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Photoelectrochemical Demonstrator Device for Solar Hydrogen Generation

Final Report Summary - PECDEMO (Photoelectrochemical Demonstrator Device for Solar Hydrogen Generation)

Executive Summary:
PECDEMO’s main aim was to develop a photoelectrochemical (PEC) water splitting device based on low-cost and abundant materials that shows a solar-to-hydrogen (STH) efficiency of 10%, a stability of 1000 hours, and an active area of at least 50 cm2.
PECDEMO has addressed these challenges by focussing its efforts on three metal oxide photoelectrode materials (Fe2O3, BiVO4, and Cu2O) and by combining them with a silicon- or perovskite-based photovoltaic (PV) cell in a tandem configuration. To improve the efficiency and stability of the metal oxides, modifications were made by doping, application of protection layers, nano-structuring, and surface functionalization with co-catalysts for hydrogen or oxygen evolution. Fe2O3 is the most stable material; lab tests showed negligible performance decrease after 1000 h operation. A new hydrogen treatment method significantly improved the performance of BiVO4 photoanodes, resulting in a 9.2% STH efficiency for a small-area dual BiVO4/Fe2O3 photoanode/Si PV tandem cell. PECDEMO’s highest efficiency achieved for small-area devices was 16.2%, obtained for a Ga2O3/Cu2O nanowire photocathode coupled to a silicon PV cell using a dichroic mirror for photon management. The highest large-area photocurrent densities were obtained for Cu2O, giving an unprecedented 3.5 mA/cm2 for a 50 cm2 photoelectrode.
Various large-area cell designs for were studied, resulting in an optimized design that features an open path for sunlight from the front to the back window, with counter electrodes placed at both sides of the central photoelectrode. CFD simulations were used to ensure an optimal flow path of the electrolyte, resulting in efficient removal of gas bubbles and good thermal management; the temperature of the cell did not increase above 55°C even under 17-suns concentrated light. Based on this design, a modular array of four PEC cells of 50 cm2 each was constructed for field tests on the SoCRatus facility at DLR in Cologne. The cell design showed limited cross-over of H2, but the efficiencies for BiVO4 and Fe2O3 were modest under concentrated sunlight – presumably due to poor carrier transport in these materials.
Two conceptually new innovations were made to further improve the PEC concept. A power management scheme that allows co-generation of electricity and hydrogen; in combination with active light management, the PEC efficiencies can exceed those of PV-electrolyzer systems. The second one is the use of auxiliary NiOOH/Ni(OH)2 electrodes, which avoids the need to separate H2/O2 reaction products within the same cell. This significantly reduces the overall complexity and costs of the concept.
Plant design studies showed that cooling is a crucial issue, especially under concentrated sunlight. Life-cycle analyses revealed that the PEC-PV approach is potentially best in class in terms of global warming potential. Economic analysis showed that PEC-PV generation can compete with PV-driven electrolysis. However, STH efficiencies higher than 8%, solar concentration factors > 30, cell temperatures above 60°C, and active areas approaching 1 m2 should be pursued.
Finally, all PECDEMO targets (10% efficiency, 1000 h stability, 50 cm2) have been individually achieved, but meeting them simultaneously with a single system remains a major challenge to be addressed.

