The development of an efficient catalyst for the oxygen evolution reaction (OER) is critical to fulfilling the mission of hydrogen generation by water splitting. Various multicomponent systems have been investigated so far for the OER, although a systematic investigation is lacking and there are discrepancies as to which formulations make the best catalyst. Here, we perform a systematic investigation of a ternary Ni-Fe-Co oxide gradient library for the OER, using a combinatorial approach. This approach allows a much faster investigation of a vast compositional space compared to the traditional step by step approach. Also, it enables a more reliable comparison of the various catalysts as they all experience the same process and measurement conditions. We used the spray pyrolysis technique in combinatorial electrocatalyst screening for the first time for the generation of a gradient library of Ni-Fe-Co oxides. Combinatorial electrochemical screening using a scanning droplet cell (SDC) set up allowed the study of 94 different points in the compositional space of the trimetallic Ni-Fe-Co oxide library, which was used to deduce a composition–activity relationship. At 10 mA cm–2, the investigated points spanned an overpotential window of 294–320 mV, with the best performing ternary oxides being confined to a compositional space of 4–15% Fe, 4–15% Co, and 80–90% Ni. This study provides a foundation for broad exploration of other mixed-metal oxide combinations.

We present a high-throughput (combinatorial) method to screen thin ceramic films as Li-ion conductors by mapping an optical effect of Li-ion conduction. The method, while qualitative, is fast and simple to implement, provides a planar (XY) map of Li-ion conductivity through different parts of the film. The effect, FTO darkening, is an optoelectrochemical one that relies on darkening of the FTO (F-doped tin oxide) substrate, onto which the investigated film is deposited. The rate of color change of the FTO reflects the rate of Li-ion migration through the film.The method is validated by testing two model systems, a Li− La−S−O film with uniform composition and varying thickness, and a Li−La−P−O film with varying thickness and lateral composition. The darkening rate, obtained from optical transmission, correlates linearly with inverse film thickness. The darkening rate map can be compared with a resistance map obtained by impedance measurements, showing that only Li conduction is measured. We discuss the conditions required to distinguish between areas with pure ion conductivity and those with mixed conductivity, the reversibility of the darkening effect and artifacts.

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