Abstract
Electrochemical hydrogen compression is a potentially high efficient, low-maintenance and silent technology to produce high pressure hydrogen. A new pressure concept with increased compression efficiency, termed intermediate differential pressure polymer electrolyte water electrolysis is proposed. With slightly pressurized oxygen and a much higher hydrogen pressure, this pressure concept profits from the advantages of the lower gas crossover of differential (only hydrogen compressed) pressure water electrolysis and the improved oxygen evolution reaction kinetics with increasing pressure of balanced pressure operation. Data for gas pressures up to 100 MPa is modeled, based on experimental results up to 5 MPa, of the three pressure concepts and validated with literature data up to 70 MPa. While differential pressure electrolyzer operation, following ideal isothermal compression, can be more efficient than today's best mechanical compressors up to 40 MPa, the intermediate pressure concept shows higher compression efficiency up to 90 MPa.
Highlights
Electricity generation from renewable sources is constantly expanding, e.g. the wind and photovoltaic (PV) power capacity installed in the European Union (EU-28) has increased from 172 GW in 2010 to 236 GW in 2015 and the concurrent electricity generation to about 400 TWh.[1,2] A major drawback of these technologies is their intermittent generation
In today’s electrolysis plants, hydrogen is produced typically at 3 MPa8,9 and is further compressed mechanically to be fed into the natural gas grid or transported to a hydrogen refueling station (HRS),[8,9] where the gas is compressed to pressure levels up to 100 MPa10 in order to refuel 70 MPa fuel cell electric vehicles (FCEVs)
With oxygen compression beneficial processes take place, reducing the apparent compression effort. These effects have been referred to improved oxygen evolution reaction (OER) kinetics, i.e. an increased apparent exchange current density with pressure.[21,26]
Summary
Two aspects are considered which determine the energetic compression losses due to pressurization. With respect to the cell voltage, from thermodynamics an isothermal compression behavior is expected according to the Nernst equation given in Equation 1:. According to Fick’s law, the gas crossover can be expressed as a molar permeation flux where the driving force is given by the partial pressure difference p for hydrogen and oxygen, respectively, as shown in Equation 4:. Here P is the gas permeability and δ is the wet thickness of the polymer electrolyte during operation Because in most energy applications only the hydrogen produced is of interest and oxygen produced is considered as a by-product, all given electrochemical compression efficiencies are calculated based on the thermodynamic cell voltage increase due to hydrogen pressurization only (see Equation 1). If oxygen would be of interest too, the cell efficiencies for balanced pressure operations would be even higher, because of an increase in the thermodynamic cell voltage with oxygen pressure
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