Electrocatalytic Water Oxidation by Single Site and Small Nuclearity Clusters of Cobalt

Abstract

Cobalt oxides are an earth abundant material that exhibits high electrocatalytic activity for the oxygen evolution reaction (OER) across a wide pH range. Recent studies suggest that OER catalysis can proceed through an active site comprised of one or two cobalt atoms but that multiple adjacent cobalt centers are preferred to stabilize high valent cobalt oxo-intermediates by delocalization. Utilizing molecular precursors to prepare single, isolated cobalt atoms (SS-Co) and small clusters of Co 3 O 4 we find that OER proceeds more efficiently on Co 3 O 4 . Using electrochemical impedance spectroscopy (EIS), these results were rationalized at an atomic level. The EIS results support a hypothesis that charge transfer related to the formation of reaction intermediates proceeds more easily on Co 3 O 4 than on SS-Co, which is attributed to the difficulty in forming Co(IV) = O and unlikely nucleophilic attack by water to form Co(II) Conversion of solar energy into a useful chemical fuel for later use represents a major scientific goal in the evolution toward a society fully powered by renewable energy.-1 Several potential fuel targets exist including hydrogen from proton reduction; methane, methanol and higher order carbon species from carbon dioxide reduction; and ammonia via nitrogen reduction. To achieve meaningful rates of fuel production, any of these potential reduction reactions must be coupled to an oxidative reaction that generates both electrons and protons. The most sensible candidate to provide these electrons and protons is water, which can be decomposed via the oxygen evolution reaction (OER):In all proposed systems, the OER occurs at the anode and is coupled to the reductive reaction occurring at the cathode. Ideally, the potential needed to drive the OER and its partner reaction is generated via a photovoltaic process. In a photoelectrochemical cell with the catalysts directly coupled to the light absorber, the efficiency of the catalyst, in particular the OER catalyst is paramount. A key figure of merit for a catalyst used in a photoelectrochemical cell is the amount of overpotential required to achieve a desired current density (typically ∼10 mA cm −2 ). 2 In addition, because photoelectrochemical cells operate at lower current densities than electrolyzers, a larger amount of catalyst is needed and so cost becomes a key consideration. Finding OER catalysts that are both active and cost effective is a major challenge. Iridium and ruthenium OER catalysts are highly active across a wide pH range but are costly.3-9 By contrast, some lower cost non-noble metals also show activity for OER. Fe x Ni 1-x OOH catalysts exhibit low overpotentials for OER (∼300 mV for 10 mA cm −2 ) in base.10-13 Natural photosynthesis utilizes a manganese oxo cluster 14 and manganese oxides show some facility for OER, albeit at large overpotentials. [15][16][17][18] Some copper complexes also demonstrate electrocatalytic OER behavior at high pH. 19,20 As an alternative to noble metal catalysts, cobalt oxides are a promising potent...

Compositional Control as the Key for Achieving Highly Efficient OER Electrocatalysis with Cobalt Phosphates Decorated Nanocarbon Florets

“…Interestingly, the τ corresponding to high‐intermediary frequencies is identical for all the three catalytic systems while the low frequency τ is the least for the Co‐DP‐MP@NCF. In addition to establishing the involvement of multiple adsorbed intermediates, the lowering of the low‐frequency τ indicates that the oxygen deficient metal centers in the case of Co‐DP‐MP@NCF enhances the kinetics of OER through the activation of water molecule through surface adsorbed OH ads . Thus, the Randles–Sevcik plot (Figure S15, Supporting Information) establishes the occurrence of the four electron relay pathway occurring in the case of Co‐DP‐MP@NCF.…”

Mono‐/Multinuclear Water Oxidation Catalysts

“…After acid leaching, the single-site catalyst can be obtained. [33] Moreover,s ome cobalt-basedc atalysts with single sites, such as the single-site cobalt on indium tin oxide (ITO) electrode preparedb yS wierk andT illey, [34] can also be synthesized through pyrolysis. [32] Am ixture of melamine and nickel chloride hexahydratei sa nnealed at ah igh temperatureu nder aN 2 atmosphere.A dditionally,amononuclearh eterogeneous Mn-NGc atalysth as been recently synthesized by using this method.…”

“…Pan et al successfully synthesized at rifunctional catalystw ith the FeÀN 4 site, which had the ability to catalyze the OER. [33] Moreover,s ome cobalt-basedc atalysts with single sites, such as the single-site cobalt on indium tin oxide (ITO) electrode preparedb yS wierk andT illey, [34] can also be synthesized through pyrolysis. In addition, bimetallic nitrogen-doped carbon materials, such as am ultifunctional catalyst with the FeÀN 4 site, Pt 1 @FeÀNÀC, have been prepared through pyrolysis.…”

Proton coupled electron transfer from Co3O4 nanoparticles to photogenerated Ru(bpy)33+: base catalysis and buffer effect

“…, both tetrahedral Co(II) and octahedral Co(III) sites of the Co 3 O 4 spinel structure could be considered. 4,25,26 Different Co(III)-OH surface sites were envisaged based on time resolved rapid-scan infrared spectroscopy, 7 .…”

“…The development of efficient water oxidation catalysts (WOCs) has been the subject of intensive research in the last decade. Since the pioneering studies by Harriman, 1 cobalt oxide nanoparticles (NPs) have been recognized as very active WOCs, displaying low operating overpotentials (η = 0.30 -0.40 V) 2,3,4,5 and residing close to the top of the Volcano plot for metal oxides, 6 while promoting fast catalysis (turnover frequency (TOF) up to 3 s -1 per surface catalytic site). 3,7 In addition, cobalt oxide NPs, in the Co 3 O 4 cubic spinel crystalline structure, have been employed as WOCs in light activated sacrificial systems employing photogenerated Ru(bpy) 3 3+ oxidant, 8,9,10,11,12 with quantum yield up to 0.07, 13 and spectroscopic evidence of Co(III)-OH moieties as the active surface sites.…”