.sup.3 and a flux of X; heating a substrate to between substantially 50° C. and 800° C.; depositing the component elements from the sources onto the heated substrate, wherein the component elements react on the substrate to form a crystalline structure of M.sup.1, M.sup.2, M.sup.3 and X, said crystalline structure consisting essentially of a perovskite structure
(124) The physical vapour deposition (PVD) method according to the invention typically involves co-depositing the component elements from the vapour sources onto a heated substrate. In one embodiment, the substrate is heated to about 150 to about 700° C. In one embodiment, the substrate is heated to about 200 to about 700° C. In one embodiment, the substrate is heated to about 500 to about 700° C. In one embodiment, the substrate is heated to about 500 to about 600° C. In one embodiment, the substrate is heated to about 530 to about 570° C.
(125) The physical vapor deposition (PVD) method according to the invention is typically carried out at a pressure of 1×10.sup.−7 to 1×10.sup.−4 Torr, preferably 1×10.sup.−6 to 5×10.sup.−5 Torr, and more preferably 5×10.sup.−6 to 2×10.sup.−5 Torr.
(126) A particularly preferred PVD method for forming the crystalline compounds of the invention is described in Guerin, S.; Hayden, B. E., “Physical Vapour Deposition Method for the High-Throughput Synthesis of Solid-State Material Libraries”, Journal of Combinatorial Chemistry 2006, 8 (1), 66-73.
(127) In the alternative, the method according to the invention may be carried out by sputtering. As is known to the person skilled in the art, sputter deposition is a physical vapour deposition (PVD) method of thin film deposition by sputtering. Sputtering involves ejecting material from one or more targets that is a source of the required element(s) and directing it onto a substrate to enable growth of the required material, typically as a film on the substrate. Typically, the sputtering targets comprise the desired element in the form combined with its respective anion in the desired material (e.g. for a desired material which is a mixed metal oxide, the sputtering targets typically comprise the oxides of each separate component metal).
(128) Therefore, in one embodiment, there is provided a method of producing the composition of the invention, wherein the method is a sputtering deposition method comprising:
(129) providing at least one sputtering target, at least one sputtering target comprising a source of M.sup.1, at least one sputtering target comprising a source of M.sup.2, at least one sputtering target comprising a source of M.sup.3, each sputtering target comprising M.sup.1, M.sup.2 or M.sup.3 combined with X; and sputtering said targets to produce a composition having a crystalline structure comprising M.sup.1, M.sup.2, M.sup.3 and X, said crystalline structure consisting essentially of a perovskite structure.
(130) In the alternative, the method according to the invention may be carried out by chemical vapour deposition (CVD). As is known to the person skilled in the art, chemical vapour deposition comprises providing a vapour source of each component element of the desired substance, wherein the sources comprise one or more precursor compounds containing the required element, and depositing the vaporised elements onto a heated substrate, typically by spraying. The component elements from the sources react on the substrate to form the desired material.
(131) Accordingly, there is further provided a method of producing the composition of the invention) wherein the method is a vapour deposition method comprising: providing a source of each component element of the compound, wherein the sources comprise one or more precursor compounds, at least one precursor compound containing M.sup.1, at least one precursor compound containing M.sup.2, at least one precursor compound containing M.sup.3, and at least one precursor compound containing X;
(132) heating a substrate to between about 200° C. and about 1000° C.;
(133) spraying the precursor compounds onto the heated substrate;
(134) wherein the component elements from the sources react on the substrate to form a crystalline composition containing M.sup.1, M.sup.2, M.sup.3 and X, said crystalline composition consisting essentially of a perovskite structure.
(135) In one embodiment, the substrate is heated to between 250 and 950° C. In one embodiment, the substrate is heated to between 300 and 600° C.
(136) In one embodiment, the CVD method according to the invention is carried out at a pressure of 0.1 to 500 Torr. In one embodiment, the CVD method according to the invention is carried out at a pressure of 1 to 100 Torr.
