Cobalt Carbonate as an Electrocatalyst for Water Oxidation

Abstract

Co II salts in the presence of HCO 3 À /CO 3 2À in aqueous solutionsa ct as electrocatalysts for water oxidation. It comprises of several key steps:( i) Ar elatively small wave at E pa % 0.71 V( vs. Ag/AgCl) owing to the Co III/II redox couple. (ii)Asecond wave is observed at E pa % 1.10 Vwith aconsiderably larger current.I nw hich the Co III undergoes oxidation to form aC o IV species. The large current is attributed to catalytic oxidation of HCO 3 À /CO 3 2À to HCO 4 À .( iii)A process with very large currents at > 1.2 Vo wing to the formation of Co V (CO 3 ) 3 À ,w hich oxidizes both water and HCO 3 À /CO 3 2À . Thesep rocesses depend on [Co II ], [NaHCO 3 ], andp H. Chronoamperometry at 1.3 Vg ives ag reen deposit. It acts as a heterogeneous catalystf or water oxidation. DFT calculations point out that Co n (CO 3 ) 3 nÀ6, n = 4, 5a re attainable at potentials similar to those experimentally observed.Scheme1.Outline of the water oxidation mechanisms. Misthe metal, n represents the highest oxidation state. [50] This figure has beenr eproduced (modified) from reference [50] with permission from the Royal Societyof Chemistry.

The Role of Carbonate in Catalytic Oxidations

“…23 The analogous systems containing other divalent first-row transition metal cations in the presence of HCO pointed out that Co II in the presence of >1 × 10 −3 M HCO 3 − / CO 3 2− is an excellent catalyst for electrocatalytic water oxidation. 3 During chronoamperometry, a precipitate is formed that serves as a heterogeneous catalyst for the same (vide infra). Electrochemically, three homogeneous processes are observed:…”

“…46 In the study of homogeneous electrocatalysis of the OER in solutions containing Co II and HCO 3 − /CO 3 2− , it was noted that during chronoamperometry a green precipitate of Na 3 [Co-(CO 3 ) 3 ] is formed. 3 This precipitate on the anode surface serves as an excellent heterogeneous catalyst in the presence of bicarbonate/carbonate. This catalytic process was shown to be in agreement with reactions analogous to reactions 22 and 26− 34 in Scheme 3.…”

“…DFT calculations suggest that the active species in these process is the Co V (CO 3 ) 3 − complex. 3 In an analogous study, 18 a precipitate was formed on an anode by electrolysis of a CO 2 -saturated solution containing Fe II (aq) and HCO 3 − . The thin film thus formed contained Fe III , O 2− , OH − , and CO 3 2− .…”

“…Therefore, it is thermodynamically easier to oxidize bicarbonate/carbonate than to oxidize water. Not surprisingly, adsorbed carbonates on semiconductors facilitate photocatalytic water oxidation. − Therefore, it is also not surprising that HCO 3 – and CO 3 2– catalyze the Fenton reaction, forming CO 3 •– and not OH • . , HCO 3 – and CO 3 2– also act as electrocatalysts for the OER process. ,,− In the absence of other substrates, CO 3 •– decomposes via ,, The CO 3 •– anion radical is considerably less reactive than the OH • radical, and its reactions are more selective . The CO 3 •– anion radical reacts in most systems via the inner-sphere mechanism. , …”

“…(c) As a strong hard base, carbonate is a very good ligand for high-valent transition metal cations and therefore stabilizes transition metal complexes with higher valence, e.g., Mn III , Fe IV,III , , Co V/IV/III , Ni IV/III , Cu IV/III , Ru IV , etc. In these complexes, the carbonate is a noninnocent ligand and is therefore involved in electrocatalytic OER processes and photoelectrocatalytic processes.…”