Proton exchange membrane fuel cells generate electrical power by breaking down molecular hydrogen at finely dispersed platinum nanoparticles on the surface of a proton-conducting membrane. Simultaneously, oxygen is reduced at the cathode, resulting in the formation of water. At high current densities, the oxygen reduction is often limited by the transport of protons through the membrane. It is not feasible to use thinner membranes, as this would make them susceptible to degradation.
A promising alternative approach involves the direct supply of protons to the cathode, thus bypassing the mass transport limitations through the membrane. This can be achieved by providing an acidic environment on the cathode, so-called acid doping, thus improving the performance of fuel cells. Here, the electrode and the ionomer – a polymer that ensures the protonic conductivity – are acidic, while the electrolyte remains alkaline.
An important role is played by surface oxides
Researchers of the Laboratory for Neutron Scattering and Imaging and the Laboratory for Electrochemical Interfaces at PSI and the Helmholtz-Zentrum Hereon have now been able to identify and characterize the processes that take place on the cathode during this so-called acid doping.
For the experiments, the researchers used two different setups: On the one hand, model experiments in a specially designed electrochemical cell allowed them to perform X-ray photoelectron spectroscopy experiments at the beamline of the Swiss Light Source SLS at PSI. On the other hand, they used operando electrochemical impedance measurements in a fuel cell test bench.
The combination of the experimental results with theoretical models developed at the University of Vienna (Austria) allowed the researchers to identify and describe the underlying mechanisms in detail.
Key role of surface oxides
The scientists were able to visualize and chemically analyze the cathode under realistic fuel cell conditions, that is, during the electrochemical oxygen reduction reaction. For the first time, they were able to show how the cathode surface is modified in the acidic environment. Specifically, they were able to demonstrate that protons from the acidic electrolyte react with the iron of the cathode to form iron oxides: These iron oxides then react further with ionomer molecules, improving the protonic conductivity of the cathode and thus the overall performance of the fuel cell.
"As the iron oxide forms on the surface of the cathode, the ionomer molecules can better anchor to the surface and are in better contact with the iron surface. They are therefore able to transport protons more easily," explains PSI researcher and first author of the study, Thomas Justus Schmidt.
The exact understanding of these complex mechanisms can provide important insights for the further development and optimization of fuel cells, in particular of highly efficient low-temperature fuel cells for the mobility sector and stationary applications.