(137) In one embodiment, the composition of the present invention is formed as a nanoparticle. In this specification, the term ‘nanoparticle’ takes its normal meaning in the art as a particle having at least one dimension (such as two dimensions, such as three dimensions) between 1 and 100 nm. Suitably, the nanoparticle has at least one dimension (such as two dimensions, such as three dimensions) between 10 and 100 nm. Preferably, the nanoparticle has at least one dimension (such as two dimensions, such as three dimensions) between 10 and 50 nm.
(138) In this embodiment, the nanoparticle is typically supported on a support. Therefore, in another aspect, there is provided a system of the present invention comprising (a) a support; and (b) a composition of the present invention in the form or one or more nanoparticles supported on the support.
(139) In one embodiment, the support comprises a current collecting material, typically a metal, a conducting metal oxide or graphite. In one embodiment, the current collecting material is selected from the group consisting of: platinum, aluminium, titanium, chromium, iron, zinc, gold, silver, nickel, molybdenum, tin oxide, indium tin oxide and stainless steel. In one embodiment, the support is a non-conducting support. In one embodiment, the substrate is selected from the group consisting of: silicon oxide, aluminium oxide, an aluminosilicate material, a glass, and a ceramic material.
(140) The present invention also provides for “core-shell” materials comprising a core particle of a material; and a shell on the core particle.
(141) In one embodiment, the composition of the present invention is formed as a core particle. In one embodiment, the particle has a diameter of 10-50 nm, more preferably 10-25 nm.
(142) In one embodiment, there is provided a core-shell composition comprising (a) a core particle comprising a first composition; and (b) a second, different composition applied as a shell on the core particle, wherein the first composition is a composition of the present invention.
(143) In another embodiment, there is provided a core-shell composition comprising (a) a core particle comprising a first composition; and (b) a second, different composition applied as a shell on the core particle, wherein the second composition is a composition of the present invention.
(144) In one embodiment, the composition which is not the composition of the invention comprises, consists essentially of or consists of a substance capable of catalysing electrochemical reactions, particularly the oxygen reduction reaction (ORR) and/or the oxygen evolution reaction (OER). In one embodiment, this composition comprises, consists essentially of or consists of a metal or conducting metal oxide. In one embodiment, this composition comprises, consists essentially of or consists of platinum or platinum alloy.
(145) In one embodiment, there is provided a core-shell composition comprising (a) a core particle comprising a first composition of the present invention; and (b) a second composition applied as a shell on the core particle, the second composition being a second, different composition of the present invention.
(146) Preferably, the second composition comprises one monolayer (ML) or more of a substance capable of catalysing electrochemical reactions. Even more preferably, the catalyst comprises 1.5 ML or more of this substance.
(147) Preferably, the second composition comprises 20 ML or fewer of a substance capable of catalysing electrochemical reactions, more preferably 15 ML or fewer, most preferably 10 ML or fewer.
(148) Preferably, the second composition is applied to the core particle in a thickness of from 1 to 5 ML; more preferably 1.5 to 5.
(149) In another embodiment, there is provided a method of producing a composition, the method comprising forming a core particle comprising a first composition; and forming a shell comprising at least one monolayer of a second composition on the core particle.
(150) In another embodiment, there is provided a method of producing a core-shell composition, the method comprising forming a core particle comprising a first composition; and forming a shell comprising at least one monolayer of a second composition on the core particle, wherein the first composition is a composition of the present invention.
(151) In another embodiment, there is provided a method of producing a core-shell composition, the method comprising forming a core particle comprising a first composition; and forming a shell comprising at least one monolayer of a second composition on the core particle, wherein the second composition is a composition of the present invention.
(152) In another embodiment, there is provided a method of producing a core-shell composition, the method comprising forming a core particle comprising a first composition; and forming a shell comprising at least one monolayer of a second composition on the core particle, wherein the first composition is a composition of the present invention, and the second composition is a composition of the present invention, the first and second compositions being different.
Applications
(153) The crystalline composition of the present invention is particularly useful as a catalyst for electrochemical reactions, particularly the oxygen reduction reaction (ORR) and/or the oxygen evolution reaction (OER).
(154) Accordingly, in a further aspect the present invention comprises use of the composition of the invention as a catalyst. In one embodiment, the invention comprises use of the composition of the invention as a catalyst for the oxygen reduction reaction (ORR) and/or the oxygen evolution reaction (OER).
