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In the power-to-gas process, hydrogen, produced by water electrolysis, is used as storage for excess, fluctuating renewable electric power. Reconversion of hydrogen back to electricity with the maximum possible efficiency is one pre-requisite to render hydrogen storage technically and economically viable. Pure oxygen is a byproduct in the electrolysis of water. The use of pure oxygen as the oxidant in a polymer electrolyte fuel cell (PEFC) is a possible way of increasing the conversion efficiency of hydrogen to power, by reducing the fuel cell's cathodic kinetic overvoltage, which is the most important energy loss process in low temperature PEFCs. As we demonstrate in this work, when using pure oxygen, either high efficiencies at current densities around 1 A cm À2 are obtained or a very high power density operation (up to 1.6 W cm À2 at cell voltages above 0.62 V) can be reached, giving the technology a broad window of operation and application. The fuel cell stack durability is assessed in accelerated longterm tests of up to 2700 h. The potential of the technology is demonstrated with the realization of a complete 25 kW prototype system delivering a peak efficiency of 69% LHV (57% HHV).

No abstract

When storing fluctuating renewable electricity by water electrolysis hydrogen and oxygen are obtained. Up to a scale in the order of Megawatt-hours the concurrent storage of oxygen and hydrogen gas is technically feasible. This opens up the possibility to use hydrogen/pure oxygen fuel cells for the efficient conversion of hydrogen back to electric power. The oxygen reduction reaction is responsible for the largest voltage loss in the operation of polymer electrolyte fuel cells. The sluggish reaction rate of the four-electron reduction, even on the best platinum based catalysts, is aggravated by the low partial pressure of oxygen in air. When using pure oxygen the partial pressure can typically be increased by an order of magnitude. On the cell level, not only the reduced electro catalytic loss but also voltage gain from lower transport overvoltage and reduced ohmic loss at same gas humidification as with air operation are observed. At the same time very high specific power densities of up to 2 W/cm2can be realized. In addition to the advantages on the cell level, low parasitic power consumption in the balance of plant (no compressor/blower) also contributes to high system efficiencies of up to 69% (LHV).

No abstract