Method for preparing a catalyst mediating H2 evolution, said catalyst and uses thereof

Abstract

The present invention concerns a method for the preparation of a catalyst onto a solid support of a (semi-)conductive material consisting in depositing said catalyst onto said support from a near-neutral aqueous solution containing at least one nickel or cobalt organic complex and at least one basic oxoanion, by a method selected in the group consisting of reductive electrodeposition, photochemical electrodeposition and photoelectrochemical deposition. The present invention also concerns said catalyst and uses thereof.

Claims

1. A method for the preparation of a catalyst onto a solid support of a conductive or semiconductive material, the method comprising: depositing said catalyst onto said solid support from a near-neutral aqueous solution containing at least one nickel or cobalt organic complex and at least one basic oxoanion, using a method selected from the group consisting of reductive electrodeposition, photochemical electrodeposition, and photoelectrochemical deposition, wherein the catalyst consists of elemental cobalt/nickel covered or coated by a cobalt/nickel oxo/hydroxo-oxoanion layer.

2. The method according to claim 1, wherein the conductive or semiconductive material of said solid support is selected from the group consisting of a metallic material, a carbon material, a semiconductor or conductor metal oxide, nitride, and chalcogenide.

3. The method according to claim 1, wherein the conductive or semiconductive material of said solid support is selected from the group consisting of silicon, brass, stainless steel, iron, copper, nickel, cobalt, aluminium, silver, gold, titanium, carbon black, single or multi-walled carbon nanotubes (CNT), fullerenic nanoparticles, graphite, glassy carbon, graphene, reduced graphene oxide, doped diamond, TiO.sub.2, NiO, ZnO, ZrO.sub.2, ITO, SnO.sub.2, WO.sub.3, Fe.sub.2O.sub.3, BiVO.sub.4, Ta.sub.2O.sub.5, Ta.sub.3N.sub.5, TaON, ZnS, ZnSe, CdS, CdSe, CdTe, ZnTe, and composites of these materials, possibly doped with other elements.

4. The method according to claim 1, wherein the near-neutral aqueous solution has a pH of between 5 and 9.

5. The method according to claim 1, wherein the near-neutral aqueous solution has a pH of between 6 and 8.

6. The method according to claim 1, wherein the near-neutral aqueous solution has a pH of between 6.3 and 7.7.

7. The method according to claim 1, wherein the near-neutral aqueous solution has a pH of between 6.5 and 7.5.

8. The method according to claim 1, wherein the near-neutral aqueous solution has a pH of between 6.7 and 7.3.

9. The method according to claim 1, wherein the near-neutral solution is an aqueous solution having a pH of 7.

10. The method according to claim 1, wherein said cobalt or nickel organic complex is selected from the group consisting of cobalt or nickel dioxime/diimine complex; and cobalt or nickel amine/imine/pyridine complex.

11. The method according to claim 1, wherein said cobalt or nickel organic complex is selected from the group consisting of [Co(DO)(DOH)pnCl.sub.2] with (DOH)(DOH)pn representing N.sup.2,N.sup.2-propanediylbis(2,3-butandione 2-imine 3-oxime); [Co(DO)(DOH)pnBr.sub.2]; [Co((DO).sub.2BF.sub.2)pnBr.sub.2] with ((DO).sub.2BF.sub.2)pn representing N.sup.2,N.sup.2-propanediylbis(2,3-butandione 2-imine 3-oximato)-N.sup.1,N.sup.1-difluoroboryl [Co(MO)(MOH)pnCl.sub.2] with (MOH)(MOH)pn representing N.sup.2,N.sup.2-propanediylbis(1,2-propandione 2-imine 1-oxime); [Ni((CO).sub.2BF.sub.2)pn](ClO.sub.4); [Co(dmgBF.sub.2).sub.2(H.sub.2O).sub.2] with dmgH.sub.2 representing dimethylglyoxime; [Co(dmgH).sub.2pyCl]; [Co(dmgH).sub.2(OH.sub.2).sub.2]; [Co(dmgBF.sub.2).sub.2(DMF).sub.2]; [Co(dmgBF.sub.2).sub.2(CH.sub.3CN).sub.2]; [Co(dpgBF.sub.2).sub.2(H.sub.2O).sub.2] with dpgH.sub.2 representing diphenylglyoxime; [Co(dpgBF.sub.2).sub.2(DMF).sub.2]; [Co(dpgBF.sub.2).sub.2(CH.sub.3CN).sub.2]; [Ni(dmgBF.sub.2).sub.2]; [Ni(dmgH).sub.2] [Ni(DO)(DOH)pn](ClO.sub.4); [Ni(MO)(MOH)pnCl]; [Ni((DO).sub.2BF.sub.2)pn](ClO.sub.4); [Co(DO)(DOH)pnBr(PPh.sub.3)]; [Co(DO)(DOH)pn(PPh.sub.3)]; [Co(dmg).sub.3(BF).sub.2].sup.+; [Co(dpg).sub.3(BF).sub.2].sup.+; [Co(dmg).sub.3(BPh).sub.2].sup.0/1+ and [Co(dpg).sub.3(BPh).sub.2].sup.0/1+.

