299
Water Dissociation Technologies for Hydrogen
electrolytes have been tested. Recent studies have used yttria-stabilized zirconia
(YSZ) electrolytes, nickel-cermet steam/hydrogen electrodes, and mixed oxide of
lanthanum, strontium, and cobalt oxygen electrodes [18,19] (Laguna-Barcero et al.,
2012, pers. comm.). Future advances in the HTE process will require materials that
can withstand high temperature, high pressure, and corrosive environment. At the
present time, the HTE process appears to be an inefficient way to generate hydrogen.
The process will become more efficient if sources such as nuclear, solar, and hydro
energy can be the source of thermal energy.
When the energy is supplied in the form of heat, such as by solar or nuclear energy,
the production of hydrogen by HTE is very attractive. Unlike in low-temperature
electrolysis, in HTE water converts more of initial thermal energy into chemical
energy (like hydrogen) by increasing the conversion efficiency. Since energy in the
HTE process is supplied in the form of heat, less of the energy must be converted
twice (from heat to electricity and then to chemical form), and so less energy is lost
and efficiency can be doubled up to 50%.
While the heat required for the HTE process can be obtained by solar energy or
nuclear energy, the latter source is more reliable and is often used. The solar form
of high-temperature heat is not consistent enough to bring down the capital cost of
HTE equipment. More research into HTE and high-temperature nuclear reactors
may eventually lead to hydrogen supply that is cost competitive with natural gas
steam reforming. This concept of coupling a high-temperature electrolyzer and a
high-temperature gas-cooled nuclear reactor (HTGR) has been demonstrated in a
laboratory but not at a commercial scale, although Idaho National Laboratory is
developing a commercial process based on this concept [17].
11.2.3 hPe ProCeSS
When electrolysis is conducted at high pressure, the produced hydrogen gas is compressed at around 120–200 bar (1740–2900 psi). By pressurizing the hydrogen in the
electrolyzer, the need for an external hydrogen compressor is eliminated. The average energy consumption for internal compression is around 3% [13].
HPE is often carried out using a solid polymer electrolyte (SPE) membrane such
as perflurosulfonic acid (Nafion) rather than classic liquid electrolyte (alkaline electrolyte) under high pressure. Laoun [13], LeRoy et al. [20,21], and Onda et al. [15]
carried out a thermodynamic analysis of such a process and showed the importance
of temperature and pressure on the entire efficiency of water electrolysis. Using the
model and analysis of LeRoy et al. [20,21], Onda et al. [15] showed that a temperature
change up to 250°C and pressure changes up to 70 atm can be carried out by polymer
electrolytic membranes. They showed that an increase in pressure and a decrease in
temperature deliver more power for water electrolysis. The increase is, however, found
to be small at pressures above around 200 atm. They also found that hydrogen can be
produced with about 5% less power using HPE than that required using atmospheric
water electrolysis.
Fateev et al. (2012, pers. comm.) showed that water electrolysis using polymeric
electrolyte membrane (PEM) has demonstrated its potential for high cell efficiency
(energy consumption of about 4–4.2 kW/Nm 3 H 2 ) and gas purity of about 99.99%.