Effect of interlayer anions on [NiFe]-LDH nanosheet water oxidation activity

Abstract

Powering the planet with sustainable, carbon-neutral fuels affects every aspect of human life. Sunlight-driven water splitting is an attractive soln. to provide environmentally benign hydrogen fuel. Global scalability demands that all photoelectrode and catalyst materials consist of earth- abundant elements. The water oxidn. half reaction requires four coupled electron and proton transfer steps, for which robust and efficient electrocatalysts are needed. We have shown previously that [NiFe] - layered double hydroxide (LDH) nanosheets are highly active water oxidn. catalysts [Hunter, Blakemore, Deimund, Gray, Winkler, Mueller, J. Am. Chem. Soc. 2014, 136, 13118]. They were synthesized by pulsed laser ablation in liqs. (PLAL), a medium-throughput method that yields small, monodisperse, surfactant- free, size and compn. controlled nanomaterials [Blakemore, Gray, Winkler, Mueller, A. M. ACS Catal. 2013, 3, 2497]. Our [NiFe]-LDH nanosheets are among the best earthabundant water oxidn. catalysts, reaching 10 mA cm^(-2) at only 280 mV overpotential on a flat electrode in 1 M aq. KOH. We have now investigated the effect of interlayer anions on water oxidn. activity. Our [NiFe]-LDH materials consisted of sheets of edge- shared nickel oxide octahedra, with 22 % of ferric iron substituting at nickel sites. The excess pos. charges of Fe^(3+) substituting for Ni^(2+) were balanced by interlayer anions; water was also present in the interlayer galleries. The small size (<20 nm) of our nanosheets made by PLAL allowed for complete exchange of interlayer anions simply by soaking in aq. solns. We synthesized twelve materials with different interlayer anions, either by anion exchange or directly by PLAL, and assessed their water oxidn. activity in strong aq. base. Const.- current electrolysis data showed that the basicity of interlayer anions matters for water oxidn. catalysis. The catalysts were most active and self- healing in self-buffered carbonate-contg. alk. electrolyte.