Project Final Report

Transcription

1 Project Final Report FCH JU Grant Agreement number: Project acronym: PECDEMO Photoelectrochemical Project title: Demonstrator Device for Solar Hydrogen Generation Funding Scheme: FP7-JTI-CP-FCH Date of latest version of Annex I against 07/07/2016 which the assessment will be made: Period covered: From Apr 2014 to Mar 2017 Name, title and organisation of the scientific representative of the project s coordinator: Helmholtz-Zentrum Berlin Prof. Dr. Roel van de Krol Tel: Fax: roel.vandekrol@helmholtz-berlin.de Project website address: PECDEMO is a Collaborative Project co-funded by FCH JU under the call SP1-JTI-FCH GA n : Start date: April 1 st, Duration: 36 months.

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3 TABLE OF CONTENTS 1.1. Executive summary Summary description of the project context and the main objectives Context Approach and main objectives Description of the main S & T results/foregrounds Work Package Work Package Work Package Work Package Work Package Work Package Potential impact (including socio-economic impact and wider societal implications) and the main dissemination activities and exploitation of results Potential impact Dissemination activities Public website and relevant contact details... 33

4 1.1. 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 cm 2. 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, nanostructuring, 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/cm 2 for a 50 cm 2 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 cm 2 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 m 2 should be pursued. Finally, all PECDEMO targets (10% efficiency, 1000 h stability, 50 cm 2 ) have been individually achieved, but meeting them simultaneously with a single system remains a major challenge to be addressed. 1

5 1.2. Summary description of the project context and the main objectives 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 largescale 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 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 smallbandgap 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 cm 2 cells, with only very few exceptions. At such small length scales, ion transport between 2

6 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 3

7 1.3. Description of the main S & T results/foregrounds Work Package 1 The first work package (WP1) focuses on the development of an efficient and chemically stable small-area hybrid PEC-PV water splitting device, by utilizing metal oxides as the PEC materials. Three metal oxides have been chosen to be our main focus: iron oxide (Fe2O3), bismuth vanadate (BiVO4) and cuprous oxide (Cu2O). Activities in the last three years include the development of: (1) small-area metal oxide photoelectrodes, (2) optically-transparent counter electrodes, (3) PEC-PV without photon management (1 st generation device), (4) effective light management strategies, and (5) PEC-PV including photon management (2 nd generation device). The summary of these activities and achievements within WP1 is as follows. Fe2O3 photoanode Prior to PECDEMO, the champion Fe2O3 photoelectrode utilized a resonant light trapping structure, by depositing hematite on Ag-coated glass substrates. 2 One of the limitation, however, is the oxidation of Ag during the high-temperature deposition of hematite, which in turn loses its specularity. In order to overcome this, within PECDEMO, we reversed the order of deposition of the films, so that the Ag layer is deposited after hematite. In short, hematite was deposited on a Si substrate, and a Ag layer was deposited on top of this hematite film. The film is then flipped over and attached to another Si substrate. Finally, the first Si substrate was removed by dry etching, thereby exposing the hematite layer at the surface. This process Figure 1. Photographs of (a) the champion ultrathin films hematite photoanode from 2013 and (b) one of the most recent photoanodes obtained by the film transfer process. (c) Off-axis φ scans of Al2O3(104), Fe2O3(104), and Pt(200) reflections. therefore allows Ag layer to be deposited at low temperature and in vacuum, which is ideal for specular mirror deposition. The improved results can be seen in the photographs shown in Figure 1a and b. In addition, we have also developed heteroepitaxial hematite photoanodes with high crystalline quality. We first deposited a platinum layer (as a bottom contact and a reflector) on top of (0001) basal plane sapphire, followed by growth of the hematite layer with pulsed laser deposition. The in-plane alignment of the film stack is investigated by azimuthal φ-scans of the off axis peaks as shown in Figure 1c and verifies the epitaxial growth of the layers. We are still in the process of optimizing the heteroepitaxial films for photoelectrochemical performance, and have achieved photocurrents of 1.8 ma/cm 2. Noteworthy, the flat-band potential of heteroepitaxial Ti-doped hematite films was found to be ~0.2 VRHE, considerably lower than reported values for polycrystalline hematite photoanodes that typical range between 0.4 and 0.6 VRHE. 3 This may open up another route to reduce the potential of hematite photoanodes. Our efforts in heteroepitaxial hematite can be found in our recent publication. 4 4