Project Context and Objectives:
1.2.1. Context
Sunlight is by far the largest sustainable source of energy, and there is little doubt that it will play a major role in any conceivable future energy scenario. One of the main challenges for the large-scale use of solar energy is its intermittent nature, which requires intermediate storage solutions. An attractive pathway to achieve this is by directly converting an abundant resource, such as water, into hydrogen using sunlight. The hydrogen can then be used directly as a fuel, or further processed into liquid hydrocarbons. These ‘solar fuels’ have up to 100 times higher energy and power densities than the best batteries and can be stored indefinitely.
PECDEMO aimed at developing a PhotoElectroChemical DEMOnstrator that splits water into hydrogen and oxygen under solar irradiation. By integrating the light absorption and electrolysis functionalities into a single device, significantly lower balance-of-systems costs than coupled photovoltaic-electrolysis systems are, in principle, possible. Efficient and cost-effective solar hydrogen production would thus solve one of the major challenges for a solar-driven society, i.e. that of efficient large-scale storage of solar energy. However, before this dream becomes reality, some hard technological and economic targets have to be met. As outlined in the call that PECDEMO addressed, solar-to-hydrogen energy conversion efficiencies of 8-10% have to be achieved and lifetimes of more than 1000 h need to be demonstrated. Only then will there be a realistic chance to meet the FCH-JU’s cost target of 5 € per kg H2 and can this technology have a significant impact on society.
1.2.2. Approach and main objectives
Building on the breakthroughs achieved in the highly successful EU project “NanoPEC”, PECDEMO partners aimed to develop a module-sized hybrid tandem device for solar water splitting based on a stable metal oxide photoelectrode as a wide-bandgap top absorber and an efficient photovoltaic solar cell as a small-bandgap bottom absorber. Based on earlier work by the partners, three candidates were selected as promising metal oxide photoelectrode materials: Fe2O3, BiVO4, and Cu2O. The stability and durability of the photoelectrodes was planned to be enhanced through functionalization with efficient electrocatalysts, by applying selective transport layers and protective coatings, and selection of suitable electrolyte solutions and operating conditions. The photovoltaic cells were to be optimized for tandem operation with the metal oxide photoelectrodes. Here, silicon-based photovoltaic cells and the emerging class of perovskite PV cells have been selected as the most suitable candidates.
The second aim was to demonstrate the scalability of this technology by combining multiple devices into a larger water splitting module. Nearly all previous efforts in the field of photoelectrochemical water splitting have been done on < 1 cm2 cells, with only very few exceptions. At such small length scales, ion transport between the electrodes is sufficiently fast. At larger length scales, however, resistive losses due to mass transport limitations in the electrolyte quickly start to dominate the overall performance. Innovative cell designs are needed to minimize these losses and to manage the transport of photons, electrons, and ions in the water splitting system.
To achieve the project goals, five science and technology objectives were defined:
1. To demonstrate a chemically stable and highly efficient stand-alone hybrid water splitting cell based on a metal oxide photoelectrode in tandem with a photovoltaic solar cell
2. To develop deposition technologies that are suited for fabricating components for large-area hybrid PEC-PV devices
3. To design, construct, and test complete large-area hybrid PEC-PV devices for water splitting
4. To carry out extensive techno-economic and life-cycle analyses based on the devices’ demonstrated performance characteristics, and evaluate the potential for large-scale commercialization
5. To build a prototype module consisting of an array of large area devices and to test this prototype in the field

Project Results:
see attached pdf
Potential Impact:
1.4.1. Potential impact
By achieving its main project goals, PECDEMO has made an important step forward in the development of efficient, stable, and scalable water splitting concepts. The small-area solar-to-hydrogen efficiencies of up to 9.2% (BiVO4/Fe2O3/Si-HIT) and even 16.2% (Cu2O/3-HIT) are amongst the highest ever reported for this concept, and have put Europe at the forefront of efforts in this field. Moreover, PECDEMO has demonstrated the very first large-area (50 cm2) metal oxide-based PEC-PV water splitting systems that are based on a true tandem design, i.e. with a wide-bandgap absorber in front of a smaller-bandgap PV cell. These activities have attracted the interest of Toyota; as a direct result of the PECDEMO project, one of the project partners (HZB) has recently started a small seed project with (and funded by) Toyota to further explore photoelectrochemical water splitting devices.

