rechargeable metal-air batteries. [1,2] This reaction demands efficient electrocatalysts that can accelerate the reaction rate, lower the overpotential, and remain stable over time. Currently, noble-metal-based compounds such as IrO 2 and RuO 2 provide good OER performance under alkaline conditions, but their large-scale application is restricted by their scarcity and high cost. [3] Accordingly, much research effort has been devoted to the development of high-performance earth-abundant OER electrocatalysts based on transition-metal elements, usually in the form of metal oxides or metal (oxy)hydroxides, that are inexpensive and stable upon prolonged exposure under oxidizing conditions. [4][5][6][7] In addition to the synergistic effects of transition metals and electrical conductivity, the intrinsic activities of these transition metal oxide or (oxy)hydroxide OER catalysts are closely connected to the number of 3d electrons of the metals; the surface transition-metal ions exhibited e g orbitals which could bond with surfaceanion adsorbates and then influence the binding of oxygenic intermediates. [8,9] The binding strength of these intermediates is thought to dictate catalytic activity. [10] Identifying the relationship between OER activity and the catalyst electronic structure can provide a simple rationale for gaining mechanistic insights and finding new design strategies for the earth-abundant OER catalysts.Among various transition metal-based OER catalysts, metal layered double hydroxides (LDHs) and oxyhydroxides have attracted much attention because of their abundance in the earth's crust and their considerable catalytic activity. [5,6,[11][12][13][14][15][16][17][18][19][20][21] NiFe LDH and more generally NiFe (oxy)hydroxides have emerged as the most active OER catalyst compared to other bimetallic earth-abundant LDHs under basic conditions, [6,15,17,22] and several studies have been directed at understanding the role of Fe in increasing the OER intrinsic activity of NiFe-containing (oxy) hydroxide materials. [23] Boettcher and co-workers demonstrated that Fe incorporation into NiOOH lattice enhances the electronic conductivity in the film and Fe exerts a partial-chargetransfer activation effect on Ni centers throughout the catalyst film, but the enhanced catalytic efficiency cannot be completely explained. [17] To better understand the role of Fe, they further studied other incorporated metal cations (Mn, Ti, Ce, Fe, and La) in NiO x H y , finding that only Fe permanently increases the OER The development of efficient and robust earth-abundant electrocatalysts for the oxygen evolution reaction (OER) is an ongoing challenge. Here, a novel and stable trimetallic NiFeCr layered double hydroxide (LDH) electrocatalyst for improving OER kinetics is rationally designed and synthesized. Electrochemical testing of a series of trimetallic NiFeCr LDH materials at similar catalyst loading and electrochemical surface area shows that the molar ratio Ni:Fe:Cr = 6:2:1 exhibits the best intrinsic OER catalytic activity ...

Developing efficient and stable earth-abundant electrocatalysts for acidic oxygen evolution reaction is the bottleneck for water splitting using proton exchange membrane electrolyzers. Here, we show that nanocrystalline CeO2 in a Co3O4/CeO2 nanocomposite can modify the redox properties of Co3O4 and enhances its intrinsic oxygen evolution reaction activity, and combine electrochemical and structural characterizations including kinetic isotope effect, pH- and temperature-dependence, in situ Raman and ex situ X-ray absorption spectroscopy analyses to understand the origin. The local bonding environment of Co3O4 can be modified after the introduction of nanocrystalline CeO2, which allows the CoIII species to be easily oxidized into catalytically active CoIV species, bypassing the potential-determining surface reconstruction process. Co3O4/CeO2 displays a comparable stability to Co3O4 thus breaks the activity/stability tradeoff. This work not only establishes an efficient earth-abundant catalysts for acidic oxygen evolution reaction, but also provides strategies for designing more active catalysts for other reactions.

The electrochemical oxidation of abundantly available glycerol for the production of value-added chemicals, such as formic acid, could be a promising approach to utilize glycerol more effectively and to meet the future demand for formic acid as a fuel for direct or indirect formic acid fuel cells. Here we report a comparative study of a series of earth-abundant cobalt-based spinel oxide (MCo2O4, M = Mn, Fe, Co, Ni, Cu, and Zn) nanostructures as robust electrocatalysts for the glycerol oxidation to selectively produce formic acid. Their intrinsic catalytic activities in alkaline solution follow the sequence of CuCo2O4 > NiCo2O4 > CoCo2O4 > FeCo2O4 > ZnCo2O4 > MnCo2O4. Using the best-performing CuCo2O4 catalyst directly integrated onto carbon fiber paper electrodes for the bulk electrolysis reaction of glycerol oxidation (pH = 13) at the constant potential of 1.30 V vs reversible hydrogen electrode (RHE), a high selectivity of 80.6% for formic acid production and an overall Faradaic efficiency of 89.1% toward all value-added products were achieved with a high glycerol conversion of 79.7%. Various structural characterization techniques confirm the stability of the CuCo2O4 catalyst after electrochemical testing. These results open up opportunities for studying earth-abundant electrocatalysts for efficient and selective oxidation of glycerol to produce formic acid or other value-added chemicals.

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Decentralized on-site production of hydrogen peroxide (H 2 O 2 ) relies on efficient, robust, and inexpensive electrocatalysts for the selective two-electron (2e − ) oxygen reduction reaction (ORR). Here, we combine computations and experiments to demonstrate that cobalt pyrite (CoS 2 ), an earth-abundant transition-metal compound, is both active and selective toward 2e − ORR in the acidic solution. CoS 2 nanomaterials drop-casted on the rotating ring-disk electrode (RRDE) showed selective and efficient H 2 O 2 formation in 0.05 M H 2 SO 4 at high catalyst loadings, with their operational stability evaluated by structural and surface analyses. CoS 2 nanowires directly grown on the high-surface-area carbon fiber paper electrode boosted the overall performance of bulk ORR electrolysis and the H 2 O 2 product was chemically quantified to yield a ∼70% H 2 O 2 selectivity at 0.5 V vs reversible hydrogen electrode (RHE), in good agreement with the RRDE results. Computations suggested the modest binding of OOH* adsorbate on the single Co site of CoS 2 and the kinetically disfavored O−O bond scission due to the lack of active site ensembles in the crystal structure, consistent with the experimentally observed activity and selectivity. CoS 2 also catalyzes 2e − ORR with less activity and selectivity in the noncorrosive neutral solution. This work opens up the exploration of diverse earth-abundant transition-metal compounds in search of highly active and selective electrocatalysts for efficient H 2 O 2 production.