Prospects and Challenges of Proton Conducting Cerates in Electrochemical Hydrogen Devices for Clean Energy Systems: A Review

2.1 Proton-Conducting Cerates

Cerate-based perovskite proton conductors, such as BaCeO₃ and SrCeO₃, exhibited high proton conductivity due to their crystal structure, high lattice volume and distortion, and low electronegativity. Different strategies, such as doping with different materials, were developed to improve the performance and properties of cerate-based proton conductors. Among all cerate-based proton conductors, BaCeO₃ and SrCeO₃ were widely studied due to their good proton conductivity and stability in the harsh atmosphere (water vapor, CO₂) at intermediate temperatures.^[67] BaCeO₃ showed the highest proton conductivity compared to the other cerate proton conductors due to their largest ionic radius cation, large lattice parameter, low grain-boundary resistance, and low electronegativity.^{[89, 90]} Figure 1 represents calculated proton conductivities of different oxides using the information on proton concentrations and mobilities. Compared to the other perovskite cerate materials, BaZrO₃ and BaCeO₃ materials are the best proton conductor materials (Figure 1).^[51] However, BaCeO₃ has low chemical stability that prevents it from being used as an electrolyte in the application of electrochemical devices.

Recent research has shown that the proton conductivity of BaCeO₃ can be enhanced by doping with trivalent rare earth material and zirconate materials. Hossain et al. exposed deuterium gas (D₂) and heavy water vapor (D₂O) at 873 or 973 K for 5 h to study the dissolution and release behavior of hydrogen in Y-doped BaCeO₃ (BaCe_0.9Y_0.1O₃) proton conductor, shown in Figure 2.^[91] In Figure 2c–f, they investigated how much hydrogen isotopes (deuterium) dissolve in a material at different gas pressures. It was found that the amount of absorbed hydrogen increased with pressure, but not as strongly as predicted by Sieverts' Law (which predicts a square root dependence). This suggests that the pressure affects the number of sites available for hydrogen storage within the material. Furthermore, the type of gas released upon heating the material depended on the exposure pressure. At lower pressures, more HD gas (containing one hydrogen and one deuterium atom) was released, while higher pressures favored D₂ gas (pure deuterium). This indicates that higher pressure promotes the exchange of lighter hydrogen atoms in the material with the incoming deuterium gas. Gu et al. showed that BaCeO₃ doped with rare earth elements such as samarium (Sm) and Gadolinium (Gd) showed higher proton conductivity compared to the other proton conductors.^[92] However, SrCeO₃ proton conductors exhibited lower proton conductivity than BaCeO₃ in reducing environments at high temperatures.^[67] For example, Iwahara et al. reported that at a temperature (800 °C), the proton conductivity of SrCeO₃ was 10⁻³–10⁻² S cm⁻¹. However, SrCeO₃ has higher chemical stability than BaCeO₃. Some studies were conducted to improve the chemical stability of BaCeO₃, where BaCeO₃ was doped with zirconium (Zr), Niobium (Nb), Tin (Sn), and Titanium (Ti).^{[67, 93, 94]} This type of reported study showed that this approach reduced the proton conductivity of BaCeO₃. Therefore, finding a suitable path to improve the ionic conductivity and stability of cerate-based proton conductors is the research center.

Table 1 represents the performance of various electrolyte materials with their conductivity, chemical stability, operating temperature, sintering temperature, and synthesis methods.^[124] The performance includes the open-circuit voltage (V) and peak power density (mW cm⁻²) of the various cerate-based materials. Table 1 demonstrates that various electrolyte materials provide better conductivity and peak power density based on their synthesis method, operating temperature, and sintering temperature. It is noted that BaZr_0.8X_0.2O_3-δ provides a peak power density of up to 340 mW cm⁻² under 650 °C operating temperature, and SrCe_0.8Y_0.2O_3-δ provides conductivity up to 12 × 10⁻³ S cm⁻¹ under 1500 °C sintering temperature. Table 2 represents the performance of different hydrogen devices using various cerate-based electrolytes, summarizing the conductivity, peak power density, and key findings of different materials under various operating temperatures and H₂ Environmental Conditions. It is noted that fuel cells using various cerate-based electrolyte materials provide outstanding performance, such as better conductivity and peak power density under different operating temperatures.

2.2 Cerate Material-Based Different Types of Electrochemical Hydrogen Devices

Cerate material-based electrochemical hydrogen devices can be classified as hydrogen pumps, hydrogen isotope separation systems, tritium recovery systems, and hydrogen sensors, which are briefly defined below.

Hydrogen pump: Hydrogen pumps based on cerate materials are electrochemical devices that use proton-conducting ceramics to transport hydrogen through solid-state membranes at high temperatures. Hydrogen splits into protons and electrons at the anode when an electric potential is applied across the ceramic membrane in these devices. Protons migrate through the ceramic electrolyte, and electrons travel through an external circuit, finally joining again to generate hydrogen gas at the cathode.^[125] These pumps' efficiency is greatly impacted by the properties of the ceramic materials that are used. For instance, doping SrCeO₃ with ytterbium improves its stability and proton conductivity, which makes it more suitable for high-temperature applications. It has been demonstrated that these modifications enhance the performance of hydrogen pumps, especially in situations where effective hydrogen transport is essential, such as tritium recovery systems in fusion reactors.^[74]

Hydrogen isotope separation systems: Hydrogen isotope separation systems are a form of electrochemical hydrogen device that uses ceramic membranes that conduct protons, usually perovskite-type oxides like BaZrO₃ or BaCeO₃, to transport hydrogen isotopes selectively when an electric field is applied. These devices separate hydrogen isotopes (tritium, deuterium, and protium) through the exploitation of the differential mobility inside the ceramic lattice.^[126] In such systems, hydrogen ions (protons) are driven through the material by applying a voltage across the ceramic membrane. They can separate because lighter isotopes, like protium, migrate more easily than heavier ones, like deuterium or tritium, due to variations in mass and contact with the lattice. Compared to conventional separation methods, this approach has the potential for continuous operation and uses less energy.^[127]