12. The method according to claim 1, wherein said basic oxoanion is selected from the group consisting of a phosphate, carbonate, arsenate, borate, vanadate, chromate, phosphonate, phosphite, nitrate, nitrite, sulphate, sulphonate, molybdate, and tungstate.

13. The method according to claim 1, wherein the reductive electrodeposition comprises applying to said solid support a potential below ?0.4 V versus Ag/AgCl.

14. The method according to claim 1, wherein the reductive electrodeposition comprises applying to said solid support a potential below ?0.6 V versus Ag/AgCl.

15. The method according to claim 1, wherein the reductive electrodeposition comprises applying to said solid support a potential below ?0.8 V versus Ag/AgCl.

16. The method according to claim 1, wherein the reductive electrodeposition comprises applying to said solid support a potential of about ?1 V versus Ag/AgCl.

17. A method for mediating hydrogen evolution comprising implementing the catalyst obtained by the preparation method according to claim 1.

18. The method according to claim 3, wherein the conductive or semiconductive material is doped with one or n ore elements other than said selected material.

Description

SHORT DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 presents different linear voltammetry experiments. In plain, linear voltammetry experiment (plain) recorded at a FTO electrode (1 cm.sup.2) in phosphate buffer (KPi, 0.5 M, pH 7) containing the water-soluble diimine dioxime cobalt(III) complex [Co(DO)(DOH)pnCl.sub.2] with (DO)(DOH)pn representing N.sup.2,N.sup.2-propanediylbis(2,3-butandione 2-imine 3-oxime) (0.1 mM) at a low scan rate (0.05 mV.Math.s.sup.?1) with simultaneous GC monitoring of H.sub.2 evolution (dotted): N.sub.2 was continuously bubbled through the electrolyte at a constant flow (5 mL.Math.min.sup.?1) during the experiment and the concentration of H.sub.2 in the output gas was determined every 2 min by gas chromatography. In bold, similar experiment carried out on a FTO electrode modified by controlled potential electrolysis for 3 h at ?385 mV vs RHE in the same solution and transferred into a cobalt-free buffer. The H.sub.2 evolution scale on the right has been adjusted so as to correspond to the current density scale on the left with a 100% faradaic yield.

(2) FIG. 2 presents SEM images of electrodes modified by electrolysis at (FIG. 2A) ?0.9 V vs Ag/AgCl for 1 h (ITO, 0.1 C.Math.cm.sup.?2.sub.geometric) and (FIG. 2B) ?1.0 V vs Ag/AgCl for 3 h (FTO, 6.5 C.Math.cm.sup.?2.sub.geometric) vs RHE in phosphate buffer (KPi, 0.5 M, pH 7) containing Co(DO)(DOH)pnCl.sub.2 (0.5 mM). FIG. 2C is a SEM image of a H.sub.2CoCat film on FTO electrode formed at ?1 V vs Ag/AgCl and then equilibrated at 1.16 V vs Ag/AgCl for 90 min in a cobalt-free 0.5 mol.Math.L.sup.?1 KPi, pH 7 electrolyte, while FIG. 2D is a SEM image of H.sub.2CoCat film on FTO electrode initially equilibrated at ?1 V vs Ag/AgCl and taken out of the solution just after a potential switch to 1.16 V vs Ag/AgCl before equilibration of the current.

(3) FIG. 3 presents SEM images of pristine ZnO nanorods (FIG. 3A) and of ZnO nanorods onto which H.sub.2CoCat nanoparticles are photochemically deposited for 3 h ([Co(DO)(DOH)pnCl.sub.2]=0.05 mM in 0.1 M KPi, UV-light) (FIG. 3B).

(4) FIG. 4 presents H.sub.2 and O.sub.2 evolution quantified through gas chromatography during photochemical deposition in 0.1 M KPi, UV-light at [Co(DO)(DOH)pnCl.sub.2]=0.05 mM (H.sub.2, open squares and O.sub.2, black circles) and at [Co(DO)(DOH)pnCl.sub.2]=0.1 mM (O.sub.2, open circles).

(5) FIG. 5 presents XPS survey (FIG. 5B), Co.sub.2p (FIG. 5C), O.sub.1s (FIG. 5A) and P.sub.2p (FIG. 5D) core levels spectra of H.sub.2CoCat deposited on FTO substrate (bottom on all panels) and commercial Co.sub.3(PO.sub.4).sub.2 xH.sub.2O (top on all panels).

