Buffered cobalt oxide catalysts

Abstract

Disclosed are electrolysis catalysts formed from cobalt, oxygen and buffering electrolytes (e.g. fluoride). They can be formed as a coating on an anode by conducting an electrolysis reaction using an electrolyte containing cobalt and an anionic buffering electrolyte. The catalysts will facilitate the conversion of water to oxygen and hydrogen gas at a range of mildly acidic conditions. Alternatively, these anodes can be used with cathodes that facilitate other desirable reactions such as converting carbon dioxide to methanol.

Claims

1. An anode suitable for generating oxygen in an electrolysis reaction, comprising: a substrate; and a catalytic coating positioned on the substrate which comprises cobalt, oxygen, and an anion selected from the group consisting of perfluoroalkyl sulfonamides, and anions of perfluorinated dialkyl ketone hydrates.

2. The anode of claim 1, wherein the catalytic coating was positioned on the substrate by electrolytic film deposition of the catalytic coating on the substrate during an electrolysis reaction in which the substrate was positioned in an aqueous solution comprising cobalt cation and the selected anion.

3. An electrolysis cell comprising the anode of claim 1 and further comprising a cathode.

4. The electrolysis cell of claim 3, wherein the cathode is suitable to generate hydrogen gas.

5. The electrolysis cell of claim 3, wherein the cathode is suitable to convert carbon dioxide to another carbon containing material.

6. The electrolysis cell of claim 5, wherein the cathode is suitable to convert carbon dioxide to methanol.

7. The anode of claim 1, wherein the selected anion is a perfluoroalkyl sulfonamide.

8. The anode of claim 7, wherein the selected perfluoroalkyl sulfonamide is trifluoromethyl sulfonamide.

9. The anode of claim 1, wherein the selected anion is an anion of a perfluorinated dialkyl ketone hydrate.

10. The anode of claim 9, wherein the selected anion of a perfluorinated dialkyl ketone hydrate is deprotonated hexafluoroacetone hydrate.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 schematically depicts a prior art system for conducting electrolysis of water.

(2) FIG. 2 compares the effect, in a FIG. 1 type system, of an electrolytic solution with no cobalt (10) with using the identical system with cobalt also added (11).

(3) FIG. 3 shows comparative experiments taken after a catalyst coating from the (11) experiments has deposited on the anode, comparing the results of that (13), with the use of that coated catalyst anode in cobalt-free solution (14).

(4) FIG. 4 shows similar experiments as with (13) at pH 3.0 (16), pH 4.5 (17), pH 5.5 (18) and pH 7.1 (19).

(5) FIG. 5 shows similar experiments where the electrolyte contained fluoride at pH 3.5 (23/24), or contained phosphate at pH 7.0 (25/26).

(6) FIG. 6 compares current density versus time effects where the electrolyte contained fluoride (20), or contained phosphate (21).

(7) FIG. 7 shows test results from the operation of a FIG. 1 type cell using fluorophosphate electrolyte.

(8) FIG. 8 shows test results from the operation of a FIG. 1 type cell using trifluoromethyl sulfonamide electrolyte.

(9) FIG. 9 shows test results from the operation of a FIG. 1 type cell using sulfate electrolyte.

DETAILED DESCRIPTION OF THE INVENTION

(10) To create one electrolyte solution we add to water cobalt cation at around 1 mM, such as by adding CoSO.sub.4, CoCl.sub.2, Co(NO.sub.3).sub.2 or the like. We also add a fluoride anion at a concentration of about 0.1 M. We preferred providing the fluoride anion in the form of a pH buffered mixture of KF and HF. In our experiments with varied pHs the pH was adjusted by the addition of KHF.sub.2 or NaOH as needed.

(11) In other electrolyte solutions we added to water cobalt cation at around 1 mM, such as by adding CoSO.sub.4, CoCl.sub.2, Co(NO.sub.3).sub.2 or the like. We also added our selected buffering electrolyte, typically at a concentration of about 0.1 M or 1 M. All potentials are given relative to the NHE reference electrode.

