Proton-Conducting Ceramic Membranes for the Production of Hydrogen via Decarbonized Heat: Overview and Prospects

: Proton-conducting ceramic membranes show high hydrogen ion conductivity in the temperature range of 300–700 ◦ C. They are attracting signiﬁcant attention due to their relevant characteristics compared to both higher-temperature oxygen ion-conducting ceramic membranes and lower-temperature proton-conducting polymers. The aim of this review is to integrate the fundamentals of proton-conducting ceramic membranes with two of their relevant applications, i.e., membrane reactors (PCMRs) for methane steam reforming (SMR) and electrolysis (PCEC). Both applications facilitate the production of pure H 2 in the logic of process intensiﬁcation via decarbonized heat. Firstly, an overview of various types of hydrogen production is given. The fundamentals of proton-conducting ceramic membranes and their applications in PCMRs for SMR and reversible PCEC (RePCEC), respectively, are given. In particular, RePCECs are of particular interest when renewable power generation exceeds demand because the excess electrical energy is converted to chemical energy in the electrolysis cell mode, therefore representing an appealing solution for energy conversion and grid-scale storage.


Introduction
The world faces fundamental choices that will determine whether in the framework of global warming the 1.5 • C path, as defined at COP26, will remain within reach.Electrification and efficiency are key resulting in a cut of nearly 37 gigatonnes in annual CO 2 emissions by 2050 [1].These reductions can be achieved through (1) increases in production and uses of renewables-based electricity; (2) improvements in energy efficiency [2]; (3) electrification of end-use sectors such as electric vehicles and heat pumps; (4) clean hydrogen and its derivatives [3,4]; (5) bioenergy coupled with carbon capture and storage [5]; and (6) the use of carbon capture and storage (Figure 1).

Introduction
The world faces fundamental choices that will determine whether in the framework of global warming the 1.5 °C path, as defined at COP26, will remain within reach.Electrification and efficiency are key resulting in a cut of nearly 37 gigatonnes in annual CO2 emissions by 2050 [1].These reductions can be achieved through (1) increases in production and uses of renewables-based electricity; (2) improvements in energy efficiency [2]; (3) electrification of end-use sectors such as electric vehicles and heat pumps; (4) clean hydrogen and its derivatives [3,4]; (5) bioenergy coupled with carbon capture and storage [5]; and (6) the use of carbon capture and storage (Figure 1).sector emissions, estimated at about 7.5 Gt of CO 2 [9], about 21% of global greenhouse gas (GHG) emissions [10], derive from the generation of over 100 EJ th of thermal energy.The combustion of three fuels, i.e., coal, natural gas and oil, generates the vast majority of this heat and it's associated with CO 2 emissions.Thiel and Stark [11] discussed four strategies for decarbonizing heat production, i.e., zero-carbon fuels, zero-carbon heat sources, the electrification of heat and better heat management, as schematized in Figure 2.
Hydrogen 2023, 4, FOR PEER REVIEW 2 In this context, the decarbonization of heat is a cross-cutting opportunity.Many recent studies have emphasized its crucial role [6][7][8].Indeed, a large portion of industrial sector emissions, estimated at about 7.5 Gt of CO2 [9], about 21% of global greenhouse gas (GHG) emissions [10], derive from the generation of over 100 EJth of thermal energy.The combustion of three fuels, i.e., coal, natural gas and oil, generates the vast majority of this heat and it's associated with CO2 emissions.
Thiel and Stark [11] discussed four strategies for decarbonizing heat production, i.e., zero-carbon fuels, zero-carbon heat sources, the electrification of heat and better heat management, as schematized in Figure 2.
Figure 2. Schematic representation of technologies to enable zero-carbon industrial heat through the utilization of zero-carbon fuels, zero-carbon heat, electrification of heat, and better heat management technologies.Reproduced from [11] with permission of Elsevier.Among these four pathways, there is electrification of heat, i.e., the generation of thermal energy from electricity.It must be emphasized that the greenhouse gas benefits of the electrification of heat are strictly related to grid decarbonization; in other words, electrified heat is decarbonized heat only when electricity is generated without carbon emissions.As the grid moves toward 100% low-carbon electricity, the GHG benefits of electrification will increase, but until then, it is important to recognize the proportion of low-carbon energy in the grid to understand the magnitude of electrification benefits in the near future [1].Even though a rapid increase in power generation from solar and wind was observed covering in summer up to 30-40% of the total energy demand at the expense of traditional energy sources from fossil fuels [12], the intermittent nature of these renewable energies has hastened the need for low-cost storage over a wide range of time scales, from seconds to days and even seasonal storage.Energy storage systems as well as decarbonizing industrial processes are two current grand challenges to be faced [7].In addition to direct electrical energy storage such as pumped hydroelectricity, batteries, flow batteries [13], compressed air and emerging options with direct ties to the grid [14], there is the indirect approach of converting variable energy like wind and solar to energy carriers (chemical energy) such as hydrogen (H2), ammonia, and ethanol, storing and transporting those energy carriers, and then using them in industry.In this context, the use of electrolysis fed by renewable energy to produce low-carbon energy carriers such as H2, and the use of those chemicals as fuels, reductants, feedstock, and for upgrading petroleum products in refining, represents a low-carbon pathway to net-zero GHG emissions for a number of industrial processes [8].
In this paper, an overview of recent advances in the production of hydrogen within the framework of decarbonizing heat by means of proton-conducting ceramic membranes Figure 2. Schematic representation of technologies to enable zero-carbon industrial heat through the utilization of zero-carbon fuels, zero-carbon heat, electrification of heat, and better heat management technologies.Reproduced from [11] with permission of Elsevier.Among these four pathways, there is electrification of heat, i.e., the generation of thermal energy from electricity.It must be emphasized that the greenhouse gas benefits of the electrification of heat are strictly related to grid decarbonization; in other words, electrified heat is decarbonized heat only when electricity is generated without carbon emissions.As the grid moves toward 100% low-carbon electricity, the GHG benefits of electrification will increase, but until then, it is important to recognize the proportion of low-carbon energy in the grid to understand the magnitude of electrification benefits in the near future [1].Even though a rapid increase in power generation from solar and wind was observed covering in summer up to 30-40% of the total energy demand at the expense of traditional energy sources from fossil fuels [12], the intermittent nature of these renewable energies has hastened the need for low-cost storage over a wide range of time scales, from seconds to days and even seasonal storage.Energy storage systems as well as decarbonizing industrial processes are two current grand challenges to be faced [7].In addition to direct electrical energy storage such as pumped hydroelectricity, batteries, flow batteries [13], compressed air and emerging options with direct ties to the grid [14], there is the indirect approach of converting variable energy like wind and solar to energy carriers (chemical energy) such as hydrogen (H 2 ), ammonia, and ethanol, storing and transporting those energy carriers, and then using them in industry.In this context, the use of electrolysis fed by renewable energy to produce low-carbon energy carriers such as H 2 , and the use of those chemicals as fuels, reductants, feedstock, and for upgrading petroleum products in refining, represents a low-carbon pathway to net-zero GHG emissions for a number of industrial processes [8].
In this paper, an overview of recent advances in the production of hydrogen within the framework of decarbonizing heat by means of proton-conducting ceramic membranes is given.In particular, after a section on hydrogen production, some general considerations on proton-conducting ceramic membranes as well as types of proton-conducting ceramic membranes and their applications in membrane reactors (MRs) and reversible protonic ceramic electrochemical cells (PCECs) are reported.

