sharply at 100 °C because of the phase transition from liquid water to steam. In addition, the electrical energy needed for the electroreduction of water and carbon monoxide decreases as the temperature is increased. Furthermore, as temperature increases, the enthalpy of the CO2 electrolysis decreases. Therefore, by increasing temperature and operating at higher temperatures, the system will work more efficiently. Reaction kinetics are also enhanced at higher temperatures, which indicates higher H2 or/and CO production at the same operation voltage.
Figure 3.6 Enthalpy and free energy of CO2 and H2O reduction reactions and water gas shift reaction (WGSR). The present figure has been taken from the Grave's review.
Source: Graves et al. [102].
If co‐electrolysis is conducted, the WGSR has to be taken into account, and the system becomes more complicated. At temperatures above the zero free energy, reverse water gas shift reaction (RWGSR) is favored. As results, co‐electrolysis is usually run by systems that work at high temperature (solid oxide electrolyzers).
3.5.1.1 Water Electrolysis
The electrolyzer can be depicted as an electrolyte placed between two electrodes (anode and cathode) connected electrically by an external circuit. In most electrolyzers, the electrolyte is an ion‐selective membrane and works due to the electrochemical gradient generated in the cell. The principle of the transport is ascribed to the compensation of charges in the system by charge‐carrying species. For polymeric membranes, a functional group is introduced in the polymer structure to induce the ionic conductivity, whereas in solid oxide materials, defects are generated into the crystalline structure to enhance the O2− or H+ transport.
Electrolyzers can be classified according to the materials used for the cell construction, operation temperature, and charge carriers. Considering the charge carrier in the electrolyte, the main electrolyzers can be divided into three groups:
Hydroxide anion (OH−).○ Alkaline electrolysis cell (AEC): Water is decomposed into hydrogen and HO− at the cathode. The hydroxide anion migrates to the anode, through the electrolyte, generating oxygen. The electrolyte solution consists of a mixture of water and NaOH or KOH. Among the electrolysis technologies, the alkaline electrolysis of water is the most mature technology and a long‐time commercial technology.○ Polymer alkaline electrolyzer cell (PAEC): The main difference with the previous electrolyzer lies in the electrolyte; instead of using a liquid electrolyte, a polymeric membrane with OH− ions conductivity is employed.
Proton (H+).○ Proton exchange membrane electrolysis cell (PEMEC): The two half cells are separated by a polymeric membrane. Protons are generated in the anode and then they pass by the membrane, generating hydrogen in the cathode. Because of the low temperature, expensive noble metals are used generally on both electrodes, the most common is platinum, and high external voltages are required to overcome the reaction kinetics.○ Proton conductor solid oxide electrolyzer cell (PC‐SOEC): All components in the cell are solids and high operation temperatures (600–1000 °C) are required for the cell operation to favor the electronic and/or ionic conductivity of materials. As in the PEM electrolyzers, the protons are generated in the cathode and are recombined in the anode to produce hydrogen. Membrane is based on a proton conductor material.
Oxygen ions (O2−).○ Solid oxide electrolyzer cell (SOEC): Oxygen ions generated in the cathode are conducted toward the anode through an oxygen ion conductor membrane. All components of the cell are solids and work at high operation temperature.
The main components and most common materials for the cell construction of each type of electrolyzer are summarized in Table 3.2.
Despite the fact that SOEC technology is a more mature technology, proton conductor electrolysis cell (PCEC) technology presents the advantage of producing directly dry pressurized H2 and subsequently the need of less separation steps. In addition, operation temperatures are lower (500–700 °C), allowing the reduction of the material cost.
Recently, a study about the first fully operational PCEC has been published [106]. A tubular supported electrolyte made of BaZr0.7Ce0.2Y0.1O2.95 and Ba1−xGd0.8La0.2+xCo2O6−δ as a steam anode was employed and a hydrogen production above 15 N ml min−1 was obtained. In addition, Faradaic efficiencies close to 100% at high steam pressures were observed.
Table 3.2 Summary of main characteristics of electrolyzers [103–105].
Source: Adapted from Millet et al. [103], Laguna‐Bercero [104], and Sakai et al. [105].
| AEC | PAEC | PEMEC | PC‐SOEC | SOEC | ||
|---|---|---|---|---|---|---|
| Anode | Reaction | 4OH−→ 2H2O + O2 + 4e− | 4OH− → 2H2O + O2 + 4e− | 2H2O → 4H+ + O2 + 4e− | 2H2O → 4H+ + O2 + 4e− | |
| O2− → 1/2O2 + 2e− | Materials | Ni–Co–Fe, Ni2CoO4, La–Sr–CoO3, Co3O4 | Ni‐based | Ir, Ru oxide | BCZY, SCZY | LaxSr1−xMnO3 + Y‐stabilized ZrO2 (YSZ) |
| Electrolyte | Charge carrier | OH− | OH− | H+ | H+ | O2− |
| Materials | Liquid: 25–30 wt% (KOH)aq | Solid: polymeric | Solid: polymeric | Solid: BCZY, BZY, BCY, proton conductors | Solid: Y2O3–ZrO2, Sc2O3–ZrO2, MgO–ZrO2, CaO–ZrO2 | |
| Cathode | Reaction | 2H2O + 4e− → 4OH− + 2H2 | 2H2O + 4e− → 4OH− + 2H2 | 4H+ + 4e− → 2H2 | 4H+ + 4e− → 2H2 |
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