Protonic Ceramic Electrochemical Cells: Materials, Cell Architecture, Testing, and Performance
Protonic ceramic electrochemical cells (PCECs) are solid-state devices that use a proton-conducting ceramic electrolyte to couple hydrogen-containing gas reactions with electrical power. Depending on the applied operating conditions, a cell can generate electricity from hydrogen, produce hydrogen by steam electrolysis, or operate reversibly in both modes.
Their central materials challenge is integration. The electrolyte must conduct protons while limiting electronic leakage; the fuel and oxygen electrodes must catalyse their respective reactions; and all layers must remain chemically, mechanically, and thermally compatible during fabrication and operation.
This guide connects the underlying proton defect chemistry to cell architecture, electrochemical measurements, performance analysis, and durability assessment.
1. What distinguishes a protonic ceramic cell?
A conventional oxygen-ion-conducting solid oxide cell transports oxide ions through its electrolyte. A protonic ceramic cell instead transports protonic defects, commonly represented as hydroxyl defects on oxygen lattice sites:
\[ OH_O^{\bullet} \]
In a typical acceptor-doped perovskite electrolyte, water vapour hydrates oxygen vacancies:
\[ H_2O(g)+V_O^{\bullet\bullet}+O_O^x \rightleftharpoons 2OH_O^{\bullet} \]
The proton is associated with a lattice oxygen and moves through the oxide by thermally activated reorientation and transfer between neighbouring oxygen sites.
The operating temperature depends on electrolyte composition, thickness, electrode kinetics, and cell design. Many studies target intermediate-temperature operation, but there is no single universal operating window for every PCEC architecture.
The practical distinction is the transported species, not simply a lower operating temperature. The cell’s gas feeds, electrode reactions, and water-production or water-consumption locations must be interpreted using proton transport.
2. Cell components and their functions
A typical electrolyte-supported or electrode-supported PCEC contains three functional layers.
2.1 Proton-conducting electrolyte
The electrolyte is a dense ceramic membrane that transports protons between the two electrodes. A commonly investigated material family is based on acceptor-doped barium cerate, barium zirconate, or mixed cerate-zirconate perovskites.
A representative composition is:
\[ BaZr_{1-x-y}Ce_xY_yO_{3-\delta} \]
This is a compositional family, not a guarantee of a single phase or optimum performance. Increasing zirconium content can improve chemical stability in some environments but may make densification more difficult. Cerium-rich compositions can offer favourable sintering and transport characteristics but require careful assessment of chemical stability and redox response.
The electrolyte should provide:
- High protonic conductivity under the intended temperature and humidity.
- Low electronic conductivity to maintain Faradaic efficiency.
- Gas-tightness and low open porosity.
- Chemical compatibility with both electrodes.
- Adequate mechanical strength and thermal-cycle resistance.
- A microstructure with sufficiently low grain-boundary resistance.
A dense electrolyte is essential. Open pores or cracks allow gas crossover, which can reduce voltage, create unsafe gas mixtures, and invalidate measured Faradaic efficiency.
2.2 Fuel electrode
The fuel electrode is exposed to hydrogen-containing fuel in fuel-cell operation and commonly to the hydrogen product side during electrolysis. It must support hydrogen oxidation and evolution, provide electronic conduction, and maintain effective contact with the electrolyte.
Candidate materials include Ni-containing cermets and mixed-conducting ceramic electrodes. Material choice depends on redox stability, thermal expansion, chemical compatibility, catalytic activity, and the ability to form a stable porous microstructure.
A useful fuel electrode needs connected pathways for: - Gas transport through open porosity. - Proton transfer at or near the electrolyte interface. - Electronic conduction to the current collector. - Surface reaction and charge transfer.
The triple-phase-boundary concept can be useful for composite electrodes, but it is not a universal description of all PCEC electrodes. Mixed-conducting phases can extend reaction zones beyond a geometrically narrow triple-phase boundary.
2.3 Oxygen electrode
The oxygen electrode is exposed to air or an oxygen-containing gas. It supports oxygen reduction in fuel-cell mode and oxygen evolution in steam-electrolysis mode.
It must combine suitable electronic conductivity with catalytic activity and, depending on the electrode design, protonic and/or oxide-ion transport. Candidate materials include perovskite-based mixed conductors and composite electrodes designed to improve reaction kinetics and compatibility with the electrolyte.
The oxygen electrode is often a major source of polarization losses. Its behaviour is sensitive to surface composition, humidity, oxygen partial pressure, cation segregation, and interfacial reactions.
3. Cell reactions in fuel-cell and electrolysis modes
The electrode reactions should be written consistently with the proton-conducting electrolyte.