Tritium recovery systems: Tritium recovery systems are electrochemical hydrogen devices that use proton-conducting ceramics, especially doped zirconates like CaZr₀.₉In₀.₁O₃₋α and SrZr₀.₉Yb₀.₁O₃₋α, to extract tritium and other hydrogen isotopes from gas mixtures or water vapor in fusion reactor environments. These systems operate by applying an electric voltage over a ceramic membrane, which forces protons across it and makes it possible to separate and recover hydrogen isotopes.^[56] A key component of these systems is the electrochemical hydrogen pump, which utilizes the strong proton conductivity and chemical stability of perovskite-type ceramics at high temperatures (usually between 873 and 1073 K). For instance, studies have demonstrated that using CaZr_₀.₉In_₀.₁O_₃₋α as the proton-conducting membrane enables effective hydrogen extraction from helium-hydrogen mixtures, with hydrogen recovery efficiency exceeding 99% in certain conditions.^[128]

Hydrogen sensors: Hydrogen sensors refer to sensors that use cerate-based materials as the primary sensing component to detect hydrogen gas. Based on electrochemical principles, these sensors measure the concentration of hydrogen by producing a quantifiable electrical signal from the interaction between the hydrogen gas and the cerate material. The hydrogen gas usually reacts with the material during the electrochemical process, changing its conductivity or potential, which can be measured with electrodes.^[129] Hydrogen sensors based on cerate operate over a broad temperature range, usually between room temperature and 500 °C. They may be used in both ambient and high-temperature industrial settings because of their broad operating temperature range.^[130] Cerate-based hydrogen sensors tend to exhibit long-term stability due to the robust nature of cerate materials, which resist degradation over time. This makes them suitable for continuous monitoring in critical applications.^[131]

3.1 Mechanism of Hydrogen Pumps

Numerous studies have been conducted to develop a technology for producing pure hydrogen, including pressure swing adsorption, cryogenic distillation, and nongalvanic hydrogen separation pumps.^[139-141] An EHP can be constructed by using proton conductors. The principal operation and construction of the EHP are represented in Figure 3.^[142] The system consists of two electrodes (anode and cathode) separated by a proton-conducting electrolyte membrane. When the direct current is applied to the proton-conducting electrolyte, hydrogen is ionized at the anode to produce protons in the electrolyte.^[71] This reaction produces protons (H⁺) and electrons (e⁻). A proton moves toward the cathode through the electrolyte layer, which converts it to hydrogen gas.^[143] This method uses separated hydrogen in a controlled way by the applied current in the electrolyte.

Hydrogen extraction and production have been achieved utilizing an EHP based on the proton conductor, for example, SrCe_0.95Yb_0.05O_3-α, BaCe_0.9Nd_0.10O_3−α, CaZr_0.96In_0.04O₃, and BaZr_0⋅1Ce_0⋅7Y_0⋅2O_3-δ.^{[140, 144]} The protonic conductor BaZr_0⋅1Ce_0⋅7Y_0⋅2O_3-δ has excellent chemical stability at high humidity and high cerate protonic conductivities.^[140] Cerium is easily reduced to its +3 and +4 oxidation states by redox processes. This redox chemistry is especially important for processes involving hydrogen since materials containing cerium can operate as redox catalysts or redox-active species. The stability and conductivity of cerate-based PCOs at high temperatures are beneficial. This makes it possible for hydrogen pumps to function at higher temperatures when traditional proton exchange membrane (PEM) technologies might not be as effective. The ceramics proton conductor SrCe_0.95Yb_0.05O_3-α, BaCe_0.9Nd_0.10O_3−α, and CaZr_0.96In_0.04O₃ are used to enhance the ion conductibility, sinterability, and material's chemical stability.^{[145, 146]} The ceramic composition becomes the purest protonic conductor at high temperatures for exposing hydrogen gas. Alternatively, at high temperatures, cerium oxide can be reduced by hydrogen gas to generate cerium hydride (CeH₂) or lower oxidation state cerium species. Because of the reversibility of this reduction process, cerium oxide can be renewed by reoxidation in environments with oxygen. However, a hydrogen pumping system can be manipulated by temperature, hydrogen pressure, and water mixture.^[140]

Cerate-based electrochemical hydrogen pumps have shown promising performances in applications involving the separation and purification of hydrogen, especially those that use doped barium cerate (BaCeO₃) materials. For instance, BaCe₀.₂Zr₀.₇Y₀.₁O₃_−δ (BCZY271)-based proton-conducting solid oxide cells (P-SOCs) have been developed and are capable of working efficiently at intermediate temperatures (350–450 °C) with current densities ranging from 150 to 525 mA cm⁻² at 1 V. These P-SOCs have benefits in thermodynamic efficiency and material stability, which makes them suitable for a variety of hydrogen-related applications.^{[147, 148]} Proton conductor electrolytes based on yttrium-doped barium cerate, as BaCe₁–_xY_xO₃_−δ (where x = 0.15), have also demonstrated improved proton conductivity in humidified conditions. According to studies, these materials' conductivity is greatly increased in moist air conditions, highlighting the role that water vapor plays in promoting proton conduction.^[149] Although their performance is affected by factors including operating temperature, humidity, and material composition, cerate-based materials usually demonstrate excellent electrochemical properties and high proton conductivity.^[150]

3.2 Mechanism of Hydrogen Isotope Separation Systems

In cerate-based electrochemical hydrogen devices, hydrogen isotope separation primarily relies on the difference in ion mobility and diffusion rates arising from the mass disparity between isotopes. This mechanism enables the selective transport of protons (H⁺) and deuterons (D⁺) across a proton-conducting ceramic membrane, typically composed of doped barium cerate.^[71] These materials demonstrate high proton conductivity at high temperatures (usually 500–800 °C).^[151] When a voltage is applied across the electrochemical cell, hydrogen isotopes from a gas mixture (e.g., H₂ and D₂ or H₂O and D₂O) are ionized and transported through the ceramic membrane. Protons, having lower mass, migrate faster than deuterons, allowing partial isotope separation.^[152] Devices based on cerates leverage the isotope effect in ionic conductivity to distinguish between isotopes. The effectiveness improves under carefully controlled temperature, pressure, and gas composition.^[153] The difference in partial pressure between the retentate and permeate sides of the membrane drives the selective diffusion of hydrogen isotopes.^[154] The proton-conducting perovskite membrane selectively allows H⁺ transport while blocking other species, enabling effective isotope separation and ultra-pure hydrogen production. The resultant hydrogen can achieve remarkable purity levels (99.999999999%).^[155] Here, Figure 4 illustrates the D–T (Deuterium–Tritium) purification system using proton-conducting oxide materials as isotope separators.^[143] A DC source drives the electrochemical separation process, where a gas mixture containing DTO (deuterium oxide), CT₄ (tritiated methane), CD₄ (deuterated methane), and He enters the system. Inside the proton-conducting membrane, protons (T⁺ and D⁺) selectively migrate toward the cathode side due to the applied potential, based on their mass-dependent mobility. On the cathode side, DT (deuterium-tritium gas), D₂, and T₂ are formed and collected. Electrons flow through the external circuit while protons are transported through the membrane, achieving isotope separation. This mechanism efficiently separates hydrogen isotopes for fusion fuel and other applications.

