# Perspectives on Cathodes for Protonic Ceramic Fuel Cells
## 6 Summarising Remarks  
The cathode is generally recognized as critical for the performance of solid oxide fuel cells, and even more so for the proton-conducting class of ceramic devices. The lower working temperature of protonic ceramic fuel cells places greater demands on the cathode with regard to high catalytic activity and electronic conductivity, even though the lower temperature favours thermal and chemical compatibility between cell components. The earliest studies of PCFC cathodes, dating back to the mid-1990s, parallel SOFC research, with the employment of simple perovskites based on ferrites and cobaltites, and, to a much lesser extent, manganites. Nevertheless, the cathode reactions differ between oxide-ion- and proton-conducting systems. The reaction steps for the overall reduction reaction occurring in a proton-conducting cathode compartment seem to be fairly intuitive, but a clear picture of the rate-determining steps has not emerged. There is widespread agreement that high electron-hole conductivity is essential for a good electrode performance, and good surface proton conductivity is also often recognized as important. Nevertheless, excellent performances have been achieved with \"proton-blocking\" electrodes, for which it is proposed that the lower level of hydration in the cathode leads to a lower water vapour partial pressure and higher cell potential. A similar argument suggests that high proton conductivity is not necessarily important provided oxide-ion conductivity is high. The different nature of the cathode reactions between the oxide-ion and proton-conducting systems has driven the pursuit of alternative approaches for PCFC cathode compositions beyond the classical SOFC perovskites. One of the most prominent strategies, focusing on the proton-conducting character of the cathode, has been to dope established proton conductors, typically Ba(Ce,Y)O\({}_{3-\delta}\), Ba(Zr,Y)O\({}_{3-\delta}\), with multivalent cations, such as Co and Pr, to enhance the electronic-transport component. However, this approach has met with only limited success, since the electronic conductivity and phase stability are generally not sufficient for electrode applications. The analogy to cathode SOFC research extends to a wide adoption of composite electrodes with different oxide phases providing the electronic and ionic requirements. Very good performances have been achieved with a wide variety of composites, despite their greater complexity in terms of transport properties, microstructure, and compatibility. High power densities have also been achieved for monophasic air electrodes, particularly more recently for so-called triple-conducting compositions, with contributions to transport from electron holes, oxide ions, and protons. Mostly, such phases are recognized mixed ionic-electronic conductors with layered perovskite and Ruddlesden-Popper-type structures, such as Pr\({}_{2}\)NiO\({}_{4+\delta}\) and PrBaCo\({}_{2}\)O\({}_{5+\delta}\), which have generally been first adopted in SOFC research. The true nature of transport with the three co-existing species has been a source of debate, since some studies indicate that the protonic contribution may arise from secondary phases or surface reactions. However, it is difficult to reach generalisations covering an entire class of materials as, in some cases, the evidence for bulk triple conduction is convincing. The evolution of maximum power densities for protonic ceramic single cells since 2008 is shown in Figure 10, where a trend toward generally higher outputs and their more frequent reports can be discerned in the last few years. Future research endeavours are likely to increasingly focus on triple-conducting phases due to their outstanding performances. The employment of infiltrated elements with high catalytic activity has proven to be successful in a limited number of studies, and is also likely to be a popular future route to achieving better cathode performance.