Separation and Characterization of Overpotentials in Electrochemical Hydrogen Pump with a Reference Electrode

Abstract

An electrochemical hydrogen pump (EHP) is a device employed for compressing hydrogen gas. The compression efficiency of the EHP, which operates by an isothermal process in principle, is theoretically higher than that of a conventional mechanical compressor, which operates by an adiabatic process. The structure of an EHP cell is similar to that of polymer electrolyte membrane fuel cell (PEMFC). In the EHP, compressing process is realized by imposing a potential difference to catalyst coated membrane (CCM) and completed by 3 steps: hydrogen oxidation reaction (HOR) at anode, conduction of hydrogen protons through membrane and hydrogen evolution reaction (HER) at cathode. Similar with PEMFC, overpotentials involved in EHP are categorized into two components: ohmic overpotential, mainly attributed to the conduction of protons through the membrane, and non-ohmic overpotential, which involves activation and concentration overpotentials. Due to the fast speed of hydrogen electrode reaction, ohmic overpotential is the main overpotential in the EHP. In order to reduce membrane resistance, bubbler humidifier same as that used in PEMFC system is conventionally utilized for humidifying membrane. However, different with PEMFC there is no water generation reaction occurs in the cathode side of EHP, the water at cathode dragged by hydrogen protons may be not enough for humidifying membrane at low current and high temperature conditions. Moreover, the effects of compression ratio (pressure of hydrogen at cathode) on overpotentials are important but not well discussed yet. In order to solve the problems which mentioned above, we supposed a different humidification method and employed a reference electrode for separating overpotentials. An internal humidification method is employed which is realized by storing liquid water in the cathode end plate shown in Fig.1. The liquid water directly humidifies membrane, leading to higher conductivity of membrane than that using a conventional bubbler humidifier [1]. This method is expected to balance water transport through membrane and keep membrane hydrated at high temperature. Electrochemical impedance spectroscopy (EIS) is introduced to clarify the effects of compression ratio on ohmic overpotential and non-ohmic overpotential. The compression ratio effects on overpotentials evaluated by EIS show following characteristics: For the part of ohmic overpotential, experimental results don’t show any dependence on compression ratio which suggests that water transport in EHP is more like diffusion process rather than convection process. On the other hand, non-ohmic overpotential decreases with increasing of hydrogen pressure of cathode. Separation results with reference electrode clearly shows that decreasing of non-ohmic overpotential occurs at cathode and it is attributed to the increasing of exchange current density of HER by increasing hydrogen pressure. Fitting I-V curves with a Volmer-Heyrovsy-Tafel expression [2] shows that the kinetic of HER and HOR are limited by diffusion process to/from electrode at anode/cathode. This is also confirmed by electrochemical impedance spectra data obtained at low frequency region. The concentration overpotential at anode is attributed to hydrogen diffusion through gas diffusion layer (GDL). In the EHP, in order to decrease hydrogen leakage, compression pressure supplied by end plates is set to 5MPa, which is much higher than common value (1~2MPa) used in PEMFC cell. At such a high compression pressure, GDL is compressed extremely, leading to small value of efficient gas diffusivity of hydrogen. Limiting current of anode calculated by hydrogen gas diffusion through GDL is less than 2A/cm2which agrees with the fitting results generally. The concentration overpotential at cathode is larger than that at anode. Frist, we suspected that water layer built at cathode may work as a mass transfer barrier for hydrogen gas and lead to large mass transfer resistance. However, an additional experiment carried out with bubbler humidification showed even larger mass transfer resistance at cathode. It is clear that mass transfer of hydrogen at cathode is not limited by water layer. It is possible that hydrogen diffusion in catalyst layer leads to the high concentration overpotential. The structure of catalyst layer is assumed to consist of gas pores and aggregates of Pt/C covered by ionomer layers [3]. Hydrogen that produced at reaction sites of cathode need to diffuse through this ionomer layer. With the liquid water humidification, water content of ionomer layer increases [4] which enhances the permeability of hydrogen and decreases mass transfer resistance. Reference: [1] T. A. Zawodzinski Jr., T. E. Springer, and S. Gottesfeld. Journal of The Electrochemical Society, 1993, 140(7) 1981-1985. [2] P.M. Quaino, A.C. Chialvo, Electrochimica Acta, 2007(52) 7396–7403. [3] Firat C, Ajay K. Prasad, Journal of The Electrochemical Society, 2014,161 (6) F803-F813. [4] T. Sakai, E. Torikai. Journal of The Electrochemical Society, 1985, 132(6) 1328-1332. Figure 1