3.1 Fuel-cell mode
At the fuel electrode, hydrogen is oxidized:
\[ H_2(g) \rightarrow 2H^{\bullet}_{(electrolyte)}+2e' \]
Here, \(H^{\bullet}_{(electrolyte)}\) is a compact representation of a protonic defect transported through the ceramic. In Kröger–Vink notation for a proton bound to lattice oxygen, it corresponds to \(OH_O^{\bullet}\) relative to the ideal oxygen site.
At the oxygen electrode, oxygen reacts with protons and electrons to form water:
\[ \frac{1}{2}O_2(g)+2H^{\bullet}_{(electrolyte)}+2e' \rightarrow H_2O(g) \]
The overall reaction is:
\[ H_2(g)+\frac{1}{2}O_2(g)\rightarrow H_2O(g) \]
The electrons travel through the external circuit and deliver electrical power, while protons cross the electrolyte.
3.2 Steam-electrolysis mode
In electrolysis mode, water is supplied to the oxygen/steam electrode, where it is oxidized:
\[ H_2O(g)\rightarrow 2H^{\bullet}_{(electrolyte)} +\frac{1}{2}O_2(g)+2e' \]
Protons migrate through the electrolyte to the fuel electrode, where they are reduced:
\[ 2H^{\bullet}_{(electrolyte)}+2e' \rightarrow H_2(g) \]
The overall reaction is:
\[ H_2O(g)+\text{electrical energy} \rightarrow H_2(g)+\frac{1}{2}O_2(g) \]
The exact electrode naming convention can vary between publications. To avoid ambiguity, state explicitly which side receives steam, which side produces hydrogen, and the direction of proton transport.
3.3 Why product water is formed on a different side
In a protonic cell operating in fuel-cell mode, protons move from the fuel electrode toward the oxygen electrode, where water is produced. In steam electrolysis, water is consumed at the steam electrode and hydrogen is produced on the opposite side.
This differs from the usual oxygen-ion-conducting electrolysis configuration, in which oxide ions move through the electrolyte and the water-splitting and hydrogen-production locations follow a different arrangement. Gas-flow diagrams should therefore accompany cell descriptions whenever possible.
4. Electrolyte composition and defect chemistry
Acceptor doping is commonly used to introduce oxygen vacancies into a perovskite oxide. For an idealized acceptor dopant \(M_B'\) substituting on a tetravalent B site, oxygen-vacancy compensation can be represented approximately as:
\[ 2[M_B']\approx[V_O^{\bullet\bullet}] \]
This is a limiting charge-compensation relation, not a universal equality under all atmospheres. Electronic defects, cation non-stoichiometry, secondary phases, and other defect species may contribute.
Hydration converts oxygen vacancies into protonic defects:
\[ H_2O(g)+V_O^{\bullet\bullet}+O_O^x \rightleftharpoons 2OH_O^{\bullet} \]
The concentration of protonic defects therefore depends on temperature, water-vapour partial pressure, acceptor concentration, and hydration thermodynamics.
A high nominal acceptor concentration does not necessarily yield a proportionally high proton conductivity. Dopant-vacancy association, proton trapping, local structural distortion, and reduced mobility can offset the increase in the number of hydration sites.
For a detailed discussion of hydration, proton transport, and competing charge carriers, see Proton-Conducting Oxides: Defect Chemistry, Transport Mechanisms, and Hydrogen Applications.
5. Electrolyte-supported and electrode-supported architectures
Cell architecture determines the balance between ohmic resistance, mechanical strength, gas transport, and fabrication complexity.
5.1 Electrolyte-supported cells
In an electrolyte-supported design, the electrolyte is relatively thick and provides much of the mechanical support.
Advantages: - Mechanically robust electrolyte substrate. - Straightforward handling during fabrication. - Potentially lower sensitivity to support-layer redox changes.
Limitations: - Higher electrolyte ohmic resistance. - Greater need for high electrolyte conductivity or elevated operating temperature. - Thicker membrane can limit power density.
5.2 Electrode-supported cells
In an electrode-supported design, a porous electrode provides mechanical support and the electrolyte is made comparatively thin.
Advantages: - Lower electrolyte ohmic resistance. - Potential for improved performance at reduced temperature. - Greater flexibility in electrolyte thickness.
Limitations: - More demanding control of electrolyte deposition and densification. - Risk of pinholes, cracks, and gas leakage. - Interdiffusion and reactions during co-sintering. - Greater sensitivity to support microstructure and shrinkage mismatch.
A thin electrolyte is not automatically a better electrolyte. Its gas tightness, phase purity, adhesion, and resistance must be verified after the complete thermal processing sequence.
6. Electrode and interface design
The electrochemical performance of a PCEC is governed by the complete electrode/electrolyte assembly rather than by isolated powder conductivity.
Important design variables include:
- Electrode porosity and pore-size distribution.
- Grain size and particle connectivity.
- Electrode thickness and current-collection geometry.