(6) FIG. 6 presents characteristic EDX spectrum acquired at 15 kV counts per second of FTO electrodes modified by H.sub.2CoCat (FIG. 6A) and H.sub.2CoCat equilibrated for 90 min at +1.16 V vs Ag/AgCl in phosphate buffer (KPi, 0.5 M, pH 7) (FIG. 6B).

(7) FIG. 7 presents Fourier-transformed EXAFS spectra collected at the Co K edge. Plain trace: H.sub.2CoCat equilibrated at ?1.0 V vs Ag/AgCl. Dotted trace: H.sub.2CoCat equilibrated at +1.16 V vs Ag/AgCl for 4 min. Dashed trace: Co metal foil (hexagonal close-packed state). The XANES spectra of the film formed at ?1.0 V vs Ag/AgCl and of metallic cobalt are shown in the inset. The arrows mark features that are assignable to the contribution (.sup.?50%, FIG. 9) of a phase of edge-sharing CoO.sub.6 octahedra to the spectrum of the film equilibrated at +1.16 V vs Ag/AgCl (dotted line).

(8) FIG. 8 presents X-ray absorption spectra of H.sub.2CoCat (bold lines) and of purely metallic Co.sup.0 (plain lines). H.sub.2CoCat was electro-deposited on a glassy carbon electrode at ?1.0 V (vs Ag/AgCl) for 3 h from an aqueous solution containing 0.5 M potassium phosphate (KP.sub.i, pH 7) and 0.5 mM Co(DO)(DOH)pnCl.sub.2, then equilibrated at ?1.0 V for 4 min in a Co-free KPi solution and rapidly frozen in liquid nitrogen. In FIGS. 8A and 8B, XANES spectra; in FIGS. 8C and 8D, Fourier-transformed EXAFS spectra. The plain line in FIG. 8D was obtained by subtraction of the appropriately weighted metal spectrum from the spectrum of H.sub.2CoCat (and renormalisation of the resulting spectrum) assuming that 59% of film consisted of purely metallic cobalt. The resulting spectrum (plain line) is assignable to a non-metallic species with light atoms (O, N, C) in the first coordination sphere of cobalt. A more quantitative analysis is prevented by noise problems and the uncertainties in the used approach to correct for contributions of the metallic cobalt.

(9) FIG. 9 presents linear voltammetry experiments recorded at a low scan rate (0.05 mV.Math.s.sup.?1) at a FTO electrode (1 cm.sup.2) in (FIG. 9, plain, formation of H.sub.2CoCat) phosphate buffer (KPi, 0.5 M, pH 7) solution containing Co(DO)(DOH)pnCl.sub.2 (0.5 mM), (FIG. 9, squared) a NH.sub.4Cl (1 M, pH 5) solution containing CoCl.sub.2 (0.5 mM) [25] and (FIG. 9, dashed) a LiClO.sub.4 (1 M, pH 5) solution containing CoCl.sub.2 (0.5 mM) [26].

(10) FIG. 10 presents the evolution of the current density at a FTO electrode (1 cm.sup.2) coated with H.sub.2CoCat in 0.5 M KPi, pH 7 electrolyte when the potential is switched from reductive (plain line, ?1 V vs Ag/AgCl) to oxidative (bold line, 1.16 V vs Ag/AgCl) conditions.

(11) FIG. 11 presents XPS survey of H.sub.2CoCat film after anodic equilibration at +1.16 V vs Ag/AgCl for 1 h in KPi (pH 7, 0.5 M).

(12) FIG. 12 presents X-ray absorption spectrum of H.sub.2CoCat formed at ?1 V vs Ag/AgCl and further equilibrated at +1.16 V for 4 min (plain lines) compared to a weighted addition (circles) of spectra from the H.sub.2CoCat (FIG. 8) and O.sub.2CoCat initially described by Kanan and Nocera [2] corresponding XAS data by Dau and coworkers (dotted line) [3]. In FIGS. 12A and 12B, XANES spectra; in FIG. 12C, Fourier-transformed EXAFS spectra. The weighting coefficients in FIGS. 12A and 12B suggest that about 50% of the Co ions of the +1.16 V equilibrated CoCat film stay in the state present in H.sub.2CoCat before application of the positive voltage, whereas 50% of the deposit restructured resulting in the same Co oxide (consisting of clusters of edge-sharing Co.sup.IIIO.sub.6 octahedra [3]) that is obtained by electro-deposition at positive potentials. The green line in FIG. 12C was determined by the subtraction of the XANES/EXAFS spectrum of H.sub.2CoCat weighted by a factor of 0.5 from the spectrum of the +1.16 V equilibrated CoCat film. The resulting spectrum was renormalized and Fourier-transformed. It is assumed that this spectrum corresponds to the O.sub.2CoCat contribution of the +1.16 V equilibrated CoCat film. The +1.16 V equilibrated CoCat film was obtained from H.sub.2CoCat equilibrated at +1.16 V for 4 min in a Co-free KP.sub.i solution and rapidly frozen in liquid nitrogen. The O.sub.2CoCat film was electro-deposited on a indium tin oxide electrode at +1.19 V vs Ag/AgCl for 70 min from an aqueous solution containing 0.1 M potassium phosphate (KP.sub.i, pH 7) and 0.5 mM Co.sup.2+ and rapidly frozen in liquid nitrogen.