(12) In the FIG. 2-FIG. 6 experiments we causes electrolytic film deposition of our catalyst by operating the FIG. 1 device using the aforesaid electrolytic solution at about 1.48 volts (e.g. 1.33 volts to 1.58 volts). Once the anode has been coated with our catalyst, it is no longer critical that the electrolyte solution contain both the cobalt or fluoride. It could continue to be operated with fluoride.

(13) FIG. 2 depicts the results of cyclic voltammetry scans of an indium tin oxide substrate anode in 0.1 M KF electrolyte with and without 1 mM CoSO.sub.4 at pH 5. The vertical axis is the log current density. The horizontal axis is voltage. In the presence of cobalt ions (11) there was an abrupt production of catalytic current. As the voltage is scanned back, there was a broad cathodic peak centered at E.sub.p,c=1.07 V.

(14) Subsequent to electrodeposition we ran the FIG. 3 experiments. Continued controlled-potential (CPE) electrolysis at 600 s 1.48 V, in 0.1 M fluoride at pH 5 with 1 mM CoSO.sub.4, and following a subsequent 600 s. CPE at 1.48 V in cobalt-containing buffer led to deposition of a film of material that showed increased catalytic current on subsequent cyclic voltammetric scans. These (13) experiments showed an anodic wave at .sup.1.2 V that blended into the catalytic current.

(15) A subsequent cyclic voltammetric scan following rinsing of the electrode and electrolysis in fresh pH 5 fluoride buffer for 10 min at 1.48 V confirmed that even without cobalt in the electrolyte solution the coated anode retained essentially the same activity (14). Note that in our experiments the catalytic effect was noted unless the electrode is held at potentials more reducing than the cathodic wave at .sup.1 V, below which dissolution of the catalyst is observed.

(16) As depicted in FIG. 4, we then compared the effect of different pHs using a graphite anode. We found that even at pHs around neutral the catalytic effects are quite efficient.

(17) We then sought to compare the efficiency of our catalyst with catalytic results using another anion besides fluoride, with cobalt. These experiments are depicted on FIG. 5. The FIG. 5 experiments confirm the superiority of the fluoride anion (23)/1 M or (24)/0.1 M versus phosphate (25) or (26) at those molarities. We compared the log of the current density versus overpotential.

(18) We then ran an experiment involving constant-potential electrolyses of fluoride-buffered cobalt solutions in a stirred, undivided cell (without the diaphragm 8). These experiments were not focused on the collection of the gases. FIG. 6 experiments were run at an initial pH of 5, and showed the pattern of current increase reflecting deposition as graphed. With the increase in current there was formation of increased visible deposit on the electrode and bubbling. Fluoride results (20) were superior to phosphate (21), and vastly superior to sulfate.

(19) In prolonged electrolyses in cobalt-free buffer at lower pH, we noted that there was a decrease in current over time. We attribute this to slight dissolution of the visible coating on the anode. This suggests that the pKa of HF is close to that of the solid. However, steady state is achieved at approximately 0.1 mM Co.sup.++. Alternatively, increasing the fluoride concentration in the electrolyte solution after anode coating formation was found to lead to a more stable deposit.

(20) In the FIG. 7 experiment we used 0.1 M fluorophosphate presented as sodium monofluorophosphate adjusted with sulfuric acid or sodium hydroxide to a pH of 4.8. Catalyst was deposited at about 1.3 V and the resulting cell then worked efficiently at about 1.6 V.

(21) In the FIG. 8 experiment we used 0.1 M of trifluoromethyl sulfonamide adjusted with sodium hydroxide to a pH of about 6.3. Catalyst was deposited on the anode at 1.05 V and the resulting cell then worked efficiently at about 1.55 V.