8 BiVO4 photoanode We have explored the possibility of depositing BiVO4 with various techniques, such as drop casting and reactive magnetron sputtering, but the performance of our Co-Pi catalysed, spray-pyrolysed, W-doped BiVO4 photoanode is still superior. 5 At the beginning of PECDEMO, in the efforts of transitioning towards large scale BiVO4, we have also improved the reproducibility of our spray pyrolysed BiVO4; we have now a sample-to-sample photocurrent reproducibility of ~5-10%. To improve the photocurrent further, we have explored several other dopant. Calcium-doped BiVO4 was first studied in order to produce a p-type conductivity, but the improvement is limited due to Figure 2. (a) AM1.5 photocurrent-voltage curves of W-doped BiVO4 (black), hydrogen-treated W-doped BiVO4 (blue), and dual hydrogentreated W-doped BiVO4 (red). The respective labels indicate the photocurrent value at 1.23 V vs RHE. The schematic of dual BiVO4 photoelectrode is shown in (b). segregation of calcium. A successful treatment is to anneal BiVO4 in a mild hydrogen atmosphere (2.4% H2 in Ar) at relatively low temperature of 300 C. The onset potential and plateau photocurrent were both improved by hydrogen treatment, as shown in Figure 2a. An AM1.5 photocurrent of 4 ma/cm 2 was achieved at 1.23 V vs RHE for a hydrogen treated W-doped BiVO4. At this point, we were limited by the absorption in our thin film BiVO4. The absorption can be simply improved by increasing the thickness of the film, but unfortunately the carrier diffusion in BiVO4 (< 100 nm) does not allow efficient carrier transport in a thick film. We therefore implemented a dual photoelectrode approach (see Figure 2b), where an additional BiVO4 photoelectrode was placed behind the same BiVO4. This simple approach which is commonly used in the field of organic PV but not to large extent in PEC water splitting resulted in further photocurrent improvement to 4.8 ma/cm 2 at 1.23 V vs RHE and > 5.4 ma/cm 2 at 1.7 V vs RHE. This satisfied the deliverable 1.1 (first generation metal oxides with a photocurrent of at least 5.4 ma/cm 2 ) of PECDEMO. The 20% photocurrent improvement (i.e., from 4 to 4.8 ma/cm 2 ) can also be replicated by implementing a photon management strategy: we deposited a distributed Bragg reflector (DBR) at the back-side of the substrate and therefore removing the necessity of having dual photoelectrode (deliverable 1.3 proof of enhanced performance using a photon management strategy). Cu2O photocathode In the case of Cu2O photocathode, we have focused on developing a transparent Cu2O photocathode toward the ultimate goal of designing an efficient PEC-PV stacked tandem configuration. For the efficient transparent Cu2O photocathode, we optimized the thicknesses of Au underlayer and Cu2O flat film (see Figure 3a for the structure of the photoelectrode), because these are crucial 5

