Role of the diffuse layer in acidic and alkaline fuel cells

A numerical model is developed to study electrolyte dependent kinetics in fuel cells. The model is based on the Poisson–Nernst–Planck (PNP) and generalized-Frumkin–Butler–Volmer (gFBV) equations, and is used to understand how the diffuse layer and ionic transport play a role in the performance difference between acidic and alkaline systems. The laminar flow fuel cell (LFFC) is used as the model fuel cell architecture to allow for the appropriate comparison of equivalent acidic and alkaline systems. We study the overall cell performance and individual electrode polarizations of acidic and alkaline fuel cells for both balanced and unbalanced electrode kinetics as well as in the presence of transport limitations. The results predict cell behavior based on electrolyte composition that strongly correlates with observed experimental results from literature and provides insight into the fundamental cause of these results. Specifically, it is found that the working ion concentration at the reaction plane plays a significant role in fuel cell performance including activation losses and the response to different kinetic rates at an individual electrode. The working ion and the electrode where its consumed are different for acidic and alkaline fuel cells. Therefore, we compare the role of the diffuse region in both acidic and alkaline fuel cells. From this we conclude that oxidant reduction at the cathode and slow fuel oxidation (such as alcohol oxidation) can be improved with an alkaline electrolyte.

Graphical abstract. This comprehensive theoretical study presents important differences between alkaline and acidic fuel cells and identifies reasons for altered electrode kinetics in acidic and alkaline media. For direct comparison between acidic and alkaline fuel cells, a laminar flow fuel cell architecture that is compatible with both acidic and alkaline fuel cells is selected to isolate the electrolyte and ionic transport effects from the physical system differences. Numerical results predict cell behavior based on electrolyte composition that strongly correlates with observed experimental results from literature and provides insight into the fundamental cause of these results. It has been found that the working ion concentration at the reaction plane plays a significant role in fuel cell performance including activation losses and the response to different kinetic rates at an individual electrode.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► In this study we compare the performance of acidic and alkaline fuel cells. ► We study how kinetic and transport limitations are affected by the electrolyte. ► We consider the role of the electric double layer and working ion transport. ► The diffuse layer affects activation losses and overall cell performance. ► Improved alcohol oxidation in alkaline cells is attributed to the diffuse layer.