Comprehensive Evaluation of Mass Transport Resistance in Pemfcs with Open Flow Field and Land/Channel Architectures

Abstract

The deployment of fuel cell-powered vehicles to the consumer market clearly showed that continued performance improvement and cost reduction are further required. Proton exchange membrane fuel cell (PEMFC) performance can be improved not only by variation of a catalyst or electrode structure but also by modification of the flow field architecture. An open flow field design from Nuvera Fuel Cells enables power density higher than 1 W cm-2 and ensures excellent mass transfer and water management properties [1, 2]. Quantifying mass transport in the cell with open flow field architecture relative to the conventional land/channel benchmark presents high value to developers of fuel cell stacks for mobile applications, considering limited data available in the literature.Previously we developed a method for determining the oxygen mass transfer coefficient which is based on measurements of limiting current distributions using O2 mixtures with different diluents (from He to C3H8) [3]. The method can be modified and be applicable for a single cell and average limiting current density (iave ) can be described by Eq. 1. iave=ie (1-exp(-nFpr/RTiefRMT)) (1)where n is the number of electrons, F is the Faraday constant (96,485 C mol-1), pr is the dry inlet reactant pressure (Pa), R is the ideal gas constant (8.31 J mol-1 K-1), T is the temperature (K), f is the diluent-to-oxygen fraction in the dry inlet stream, and ie is the current density equivalent to the dry inlet oxygen concentration (A m-2), RMT is oxygen mass transport resistance (s m-1).In this work, a serpentine land/channel and single cell open flow field (SCOF) hardware from Nuvera Fuel Cells were employed to evaluate the feasibility of the methodology to determine the oxygen mass transport resistance, separate its components and compare two flow field designs.In this work, we evaluated 50 cm2 SCOF and 100 cm2 cell with a 10-channel serpentine configuration described in [4]. We employed commercially available MEAs from Gore with Pt loadings of 0.1 and 0.4 mgPt cm-2 for the anode and cathode, respectively. A GDL from Freudenberg with a thickness of 170 mm was used for the SCOF, while 25BC was applied for the serpentine design. The cells performance was studied at subsaturated conditions with 50% RH of feed gases and cell temperature of 80°C and at oversaturated conditions with 100% RH at cell temperature of 60°C.The serpentine cell showed better polarization performance at low current density (< 0.5 A cm-2), while at high current conditions, the SCOF demonstrated superior properties (Fig. 1 a). The oversaturated conditions caused a drastic drop in the serpentine cell potential with increasing current density, while a low humidity and higher cell temperature improved the cell performance. Polarization performance of the SCOF was similar under oversaturated and subsaturated conditions, and the cell reached 2.5 A cm-2 at a cell voltage of 0.58-0.6 V and generated a power density of 1.5 W cm-2.Using our method (Eq. 1 and Ref. 3), we determined the O2 mass transport resistances for SCOF and serpentine flow fields and deconvoluted them to contribution of a gas phase (Rm, N2 ) and a combination of Knudsen diffusion and transport through ionomer/water films in cathode electrode (RK+film ) (Fig. 1 b). Analysis of the the SCOF under subsaturated conditions showed that RK+film and Rm, N2 were 74.85 and 35.65 s m-1, respectively. These values were found to be lower than those for the serpentine cell and explained the lowest voltage losses and superior performance of the SCOF. The open field architecture also had better performance under oversaturated conditions, although it was characterized by the highest RK+film value of 144.09 s m-1 vs. 75.76 s m-1 for the serpentine cell. The SCOF had a lower Rm, N2 than the serpentine architecture, and the superior mass transport of the SCOF in the gas phase likely compensated for the hindered Knudsen diffusion and dissolution though films in the electrode. Detailed analysis of the obtained results will be presented and discussed.ACKNOWLEDGEMENTSWe gratefully acknowledge funding from ONR (N00014-19-1-2159) and ARO (W911NF-15-1-0188). The authors thank K. Bethune and J. Huizingh for valuable help and support in system operation.References K. Srouji, L.J. Zheng, R. Dross, A. Turhan, M.M. Mench, J. Power Sources, 218, 341-347 (2012).K. Srouji, L.J. Zheng, R. Dross, D. Aaron, M.M. Mench, J. Power Sources, 364, 92-100 (2017).V. Reshetenko, J. St-Pierre, J. Electrochem. Soc., 161, F1089-F1100 (2014).V. Reshetenko, G. Bender, K. Bethune, R. Rocheleau, Electrochim. Acta 88, 571-579 (2013). Figure 1