Hydrogen Production
The energy industry uses color codes to differentiate hydrogen on the basis of the type production.
Across a spectrum of carbon intensity (Figure 3), there is black and brown hydrogen using black coal or lignite (brown coal) in the hydrogen-making process and green hydrogen, which is made by using clean electricity from surplus renewable energy sources, such as solar or wind power, in water electrolysis [15][16][17] (discussed below).The black and brown hydrogen, which are the opposite of green hydrogen, are the most environmentally damaging.
ramic membranes and their applications in membrane reactors (MRs) an tonic ceramic electrochemical cells (PCECs) are reported.

Hydrogen Production
The energy industry uses color codes to differentiate hydrogen on type production.
Across a spectrum of carbon intensity (Figure 3), there is black and using black coal or lignite (brown coal) in the hydrogen-making process a gen, which is made by using clean electricity from surplus renewable ene as solar or wind power, in water electrolysis [15][16][17] (discussed below brown hydrogen, which are the opposite of green hydrogen, are the most damaging.Currently, gray hydrogen is the most common form of hydrogen obtained from natural gas, or methane, using steam methane reforming ( below), but without capturing the greenhouse gases made in the proces is essentially the same as gray hydrogen, but includes the use of carbon c age (CCS) to trap and store the CO2 produced in the process.Moreover, th new hydrogen codes such as yellow hydrogen, turquoise hydrogen and Yellow hydrogen indicates hydrogen made through electrolysis using s quoise hydrogen is made using a methane pyrolysis to produce hydrog bon.In the future, such type of hydrogen may be valued as a low-emiss the thermal process is powered with renewable energy and the carbon be stored.Pink hydrogen is generated through electrolysis powered by nu addition, the very high temperatures from nuclear reactors could be used gen productions by producing steam for more efficient electrolysis or steam methane reforming [18].
The current cost for green H2 is five to seven times that of gray H2 that of blue H2, but such cost difference is expected to narrow with increas and scale [19].The CO2 reduction between an electrolytic H2 via grid-su and gray H2 is around 20-30%.For industry, H2 via electrolysis using low could drive a much larger drop in CO2 emissions [8].

SMR
Today, industrial-scale H2 is produced via SMR, which supplies a r industries including synthesis of ammonia via the Haber-Bosch process anol.In SMR, natural gas (methane) reacts with steam according to the tions: Currently, gray hydrogen is the most common form of hydrogen production: it is obtained from natural gas, or methane, using steam methane reforming (SMR) (discussed below), but without capturing the greenhouse gases made in the process.Blue hydrogen is essentially the same as gray hydrogen, but includes the use of carbon capture and storage (CCS) to trap and store the CO 2 produced in the process.Moreover, there are relatively new hydrogen codes such as yellow hydrogen, turquoise hydrogen and pink hydrogen.Yellow hydrogen indicates hydrogen made through electrolysis using solar power.Turquoise hydrogen is made using a methane pyrolysis to produce hydrogen and solid carbon.In the future, such type of hydrogen may be valued as a low-emission hydrogen, if the thermal process is powered with renewable energy and the carbon being permanently stored.Pink hydrogen is generated through electrolysis powered by nuclear energy.In addition, the very high temperatures from nuclear reactors could be used in other hydrogen productions by producing steam for more efficient electrolysis or fossil-gas-based steam methane reforming [18].
The current cost for green H 2 is five to seven times that of gray H 2 and three times that of blue H 2 , but such cost difference is expected to narrow with increased development and scale [19].The CO 2 reduction between an electrolytic H 2 via grid-supplied electricity and gray H 2 is around 20-30%.For industry, H 2 via electrolysis using low-carbon sources could drive a much larger drop in CO 2 emissions [8].

SMR
Today, industrial-scale H 2 is produced via SMR, which supplies a range of chemical industries including synthesis of ammonia via the Haber-Bosch process, fuels and methanol.In SMR, natural gas (methane) reacts with steam according to the following equations: The synthesis gas known as syngas (CO + H 2 ) composition is governed by the methane reforming reaction (1) and the water-gas shift (2).Syngas conversion processes occur at high pressure and therefore, process economy favors operating the SMR at high pressure to decrease compression cost in the plant design.However, looking at the stoichiometry of the reaction (1), SMR is adversely impacted by high pressure due to Le Chatelier principle, shifting the equilibrium to left.Consequently, high temperatures are required to drive the reaction.Typical conditions are 30 bar and 900 • C [20].A large-scale industrial reformer can contain up to several hundred tubular reactors in a large furnace, favored for uniform distribution of heat, which is produced by combustion of fossil fuels [20].For most largescale industrial reformers, radiation is the primary heat-transfer mechanism.Therefore, the combustion process must occur several hundred degrees above the reaction temperature to supply the necessary heat flux for the catalyst [20][21][22].
A typical large-scale industrial syngas plant for pure H 2 production has several reaction units, being steam-reforming equilibrium-limited and even, in the case of complete fuel conversion, a hydrogen-rich gas mixture containing carbon oxides and other byproducts is produced.Typically, a high-temperature reformer, high-and low-temperature shift reactors with subsequent separation and compression downstream via pressure swing adsorption to separate H 2 from CO 2 feature an industrial steam reforming plant (Figure 4).
occur at high pressure and therefore, process economy favors operating the SMR pressure to decrease compression cost in the plant design.However, looking at chiometry of the reaction (1), SMR is adversely impacted by high pressure due to telier principle, shifting the equilibrium to left.Consequently, high temperature quired to drive the reaction.Typical conditions are 30 bar and 900 °C [20].A lar industrial reformer can contain up to several hundred tubular reactors in a large favored for uniform distribution of heat, which is produced by combustion of fos [20].For most large-scale industrial reformers, radiation is the primary heat mechanism.Therefore, the combustion process must occur several hundred above the reaction temperature to supply the necessary heat flux for the catalyst A typical large-scale industrial syngas plant for pure H2 production has seve tion units, being steam-reforming equilibrium-limited and even, in the case of c fuel conversion, a hydrogen-rich gas mixture containing carbon oxides and other b ucts is produced.Typically, a high-temperature reformer, high-and low-temperat reactors with subsequent separation and compression downstream via pressur adsorption to separate H2 from CO2 feature an industrial steam reforming plant 4).[23] with permission of Elsevie A typical fired SMR process based on natural gas emits 7-11 kg CO2/kg H2 [ global production of syngas accounts for nearly 3% of global CO2 emissions [25 heat source can be a variety of hydrocarbons with a high to low carbon content, i coal or coke to natural gas to renewables.In the context of decarbonized heat, i. place the fossil fuel combustion, excess electric power electricity from renewable transformed into heat via microwaves, induction, plasma, or joule heating (Figur  A typical fired SMR process based on natural gas emits 7-11 kg CO 2 /kg H 2 [24].The global production of syngas accounts for nearly 3% of global CO 2 emissions [25,26].The heat source can be a variety of hydrocarbons with a high to low carbon content, i.e., from coal or coke to natural gas to renewables.In the context of decarbonized heat, i.e., to replace the fossil fuel combustion, excess electric power electricity from renewables can be transformed into heat via microwaves, induction, plasma, or joule heating (Figure 5).
Hydrogen 2023, 4, FOR PEER REVIEW 4 occur at high pressure and therefore, process economy favors operating the SMR at high pressure to decrease compression cost in the plant design.However, looking at the stoichiometry of the reaction (1), SMR is adversely impacted by high pressure due to Le Chatelier principle, shifting the equilibrium to left.Consequently, high temperatures are required to drive the reaction.Typical conditions are 30 bar and 900 °C [20].A large-scale industrial reformer can contain up to several hundred tubular reactors in a large furnace, favored for uniform distribution of heat, which is produced by combustion of fossil fuels [20].For most large-scale industrial reformers, radiation is the primary heat-transfer mechanism.Therefore, the combustion process must occur several hundred degrees above the reaction temperature to supply the necessary heat flux for the catalyst [20][21][22].
A typical large-scale industrial syngas plant for pure H2 production has several reaction units, being steam-reforming equilibrium-limited and even, in the case of complete fuel conversion, a hydrogen-rich gas mixture containing carbon oxides and other by-products is produced.Typically, a high-temperature reformer, high-and low-temperature shift reactors with subsequent separation and compression downstream via pressure swing adsorption to separate H2 from CO2 feature an industrial steam reforming plant (Figure 4).A typical fired SMR process based on natural gas emits 7-11 kg CO2/kg H2 [24].The global production of syngas accounts for nearly 3% of global CO2 emissions [25,26].The heat source can be a variety of hydrocarbons with a high to low carbon content, i.e., from coal or coke to natural gas to renewables.In the context of decarbonized heat, i.e., to replace the fossil fuel combustion, excess electric power electricity from renewables can be transformed into heat via microwaves, induction, plasma, or joule heating (Figure 5).Several studies have investigated relevant endothermic processes proposing electrified heat as listed in Table 1.In particular, joule heating instead of heating from fossil fuel combustion has been proposed for highly endothermic chemical synthesis such as SMR (Table 1) as discussed below.Noteably, electrified heat is only decarbonized heat when the electricity is generated without carbon emissions [11].Therefore, hydrogen synthesis from methane should include carbon capture converting emitted carbon dioxide into valuable commodity chemicals (Figure 6).
Hydrogen 2023, 4, FOR PEER REVIEW 5 Several studies have investigated relevant endothermic processes proposing electrified heat as listed in Table 1.Decomposition of ammonia Microwave [45,46] In particular, joule heating instead of heating from fossil fuel combustion has been proposed for highly endothermic chemical synthesis such as SMR (Table 1) as discussed below.Noteably, electrified heat is only decarbonized heat when the electricity is generated without carbon emissions [11].Therefore, hydrogen synthesis from methane should include carbon capture converting emitted carbon dioxide into valuable commodity chemicals (Figure 6).