(155) In a further aspect the present invention comprises a catalyst comprising the composition of the invention. In one embodiment, the invention comprises use of the composition of the invention as a catalyst for the oxygen reduction reaction (ORR) and/or the oxygen evolution reaction (OER).
(156) In particular, the composition of the invention can be used as a catalyst for use in electrodes, particularly for use in cells such as electrochemical cells and fuel cells.
(157) Accordingly, according to a further aspect the invention further provides an electrode comprising the composition of the invention.
(158) Typically, in the electrode of the invention, the composition is supported on a support. In one embodiment, the composition forms a layer on the support. In one embodiment, the composition forms a film on the support.
(159) Typically, the method involves forming the crystalline composition in the form of a film, typically on a substrate (as defined in more detail below). In one embodiment, the thickness of the film is from 1 nm to 10 μm. In one embodiment, the thickness of the film is from 2 nm to 5 μm. In one embodiment, the thickness of the film is from 5 nm to 2 μm. In one embodiment, the thickness of the film is from 10 nm to 1 μm. In one embodiment, the thickness of the film is from 20 nm to 500 nm. In one embodiment, the thickness of the film is from 50 nm to 200 nm.
(160) Typically, the support is a substrate on which a current collecting material is supported. In one embodiment, the current collecting material is a metal or a conducting metal oxide. In one embodiment, the current collecting material is selected from the group consisting of: platinum, aluminium, titanium, chromium, iron, zinc, gold, silver, nickel, molybdenum, tin oxide, indium tin oxide and stainless steel.
(161) Typically, the substrate is an inert substrate. In one embodiment, the substrate is selected from the group consisting of: silicon, silicon oxide, aluminium oxide, an aluminosilicate material, doped silicon oxide, a glass, a metal, and a ceramic material.
(162) In one embodiment, the substrate is a silicon substrate. In one embodiment, the silicon substrate is covered by one or more passivation layers. In one embodiment, at least one passivation layer comprises silicon dioxide. In one embodiment, at least one further passivation layer comprises silicon nitride.
(163) In one embodiment, an adhesion layer is present between the current collector and substrate. In one embodiment, the adhesion layer is selected from a metal and a metal oxide. In one embodiment, the adhesion layer is selected from the group consisting of: titanium oxide, titanium, zirconium and chromium.
(164) The composition of the present invention may usefully be used in a cell, particularly an electrochemical cell.
(165) Accordingly, to a further aspect of the invention there is provided an electrochemical cell comprising: an electrolyte; an anode; and a cathode; wherein the anode and/or the cathode comprises an electrode according to the present invention.
(166) In one embodiment, the cell is a metal-air cell. As is known to the person skilled in the art, in a metal-air electrochemical cell, a metal is oxidised at the anode and oxygen is reduced at the cathode to induce a current flow. Typical metal-air cells include lithium-air, sodium-air, potassium-air, magnesium-air, calcium-air, aluminium-air, iron-air, zinc-air, aluminium-air, silicon-air, germanium-air and tin-air.
(167) In one embodiment, the cell is a lithium-air cell. As is known to the person skilled in the art, the lithium-air cell is a metal-air electrochemical cell in which lithium is oxidised at the anode and oxygen is reduced at the cathode to induce a current flow.
Fuel Cells
(168) The composition of the present invention may usefully be used in a fuel cell, particularly an alkaline fuel cell.
(169) Therefore, in a further aspect, the invention comprises a fuel cell comprising: an anode;
(170) a cathode;
(171) an electrolyte;
(172) wherein the anode and/or the cathode includes a catalyst comprising a composition according to the present invention.
(173) Typically, the fuel cell further comprises a source of fuel; and/or a source of oxidant.
(174) The fuel may be any fuel commonly used in fuel cells. In one embodiment, the fuel comprises hydrogen. In one embodiment, the fuel comprises an organic compound. Typical examples of organic compounds used in fuel cells include alkanes such as methane and ethane; alcohols such as methanol and ethanol, and carboxylic acids such as formic acid.
