Mixed-metal nanosheet water oxidation catalysts made by pulsed-laser ablation in liquids - Part 1: Synthesis, characterization, and electrocatalysis

Global sustainable energy solns. remain one of the greatest challenges of the 21st century. Health and climate risks assocd. with combustion of fossil fuels threaten global stability, development and national security. The sun is the most abundant and cleanest source of energy, but its intermittence on earth and unequal local energy needs require conversion into stored fuels. Chem. can meet this challenge by solar-driven water splitting with earth-abundant, efficient and robust materials. Water oxidn. is central to the prodn. of storable chem. fuels, since ample supply of cleanly, efficiently, and affordably generated protons and electrons is a prerequisite for all sustainable chem. transformations. Pulsed-laser ablation in liqs. (PLAL) is a flexible synthetic strategy to prep. earth-abundant, surfactant-free, mixed-metal (hydrous) oxide nanoparticle water oxidn. catalysts [Blakemore, Gray, Winkler, Mueller, ACS Catal. 2013, 3, 2497]. It offers size and compn. control through multiple tuneable parameters (e.g. laser pulse energy and elemental content in the ablation target and liq.). With PLAL, many different nanocatalysts can readily be synthesized and screened for water oxidn. activity, rendering PLAL a medium-throughput method for catalyst design. We prepd. a series of Ni-Fe materials and systematically varied Fe content. Oxygen evolution activity in basic electrolyte increased as Fe content decreased to 22%. Addn. of Ti^(4+) and La^(3+) ions further enhanced electrocatalysis, reaching 10 mA cm^(-2) at 260 mV overpotential; on a flat working electrode, this is the lowest overpotential to date for Fe-Ni catalysts. We spectroscopically identified [Ni-Fe]-layered double hydroxide nanosheets with intercalated nitrate and water, [Ni_(1-x)Fe_x(OH_2)](NO_3)_y(OH)_(x-y)•nH_2O, as the most active precatalyst. Higher turnover frequencies were obsd. with a greater relative proportion of a 405.1 eV N 1s (XPS binding energy) species in the nanosheets [Hunter, Blakemore, Deimund, Gray, Winkler, Mueller, J. Am. Chem. Soc. 2014, 136, 13118]. The effect of different intercalated anions on water oxidn. activity was investigated