Reversible Polymer Electrolyte Membrane Fuel Cells - Basic Membrane Electrode Assembly Design
Authors: Chuluunbandi Khajidkhand , Thiele Simon , Freiberg Anna T.S. 
Published: December 2024

A reversible fuel cell (r-FC) is a cell that can be operated both in fuel cell (FC) and water electrolysis (WE) mode. Having both modes in a single cell offers several advantages: lower cost of the system, higher operating ratio, and smaller footprint.1 There are two types of r-FC system. The first one is where the electrodes do not change their redox function in both modes of operation. The second one is where gases are fixed, in which one electrode is dedicated to dealing with hydrogen (H2-electrode), while the other electrode deals with oxygen (O2-electrode).2 The latter configuration is preferred due to its simpler balance of plant design, faster mode switches, and enhanced safety by keeping hydrogen and oxygen in separate compartments. In this configuration, hydrogen oxidation reaction (HOR) and hydrogen evolution reaction (HER) take place on the H2-electrode, and on the O2-electrode, oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) take place.

In this work, we start from a proton exchange membrane fuel cell (PEMFC) membrane electrode assembly (MEA) and modify MEA components one by one to establish a basic MEA design for r-FC in acidic environment. The H2-electrode design for both PEMFC and PEM water electrolysis is the same. Therefore, it is fixed, and the Pt/C catalyst, which is a state-of-art catalyst for both HOR and HER, is used on the H2-electrode. On the other hand, the O2-electrode must fulfill both ORR and OER functionalities. Therefore, a bifunctional catalyst must be used; hence, Pt and Ir both must be present. The operation potential window for the O2-electrode increases to roughly 0.5V-2V vs. RHE. Therefore, Pt/C and carbon based gas diffusion layer (GDL) materials can not be used as carbon corrosion takes place at high potentials (>1.4V vs. RHE).3 Consequently, WE based O2-electrode comprising of only metal based catalyst and a titanium porous transport layer (PTL) must be implemented on the O2-electrode.

First, the Pt/C catalyst on the O2-electrode is replaced with a Pt black catalyst, taking a step back to the historic start of PEMFC electrode design.4 The performance of the cell drops due to a much lower electrochemically active surface area of Pt black compared to Pt/C, which can be quantified by hydrogen under potential deposition. In the next step, the carbon-based GDL is exchanged with a titanium PTL on the O2- electrode. In this case, the performance drops drastically due to the utilization of the catalyst-coated substrate (CCS) method for preparing the electrode. As the sprayed catalyst penetrates deeper into the big porous structure of the Ti PTL, a significant amount of catalyst is not connected to the ionic network and therefore inactive. Hence, the catalyst-coated membrane (CCM) method is used for the next steps.

Furthermore, a physical mixture of Pt black and IrO2 is compared with a physical mixture of Pt black and Ir black for implementation on the O2-electrode. Accelerated stress tests5 (1000 cycles) in single cell fuel cells are performed, and begin of test (BoT) and end of test (EoT) performances are compared. Pt black and Ir black mixture implemented O2-electrode show a lower Tafel slope (69 mV/dec) than Pt black and IrO2 implemented O2-electrode (86 mV/dec) at BOT. At EoT, the Tafel slopes increase (20-30%) in both cases, indicating the degradation of the CL probably caused by the low voltage operation of the OER catalyst.

Several crucial components, including efficient and stable bifunctional catalysts, O2-side GDLs, and bipolar plates suitable for both modes, need to be developed for r-FCs to compete with their single mode equivalents effectively.

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Figure 1