- Electrolyte surface chemistry and roughness.
- Chemical interdiffusion during firing.
- Thermal expansion mismatch.
- Chemical expansion under humid or reducing conditions.
- Contact resistance at the current collector.
Interfacial reactions can form resistive secondary phases even when both starting materials are individually stable. A practical compatibility study should therefore include powder mixtures or diffusion couples subjected to the intended firing schedule, followed by phase and microstructure analysis.
For the oxygen electrode, high humidity and oxygen activity can change surface composition and promote segregation or reconstruction. Long-term stability should be assessed under realistic gas composition rather than inferred from dry-air screening alone.
7. Electrochemical characterization
7.1 Current-voltage curves
A current-voltage (I–V) curve records cell voltage as a function of current density. In fuel-cell mode, the open-circuit voltage (OCV) and loaded voltage are commonly reported. In electrolysis mode, the applied voltage and resulting current density are measured.
The power density in fuel-cell mode is:
\[ P= jV \]
where \(j\) is current density and \(V\) is cell voltage. If \(j\) is in A cm\(^{-2}\) and \(V\) is in volts, \(P\) is in W cm\(^{-2}\).
Report the active electrode area, whether current is normalized to geometric or another area, gas composition, flow rate, humidity, temperature, and heating protocol. A peak power density without these conditions is not a transferable material metric.
7.2 Open-circuit voltage
The OCV provides a useful check of gas separation and electrochemical reversibility. A value substantially below the expected Nernst voltage may indicate gas leakage, electronic leakage through the electrolyte, mixed potentials, electrode non-equilibrium, or measurement errors.
For a hydrogen/steam electrode and an oxygen electrode, the equilibrium voltage depends on temperature and the relevant gas activities. The Nernst equation must be written for the actual overall cell reaction and gas composition.
Do not use OCV alone to claim proton transference number. A high OCV is consistent with good gas tightness and predominantly ionic transport, but independent measurements are needed to quantify carrier contributions.
7.3 Electrochemical impedance spectroscopy
EIS separates the total cell response into contributions that may include: - Electrolyte and contact ohmic resistance. - Fuel-electrode polarization. - Oxygen-electrode polarization. - Interfacial and gas-transport processes.
The area-specific resistance (ASR) is commonly calculated from a resistance contribution multiplied by the active cell area:
\[ ASR=R\,A \]
where \(R\) is in ohms and \(A\) is in cm\(^2\), giving ASR in \(\Omega\,cm^2\).
For a symmetric cell with two nominally identical electrodes, the measured polarization resistance contains contributions from both electrodes. The single-electrode ASR is often obtained by dividing the total polarization contribution by two, but only when symmetry and comparable electrode behaviour are justified.
Equivalent-circuit assignments should be supported by frequency dependence, capacitance estimates, temperature trends, electrode symmetry tests, and changes in gas composition. A fitted semicircle is not automatically a unique physical process.
See Electrochemical Impedance Spectroscopy: A Practical Guide to Measurement and Interpretation.
7.4 Separating protonic and electronic transport
Total conductivity alone does not establish proton conductivity. Useful approaches include: - Comparing dry and humid atmospheres at controlled oxygen partial pressure. - H/D isotope substitution, with careful control of water activity and equilibration. - Concentration-cell and electromotive-force measurements. - Transport-number measurements where the cell configuration permits. - Combining conductivity with independent oxygen-content or hydration measurements.
Each method has assumptions and possible artefacts. For example, a conductivity increase in wet gas may reflect proton incorporation, but changes in surface chemistry or electrode response can also affect a measured cell signal.
8. Faradaic efficiency and hydrogen production
Faradaic efficiency compares the experimentally measured hydrogen production with the amount expected from the total charge passed.
For hydrogen production involving two electrons per molecule:
\[ n_{H_2,\,theoretical}=\frac{Q}{2F} \]
where \(Q\) is the total charge and \(F\) is the Faraday constant. The Faradaic efficiency is:
\[ FE=\frac{2F n_{H_2,\,measured}}{Q}\times100\% \]
The measured hydrogen amount must be corrected for gas flow, pressure, temperature, water vapour, sampling losses, and instrument calibration as appropriate.
A low Faradaic efficiency may arise from electronic leakage, gas crossover, parasitic reactions, leaks, or errors in product quantification. It should not automatically be attributed to one mechanism.
In a proton-conducting electrolyte, electronic conductivity is particularly important: electronic leakage can carry part of the applied current without contributing to the intended proton-mediated hydrogen production.
9. Performance limitations and degradation
PCEC performance can decline through several coupled mechanisms.
10. A practical workflow for developing and testing a PCEC
- Select the electrolyte composition. Define the target protonic conductivity, chemical stability, and sintering route.