SrCeO₃ is preferred over BaCeO₃ for membrane materials due to its stability. Though SrCeO₂-based membranes have lower hydrogen permeability, it can be enhanced through high-temperature sintering (≈1400 °C).^[156] The low electronic conductivity of proton conductors can be offset by incorporating metallic or electronically conductive oxides.^[157] These membranes maintain chemical stability under hydrogen isotope exposure.^[158] Cermet membranes that combine ceramics with metals like Ni offer both proton conductivity and fast electron conduction, enhancing membrane performance.^[78] Cerate-based proton conductors, such as BaCe₀.₉Yb₀.₁O₂.₉₅ and SrCe₀.₉₅Yb₀.₀₅O₂.₉₇₅, are promising for hydrogen isotope separation due to their high proton conductivity and stability in water and CO₂ atmospheres.^{[159, 160]} A 2025 study demonstrated the use of cerate materials in a proton-conducting fuel cell (PCFC) to achieve a separation factor of ≈3.8 for hydrogen and deuterium at 600 °C and 0.7 V, indicating strong potential for nuclear energy applications.^[126]

3.3 Mechanism of Tritium Recovery Systems

The research on fusion reactors that use nuclear fusion of heavy hydrogen with tritium is ongoing to create a clean energy source in the future. In a fusion reaction, the exhaust gas processing system must attain a high tritium decontamination factor, for example, using hydrogen recovery devices. Regarding the safety and security of a nuclear fusion reactor, a hydrogen isotope, tritium, loss from the exhaust gas processing system is a significant concern.^[91] Therefore, Perovskite PCOs are a promising candidate material in nuclear fusion reactors’ different electrochemical devices like the hydrogen sensor, tritium recovery system, hydrogen isotope separation, and hydrogen pump due to their excellent thermal and chemical stability and high H⁺ conductivity at high temperatures.^{[91, 143]} A conceptual diagram of the tritium recovery system is presented in Figure 5.^[91] Figure 5 shows a tritium recovery system using proton-conductive oxides.^[91] A gas mixture containing hydrogen isotopes (H, D, T) and other species is introduced into the chamber. Under applied voltage, protons (including tritons) migrate through the proton-conducting oxide membrane. The recovered tritium exits as purified gas, while other gases like He, CO₂, and O₂ are removed.

The tritium recovery system that uses the same principles as a hydrogen pump can separate the hydrogen isotope from the plasma exhaust gas, including contaminants such as DTO, He, CD₄, and CT₄.^[91] When DC power is delivered between the electrodes of this tritium recovery system (Figure 5), purified deuterium (D), tritium (T), DT, D₂, and T₂ are produced.^[161] The production of these elements can be recycled as fuel in the fuel circulation system of a nuclear fusion reactor.^[71] Cerate-based perovskite proton conductors are widely used in tritium purification and recycling due to their excellent mechanical and electronic properties. Compared to the other cerate-based proton conductor, doped strontium cerates with ytterbium like SrCe_0.95Yb_0.05O_3-α are used as an electrochemical cell for the tritium recovery system application due to their high tritium conductivity with good chemical stability at high temperature.^[162] A high temperature is expected for tritium extraction in the tritium recovery system of a fusion reactor.

Cerate-based electrochemical hydrogen devices have demonstrated promising results in tritium recovery systems. Perovskite-type ceramics, such as CaZr₀.₉In₀.₁O₃_−α and SrZr₀.₉Yb₀.₋O₃_−α, is suitable for tritium extraction in fusion reactors because they have strong proton conductivity and good chemical stability at high temperatures.^[56] In CaZr₀.₉In₀.₁O₃_−α, one-end closed tubes have been used in experiments to recover hydrogen with efficiency above 99%. Almost 100% current efficiency was also demonstrated in these tests, which were carried out at 1023 K and 1.15 V, demonstrating the material's ability to separate hydrogen isotopes from gas mixtures that are rich in helium.^[128] The choice of electrode material greatly affects performance. For example, plated platinum electrodes work best with gas mixtures containing hydrogen, water vapor, and methane, while porous pasted platinum electrodes are more effective for water vapor electrolysis, as shown in studies under different conditions.^[163] The design of electrochemical cells is important for improving tritium recovery. Larger electrode areas allow for higher current densities, which lead to better extraction efficiency.^{[56, 164]}

3.4 Mechanism of Hydrogen Sensors

Hydrogen has immense combustion heat (142 KJ g⁻¹ H₂⁻¹), minimum flammable energy (0.017 mJ), a broad range of flames (4–75%), high ignition temperature (560 °C),  and maximum flaming velocity. Humans cannot identify hydrogen gas since it has no taste, color, or odor. Due to its low ignition energy and broad flammable range, hydrogen is easily flammable and explosive. As a result, developing a reliable and accurate hydrogen sensor is necessary to prevent damage and explosions in several applications. A hydrogen sensor is a device that can detect hydrogen at concentrations close to 100% and at parts per million (ppm) levels that are close to the lower explosive limit (4% H₂ in the atmosphere). Hydrogen sensors are available in electrochemical devices, semiconductors, thermoelectrics, etc. A schematic diagram of the hydrogen sensor is demonstrated in Figure 6.^[165] This figure illustrates a hydrogen sensor setup based on a proton-conducting ceramic electrolyte disk. It features two compartments: Compartment I with H₂ reference gas and Compartment II with the H₂ working gas. Pt wires and electrodes are used to establish electrical contact and facilitate proton conduction across the ceramic disk. A thermocouple is included to monitor the operating temperature, which is crucial for proton conductivity. Al₂O₃ cement and tubes are used to support and seal the structure, ensuring gas-tight operation and stability. The development of highly proton-conductive ceramics is essential for efficient hydrogen sensors. Cerate-based oxides like BaCe₀.₇Zr₀.₁Y₀.₂O₃−δ, BaCe₀.₈Gd₀.₂O₃ (BCG), and BaZr₀.₄Ce₀.₄In₀.₂O₃ (BZCI) exhibit high proton conductivity and thermal stability, making them suitable electrolytes for potentiometric and amperometric hydrogen sensors.^[166-168] Materials such as SrCeO₃ and CaZrO₃ also perform well at high temperatures in hydrogen-rich environments.^[169] These oxides quickly respond to H₂ by proton generation and conduction, enabling real-time, sensitive detection of hydrogen. Their compactness and affordability allow usage in portable sensors for applications like refueling stations, hydrogen production, and automotive monitoring.

