Pressurized electrolysis

The current efficiency of Traube s method was pushed to 92% [94] with the introduction of pressurized electrolysis. In 1939, Rubio [93] developed another process for producing H202 by the reduction of 02 in 50% KOH using an active carbon cathode and a Ni anode, with no industrial success. Today, most H202 are produced chemically by the anthraquinone process, which is unsuitable for small-scale production [91]. Another electrochemical process involving 02 reduction on carbon-based cathodes developed by Dow Chemical [2,95] has found a marketplace for on-site production of alkaline H202 for pulp bleaching. 

If only operated during low off-peak rate times would have low ann load factor need more expensive H2 storage Assume high pressure electrolysis cell with drastically lower capital cost of mass production cost higher efficiency Assumed oxygen recovery for by-product sales with large central plant case, but only minor economic impact... 

Enthalpy is an intrinsic property of a substance and a function of temperature and pressure.19 In practice the Gibbs free energy is the net internal energy available to do work, less work done by changes in pressure and temperature.20 Exergy, on the other hand, is defined as the total amount of work that can be harnessed and becomes more relevant in high-temperature and high-pressure electrolysis. 

As previously mentioned, high pressure electrolysis is effective in activation of CO2 reduction at Pt electrode. Hara et al. reported that CO2 can be reduced in aqueous 0.1-M KHCO3 under 30 atm mainly to HCOO and CO with the faradaic efficiency of 50.4% and 6.1% respectively at the current density 163 mA cm . ... 

In an advanced plant concept for the low-power range, PHOEBUS 2, high-pressure electrolysis is employed to avoid the compression step. Other new components are an air-driven PEM fuel cell and optionally metal hydride as an alternative storage concept. The goal is a simplification of the whole system. 

The PEM fuel cells utilize environmentally friendly fuel—oxygen and hydrogen that can be produced via electrolysis of water. Electrolysis of water when using renewable resources (i.e., solar or wind) provides a clean source of fuel that is then reverted to the water by the fuel cell. Currently however, the majority of the world s hydrogen is generated from hydrocarbon fuels since the economics are more favorable than electrolysis of water. The current high cost of electrolysis is related to the raw material cost of cell hardware and the limited volume of cell hardware within the market. If widely adopted, low pressure electrolysis can be reasonably cheap, if auxiliary systems are utilized for gas cleanup (humidity removal) and gas compression. 

Generally, gas reactions are enhanced at elevated pressure. This was proved for ODC chlor-alkali electrolysis in laboratory scale at pressures up to 5 bar [11]. Operaticai with air was possible. Even though pressurized electrolysis producing the dangerous gas chlorine is risky, it could be interesting to apply new technical know-how for this challenge. 

If hydrogen should be produced at higher pressures, we have two options (i) high-pressure electrolysis and pumping of water or (ii) atmospheric electrolysis and H2 is compressed to the desired pressure by a compressor [see Onda et al. (2004) and Laoun (2007)]. 

Table 6.19.8 indicates that the energy required for high-pressure electrolysis including water pumping is lower than the energy required for atmospheric electrolysis and H2 compression, since the compression energy for water is much less... 

What fraction of compression work would be saved by using high-pressure electrolysis at 20 MPa followed by regular compression to fill a compressed hydrogen tank to 70 MPa, compared to only compression from 0.1 MPa ... 

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