Transparent Dual-Conductivity Membrane Composites as Current Distributors for Diffuse Electrocatalysts

Integrated photoelectrochemical (PEC) devices are commonly defined by a need to permit carrier flow between a photovoltaic (PV) element and the catalyst layer, while preventing light attenuation between the illumination source and PV by any intervening membrane, catalyst, or other structural component. Many integrated PEC structures must contend with the specific challenge of light having to pass through a catalyst film before being absorbed by a PV. Such device architectures necessitate tradeoffs between catalyst layer thickness (along with catalyst robustness) and catalyst layer transparency. This article details a material composition for potentially mitigating such tradeoffs. It is shown that deposition of the electrically conductive polymer polyethlyenedioxythiophene:polystyrenesulfonic acid (PEDOT:PSS) on the commercial ionomer substrate Nafion (PEDOT:PSS/Nafion) results in a material structure displaying the ionic and electrical conductivities required for driving electrocatalysis, while retaining a high degree of light transparency. This material concept is examined using a highly diffuse (∼5 nm) deposition of Au on a PEDOT:PSS/Nafion membrane as a cathode catalyst in a CO₂ electrolyzer. While direct deposition of 5 nm Au on unmodified Nafion yields a catalyst layer that is electrically insulating, configuring the same Au film on PEDOT:PSS/Nafion results in a composite electrical/ionic conductor that is capable of supporting electrochemical CO₂ reduction to a mixture of CO and hydrogen.