- Verify powder chemistry and phase purity. Use XRD and appropriate chemical analysis before cell fabrication.
- Evaluate electrolyte densification. Measure relative density, porosity, and microstructure; confirm gas tightness after firing.
- Screen electrode compatibility. Test electrode/electrolyte mixtures and diffusion couples using the intended thermal schedule.
- Fabricate a symmetric cell. Use it to compare electrode polarization under controlled atmospheres before assembling a complete cell.
- Build the full cell. Document layer thicknesses, active area, support architecture, and firing conditions.
- Check sealing and OCV. Confirm gas separation before interpreting electrochemical performance.
- Perform EIS and I–V measurements. Use controlled gas composition, flow, humidity, and temperature.
- Quantify hydrogen production. Calculate Faradaic efficiency from calibrated product analysis and integrated charge.
- Run durability tests. Track voltage or current, ASR, gas composition, and post-test microstructure.
- Characterize after operation. Compare XRD, SEM/EDS, and relevant surface or chemical analyses before and after testing.
For meaningful comparison between studies, report not only peak performance but also the cell architecture, electrode area, electrolyte thickness, gas utilization, humidification, current-collection method, and test duration.
11. Common interpretation errors
- Describing a PCEC as an oxygen-ion cell because it is tested using conventional solid-oxide-cell equipment.
- Reversing the steam and hydrogen sides when drawing the electrolysis configuration.
- Assuming that nominal acceptor content equals the concentration of mobile protons.
- Treating total conductivity as proton conductivity without transport-number evidence.
- Reporting an OCV as proof of a specific ionic transference number.
- Comparing power densities without matching active area, gas composition, temperature, and cell architecture.
- Assigning each EIS arc to a physical process without independent support.
- Reporting Faradaic efficiency without calibrated hydrogen quantification and leak checks.
- Inferring long-term stability from short-duration screening.
- Ignoring interfacial reactions introduced during co-sintering.
12. Research opportunities
Important research directions include: - Electrolytes that combine high proton conductivity with improved tolerance to CO\(_2\) and water vapour. - Low-temperature densification routes that preserve chemical compatibility. - Electrode materials with fast oxygen exchange under humid conditions. - Stable electrode/electrolyte interfaces with limited cation interdiffusion. - Operando methods that track proton defects and electrode reconstruction under load. - Reversible operation with quantified degradation under fuel-cell/electrolysis cycling. - Scalable fabrication of thin, dense electrolytes and mechanically robust cells.
The most useful advances will connect defect chemistry and transport measurements to full-cell operation. Improvements in a powder-level conductivity value are valuable only when they translate into lower cell resistance, higher Faradaic efficiency, and stable operation.
13. Key takeaways
- PCECs use protonic defects as the principal electrolyte charge carrier.
- The fuel and oxygen electrodes perform different reactions in fuel-cell and steam-electrolysis modes.
- Electrolyte conductivity, gas tightness, electrode kinetics, and interface compatibility must be optimized together.
- EIS, I–V curves, OCV, and product analysis provide complementary information; none alone is a complete performance assessment.
- Faradaic efficiency requires calibrated hydrogen quantification and a reliable charge balance.
- Durability depends on chemical stability, microstructural evolution, interfaces, and operating atmosphere.
References
- Y. Wang, Y. Ling, B. Wang, G. Zhai, G. Yang, Z. Shao, R. Xiao, and T. Li, “A review of progress in proton ceramic electrochemical cells: material and structural design, coupled with value-added chemical production,” Energy & Environmental Science 16 (2023), 5721–5770. https://doi.org/10.1039/D3EE03121G
- H. Su and Y. H. Hu, “Degradation issues and stabilization strategies of protonic ceramic electrolysis cells for steam electrolysis,” Energy Science & Engineering 10 (2022), 1706–1725. https://doi.org/10.1002/ese3.1010
- S. Oh, H. Kim, I. Jeong, D. Kim, H. Yu, and K. T. Lee, “Recent progress in oxygen electrodes for protonic ceramic electrochemical cells,” Journal of the Korean Ceramic Society 61 (2024), 224–249. https://doi.org/10.1007/s43207-023-00360-y
- X. Hu et al., “Data-driven discovery of electrode materials for protonic ceramic cells,” Energy & Environmental Science 17 (2024), 9335–9345. https://doi.org/10.1039/D4EE03762F
- M. Li, F. Liu, and D. Ding, “Critical insights into the steam electrolysis electrode in protonic ceramic cells for hydrogen production,” Nature Catalysis 8 (2025), 293–300. https://doi.org/10.1038/s41929-025-01313-w
- D. Kim, S. Oh, and H. Kim, “Processing-Centric Innovative Strategies for Protonic Ceramic Electrochemical Cells,” Advanced Energy Materials (2026). https://doi.org/10.1002/aenm.71241