(Invited) Parametric Investigation of Electrochemical Hydrogen Compression: An Experimental Study

Abstract

The rising global demand for energy has been primarily met by fossil fuels resulting in environmental threats like global warming and climate change. Hydrogen can play a key role in combating climate change because it represents a carbon-free energy pathway when it is produced renewably. The chemical energy in hydrogen can be converted with high efficiency to electricity using fuel cells with water as the only byproduct. However, the establishment of the hydrogen infrastructure still poses significant challenges, particularly in the area of storage. Due to its low density, hydrogen is difficult to store either as a compressed gas or a liquid. Mechanical compressors are typically used to compress hydrogen, but their complex design and maintenance requirements add to their cost.Electrochemical compression (ECC) represents a viable alternative to mechanical compressors due to its higher efficiency, lack of moving parts, noiseless and vibration-free operation, and modularity. ECC is an electrochemical device that employs a proton exchange membrane (PEM) to compress hydrogen. It operates by applying a voltage across the PEM while supplying low-pressure hydrogen to the anode. The applied voltage initiates the oxidization of hydrogen into protons and electrons. The protons migrate through the PEM towards the cathode, while the electrons are directed towards the cathode through the external circuit by the power source. Upon reaching the cathode, the protons and electrons recombine to produce hydrogen at a higher pressure. Furthermore, ECC offers additional advantages such as the ability to separate and purify hydrogen from a gaseous mixture.For the successful commercialization of ECC, it is essential to achieve high performance with minimal energy consumption. This work presents a detailed parametric investigation of electrochemical hydrogen compression conducted experimentally using a single cell ECC apparatus. The effects of key parameters including voltage, temperature, membrane thickness, and relative humidity on ECC performance are examined for two outlet variables, viz. discharge pressure and gas throughput. We also elucidate the adverse effect of back diffusion due to the high pressure differential between cathode and anode compartments on the overall ECC performance. Various ECC performance metrics such as output pressure, current density, net flux, and compression efficiency are investigated as a function of the operating parameters. The results show that tradeoffs are required to achieve different objectives. For example, operating at a higher voltage is favorable for both discharge pressure and gas throughput but at the cost of reduced ECC efficiency due to increased back diffusion. The results also show that a lower operating temperature favors higher discharge pressures, whereas a higher temperature favors higher gas throughput. Similarly, thinner membranes facilitate a higher gas throughput due to reduced ohmic resistance, while thicker membranes can mitigate back diffusion thereby yielding higher discharge pressures. Furthermore, operating at higher RH is beneficial for both the discharge pressure and gas throughput.These results provide valuable insights on the effect of various operating parameters on ECC performance. This study confirms that back diffusion plays a deleterious role in ECC operation and must be given careful consideration in stack design. Also, novel PEMs with low hydrogen permeability should be identified to optimize ECC performance. The results suggest that ECC is a promising alternative to conventional technologies for hydrogen compression.