(175) In one embodiment, the fuel cell is a hydrogen fuel cell. In hydrogen fuel cells, hydrogen gas is oxidised and oxygen gas reduced to form water, with an electrical current produced as a by-product of the redox reaction. In this embodiment, the fuel cell further comprises a source of hydrogen-containing fuel, and a source of oxygen.
(176) In one embodiment, the fuel cell is an alkaline fuel cell. As with other hydrogen fuel cells, hydrogen gas is oxidised and oxygen gas reduced to form water, with an electrical current produced as a by-product of the redox reaction. In this embodiment, hydroxide anions OH.sup.− flow through the electrolyte from the cathode to the anode.
(177) Preferably, the fuel cell is a regenerative fuel cell, that is, it is capable of functioning both as a fuel cell, which generates electricity from fuel and oxidant, and as an electrolyser, which consumes electricity to generate fuel and oxidant.
EXAMPLES
(178) To understand better the role of A and B-site substitution, and the role of A-site and B-site sub-stoichiometry, in Mn based perovskites, we have investigated both the ORR and OER activities of the La.sub.1−xMn.sub.yNi.sub.1−y+xO.sub.3−δ and La.sub.x−zCa.sub.1−x+wMn.sub.y+z+wO.sub.3−δ perovskites. The intrinsic activity of continuous thin film catalysts has been measured to exclude any effects of a carbon-based support, which may influence the reaction in the alkali environment. We have applied a combinatorial methodology employing metal evaporative sources combined with an oxygen plasma atom source to produce perovskite structures which can be screened on chip for ORR and OER activity.
General Experimental
(179) Compositional gradient thin film libraries were deposited using a High Throughput Physical Vapour Deposition (HT-PVD) system described in more detail in Guerin, S.; Hayden, B. E., Physical Vapor Deposition Method for the High-Throughput Synthesis of Solid-State Material Libraries. Journal of Combinatorial Chemistry 2006, 8 (1), 66-73.
(180) The constituent elements were deposited simultaneously using multiple off-axis Knudsen cell (k-cell) or electron beam (e-beam) sources, each source independently shadowed by a partial “wedge” shutter which controls the flux distribution across the substrate. The position of the wedge shutters controls the compositional region being deposited on the substrate. Lanthanum (Absco 99.95%) was deposited from either a high temperature k-cell or an e-beam, nickel (Testborne 99.99%) was deposited from an e-beam: calcium (Alfa Aesar 99.5%) and manganese (Alfa Aesar 99.95%) were deposited from a k-cell. Oxygen atoms were co-deposited at the substrate by means of a plasma atom source (Oxford Applied Research or Mantis). Simultaneous mixing of the elements provides a low kinetic energy pathway to solid state oxide formation enabling a low temperature route to amorphous and crystalline stoichiometric oxides. Shadow masks were used to define confine oxide growth to discrete fields on the electrochemical chip.
(181) The thin film libraries were deposited onto 35×35 mm Si/SiO substrates (<100> silicon with 100 nm thermal silicon oxide) for composition and X-ray diffraction measurements. For electrochemical measurements, depositions were made on an electrochemical screening chip based on the design described in Guerin, S. et al. J Phys Chem B 2006, 110 (29), 14355-62, but with indium tin oxide (ITO) tracks and pads. The ITO conductive layer was chosen for stability and electrochemical inactivity, and the connecting tracks were passivated by a layer of silicon oxide. All depositions were made with the substrate at 550° C. which was sufficient to produce the crystalline perovskite phases.
(182) Elemental composition was measured by Energy-Dispersive X-ray Spectroscopy (EDX) (Tescan Vega 3 LMU SEM incorporating an Oxford Instruments X-Max 50, 50 mm.sup.2 large area SDD detector). The composition and thickness of the films were derived using LayerProbe software from Oxford Instruments. Crystal structures were determined by X-ray diffraction (XRD) (Bruker D8 diffractometer with GADDS detector and an Incoatec Cu K.sub.α spot X-ray source).
(183) Electrochemical measurements were carried out using a glass cell, instrumentation and software specifically developed for high-throughput electrochemical screening (as described in Guerin, S.; et al. Journal of Combinatorial Chemistry 2004, 6 (1), 149-158).
