Patterning Metal Oxides Using Electrochemical Methods and Exploring the Dynamics of Water-Splitting Catalysts

Abstract

Artificial photosynthesis is the electrochemical decomposition of water into oxygen and hydrogen driven by solar energy, allowing for the storage of solar energy in chemical bonds. Integrated photoelectrochemical cells (PECs), such as the “artificial leaf,” have the potential to reduce the cost of artificial photosynthesis. In a buried-junction PEC configuration, a catalyst for the oxygen evolution reaction (OER) is deposited onto one surface of a photovoltaic, with a catalyst for the hydrogen evolution reaction (HER) deposited on the opposite side. Such designs eliminate the need for metallic wires, thereby reducing the material cost of the device. A problem faced by the artificial leaf is a loss in efficiency resulting from the absorption of sunlight by the catalyst. Catalysts can be appropriately distributed using lithography, but at a significant expense of time and material. In view of this challenge, simple electrochemical methods can be used to control the distribution of catalyst on a substrate. Here, we examine an electrochemical method that allows for the patterning of metal oxides over conductive surfaces. Reactive interfacial patterning promoted by lithography, or “RIPPLE,” allows for the formation of sub-micron ridges of oxide by employing cyclic voltammetry. The method is expanded to include oxides that are known as efficient OER catalysts. Furthermore, the tolerance of this method to pattern electrochemically-deposited metal films, as well as metal films on rough substrates, suggests it is well suited for patterning catalysts in a scalable fashion. We find that the patterns generated by RIPPLE using a linear potential scan (cyclic voltammetry) can be identically reproduced by utilizing a square wave function (binary-potential step method). The reduction of applied potentials not only simplifies the method, but also sheds light on the mechanism by which the patterns are formed. A scaling analysis reveals the mass transport limitations that govern pattern formation. We further investigate how the lithographic resist influences mass transport. Understanding this relationship inspires a new patterning technique that allows us to substitute the lithographic resist with an emulsion. Finally, we attempt patterning at a triple-phase interface, thereby obviating the masking requirement altogether. The dynamics of metal oxides under OER catalysis is of relevance to the preservation of oxide patterns. An oxide undergoing dynamic changes to its surface would result in a homogenously distributed film. We explore the dynamics of a Co oxide-based OER catalyst as a function of pH, which elucidates fundamental behavior of the catalyst during deposition and OER operation. Furthermore, we investigate dynamic catalysts that exploit the equilibria observed under dynamic conditions.