(13) FIG. 13B presents the charge passed through a FTO electrode (1 cm.sup.2) during controlled potential coulometry initially at ?1.0 V vs Ag/AgCl (3 h, H.sub.2CoCat deposition) in 0.5 mol.Math.L.sup.?1 KPi, pH 7 electrolyte containing Co(DO)(DOH)pnCl.sub.2 (0.5 mM) and after transfer to a cobalt-free 0.5 mol.Math.L.sup.?1 KPi, pH 7 electrolyte, with potential switching between oxidative (bold, 1.16 V vs Ag/AgCl) and reductive conditions (plain, ?1 V vs Ag/AgCl), while FIG. 13A presents the hydrogen (plain) and oxygen (bold) evolution quantified through gas chromatography measurements during the same experiment. N.sub.2 was continuously bubbled through the electrolyte at a constant flow (5 mL.Math.min.sup.?1) during the experiment and the concentration of O.sub.2 and H.sub.2 in the output gas was determined every 2 min by gas chromatography.

DETAILED DISCUSSION OF PARTICULAR EMBODIMENTS

I. Experimental Section

(14) I.1. Materials.

(15) The cobalt complexes used hereinafter to prepare H.sub.2CoCat are [Co(DO)(DOH)pnCl.sub.2] [15] and [Co(dmgBF.sub.2).sub.2(H.sub.2O).sub.2] with dmgH.sub.2 representing dimethylglyoxime [27] were prepared according to previously described procedure.

(16) Fluorine doped tin oxide (FTO) coated glass slides with 7-10 ?/sq surface resistivity and a thickness of 6000 ? were purchased from Solems (France).

(17) KH.sub.2PO.sub.4 (98-100.5%) and K.sub.2HPO.sub.4 (99%) were purchased from Carlo Erba and Acros Organics respectively. ZnCl.sub.2 (Fluka, purity >98%)) and KCl (Fluka, purity 99.5%) analytical reagent grade were used for ZnO preparation without further purification.

(18) A three-electrode electrochemical cell was used for ZnO nanorods deposition on FTO substrates. The electrochemical growth was done at E=?1.0 V vs SCE, passed charge density 10 and 30 C.Math.cm.sup.?2 and T=80? C.

(19) Further details can be found in [28].

(20) I.2. Methods.

(21) X-ray photoemission spectroscopy (XPS) analyses were performed with a Kratos Axis Ultra DLD using a high-resolution monochromatic AlK? line X-ray source at 1486.6 eV. Fixed analyzer pass energy of 20 eV was used for core level scans. Survey spectra were captured at pass energy of 160 eV. The photoelectron take-off angle was normal to the surface, which provided an integrated sampling depth of approximately 15 nm. All spectra were referenced with an external gold substrate with a binding energy of 84.0 eV for Au 4f. For quantification, relative sensitivity factors issued from Wagner's publication were used [29]. The amount of carbon varies from samples and corresponds to carbon contamination from the atmosphere and/or ethanol washing after electrodeposition.

(22) SEM images and EDX spectra were recorded with a FEG-SEM (Leo 1530) operating at 5 kV and equipped with a Princeton Gamma-Tech EDX system operating at 15 kV.

(23) Electrochemical analysis was done using a Bio-logic SP300 potentiostat. FTO was preferred to graphite and ITO (indium-tin oxide) as the electrode material to minimize background H.sub.2 evolution and avoid reductive degradation of the ITO substrate [30,31] respectively.

(24) Cyclic voltammograms are shown using two different potential scales: first, potential is quoted against the Ag/AgCl reference electrode. Second, to ease reading the graphs in terms of overpotentials, potentials are also quoted against the Reversible Hydrogen Electrode (ie the apparent standard potential of the H.sup.+/H.sub.2 couple at the given pH).

(25) The potential of the Reversible Hydrogen Electrode (RHE) is defined as E.sup.RHE=?0.059 pH. Thus potentials measured versus the Ag/AgCl electrode can be converted versus the RHE by using the following formula: E.sub.vs RHE=E.sub.vs Ag/AgCl+E.sup.o Ag/AgCl+0.059 pH. The [Fe(CN).sub.6].sup.3?/[Fe(CN).sub.6].sup.4? couple (E.sup.0=0.20 V vs Ag/AgCl in phosphate buffer at pH=7) has then been used for the standardisation of the measurements.