Although the project represents a significant step forward, we are still far away from a viable PEC-based technology for solar water splitting. Specifically, the efficiencies for the large-area devices are still modest. Moreover, fulfilling all three requirements (efficiency, stability, and scalability) within a single system remains a major challenge. Nevertheless, with our scaling work we pushed the limit for real application and performed important pioneering work to reveal (and overcome) limitation mechanisms and paved the way for solutions, which are of great importance for future work and coming projects towards commercial PEC-PV applicability.
On the systems level, cooling turned out to be an important aspect that has received little attention in the field. While all these technical issues can be addressed, the inherent complexity of the overall process tends to drive up the costs, and makes it challenging to compete with alternative approaches that make use of mature technologies, such as PV-driven electrolysis. While this can be partly remedied by developing more efficient materials, especially light absorbers, innovative new concepts may be needed in order to achieve the necessary breakthroughs.

PECDEMO has proposed several innovative solutions that may help achieve these breakthroughs. Examples are the PEC-PV power management strategy (i.e. co-generation of electricity and hydrogen) and the auxiliary electrode concept. These concepts have been published in high-ranking journals and are likely to have a significant impact on future efforts in the field. Continued efforts by multi-disciplinary teams consisting of materials scientists, chemical engineers, plant designers, and business developers are needed to further develop photoelectrochemical water splitting into a viable technology that has a substantial impact on society.
1.4.2. Dissemination activities
Dissemination activities concentrated on four tasks:
- To effectively communicate PECDEMO’s innovative research
- To establish and maintain a web database to foster communication within the consortium
- To organize two international workshops
- To conduct outreach activities
For Task 1 the following list compiles some relevant publications from PECDEMO
• J. Luo et al. (2014), Water photolysis at 12.3% efficiency via perovskite photovoltaics and Earth-abundant catalysts, Science Vol. 345/Issue 6204, 26/09/2014 1593-1596
• J.H. Kim et al. (2016), Hetero-type dual photoanodes for unbiased solar water splitting with extended light harvesting, Nature Communications Vol. 7 Nature Publishing Group, 14/12/2016 13380
• Landman et al. (2017), Photoelectrochemical water splitting in separate oxygen and hydrogen cells, Nature Materials N/A Nature Publishing Group, 13/03/2017
• J- Luo et al. (2016), Cu 2 O Nanowire Photocathodes for Efficient and Durable Solar Water Splitting, Nano Letters Vol. 16/Issue 3, American Chemical Society, 09/03/2016 1848-1857
• M-K Son et al. (2017), A copper nickel mixed oxide hole selective layer for Au-free transparent cuprous oxide photocathodes, Energy and Environmental Science Vol. 10/Issue 4, Royal Society of Chemistry, 01/01/2017 912-918
• Tin oxide as stable protective layer for composite cuprous oxide water-splitting photocathodes
• J. Azevedo et al. (2016), Nano Energy Vol. 24, Elsevier Netherlands 01/06/2016, 10-16
• P. Dias et al. (2015), Transparent Cuprous Oxide Photocathode Enabling a Stacked Tandem Cell for Unbiased Water Splitting, Advanced Energy Materials Vol. 5/Issue 24, Wiley 01/12/2015
• J- Luo et al. (2015), Targeting Ideal Dual-Absorber Tandem Water Splitting Using Perovskite Photo voltaics and CuIn x Ga 1- x Se 2 Photocathodes, Advanced Energy Materials Vol. 5/Issue 24, Wiley 01/12/2015
• J. Luo et al. (2015), Solution Transformation of Cu 2 O into CuInS 2 for Solar Water Splitting, Nano Letters Vol. 15/Issue 2, American Chemical Society, 11/02/2015 1395-1402
• L. Steier et al. (2015), Low-Temperature Atomic Layer Deposition of Crystalline and Photoactive Ultrathin Hematite Films for Solar Water Splitting, ACS Nano Vol. 9/Issue 12, American Chemical Society, 22/12/2015 11775-117 83
• J. Azevedo et al. (2014), On the stability enhancement of cuprous oxide water splitting photocathodes by low temperature steam annealing, Energy and Environmental Science Vol. 7/Issue 12, Royal Society of Chemistry, 01/01/2014 4044-4052
• G. Morales-Guio et al. (2015), An Optically Transparent Iron Nickel Oxide Catalyst for Solar Water Splitting, Journal of the American Chemical Society Vol. 137/Issue 31, American Chemical Society, 12/08/2015, 9927-9936
• J. Luo et al. (2016), Bipolar Membrane-Assisted Solar Water Splitting in Optimal pH, Advanced Energy Materials Vol. 6/Issue 13, Wiley, 01/07/2016
• P. Dias, Extremely stable bare hematite photoanode for solar water splitting, Nano Energy Vol. 23 Elsevier, 01/05/2016 70-79
• J. D. Costa et al. (2016), The effect of electrolyte re-utilization in the growth rate and morphology of TiO2 nanotubes, Materials Letters Vol. 171 Elsevier, 01/05/2016 224-227
• Rothschild et al. (2017), Beating the Efficiency of Photovoltaics Powered Electrolysis with Tandem Cell Photoelectrolysis, ACS Energy Letters Vol. 2/Issue 1, American Chemical Society, 13/01/2017 45-51
• G. Segev et al. (2016), High Solar Flux Concentration Water Splitting with Hematite (#-Fe 2 O 3 ) Photoanodes, Advanced Energy Materials Vol. 6/Issue 1, Wiley 01/01/2016
• H. Dotan On the Solar to Hydrogen Conversion Efficiency of Photoelectrodes for Water Splitting, Journal of Physical Chemistry Letters Vol. 5/Issue 19, American Chemical Society, 02/10/2014 3330-3334
• S. Kirner et al. (2016), Architectures for scalable integrated photo driven catalytic devices-A concept study, International Journal of Hydrogen Energy Vol. 41/Issue 45, Elsevier, 01/12/2016 20823-20831
• S. Kirner et al. (2015), Quadruple-junction solar cells and modules based on amorphous and microcrystalline silicon with high stable efficiencies, Japanese Journal of Applied Physics Vol. 54/Issue 8S1, Japan Society of Applied Physics, 01/08/2015 08KB03
• F. F. Abdi et al. (2014), Plasmonic enhancement of the optical absorption and catalytic efficiency of BiVO4 photoanodes decorated with Ag@SiO2 core–shell nanoparticles, Physical Chemistry Chemical Physics Vol. 16/Issue 29, Royal Society of Chemistry, 01/01/2014 15272
• S. Kirner et al. (2016), Wafer Surface Tuning for a-Si:H/µc-Si:H/c-Si Triple Junction Solar Cells for Application in Water Splitting, Energy Procedia Vol. 102 Elsevier BV Netherlands 01/12/2016 126-135
• Zachäus Photocurrent of BiVO 4 is limited by surface recombination, not surface catalysis, Chemical Science Vol. 8/Issue 5 Royal Society of Chemistry, 01/01/2017 3712-3719

