Comparing Effects of Back Diffusion and Membrane Resistance on the Performance of a Low Temperature Electrochemical Hydrogen Pump
Authors: Bagacki Rory , Reinhardt Maximilian , van de Krol Roel , Schlatmann Rutger , Calnan Sonya 
Published: August 2024

“Green” hydrogen produced via electrolysis using electricity from renewable energy sources is seen as a promising energy carrier to aid in the transition from fossil fuels because its production emits no carbon and consumes only water as a reactant. However, the high energy demand required to compress hydrogen to higher energy densities required for efficient transport and storage remains a bottleneck for its deployment1-4. Electrochemical hydrogen pumps (EHPs) are an attractive solution since they operate via isothermal compression and are thus more efficient than typical mechanical compressors which operate adiabatically1,2,5. EHPs also have the advantage of low noise, absence of moving parts, and ability to simultaneously purify hydrogen during compression1-4.
In this study, an in-house built EHP test cell with an active geometric area of 25 cm2 was constructed using membrane electrode assemblies (MEA) based on a proton exchange membrane (PEM) and a platinum catalyst. The cell was used to pressurize hydrogen from atmospheric pressure to 10 barg. The net performance of EHPs is a result of the tradeoff between the two major losses that is, back diffusion of H2 and membrane resistance3-5. Thus, we compared the effect of varying temperature, in the range of 30-75°C, on cell performance. Current densities reaching 1.0 A cm-2 (corresponding to a specific hydrogen flow rate of 7.45 mLs min-1 cm-²) were achieved during testing. Linear sweep voltammetry was used to quantify the back diffusion of hydrogen with temperature, see Figure 1(a). Electrochemical impedance spectroscopy was used to determine the series resistance of the EHP. Figure 1(b) shows the increase in voltage with increasing high frequency resistance, whose largest contribution comes from the membrane, with reducing cell temperatures. Stepped chronopotentiometry measurements were used to characterize the overall cell performance. Figure 1 (c) and (d) show the effect of temperature on the polarization curves at a cathode pressure of 5 and 10 barg, respectively, of the EHP operating with an Aquivion E98-09S membrane with 0.25 mg cm-2 Pt per electrode. At 0.5 A cm-2, an applied voltage of 195 mV was required to compress hydrogen at atmospheric pressure in one step to 10 barg operating at 30°C, corresponding to an overall efficiency of 16%. When the cell temperature is raised to 75°C, a lower voltage of 132 mV is required at 0.5 A cm-2 due to the stronger effect of the reduced series resistance that leads to a higher cell efficiency of 27%. This work shows that membrane resistance has a greater effect than back diffusion on the overall EHP performance to pressures of 10 barg.
Future work is focused on coupling this EHP with a photovoltaic driven electrolyser and further decreasing the energy demand required to compress hydrogen.

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Figure 1