Electrochemical Hydrogen Compression and Its Optimization through Computational Modeling

Abstract

Climate change resulting from the heavy use of fossil fuels poses a serious threat to our environment. It is critically important to find alternatives to combustion-based energy technologies and promote renewable energy production. Hydrogen is a leading candidate for the transportation sector as a carbon-free fuel with high energy density. Hydrogen can be produced in a carbon-neutral manner via water electrolysis and other means. However, the establishment of the hydrogen infrastructure still faces serious challenges such as storage. Due to its lightweight nature, hydrogen is very difficult to store either as a compressed gas or as a liquid. Typically, hydrogen is compressed using mechanical compressors which contain moving parts that require frequent maintenance adding to their cost and complexity. Electrochemical compression (ECC) is a promising alternative to mechanical compressors. ECC is an electrochemical device that uses a proton exchange membrane (PEM) to compress the gas. When a voltage is applied across the PEM, low-pressure hydrogen supplied to the anode is oxidized to generate protons and electrons. The electrons are driven through the external circuit by the power source, whereas the protons travel through the PEM towards the cathode. The protons and electrons recombine via the cathodic reduction to obtain hydrogen at elevated pressure. The currently available knowledge base regarding electrochemical compressors has mainly resulted from experimental studies and less attention has been paid to modeling. The current work presents a comprehensive three-dimensional model of a single cell ECC that incorporates the relevant electrochemical phenomena as well as physical processes including water transport. It also takes into account the important phenomenon of back diffusion due to the pressure difference between the cathode and anode that limits the achievable pressure in the cathode compartment. Simulations were conducted for the pressurized cathode case . It was observed that back diffusion limits the compression ratio to a value well below that predicted by Nernst equation. Additionally, the effects of membrane thickness, temperature, and applied voltage on ECC performance are analyzed. The results show that there is a tradeoff between different objectives that must be addressed to optimize the ECC process. For example, thinner membranes permit faster compression rates but at the cost of increased back diffusion. Similarly, higher temperatures improve the pressure efficiency and the compression rate but at a higher operational cost. The results also show that ECC is more efficient at lower voltages because the associated compression ratio is smaller which consequently reduces back diffusion; however, such small voltages do not allow higher pressures. As a result of these tradeoffs, all design parameters need to be optimized simultaneously. This study offers valuable insights on the effect of different parameters on ECC performance. The results suggest that ECC is a viable alternative to conventional technologies for hydrogen compression. Back diffusion significantly reduces ECC efficiency; therefore, ECCs would benefit from membranes with low hydrogen diffusivity. The results provide a foundation for the modeling, analysis, and optimization of a full scale ECC system. Keywords—Hydrogen; Computational Model; Back Diffusion; Electrochemical Compression; Optimization;