Hydrogen sensors utilizing cerate-based materials have demonstrated significant advancements in electrochemical hydrogen detection. For instance, a study used polyaniline/cerium oxide (PANI/CeO₂) nanocomposites to develop an effective electrochemical sensor. These nanocomposites demonstrated outstanding electrocatalytic oxidation properties toward hydrogen peroxide, indicating that these materials could be used in hydrogen sensing applications.^[170] Another study examined glassy carbon electrodes modified with nano-structured CeO₂. The increased surface area and oxygen vacancies in the nanostructured material have been demonstrated to be responsible for the improved electrochemical sensing behavior for hydrogen peroxide reduction.^[171] Furthermore, a cerium oxide nanocube-based nonenzymatic electrochemical sensor demonstrated rapid and precise detection of hydrogen peroxide residues in food samples, emphasizing the adaptability of cerate materials in sensor applications.^[172]

4.1 Performance Analysis of Cerate-Based Hydrogen Pumps

Doped BaCeO₃ materials have been investigated as solid electrolytes in hydrogen pump applications because of their strong proton conductivity and chemical stability in harsh environments. Patki et al. investigated a BaZr_0.8Ce_0.1Y_0.1O_3-δ-based membrane for galvanic hydrogen pumping in the methane dehydroaromatization reaction for in-situ separation of product hydrogen from the system.^[82] They used a BaZr_0.8Ce_0.1Y_0.1O_3-δ -cermet as a cathode, and electroless plated Cu onto perovskite materials to serve as an anode electrode on one side. The performance of such a system under a gas mixture containing methane, ethane, CO, and H₂ demonstrates that the proposed system is capable of pumping H₂ at 40 mA cm² through Cu─Cu electrodes using 268 mW/(NmL H₂ min⁻¹); and the faradaic efficiency decreases with increasing cerium content in the solid membrane structure, possibly due to cerium migration from cermet structure. Recently, Mushtaq et al. developed a reliable and efficient hydrogen pump with BaZr_0.7Ce_0.2Y_0.1O_3-δ (BZCY271) electrolyte by using a vacuum-assisted dip coating approach on a porous Ni-BZCY support.^[147] Another hydrogen pump demonstrated by Tong et al. using porous BaZr_0.1Ce_0.7Y_0.2O_3-δcermet instead of a metal electrode significantly enhanced the H₂ flux (15.6 mL cm⁻² min⁻¹) at 500 ⁰C and made them suitable for low-temperature (350–500 ⁰C) hydrogen pumping applications.^[140] Figure 7a,c displays Nyquist plots and Figure 7b,d displays the distribution of Relaxation Time (DRT) of impedance data obtained under the gradient of 10% H₂/N₂||100% H₂ for the ceramic hydrogen pump with wet in both sweep and feed gases, and dry in the feed side. The dynamics of hydrogen adsorption and desorption are better understood in the context of a hydrogen pump by DRT spectra. For understanding the surface reactions, charge transfer, and diffusion processes related to hydrogen pumping by examining the distribution of relaxation times.^[173] Impedance measurements here revealed that under these conditions, the resistance at the electrode/electrolyte interface increases significantly at high current densities. This suggests a decrease in the concentration of a key component (OH: O) at this interface. Additionally, for cells with a limited hydrogen supply, the overall resistance jumps at a critical current density, potentially due to a switch in the reaction mechanism at the anode. These findings suggest that a sufficient supply of hydrogen is crucial for maintaining efficient fuel cell operation. Figure 7e shows the measurement of the short-term stability of the ceramic hydrogen pump as it extracts pure H₂ at 500 °C from a mixture of 50% H₂/N₂. A cross-section of the SEM micrograph for the ceramic hydrogen pump is shown in Figure 7d.^[140]

Complex oxide conducting materials having dominant ion transportation characteristics compared to electronic transportation have seen prospective applications as electrolyte materials while designing an EHP.^[174] The infiltration technique has opted to design an efficient EHP possessing the general structure Ba(Zr_0⋅85Y_0.15)O_3-δ-(La_0⋅7Sr_0⋅3)V_0.9O_3-δ@CeO₂-Pd|Ba(Zr_0.30Ce_0.54Y_0.15Cu_0.01)O_3-δ|Ba(Zr_0⋅85Y_0.15)O_3-δ-(La_0⋅7Sr_0⋅3)V_0.9O_3-δ@CeO₂-Pd.^[145] Here, the Ba (Zr_0.30Ce_0.54Y_0.15Cu_0.01)O_3-δ ceramic acts as the electrolyte material, whereas the Ba(Zr_0⋅85Y_0.15)O_3-δ and (La_0⋅7Sr_0⋅3)V_0.9O_3-δ ceramics perform as the protonic and electron conducting composite electrodes, respectively, with CeO₂ and Pd working as the catalysts. The hydrogen pump exhibited a significant 2 Acm⁻² current density, having a meager 1.1 V overpotential owing to the presence of the composite ceramic electrodes. The electron-conducting anode (La_0⋅7Sr_0⋅3)V_0.9O_3-δ opted in this hydrogen pump is suitable for operation under oxygen partial pressure <10⁻¹⁷ and has been utilized in galvanic hydrogen pumps and PCFC as well.^[175] Another PCO, SrCe_0.95Yb_0.05O_3-α has been utilized for hydrogen extraction through electrochemical pumps.^[176] A direct current having approximately unity efficiency in this system has been observed to pump pure hydrogen entirely from the anode to the cathode side from a synthesis gas mixture of H₂ and CO at a high temperature of 800 °C. The water vapor electrolysis increased the current density and decreased the ionic transportation number due to partial n-type conductivity.^[145] By comparing the performances of SrCe_0.95Yb_0.05O_3-α with SrZr_0.95Yb_0.05O_3-α, SrCe_0.95Yb_0.05O_3-α demonstrated superior electrochemical performances in terms of proton conduction in hydrogen pumps.^[83] Matsumoto et al. also observed that proton conductivity through SrCe_0.95Yb_0.05O_3-α electrolyte was enhanced due to humidity in the feed gas at the cathode within a temperature range of 700–900 ⁰C.^[88]

