Protonic Ceramic Electrochemical Cells for Synthesizing Sustainable Chemicals and Fuels

Abstract

Protonic ceramic electrochemical cells (PCECs) have been intensively studied as the technology that can be employed for power generation, energy storage, and sustainable chemical synthesis. Recently, there have been substantial advances in electrolyte and electrode materials for improving the performance of protonic ceramic fuel cells and protonic ceramic electrolyzers. However, the electrocatalytic materials development for synthesizing chemicals in PCECs has gained less attention, and there is a lack of systematic and fundamental understanding of the PCEC reactor design, reaction mechanisms, and electrode materials. This review comprehensively summarizes and critically evaluates the most up-to-date progress in employing PCECs to synthesize a wide range of chemicals, including ammonia, carbon monoxide, methane, light olefins, and aromatics. Factors that impact the conversion, selectivity, product yield, and energy efficiencies are discussed to provide new insights into designing electrochemical cells, developing electrode materials, and achieving economically viable chemical synthesis. The primary challenges associated with producing chemicals in PCECs are highlighted. Approaches to tackle these challenges are then offered, with a particular focus on deliberately designing electrode materials, aiming to achieve practically valuable product yield and energy efficiency. Finally, perspectives on the future development of PCECs for synthesizing sustainable chemicals are provided.

Keywords: CO2 reduction; ammonia synthesis; natural gas upgrading; protonic ceramic electrochemical fuel cells; sustainable chemical synthesis.

Figure 1

Applications of PCECs for sustainable chemical synthesis.

Figure 2

Schematic illustration of four primary PCEC configurations developed for ammonia synthesis. A) Reactor 1 that converts H₂ and N₂ to ammonia in a two‐chamber configuration, B) single‐chamber Reactor 2 that electrochemically promotes ammonia synthesis, C) Reactor 3 that intensifies steam methane reforming with ammonia synthesis, and D) Reactor 4 that converts H₂O and N₂ to ammonia. OER: oxygen evolution reaction. HOR: hydrogen oxidation reaction. HER: hydrogen evolution reaction. NRR: nitrogen reduction reaction. SMR: steam methane reforming. WGSR: water gas shift reaction.

Figure 3

Thermodynamics and corresponding reversible electrochemical potential of three ammonia synthesis reactions calculated under the standard pressure.

Figure 4

Electrical energy consumption of synthesizing ammonia in PCECs.^{[ 14} , ⁴⁸ , ⁵⁰ , ⁵¹ , ⁵² , ⁵⁵ , ⁵⁶ , ^{59 ]} The electrical energy consumption is determined based on the experimental results reported in papers published in the last two decades. Cross and star symbols represent the PCECs with Reaction 1 configuration. Open symbols represent single‐chamber PCECs with Reactor 2 configuration. Solid symbols denote PCECs with Reactor 3 configuration. Half‐solid and half‐open symbols stand for PCECs with Reactor 4 configuration. Next‐generation Reactor 4 indicates the targets of synthesizing ammonia in PCECs. The green dash line represents the total energy consumption for the Haber–Bosch process.

Figure 5

Potential NRR pathways in PCECs. A) Dissociative electrochemical mechanism (Pathway 1). B) Associate thermochemical mechanism (Pathway 2).

Figure 6

Summary of the strategies proposed for enhancing ammonia production in PCECs.

Figure 7

The ammonia production rate and Faradaic Selectivity toward ammonia as a function of the current density of PCECs.

Figure 8

Schematic illustration of PCECs for carbon‐containing chemical synthesis via CO₂ reduction reaction.

Figure 9

Thermodynamics of CO₂ reduction in PCECs under standard atmosphere. A) The Gibbs free energy change of half‐cell reaction and corresponding CO₂ reduction potentials versus the potential of HER (i.e., 2H⁺ + 2e ⁻ = H₂(g)). The product of CO₂ reduction is CH₄. B) The Gibbs free energy change of half‐cell reaction and corresponding CO₂ reduction potentials versus the potential of HER. The product of CO₂ reduction is CO. The assumption in Figure 9A,B is that the potential of HER is zero and it does not change with operating temperatures. The equilibrium potential of CO₂ reduction is calculated based on E = − ΔG/nF. C) The thermodynamics of CO₂ reduction with various feedstocks and CO as the final product. D) The thermodynamics of CO₂ reduction with various feedstocks and CH₄ as the final product.

