Technology Unlocking the Potential of Planar Proton-Conducting Electrolysis

PEPPER pioneers the development of planar proton-conducting ceramic electrolysis cells (PCCELs) — a next-generation electrolysis technology operating at intermediate temperatures (400°C–600°C). This temperature range enables high energy efficiency and integration with industrial waste heat, making it ideal for decarbonizing energy-intensive sectors such as steel, chemicals, and refining.

What Are PCCELs?

PCCELs are solid-state electrochemical devices that split steam (H₂O) into hydrogen (H₂) and oxygen (O₂) using electricity. Unlike Solid Oxide Electrolysis Cells (SOECs) that conduct oxygen ions (O²⁻), PCCELs transport protons (H⁺) through a dense ceramic electrolyte. This fundamental difference has powerful implications:

  • Higher hydrogen purity: Because steam and hydrogen streams are separated, the output is undiluted.
  • Electrochemical hydrogen compression: Hydrogen can be produced at pressure, reducing or eliminating the need for mechanical compressors.

Lower operating temperatures than SOECs, which eases materials constraints and simplifies system integration.

How PCCELs Work – In Brief

  • Steam is introduced at the oxygen electrode.
  • An electrical current splits water molecules into oxygen gas and protons.
  • Protons migrate through the ceramic electrolyte.

On the other side (fuel electrode), they combine with electrons to form pure hydrogen gas. Because of the cell architecture and chemistry, PCCELs operate more efficiently than low-temperature electrolysis and offer a more compact and modular design than traditional SOECs.

Why PEPPER Focuses on Planar PCCELs

Historically, PCCELs have been built using tubular geometries, which, although robust, suffer from scalability, limited current density, and complex manufacturing. PEPPER replaces tubular designs with planar architectures, unlocking multiple benefits:

  • Larger active areas (~100 cm²) for industrial relevance.
  • Higher current densities (>0.75 A/cm² at 600°C).
  • Compact and stackable reactor designs, reducing footprint and cost.
  • Fewer repeating units per stack, minimizing materials and assembly complexity.

This shift dramatically improves the feasibility of scaling up for commercial deployment.

Dual Technology Approach: Two Types of Planar Cells

PEPPER is developing and validating two advanced planar cell types:

Cermet-Supported Planar PCCELs (CS-PCC):

  • Use ceramic-metal composites for structural support.
  • Offer high mechanical strength and resilience during transient operations.
  • Employ advanced electrodes and thin-film electrolytes for performance and durability.

Metal-Supported Planar PCCELs (MS-PCC):

  • Replace ceramic supports with porous stainless-steel substrates.
  • Cut down critical raw material (CRM) use by over 90%.
  • Enable lower manufacturing costs and improved thermal robustness.

Both cell types are being scaled and integrated into custom short-stack configurations, specifically tailored for pressurized hydrogen production and long-term operation.

Advanced Materials and Manufacturing for Scalability

To ensure sustainability and industrial relevance, PEPPER integrates state-of-the-art materials science and manufacturing processes:

  • CRM reduction: Significant decrease in reliance on rare earth elements, Ni, Co, and other critical materials.
  • Electrode optimization: Functionalized oxygen electrodes boost resistance to contaminants like chromium and silicon.
  • Thin-film electrolyte layers (<5 μm): Achieved through magnetron sputtering (MS-PVD), improving gas tightness and lowering sintering temperatures.
  • Scalable processes: Techniques like tape casting, screen printing, infiltration, and wet powder spraying are used to build up layers and ensure reproducibility at industrial scale.

Integrated Stack Design and Validation

One of PEPPER’s core innovations is a dedicated stack design optimized for PCCELs. Unlike typical SOEC stacks that rely on external manifolding, PEPPER’s design features internal manifolds for both electrodes to:

  • Prevent corrosion from steam bypass or chromium release.
  • Enable uniform steam and gas flow across large cell areas.
  • Support integration with pressurized systems, while stack operation itself remains at ambient pressure.

The stacks are being rigorously tested for:

  • Performance and durability testing of stacks under ambient pressure.
  • Pressurized testing (up to 10 bar) at the individual cell level.
  • Electrochemical efficiency (targeting >90% faradaic efficiency).
  • Long-term stability (aiming for >5,000 hours of continuous operation).

Challenges and the Road Ahead

Despite their potential, PCCELs remain a maturing technology. PEPPER is accelerating their advancement from Technology Readiness Level (TRL) 2 to TRL 4, transitioning from lab-scale concepts to validated stack-level prototypes. Major technical challenges include:

  • Material compatibility at elevated temperatures.
  • Reliable sealing solutions for steam and gas.
  • Stability under pressurization and cycling.

By tackling these hurdles, PEPPER sets the foundation for commercial-scale electrolysis reactors that are efficient, scalable, and environmentally responsible.

In Conclusion

The PEPPER project is not just improving electrolysis — it’s redefining it. By combining:

  • High energy efficiency (up to 20% less electricity use),
  • Smart waste heat integration,
  • Cleaner material sourcing, and
  • A pathway to industrial upscaling,

PEPPER is positioning planar PCCEL technology as a pillar of Europe’s future hydrogen economy — one that is more efficient, sustainable, and tailored to the needs of heavy industry.

Clean Hydrogen Partnership

The project is supported by the Clean Hydrogen Partnership and its members.

Co-Funded by the European Union

Co-funded by the European Union. Views and opinions expressed are however those of the authors only and do not necessarily reflect those of the European Union or Clean Hydrogen Partnership. Neither the European Union nor the granting authority can be held responsible for them.