(184) Combining the use of a single channel potentiostat and a multi-channel current follower, fast sequential monitoring of the electrode currents is achieved. The electrochemical measurements were performed in a three-compartment glass cell, especially designed to accommodate the high throughput array. A reference electrode (Hg/HgO, Sentek) was mounted in a Luggin capillary with a tip situated a few millimetres away from the electrochemical array. The counter electrode (a platinum mesh of area˜4 cm.sup.2 and pitch 0.5 mm) was separated from the array under test by a glass sinter. All experiments were conducted in 300 ml of 0.1 M KOH (>99%, Sigma-Aldrich) solution the solution was either used saturated by oxygen or deoxygenated using argon bubbled through the cell.
Example 1—LaMnNiO Compositions
(185) X-ray diffraction patterns were recorded (2θ values between 21° and)52° on both Si/SiO and electrochemical substrates: The structure for similar compositions was found to be the same irrespective of the substrate. XRD patterns were collected with 2θ values between 21° and 52°, within this range only 6 clearly defined peaks were found. A series of XRD measurements taken along the pseudo-binary line indicate that the perovskite is synthesised exclusively at all compositions and characterised by the four Bragg peaks (100), (110), (111) and (200) (
(186)
(187) It is evident that the La.sub.1−xMn.sub.yNi.sub.1−y+xO.sub.3−δ perovskites can accommodate a wide range of Mn/La sub-stoichiometry.
(188) As described below, it is this compositional region according to the present invention in which both the Mn.sup.3+/Mn.sup.4+ redox is evident and the perovskites exhibit the most reversible ORR/OER activity. In the absence of dissolved oxygen, cyclic voltammetry (0.1 M KOH electrolyte, 5 mVs.sup.−1 scan rate and room temperature) was used to identify the main redox couples associated with the La.sub.1−xMn.sub.yNi.sub.1−y+xO.sub.3−δ perovskites. Some examples of these are shown in
(189) The compositional dependence of the charge associated with the Mn.sup.3+/Mn.sup.4+ couple centred at 1.0V.sub.RHE (manifested as broad anodic and cathodic peaks separated by 200 mV) was assessed by integrating the anodic peak over the potential range 0.9<V.sub.RHE<1.15 and is shown in
(190) The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) were measured on the catalyst libraries using cyclic voltammetry. Examples of ORR activities in cathodic sweeps of a series of catalysts with pseudo-binary compositions measured in oxygen saturated electrolyte are shown in
(191) Examples of OER activities in anodic sweeps of a series of catalysts with pseudo-binary compositions are shown in
(192) The compositional dependence of the ORR and OER activities along the pseudo-binary compositions La.sub.1−xMn.sub.yNi.sub.1−y+xO.sub.3−δ highlight that there is, in general, an anti-correlation in the activities of the catalysts: Overpotentials for OER are lowest for Ni rich compositions, and overpotentials for ORR are lowest for Mn rich compositions. It is also clear, by comparison with the main redox activities of Ni and Mn, shown in
(193) There is, however, a region of compositional space according to the present invention that evidently does not show this anti-correlation in the ORR and OER activities (
(194)
(195) Striking is the correlation between catalyst compositions which exhibit the most reversible OER/ORR activities (
(196) The association of the Mn.sup.3+/Mn.sup.4+ redox couple with ORR activity is consistent with the observation that the oxidation state on manganese oxides which can result in this couple, and increase ORR activity. The uniqueness of the ability of the Mn.sup.3+/Mn.sup.4+ to also sustain reversible ORR/OER is also consistent with the arguments that it is OH adsorbed at the Mn.sup.3+ site which is active in ORR, and O adsorbed at Mn.sup.4+ for the OER activity. It will therefore be the proximity of the Mn.sup.3+/Mn.sup.4+ to the ORR/OER equilibrium potential together with the reversibility of the surface redox which will determine the ultimate reversibility of this reaction at such a bifunctional site.
