Development of an O2-sensitive fluorescence-quenching assay for the combinatorial discovery of electrocatalysts for water oxidation.

High kinetic barriers associated with the oxidation of water to O2 [4,5] and the common use of high-cost electrocatalytic materials are among the challenges that limit the utility of photoelectrochemical energy storage. Improved electrocatalysts that operate at lower overpotential and avoid the use of expensive precious-metal or rare-earth elements are needed. While valuable progress is being made toward this goal, the rational design of optimal catalysts from first principles remains infeasible, and the number of possible catalyst compositions, even those with well-defined metal stoichiometry, far exceeds the number that can be tested in a traditional sequential fashion. Combinatorial methods can play an important role in the discovery of new electrocatalysts, and we describe herein a fluorescence-based assay for spatially resolved, direct detection of O2 across an array of metal-oxide electrocatalysts. Initial implementation of this technique has led to the identification of new electrocatalysts, which are composed entirely of earth-abundant elements (e.g., Ni/Al/ Fe) and warrant further investigation. Combinatorial methods for the discovery of electrocatalysts have been pursued previously. For example, a soluble fluorescent pH indicator has been used to screen electrocatalysts for reactions that consume or generate protons, including water oxidation mediated by platinum-group-metal electrocatalysts. Potential complications with this method include the use of poorly buffered electrolytes to ensure sensitivity to pH changes, and the instability of organic pHsensitive fluorophores under conditions required for water oxidation. Various combinatorial approaches have been used to probe photoelectrocatalytic performance of mixed-metaloxide materials. In most of these assays, the oxides are required to act simultaneously as a light-harvesting semiconductor and as the electrocatalyst for one or both watersplitting half-reactions. The most efficient PECs, however, will probably integrate separate photovoltaic (PV) and catalytic materials. Therefore, we targeted an assay that would enable rapid assessment of electrocatalysts for water oxidation independent of the other PEC functions. Catalysts discovered by such methods could then be used in indirect PECs (Figure 1A) or developed further for integration with PV semiconductors in direct PECs (Figure 1B). The essential feature of electrocatalytic water oxidation is O2 production, and an ideal catalyst-screening assay would directly monitor O2 evolution. In addition, a fluorescencebased assay seemed appealing because such methods are often compatible with parallel, rather than serial, analysis of activity, and they avoid the need for costly specialized analytical instrumentation. Fluorescent pressure-sensitive paints are well suited to meet these criteria. These paints are used in the automobile and aerospace industries to study aerodynamics in wind tunnels, and their utility arises from the sensitivity of their fluorescence intensity to the partial pressure of O2 (pO2). [23] Quantitative measurements are improved by incorporating two fluorophores into the paint, one that is insensitive to O2 as a background reference, and another that exhibits fluorescence quenching in proportion to the pO2. Our assay takes advantage of a commercially Figure 1. Schematic representations of indirect (A) and direct (B) PEC configurations for water splitting. The former employs a PV solar cell coupled to an electrolysis cell, whereas the latter features direct integration of the electrocatalysts with the charge-separating PV semiconductor.