(26) The electrochemical experiments were carried out in a three electrode cell consisting of two compartments separated by a glass frit. The catalyst was assembled on a working electrode of FTO-coated glass of 1 cm.sup.2 and rinsed with acetone and deionized water prior to use. Connection of the FTO to the potentiostat was made via an alligator clip. The platinum-grid counter electrode was placed in a separate compartment connected by a glass-frit and filled with the electrolytic solution. The potential has been calibrated after each experiment by adding potassium ferrocyanide in the solution and measuring its half-wave potential.

(27) During controlled-potential coulometry and linear sweep voltammetry experiments, the cell was flushed with nitrogen (5 ml.Math.min.sup.?1) and the output gas was sampled (100 ?l) every 2 min and analysed in a Perkin-Elmer Clarus 500 gas chromatograph equipped with a porapack Q 80/100 column (6?) thermostated at 40? C. and a TCD detector thermostated at 100? C.

(28) I.3. Deposition Procedure.

(29) Before starting the deposition, the FTO substrate is cycled hundreds of times between ?1 V and +1 V in 0.5 M potassium phosphate buffer (KPi), pH 7.0 to ensure the stability and reproducibility of experiments.

(30) Catalyst films were grown by controlled potential electrolysis of freshly prepared 0.1 mM Co(DO)(DOH)pnCl.sub.2 solution and in 0.5 M KPi, pH 7.0. Performing the electrolysis at ?1 V (vs Ag/AgCl) gives rise to a catalyst film of several micrometer thickness formed by nanoparticles after a course of around three hours.

(31) During this time, a film is formed on the working electrode surface. After the film formation is completed, the substrate is transferred to a cobalt-free 0.5 M KPi, pH 7.0, with potential set to reductive conditions (?1 V vs Ag/AgCl).

(32) Photodeposition was carried out by immersing ZnO electrode in a UV cell of freshly prepared 0.5 mM or 0.1 mM Co(DO)(DOH)pnCl.sub.2 solution and in 0.1 M KPi, pH 7.0 and illuminating the samples using a UV-handlamp for 3 h (VL-6LC, ?=254 nm).

(33) I.4. XAS Sample Preparation and Data Collection.

(34) The glassy carbon substrates (SIGRADUR K, thickness 100 ?m) had a surface resistivity of 2-15 ?/sq and were from HTW Hochtemperatur-Werkstoffe GmbH. KH.sub.2PO.sub.4 (99.5%) and K.sub.2HPO.sub.4 (99.5%) were purchased from AppliChem.

(35) For preparing the XAS samples, the inventors employed a single compartment, three electrode setup driven by an SP-200 potentiostat from BioLogic. The electrochemical cell consisted of a sample frame for XAS measurements glued (Momentive, SCNC silicone glue) on a glassy carbon substrate (working electrode) and a Pt wire (counter electrode) attached on the inner side of the frame. An Hg/HgSO.sub.4 reference electrode (?440 mV vs Ag/AgCl) was approached close to the center of the frame.

(36) Aqueous solutions of KH.sub.2PO.sub.4 (0.5 M) and K.sub.2HPO.sub.4 (0.5 M) were mixed until the KP.sub.i mixture reached a pH of 7.0. This electrolyte was pipetted into the sample frame until the liquid reached the reference electrode. The overall ohmic resistance of the electrochemical cell was determined by impedance spectroscopy and then used as set value for the applied IR compensation. The electrochemical background of the glassy carbon substrate was low, as verified by cyclic voltammetry.

(37) For the formation of the Co film, 0.5 mM Co(DO)(DOH)pnCl.sub.2 was added and a voltage of ?1.0 V vs Ag/AgCl was applied for 3 h. Then the cobalt solution was replaced by (cobalt-free) KP.sub.i via pipetting. Four CVs from ?1.21 V to ?0.56 V vs Ag/AgCl (scan rate: 20 mV/s) were performed to characterize the Co deposit.

(38) Finally, the Co film was equilibrated at ?1 V vs Ag/AgCl for 4 min and then rapidly frozen in a bath of liquid nitrogen. The reference electrode was removed before freezing to avoid any damage to the electrode by immersion in liquid nitrogen. However the potential between the working and the counter electrode was kept constant by applying the same voltage that had been present before removing the reference electrode (using a standard DC power supply connected to working and counter electrode). The power supply was disconnected after freezing in liquid nitrogen.