Figure 10

CO₂ reduction in CO₂‐PCEC 2 with BaCe_0.7Zr_0.1Y_0.1Yb_0.1O₃+Ni (BCZYYb7111+Ni) as the negative electrode. A) Production rate of chemicals produced in a button cell.^{[ 5 ]} Copyright 2019, The Authors, published by Springer Nature. B) CO₂ conversion in a button cell. Reproduced under the terms of the Creative Commons Attribution 4.0 International License.^{[ 5 ]} Copyright 2019, The Authors, published by Springer Nature. C) CH₄ selectivity in a button cell.^{[ 5 ]} Copyright 2019, The Authors, published by Springer Nature. D) CH₄ selectivity in a large area cell (5 cm²). Reproduced with permission.^{[ 98 ]} Copyright 2022, Elsevier.

Figure 11

CO₂ reduction in CO₂‐PCEC 1 with three different negative electrodes. Reproduced with permission.^{[ 28 ]} Copyright 2021, Springer Nature. A) I‐V curve of CO₂ reduction. B) CO Faradaic selectivity and CO production rate. C) CH₄ Faradaic selectivity and CH₄ production rate.

Figure 12

The selectivity of CO₂ reduction toward CH₄ and CO in PCECs has been recently demonstrated.^{[ 5} , ²⁸ , ⁸⁶ , ⁸⁷ , ^{90 ]} A) The selectivity of CO as a function of operating temperature. B) The selectivity of CH₄ as a function of operating temperature. C) The Faradaic selectivity of H₂ as a function of operating temperature.

Figure 13

Schematic illustration of the proposed CO2RR mechanisms in PCECs. The blue rectangle is the proton‐conducting electrolyte membrane. The brown dots are the oxide supports and the green dots are metallic particles, which act as the CO₂ electrode.

Figure 14

Summary of challenges and future perspectives to improve the chemical synthesis in PCECs via CO2RR.

Figure 15

Pathways of synthesizing chemicals from natural gas/methane. NG: natural gas; FTS: Fischer–Tropsch synthesis; MTO: methanol to olefins; MTA: methanol to aromatics; OCM: oxidative coupling of methane; AMO: Aromatization of methane under oxidizing atmosphere; MDA: methane dehydroaromatization.

Figure 16

Schematic illustration of PCECs for producing aromatics from natural gas. A) PCECs with a mixed proton and oxygen‐ion conductor as the membrane. B) PCECs with a mixed proton, oxygen‐ion, and electronic conductor as the membrane. MA: methane aromatization; MIEC: mixed ionic and electronic conductor; MDA: methane dehydroaromatization.

Figure 17

Impacts of H₂ extraction and O₂ injection on methane aromatization.

Figure 18

Converting CH₄ to aromatics in PCECs. Reproduced with permission.^{[ 23 ]} Copyright 2016, AAAS. A) A Tubular reactor with the proton‐conducting membrane. B) The aromatic yield and CO yield as a function of H₂ extracted and O₂ injected. C) Deactivation rate constant as a function of H₂ extracted and O₂ injected.

Figure 19

Converting C₂H₆ to light olefins in PCECs. Reproduced with permission.^{[ 157 ]} Copyright 2021, American Chemistry Society. A) Planar reactor with a proton‐conducting membrane for converting C₂H₆ to light olefins. B) Conversion of C₂H₆ and C₂H₄ yield as a function of applied current density. C) Product selectivity as a function of applied current density.

Figure 20

Direct nonoxidative methane conversion (DNMC) transforms CH₄ to higher (C2+) hydrocarbons and H₂ in an MIEC membrane. Reproduced with permission.^{[ 158 ]} Copyright 2021, Wiley‐VCH. A) Tubular reactor with a MIEC as the membrane. B) CH₄ conversion and product yield achieved in a fixed‐bed reactor and the tubular membrane reactor using different gas to seep the H₂. C) Product selectivity achieved in a fixed‐bed reactor and the tubular membrane reactor using different gas to seep the H₂.