Electrochemical hydrogen separation and compression: experimental and modeling study

Abstract

Hydrogen can play a key role in combating climate change because it represents a carbon-free energy pathway when it is produced renewably. Hydrogen powered fuel cells have already demonstrated their potential to decarbonize the mobility and stationary power sectors with advantages such as high efficiency and zero emissions. Storing renewable energy in the form of green hydrogen can also help balance fluctuations due to the inherently intermittent nature of renewal energy production. Although hydrogen has a high gravimetric energy density, it has a low volumetric energy density as it is the lightest element. The potential benefits of hydrogen as an energy carrier can only be realized when its production, storage, and distribution are accomplished in a sustainable, safe, and efficient manner. This work focuses on developing the science and technology related to hydrogen distribution and its storage as a compressed gas. ☐ Currently, the separation/purification and compression of hydrogen are achieved by two independent processes. Here, we propose to combine both processes using electrochemical hydrogen separation and compression (ECHSC). ECHSC is a membrane-based alternative that purifies and compresses hydrogen in one single step and is superior to conventional technologies because of its simple design, high efficiency, lack of moving parts, and noiseless operation. ☐ ECHSC is first demonstrated in the electrochemical compression (ECC) mode for pure hydrogen and the performance of a single cell ECC is evaluated experimentally. ECC performance parameters such as discharge pressure, current density, net flux, and efficiency are investigated as a function of operating voltage, temperature, membrane thickness, and relative humidity. This work also highlights the loss in ECC efficiency due to back-diffusion. This study provides insights on optimizing ECC operating efficiency for a desired output flux or output pressure while varying the operating parameters, and provides a foundation for the design of a full-scale ECC system. ☐ Next, we present a comprehensive 3D ECC model developed for a single cell using COMSOL Multiphysics 5.6 that incorporates all relevant physical and electrochemical processes, and examines the effect of key parameters on ECC performance. It also considers the important phenomenon of back diffusion resulting from the high-pressure differential between the cathode and anode during compression. The study reveals that three parameters in particular viz. membrane thickness, operating temperature, and voltage must be carefully selected to optimize ECC operation. ☐ Finally, we experimentally investigate the application of ECHSC to gas mixtures such as H2-N2, H2-CH4, and H2-CO2. The performance of ECHSC is evaluated in pumping, separation, and compression modes. Results indicate that ECHSC performance for the three gas mixtures is in the order: H2-N2 > H2-CH4 > H2-CO2. The separated and compressed hydrogen gas is analyzed using a MicroGC and a high purity level of hydrogen is confirmed. The results of this study provide important insights into the effect of various gas mixtures on ECHSC performance. Overall, it is shown that ECHSC is a promising alternative for the separation and compression of hydrogen gas with high efficiency and purity.