In addition, PECDEMO was represented at conferences with oral and poster presentations as listed below (most important)
• HZB, Oral presentation to a scientific event, Direct current magnetron sputtering of photoactive BiVO4: Role of stoichiometry on grain size, structure, carrier mobility and lifetime, 28/11/2016 MRS Fall 2016,Boston, USA
• HZB, Oral presentation to a scientific event, Photoelectrochemical Water Oxidation of BiVO4 Photoanodes with 50 cm2 Active Area 18/04/2017 MRS Spring 2017,Phoenix, USA
• HZB, Oral presentation to a scientific event, Surface and bulk recombination in spraydeposited BiVO4, 07/04/2015 MRS Spring 2015, San Francisco, USA
• EPFL, Oral presentation to a scientific event, Large Scale Cuprous Oxide Photocathode toward PEC-PV Tandem Demonstrator for Solar-Driven Water Splitting – From Design to Characterization, 28/11/2016 MRS Fall 2016, Boston, USA
• EPFL, Oral presentation to a scientific event, Using potential-dependent quantum efficiency measurements to probe device characteristics in photoelectrodes for solar fuels generation, 01/04/2016 MRS Spring 2016, Phoenix, USA
• UPORTO, Oral presentation to a scientific event, Solar Photoelectrochemical Hydrogen – Technological Advancements, 02/12/2016, MRS Fall 2016, Boston, USA
• UPORTO, Oral presentation to a scientific event, Up Scaled Photoelectro chemical Device for Solar Water Splitting-Development and Characterization of a New Design, 02/12/2016, MRS Fall 2016, Boston, USA

