Novel modular stack design for high PREssure PEM

water elecTrolyZer tEchnoLogy with wide operation range and reduced cost (PRETZEL)



Green hydrogen produced

by electrolysis will become a key energy carrier for the implementation of renewable energy as a cross-sectional connection between the energy sector, industry and mobility. Proton exchange membrane electrolysis (PEMEL) is the preferred technology for this purpose, yet large facilities can hardly achieve FCH-JU key performance indicators (KPI) in terms of cost, efficiency, lifetime and operability. Consequently, game changers in the technology are direly needed.

PRETZEL has been successfully concluded!



The overall goal of PRETZEL was to develop an innovative polymer electrolyte membrane electrolyzer (PEMEL) that provides significant increases in efficiency and operability to satisfy emerging market demands.

The main challenges in PEMEL design are the CAPEX-reduction by minimizing the use of critical raw materials (precious metal catalysts and coatings and titanium structures) and the increase of the operating pressure to reduce mechanical compression requirements, reducing system complexity and increasing system efficiency.

The central target of the PRETZEL project has been to design and manufacture a 25 kW PEMEL stack that reaches an operation temperature of 90°C, pressure of 100 bar and current density of 4 A/cm² (6 A/cm² in overload mode), while maintaining above 70 % efficiency and fast system response times. This has been demonstrated by reaching the main objectives of developing and evaluating optimized PEMEL components and a stack design based on hydraulic compression. Following the objective to disseminate and exploit the PRETZEL archivements impactfully, results have been published in scientific journals and several novel components designed during PRETZEL have been commercialized.


The project PRETZEL innovated in the following areas:

Reduction of precious metalsIncreased availability, decreased CAPEX
Production of optimized current collectors with low cost coatings that will allow operation at high current densitiesIncrease of efficiency and reduction of operative costs, reduction of bipolar plates (BPP) production costs while taking over the water distribution, increasing the power density without mass transport limitation
Corrosion resistant (BPP) without flow fieldReduction of Ti and reduction of manufacturing costs
High pressure operation > 100 barReduction of compressor costs, increased system efficiency
Innovative hydraulic concept for pressurizing and coolingIncreased life time and efficiency, enableing high current density, enableing durable large active area cells

The project PRETZEL

Project achievements

The hydraulically compressed design has proven itself as an innovative design with several advantages. The large heat exchange surface area that is attained in this design, in combination with the high thermal conductivity developed pole plates, minimizes lateral temperature gradients and hot-spots, reducing thermal stress and prolonging stack lifetime. Furthermore, the inevitable long term dimensional change of the membrane electrode assembly caused by membrane or electrode thinning is compensated by the hydraulic compression preventing the loss of electrical contact. The PRETZEL-developed pore graded porous current distributor increases electrical efficiency of the electrolysis process by over 20 % at the target current densities, reducing operational cost significantly. The highly corrosion resistant niobium, titanium and gold coated copper polar plates designed and manufactured in the PRETZEL project provide two distinct improvements. Firstly, the material costs are significantly lower than state-of-the-art high-grade titanium and secondly the high conductivity copper ensures low electrical and heat transport resistance.

The PRETZEL innovations, commericalized stack design and components will directly enable the additional commerical roll-out of PEMEL systems in the future. Furthermore, new knowledge with respect to operating PEMEL systems at high pressure, current density and temperature as well as on all developments of the PRETZEL components was gained and shared with the scientific community, presenting the lessons learned. This will accelerate further technological advancements. In this, the PRETZEL results will have a signficant impact on both a scientific as well as on an economic level.

The PRETZEL achievements will, together with the numerous impactful innovations realized in the scientific community, drive the large-scale commercial PEM electrolysis adaptation forward, accelerating the green revolution that is direly needed to combat a catastrophic climate change.

The project PRETZEL was divided into the following seven work packages:

WORK PACKAGE 1 – Coordination

This work package was concerned with the general activity of coordination and steering of the activities in view of the time-schedule, project finances and the quality of the results.

WORK PACKAGE 2 – Definition of Specifications and Requirements

This work package validated the component development tasks. Here the requirements and specifications on the new developed components such as catalysts, membrane electrode assembly (MEA), current collectors, macroporous layers (MPL), bipolar plates (BPP) and especially the design and tolerances were defined. Also the measurement protocols were determined and defined in collaboration with the existing standardization activities of the FCH JU and JRC.