Water Electrolysis
Another relevant strategy to produce hydrogen via decarbonized heat is by water electrolysis fed by renewable electric energy, which in other words is transforming renewable electric energy into chemical energy by producing feedstocks such as H 2 (Figure 6).
By sourcing hydrogen exclusively by electrolysis instead of by SMR would avoid the generation of CO 2 altogether.
Hydrogen produced by electrolysis can be used for several purposes ranging from lowcarbon heating to the synthesis of chemicals, representing a relatively new opportunity to reduce our dependence on carbon-based sources of heat and feedstock.Water electrolysis is a well-established mature technology used in special applications [47].The overall electrolysis reaction is the electrochemical splitting of water molecules, owing to an electric current between cathode and anode, separated by electrolyte.
Electrolytic hydrogen production technology can be classified on the basis of the employed ionic conductor, i.e., alkaline electrolysis cells (AECs), polymer electrolyte membrane electrolysis cells (PEMECs), protonic ceramic electrolysis cells (PCECs), and solid oxide electrolysis cells (SOECs).An overview of the four types of electrolytic hydrogen production technology is given in Table 2.In particular, AECs conventionally operate at lower temperatures in the range of 25−100 • C and utilize a concentrated alkaline solution as a medium for the conduction of hydroxide ions (OH − ), which is fed to the cathode and anode for the corresponding hydrogen evolution reaction (HER) (Equation ( 4)) and oxygen evolution reaction (OER) (Equation ( 5)) To avoid the mixing of the produced H 2 and O 2 (Equations ( 4) and ( 5)), a separator is typically placed between the electrodes, generally a polyphenylene sulfide (PPS)-based diaphragms [48] for which the main drawbacks are a limited current density (normally below 400 mA•cm −2 ) and an insufficient energy efficiency [49].
In the case of PEMECs, the conducted ion is proton (H + ) transported through a polymer electrolyte membrane, while the deposited electrocatalysts promote the HER and OER on both sides as reported in Equations ( 6) and ( 7): Additionally, PCECs use H + as the conducted ion in electrolyte materials (with identical HER and OER expressions to those shown in Equations ( 6) and ( 7)), whilst the operating temperatures are in the range of 550−700 • C, requiring very different cell structures and material selections.The electrolyte materials mainly comprise proton conducting ceramic membranes as discussed below in this review, whereas cermet materials incorporating metals are utilized as electrocatalysts for remarkable catalytic activity.
SOECS have similar structures and materials to PCECs with a further increase in the working temperature to >700 • C, whereas the ion conduction in the electrolyte is dominated by the migration of oxygen ions (O 2− ), which requires a water supply to the hydrogen electrode (cathode) and involves distinct HER and OER expressions, as reported in Equations ( 8) and ( 9): Table 2. Specification of AECs, PEMECs, PCECs and SOECs (adapted from [49] with permission of American Chemical Society).

Proton Conducting Ceramic Membranes: General Considerations
Membrane technologies have all the features of process intensification strategy, i.e., low energy consumption, easy scalability, modularity [50].In particular, H 2 separation using membranes can operate continuously using potentially less than half the energy required for the PSA process at the downstream of SMR (Figure 4) [51].Among the four kinds of hydrogen separation membranes (polymeric, metallic, carbon, and ceramic [52]), only polymeric membranes have been in commercial use to any considerable extent at the current state of technology.Among the three inorganic categories, i.e., metallic, carbon and ceramic, proton-electron conducting ceramic membranes are the most promising due to their two main advantages, i.e., high selectivity and low cost [53].Indeed, proton-electronconducting ceramic membranes are permeable only to hydrogen ions, i.e., protons allowing the extraction of hydrogen from gas mixtures with a theoretical selectivity of 100%.
Overall, the process of H 2 transport through a proton-electron-conducting ceramic membrane mainly involves two steps, as schematized in Figure 7: (i) H 2 is first adsorbed onto the surface of the membrane, then dissociates into protons and electrons (surface process); (ii) protons and electrons diffuse together to the other side of the membrane surface where they reassociate to form molecular H 2 again (bulk diffusion) [54].
American Chemical Society).

