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Engineering Solutions for CO2 Conversion. Группа авторовЧитать онлайн книгу.

Engineering Solutions for CO2 Conversion - Группа авторов


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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].

      3.5.1.1 Water Electrolysis

      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.

      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.

      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

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