Water Oxidation Catalysts: The Quest for New Oxide-Based Materials

Polar Layered Intermetallic LaCo2P2 as a Water Oxidation Electrocatalyst

We investigate LaCo₂P₂ as an electrocatalytic material for oxygen evolution reaction (OER) under alkaline and acidic conditions. This layered intermetallic material was prepared via Sn-flux high-temperature annealing. The electrocatalytic ink, prepared with the ball-milled LaCo₂P₂ catalyst at the mass loading of 0.25 mg/cm², shows OER activity at pH = 14, reaching current densities of 10, 50, and 100 mA/cm² under the overpotential of 400, 440, and 460 mV, respectively. Remarkably, the electrocatalytic performance remains constant for at least 4 days. Transmission electron microscopy reveals the formation of a catalytically active CoO_x shell around the pre-catalyst LaCo₂P₂ core during the alkaline OER. The core serves as a robust support for the in situ-formed electrocatalytic system. Similar studies under pH = 0 reveal the rapid deterioration of LaCo₂P₂, with the formation of LaPO₄ and amorphous cobalt oxide. This study shows the viability of layered intermetallics as stable OER electrocatalysts, although further developments are required to improve the electrocatalytic performance and increase the stability at lower pH values.

Self-healing oxygen evolution catalysts

Electrochemical and photoelectrochemical water splitting offers a scalable approach to producing hydrogen from renewable sources for sustainable energy storage. Depending on the applications, oxygen evolution catalysts (OECs) may perform water splitting under a variety of conditions. However, low stability and/or activity present challenges to the design of OECs, prompting the design of self-healing OECs composed of earth-abundant first-row transition metal oxides. The concept of self-healing catalysis offers a new tool to be employed in the design of stable and functionally active OECs under operating conditions ranging from acidic to basic solutions and from a variety of water sources.

Large scale sustainable energy storage by water splitting benefits from performing the oxygen evolution reaction under a variety of conditions. Here, the authors discuss self-healing catalysis as a new tool in the design of stable and functionally active catalysts in acidic to basic solutions, and a variety of water sources

Molecular and heterogenized dinuclear Ir-Cp* water oxidation catalysts bearing EDTA or EDTMP as bridging and anchoring ligands

The development of efficient water oxidation catalysts (WOCs) is of key importance in order to drive sustainable reductive processes aimed at producing renewable fuels. Herein, two novel dinuclear complexes, [(Cp*Ir)₂(μ-κ³-O,N,O-H₄-EDTMP)] (Ir-H₄-EDTMP, H₄-EDTMP^4- = ethylenediamine tetra(methylene phosphonate)) and [(Cp*Ir)₂(μ-κ³-O,N,O-EDTA)] (Ir-EDTA, EDTA^4- = ethylenediaminetetraacetate), were synthesized and completely characterized in solution, by multinuclear and multidimensional NMR spectroscopy, and in the solid state, by single crystal X-Ray diffraction. They were supported onto rutile TiO₂ nanocrystals obtaining Ir-H₄-EDTMP@TiO₂ and Ir-EDTA@TiO₂ hybrid materials. Both molecular complexes and hybrid materials were found to be efficient catalysts for WO driven by NaIO₄, providing almost quantitative yields, and TON values only limited by the amount of NaIO₄ used. As for the molecular catalysts, Ir-H₄-EDTMP (TOF up to 184 min^-1) exhibited much higher activity than Ir-EDTA (TOF up to 19 min^-1), likely owing to the higher propensity of the former to generate a coordination vacancy through the dissociation of a Ir-OP bond (2.123 Å, significantly longer than Ir-OC, 2.0913 Å), which is a necessary step to activate these saturated complexes. Ir-H₄-EDTMP@TiO₂ (up to 33 min^-1) and Ir-EDTA@TiO₂ (up to 41 min^-1) hybrid materials showed similar activity that was only marginally reduced in the second and third catalytic runs carried out after having separated the supernatant, which did not show any sign of activity, instead. The observed TOF values for hybrid materials are higher than those reported for analogous systems deriving from heterogenized mononuclear complexes. This suggests that supporting dinuclear molecular precursors could be a successful strategy to obtain efficient heterogenized water oxidation catalysts.

Perovskite Oxides Prepared by Hydrothermal and Solvothermal Synthesis: A Review of Crystallisation, Chemistry, and Compositions

Perovskite oxides with general composition ABO₃ are a large group of inorganic materials that can contain a variety of cations from all parts of the Periodic Table and that have diverse properties of application in fields ranging from electronics, energy storage to photocatalysis. Solvothermal synthesis routes to these materials have become increasingly investigated in the past decade as a means of direct crystallisation of the solids from solution. These methods have significant advantages leading to adjustment of crystal form from the nanoscale to the micron-scale, the isolation of compositions not possible using conventional solid-state synthesis and in addition may lead to scalable processes for producing materials at moderate temperatures. These aspects are reviewed, with examples taken from the past decade's literature on the solvothermal synthesis of perovskites with a systematic survey of B-site cations, from transition metals in Groups 4-8 and main group elements in Groups 13, 14 and 15, to solid solutions and heterostructures. As well as hydrothermal reactions, the use of various solvents and solution additives are discussed and some trends identified, along with prospects for developing control and predictability in the crystallisation of complex oxide materials.

Ir- and Ru-doped layered double hydroxides as affordable heterogeneous catalysts for electrochemical water oxidation

Doping low-cost LDHs with noble metal atoms represents a promising approach to develop effective heterogeneous Water Oxidation Catalysts.

Cobalt Carbonate as an Electrocatalyst for Water Oxidation

Co^II salts in the presence of HCO₃ ^- /CO₃ ^2- in aqueous solutions act as electrocatalysts for water oxidation. It comprises of several key steps: (i) A relatively small wave at E_pa ≈0.71 V (vs. Ag/AgCl) owing to the Co^III/II redox couple. (ii) A second wave is observed at E_pa ≈1.10 V with a considerably larger current. In which the Co^III undergoes oxidation to form a Co^IV species. The large current is attributed to catalytic oxidation of HCO₃ ^- /CO₃ ^2- to HCO₄ ^- . (iii) A process with very large currents at >1.2 V owing to the formation of Co^V (CO₃ )₃ ^- , which oxidizes both water and HCO₃ ^- /CO₃ ^2- . These processes depend on [Co^II ], [NaHCO₃ ], and pH. Chronoamperometry at 1.3 V gives a green deposit. It acts as a heterogeneous catalyst for water oxidation. DFT calculations point out that Coⁿ (CO₃ )₃ ^n-6 , n=4, 5 are attainable at potentials similar to those experimentally observed.