Regarding Task 2, the web domain was obtained and a comprehensive project website was built, hosted by EPFL. The site went live on July 15th, 2014. The website features many sections, including “About PECDEMO” (Project Details, Project Description, Consortium), “Partners”, “Activities” (Meetings, Deliverables, Demonstrations), and “Dissemination” (Publications, Presentations). The website was continuously updated with current news and publications from the project. Pictures of the demonstrator device were published on the website on November 30th 2016.

For Task 3, the first goal of organizing an international conference was successfully accomplished by realizing the IPS-20 meeting in Berlin in 2014. The meeting, titled “20th International Conference on Photochemical Conversion and Storage of Solar Energy” was organized by HZB and chaired by Prof. Roel van de Krol. The conference was a great success, attracting over 430 participants from 36 countries and featuring 14 plenary speakers, 19 keynote speakers, and hundreds of contributed talks and posters.
The second part of the task was to organize a symposium on solar fuels conversion at a large international conference. To this end, members of the PECDEMO consortium have co-organized the “Symposium EC4 – Materials, Devices and Systems for Sustainable Conversion of Solar Energy to Fuels” at the “2016 Materials Research Society Fall Meeting” in Boston. The five-day symposium took place November 28 – December 2, 2016, and featured 21 invited speakers, 73 contributed oral presentations, and 21 poster presentations. The four co-organizers were Roel van de Krol (HZB), Avner Rothschild (Technion), Matthew Mayer (EPFL), and Todd Deutsch (NREL), which were able to recruit symposium support by ACS Energy Letters, ACS Publications, Helmholtz-Zentrum Berlin für Materialien und Energie, Journal of Physics D: Applied Physics, IOP Publishing, Nature Energy, and Macmillan Publishers Ltd. The symposium was well-attended and during the presentation of Harry Atwater, the meeting room was even filled beyond capacity. Especially the PECDEMO project was well-represented within the symposium, with 16 oral presentations and 5 posters contributed by members of the project. For detailed information, see the links:
call for papers

Outreach activities were mainly undertaken in the form of teaching. PECDEMO’s main materials of interest (Fe2O3, BiVO4, and Cu2O) and overall approach were extensively discussed during the following courses and summer/winter schools:
• MSc course on “Photo-Electrochemical Energy Conversion”, taught at the TU Berlin in the winter semester of 2014, 2015, and 2016
• Two-hour seminar on “Solar Fuels and Photocatalysis” as part of the MSc course on “Modern Developments in X-Ray and Neutron Methods for Science and Technology“, taught at the Free University of Berlin in 2015, 2016, and 2017
• Seminar (1/2 day) taught in August 2015 for students of the German Academy for Renewable Energy and Environmental Technology (
• QuantSol Summer School, Hirschegg, Austria (September 2015)
• EPFL hosted a one-day research symposium “SwissPEC” on the topic of photoelectrochemical energy conversion, hosted by EPFL on 11 November 2016 In Lausanne

List of Websites:

Prof. Roel van de Krol
Helmholtz-Zentrum Berlin für Materialien und Energie
Institute for Solar Fuels (EE-IF)
Hahn-Meitner-Platz 1,
14109 Berlin, Germany

Tel. +49 30 8062 - 43035
Fax: +49 30 8062 - 42434