Proton Conducting Ceramic Membranes: General Considerations
Membrane technologies have all the features of process intensification strateg low energy consumption, easy scalability, modularity [50].In particular, H2 sepa using membranes can operate continuously using potentially less than half the e required for the PSA process at the downstream of SMR (Figure 4) [51].Among th kinds of hydrogen separation membranes (polymeric, metallic, carbon, and cerami only polymeric membranes have been in commercial use to any considerable exten current state of technology.Among the three inorganic categories, i.e., metallic, c and ceramic, proton-electron conducting ceramic membranes are the most promisin to their two main advantages, i.e., high selectivity and low cost [53].Indeed, proton tron-conducting ceramic membranes are permeable only to hydrogen ions, i.e., p allowing the extraction of hydrogen from gas mixtures with a theoretical selecti 100%. Overall, the process of H2 transport through a proton-electron-conducting c membrane mainly involves two steps, as schematized in Figure 7: (i) H2 is first ad onto the surface of the membrane, then dissociates into protons and electrons (s process); (ii) protons and electrons diffuse together to the other side of the membran face where they reassociate to form molecular H2 again (bulk diffusion) [54].The permeation of hydrogen through the membrane is primarily controlled surface process and bulk diffusion.
Regarding bulk diffusion, the transport of protons through the membrane is th determining step if the electron conductivity is much higher than the proton conduc Taking into account the chemical/electrochemical potentials and conductivities o protons and electrons within the membrane, the H2 flux (JH2) can be described by th ner equation as follows [55]: The permeation of hydrogen through the membrane is primarily controlled by the surface process and bulk diffusion.
Regarding bulk diffusion, the transport of protons through the membrane is the ratedetermining step if the electron conductivity is much higher than the proton conductivity.Taking into account the chemical/electrochemical potentials and conductivities of both protons and electrons within the membrane, the H 2 flux (J H2 ) can be described by the Wagner equation as follows [55]: σ H+ σ el σ H+ +σ el ln P H2 P H2 (10) where R is the universal gas constant, T is the temperature, F is the Faraday constant, l is the thickness of the ceramic membrane, σ H+ is the proton conductivity within the membrane, σ el is the sum of the electron and hole conductivities, P H2 is the H 2 pressure at upstream membrane and P H2 is the H 2 pressure at downstream membrane (Figure 7).Proton and electron conductivities as well as the temperature and pressure at the membrane upstream and downstream as well as membrane thickness control the H 2 flux.Two opposite cases can occur.
The first is when the proton conductivity σ H+ within the membrane is much smaller than the electron conductivity σ el ; that is σ H+ << σ el , Equation ( 10) can be rewritten as: In this case, the development of highly conductive proton-conducting ceramic membranes is very important for achieving high H 2 flux as discussed below.
The second case is if the proton conductivity within the membrane is much larger than the electron conductivity, i.e., σ H+ >> σ el .So, Equation ( 10) can be rewritten as: In this case, ceramic membranes with high electron conductivity become crucial in achieving high H 2 flux during the separation process.
Normally, it is difficult to obtain both high proton and high electron conductivities.This depends on the membrane materials.
Different types of ceramic H 2 separation membranes are reported in the literature: (1) Cermet membranes, consisting of a combination of a ceramic phase and a metallic phase.The metal is used as the electron-conducting phase and the ceramic oxide serves as the proton-conducting phase.Combining these two phases together may lead to high h 2 permeation because both σ h+ and σ el are high; (2) Single-phase ceramic oxides membranes, which simultaneously transport protons and electrons.(3) Cer-cer membranes, i.e., dual-phase ceramic oxides membranes, which combine a protonic conducting perovskite phase and electronic (non-perovskite) phase.
Single-phase ceramic membranes in turn may be classified into two subcategories: perovskite-type and non-perovskite-type membranes.
A scheme of the various types of proton conducting ceramic membranes is shown in Figure 8.In Table 3 a comparison of thickness normalized H 2 permeation flux of ceramic membranes from the literature is given.

Perovskite-Type Ceramic Membranes
The term perovskite originally refers to a mineral calcium titanium oxide, CaTiO 3 .Today perovskite-type metal oxides is the name given to a group of materials with general formula ABO 3 (Figure 9) having the same structure as the mineral calcium titanate (CaTiO 3 ) [56].In this general formula, A = Ca, Ba, Sr; B = Ce, Zr.The distance between the oxygen ions, which plays an important role in proton conductivity, can be changed by doping a trivalent M element into the structure leading to a significant improvement [57].The formula of these high-temperature, proton-conducting perovskite oxides can be written as AB 1−x M x O 3−δ .Doping a trivalent M cation into the B site can increase the vibration distance between the oxygen ions and also create more oxygen vacancies, leading to higher proton conductivity [58][59][60].The most studied high temperature perovskite-type oxides are SrCeO 3 and BaCeO 3 , which have high protonic conductivity but poor electronic conductivity.Electron conductivity has been improved by doping the B site in SrCeO 3 and BaCeO 3 with a multivalent cation [61][62][63].Moreover, BaCeO 3 and SrCeO 3 are chemically unstable, easily reacting with CO 2 and H 2 O [64][65][66].In particular, formation of undesired phases such as carbonates would be formed because of the reaction between alkali earth ions, i.e., Ba 2+ and Sr 2+ and CO 2 at elevated temperature.This is the weak point of ceramic-based membranes, while the drawback of Pd-based membranes (not discussed in this paper) is hydrogen-induced embrittlement and sulfur poisoning.
phase.The metal is used as the electron-conducting phase and th serves as the proton-conducting phase.Combining these two phase lead to high h2 permeation because both σh+ and σel are high; (2) Single-phase ceramic oxides membranes, which simultaneously tra and electrons.(3) Cer-cer membranes, i.e., dual-phase ceramic oxides membranes, w protonic conducting perovskite phase and electronic (non-perovskite Single-phase ceramic membranes in turn may be classified into two perovskite-type and non-perovskite-type membranes.
A scheme of the various types of proton conducting ceramic membra Figure 8.In Table 3 a comparison of thickness normalized H2 permeation membranes from the literature is given.

Perovskite-Type Ceramic Membranes
The term perovskite originally refers to a mineral calcium titanium oxi Today perovskite-type metal oxides is the name given to a group of materials w formula ABO3 (Figure 9) having the same structure as the mineral calciu (CaTiO3) [56].In this general formula, A = Ca, Ba, Sr; B = Ce, Zr.The distance b oxygen ions, which plays an important role in proton conductivity, can be doping a trivalent M element into the structure leading to a significant improv The formula of these high-temperature, proton-conducting perovskite oxides ten as AB1−xMxO3−δ.Doping a trivalent M cation into the B site can increase th distance between the oxygen ions and also create more oxygen vacancies, higher proton conductivity [58][59][60].The most studied high temperature pero oxides are SrCeO3 and BaCeO3, which have high protonic conductivity but poo conductivity.Electron conductivity has been improved by doping the B site in BaCeO3 with a multivalent cation [61][62][63].Moreover, BaCeO3 and SrCeO3 are unstable, easily reacting with CO2 and H2O [64][65][66].In particular, formation o phases such as carbonates would be formed because of the reaction between ions, i.e., Ba 2+ and Sr 2+ and CO2 at elevated temperature.This is the weak point based membranes, while the drawback of Pd-based membranes (not discusse per) is hydrogen-induced embrittlement and sulfur poisoning.In contrast, zirconate-based proton-conducting perovskite oxides are stab conductivity is limited by highly resistive grain boundaries [68,69].Indeed, th ductivity of Y-doped BaZrO3, the most promising zirconate in H2O and CO2 atmospheres, decreases sharply, owing to the blocking effect of grain boun In contrast, zirconate-based proton-conducting perovskite oxides are stable, but their conductivity is limited by highly resistive grain boundaries [68,69].Indeed, the total conductivity of Y-doped BaZrO 3 , the most promising zirconate in H 2 O and CO 2 -containing atmospheres, decreases sharply, owing to the blocking effect of grain boundaries combined with the small grain sizes in these materials [69].In order to develop advanced membrane materials, solid solutions of cerate and doped zirconate (Y-doped BaZrO 3 -BaCeO 3 ) have been investigated.Yttrium-doped barium zirconate/cerate materials (BaCe x Zr 1−x−y Y y O 3−d , BCZY) represent the best compromise between the high proton conductivity of cerate-based and high stability of zirconate-based proton conducting perovskite oxides.They are stable over a wide range of temperatures (300-800 • C) and chemical environments and are among the most studied materials for hydrogen separation membranes in membrane reactors (MRs) [70][71][72][73][74] and reversible protonic ceramic electrochemical cells (PCECs) [75,76] as discussed below.

Non-Perovskite-Type Ceramic Membranes
The non-perovskite type membranes for hydrogen separation are mainly doped rare earth metal oxides and fluorite-structured metal oxide.Alkali earth ion-free oxides such as LaNbO 4 , Ln 6 WO 12 , Y 2 O 3 , CeO 2 were developed in order to overcome the drawback of other phase formation such as carbonates under a CO 2 rich atmosphere.Ln 6 WO 12 presents relatively high mixed protonic and electronic conductivities and reasonable stability under CO 2 and H 2 O [77].Moreover, among the Ln 6 WO 12 -based membrane materials, Modoped, Nd 5.5 WO 11.25−d , and Re-doped materials showed almost the highest ambipolar conductivity and hydrogen permeation flux.

