Interaction of Catalysts for Unitized Regenerative Fuel Cells

Abstract

There is a need for flexible and efficient storage facilities so that the integration of renewable energies and thus the energy transition can succeed. Intermediate storage of energy in the form of hydrogen by electrolysers and effective reconversion to electricity by fuel cells is a possibility. The unitized regenerative fuel cell (URFC) is a single-unit device that combines an electrolyser and a fuel cell, which makes the system costs much cheaper compared to two separate devices. Currently, the focus of URFC research, similar to proton exchange membrane (PEM) electrolysers, is on the development of new catalysts and electrodes with the aim of reducing costs through manufacturing and design optimization [1,2]. In this context, equally important is the increase of electrochemical activity on the oxygen side since oxygen evolution (OER) and oxygen reduction (ORR) are the speed-determining steps comparatively to their counter reactions on the hydrogen side. Due to high potentials and the corrosive oxygen atmosphere, there are additional requirements for electrode stability on the oxygen side. According to the current state of the art, the materials currently used as OER and ORR catalysts in single cell systems are resorted to and combined for the bifunctional oxygen electrode [3,4]. An effective working catalyst layer is crucial for high power densities in both operating directions and thus a research focus for URFCs.To further improve the stability and activity of the catalysts, support materials are used [5]. Important for support materials are good electrical conductivities and a high surface area, to allow a fine dispersion. This maximizes the relative electrochemical catalyst surface area (ECSA), thus leading to a reduction in costs and catalyst layer thickness, which in turn can lead to a reduction in mass transport resistances. A thin catalyst layer would also have advantages for alternating operation between electrolysis and fuel cell mode, since improved removal of liquid water from the catalyst layer can be ensured here.In this study we prepared supported and unsupported Ir and Pt catalysts. We investigated the distribution of catalyst particles on the support, measured by transmission electron microscope (TEM), and the crystallite sizes and structures by X-ray diffraction (XRD). Bifunctional electrodes consisting of supported and unsupported catalysts were investigated by rotating disk electrode (RDE) and single cell measurements. In this talk, we will take a closer look at interactions between Pt and Ir and their influence on OER and ORR.