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Efficiency analysis of the electrochemical hydrogen compressor

Electrochemical hydrogen compressor is a combination of a PEM fuel cell anode and PEM electrolyzer cathode. By application of direct current hydrogen is decomposed into protons and electrons on the anode and recombined on the cathode, which can be closed and compressed up to a pressured that its construction may withstand. Power required for this process theoretically corresponds to isothermal compression, which in theory is better than a piston compressor whose power corresponds to adiabatic compression. Indeed, the Nernst equation that gives the theoretical voltage required for a given pressure ratio multiplied by current is the same as the equation for isothermal work. However, there are voltage losses associated with processes in an electrochemical processor, namely activation losses on both anode and cathode, resistive losses associated with electrical resistance in the cell structures and ionic resistance through a polymer electrolyte membrane. However, due to very fast kinetics the activation losses are about an order of magnitude lower than the resistive losses, and they increase with current logarithmically. In a well designed device, the electric losses should be minimized, however they may increase at very high pressures due to the pressure forces which would decrease the contact forces between the cell layers, thus increasing the contact resistance. Ionic resistive losses increase with the membrane thickness and are usually fairly proportional to current. Losses of hydrogen through the polymer membrane are not a function of current density, but are directly proportional to pressure and inversely proportional to membrane thickness. The efficiency, actually the specific energy consumption, in terms of kWh/kg, of an electrochemical compressor is compared to that of a multistage piston compressor for the entire range of current densities, various pressures, and various membrane thicknesses. The specific energy consumption at very low current densities is affected by hydrogen crossover, and at high current densities it is affected by resistive losses. Thus the specific energy consumption vs. current density curve has a minimum which varies with the membrane thickness and pressure ratio. However, selection of current density at which an electrochemical compressor should be designed to operate also depends on the economics. Too low current density means larger active area, and thus a more expensive device for the same hydrogen throughput. The principle of electrochemical compression of hydrogen can be applied directly to the process of electrolysis, thus avoiding activation and resistive losses and eliminating an additional device. However, in an electrolyzer hydrogen and oxygen permeation through a polymer membrane may pose a significant safety hazard, especially at very high pressures.

Polymer electrolyte membrane ; Electrochemical hydrogen compressor ; Voltage losses ; Hydrogen crossover

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