This of course implies that WP 3 and 4 ran in parallel in order to guarantee a deep communication between the component development of WP 3 followed by characterization and testing (WP 4).

WORK PACKAGE 3 – Component Manufacture

This work package was the heart of the project and the basis for reaching the FCH2 JU impacts in WP 5 and 6.

DLR and WHS produced the PRETZEL pole plates by coating high conductivity copper plates with corrosion resistant titanium or niobium via VPS. Corrosion test performed by UPT showed, that both coatings are totally effective in protecting the copper plate from corrosion at harsh anodic conditions. Interfacial contact resistance tests showed that the niobium coating provides better electrical contact than titanium. The research results were published in the International Journal of Electrochemical Science.

Additionally, GKN and DLR developed novel porous current distributors (PCDs) by sintering a titanium powder micro porous layer onto titanium expanded metal sheets. Selective laser melting was identified as a rapid, low-cost technique, but additional optimisation and testing is needed to perfect and upscale this approach. Tests at DLR have shown that the developed Ti-GKN PCDs increase PEMEL efficiency very significantly by over 20% at 4 A/cm² compared to state-of-the-art mesh type PCDs. This innovation was publicized in the high impact scientific journal Advanced Energy Materials. The Ti-GKN PCD component was commercialised and is now available as a product, sparking the interest of several potential customers.

In a second parallel approach DLR manufactured another novel, low-cost corrosion resistant PCDs, by coating commercial stainless-steel mesh PCDs with thin layers of titanium and niobium via VPS. 1000-hour accelerated stress test have shown, that this layer is effective in preventing catastrophic surface corrosion and MEA poisoning completely and reduces mass transport limitations significantly (12% efficiency increase at 2 A/cm²). This novel approach was published in the Journal Energy & Environmental Science and has led to additional industry cooperation developing new and optimised coated PCD structures.

High activity antimony doped tin oxide (ATO) supported iridium catalysts were developed by IBERCAT and ARMINES and were synthesised at IBERCAT exploring several synthesis routes, iridium loadings and ATO support types. The highest mass activity, measured in RDE at CERTH, up to eight times and three times higher than commercial iridium oxide and iridium metal, respectively, was achieved by synthesising a commercial ATO supported 30 wt.-% iridium catalyst via the Lettenmeier synthesis route. Research results were published in ACS Catalysis and the International Journal of Hydrogen Energy.

ADAMANT and CERTH prepared MEAs exploring different catalyst types, catalyst loadings, coating procedures and pressing procedures. Small scale PEMEL cell characterisations at CERTH and DLR identified the developed doctor blade coating and roll-to-roll pressing, which could enable low cost and continuous mass production, as a promising technique. After comparison of different manufacturing techniques for CCM production, spraying was selected as a suitable technique especially for large-scale double CCM manufacture in the framework of the project. The work performed was presented in the Patras IQ exhibition through PRETZEL’s brochure and poster created by ADAMANT. The double-CCMs and their developed manufacturing process will be exploited for using in further research activities, developing products, and providing service to stakeholders.

WORK PACKAGE 4 – Compliance Testing and Characterization

This work package was responsible for the pre-qualification, characterization and testing of all developed and adjusted materials and components from Work Package 3. Universitatea Politehnica Timisoara (UPT), CERTH and DLR were responsible for electrochemical and physical characterization. Physical characterisations at CERTH, UPT and DLR of catalysts, MEAs and PPs provided valuable insight and enabled the connection of observed phenomena and trends to their physical cause.

WORK PACKAGE 5 – High Pressure Stack

In this work package, the design of the cell frames developed in a past project at WHS have been adjusted to suit the cell components developed in Work Package 3. Major tasks have been the integration of the bipolar plates (BPPs) and the porous current distributors (PCDs) in combination with an adequate process media and hydraulic medium conduction. This was crucial in order to guarantee a homogeneous media transport through the single cells as well as a homogeneous temperature distribution. Therefore, the respective flow channels have been designed according to the specifications of the inner cell components (BPPs and PCDs). The PRETZEL stack design approach was presented in the International Journal of Hydrogen Energy and the concept is licenced to a WHS spin-off company offering a commercial product.

