Substrate Selection for Fundamental Studies of Electrocatalysts and Photoelectrodes: Inert Potential Windows in Acidic, Neutral, and Basic Electrolyte

and Basic Electrolyte. PLoS ONE 9(10): e107942. doi:10.1371/journal.pone.0107942 Substrate Selection for Fundamental Studies of Electrocatalysts and Photoelectrodes: Inert Potential Windows in Acidic, Neutral, and Basic Electrolyte Jesse D. Benck. 0 Blaise A. Pinaud. 0 Yelena Gorlin 0 Thomas F. Jaramillo 0 Vipul Bansal, RMIT University, Australia 0 Department of Chemical Engineering, Stanford University , Stanford, California , United States of America The selection of an appropriate substrate is an important initial step for many studies of electrochemically active materials. In order to help researchers with the substrate selection process, we employ a consistent experimental methodology to evaluate the electrochemical reactivity and stability of seven potential substrate materials for electrocatalyst and photoelectrode evaluation. Using cyclic voltammetry with a progressively increased scan range, we characterize three transparent conducting oxides (indium tin oxide, fluorine-doped tin oxide, and aluminum-doped zinc oxide) and four opaque conductors (gold, stainless steel 304, glassy carbon, and highly oriented pyrolytic graphite) in three different electrolytes (sulfuric acid, sodium acetate, and sodium hydroxide). We determine the inert potential window for each substrate/electrolyte combination and make recommendations about which materials may be most suitable for application under different experimental conditions. Furthermore, the testing methodology provides a framework for other researchers to evaluate and report the baseline activity of other substrates of interest to the broader community. - Funding: JDB and YG were supported as part of the Center of Nanostructuring for Efficient Energy Conversion, an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES), under Award No. DE-SC0001060. Funding source websites: (http://cneec.stanford. edu/) and (http://science.energy.gov/bes/). BAP was supported by CCI Solar Fuels, a National Science Foundation Center for Chemical Innovation under Grant No. CHE-1305124. Funding source websites: (http://www.ccisolar.caltech.edu/) and (http://www.nsf.gov/). JDB received support from the National Science Foundation Graduate Research Fellowship Program and a Stanford Graduate Fellowship. Funding source websites: (http://www.nsfgrfp.org/) and (http://sgf. stanford.edu/). BAP received funding from a United Technologies Research Center fellowship in Sustainable Energy and a Natural Sciences and Engineering Research Council of Canada graduate award. Funding source websites: (http://www.utrc.utc.com/about-fellows.html) and (http://www.nserc-crsng.gc.ca/index_ eng.asp). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. . These authors contributed equally to this work. The selection of an appropriate substrate is an important preliminary step in accurately evaluating electrochemically active materials including electrocatalysts, photoelectrodes, and electrochemical capacitors [1]. The substrate is typically defined as an inert, electrically conductive support onto which a material of interest can be deposited [2], but the substrate may also need to fulfill a variety of additional requirements for specialized studies. Key properties of the substrate may include optical transparency, thermal stability, mechanical strength, and chemical stability, among others. Thus, the selection of an appropriate substrate can be challenging, as an experimentalist must consider many different requirements for the substrate material, and the relevant properties will vary depending on the testing parameters. The electrochemical reactivity of the substrate is a key criterion which is particularly important when choosing a substrate for electrochemical applications. In most cases, an inert substrate that exhibits no electrochemical activity under the testing conditions is preferred. Such a substrate facilitates a straightforward analysis because all electrochemical features can be attributed to the active material. In practice, this ideal is never perfectly attained, as the substrate almost always contributes some electrochemical features through capacitance, surface phase changes, or background electrocatalysis [1,2]. In some cases, the substrate can also modify the properties of the electrocatalyst or photoelectrode material [3,4]. This type of interaction can be either beneficial or detrimental to the performance of the system, and as these interactions can be difficult to predict and control, they are not routinely desired for evaluating electrocatalysts or photoelectrodes. Thus, for the majority of electrocatalyst or photoelectrode evaluations, the best strategy is to choose a substrate which approximates an ideal inert support as closely as possible under the given testing conditions. Assessing the electrochemical reactivity of a substrate, however, can be a challenge in its own right because the observed behavior depends not only on the properties of the substrate, but also on the electrolyte, voltage range, temperature, gas purge, and other testing conditions [5]. The difficulties associated with selecting an appropriate substrate are confounded by the wide array of potential substrate materials and the lack of systematic published data aimed at aiding in the selection. The electrochemical reactivity of many individual candidate substrate materials such as indium tin oxide and gold has been studied extensively [614], but these studies have been performed under widely different conditions, and applying these data with the aim of selecting an appropriate substrate is not straightforward. In contrast, there are few reports about the electrochemical reactivity of many other substrate materials such as aluminum-doped zinc oxide. While there have been a few efforts to address this issue over the past several decades [15], to the best of our knowledge, there has been no comprehensive and systematic experimental study of electrochemical substrate materials with the aim of developing guidelines for appropriate substrate selection. In this work, we employ a consistent experimental methodology to examine the electrochemical reactivity and stability of several transparent and opaque conductive materials that are frequently used as substrates in the evaluation of electrocatalyst and photoelectrode materials. We evaluate three transparent conducting oxide substrates (indium tin oxide [13,1622], fluorine-doped tin oxide [17,19,2224], and aluminum-doped zinc oxide [2529]) and four opaque substrates (gold [68,11,3039], stainless steel 304 [5,4048], glassy carbon [15,4969], and highly oriented pyrolytic graphite [7080]). We use testing parameters that approximate the conditions commonly employed in the evaluation of electrocataly (...truncated)