Cer-Cer Dual-Phase Ceramic Membranes
As reported by Elangovan et al. [78], H 2 permeating material with high performance can be obtained by combining (i) a protonic conducting perovskite phase BaCe 1−x M x O 3−d and (ii) an electronic conducting fluorite phase Ce 1−y M y O 2−d (M is the metal dopant).Such combination is named as cer-cer composite.It has certain degree of percolation, which provides efficient pathways for proton and electronic transport through the membrane.
In such systems (Table 3), the addition of the doped ceria phase enhances the stability of BaCeO 3 phase, which otherwise, in the presence of CO 2 and H 2 O, would decompose to BaCO 3 , Ba(OH) 2 and CeO 2 .

Cermet Membranes
In cermet membranes, a ceramic phase is combined with a metallic phase in order to enhance H 2 permeation because the ceramic phase is a proton conductor and the metallic phase is a highly electron conductor.Depending on the degree of hydrogen conductivity in the metal phase, mainly three different combinations can be realized [23]: (i) a metal with low hydrogen conductivity in combination with a highly proton-conductive oxide; (ii) a metal or an alloy with high hydrogen permeability such as Pd, Pd/Ag, Pd/Cu, Pd/Cu, combined with a ceramic of low hydrogen permeability; (iii) a combination where both the metallic and ceramic phases conduct hydrogen.Table 3. Thickness-normalized values of H 2 permeation rates (J H2 ) through ceramic membranes (adapted from [53] under CC-BY 4.0).

Material
Gas Atmosphere Feed-Sweep T ( Ref.

PCMRs
Membrane reactors can be classified on the basis of the membrane role [113,114].The membrane can have three different functions (Figure 10): (i).It can be used to extract a reaction product from the reaction zone in the so-called extractor.(ii).It can control the introduction of one of the reactants into the reaction zone in the so-called distributor.(iii).It can facilitate the contact between reactants and catalyst in the so-called contactor.All the case studies discussed below involve decarbonized hydrogen production concern extractor type MRs for SMR (Table 4, entries 1-3).Indeed, the reactions involved are equilibrium reactions and the membrane, which allow continuous removal of the H2 product, i.e., its extraction; as a result increases the reaction conversion according to the Le Chatelier principle.In the specific case of SMR, with respect to a classical configuration consisting of a reaction unit in series with a separation unit, i.e., PSA (Figure 4), in an MR, the integration of a reaction to produce H2 with its concomitant separation by the membrane reduces capital costs, improves yields and selectivities, and drastically reduces downstream separation costs [116].In most studies, metallic membranes, predominantly based on Pd or Pd-Ag alloy are used [23,116].The difference in hydrogen's chemical potential constitutes the driving force for hydrogen transport through the Pd membrane, from which it follows that even though complete methane conversion has been obtained, high pressures upstream are necessary and the pressure of the produced H2 is low, and further compression requires multistage compressors, increasing energy consumption.In PCMR, protons are driven through the protonic ceramic membrane through the application of a voltage (or current) (Figure 11), which indirectly drives the flux of hydrogen gas (Figure 7) avoiding higher pressure.A gas mixture of methane saturated with steam is fed over the electrocatalyst (anode) where catalytic reactions (1)-( 2) occur.With the use of an external power source, the produced hydrogen is extracted thanks its conversion at the anode in protons which are transported through the proton conducting ceramic membrane to the cathode according to the mechanism schematized above (Figure 7).At the cathode, protons recombine with electrons, forming molecular H2.All the case studies discussed below involve decarbonized hydrogen production concern extractor type MRs for SMR (Table 4, entries 1-3).Indeed, the reactions involved are equilibrium reactions and the membrane, which allow continuous removal of the H 2 product, i.e., its extraction; as a result increases the reaction conversion according to the Le Chatelier principle.In the specific case of SMR, with respect to a classical configuration consisting of a reaction unit in series with a separation unit, i.e., PSA (Figure 4), in an MR, the integration of a reaction to produce H 2 with its concomitant separation by the membrane reduces capital costs, improves yields and selectivities, and drastically reduces downstream separation costs [116].In most studies, metallic membranes, predominantly based on Pd or Pd-Ag alloy are used [23,116].The difference in hydrogen's chemical potential constitutes the driving force for hydrogen transport through the Pd membrane, from which it follows that even though complete methane conversion has been obtained, high pressures upstream are necessary and the pressure of the produced H 2 is low, and further compression requires multistage compressors, increasing energy consumption.In PCMR, protons are driven through the protonic ceramic membrane through the application of a voltage (or current) (Figure 11), which indirectly drives the flux of hydrogen gas (Figure 7) avoiding higher pressure.A gas mixture of methane saturated with steam is fed over the electrocatalyst (anode) where catalytic reactions (1)-( 2) occur.With the use of an external power source, the produced hydrogen is extracted thanks its conversion at the anode in protons which are transported through the proton conducting ceramic membrane to the cathode according to the mechanism schematized above (Figure 7).At the cathode, protons recombine with electrons, forming molecular H 2 .
Kyriacou et al. [74] (Table 4, entry 1) used BaZr 0.7 Ce 0.2 Y 0.1 O 2.9 perovskite as a protonconducting membrane, a tubular NiBZCY72 cermet as anodic electrode, and a film of Cu as the cathode.The results showed that hydrogen migration was beneficial for methane conversion and hydrogen yield. Figure 12 shows the effect of proton removal from the anode on the increase in methane consumption.Initially, reacting methane correlates linearly with removed protons (under closed circuit operation).The experiments carried out to investigate the stability indicate suitable activity thanks to the outstanding BaZr x Ce 1−x Y 0.1 O 3−δ perovskite membrane stability for x > 0.3 without the of formation barium carbonate as reported in the literature [117][118][119].
, FOR PEER REVIEW 13 Figure 11.SMR concept in a PCMR.Reproduced from [74] with permission of Elsevier.
Kyriacou et al. [74] (Table 4, entry 1) used BaZr0.7Ce0.2Y0.1O2.9 perovskite as a protonconducting membrane, a tubular NiBZCY72 cermet as anodic electrode, and a film of Cu as the cathode.The results showed that hydrogen migration was beneficial for methane conversion and hydrogen yield. Figure 12 shows the effect of proton removal from the anode on the increase in methane consumption.Initially, reacting methane correlates linearly with removed protons (under closed circuit operation).The experiments carried out to investigate the stability indicate suitable activity thanks to the outstanding Ba-ZrxCe1−xY0.1O3−δ perovskite membrane stability for x > 0.3 without the of formation barium carbonate as reported in the literature [117][118][119].Malerød-Fjeld et al. [72] proposed a PCMR operating at 800 °C and using a BaZrO3based proton conducting membrane (Table 4, entry 2).They simultaneously realized the extraction and shift of a thermodynamically limited reaction towards full conversion of  Kyriacou et al. [74] (Table 4, entry 1) used BaZr0.7Ce0.2Y0.1O2.9 perovskite as a conducting membrane, a tubular NiBZCY72 cermet as anodic electrode, and a fil as the cathode.The results showed that hydrogen migration was beneficial for m conversion and hydrogen yield. Figure 12 shows the effect of proton removal f anode on the increase in methane consumption.Initially, reacting methane corre early with removed protons (under closed circuit operation).The experiments car to investigate the stability indicate suitable activity thanks to the outstand ZrxCe1−xY0.1O3−δ perovskite membrane stability for x > 0.3 without the of formation carbonate as reported in the literature [117][118][119].Malerød-Fjeld et al. [72] proposed a PCMR operating at 800 °C and using a based proton conducting membrane (Table 4, entry 2).They simultaneously real extraction and shift of a thermodynamically limited reaction towards full conve methane and the production of high-purity H2 as in MRs based on Pd membra differently; on the latter, PCMR delivers heat to the strongly endothermic rea through the electrical operation of the membrane, which acts as separator and c sor, compressing hydrogen directly at the sweep side of the membrane.In Table parison of hydrogen production rate (JH2), methane conversion (XCH4), CO2 se Malerød-Fjeld et al. [72] proposed a PCMR operating at 800 • C and using a BaZrO 3based proton conducting membrane (Table 4, entry 2).They simultaneously realized the extraction and shift of a thermodynamically limited reaction towards full conversion of methane and the production of high-purity H 2 as in MRs based on Pd membranes, but differently; on the latter, PCMR delivers heat to the strongly endothermic reaction (1) through the electrical operation of the membrane, which acts as separator and compressor, compressing hydrogen directly at the sweep side of the membrane.In Table 5 comparison of hydrogen production rate (J H2 ), methane conversion (X CH4 ), CO 2 selectivity (S CO2 ), and hydrogen recovery (HR) of PCMR with Pd-based membrane reformers is given.Malerød-Fjeld et al. [72] showed that the reaction heat required to form H 2 from CH 4 and H 2 O scales linearly with the production rate.The heat evolved from the galvanic operation of the membrane distributes along the length of the membrane and originates from compression and Joule contributions [72].The complete reformer plant was simulated as a function of H 2 production rate by changing the applied current in order to evaluate system heat integration (Figure 13).
(SCO2), and hydrogen recovery (HR) of PCMR with Pd-based membrane reformers is given.Malerød-Fjeld et al. [72] showed that the reaction heat required to form H2 from CH4 and H2O scales linearly with the production rate.The heat evolved from the galvanic operation of the membrane distributes along the length of the membrane and originates from compression and Joule contributions [72].The complete reformer plant was simulated as a function of H2 production rate by changing the applied current in order to evaluate system heat integration (Figure 13).Noteably, in order to assess the practical implications of PCMR technology, Malerød-Fjeld et al. made a techno-economic analysis, comparing the operating energy costs of PCMR with water electrolysis and SMR (Figure 14).The analysis reveals that PCMR and SMR are the less sensitive technologies to energy prices.The PCMR technology allows significant process intensification.High energy efficiency together with its hybrid nature, which uses 1/3 of electricity and 2/3 of natural gas, decreases the carbon footprint especially when the sources of electricity are renewable energies.Moreover, PCMR can benefits from producing a nearly pure stream of CO2, enabling carbon capture, use, and storage for industrial scale H2 production at locations with access to infrastructure for CO2 storage and use.Noteably, in order to assess the practical implications of PCMR technology, Malerød-Fjeld et al. made a techno-economic analysis, comparing the operating energy costs of PCMR with water electrolysis and SMR (Figure 14).The analysis reveals that PCMR and SMR are the less sensitive technologies to energy prices.The PCMR technology allows significant process intensification.High energy efficiency together with its hybrid nature, which uses 1/3 of electricity and 2/3 of natural gas, decreases the carbon footprint especially when the sources of electricity are renewable energies.Moreover, PCMR can benefits from producing a nearly pure stream of CO 2 , enabling carbon capture, use, and storage for industrial scale H 2 production at locations with access to infrastructure for CO 2 storage and use.Recently, Clark et al. [115] (Table 4, entry 3) proposed an optimized PCMR which retains the energy efficiency and H2 recovery of single cells while achieving a 36-fold Recently, Clark et al. [115] (Table 4, entry 3) proposed an optimized PCMR which retains the energy efficiency and H 2 recovery of single cells while achieving a 36-fold increase in H 2 production capacity.The PCMR stacks showed high-pressure H 2 at high purity and a CO 2 -rich effluent at a hydrogen recovery and methane conversion > 99%.These results are very promising compared to Pd based membrane reformers.

