Combinatorial Discovery and Optimization of a Complex Oxide with Water Photoelectrolysis Activity

Abstract

A ternary oxide containing cobalt, aluminum, and iron and not previously known to be active for the photoelectrolysis of water was identified using a high throughput combinatorial technique. The technique involves ink jet printing overlapping patterns of oxide precursors onto fluorine-doped tin oxide conductive glass substrates. Subsequent pyrolysis yields patterns of mixed oxide compositions that were screened for photoelectrolysis activity by scanning a laser over the material while it was immersed in an electrolyte and mapping the photocurrent response. The composition and optimum thickness for photoelectrochemical response of the newly identified material was further refined using quantitative ink jet printing. Chemical analysis of bulk and thin film samples revealed that the material contains cobalt, aluminum, and iron in a Co 3 O 4 spinel structure with Fe and Al substituted into Co sites with a nominal stoichiometry of Co 3-x-y Al x Fe y O 4 where x and y are about 0.18 and 0.30, respectively. The material is a p-type semiconductor with an indirect band gap of around 1.5 eV, a value that is nearly ideal for an efficient single photoelectrode for water photoelectroylsis. Photoelectrochemical measurements indicate that the onset of hydrogen evolution is about 0.9 V positive of the thermodynamic value but the photocurrent is limited by slow kinetics for hydrogen evolution.

Recent developments in solar water-splitting photocatalysis

“…We have used this approach to discover several new promising oxide semiconductors. 27 Since there are so many possible combinations to produce and screen, we have developed a project to enlist a multitude of undergraduates and high school students to help with the effort to discover these most important materials. The Solar Hydrogen Activity research Kit, or SHArK Project, uses simple materials such as Lego Mindstorms to provide the students with everything they need to produce libraries of metal oxides and screen them for photoelectrolysis activity.…”

Solar fuels photoanode materials discovery by integrating high-throughput theory and experiment

“…Since their advent just a few years ago, these DFT-based databases and analytics tools have already been used to identify more than 20 new functional materials that were later confirmed by experiments across a number of applications (8), motivating concerted efforts to validate theory predictions with experiments (11). However, in photoelectrochemistry for the renewable synthesis of solar fuels, efficient metal-oxide photoanode materials--photoelectrocatalysts for the oxygen evolution reaction (OER)--remain critically missing (12). Forty years of experimental research has yielded just 16 metal-oxide photoanode compounds with band-gap energy in the desirable 1.2-2.8-eV range that strongly overlaps with the solar spectrum.…”

Nanoscale Effects in Water Splitting Photocatalysis

“…Another approach involves the development of new metal oxide materials that combine suitable properties (visible band gap, high carrier mobilities, long carrier lifetimes) with greater chemical stability for photoelectrochemical water splitting. Such materials can be made by directed synthesis, sometimes guided by theory [47], or they can be made by combinatorial approaches, as described by Bruce Parkinson [48][49][50], Eric McFarland [51], and Nathan Lewis [52]. The third strategy is to exploit scaling laws and specific effects at the nanoscale [53][54][55][56] to overcome the limiting problems of metal oxide absorbers, such as their short electron-hole lifetimes and low mobility.…”