Challenge of Membrane Electrode Assembly to Polymer Electrolyte Fuel Cell Systems Having Higher Power Density
Authors: Shibata Masao , Suzuki Toshiyuki , Suzuki Takahisa , Asaoka Takahiko 
Published: February 2020

Air-cooled polymer electrolyte fuel cells (PEFCs) are considered potentially to simplify the system design [1, 2]. Enhancement of the membrane properties was proposed for higher performances from the analysis and optimization of the system [3]. In this work, requirements for the properties of the MEA components are discussed.
The two diamonds labeled A and B in Fig. 1 represent the MEA performance required for the air-cooled PEFC system. An open-cathode PEFC is assumed for model calculation. The heat from the cell is assumed to be removed only by the gas flows through the channels. Operating point A can be arbitrarily determined to satisfy the specified output power, and a current density of 0.128 A·cm−2 at a cell potential of 1.15 V was selected. Operating point B was determined so that the heat from the MEAs in the stack balances the heat removed by the exhaust gas.
Curve 1 in Fig. 1 is the calculated performance of the MEA with a Pt loading of 0.1 mg·cm−2 (denoted MEA 1) for comparison, where the parameters for calculation assumes one of the state-of-the-art MEAs. The calculation was performed using a through-plane model of the MEA with an in-plane reaction distribution along the flow channels.
Curve 2 in Fig. 1 shows the calculated performance when state-of-the-art MEA component materials reported in the literature can be employed without trade-offs (denoted MEA 2). The performance enhancement was mainly due to improvements in the cathode catalyst (Pt nanowire), ionomer (of high oxygen permeability), and gas diffusion layer (without substrate fibers). Activation, ohmic, and mass-transport losses were significantly reduced, but the cell potential loss was still mainly caused by the activation loss. Although MEA 2 exhibits dramatically higher performance than MEA 1, it does not achieve the required performance.
Curve 3 in Fig. 1 represents the calculated performance when a Pt monolayer having oxygen reduction reaction (ORR) activity at the top of the volcano plot is assumed. (The MEA is denoted MEA 3.) The operating temperature was raised to 150°C. The resistance of the membrane at 0% relative humidity (RH) and 150°C was assumed to be only 20 times lower than that at 30% RH and 60°C. Under the assumptions and change in operating temperature, MEA 3 reached the required performance.
The importance of studies on catalysts having high ORR activity and ionomers having high proton conductivity at lower relative humidity was reconfirmed. Balance-of-plant components as well as MEA materials require new innovations to downsize fuel cell systems. Although extremely idealized properties pose big challenges in material development, advance in the component materials would greatly benefit the cell performance.
References
[1] Q. Meyer et al., J. Power Sources, 291, 261 (2015)
[2] A. de las Heras, F. J. Vivas, F. Segura, and J. M. Andúlar, Int. J. Hydrogen Energy, 42, 12841 (2017)
[3] Q. Meyer et al., Int. J. Hydrogen Energy, 40, 16760 (2015)

Figure 1