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Electrochemical Hydrogen Compression: Modeling, Internal States Estimation and System Control

Hydrogen is a clean and flexible energy carrier that can be produced from diverse renewable energy sources (e.g., wind, solar and biomass) and used in a broad range of applications (e.g., transportation and power generation). Due to its high efficiency and carbon-free emission, hydrogen will play a key role in transition to Net Zero Emissions by 2050. However, for a successful hydrogen economy to develop, the compression and storage costs of hydrogen must be lowered to overcome its low volumetric energy density (i.e., 0.01079 MJ/L at STP). Currently， conventional mechanical compression accounts for the largest percentage of operating costs in hydrogen refueling stations [1].
 Electrochemical hydrogen compression (EHC) is an innovative non-mechanical technology that compresses hydrogen through application of voltage across membrane separators. Without the vulnerable moving parts of mechanical compressors, EHC is vibration-free, and the noise and possibility of compressor failure are reduced. Electrochemical compression is isothermal, in theory, and thus a higher efficiency is obtained than the adiabatic process of conventional mechanical compression. Consequently, EHC technology is attracting more and more attention as the hydrogen economy takes off.
 The proton conductivity of the proton exchange membrane (PEM) used in EHC is dependent on its water content and has a significant impact on the EHC performance. Unlike in PEM fuel cells, water is not a reaction product in EHC. Therefore, it is necessary to develop a control system to monitor the hydration degree of the membrane. Besides water management, thermal management is also important. Since high temperature can affect the stability of perfluoro-sulfonate polymers like Nafion®, their operating temperatures are typically below 353 K [2]. Reasonable optimization of operating conditions including current density, back pressure, gas flow, and inlet relative humidity, can effectively manage the temperature and moisture inside the EHC. However, the real-time internal states (e.g., temperature and water content) of EHC must be determined. Mathematical model of EHC with detailed geometries can be built to reveal the multiple coupling phenomena for mass/heat transfer, fluid flow and electrochemical reaction inside the EHC. Also, the EHC model can be used for system control and fault diagnosis.
 In this study, a high-fidelity dynamic model of EHC is developed with due consideration of two-dimensional distributed mass/heat transfer coupled with electrochemical kinetics. In the EHC model, the dynamics inside the anode and cathode gas channels are characterized by the discretized conservation equations of mass and energy balances, and a lumped parameter model is used to describe the transport and electrochemical reaction in the membrane electrode assembly (MEA). A numerical approach is implemented to solve the spatial derivatives of the gas channel model. Using appropriate boundary conditions, forward or backward differences are employed to realize finite-difference discretization. In the n-discretized finite elements model of EHC, the inlet and outlet flow of cathode and anode gas channels are considered as the boundary conditions. The reaction gas and water transport terms are considered as input disturbances for the discretized model, which can be determined based on the lumped MEA model.
 The EHC model can be used to detect local dehydration of PEM due to the unbalanced contribution of the electro-osmotic flow and the back diffusion of water across the membrane. The heterogeneities significantly affect the overall efficiency of an EHC. The physical parameters, such as the inlet relative humidity, discharge pressure, and PEM thickness, are optimized to enhance the overall efficiency of the system.
 [1] Parks, G., Boyd, R., Cornish, J., & Remick, R. (2014). Hydrogen station compression, storage, and dispensing technical status and costs: Systems integration (No. NREL/BK-6A10-58564). National Renewable Energy Lab. (NREL), Golden, CO (United States).
 [2] Sdanghi, G., Maranzana, G., Celzard, A., & Fierro, V. (2019). Review of the current technologies and performances of hydrogen compression for stationary and automotive applications. Renewable and Sustainable Energy Reviews, 102, 150-170.