# Perspectives on Cathodes for Protonic Ceramic Fuel Cells
## 1 Introduction  
The dependence of contemporary society on the combustion of fossil fuels for its energy requirements has created the urgent and overriding challenge of arresting the climate change that they promote by transitioning to a carbon-neutral, sustainable energy economy. Hydrogen is likely to play an expanding role as an energy vector in a sustainable and clean future economy, in which its chemical energy is converted to electrical energy in fuel cells [1]. Solid oxide fuel cells (SOFCs; commonly used abbreviations are listed in Table 1) based on oxide-ion conducting electrolytes have been intensely developed due to their high combined heat and power efficiency, long-term stability, low emissions, and relatively low cost [2, 3]. The high operating temperature (typically in the range 800-1000 \({}^{\circ}\)C) provides the advantage of fuel flexibility, and their operation with simple hydrocarbons is often touted as a gateway to their widespread employment with hydrogen as fuel. Nevertheless, the high operating temperature is associated with long start-up times and problems with mechanical and chemical compatibility. Consequently, there has been much research effort dedicated to lowering the operation temperature of SOFCs to better address small-scale applications (micro SOFCs and auxiliary power units) and reduce the costs associated with fabrication and operation [4]. In particular, a running temperature of 600 \({}^{\circ}\)C would allow the use of stainless-steel balance-of-plant components and cheaper interconnect materials. Moreover, the thermochemical efficiency of combustion of natural gas is maximum at this temperature [5]. Certainic fuel cells with a proton-conducting electrolyte (protonic ceramic fuel cells, PCFCs, or proton-conducting solid oxide fuel cells, H\({}^{+}\)-SOFCs) have been of ever-growing interest since the pioneering work of [PERSON] starting in the 1980s on proton-conducting ceramic materials [6; 7; 8; 9]. Such cells may also be operated in reverse to affect the electrolysis of water (protonic ceramic electrolysis cells, PCECs) [10; 11]. These electrochemical cells may operate within an intermediate temperature (400-800 \({}^{\circ}\)C), which is partly due to the higher mobility and lower activation energy of protons in comparison to oxide ions in this range. PCFCs have additional advantages: the water in the electrochemical reaction is generated in the cathode, so no further fuel recycling is required, and Ni in the fuel electrode remains at a suitably low oxygen partial pressure [5]. However, lowering the operating temperature creates much larger overpotentials at the electrode-electrolyte interface [12]. Specifically, the reaction kinetics related to oxygen reduction occurring in the air electrode are slower than fuel oxidation in the anode or ion migration in the electrolyte [13]. Hence, the cathode plays a major role in determining the efficiency of fuel-cell operation in the intermediate-temperature range. Electrochemical modelling has shown that, whereas lower ohmic resistance offered by thinner electrolytes is the easiest way to improve the performance of PCFCs, the performance is otherwise restricted by the attempted current density at the cathode, meaning that the cathode limits the performance of the whole cell [13]. A number of studies of protonic ceramic single cells, particularly earlier studies, employed a single metallic phase as an air electrode, most commonly platinum [5; 14; 15]. The elementary reaction steps occurring in a Pt cathode in contact with a protonic ceramic membrane in fuel-cell mode for the overall oxidation reaction have been described by [PERSON] et al.