Kawamura et al. proposed a novel technique to enhance the transportation of hydrogen by using a triple-phase boundary (region among gas phase, electrode, and electrolyte) system between electrolyte and electrodes. The sputtering technique was implemented to fabricate Pt and Pd electrode layers and subsequent attachment to the disk-shaped ceramics (SrCe_0.95Yb_0.05O_3-α) as electrolytes. In comparison between the two electrode materials, due to the high hydrogen permeability, the current density in the nonporous Pd electrode is much higher (4–5 times) than in the traditional Pt electrode. In addition, using a sputtering electrode compared to a paste electrode was reported to be beneficial for enhancing the current density, resulting in hydrogen transportation capability. A comparison of an electrode, gas flow rate, applied voltage, and temperature between Pt and porous Pt type electrode with SrCe_0.95Yb_0.05O_3-α electrolyte are presented in Table 3.^{[162, 177]} It is observed that H₂, H₂/He, and helium flow rates are 0.4 L min⁻¹ with 0 to 2.0 V at 873 K when the Pt electrode is used, whereas H₂–D₂/He, HT–H₂/He flow rates are 0.2 and 0.4 0.4 L min⁻¹, with 0 to 1000 mV at 873 K when porous Pt electrode is used in the hydrogen pump (Table 3).^[177] Matsumoto et al. have investigated the electrochemical hydrogen pumping efficiency of pure hydrogen gas for SrCe_0.95Yb_0.05O_3-α as a proton-conducting solid electrolyte at high temperature (900 °C).^[178] The current efficiency decreased at high current density due to the critical current density reducing efficiency, and decreased with the rising amount of water vapor in the anode hydrogen gas. The hydrogen inadequacy and the increase of oxygen partial pressure at the anode in sending high current typically account for the partial hole conduction in the electrolyte and the reduction of the current efficiency of the hydrogen pump.^[179]

4.2 Performance Analysis of Cerate-Based Hydrogen Isotope Separation Systems

The effective way of promoting hydrogen permeation flux is by reducing the membrane thickness and the sintering process of the membrane. Xiaoyao Tan et al. have fabricated a gas-tight hollow fiber membrane consisting of BaCe_0.95Tb_0.05O_3-α (BCTb) perovskite oxides by the combined use of phase inversion and steering technique at elevated temperature.^[180] Based on the final crystal morphology and porosity investigated by SEM images, the sintering temperature range of 1300–1500 °C is ideal for fabricating a BCTb-based gas-tight membrane. The hydrogen permeability increased steadily with the measurement temperature (700–1000 °C) and sweep gas (N₂) flow rates. However, Transition metal cations such as cobalt, having variable valance properties, have been introduced into the ceramic membrane to achieve higher electronic conductivity and improve the hydrogen permeation flux.^[181] Through a combination of phase inversion and sintering methods, cobalt-doped BaCe_0.85Tb_0.05Co_0.1O_3-δ hollow membranes were synthesized. Since the sintering properties of perovskite ceramic and transition metal phases differ significantly, synthesizing hollow tubular fibers through this technique is complex and requires additional attention.^[182] The cobalt-doped BaCe_0.85Tb_0.05Co_0.1O_3-δ exhibited higher electrical conductivity than the BaCe_0.95Tb_0.05O_3-δ oxide, which may be attributed to the rise in oxygen ionic as well as electronic conductivity. The maximal hydrogen permeation flux through BaCe_0.85Tb_0.05Co_0.1O_3-δ hollow fiber membranes can reach 0.385 mL cm⁻² min⁻¹ at 1000 °C when the flow rates the 50% H₂-He/N₂ are 60 and 100 mL min⁻¹, respectively (Table 4).^[181] The development of the BaCe_0.95Tb_0.05O_3-δ hollow fibers' microstructure with the sintering temperature is shown in Figure 8.^[181] It is observed that the general structure of ceramic hollow fibers does not change significantly at a high sintering temperature (Figure 8). The inner surface micrographs, however, clearly show that the sintering temperature considerably impacted the grain growth in the membrane.

The Ni-BZCY (Ni–BaZr_0.1Ce_0.7Y_0.2O_3−δ) exhibited excellent stability and performance in the presence of H₂O and CO₂ at high temperatures (900 °C), whereas unstable in the case of acidic H₂S. Fank et al. reported the chemical stability of Ni-BZCY and the hydrogen permeation flux in the presence of H₂S concentrations of 30–300 ppm at 900 °C to overcome the instability mentioned above.^[184] The steady hydrogen permeation flux (HPF) of Ni-BZCY cermet (ceramic-metal composite) was found in the presence of 30 ppm H₂S; however, it started to decrease in the presence of 60 ppm H₂S. Liu et al. have fabricated a thin Ni-BZCCYb (Ni incorporated on BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3-δ) membrane on porous Ni-BZCYYb substrate support via a co-firing process.^[78] The hydrogen separation was at temperature ranges from 700 to 900 °C, and hydrogen permeation fluxes under different flux conditions of 1.12 and 0.49 mL min⁻¹ cm⁻² at 900 and 700 °C, respectively. Figure 9a,b shows a cross-sectional view of a dense Ni-BZCYYb membrane on a porous Ni-BZCYYb substrate.^[78] Figure 9c presents dense Ni-BZCYYb membranes, and Figure 9d shows a backscattered electron image of a dense Ni-BZCYYb membrane. The permeation flux with temperature (Figure 9e) and the permeation flux with H₂ partial pressure (Figure 9f) are presented. It is noticed in Figure 9a–d that the metal phase in the BZCYYb matrix is randomly dispersed, and the membrane is dense. The nickel phase in this cermet membrane significantly increases the cermet's H₂ permeability through enhancement of the cermet's electronic conductivity, surface exchange kinetics, and mechanical stability. Figure 9e presents that the H₂ permeation rate is higher than the cermets' previously published H₂ permeation rate.^[78]

