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FP7

SOLAROGENIX Report Summary

Project ID: 310333
Funded under: FP7-NMP
Country: Germany

Final Report Summary - SOLAROGENIX (Visible-Light Active Metal Oxide Nano-catalysts for Sustainable Solar Hydrogen Production)

Executive Summary:
On the way to a hydrogen economy replacing fossil fuel energy carriers such as oil, coal and natural gas new complementary methods for a sustainable hydrogen production need to be investigated and tested for their commercial viability. The current production of hydrogen relies to the biggest extent on steam reforming of fossil fuels with CO2 as a side product. For reducing the carbon footprint water splitting via electrolysis using electrical energy from renewable energy sources like wind, photovoltaic and hydro power is an already established technology in commercial scale with different electrolyser designs for varying application conditions. Electrolysis is for example feasible for load leveling in a fluctuating electrical grid by storing excess energy by producing hydrogen, which can be reconverted to electrical energy using fuel cell technology or directly feeding it into the gas pipeline system. Nonetheless with an expected increased hydrogen demand alternative and complimentary technologies need to be brought to a commercial scale as renewable energies cannot supply the baseline of future hydrogen demand.

Photoelectrochemical (PEC) water splitting is able to directly convert solar energy to chemical energy by splitting water in its constituents hydrogen and oxygen with little to none external electrical power, thus providing a potential alternative route for hydrogen production. The main concern in commercializing this technology is to identify efficient, stable, abundant and non-toxic photocatalysts, which can be integrated in a scalable reactor concept and has hindered commercial uptake of PEC technology since its initial discovery more than 30 years ago. The SOLAROGENIX project identified metal oxide photocatalysts consisting of iron, titanium and tungsten, which can be integrated in a PEC module and produce hydrogen and oxygen with sufficient efficiency and good stability at low external bias potential, which can be integrated in a 100 cm2 modular reactor (10-100 times the size of current state of the art photoelectrodes). Besides the synthesis and optimization of materials performance an in-depth analysis of chemical and (photo-)physical properties accompanied with computer simulations and modeling provided fundamental understanding of the photocatalystic properties of the materials. In addition industrial partners provided the module engineering, as well as scaled-up wet-chemical production of identified photocatalysts. These extensive efforts resulted in a conclusive and comparable library of potential binary and ternary metal oxide photocatalysts for future analysis and studies in this field. The availability of larger photoanode modules enabled long-term stability studies as well as estimations on the levelized cost of energy (LCOE) for solar hydrogen produced via PEC water splitting.

With the availability of first modules further optimization of module layout as well as photocatalyst performance need to be conducted and larger PEC reactor assemblies need to be tested for further commercial validation of this technology for a sustainable hydrogen supply.

Project Context and Objectives:
The overall objective of the project was to identify and evaluate stable, abundant and efficient photocatalysts with a low toxicity for solar hydrogen production using photoelectrochemical (PEC) water splitting technology resulting in a solar-to-hydrogen efficiency of more than 5% (photocurrent density: 5 mA cm-2; hydrogen production rate: 20 l m-2 h-1). Therefore metal oxides like TiO2 (anatase, rutile), Fe2O3 (hematite), WO3 and mixtures thereof have been selected as stable, abundant and low- to non-toxic photoanode materials in either basic or acidic conditions. The research activities were limited to n-type photoanode materials and no (photo-) cathodes were optimized but standard platinum and/or nickel counter electrodes have been used. TiO2 was selected as a reference material against which all other materials have been evaluated.

The project was structured in several interlinked work packages to address the different aspects of finding stable, efficient and scalable photoelectrode materials:

- Synthesis of photocatalysts
Nanostructures photocatalysts were produced by either liquid phase (sol-gel) or gas phase synthesis (PECVD). These resulting powders or thin films were assembled on metal plates or transparent conductive materials (FTO, ITO) to photoelectrodes. In addition a scaled-up production of lab-scale optimized photocatalysts was conducted with a pigment manufacturer for testing the photocataltical performance of these "pre-commercial" materials. Besides nanoparticles and thin-films also nanowires and electrospun nanofiber mats have been tested due to their inherent larger surface area. The mechanical as well as electrical integration of such anisotropic structures on electrode substrates (metals, conductive glass) has been proven to be inferior compared to particles and thin-films resulting in lower photocatalystic efficiencies as well as stability.