9 Figure 3. (a) False-color cross-section SEM image of a Cu2O-based photocathode, indicating the different underlayers and overlayers. (b) PEC performance of Cu2O photocathode with different thicknesses on 3 nm Au underlayer substrates. (c) Scanning electron microscope image of Cu2O nanowire with AZO/TiO2 overlayers. Inset shows transmission electron image of a single composite of Cu2O/AZO/TiO2/RuO2 nanowire. (d) Linear sweep voltammetry scan under chopped illumination (1 sun) in the ph 5 electrolyte of Cu2O nanowire photocathode with AZO (black) and Ga2O3 (red) overlayers. parameters affecting the incident light to the PV device placed behind the photocathode. Consequently, a photocurrent density was reached up to 5.4 ma cm - 2 at 0 V vs RHE (deliverable 1.1 PECDEMO) in ph 5 electrolyte under 1 sun condition using 3 nm Au underlayer and 260 nm Cu2O thickness (Figure 3b). This performance was comparable to that achieved on standard photoelectrodes on thick Au substrates, with the advance that the device showed transmittance of around 35 % for wavelengths longer than 550 nm. We improved the transparency further by developing an alternative underlayer in the form of Cu-doped NiO. The transmittance was further increased by ~10-20 % for wavelengths longer than 550 nm, while maintaining the PEC performance. The performance of transparent Cu2O photocathode was still low compared to the theoretical performance. It is mainly attributed to the imbalance of the carrier diffusion length and the light absorption depth of Cu2O. 6,7 Nanostructuring is a promising way to solve this problem, enabling the further improvement of Cu2O photocathode performance. We recently succeeded to prepare well characterized Cu2O nanowire array photocathodes through electrochemical anodization and thermal annealing. The Cu2O nanowire arrays were high-quality and pure and were adapted into the photocathode device structure by depositing AZO/TiO2 overlayers and RuOx catalyst (Figure 3c and d). Remarkably, photocurrent densities up to 8 ma cm -2 at 0 V vs RHE were reached in ph 5 electrolyte under 1 sun condition, with 6

10 photocurrents exceeding 10 ma cm -2 at more negative potentials. With the photocurrent level improved, we finally enhanced the performance of Cu2O photocathode by replacing the AZO overlayer with Ga2O3. Ga2O3 has a minimal conduction band offset with Cu2O, resulting in a maximized built-in potential and an anodic shift of the onset potential up to 1.0 V vs RHE, as shown in Figure 3d. Our improvement efforts on Cu2O photocathodes can be found in our recently published articles PEC and PV stability The stability of our photoelectrodes have also been investigated under long-term AM1.5 exposure. Fe2O3 photoanode is known to be highly stable, and we show for the first time a stability data of up to 1000 hours (See D1.4), achieving our deliverable 1.4 target (less than 10% performance decrease after 100 hours of operation). No noticeable degradation was observed; full details on this study were recently published. 11 BiVO4 photoanode is expected to be stable in neutral ph electrolyte, but we observed a photocurrent decrease within the 100-hour measurement period (See D1.4). The decrease is, however, not related to material degradation, but due to suboptimal PEC cell design. Bubbles formed rapidly and trapped at the surface of BiVO4, causing decreased effective surface area. More optimal cell design is expected to fully resolve this issue. In alkaline environment, protection layer consisting of TiO2 and Ni successfully improves the stability, although it is still limited to less than one hour. Cu2O photocathodes stability is shown to be enhanced with the protection layer strategy that we developed. Although 10% performance decrease is observed within ~55-60 hours, the improvement in stability is unprecedented for Cu2O photocathodes. For the PV, perovskite solar cell shows increasing efficiency within the first 500 hours of measurement, with no noticeable change of efficiency afterwards, up to more than 2000 hours of operation. HIT silicon solar cell shows stable short-circuit current and open-circuit potential within 100 hours, and a slight decrease of fill factor is observed. Overall, the efficiency decreases only by less than 4%. Detailed description of our stability tests and results have been published in a public deliverable 1.4 report. PEC-PV devices We then combined the photoelectrodes and PV cells developed within PECDEMO to form a hybrid tandem water splitting device. First, for the first generation device (i.e., stacked tandem configuration), we have initially simulated the expected Figure 4. (a) Schematic setup of our 2 nd generation PEC-PV tandem configuration, consisting of a Cu2O photocathode, an IrO2 anode, a HIT solar cell and a dichroic mirror. (b) Chopped AM 1.5 shortcircuit photocurrent density and calculated STH efficiency of the real PEC-PV tandem device. The green dashed line is the 10% STH efficiency target. 7