RePCECs
As reported above, water electrolysis or water splitting via electricity enables energy storage in the H 2 molecule, which can be converted back into electricity through fuel cells or used in chemical processing (Figure 6, above).Among the different types of electrolyzers discussed above, PCECs in which electron flow through the device is accompanied by a flow of protons through a ceramic electrolyte are promising because of the potential for high energy efficiency and the possibility to produce dry pressurized H 2 [122].RePCECs allow the seasonal energy storage challenge of intermittent renewable electricity generation, favoring the paradigm based on decarbonized heat, to be addressed.Indeed, conventional batteries are less suited for seasonal energy storage due to self-discharge and economic constraints than they are for short-term energy storage [119,[123][124][125][126]. A. RePCEC produces H 2 from water when there is an excess of renewable electricity on the grid that might otherwise go to waste; otherwise, it can work in fuel cell mode using the stored H 2 to produce electricity when there is not enough electricity available on the grid [127].In particular, in an RePCEC, in the electrolysis (EC) mode, the applied external voltage drives H + from the air electrode to which steam, i.e., H 2 O, is fed toward the electrode where H 2 is produced; in the fuel cell mode (FC), electric power is generated as a result of the oxidation of H 2 , which is fed to the fuel side of the cell (Figure 15).
Hydrogen 2023, 4, FOR PEER REVIEW increase in H2 production capacity.The PCMR stacks showed high-pressure H purity and a CO2-rich effluent at a hydrogen recovery and methane conversio These results are very promising compared to Pd based membrane reformers.