- Modification of photocatalysts
The resulting binary or ternary photocatalysts were functionalitzed using plasma treatment for creating surface defects and disorder as well as reduced surface states. In addition metallic nanoparticles (plasmonic structures) as wll as dopants have been introduces to enhance the absorption as well as electronic structure of the photoanode materials thus reducing the charge carrier recombination.


- Characterization of photocatalyst materials
A complete and thorough materials characterization has been carried out on all catalyst materials providing essential insight into structure, morphology, electrical and electronic properties as well as composition an impurity levels. These results were also used for comparison with modeling results

- Efficiency and activity determination of photoelectrodes
Using ex-situ.in-situ asn well as in-operando photophysical characherization techniques (e.g. femtosecond spectroscopy, transient absorption spectrioscopy, etc.) a in-depth analysis on the charge carrier life times and recombination methods could be obtained.

- Modeling of materials and materials interfaces
Ab-inito DFT modeling of bulk metal oxides, dopants, defects and interfaces in heterostructures has proven the charge carrier conduction mechanism and effect of different dopants on the light absorption and electron/hole conduction mechanisms.

- Module engineering and testing
A PEC module with an absorber area of 100 cm2 (10 cm x 10 cm) was engineered and fabricated, which allowed to evaluate the device performance at a larger scale (compared to state of the art 1-5 cm2 samples) and adress a potential business case with levelized cost of energy modelling.
Project Results:
- Synthesis of photocatalysts
Nanostructures photocatalysts were produced by either liquid phase (sol-gel) or gas phase synthesis (PECVD). These resulting powders or thin films were assembled on metal plates or transparent conductive materials (FTO, ITO) to photoelectrodes. In addition a scaled-up production of lab-scale optimized photocatalysts was conducted with a pigment manufacturer for testing the photocataltical performance of these "pre-commercial" materials. Besides nanoparticles and thin-films also nanowires and electrospun nanofiber mats have been tested due to their inherent larger surface area. The mechanical as well as electrical integration of such anisotropic structures on electrode substrates (metals, conductive glass) has been proven to be inferior compared to particles and thin-films resulting in lower photocatalystic efficiencies as well as stability.

- Modification of photocatalysts
The resulting binary or ternary photocatalysts were functionalitzed using plasma treatment for creating surface defects and disorder as well as reduced surface states. In addition metallic nanoparticles (plasmonic structures) as well as dopants have been introduces to enhance the absorption as well as electronic structure of the photoanode materials thus reducing the charge carrier recombination.

- Characterization of photocatalyst materials
A complete and thorough materials characterization has been carried out on all catalyst materials providing essential insight into structure, morphology, electrical and electronic properties as well as composition an impurity levels. These results were also used for comparison with modeling results

- Efficiency and activity determination of photoelectrodes
Using ex-situ.in-situ and well as in-operando photophysical characherization techniques (e.g. femtosecond spectroscopy, transient absorption spectroscopy, etc.) a in-depth analysis on the charge carrier life times and recombination methods could be obtained.

- Modeling of materials and materials interfaces
Ab-inito DFT modeling of bulk metal oxides, dopants, defects and interfaces in heterostructures has proven the charge carrier conduction mechanism and effect of different dopants on the light absorption and electron/hole conduction mechanisms.

- Module engineering and testing
A PEC module with an absorber area of 100 cm2 (10 cm x 10 cm) was engineered and fabricated, which allowed to evaluate the device performance at a larger scale (compared to state of the art 1-5 cm2 samples) and adress a potential business case with levelized cost of energy modelling.