Following the validation of the functionality of the component and cell design, a 25 kW stack was constructed. This includes the modification of the existing high-pressure housing to fulfil the requirements concerning media and current conduction. The stack was integrated into the test system at WHS.

WORK PACKAGE 6 – 25 kW System Integration

In this work package, the constructed stack from WP5 was integrated in an existing test facility for test runs with the operating parameters. The system integration comprised, among other things, the elevated temperature and pressure. The engineering for this upgrade was carried out by iGas Energy. Bringing together all individual performance and durability optimisations on the PEMEL components of the PRETZEL project, the long-term 2000-hour electrochemical characterisation of the PRETZEL stack is currently being carried out by iGas and WHS. Tests with a prototype stack at near target conditions demonstrated the successful stack design. Characterisation of the final 25 kW stack is ongoing, and results will be shared subsequently.

WORK PACKAGE 7 – Dissemination and Exploitation

IBERCAT managed and planned the dissemination and exploitation actions of the relevant PRETZEL results. This website and social media presence was established. Among many other successful communication and dissemination activities, a NEPTUNE-PRETZEL joint workshop was organised.


Concept and approach

Schematic drawing of a PEMEL system container solution by iGas energy.

Recent developments in the field of PEMEL technology strive towards the reduction of overall system costs in order to compete with state of the art alkaline electrolysers. Major optimization aspects refer to an increase in efficiency as well as the achievement of high-pressure operation. Both issues concern the PEMEL stack, which needs to generate hydrogen with high production rates (high current density) at a reasonable cell voltage (efficiency) and high output pressure so that raw material deployment can be reduced (e.g. noble metals, proton conductive polymers and titanium) and balance of plant components can be omitted (especially compressor stages).


Concept of PRETZEL project with component inputs and expected outcome.

Furthermore, the stack components such as the membrane electrode assembly (MEA), the porous current distributor (PCD) and the bipolar plates (BPP) must be mass produced and designed with cost saving measures in mind in order to improve efficiency in use of expensive raw materials.

The PRETZEL concept addresses the above aspects to realize the next generation electrolyzer technology, whilst meeting the needs of industrial scale hydrogen production in the near future. Beyond these challenges, a significant increase of lifetime and improved operability can be achieved to cope with intermittent electricity supply from renewable energy sources.

This approach is encapsulated in the full title of the project: “Novel modular stack design for high PREssure PEM water elecTrolyZer tEchnoLogy’’ with wide operation range and reduced cost. In short: ‘’PRETZEL

Each of these innovations have already proven in a small laboratory scale and can be considered being in the range of TRL 3. Hence, the overall goal of this project was to prove the functionality of these proven technologies in a realistic environment in order to advance them to TRL 5. In practice, this meant integrating them into a fully functioning 25 kW, 100 bar electrolyzer system that reflects the realistic and highly dynamic conditions required by the market.


The overall goal of PRETZEL was to develop an innovative polymer electrolyte membrane electrolyzer (PEMEL) that provides significant increases in efficiency and operability to satisfy emerging market demands. Such electrolyzers are urgently needed in the context of the increased demands of the grid balancing market. PRETZEL is offering a break-through in becoming a game changer in the field of water electrolyzers.

The project PRETZEL innovated in the following areas:

Reduction of precious metalsIncreased availability, decreased CAPEX
Production of optimized current collectors with low cost coatings that will allow operation at high current densitiesIncreased efficiency and reduction of operative costs, reduction of bipolar plates (BPP) production costs while taking over the water distribution, increased power density without mass transport limitation
Corrosion resistant (BPP) without flow fieldReduction of Ti and reduction of manufacturing costs
High pressure operation > 100 barReduction of compressor costs, increased system efficiency
Innovative hydraulic concept for pressurizing and coolingIncreased life time and efficiency, enabling high current density, enabling durable large active area cells




The project PRETZEL had the following objectives:

  1. Develop and manufacture high pressure polymer electrolyte membrane electrolyzer (PEMEL) to operate at increased temperatures.
  2. Develop and manufacture the high pressure PEMEL stack based on the novel principle of hydraulic compression.
  3. Set-up and undertake continuous procedures to evaluate the development process through all phases against PRETZEL specifications.
  4. Integrate the innovative PEMEL stack into a high pressure PEMEL test facility and validate the overall performance and operational criteria.
  5. Disseminate and exploit the innovations in PRETZEL in order to prepare the market penetration of this new technology.