RePCECs
As reported above, water electrolysis or water splitting via electricity enable storage in the H2 molecule, which can be converted back into electricity through f or used in chemical processing (Figure 6, above).Among the different types of el ers discussed above, PCECs in which electron flow through the device is accomp a flow of protons through a ceramic electrolyte are promising because of the pote high energy efficiency and the possibility to produce dry pressurized H2 [122].R allow the seasonal energy storage challenge of intermittent renewable electricity tion, favoring the paradigm based on decarbonized heat, to be addressed.Inde ventional batteries are less suited for seasonal energy storage due to self-discha economic constraints than they are for short-term energy storage [123][124][125][126][127]. A. produces H2 from water when there is an excess of renewable electricity on the might otherwise go to waste; otherwise, it can work in fuel cell mode using the s to produce electricity when there is not enough electricity available on the grid particular, in an RePCEC, in the electrolysis (EC) mode, the applied external voltag H + from the air electrode to which steam, i.e., H2O, is fed toward the electrode w is produced; in the fuel cell mode (FC), electric power is generated as a result of dation of H2, which is fed to the fuel side of the cell (Figure 15).Dailly et al. [76] used BaCe 0.8 Zr 0.1 Y 0.1 O 3−d (BCZY81) as a proton conducting ceramic membrane.Several reversible cycles into fuel cell/steam electrolysis were performed with an accelerated electrical degradation observed (+5%/kh under EC and −8%/kh under FC mode over 800 h).However, the authors of this study reported that deeper investigations in terms of operation protocols and the optimization of cell architecture (in particular the air electrode coating) are needed in order to improve electrical Faradaic efficiency (FE).
In a subsequent study, Duan et al. [75] correlate the poor FE observed in electrolysis mode with the electronic leakage due to minority p-type electronic conduction in common proton-conducting ceramic membranes.Indeed, electron holes are produced via the incorporation of molecular oxygen into oxygen vacancies as follows: Instead, the desired hydroxide defect, OH .O , is formed via the incorporation of water into oxygen vacancies via the hydration reaction: So, a competition between oxidation (Equation ( 13)) and hydration (Equation ( 14)) for oxygen vacancies exists [111].Increasing the favorability of reaction ( 14) suppresses reaction (13) by simultaneously increasing the proton transference number and decreasing the hole transference number, which in turn decreases electronic leakage.These relationships present the opportunity to tune the composition of proton conducting ceramic membrane and cell operating conditions to achieve higher FE.The equilibrium constant of the hydration reaction ( 14), i.e., K hydration , can be expressed in terms of thermodynamic parameters [111]: where ∆S • hydration and ∆H • hydration are the standard hydration entropy and enthalpy, respectively.Equation 15 correlates proton concentration with hydration thermodynamics and provides a tool for the design of materials with high proton concentration at relatively high temperatures.For ABO 3 -based simple perovskites, prior investigation [129][130][131] has suggested that ∆H • hydration is correlated with the electronegativities of the cations, which relate to the basicity of the oxide.At 600 • C, cerates such as barium cerates, i.e., BCY exhibits the largest hydration equilibrium constant, which both favors high proton concentration and suppresses the parasitic oxidation reaction.At the opposite end, the lower K hydration value observed in zirconates (such as BZY) is the cause of low proton concentration and conductivity, and also of more oxygen vacancies available for oxidation resulting in higher hole concentrations and greater electronic leakage.
The most common proton-conducting ceramic membranes used in RePCECs employ yittrium-doped barium zirconate (BZY), barium cerates (BCY), and solid solutions thereof (BCZY).These ceramic membranes are mixed ionic-electronic conductors and not pure proton conductors.Even though electronic conduction does not greatly affect device performance in fuel cell mode, because transport across the membrane is dominated by protons and oxygen vacancies, in electrolysis mode, electronic conduction transport of delocalized small polarons O .
O is not negligible.BZY20 and BCZYYb show significantly higher electronic transport with severe electronic leakage in BZY20 more than in BCZYYb, which has a high Ce content at the B site, and therefore lower FE in electrolysis mode.So, the findings by Duan et al. [75] suggest that the preferred ceramic membrane composition for RePCECs and proton-conducting fuel cells (PCFCs) should be different.Indeed, one of the main challenges in the development of RePCECs is to enhance their performance by tailoring the composition of the ceramic membrane acting as electrolyte [111,132].Putilov and Tsidilkovski [128] investigated theoretically the impact of acceptor-bound states of ionic defects in a proton-conducting oxide with proton and hole conductivity on the performance of protonic ceramic fuel and electrolysis cells operating on humidified hydrogen.The results are presented for the electrochemical cell based on BaZr 0.8 M 0.2 O 3−δ .Acceptor doping of a proton conducting oxide is necessary to create oxygen vacancies required for the oxide hydration.To analyze the effect of acceptor doping, Putilov and Tsidilkovski [128] have considered proton hopping over bound and free oxygen sites for two types of potential energy landscape (Q) (Figure 16): (i) ∆Q = 0, which corresponds to a weak effect of acceptor impurities; (ii) ∆Q = ∆E H where ∆E H is the trapping energy of protons, which represents the situation where impurities considerable reduce both the proton bound-state energy and the saddle point energy for transition between neighboring bound sites.Fuel cells generate less electric power at a given current, and, conversely, higher applied electric power is required for electrolyzers to maintain the same current when impurities that provide deeper traps for protons and oxygen vacancies are present.Such an effect of acceptor impurities on the power density (P) of a PCEC is shown in Figure 17a, where the values of the power density calculated for Y-, Sc-, Gd-and In-doped BaZrO3 with the corresponding trapping energies of protons (ΔEH) and vacancies (ΔEV) are given.It can be observed that the reduction in the P for the considered dopants is minimal for Ydoped BaZrO3 (ΔEH = 0.11 eV, ΔEH = 0.31 eV), while for In-doped BaZrO3, the largest power loss (ΔEH = 0.17 eV, ΔEH = 0.52 eV) can be obtained.Figure 17b shows U-j and P-j curves at different trapping energies.

Conclusions and Outlook
Proton-conducting ceramic membranes play a relevant role in energy conversion and storage applications including fuel cells for power generation, electrolyzers for hydrogen production, reversible electrochemical cells for long-term energy storage, electrochemical cells/membrane reactors for natural gas conversion.In particular, among the types of Fuel cells generate less electric power at a given current, and, conversely, higher applied electric power is required for electrolyzers to maintain the same current when impurities that provide deeper traps for protons and oxygen vacancies are present.Such an effect of acceptor impurities on the power density (P) of a PCEC is shown in Figure 17a, where the values of the power density calculated for Y-, Sc-, Gd-and In-doped BaZrO 3 with the corresponding trapping energies of protons (∆E H ) and vacancies (∆E V ) are given.It can be observed that the reduction in the P for the considered dopants is minimal for Y-doped BaZrO 3 (∆E H = 0.11 eV, ∆E H = 0.31 eV), while for In-doped BaZrO 3 , the largest power loss (∆E H = 0.17 eV, ∆E H = 0.52 eV) can be obtained.Figure 17b shows U-j and P-j curves at different trapping energies.Fuel cells generate less electric power at a given current, and, conversely, higher applied electric power is required for electrolyzers to maintain the same current when impurities that provide deeper traps for protons and oxygen vacancies are present.Such an effect of acceptor impurities on the power density (P) of a PCEC is shown in Figure 17a, where the values of the power density calculated for Y-, Sc-, Gd-and In-doped BaZrO3 with the corresponding trapping energies of protons (ΔEH) and vacancies (ΔEV) are given.It can be observed that the reduction in the P for the considered dopants is minimal for Ydoped BaZrO3 (ΔEH = 0.11 eV, ΔEH = 0.31 eV), while for In-doped BaZrO3, the largest power loss (ΔEH = 0.17 eV, ΔEH = 0.52 eV) can be obtained.Figure 17b shows U-j and P-j curves at different trapping energies.

Conclusions and Outlook
Proton-conducting ceramic membranes play a relevant role in energy conversion and storage applications including fuel cells for power generation, electrolyzers for hydrogen production, reversible electrochemical cells for long-term energy storage, electrochemical cells/membrane reactors for natural gas conversion.In particular, among the types of Reproduced from [128] with permission of Elsevier.

Conclusions and Outlook
Proton-conducting ceramic membranes play a relevant role in energy conversion and storage applications including fuel cells for power generation, electrolyzers for hydrogen production, reversible electrochemical cells for long-term energy storage, electrochemical cells/membrane reactors for natural gas conversion.In particular, among the types of reactors for extracting hydrogen from natural gas, electrochemical membrane reactors based on proton ceramic membranes offer a promising opportunity for sustainable industrial growth, possessing unique characteristics such as 100% selectivity gas separation avoiding high pressures at the membrane upper stream, facilitating the use of joule heating instead of thermal heating based on fossil fuels.However, despite their potential in various fields, commercialization of proton conducting ceramic membranes remains a challenge: their performance and stability should be improved and large-scale membrane fabrication strategies are needed.To address such challenges, some key points should be considered: (i).The defect chemistry and transport properties of proton-conducting ceramic membranes and positive electrode materials as well as the relationship of defect chemistry and transport properties of these materials with composition, stoichiometry, microstructure, and operating conditions.(ii).The design of positive electrodes for PCECs and RePCECs to enhance bulk proton conductivity.(iii).The development of appropriate catalysts with specific activities for natural gas reforming and compatibility with other components of protonic ceramic devices.(iv).The engineering of the interface between the catalyst and the negative electrode to tune the electrochemical reactions.