(197) The explanation for the appearance of the Mn.sup.3+/Mn.sup.4+ redox couple and the concomitant ORR/OER reversible activity at an A-site sub-stoichiometry compositions in La.sub.1−xMn.sub.yNi.sub.1−y+xO.sub.3−δ can be explained for this B-site substituted perovskite if one assumes that Ni.sup.3+ can not only be incorporated at the B-site, but can be incorporated as Ni.sup.2+ at the A-site (Krishnan et al. above). This is equivalent to the classical divalent substitution of the La A-site which is known to achieve charge balance by inducing similar concentrations of Mn.sup.4+ (Zener, C., Physical Review 1951, 82 (3), 3). It is apparently the creation of Mn.sup.4+ in addition to Mn.sup.3+ in a mixed valence system that is important in introducing the Mn.sup.3+/Mn.sup.4+ redox couple. This model is also consistent with the observation that the optimum catalyst has a composition around La.sub.0.85Mn.sub.0.7Ni.sub.0.45O.sub.3−δ:
Example 2—LaMnCaO Compositions
(198) In order to test the thesis that A-site sub-stoichiometry of Mn based perovskites can lead to the appearance of the Mn.sup.3+/Mn.sup.4+ redox couple, and a concomitant increase in the ORR/OER reversible activity, we have also investigated the La.sub.x−zCa.sub.1−x+wMn.sub.y+z+wO.sub.3−δ perovskites. Without wishing to be bound by theory, it is thought that the inclusion of Ca should result in a direct substitution of the La A-site by the divalent cation.
(199) A series of the La.sub.x−zCa.sub.1−x+wMn.sub.y+z+wO.sub.3−δ perovskite libraries were synthesised, and characterised by X-ray diffraction on both Si\SiO and electrochemical substrates for the La.sub.x−zCa.sub.1−x+wMn.sub.y+z+wO.sub.3−δ. The XRD patterns were collected with 2θ values between 21° and 52°: within this range only 4 clearly defined peaks were found (
(200)
(201) In the absence of dissolved oxygen, three manganese redox couples can be identified in the cyclic voltammetry (
(202) The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) were measured on the catalyst libraries using cyclic voltammetry. Examples of ORR activities (measured in cyclic voltammetry) in cathodic sweeps of a series of catalysts with pseudo-binary compositions measured in oxygen saturated electrolyte are shown in
(203) Examples of OER activities (measured in cyclic voltammetry) in anodic sweeps of a series of catalysts with pseudo-binary compositions measured in oxygen free electrolyte are shown in
(204) However, a decrease in the overpotential for the OER is observed in La.sub.x−zCa.sub.1−x+wMn.sub.y+z+wO.sub.3−δ for concentrations above ca. 20% Ca: a continuing reduction in ORR overpotential is also observed. This results in a region of La.sub.x−zCa.sub.1−x+wMn.sub.y+z+wO.sub.3−δ compositions according to the present invention which exhibit a high activity for both OER and ORR reactions. This effect is again best exemplified in a plot of the difference between the OER and ORR potential as a function of composition: this reflects the effectiveness of the catalyst in the reversible reaction. This is shown in
(205) The thin film combinatorial synthetic methodology as described above can therefore be successfully to synthesise libraries of hundreds of thin film La.sub.x1−xMn.sub.yNi.sub.1−y+xO.sub.3−δ (Example 1) and La.sub.x−zCa.sub.1−x+wMn.sub.y+z+wO.sub.3−δ (Example 2) perovskite electrocatalysts. These two ternary perovskites are examples of B-site and A-site substitution along the pseudo-binary composition lines, respectively.
(206) X-ray diffraction has been used to characterise the perovskite structures of both of the above Examples. In both cases, XRD shows there is a large region of compositions away from these lines where a perovskite structure is observed. In the case of La.sub.1−xMn.sub.yNi.sub.1−y+xO.sub.3−δ there appears to be a continual distortion of the LaMnO.sub.3 cubic structure to a rhombohedral LaNiO.sub.3 structure along the tie-line. There is, however, evidence of some A-site substitution of Mn.sup.2+ in the LaMnO.sub.3, and more extensive A-site substitution of Ni.sup.2+ in La sub-stoichiometric compositions of La.sub.1−xMn.sub.yNi.sub.1−y+xO.sub.3−δ. In the case of the La.sub.x−zCa.sub.1−x+wO.sub.3−δ perovskites, there appears to be a continual distortion of the LaMnO.sub.3 cubic structure (Pm-3m) to an orthorhombic structure (Pnma) in La.sub.xCa.sub.1−xMn.sub.yO.sub.3−δ, and with a change back to a cubic (Pm-3m) structure in CaMnO.sub.3.