11 solar-to-hydrogen (STH) efficiency from a combination of our photoelectrodes and multiple multi-junction solar cells. Based on the simulation, we then fabricated a solar water splitting device consisting of the dual BiVO4 photoelectrode described above (see Figure 2a) and a 3-HIT series silicon solar cell delivering an STH efficiency of 7.5% (Figure 5a). To improve the efficiency further, we attempted to address the fundamental limitation of BiVO4, which is the relatively large bandgap of 2.4 ev. While our efforts in decreasing the bandgap of BiVO4 is still ongoing, we have simply extended the light harvesting ability of the dual photoelectrode by combining a BiVO4 and a Fe2O3 photoanode (Figure 5b). As a result, we obtained STH efficiency of 9.2%, which fulfils the deliverable 1.2 target (1 st generation device showing 8% efficiency) and is already very close to the deliverable 1.5 target (2 nd generation device showing 10% efficiency). Detailed results on this dual BiVO4-Fe2O3 photoanode can be found in our recent publication. 1 Figure 5. Schematic illustration of a dual (a) BiVO4-BiVO4 and (b) BiVO4-Fe2O3 photoelectrode used in tandem configuration with a silicon solar cell. STH efficiencies of 7.5% and 9.2% 1 have been achieved with these configurations, respectively. Finally, for our 2 nd generation device, we assembled a hybrid PEC-PV tandem cell employing a Cu2O Ga2O3 NW photocathode (see Figure 3d), a HIT PV cell and an IrO2 anode. For this demonstration, we adopted the tandem configuration with a 600 nm cut-off wavelength dichroic mirror (as shown in Figure 4a). As a result, we observed operating AM1.5 short-circuit photocurrent of 6.98 ma/cm 2 with 2 HIT PV cell and ma/cm 2 with 3 HIT PV cell (Figure 4b). The corresponding STH efficiencies are 10.3 % (2 HIT PV) and 16.2% (3 HIT PV). This result therefore successfully fulfil our deliverable 1.5 target, as well as the final PECDEMO WP1 target of a device showing efficiency larger than 10%. Additional information on these PEC-PV devices can be found in our public deliverable 1.5 report (see PECDEMO website) Work Package 2 The second work package (WP2) aims to guide the optimization efforts of PEC-PV tandem cells (WP1) and modules (WP4) by identifying material degradation processes and efficiency losses; quantifying their effect on the long-term stability and efficiency; scrutinizing materials compatibility for stable long-term operation with minimal degradation and efficiency losses; and optimizing the optical and electrical coupling of the PEC and PV cells. WP2 has five main tasks. 8

12 Task 2.1: Develop and implement diagnostic methods to identify materials degradation processes. This task involved work on Cu2O photocathodes which are known to be intrinsically unstable in aqueous electrolyte solutions, as well as on BiVO4 photoanodes which, although kinetically stable in neutral aqueous electrolyte solutions, are unstable in either acid or base solutions. Fe2O3 photoanodes are thermodynamically stable in base (alkaline) aqueous electrolyte solutions and typically they do not degrade even after repeated long-term operation in alkaline solutions. The efforts to stabilize Cu2O photocathodes and BiVO4 photoanodes were carried out by EPFL and HZB, using passivation overlayers deposited by ALD, as described in the WP1. Here in WP2 we present our efforts to diagnose the effectiveness of the passivation overlayers. In order to identify the root cause for Cu2O and BiVO4 photoelectrode degradation, different techniques were used such as: electrochemical measurements, SEM, TEM, XPS and more. An example of Cu2O photocathode characterization is presented as Figure 6. The TiO2 overlayer is shown by the green color in the TEM image. Figure 6. Cu2O photocathode characterisation by SEM (Left), TEM (right) and electrochemical measurement (inset). These characterisation techniques enable us to develop efficient passivation overlayers, which supress the degradation process in Cu2O and BiVO4 photoelectrodes and achieve high stability as published recently in Nano Letters 9 and Nature Communication 5. Further information on these characterization techniques can be found in these publications. Task 2.2: Develop and implement diagnostic methods to identify and quantify efficiency losses. Within this task, two new diagnostic methods were developed in order to identify and quantify efficiency losses due to charge separation and recombination processes. Operando diagnostics of Fe2O3 photoanodes, carried out at IIT, provides the photocurrent and photovoltage generated by the photoanode, as presented in Figure 7. 9