Tan et al. synthesized BaCe_0.8Y_0.2O_3-δ hollow membranes with the addition of 1 wt% sintering aid (Co₂O₃) by following phase inversion and sintering techniques.^[183] BaCe_0.8Y_0.2O_3−δ is a composite material belonging to the rare-earth perovskite family. It's made by partially substituting cerium (Ce) with yttrium (Y) in barium cerate (BaCeO₃). The -δ notation signifies oxygen vacancies, which can be adjusted to influence their properties. The hollow membranes are specifically designed with a hollow fiber structure. Imagine a long, thin tube with a hollow center. This design increases the surface area for gas permeation, making them efficient for gas separation processes. The best mechanical strength of the BaCe_0.8Y_0.2O_3−δ hollow fiber membrane was found at the ideal sintering temperature of 1400 °C. In addition, the maximum H₂ permeation flow through the BaCe_0.8Y_0.2O_3−δ hollow fibers reached up to 0.38 mL cm⁻² min⁻¹ at 1050 °C (Table 4), which is noticeably in contrast to the lower value of less than 0.01 mL cm⁻² min⁻¹ of BaCe_0.8Y_0.2O_3−δ disk-shaped membrane.^[183] To meet the requirement of high electronic conductivity, Wang et al. have proposed metal oxide (Zinc Oxide- ZnO) mixed with the protonic conductor phase of SrCe_0.95Y_0.05O₃ to form a dual phase membrane for permeation of hydrogen in high-temperature.^[185] The following reason is mentioned for choosing ZnO: i) Zinc oxide is expected to lower the sintering temperature rather than the expense of proton conductivity, and ii) As a dopant, to improve electronic conductivity of oxygen ionic conductor, the ZnO is used over a wide range of temperature and partial pressures of oxygen,^[186] iii) The thermal expansion coefficient of ZnO is almost identical in comparison with SrCeO₃ that reduced the formation of crack, and it is low cost, nontoxic and stable material in chemically with availability in abundance at nature, and finally; (iv) ZnO increases the sintering ability of SCY perovskites which reduced the sintering temperature. A variation of the ZnO amount in the composite was carried out. A composite having 90% SCY-10% ZnO was found to be optimum, as it shows good stability after 48 h at 900 °C, and the peak flux of hydrogen permeation is found to be 0.039 mL (STP) cm⁻² min⁻¹.

Kawamura et al. have studied the EHP where SrCe₀.₉₅Yb_0.05O_3-α proton conductor is used as a membrane within a blanket breeder system to extract the isotopes of hydrogen, including tritium, from water decomposition.^[177] The EHP has the advantage of selectively extracting hydrogen isotopes from sweep gas due to its electrostatic potential distinction driving force.^[73] It is reported that a balanced H₂-D₂/He or HT-H₂/He sample gas mixture was supplied to EHP at an operating condition of 600 °C with a varied applied voltage between 0 and 1000 mV. Mukundan and his coworker investigated BaCe_0.9Yb_0.1O_2.95, SrZr_0.9Yb_0.1O_2.95, and SrCe_0.95Yb_0.05O_2.975 PCOs by using ac impedance analysis.^{[160, 187]} The BaCe_0.9Yb_0.1O_2.95 proton conductor exhibited the highest proton occupancy compared to the SrZr_0.9Yb_0.1O_2.95 and SrCe_0.95Yb_0.05O_2.975 proton conductors. The SrCe_0.95Yb_0.05O_2.975 proton conductor exhibited the lowest protonic conductivity and thereby 2.5% vacancies for oxygen, where protons may be incorporated in a wet state. The activation energy was observed to demonstrate a large scatter, which may be attributed to the varying proton concentration with varying temperature and grain boundary conductivity. The high tritium conductivities observed here for BaCe_0.9Yb_0.1O_2.95 and SrZr_0.9Yb_0.1O_2.95 proton conductor membranes make them especially suitable for use in tritium separation. Table 4 represents the different types of cerate-based electrolytes with synthesis process, membrane structure, temperature, operating atmosphere, and maximum hydrogen flux.^{[78, 180, 181, 183-185]}

4.3 Performance Analysis of Cerate-Based Tritium Recovery Systems

Membrane permeation, such as Pd-Ag, has been utilized in exhaust treatment and processing while designing tritium recovery systems.^[188] However, using Pd-Ag membrane permeation faces difficulty in the system involving low tritium partial pressure, incorporating water vapor processing.^[189] Kawamura et al. have utilized a hydrogen pump with proton-conducting SrCe_0.95Yb_0.05O_3-α ceramic membrane to extract the hydrogen isotopes from He sweep gas only for an H₂–H₂O gas mixture in a blanket tritium recovery system.^[162] Sufficient electrical potential difference enables selective extraction of hydrogen isotopes from the blanket sweep gas through low partial to high partial pressure, facilitating tritium recovery from tritiated hydrocarbons and water molecules. Such a compact system is effective in low tritium partial pressure environments where He sweeps gas has a relatively large flow rate. Furthermore, Kawamura et al. studied the transport properties of the hydrogen isotopes for multicomponent sweep gas in a later work.^[177] It is observed that the hydrogen isotopes are dissociated and then absorbed on the electrode, followed by diffusion through the lattice structure. Such a system demonstrated ≈40% bred tritium extraction ability through the SrCe_0.95Yb_0.05O_3-α ceramic proton conducting permeator upon application of an electric potential in the range of 800 mV. Figure 10 shows a diagram of experimental equipment.^[177] A voltage (from 0 to 100 mV) was applied between the electrodes to transport hydrogen isotopes from the interior to the exterior of the SrCe_0.95Yb_0.05O_3-α tube. In the HT-H₂ system, the sample gas was supplied by flowing He gas through an alloy bed that was used to contain a mixture of tritium and hydrogen. Figure 11a,b represents the Tritium recovery ratio and decontamination factor for different voltage applications and Tritium enrichment for various applied voltages.^[177] The comparison between current-voltage properties and conductivity of different electrodes is shown in Figure 11c–f.^[74]

The use of S_rCe_0.95Yb_0.05O_3-α as electrolyte for tritium pump/separation process in nuclear fusion technology due to excellent tritium recovery performance from low concentration tritium-containing water vapor in blanket sweep gas.^[162] A comparative study based on both experimental and empirical investigation was carried out by Fukuda et al. to compare properties such as total impedance and diffusion impedances.^[190] In such an application, the transfer of both mass and charge is generally explained by the total impedance plot, termed the cole-cole plot. However, Fukada et al. studied Sr-Ce-Yb oxide-based CH₄+H₂O|Ni|SrCe_0.95Yb_0.05O_3-α|NiO|O₂+H₂O system, having a porous Nickel electrode as an internal reformer without an additional methane reformer.^[191] The principal process on the electrode surface was stated to be the steam reforming reaction, and no traces of carbon deposits were found on the Ni electrodes. Though only H₂ diffused to the cathode side through the SrCe_0.95Yb_0.05O_3-α ceramic, specific volumes of CH₄ underwent direct decomposition at the ceramic-electrode interface following diffusion through the porous Ni electrode. Compared to the H₂+H₂O supply, protonic conductivity for the CH₄+H₂O supply was ten-fold smaller. Here, the low electric conductivity of the ceramic impeded the conclusive identification of carbon localized on the ceramic surface. The relatively easy recovery of oxidized tritium as tritium water vapor on the anode side and future H₂ or CH₄ sensor applications make this system particularly advantageous.^[192]

