Analysis of Hydrogen Feeding to the Anode of a PEMFC By a Transport Impedance Technique

Hydrogen feeding to the catalyst layer of the anode of a proton-exchange membrane fuel cell (PEMFC) must be fast enough to allow for high power response and avoid starvation events. However, there are limitations to the flow rate posed by the anode architecture (manifold, inlet port, flow field), and the gas diffusion layer (GDL). Within the GDL, hydrogen transport conditions may change in-operando as a result of water generation and saturation of pores. Delays in hydrogen feeding will give rise to a decrease in power response, and transitory sub-stoichiometric conditions, that may damage the electrode and decrease fuel cell durability. Therefore, it is of high interest to probe hydrogen feeding conditions when designing new anode architectures, and during operation of the fuel cell. Mass transport conditions in the fuel cell and electrochemical systems can be probed by techniques based on the impedance concept [1,2,3]. Among them, one recently applied in our group is the current modulated H2 flow-rate spectroscopy (CH2S), which provides the transfer function H [4,5]: H(j w) = nF QH2 / I (Eq. 1) Where QH2 is the modulated hydrogen inlet flow, I the modulated cell current, n(=2) the electron exchanges per H2 molecule, and F(=96485 C mol-1) the Faraday constant. A typical response in a PEMFC with dead-end anode is shown in Fig. 1. The H function normally presents two or more semicircles in Nyquist plots, extending in the real axis from H'=0 to H'=1 (stoichiometric modulation). The high frequency semicircle is normally ascribed to the set-up time response limitation, mostly the flow meter. At higher frequencies, the H function shows characteristics of the time response of hydrogen flow up to the anodic catalyst layer. In this communication, the CH2S technique is applied in conventional single cells and in passive portable feeding PEMFCs. Some properties of H2 transport path towards the anodic catalyst layer are analyzed, like conduits length, inlet port type, anode flow field, liquid water contents, hydrogen stoichiometry, and anode hydrophobicity. Acknowledgement: The work is partially financed by the ELHYPORT project (PID2019−110896RB-I00), Spanish Ministry of Science and Innovation. [1] C. Deslouis, I. Epelboin, C. Gabrielli, P.S.-R. Fanchine, B. Tribollet, Relationship between the electrochemical impedance and the electrohydrodynamical impedances measured using a rotating disc electrode, J. Electroanal. Chem. Interfacial Electrochem. 107 (1980) 193–195. [2] A. Sorrentino, T. Vidakovic-Koch, R. Hanke-Rauschenbach, K. Sundmacher, Concentration-alternating frequency response: A new method for studying polymer electrolyte membrane fuel cell dynamics, Electrochim. Acta. 243 (2017) 53–64. [3] D. Grübl, J. Janek, W.G. Bessler, Electrochemical Pressure Impedance Spectroscopy (EPIS) as Diagnostic Method for Electrochemical Cells with Gaseous Reactants: A Model-Based Analysis, J. Electrochem. Soc. 163 (2016) A599–A610. [4] M.A. Folgado, H. Moreno, A. Molinero, J.C. Oller, J.M. Barcala, A.M. Chaparro Hydrogen Transport Impedance for the Study of Anodes in PEMFCs, European Fuel Cell Forum 2021, A0704 (Extended Abstract). Lucerne (Switzerland). [5] A. Molinero, J.C. Oller, J.M. Barcala, H. Moreno, M.A. Folgado, A.M. Chaparro, Experimental Set-Up for Transport Studies of Anodes in PEMFCs. European Fuel Cell Forum 2021, B0207 (Extended Abstract). Lucerne (Switzerland). Fig. 1. Nyquist plot of the H function according to Eq. 1, for a PEMFC cell with commercial electrodes (Pt/C 0.3mg·cm-2) and Nafion 212NR membrane, working with hydrogen feeding in dead-end mode, and air feeding in cathode. a) Full signal; b) Low frequency detail. Numbers are modulation frequencies. Figure 1