Improving the Oxygen Evolution Reaction: Exsolved Cobalt Nanoparticles on Titanate Perovskite Catalyst.

Perovskites are an important class of oxygen evolution reaction (OER) catalysts due to highly tunable compositions and adaptable characteristics. However, perovskite-based catalysts can have limited atom utilization efficiency due to large particle size, resulting in low mass activity. Herein, Cobalt nanoparticles are exsolved from La0.2+2x Ca0.7-2x Ti1-x Cox O3 perovskite and applied in OER. Upon reduction in the 5% H2 /N2 atmosphere at 800 °C for 2 h, the Co exsolved perovskite catalyst (R-LCTCo0.11) exhibits optimal OER performance. The mass activity of R-LCTCo0.11 reaches ≈1700 mA mg-1 at an overpotential of 450 mV, which is 17 times and 3 times higher than that of LCTCo0.11 (97 mA mg-1 ) and R-Mix (560 mA mg-1 ) catalysts respectively, surpassing the benchmark catalyst RuO2 (42.7 mA mg-1 of oxide at η = 470 mV). Electrochemical impedance spectroscopy (EIS) data reveals that R-LCTCo0.11 has the lowest charge transfer resistance (Rct  = 58 Ω), demonstrating the highest catalytic and kinetic activity for OER. Furthermore, this catalyst shows high stability during an accelerated durability test of 10 h electrolysis and 1000 cycles cyclic voltammetry (CV). This work demonstrates that nanoparticle exsolution from a doped perovskite is an effective strategy for improving the atom utilization efficiency in OER.

Walnut-like Transition Metal Carbides with Three-Dimensional Networks by a Versatile Electropolymerization-Assisted Method for Efficient Hydrogen Evolution.

Mo2C@NPC (N,P-doped carbon) electrocatalysts are developed on carbon cloth (CC) as binder-free cathodes for efficient hydrogen evolution through a facile route of electropolymerization followed by pyrolysis. Electropolymerization of pyrrole to form polypyrrole occurs with the homogeneous incorporation of PMo12, driven by Coulombic force between the positively charged polymer backbone and PMo12 anions. This electrochemical synthesis is easily scaled up, requiring neither complex instrumentation nor an intentionally added electrolyte (PMo12 also acts as an electrolyte). After pyrolysis, the resultant Mo2C@NPC/CC electrode exhibits a unique interconnected walnut-like porous structure, which ensures strong adhesion between the active material and the substrate and favors electrolyte penetration into the electrocatalyst. This method is effective with other monomers such as aniline and is readily extended to fabricate other metal carbide electrodes such as WC@NPC/CC. These carbide electrodes exhibit high catalytic performance for hydrogen production, for example, WC@NPC/CC can deliver an unprecedented current density of 600 mA cm-2 at an overpotential of only 200 mV either in an acidic or an alkaline solution. Considering the simplicity, scalability, and versatility of the synthetic method, the unique electrode structure, and the excellent catalysis performance, this study opens up new avenues for the design of various novel binder-free metal carbide cathodes based on electropolymerization.

Hierarchically structured multi-shell nanotube arrays by self-assembly for efficient water oxidation.

Photosynthesis in plants occurs at structures which form by self-assembly under ambient conditions, while catalysts used for artificial photosynthesis normally need special conditions like high pressure or temperature. Herein, a facile and cost effective way for the synthesis of a highly complex and efficient oxygen evolution reaction (OER) catalyst, formed solely by self-assembly in solution, is presented. Without the need for any instrumentation except for a glass beaker, highly active nickel-iron-copper multi-shell nanotube arrays are produced by immersion of a copper plate in three different solutions. Cu(OH)2 nanowires are first self-grown on a copper substrate in a basic solution and subsequently converted to novel iron-copper hydroxide nanotubes by immersion in an Fe3+ solution by a sacrificial template-accelerated hydrolysis mechanism. Finally, an additional layer of nickel nanosheets is added by treating in a nickel chemical bath. The resulting electrode shows a current density as high as 100 mA cm-2 at an overpotential of 320 mV with a Tafel slope of 32 mV dec-1, while also exhibiting long time stability. The use of inexpensive first-row transition metals, simple preparation methods with no energy consumption, the unique hierarchical structure of the nanosheet covered nanotubes, and the high catalytic performance are remarkable, and this study may therefore lead to more convenient and competitive routes for water splitting.

Cs(I) Cation Enhanced Cu(II) Catalysis of Water Oxidation.

We report here a new catalytic water oxidation system based on Cu(II) ions and a remarkable countercation effect on the catalysis. In a concentrated fluoride solution at neutral to weakly basic pHs, simple Cu(II) salts are highly active and robust in catalyzing water oxidation homogeneously. F(-) in solution acts as a proton acceptor and an oxidatively robust ligand. F(-) coordination prevents precipitation of Cu(II) as CuF2/Cu(OH)2 and lowers potentials for accessing high-oxidation-state Cu by delocalizing the oxidative charge over F(-) ligands. Significantly, the catalytic current is greatly enhanced in a solution of CsF compared to those of KF and NaF. Although countercations are not directly involved in the catalytic redox cycle, UV-vis and (19)F nuclear magnetic resonance measurements reveal that coordination of F(-) to Cu(II) is dependent on countercations by Coulombic interaction. A less intense interaction between F(-) and well-solvated Cs(+) as compared with Na(+) and K(+) leads to a more intense coordination of F(-) to Cu(II), which accounts for the improved catalytic performance.