13 Figure 7. A typical current density (J) vs. potential (U) voltammogram of thin film Fe2O3 photoanodes measured in 1M NaOH aqueous solution in the dark (dashed) and under AM1.5G solar simulated illumination (full). This information is used to extract the maximum power point and the intrinsic solar to chemical conversion (ISTC) efficiency of the photoanode. This information is important for the design of the electrical coupling between the PEC and PV cells (Task 2.4). Further information about this diagnostics can be found in the Journal of Physical Chemistry Letters 12. Photoelectrochemical spectroscopy combines PEIS (Photo Electrochemical Impedance Spectroscopy), IMPS (Intensity Modulated Photocurrent Spectroscopy) and IMVS (Intensity Modulated Photovoltage Spectroscopy) to provide new insights on the charge carrier dynamics involved in the water photo-oxidation process. This method was applied at IIT to investigate charge carrier dynamics in Fe2O3 photoanodes and allowed us to quantify the hole current reaching the hematite electrolyte interface and the recombination current at interface as presented in Figure 8. Further information on this method can be found in Physical Chemistry Chemical Physics 13. Similar analysis was carried out for BiVO4 photoanodes at HZB. We investigated the effect of Co-Pi catalysts on the surface of BiVO4. It has been shown by many research groups including HZB that Co-Pi effectively improves the performance of BiVO4 photoanode, due to an increased charge injection efficiency. However, the true nature of the improvement is not clear. Our IMPS study revealed that the surface recombination rate decreases by more than 1 order of magnitude upon deposition of Co-Pi on BiVO4 surface. Surprisingly, the charge transfer rate is not really affected by the introduction of Co-Pi; in fact, it is slightly decreased. This result is intriguing since it implies that Co-Pi does not function as a true catalyst when deposited on the surface of BiVO4. Instead, it acts as a surface passivation layer, Figure 8. Deconvolution of the water photo-oxidation current (black dots) of Fe2O3 photoanode into the hole current (red dots) and recombination current (blue dots). 10

14 reducing the surface recombination. Further information on this study and the implication can be found in our recent Chemical Science article 14. Task 2.3: Modeling the optical coupling of the PEC and PV cells in PEC-PV tandem cells. Detailed modeling of the optical coupling between the PEC and PV cells in PEC-PV tandem cells was carried out for the conventional stacked cells configuration as well as for advanced configurations that employ spectral splitting between the PEC and PV cells in order to improve the solar to hydrogen conversion efficiency of the tandem cell. For example, at HZB we modeled and tested BiVO4 photoanodes coupled in tandem Figure 9. The EQE curves of the a-si:h/nc-si:h solar cell, the a-si:h top junction (black), and the nc-si:h bottom junction (red). Dashed curves indicate EQE spectra of the PV junctions under full AM 1.5 simulated solar illumination; solid curves indicate the EQ with amorphous and nano-crystalline silicon PV cells. The results are presented in Figure 9. Further information can be found in recent articles published by HZB 1,15,16 and EPFL 17. Several spectral splitting schemes with passive or active light management were explored in the PECDEMO project, in order to tailor the degrees of freedom of the tandem system to make it more efficient (but also more complicated). One example Figure 10. Active light management design in a two-absorber tandem system. A) Absorber 1 (PEC) is active ( on ). B) Absorber 1 (PEC) is inactive ( off ). of an active light management design is presented in Figure 10. When the PEC cell is active, during hydrogen generation, the incident light is splitted between the PEC (Absorber 1) and the PV (Absorber 2) as presented in the left figure (A). On the other hand, if only power generation is required the system turns to its off state, presented in the right figure (B). This active light management design allows the system to actively control the portions of the light that go to the PEC and PV cells in order to tailor hydrogen and power productions (see Task 2.4 below). This scheme is especially attractive for tandem cells that co-generate both hydrogen and electrical power as explained in ACS Energy Letters

