Influence of Gas Humidity and Flow Rate on the Water Movement Direction through an Anion Exchange Membrane Fuel Cell

Abstract

Environmental concerns lead to a continually growing demand for clean energy generation. Thus, alternative energy sources or green fuels attract attention. The anion exchange membrane fuel cell (AEMFC) emerged as an excellent alternative due to its air CO2-free emissions, low-cost electrocatalyst used at the cathode, and industrial-scale membrane production development [1-3].Many efforts have been made to have AEMFC performances like those already shown by the proton exchange membrane fuel cell (PEMFC). Excellent results have been obtained for the lasted technology regarding power densities and operational cell life [4]. It should be recognized that due to the nature of the electrode reactions in AEMFC, Eqs. 1-2, the performance enhancement of the overall cell reaction, Eq. 3, depends not only on the selected catalysts and ionomers/membranes but also on the cell's water management. The complexity of water management relies on the simultaneous production and consumption of water in anode and cathode, respectively, where the production is two-fold higher than the consumption. This effect might lead to anode flooding during the hydrogen oxidation reaction (HRR), Eq. 1. In contrast, during oxygen reduction reaction (OER), Eq. 2, the risk of membrane drying is latent as long as a water imbalance arises between both anodic and cathodic sides. It has been confirmed quantitatively that water direction movement could go from anode to cathode in a hydrophilic membrane (diffusion transport mode); however, using a membrane with hydrophobic properties promotes the water to travel from cathode to anode (electro-osmotic drag, EOD mode). In this context, it is convenient to identify the water direction trend under variable selected operating conditions. In particular, achieving the anode-to-cathode water direction would improve the AEMFC performance by avoiding flooding on the anode by cathode hydration through water diffusion through the membrane. Therefore, this work tested the influence of symmetric and asymmetric flow rates and relative humidities within an AEMFC using a hydrophilic membrane. According to both modes of transport, the AEMFC performance was studied using polarization curves.The experimental procedure involved trapping water out of each half-cell during a chronoamperometry in steady-state conditions. Also, the water incoming was quantified independently for each selected operating condition in order to make a mass balance (Figure 1a). The variable operating conditions were the reactant flow rate, low (125 cm3 min-1) and high (250 cm3 min-1), and the reactant humidity level.In Figure 1b, the results demonstrated that the EOD predominance was promoted when low symmetric flow rates (125 cm3 min-1 for anode and cathode) were used in the AEMFC, suggesting membrane dehydration. On the contrary, the water diffusion dominance appears under high (250 cm3 min-1) symmetric and asymmetric flow rates between anode and cathode; under these conditions, the membrane dehydration might be suppressed by anode flooding alleviation. Once the flow conditions are given, no changes in water transport mode were detected for changes in the humidity level (Figure 1c-d). The AEMFC performance was affected by the water transport mechanism, giving lower peak power densities for EOD-dominated systems (35.2 mW cm-2 at 60°C, Figure 1e) than those obtained by diffusion (62.8 mW cm-2 at 60°C, Figure 1f). The most suitable operating condition is under water-diffusion at non-saturated streams (below 65%), where a decrease in performance is experienced as the humidifier temperature increases from 60°C to 80°C (supersaturated flow), with peak power densities of 62.8 and 3.7 mW cm-2, respectively.