A central objective of this project was the development of a novel PEMEL system with a maximum 25 kW electrical power consumption that generates 4.5 mh-1 H2 at rated power, at an output pressure of 100 bar and feed water temperature of maximum 90 °C.

At the system level, the specific energy demand at rated production rate is below 25 kWh kg-1 H2 and 70 % on the basis of higher heating value (HHV). Furthermore, this system is able to operate in overload mode referring to a production rate as high as 6.8 mh-1 H2 (1.5 times overload). Rapid response of 1 second for a hot start and 10 seconds for a cold start are the operating targets of the system.

At the stack level, the project implemented a patented design approach based on hydraulic cell compression. This design allows for large planar cell components, which is required for future mass production, and effective cooling at very high production rates and temperature levels. Regarding sufficient stack conditioning, a cooling system was developed for voltages of maximum 2.0 V per cell at rated power and of 2.3 V per cell in overload modus. Additionally, the target of PRETZEL was the development of a high pressure PEMEL stack, which opens a perspective for specific stack costs of below 500 € kW-1. As for the production at 100 bar, an additional compressor was omitted, for the targeted system specific systems costs are possible in the range of 750 € kW-1.

Facts and figures

General Information

  • PROJECT FULL NAME: Novel modular stack design for high PREssure PEM water elecTrolyZer tEchnoLogy with wide operation range and reduced cost
  • TOPIC: FCH-02-1-2017: Game changer Water Electrolysers
  • DURATION: 36 months
  • START DATE: January 1st 2018
  • END DATE: December 31st 2020
  • PROJECT FUNDING: € 1,999,088.75
  • COORDINATOR: Deutsches Zentrum für Luft – und Raumfahrt (DLR) (Germany)
  • PARTICIPATING ORGANIZATIONS: PRETZEL comprises a consortium of 9 partners from 5 European countries

Performance Targets

The main performance targets of the project PRETZEL was the development of a novel polymer electrolyte membrane electrolyzer (PEMEL) system with the following benefits:

  • High system efficiency by 70 %
  • H2 output pressure of 100 bar
  • Rapid response of below 1 second for a hot start and below 10 seconds for a cold start
  • Dynamic operation range between 4 and 6 A cm-2, ability for 1.5 times overload
  • Water temperature of 90°C
  • Higher than 2000 h operation


Our Partners

DLR w-hs Armines UPT Adamant Composites GKN CERTH IberCats Igas Energy


Our Latest News

February 3, 2022

The PRETZEL project has been successfully concluded

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May 11, 2021

Game Changer Proton Exchange Membrane (PEM) Water Electrolysers

The workshop “Game Changer Proton Exchange Membrane (PEM) Water Electrolysers” comes from an initiative of the FCH JU Neptune and Pretzel projects to discuss next generation polymer electrolyte membrane electrolyzers. The “Game Changer” aims to identify a major step improvement for water electrolysis looking to novel solutions mainly developed at intermediate Technological Readiness Levels to […]

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July 16, 2019 PDF

2019 Semiannual Meeting of PRETZEL project at Ibercat, Parque Científico of Madrid

On February 28th and March 1st 2019, project partner IBERCAT, located at the Parque Científico of Madrid (FPCM), hosted the latest official meeting of PRETZEL project.

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January 3, 2019 PDF

2018 Semiannual Meeting of PRETZEL Project at Universitatea Politehnica Timisoara (UPT)

On September 25th 2018, the Universitatea Politehnica Timișoara (UPT) hosted the official Semiannual meeting of PRETZEL project at the Faculty of Industrial Chemistry and Environmental Engineering.

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September 27, 2018 PDF

08/2018 D2.2 deliverable – Compliance test protocols and analytics

In August 2018, the PRETZEL consortium published the deliverable D2.2 entitled: Compliance test protocols and analytics.

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German Aerospace Center Deutsches Zentrum für Luft- und Raumfahrt e.V. (DLR)


Institute of Engineering Thermodynamics
Electrochemical Energy Technology
Pfaffenwaldring 38-40
70569 Stuttgart

Pfaffenwaldring 38-40, 70569, Stuttgart, Niemcy