Kawamura et al. (2002)^[73] have investigated transfer characteristics of ionic deuterium versus hydrogen of the H₂-D₂ mixture in a blanket tritium recovery (BTR) system consisting of SrCe_0.95Yb_0.05O_3−α membrane-based hydrogen pump. Similar work was reported by Kakuta et al.^[193] on the use of a protonic conductor membrane (PCM) for the BTR system. In such a system, the electric potential difference is the driving force of hydrogen transportation through PCM. Therefore, EMF is measured from electrodes after attaching them to the PCM membrane's anode and cathode sides, with the addition of sensing and recording the different hydrogen partial pressures, applying the Nernst equation.^{[193, 194]} For the experiment, the SrCe_0.95Yb_0.05O_3-α membrane is used as PCM, where a disk-type membrane is utilized for impedance and a test tube-type membrane for transportation properties measurement at 873 and 1073 K. Although the SrCe_0.95Yb_0.05O_3-α membrane can be successfully used in such cases, an analytical model-based study classifies the mass transportation phenomena through electrolyte membranes. Kawamura et al.^[195] have proposed the equation of the basic mass transfer rate of hydrogen by using the conductivity of apparent proton as a coefficient of mass transfer, where the estimation of the basic mass transfer of hydrogen was formulated with the help of apparent proton conductivities, with the implementation of the proposed equations and experimental data. The characteristics of ionic hydrogen regarding perovskite-type material, SrCe_0.95Yb_0.05O_3-α, are one of the contenders for good proton conductors of hydrogen pumping systems that have been explored. Table 5 represents the fabrication method, sintering temperature, gas mixture, tritium activation energy for conduction, and tritium conductivity of a cerate-based tritium recovery system.^[187]

4.4 Performance Analysis of Cerate Based Hydrogen Sensors

Borland et al. reported the different synthesized techniques and the performance of BaZr_0.9Y_0.1O₃, BaCe_0.6Zr_0.3Y_0.1O_3-δ, Sr(Ce_0.6-Zr_0.4)_0.9Y_0.1O_3-δ, and Sr₃Fe_1.8Co_0.2O_7-δ proton conductors as electrolytes in a hydrogen sensor.^[165] The solid-state reaction method has been preferred to synthesize BaZr_0.9Y_0.1O₃ and BaCe_0.6Zr_0.3Y_0.1O_3-δ proton conducting membranes, while the citrate method has been used for the synthesis of Sr(Ce_0.6-Zr_0.4)_0.9Y_0.1O_3-δ and Sr₃Fe_1.8Co_0.2O_7-δ proton conducting membranes. Hydrogen sensors synthesized using BaCe_0.6Zr_0.3Y_0.1O_3-δ and Sr₃Fe_1.8Co_0.2O_7-δ proton exchange membranes exhibit the best performances with stable output potentials having minimal ≈50 mV deviation from the theoretical value obtained from the Nernst relation. A slightly higher (≈76 mV) potential deviation is observed from BaZr_0.9Y_0.1O_3, which may be attributed to noisy signals from the experimental setup. However, Sr(Ce_0.6-Zr_0.4)_0.9Y_0.1O_3-δ perovskite ceramic does not exhibit significant promise as a proton exchange membrane. Still, it is helpful for electrochemical sensing owing to the enormous potential deviations between the observed and the theoretical values.^[196] Table 6 demonstrates the cerate-based hydrogen sensor (Part-1) with the synthesis process and different parameters, including thickness, density, electrode, proton conductivity, temperature, atmosphere, and hydrogen concentration.^{[166, 169]} Cerate-based hydrogen sensor (Part-2) with synthesis process, electrode, working temperature, theoretical and experimental electrochemical potential, and deviations are shown in Table 7.^[165]

Furthermore, Borland et al. designed a lab-scale hydrogen sensor unit and injected two different gas mixtures (Ar and H₂) of known H₂ partial pressure into both compartments. Knowing the theoretical electrochemical cell potential (−176 mV), the authors observed the deviation in electrochemical cell potential for different proton conductors at 500 ⁰C, 0.0001 bar P_{H2, WE,} and 0.02 bar P_{H2, RE}. Although the observed electrochemical cell potential for BaZrY (−100 mV) and BaCeZrY (−120 mV) was stable and very close to the theoretical value (−176 mV), the electrochemical cell potential for SrCeZrY (−30 mV) showed significant deviation. Also, it required comparatively more time to yield a stable cell potential. Any modification in the structural composition of perovskites with dopants changes the activation limit of oxygen vacancy formation; hence, it is crucial to investigate the deviation of the electrochemical cell potential of the perovskite candidates within a series of elevated temperatures (600–1000 ⁰C).^[197] Kalyakin et al. designed a sensor using BaCe_0.7Zr_0.1Y_0.2O_3-δ as cell material and tested the electrochemical properties in different operating conditions (H₂ content and temperature).^[169] BaCe_0.7Zr_0.1Y_0.2O_3-δ experienced an increase in ionic conductivity primarily at ≈620 ⁰C due to their phase transition toward a higher symmetric structure and dissociation of weak bonds between point defects such as $${{\{ {V_O^. - Y_{Ce,\ Zr}^{\prime}} \}}^ \cdot }$$ and $${{\{ {OH_O^{\ .} - Y_{Ce,\ Zr}^{\prime}} \}}^x}$$. For this reason, BaCe_0.7Zr_0.1Y_0.2O_3-δ materials could become more efficient proton conductors between 450 and 550 ⁰C, as demonstrated in their proton conductivity test. This study also examines the BaCe_0.7Zr_0.1Y_0.2O_3-δ as an electrolyte for a hydrogen sensor that operates in both potentiometric and amperometric modes.

