(Invited) Nanomembrane Coating for Advanced Electrocatalysis

Abstract

Advanced electrocatalysis that possesses higher selectivity and durability is more and more important for the electrification of chemical processes, which is expected in the renewable energy society in the future. Electrochemical reactions are often determined by the competition between the thermodynamic redox potential and the applied potential. Because the redox potential is characteristic for determining the product, it is often difficult to adjust the selectivity just by surface chemistry. Against this background, surface modification of electrodes is a well-studied field. Surface modification improves membrane functionality and selectivity, resulting in electrocatalysts with higher productivity and selectivity. Surface protective layers are also associated with material durability, as they prevent toxic and harmful substances from approaching the surface and prevent dissolution of the electrode material into the electrolyte. We are investigating the use of "nanomembrane" coatings on electrodes to achieve better selectivity and durability.Electrochemical reduction deposition of ultrathin layers of Cr and Mo on Pt cathodes has been shown to yield O2-tolerant hydrogen-evolving electrodes in the appropriate pH range. Electrochemical measurements indicate that these surface layers act as a membrane film. Many bulky ions and O2 molecules cannot pass through these surface layers, but H2 molecules can, consistent with electrochemical and spectroscopic measurements. First, the electrochemical properties of Pt modified with other elements such as Cr and Mo were investigated and monitored for catalytic activity under different atmospheres. Next, in-operando X-ray absorption spectroscopy (XAS) was used to investigate the working state of the material: electrochemical measurements in H2 and O2 confirmed that the oxygen reduction reaction (ORR) was severely suppressed and the hydrogen evolution reaction (HER) was maintained. XAS results also showed that the oxidation state of Pt can be freely changed by a potential sweep with or without a coating layer. We extended this work to nanofilm coating on oxygen-evolving anodes for water splitting, and succeeded in anodic deposition of a thin Ce layer on Ni-Fe electrodes; the Ce layer not only prevents unwanted ions from reaching the electrode surfaces, but also prevents Fe ions from leaching from the electrodes, thus improving the durability of the electrodes. In this presentation, key findings of these nanomembrane coatings and future prospects for selective and durable electrodes will be discussed.