Figure 1 .
Figure 1.Reducing emissions by 2050 through six technological avenues [1].In this context, the decarbonization of heat is a cross-cutting opportunity.Many recent studies have emphasized its crucial role [6-8].Indeed, a large portion of industrial

Figure 3 .
Figure 3. Spectrum of molecular hydrogen on the basis of its production.

Figure 3 .
Figure 3. Spectrum of molecular hydrogen on the basis of its production.

Figure 4 .
Figure 4. Conventional steam reforming reaction scheme.HT shift and LT shift are hightemperature shift reactors, respectively.Reproduced from [23] with permission of Elsevie

Figure 5 .
Figure 5. Converting renewable energies power into heat can decarbonize industrial proc the production of chemicals.

Figure 4 .
Figure 4. Conventional steam reforming reaction scheme.HT shift and LT shift are high-and low-temperature shift reactors, respectively.Reproduced from [23] with permission of Elsevier.

Figure 4 .
Figure 4. Conventional steam reforming reaction scheme.HT shift and LT shift are high-and lowtemperature shift reactors, respectively.Reproduced from [23] with permission of Elsevier.

Figure 5 .
Figure 5. Converting renewable energies power into heat can decarbonize industrial processes for the production of chemicals.

Figure 5 .
Figure 5. Converting renewable energies power into heat can decarbonize industrial processes for the production of chemicals.

Figure 6 .
Figure 6.Current and envisioned coupling of the chemical and energy industry.Reproduced from [6] with permission of Elsevier.

Figure 6 .
Figure 6.Current and envisioned coupling of the chemical and energy industry.Reproduced from [6] with permission of Elsevier.

Figure 7 .
Figure 7. Schematic of transport through a protonic conducting ceramic membrane.

Figure 7 .
Figure 7. Schematic of transport through a protonic conducting ceramic membrane.

Figure 8 .Figure 8 .
Figure 8. Schematic of the various types of proton-conducting ceramic membrane Figure 8. Schematic of the various types of proton-conducting ceramic membranes.

Figure 10 .
Figure 10.Different MRs in relation to membrane role.

Figure 10 .
Figure 10.Different MRs in relation to membrane role.

Figure 12 .
Figure 12.Dependence of the net closed-circuit methane consumption rate on proton flux through the BZCY72 membrane.Reproduced from [74] with permission of Elsevier.

Figure 11 .
Figure 11.SMR concept in a PCMR.Reproduced from [74] with permission of Elsevier.

Figure 11 .
Figure 11.SMR concept in a PCMR.Reproduced from [74] with permission of Elsevier.

Figure 12 .
Figure 12.Dependence of the net closed-circuit methane consumption rate on proton flux the BZCY72 membrane.Reproduced from [74] with permission of Elsevier.

Figure 12 .
Figure 12.Dependence of the net closed-circuit methane consumption rate on proton flux through the BZCY72 membrane.Reproduced from [74] with permission of Elsevier.

Figure 13 .
Figure 13.(a) Schematic of the sequential model and (b,c) heat model for the PCMR system investigated by Malerød-Fjeld et al. [72].

Figure 13 .
Figure 13.(a) Schematic of the sequential model and (b,c) heat model for the PCMR system investigated by Malerød-Fjeld et al. [72].

Figure 14 .
Figure 14.Techno-economic evaluation of PCMR technology investigated by Malerød-Fjeld et al. [72].(a) Schematic representation of techno-economic evaluation of hydrogen production technologies; (b) summary of inputs and assumptions for the evaluation; (c) schematic representation of well-to-wheel calculation; (d) breakdown of well-to-wheel analysis in terms of efficiencies and GHG emissions for battery electric vehicle (BEV) and internal combustion engine (ICE); (e) GHG emissions of the expended energy for the production of hydrogen in centralised plants using grid electricity, renewable electricity (RE) or RE with carbon capture (RE + CC) for the SMR and PCMR plants; (f) electricity to natural gas price ratio in different regions and markets.

Figure 14 .
Figure 14.Techno-economic evaluation of PCMR technology investigated by Malerød-Fjeld et al. [72].(a) Schematic representation of techno-economic evaluation of hydrogen production technologies; (b) summary of inputs and assumptions for the evaluation; (c) schematic representation of wellto-wheel calculation; (d) breakdown of well-to-wheel analysis in terms of efficiencies and GHG emissions for battery electric vehicle (BEV) and internal combustion engine (ICE); (e) GHG emissions of the expended energy for the production of hydrogen in centralised plants using grid electricity, renewable electricity (RE) or RE with carbon capture (RE + CC) for the SMR and PCMR plants; (f) electricity to natural gas price ratio in different regions and markets.

Figure 15 .
Figure 15.Schematic image of a RePCEC.Reproduced from [129] with permission of Else

Figure 15 .
Figure 15.Schematic image of a RePCEC.Reproduced from[128] with permission of Elsevier.

Figure 17 .
Figure 17.Characteristics of the PCEC operating under humidified H2 (2% H2O) and humidified air (50% H2O) supplied, respectively, to the fuel and air electrodes (T = 600 °C).(a) Normalized power density P of the fuel cell as a function of the proton trapping energy ΔEH and the ratio ΔEV/ΔEH (U = 0.7 V).(b) Cell voltage (left axis) and power density (right axis) versus current density dependences calculated at different trapping energies ΔEH and ΔEV for the PCEC operating in the fuel and electrolysiscell modes (ΔQ = ΔEH, ΔEV/ΔEH = 2).Reproduced from [129] with permission of Elsevier.

Figure 16 .
Figure 16.Proton transfer in an acceptor-doped perovskite AB 1-x R x O 3-δ .(a) Schematic representation of the potential energy landscape for proton hopping; (b) Proton mobility u H normalized to µ 0 H as a function of the proton trapping energy ∆E H /kT and dopant content c R calculated for ∆Q = 0 (lower surface) and ∆Q = ∆E H (upper surface).µ 0H is the proton mobility at ∆E H = ∆E V = ∆Q = 0. Reproduced from[128] with permission of Elsevier.

Figure 17 .
Figure 17.Characteristics of the PCEC operating under humidified H2 (2% H2O) and humidified air (50% H2O) supplied, respectively, to the fuel and air electrodes (T = 600 °C).(a) Normalized power density P of the fuel cell as a function of the proton trapping energy ΔEH and the ratio ΔEV/ΔEH (U = 0.7 V).(b) Cell voltage (left axis) and power density (right axis) versus current density dependences calculated at different trapping energies ΔEH and ΔEV for the PCEC operating in the fuel and electrolysiscell modes (ΔQ = ΔEH, ΔEV/ΔEH = 2).Reproduced from [129] with permission of Elsevier.

Figure 17 .
Figure 17.Characteristics of the PCEC operating under humidified H 2 (2% H 2 O) and humidified air (50% H 2 O) supplied, respectively, to the fuel and air electrodes (T = 600 • C).(a) Normalized power density P of the fuel cell as a function of the proton trapping energy ∆E H and the ratio ∆E V /∆E H (U = 0.7 V).(b) Cell voltage (left axis) and power density (right axis) versus current density dependences calculated at different trapping energies ∆E H and ∆E V for the PCEC operating in the fuel and electrolysiscell modes (∆Q = ∆E H , ∆E V /∆E H = 2).Reproduced from [128] with permission of Elsevier.

Table 1 .
Selected studies on strategies of electrification of endothermic processes to produce H 2 .

Table 1 .
Selected studies on strategies of electrification of endothermic processes to produce H2.

Table 4 .
Selected case studies of PCMRs.

Table 4 .
Selected case studies of PCMRs.