Evaluation of Hydrogen Crossover in Pemfcs at Intermediate Temperature (80 - 120 °C)

Abstract

Proton Exchange Membrane Fuel Cells (PEMFCs) are promising for heavy-duty vehicles (HDV) in the strive to make the transportation sector more sustainable. However, because of the small temperature gradient between the cell (typically run around 80°C) and the ambient temperature, these cells require a large cooling system. Raising the operating temperature to an intermediate temperature range (IT: 80 – 120 °C) would reduce the cooling system size required in HDVs and improve the tolerance towards contaminants, although higher temperature may accelerate degradation [1,2]. To be able to withstand IT operation, reinforcements and chemical modifications have been implemented in commercial PFSA polymers. Unfortunately, most of the data available for PEMFCs are referred to non-reinforced membranes and only few studies analysed the latter under realistic operating conditions. Among some possible issues regarding proton exchange membranes (PEMs), hydrogen crossover is an undesirable, but inevitable phenomenon. The membrane is not perfectly impermeable to hydrogen gas and this results in a safety concern, lower cell efficiency and can lead to faster cell degradation.This work investigates the influence of the reinforcement in a PEM on the hydrogen crossover. Specifically, Nafion HP (reinforced) and Nafion 211 (non-reinforced) are compared under different operating conditions. The purpose of the reinforcement is to improve thermo-mechanical stability, and its physical properties can differ from the rest of the membrane, which can influence hydrogen crossover.Using electrochemical methods, the hydrogen crossover is measured in-situ in a PEMFC with hydrogen on one side and inert gas on the other side. The measurements have been performed between 80 and 120 °C, at different cell relative humidity (RH) conditions (20-90 %) and multiple levels of pressure for both reinforced and non-reinforced membranes. Particular attention has been paid to the case in which the hydrogen side of the cell is more pressurized than the inert side, as this is most often the case in HDV applications. In such conditions, hydrogen permeation increases considerably as a result of the total pressure gradient over the membrane, although this factor is rarely considered in the literature [3]. By adapting the methodology established in our previous work [4], here the effects of the differential total pressure and hydrogen partial pressures are decoupled through a systematic dilution of the hydrogen gas stream. Special points are measured in which the hydrogen partial pressure is kept equal to the ambient case, while the total pressure on the hydrogen side is increased by adding the inert gas, as shown in Figure 1, plot I.Results show that the presence of a reinforcement affects the amount of hydrogen that crosses through the membrane, as seen in Figure 1. Moreover, among the different conditions tested, higher total pressures, especially on the hydrogen side of the cell, are affecting hydrogen crossover the most, while a more modest change is observed for different RH and cell temperatures.The observation that pressure is the most important factor for hydrogen crossover has noteworthy implications and should have an influence when deciding at which pressure the PEMFC is operated in transportation applications. Minimizing hydrogen crossover, while still ensuring adequate cell performance, should be a priority to improve the overall fuel consumption and the electric power obtained from the fuel cell.Figure 1: Hydrogen crossover due to concentration gradients and, in certain conditions, to a differential total pressure. Three different conditions are used to measure crossover, whose results are reported in the right figure: I) the total pressure on the hydrogen side is increased by diluting with inert gas, while the hydrogen partial pressure is kept constant, the inert side is unvaried; II) the total pressure is increased symmetrically on both sides, the hydrogen side is not diluted; III) the total pressure on the hydrogen side is increased and no dilution is employed.[1] Cullen et al., “New roads and challenges for fuel cells in heavy-duty transportation,” Nat. Energy, 2021, doi:10.1038/s41560-021-00775-z.[2] Akitomo et al., “Investigation of effects of high temperature and pressure on a polymer electrolyte fuel cell with polarization analysis and X-ray imaging of liquid water,” J. Power Sources, 2019, doi:10.1016/j.jpowsour.2019.04.115.[3] Kreitmeier et al., “Investigation of membrane degradation in polymer electrolyte fuel cells using local gas permeation analysis,” J. Power Sources, 2012, doi:10.1016/j.jpowsour.2012.03.071.[4] Butori et al., “The Effect of Oxygen Partial Pressure and Humidification in Proton Exchange Membrane Fuel Cells at Intermediate Temperature,” J. Power Sources, 2023, doi:10.1016/j.jpowsour.2023.232803. Figure 1