15 Task 2.4: Modeling the electrical coupling of the PEC and PV cells. Modeling and testing the electrical coupling of the PEC and PV cells started from simple coupling by series connection of the two cells 1, This scheme leads to a large loss of electric power generated by the PV cell, which is typically much larger than the power used by the PEC cell. In order to rectify this loss, we invented a new design that splits the power generated by the PV cell between the PEC cell and another consumer, i.e., co-generation of hydrogen and power. The electrical power can power auxiliary systems such as cooling, flow and compression, or be sold to the grid. Further optimization can be achieved using power convertors that enable continuous tracking of the maximum power point of the PEC-PV tandem system. Applying the power splitting approach enables to Figure 11. Calculated figure of merit (FOM, the ratio between the total power produced by the PEC-PV tandem system and the power produced by the PV system alon) as a function of the fraction of the total power production that goes toward chemical power generation overcome the efficiency of PVelectrolysis systems, as presented in Figure 11 and explained in ACS Energy Letters 18 Task 2.5: Predictive modeling of large-area PEC-PV tandem cells. The series resistance loss due to the resistance of the transparent electrode that collects the photocurrent from the photoelectrode in the PEC cell becomes critical in large-area PEC-PV tandem cells. Figure 12 presents an analysis of the effect of the series resistance on the photocurrent as a function of the size of the photoelectrode. The analysis was done for a transparent electrode with a sheet resistance of 15 /square, which is typical for FTO-coated glass substrates (TEC15). It clearly shows the adverse impact of the series resistance on large area cells. This effect must be rectified using metallic grid lines, as implemented in WP4. Solar plant design. One of the greatest challenges in large-scale solar water splitting plants is the separation of the hydrogen from the oxygen and the collection and transport of the hydrogen from millions of PEC cells distributed in the solar field to a central hydrogen distribution facility. This involves an immense sealing and piping constructions that puts a heavy J (U=1.5 V vs RHE) [ma] Photoanode Radius [cm] Figure 12. Calculated photocurrent density as a function of the photoanode radius for a series resistance of 15 /square. 12

16 burden on the hydrogen production economy. To overcome this challenge we invented a new PEC cell design with separated oxygen and hydrogen cells. According to this design, the PEC solar cells produce oxygen that is simply discharged to the atmosphere, whereas the hydrogen is produced elsewhere in another cell. The ion exchange between the two cells is mediated with a set of auxiliary electrodes made of NiOOH and Ni(OH)2 that undergo a reversible redox reaction exchanging OH - ions with the primary electrodes (i.e., the photoanode and cathode) in the oxygen and hydrogen cells. This enables centralized hydrogen production far away from the solar field, as explained in Nature Materials Work Package 3 Fabrication of large area TCO-coated glass substrates. In order to fabricate large-area transparent conducting oxide (TCO) electrodes spray pyrolysis based fluorine-doped tin oxide (FTO) deposition was developed. This process allows to prepare custom-made conductive substrates with the resistivity and transmittance directly related to the FTO thickness deposited (Figure 13). It is a flexible process that allows coating a large variety of substrates as long as the raw material is able to withstand the high temperatures. TCOs on up to 35 x 35 cm² were deposited and characterized including double-side coating, e.g. suitable for tandem devices. Fabrication of large-area photovoltaic (PV) cells A mesoscopic methylammonium lead iodide perovskite/tio2 heterojunction solar cell with low-cost carbon counter electrode and full screen printable process was built on a monolithic design Figure 13. Sheet resistance of FTO with various thicknesses. Insert: Examples of FTO coated glass. without any extra organic hole conducting material (Figure 14). These cells were claimed to be air stable under illumination. Such a perovskite, fully printable mesoscopic solar cell was deposited on an FTO covered glass. The mesoporous layers were infiltrated with perovskite by drop-casting from solution through an 8 µm thick carbon layer printed on top of the ZrO2. The dense TiO2 layer deposited on the FTO Figure 14. Left: SEM cross section of a monolithic perovskite test cell. Right: Perovskite PV Module conducting glass prevents the valence band holes from reaching the FTO-covered front electrode. With a complete set of new screen printable materials: (1) Ti-Nanoxide 13