(207) High throughput electrochemical screening has been undertaken a systematic study of the surface redox behaviour, and the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) on the synthesised perovskites. In the case of La.sub.1−xMn.sub.yNi.sub.1−y+xO.sub.3−δ compositions along the tie-line (Example 1), there is an anti-correlation in the ORR and OER activities. We identify, however, a highly preferred region of catalyst compositions according to the present invention with lanthanum A-site sub-stoichiometry where both high ORR and OER activity is observed, and this also correlates with the appearance of the Mn.sup.3+/Mn.sup.4+ redox couple. The optimal reversible ORR/OER activity is observed for La.sub.0.85Mn.sub.0.7Ni.sub.0.45O.sub.3−δ. This compositional region of reversible ORR/OER electro-catalytic activity and Mn.sup.3+/Mn.sup.4+ redox activity also corresponds to compositions where A-site substitution of La.sup.3+ by Ni.sup.2+ was suggested based on the X-ray diffraction results.
(208) Without wishing to be bound by theory, it is suggested that the appearance of the Mn.sup.3+/Mn.sup.4+ couple is associated with the creation of Mn.sup.4+ in the bulk as Mn.sup.3+ is substituted by Ni.sup.2+ at the A-site to create a mixed valent system.
(209) To support this interpretation, we also show (Example 2) that a direct A-site substitution of La.sup.3+ by the divalent Ca.sup.2+ cation in La.sub.x−zCa.sub.1−x+wMn.sub.y+z+wO.sub.3−δ perovskites, which is expected to result in the mixed valent Mn.sup.2+ and Mn.sup.4+ perovskite, also results in the appearance of the Mn.sup.3+/Mn.sup.4+ redox couple in cyclic voltammetry centred near to the tie-line, with the maximum charge observed for La.sub.0.3Ca.sub.0.6Mn.sub.1.1O.sub.3−δ. We also show that the compositional distribution of catalysts which exhibit the most reversible ORR/OER electrocatalysts also correlates with the presence of the Mn.sup.3+/Mn.sup.4+ redox couple.
(210) In these two La-Mn based perovskite systems investigated, therefore, La A-site substitution by a divalent cation (either Ni.sup.2+ or Ca.sup.2+) which results in the creation of a mixed valent Mn.sup.3+ and Mn.sup.4+ bulk lattice also results in the appearance of the Mn.sup.3+/Mn.sup.4+ redox couple. The appearance of the Mn.sup.3+/Mn.sup.4+ redox couple is also shown to be responsible for the creation of the most active reversible ORR/OER electrocatalysts according to the present invention. These observations are consistent with ab initio calculations on Mn oxides which predict the active site for ORR being HO* on Mn.sup.3+ and for OER is O* on Mn.sup.4+. It is this single bifunctional site mediating the Mn.sup.3+/Mn.sup.4+ redox couple in the perovskites resulting in the most reversible electrocatalysis. Mixed valent Mn.sup.3+ and Mn.sup.4+ in the bulk lattice of the perovskite appears to be sufficient to generate this active site. It is the proximity of the potential of the Mn.sup.3+/Mn.sup.4+ redox couple to the reversible ORR/OER potential in the alkali environment which results in the reversible activity. Without wishing to be bound by theory, it is suggested that a similar strategy of A-site substitution could be more widely used in producing mixed cation occupation at the B-site of perovskites generally, and the resulting couple tuned to support single (bifunctional) site reversible ORR/OER activity.
(211) All publications mentioned in the above specification are herein incorporated by reference. Various modifications and variations of the described methods and system of the present invention will be apparent to those skilled in the art without departing from the scope and spirit of the present invention. Although the present invention has been described in connection with specific preferred embodiments, it should be understood that the invention as claimed should not be unduly limited to such specific embodiments. Indeed, various modifications of the described modes for carrying out the invention which are obvious to those skilled in chemistry and materials science or related fields are intended to be within the scope of the following claims.