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target="_blank" rel="nofollow" href="#ulink_f6fc8952-c79d-5351-887e-09a2466b81b6">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 BaZr_0.7Ce_0.2Y_0.1O_2.95 and Ba_1−xGd_0.8La_0.2+xCo₂O_6−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	H2O + 2e− → O2− + H2
	Materials	Nickel foam/Ni alloys; Ni–Mo/ZrO2–TiO2	Ni, Ni–Fe, NiFe2O4	Pt/C, MoS2	Ni cermets	Ni‐YSZ
Operation temperature		20–80 °C	20–200 °C	20–200 °C	600–1000 °C	600–1000 °C

Table

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