Co-doping Indium was introduced into BZC composites, as such composites have better stability under steam conditions.^[198] A detailed study of their conduction behavior is reported by N. Taniguchi et al. (2005).^[166] The conduction properties of BZCI were investigated based on the cell's EMF response to hydrogen concentration, and electrochemical H₂ permeation was carried out. Their study reveals that BZCI ceramic showed good proton conduction properties at 300–700 °C in the presence of hydrogen, and as a potential application in limiting current-type hydrogen sensors, exhibits good sensing characteristics in a reducing atmosphere. Furthermore, the cyclic study (100 cycles) revealed that the material has robust mechanical stability. In addition, the impact of adding contaminant gases like CH₄, C₃H₈, and C₄H₁₀ in hydrogen sensors with BZCI ceramics was observed. Figure 12a depicts the relationship between the sensing cell's EMF and the voltage applied to the pumping cell while different vapor pressures are considered.^[199] At voltages exceeding 2.0 V above the applied voltage, a constant EMF was seen for each water vapor pressure in the test gas. Figure 12b represents the EMF responses of the sensor to air with different pumping voltages. The measured EMF was a monotone logarithmic function of $${{P}_{{{H}_2}O}}$$ and was unaffected by applied voltages greater than 2.0 V. A plot of the sensing cell's EMF and the pumping cell's current density versus the voltage supplied to the pumping cell is shown in Figure 12c,d; the water vapor pressure in the test gas was 4.6 Torr. Current density started to increase at ≈1.25 V when the pumping cell was exposed to the applied voltage. At this voltage, the EMF of the sensor cell increased sharply and nearly stabilized.^[199]

5.1 Challenges and Limitations of Cerate Materials in Electrochemical Hydrogen Devices

The challenges and limitations of cerate material-based electrochemical hydrogen devices are discussed in the following categories.

5.2 Prospects of Cerate Materials in Electrochemical Hydrogen Devices

The potential of cerate-based materials, particularly PCOs, in electrochemical hydrogen devices, is drawing attention because of their superior electrochemical properties and outstanding proton conductivity. These materials are being explored for application in energy conversion systems, sensors, and hydrogen pumps. Recent studies have demonstrated the effectiveness of PCECs utilizing cerate-based electrolytes for efficient hydrogen separation. For instance, a novel PCEC was reported to achieve Faraday efficiencies exceeding 96% when properly separating pure hydrogen from diluted streams at temperatures ranging from 350 to 500 °C. The well-bonded electrochemical interfaces and nanoporous nickel catalysts produced by carefully controlled in situ reduction of nickel oxides are responsible for this performance.^[230] PCECs using cerate-based materials have demonstrated remarkable stability and reversibility in terms of producing energy and hydrogen. According to a study, researchers used a cerate-based electrolyte, which conducts protons extremely well, in combination with a unique air electrode that allows protons to flow through. This configuration had minimal performance loss over a long period of time and operated effectively in both fuel cell and electrolysis modes.^[231] Improvements in material synthesis have also helped cerate-based electrochemical devices perform better. For example, using the molten salt method improved the hydrogen evolution reaction (HER) performance in both acidic and COS electrolytes for cerium-modified rhenium disulfide (Ce-ReS₂). The inclusion of cerium accelerated the dynamics of reactions and enhanced the number of active sites, providing insights into the development of effective electrocatalysts.^[232]

The commercialization readiness of cerate material-based electrochemical hydrogen devices is advancing, with notable progress in PCECs utilizing cerate-based electrolytes. These devices perform smoothly in steam electrolysis at intermediate temperatures (500–650 °C) and demonstrate remarkable reversibility, stability, and Faradaic efficiency. These systems are suitable for scalable hydrogen production as the incorporation of cerate materials improves their longevity and performance. In addition, the advancement of electrochemical hydrogen compressors (EHCs) provides a compact and efficient approach to purifying and compressing hydrogen, resolving issues with traditional techniques. These advancements indicate a significant step toward the commercialization of cerate material-based electrochemical hydrogen devices, paving the way for their integration into sustainable energy systems.^{[231, 233]} Recent developments demonstrate the potential of these technologies, such as Ceres Power's development of a 1 MW solid oxide electrolyzer. This electrolyzer produces one kilogram of hydrogen with less than 40 kWh of electricity. At Shell's R&D Center in Bangalore, it is currently being prepared for use for further testing and is up to 25% more efficient than the current technology.^[234] Researchers have developed PCECs utilizing cerate-based materials, specifically employing BaZr₀.₄Ce₀.₄Y₀.₁Yb₀.₁O₃ (BZCYYb4411) as the electrolyte. These cells demonstrate remarkable durability and reversibility, maintaining performance for 500 h at 550 °C and achieving a 76% Faradaic efficiency. These developments demonstrate the potential of cerate-based PCECs in producing power and hydrogen efficiently.^[231]

Cerate-based electrochemical hydrogen devices, especially those utilizing proton-conducting ceramics such as BaZr₀.₄Ce₀.₄Y₀.₂O₃ (BZCY), offer attractive hydrogen production possibilities because of their strong ionic conductivity and ability to operate at intermediate temperatures (500–700 °C). Significant improvements in current densities have been demonstrated in recent developments; values as high as −1.92 A cm⁻² at 1.3 V and 600 °C indicate improved performance in steam electrolysis applications.^{[231, 235]} In comparison, PEM electrolyzers function at lower temperatures (20–80 °C) and are distinguished by their high-purity hydrogen output, rapid startup, and compact construction. However, they are sensitive to water purity and frequently need costly noble metal catalysts, which might affect their longevity and long-term operating costs. Additionally, hybrid systems attempt to find a balance between operating flexibility and efficiency by integrating elements of both low-temperature and high-temperature technology. However, they have difficulties with system complexity and material compatibility. Overall, even though cerate-based devices are still in the early stages of development, they present a viable alternative to current hydrogen production technologies due to their potential for high efficiency and operating at intermediate temperatures.^{[236, 237]}

Proton-conducting materials such as barium cerate (BaCeO₃) are used in cerate-based electrochemical hydrogen devices. These devices are becoming more important in fusion reactors and hydrogen systems because they work efficiently and reliably.^[56] These materials perform well at intermediate temperatures (350–600 °C), which are aligned with the thermal profiles of nuclear fission and fusion reactors, allowing for the effective production of hydrogen and the generation of energy. Such compatibility facilitates the integration of these devices into existing energy systems, enhancing overall efficiency.^[238] In fusion reactors, hydrogen pumps using proton-conducting ceramics are being studied for important tasks like separating hydrogen isotopes and recovering tritium. These functions help keep the reactor safe and make it more cost-effective.^[239] In addition, developments in pressurized protonic ceramic electrolyzers have shown enhanced electrode activity and lower power needs for water electrolysis, which makes them suitable for fusion energy systems and scalable hydrogen production. These developments highlight the critical role of cerate-based electrochemical devices in developing the hydrogen economy as well as fusion energy applications.^[240]