Combined effects of flow channel configuration and operating conditions on PEM fuel cell performance

The performance of proton exchange membrane (PEM) fuel cells is dominated by the interactions of mass transfer and reaction. In this work, these interactions are comprehensively revealed via a "flow field analysis scheme" that combines theoretical analysis and numerical simulation. In this analysis, three operation regimes are proposed given the features of reacting flow: reaction dominated, mass transfer dominated, and axial flow dominated. Based on a detailed study of mass transfer mechanisms, including Taylor dispersion introduced to the gas channel, two dimensionless numbers are further proposed to quantify the transport-reaction interactions inside PEM fuel cells. The modified Damkohler number, defined as the ratio of the reaction rate constant and mass transfer coefficient, clarifies the relative dominance of the external mass transfer and reaction, and its critical value equals 5. Furthermore, a new dimensionless number, beta, is defined as the ratio of the characteristic time of mass transfer and that of the axial flow in the gas channel. The combined effects of flow channel configuration and operating parameters affecting the transport-reaction are covered in beta number with a threshold of 1. The incoordination of those effects causes starved or excessive flows that impair performance, while a positive and minor deviation from the beta threshold is optimum. Adopting three classic flow channel layouts (i.e., parallel, serpentine, and pin-type) as case studies via numerical simulation, the theoretical analysis is further validated and quantified with specific values.