(39) X-ray absorption spectra were collected at beamline KMC1 of the BESSY, a synchrotron radiation source operated by the Helmholtz Zentrum Berlin (HZB). Spectra were collected at 20 K in absorption and fluorescence mode as described elsewhere [3].

II. Results

(40) II.1. Preparation of Catalytic Material.

(41) A. By Reductive Electrode Position.

(42) FIG. 1 shows the results obtained during a linear sweep voltammetry experiment (plain black trace, 0.05 mV.Math.s.sup.?1) of a Co(DO)(DOH)pnCl.sub.2 solution (0.1 mM) in KPi (0.5 M, pH 7) at an FTO electrode. A reductive process is observed with onset at ?0.9 V vs Ag/AgCl. Simultaneous chromatographic monitoring of H.sub.2 production (dotted black trace) indicates that no hydrogen is produced at this point. If the electrode potential is switched to more negative values (below ?0.95 V vs Ag/AgCl) H.sub.2 is produced.

(43) To provide more insights into the reductive process at work at the onset of the wave, the inventors carried out an electrolysis experiment at ?0.9 V vs Ag/AgCl for 1 h (Q=0.1 C.Math.cm.sup.?2.sub.geometric), which resulted in a grey coating of the electrode. The catalytic material thus obtained is hereafter named H.sub.2CoCat.

(44) The scanning electronic micrograph (FIG. 2A) shows isolated nanoparticles with an average .sup.?10 nm in diameter. Performing the same electrolysis but at ?1.0 V vs Ag/AgCl for 3 h yields a film of .sup.?2 m thickness made from larger particles (100 nm) as shown by FIG. 2B. During this experiment the current density stabilizes to a value of 2 mA.Math.cm.sup.?2.sub.geometric. Neither the use of a longer electrolysis time nor a second electrolysis experiment in a new solution of the cobalt complex could increase the current density.

(45) The modified electrode was then transferred to a Co-free KPi electrolyte and its electrocatalytic properties were quantified using gas chromatography (bold traces in FIG. 1). Hydrogen evolution could be detected for overpotential values as low as 50 mV and overpotential values of 270 mV and 385 mV are required to reach current density values of 0.5 mA.Math.cm.sup.?2.sub.geometric and 2 mA.Math.cm.sup.?2.sub.geometric respectively. Overpotential values can be directly obtained from figures plotting current densities or the amount of evolved H.sub.2 as a function of the electrochemical potential values scaled versus the Reversible Hydrogen electrode (RHE). Indeed, H.sub.2 evolved with quantitative faradic yield within the experimental accuracy. It should be noted that these overpotential values are much lower than those reported with cobalt-based molecular catalysts assayed under aqueous solutions. For instance, Co macrocycles generally require overpotentials of 500 mV to 700 mV [13] and a recently described pentadentate polypyridyl cobalt complex catalyzes H.sub.2 evolution with onset of the catalytic current occurring for an overpotential of 660 mV [32].

(46) H.sub.2CoCat appears to be stable as long as the electrode is kept at a potential more negative than about ?0.6 V vs Ag/AgCl. When poised at a more positive potential, or left at open-circuit potential, the catalytic film readily dissolves in the electrolyte yielding Co(II) species. By ICP-MS titration of Co(II) ions in the electrolyte after full redissolution, the inventors determine that 1.0 10.sup.?6 mol of cobalt are deposited per geometric cm.sup.?2, from which the inventors derive a H.sub.2 evolution turnover frequency of 80 h.sup.?1 per Co center at 385 mV overpotential.

(47) B. By a Photochemical Deposition.

(48) Finally the inventors observed that H.sub.2CoCat can also be deposited through a photochemical procedure.

(49) The inventors chose ZnO because the energy level of its conduction band is suitable for H.sub.2 evolution from neutral aqueous solutions. ZnO nanorods with high specific surface area (FIG. 3A) were irradiated by UV-light in Co(DO)(DOH)pnCl.sub.2 (0.1 mM) in 0.1 M KPi, resulting in the deposition of particles (FIG. 3B).

(50) Gas chromatographic analysis of the headspace of the vial in the course of this experiment (FIG. 4) shows that only O.sub.2 is evolved, consistent with a light driven reductive deposition of H.sub.2CoCat thanks to electrons provided by water oxidation.

(51) II.1. Characterization of the Catalytic Material.

(52) The structure of the new material was characterized by X-ray photoelectron spectroscopy (XPS), Energy Dispersive X-ray (EDX) spectroscopy, X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine-structure (EXAFS) spectroscopies.