(22) In the FIG. 9 experiment we used 1 M sulfate presented at a 50/50 mix of sodium sulfate and sodium bisulfate adjusted with sulfuric acid and sodium hydroxide to a pH of 2.2. Catalyst was not deposited on the anode.

(23) Our preliminary experiments with chromate indicate similar utility. Thus, as yet another alternative we are proposing 1 M chromate presented as a mix of sodium chromate and chromium trioxide adjusted with sodium hydroxide to a pH of about 6.5.

(24) As a further alternative we are proposing 1 M trifluoromethyl phosphonate or other perfluoroalkyl phosphonate presented as the perfluoroalkyl phosphonic acid adjusted with sodium hydroxide to a pH of about 6.5.

(25) As yet another alternative we are proposing 1 M perfluoro-tert-butoxide or other perfluorinated tertiary alkoxides, deprotonated hexafluoroacetone hydrate or other anions of perfluorinated dialkyl ketone hydrates presented as the perfluorinated alcohol or ketone adjusted with sodium hydroxide to a pH of about 4.5.

(26) The cathode (6) can be any cathode suitable for use in water electrolysis under the conditions we are exposing the cathode to. Particularly preferred cathodes are platinum or platinized graphite cathodes.

(27) The anode (4) begins with a substrate (5), which again can be any anode suitable for use in water electrolysis under the conditions we are exposing the anode to. Particularly preferred substrates for the anode are materials such as tin oxides, particularly indium tin oxide or fluorine tin oxide.

(28) Once the anode has been coated with our catalyst, it is no longer critical that the electrolyte solution contain both the cobalt and the anion. It could continue to be operated without the cobalt, using the anion.

(29) One can generate oxygen gas using our improved anode (along with hydrogen at the cathode). An electrode prepared by the constant-potential deposition can be placed in 0.1 M-1 M anion, in a closed, divided cell like that of FIG. 1, and linked to a pressure transducer. The presence of gas generation at both the anode and cathode can be confirmed.

(30) Further, we note that we ran some studies of the nature of the catalysts. In one experiment we determined that the catalyst contained cobalt, oxygen, and fluorine, in about the ratio of one fluorine, to 4.24 cobalt, to about 8.9 oxygen. We believe that the fluorine is present as fluoride in the material. SEM images of the deposit show a layer of fused spherical nodules. The catalyst appears yellow-brown.

(31) We believe that with this catalyst F acts as a proton acceptor during oxidation of cluster sites bearing either a Co(H.sub.2O) or CoOH moiety en route to OO bond formation, with either subsequent proton transfer to or exchange of the formed HF with F in solution. The inability of catalytically competent deposits to form anywhere near as well in sulfate electrolyte solutions at low cobalt concentration suggests that SO.sub.4.sup.2 is too weak of a base.

(32) Our experiments with fluoride suggest that the fluoride is acting in some more complicated role than phosphate does. We believe that it is not just acting as a base. Fluoride can act as a ligand on cobalt, and fluoride is also a strong hydrogen-bond acceptor that may play a role in activating water molecules towards reaction with the catalytic center.

(33) As cobalt oxyfluoride compounds are readily produced, we favor the explanation that a cobalt oxide cluster containing at least one fluoride ligand is formed to create the claimed catalyst, and that this undergoes exchange with water to form an aqua-complex which engages in electron-coupled proton transfer to outer-sphere fluoride to yield clusters containing a Co(O) species which produces the observed water oxidation.

(34) While a number of embodiments of the present invention have been described above, the present invention is not limited to just these disclosed examples. There are other modifications that are meant to be within the scope of the invention and claims. Thus, the claims should be looked to in order to judge the full scope of the invention.

INDUSTRIAL APPLICABILITY

(35) The present invention provides catalytic materials for use in water electrolysis and other reduction reactions, where the catalyzed reaction can be conducted at mildly acidic conditions. It also provides anodes useful in these methods, methods of forming these anodes, and methods of generating a fuel and oxygen gas using them, thereby providing a more practical way of storing renewable energy.