PGM-Free Electrode Development and Optimization Using H2 Limiting Current

Abstract

PEMFCs with platinum group metal (PGM) catalysts can meet performance targets, but the main obstacle for widespread commercialization is the high cost. Alternatively, precious group metal-free (PGM-free) catalyst can meet cost targets and have the potential to reach DOE performance goals. However, successful PGM-free PEMFC development must include device-level studies and microstructure optimization, especially for minimizing the concentration overpotential in the thick PGM-free electrode layers. To date, limited work has been done to decouple the gas phase transport losses from those associated with stability1, active-site density2 and ion transport at the MEA level. Elucidating the voltage loss contributions or resistances stemming from gas transport and proton conduction at the MEA level along with subsequent correlation to MEA performance will help identify phenomena currently limiting the inception of PGM-free electrode performance and identify paths to improved electrode optimization. Baker et al. and Beuscher first suggested the use of limiting current for mass transport characterization in 2006.3,4 The technique has been developed over the past decade for operando measurement, deconvolution, and quantification of the discrete contributions to the concentration overpotential, often expressed as a mass transport resistance, arising from constituent materials and cell components.5,6 However, this method can be difficult to apply to PGM-free based electrodes, where ohmic contributions may dominate over transport phenomena. In this condition, hydrogen could be used as the molecular probe for mass transport resistance instead of oxygen. Spingler et al. first attempted the use of hydrogen as a probe molecule for PGM based electrodes.7 In this work, we integrated a platinum black (PtB) sensor layer into the MEA., enabling in situ determination of mass transport resistance throughout the entire PGM-free catalyst layer. These experiments were performed using a 5 cm2 differential cell, operated at differential conditions in all experiments such that the reactant concentration gradient down the channel was negligible. The total transport resistance ( ) is written in Equation 1 and was measured for various PGM-free electrodes with different ionomer loadings. Rtotal = nFCH2, channel / id Here, we will present 1) a limiting current measurement approach to characterize in situ mass transport resistances in fuel cell materials through bulk catalyst layer at an MEA device level; 2) PGM-free catalyst performance with different ionomer loading in the cathode; 3) how to balance mass transport and ionomer loading to achieve the best FC performance. The results from this experimental approach not only offer the guidance of transport properties and the role of ionomer loading played in the PGM-free electrodes but also increase our understanding of achieving an optimized performance for a PGM-free catalyst at an MEA level. Improved device-level understanding of mass transport limitations in the catalyst coated layer can help accelerate deployment of low cost, PGM-free PEMFCs and be useful to guide ongoing research and development in both catalyst synthesis and membrane electrode assembly fabrication.