(53) The XPS spectrum of freshly electro-deposited H.sub.2CoCat, recorded under limited air exposure conditions, shows the presence of cobalt, phosphorus and oxygen (FIG. 5) and was comparable to that of commercially available Co.sub.3(PO.sub.4).sub.2 xH.sub.2O. No significant signals are found in the N1s region confirming that the organic ligand is completely split up from with the cobalt ion during the cathodic deposition process. The analysis of the P.sub.2p region of both samples shows two sharp peaks with 133.4 eV and 134.4 eV binding energies (ratio of 2:1) corresponding to the 2p.sub.3/2 and 2p.sub.1/2 core levels of central phosphorus atom in phosphate species [33]. In the Co region, two broad sets of signals corresponding to 2p.sub.3/2 (782 eV) and 2p.sub.1/2 (798 eV) core levels are observed, excluding the presence of metallic cobalt (778.0 eV) at the surface of the H.sub.2CoCat coating. O.sub.1s signals are centered for both materials at 531.7 eV. P/Co/O ratios are however significantly different with a slight excess of cobalt and oxygen for H.sub.2CoCat (2:3.9:11.6) as compared to Co.sub.3(PO.sub.4).sub.2 xH.sub.2O (2:3:11.1).

(54) As the Co.sub.2p and O.sub.1s core levels binding energies [34] of cobalt oxides and hydroxides are in the same range as those of cobalt phosphate, the inventors tentatively describe the surface of electrodeposited H.sub.2CoCat as a combination of cobalt(II) phosphate with a cobalt oxo/hydroxo species Co.sub.xO.sub.y(OH).sub.z, probably in the Co(II) state as observed for native cobalt oxide/hydroxide that forms at the surface of metallic cobalt.

(55) EDX spectra (FIG. 6) confirm the presence of Co, P and O together and additional signals arise from silicon and tin from FTO coated glass substrate. Since EDX, which probes deeply into the cathodically deposited film, indicates a Co/P ratio of 5:1, whereas XPS, which probes only a few nanometers below the surface, indicates a 1:2 Co/P ratio, the inventors conclude that H.sub.2CoCat is not homogeneous in composition between bulk and surface.

(56) The absence of crystalline features in the powder low-angle X-ray diffraction pattern recorded at low angles and in the electron diffraction patterns recorded in a transmission electron microscope indicates the amorphous nature of H.sub.2CoCat. For insight into the atomic structure, X-ray absorption spectra were collected at the Co K-edge after H.sub.2CoCat formation and fast freezing of the still-wet electrode in liquid nitrogen (quasi-in situ measurements) as described earlier [3]. When deposition (paralleled by catalytic H.sub.2 evolution) was achieved at ?1.0 V vs Ag/AgCl, XANES and EXAFS measurements suggest a dominating contribution of the hexagonal close-packed phase of metallic cobalt (FIG. 7 and FIG. 8). However, the magnitude of EXAFS oscillation is by about 40% smaller than observed for a Co metal foil and furthermore the edge spectra (insert in FIG. 7) suggest a sizeable non-metallic contribution. An appropriate subtraction of the metallic contribution results in an EXAFS spectrum that suggests the presence of light atoms (O, N, C) in the first Co coordination sphere (FIG. 8D), but the determination of bond lengths by EXAFS simulations is prevented by noise problems in conjunction with uncertainties in the approach used to correct for the dominating contributions of the metallic cobalt.

(57) All these data indicate that the new material is made of nanoparticles with a cobalt oxo/hydroxo phosphate component mostly located at the surface and metallic cobalt in the bulk.

(58) II.3. Transformation in Water Oxidation Catalyst.

(59) Poising the H.sub.2CoCat electrode at a fairly positive potential (+1.16 V vs Ag/AgCl) resulted in a stable anodic current density of 1 mA.Math.cm.sup.?2 (FIG. 10) and concomitant oxygen evolution with quantitative faradaic yield. SEM observations of the electrode after 90 min (FIG. 2C) shows a homogeneous thin film, with cracks originated from drying, very similar to those obtained for the O.sub.2CoCat material reported in 2008 by Kanan and Nocera [2,3].

(60) In order to characterize the electrocatalytic material after this redox shift a detailed analysis by EDX, XPS and X-ray absorption spectroscopy has been carried out. According to EDX spectra (FIG. 6B) the Co:P ratio is 1:2.5, which differs significantly from the 2:1 ratio previously reported for the anodically deposited O.sub.2CoCat [2]. XPS analysis confirms the large phosphorus accumulation in the oxidized film (FIG. 11).

(61) On the other hand, the XANES and EXAFS spectra (FIGS. 5 and 12) of H.sub.2CoCat films equilibrated at +1.16 V vs Ag/AgCl, indicate that already about 50% of the Co film has enjoyed a transformation resulting in a Co oxide, consisting of clusters of edge-sharing Co.sup.IIIO.sub.6 octahedra [3,4], similar to that found in O.sub.2CoCat. The coexistence of metallic cobalt with O.sub.2CoCat in the anodically equilibrated material clearly shows that the catalytic response occurs outside of an electrochemical equilibrium, with the oxidizing equivalents being used for water oxidation preferentially, protecting the major part of the cobalt coating from oxidation.

