Materials and Design Selection to Improve High Current Density in PEMFC

Abstract

Proton-exchange-membrane fuel cells (PEMFC) have been greatly advanced towards commercially viable hydrogen-powered fuel cell systems for automotive application in recent years. Significant reduction in cost has been achieved by reducing the loading of Pt catalyst in the electrodes. While extensive research into the development of Pt-alloy nanoparticle catalysts has led to improvements in the low current density (LCD) kinetic region, the current state of the art cathode electrodes suffers from a poor high current density (HCD) performance at low Pt loadings which leads to an increase in stack area and cost. While the LCD behavior is largely dependent on the intrinsic O2-reduction kinetic activity of the Pt/Pt-alloy nanoparticles, the HCD behavior is dependent on reactant/product transport within the micro-/nano-structure of the catalyst layer. This brings into focus the need to fundamentally understand the functional role and microstructure-property relationships of the various components in the cathode catalyst layer towards mitigating the HCD mass transport losses at low PGM loadings1. In addition to the kinetic activity of the catalyst, the HCD performance is affected by mass transport losses (both proton and oxygen) and ohmic losses. To quantify various voltage loss terms and thus identify the opportunity for improvement, mathematical models with parameters extracted from experiments are commonly employed. Experimental measurements are usually carried out under differential cell conditions operating at high stoichiometry flow with minimal pressure drop down the channel, making it almost one-dimensional across the cell. The cell performance can be described by: E cell = E rev -iRΩ - ηHOR - ιηORR ι - i (RH+,a + RH+c) - ηtx,O2 (1) where E cell is the cell voltage; E rev is the reversible cell voltage, i is the current density; RΩ is the sum of the Ohmic resistances of proton conduction through the membrane and of electron conduction (commonly referred to as high frequency resistance or HFR); ηHOR and ηORR are the charge transfer overpotentials for the hydrogen oxidation reaction (HOR) and the oxygen reduction reaction (ORR), respectively; RH+ is the effective resistance to proton conduction in an electrode with subscripts a and c denoting anode and cathode, respectively; and lastly, ηtx,O2 is the oxygen transport loss which consists of two parts: one results from oxygen transport through gas phase, and the other is associated with the oxygen transport resistance local to the Pt surface, namely ηtx,O2 = ηtx,O2(gas) + ηtx,O2 (Pt-local) (2) In this study, we analyze the impact of various parameters affecting HCD performance via systematic evaluation of different material and catalyst layer design. Materials selection involves Pt– alloy catalysts on various carbon supports, synthesis routes, ionomer structures. Similarly, design selection would investigate, ionomer content and solvent formulation etc. Measurements in 5 cm2 differential cell with diagnostics such as electrochemical surface area (ECA), mass activity measurements in 100% oxygen, limiting current measurements (to evaluate bulk 2 and local oxygen transport resistance3), electrochemical impedance spectroscopy (EIS) in H2/N2 (to evaluate proton transport losses)4, CO stripping as a function of RH, etc., will be conducted. Measured transport resistances will be correlated to the ex situ properties of catalyst and ionomer microstructures. The measured differential cell data will be used in 1-D model to quantify the performance loss terms. References 1) A. Kongkanand, M. F. Mathias, J. Phys. Chem. Lett., 7 (7), 1127 (2016). 2) D. R. Baker, D. A. Caulk, K. C. Neyerlin, and M. W. Murphy, J. Electrochem. Soc., 156, B991 (2009). 3) T.A. Greszler, D.A. Caulk, P. Sinha, J. Electrochem. Soc., 159, F831 (2012). 4) R. Makharia, M. F. Mathias, D. R. Baker, J. Electrochem. Soc., 152 (5), A970 (2005).