Conclusion:
- Metal oxide photocatalysts can be scaled up to module level with sufficient efficiency
- The efficiency as well as stability can be increased dramatically by surface as well as bulk engineering via dopants and core/shell structures
- Validation of published PEC results is necessary due to lacking international standards for publication of measurement data
- A database of PEC data of various metal oxide photoanodes is available for further use
- For commercialization further research and development need to be conducted with larger absorber areas, as well as optimized reactor setups interlinking several PEC cells
Potential Impact:
A stable, abundant, affordable and sustainable energy supply with a low or even non existent carbon footprint is the key for future societal development in order to stabilize and reduce the adverse effects of global warming caused by unobstructed CO2 emissions. All aspects of modern life are directly or indirectly connected to an available energy supply like transportation, health care system, drinking water and/or food supply. Therefore a distributed energy network with diverse renewable primary energy sources is needed, which is capable to buffer fluctuations of individual sources like wind, solar or hydro power, and simultaneously provides sufficient storage capacities for stable grid operation.

As hydrogen is discussed as the main energy carrier replacing fossil fuels like coal, oil and natural gas different alternative methods need to be investigated, evaluated, and validated for commercial application. Currently 60,000,000 metric tons of hydrogen are being produced annually worldwide, and with increasing uptake of fuel cell powered transportation systems and devices this amount is going to increase drastically in the near future. Looking at the present alternatives of hydrogen production 96% of all hydrogen is produced via steam reforming from natural gas, oil and coal with a corresponding carbon footprint:

1. CH4 + H2O --> CO + 3 H2 (steam reforming)
2. CO + H2O --> CO2 + H2 (water gas shift reaction)

The remaining 4% of hydrogen are produced via electrolysis, a potential way for low carbon footprint production when renewable energy sources are used as primary energy sources. Although this mature technology has a stable niche market share for high purity hydrogen production, it cannot be the sole alternative replacing steam reforming as nearly all current renewable energy installations (including hydro power) worldwide would be needed in order to produce the current demand of hydrogen (electrolyzer with 75% efficiency), not speaking of the expected need of an increased hydrogen production capacity in the near future.

Photoelectrochemical (PEC) water splitting allows the direct conversion of solar energy to chemical energy by splitting water in hydrogen and oxygen using suitable photocatalysts with little to none external electrical power (bias). This technology can supplement other hydrogen production pathways if the commercial application can be demonstrated, as in the last 30 years of research a plethora of different photocatalysts have been tested, which have not been capable of combining a high efficiency with long-term stability and suitable abundance (availability and cost). Therefore the SOLAROGENIX project focuses of stable and abundant metal oxide photocatalysts as anode materials of PEC water splitting with the aim of improving the low intrinsic efficiency of these material classes and fabricate a modular PEC reactor design for further scaled-up demonstration in commercial scale.

Due to the nature as an high risk R&D project the main focus regarding dissemination activities lies in publication of scientific results as well as organizing scientific workshops and participate in trade fairs to inform the interested public and experts about the potential use and application of the developed photocatalysts and reactor design.

In total 36 peer reviewed publications have been published till now (available trough Thompson Reuters Web of Science), and few more are in preparation, which will be published in the following 12 months. With an h-index of 7, all published articles combined have been cited more than 160 times from other publications

In addition the involved researchers (PIs, Postdocs and Ph.D. students) have presented SOLAROGENIX results at more than 40 national as well as international conferences and workshops.

The main exploitable result is a comprehensive and complete data collection of engineered photocatalysts (binary and ternary metal oxides, composites and surface modifications), which can be made available to other projects and research activities in the field of PEC water splitting. In addition selected photocatalysts have ben scaled-up on either material synthesis or electrode fabrication level, which is of high importance for further commercialization of this technology.
List of Websites:
Public Website: http://www.solarogenix.eu

Coordinator contact details:
Prof. Dr. Sanjay Mathur
Chair, Inorganic and Materials Chemistry
Department of Chemistry, University of Cologne
Address: Greinstr. 6, 50939 Cologne, GERMANY
Phone: +49 221 470-4107
Fax: +49 221 470-4899
E-Mail: sanjay.mathur@uni-koeln.de
Homepage: http://mathur.uni-koeln.de

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