(62) This observation parallels the recent report by Nocera et al. regarding the anodic oxidation of a 800 nm sputtered cobalt film for the preparation of O.sub.2CoCat [35].

(63) After cathodic deposition, alternate switches between oxidative (+1.71 V vs RHE, bold traces in FIG. 13) and reductive conditions (?385 mV vs RHE, plain traces in FIG. 13) show that the deposited material can catalyze both water oxidation and H.sub.2 evolution respectively.

(64) Importantly the inventors could not evidence any decrease in activity for both H.sub.2 and O.sub.2 evolution after several switches. The inventors conclude that the H.sub.2CoCat film rapidly, reversibly and reproducibly commute with the O.sub.2CoCat form.

(65) From the current densities and chromatographic measurements, the inventors derive turnover frequencies of 10 and 80 h.sup.?1 per Co center at 480 and 385 mV overpotential for O.sub.2 and H.sub.2 evolution respectively. In order to determine whether this reversible transformation proceeds through the complete dissolution of one form of the CoCat film followed by the electrodeposition of the other form, the inventors stopped the experiment before current stabilization after a switch from reductive to oxidative conditions.

(66) SEM analysis then reveals the coexistence of different domains at the surface of the electrode (FIG. 2D) corresponding to both morphologies. This clearly stands for a progressive and local transformation of the material.

(67) This last observation finally provides a rationale for the fact that photochemically deposited H.sub.2CoCat on ZnO nanorods can be used as an electrocatalytic material for water oxidation, as previously described by Steinmiller and Choi [36]. Actually these authors initially interpreted the photodeposition of Co(II) salts on ZnO nanorods as the formation of O.sub.2CoCat. The inventors' results establish that O.sub.2 and not H.sub.2 evolved during the photochemical process which definitively demonstrates that the deposition of the material does not proceed through the oxidation of the Co(II) ions (equation 1) with concomitant O.sub.2 or water reduction as previously stated. Rather H.sub.2CoCat is formed through the reduction of the Co(II) ions (equations 2) thanks to photogenerated electrons in the conduction band paralleled by water oxidation (equation 3) by holes remaining in the valence band. Both the observed nanoparticle-based morphology and the similar Co/P ratio derived from EDX analysis support this conclusion. When no more Co(II) ions are present in the solution, light is used by the H.sub.2CoCat coated ZnO nanorods to split water and produce both O.sub.2 and H.sub.2 (FIG. 4, [Co.sup.2+]=0.05 mM).

(68) However, if this material is electrochemically poised at an anodic potential and irradiated, as in the study by Steinmiller and Choi [36], H.sub.2CoCat equilibrates with the valence band potential of ZnO, transforms into O.sub.2CoCat and finally mediates water oxidation.
Co.sup.2+.fwdarw.O.sub.2CoCat+e.sup.?Eq. 1
Co.sup.2++2e.sup.?.fwdarw.H.sub.2CoCatEq. 2
2H.sub.2O.fwdarw.O.sub.2+4H.sup.++4e.sup.?Eq. 3

III. Conclusion

(69) The inventors have discovered that cobalt complexes can be used for the deposition of a new nanoparticulate cobalt-based material (H.sub.2CoCat) consisting of elemental cobalt covered by a cobalt oxo/hydroxo phosphate compound. The latter proves active as a catalyst material for hydrogen evolution under strictly neutral conditions, which bridges the gap between noble metal nanoparticles, active under highly acidic conditions, and classical Ni or Co-based metallic compounds [37] and catalysing H.sub.2 evolution under strongly alkaline conditions.

(70) H.sub.2CoCat can be reversibly transformed through anodic equilibration into a cobalt oxide material catalyzing water oxidation (O.sub.2CoCat). An important finding is that the switch between the two catalytic phasesH.sub.2 evolution and water oxidationis highly reversible and corresponds to a local interconversion between two morphologies at the surface of the electrode. After deposition, the coating thus functions as a robust, bifunctional and switchable catalyst.

(71) To the best of the inventors' knowledge, such a property is not found except for noble metal catalysts such as Pt. These results open new pathways for the deposition of cobalt oxide film onto materials, such as ZnO or quantum dots, that do not withstand harsh anodic conditions.

(72) Additionally, they now make it possible to design an electrocatalytic water-splitting cell working under neutral conditions and based on cobalt ions as the sole precursors. In such a device, the catalytic materials will spontaneously self-assemble on both electrodes from a cobalt or nickel organic complex upon switching on. In addition, it will be insensitive to material cross-transfers processes when the system is switched off and on.

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