17 BL/SP (50 nm TiO2 based electron blocking layer); (2) Ti- Nanoxide T600/SP (600 nm nano-tio2 scaffolding layer); (3) spacer layer: Zr-Nanoxide ZT/SP (1 µm nano-zro2 layer); (4) highly conducting carbon layer: Elcocarb B/SP (5 ~10 Figure 15. Cross section of PE + PV tandem device with two interconnected Silicon heterojunction cells. µm, 10 ~ 20 ohm/sq resistivity) cell efficiencies over 13 % were achieved on lab scale (1.2 cm 2 ). Monolithic modules (64 cm 2 aperture area) with an optimized design to limit the electrical losses showed an efficiency of 11% (Jsc =16.3 ma/cm 2, Voc = 7.1V). The perovskite deposition process was automated to improve safety, reproducibility and productivity. By means of a DC-DC converter the appropriate voltage for biasing the photo-electrode can be obtained. For direct integration in PEC+PV tandem devices silicon heterojunction (SHJ) modules were developed, fabricated and analyzed, e.g. upon various illumination condition conditions. Such SHJ based modules offer the advantages that high PV efficiencies exceeding 20% are possible and the excellent near-infrared light absorption and high voltage makes it appropriate for application as bottom cell in such a tandem configuration with a wide-gap photo-electrode (PE) in front. In order to match the required voltage to bias the PE two PV cells have been interconnected (Figure 15). Modules reached efficiencies of 15.7% under one sun (Figure 16), which allow for STH efficiency of complete PEC-PV tandems devices exceeding 8%. area Sample (cm²) (%) J sc V oc FF V mpp I mpp (ma/cm²) (V) (%) (V) (ma) Median of 14 cells (reference) Best 5x5 cm² cell cut out of wafer Best 2-cell module Module with hematite PE Figure 16. Left: 50 cm² PV module consisting of two Si cells. Right: IV date of SHJ cells and modules Evaluation of deposition techniques for fabrication of counter-electrodes Spray coating, a well-known method to prepare thin particle layers on substrates (e.g. varnishing), was adapted for fabrication of counter-electrodes for the electrochemical hydrogen and the oxygen evaluation reaction (HER/OER). An automated setup was built with an airbrush moveable in two axis. The developed ink contains the catalyst, a volatile solvent mixed of water/ethanol and an additional ionconductive binder on basis of Nafion. Different catalysts (iridium dioxide, platinum on carbon, Evonik OER rare metal free) were deposited in different loadings on a porous nickel foam (1.6 mm thickness) as substrate. Ex-situ testing was done in a threeelectrode setup similar to the common rotating disk electrode (RDE) technique. For measurements at high current densities, electrodes were mounted to a motor, so that the catalyst coated porous substrate can rotate around its own axis. This leads to an efficient electrolyte flow right through the electrode enhancing the mass transport, especially the removal of the produced gas. The polarization curves in Figure 17 show that the spray-coated electrodes are suitable for the electrochemical water splitting. 14

18 Beside the known good activities of state-of-the-art catalysts is the activity of a rare metal free catalyst similar to IrO2. Figure 17. Polarization curves of deposited counter-electrodes with OER-catalysts (left) and HERcatalyst (right). Electrodes were made by spraying a catalyst ink on a nickel foam as porous substrate. The measurements were done in a potentiostatic mode vs. an Ag AgCl 3M KCl reference in 1M KOH at 25 C. At each point, the ohmic cell resistance was determined and IR correction of applied potential was performed. The geometrical area of the deposited electrode was 1 cm². Large-scale Cu2O photocathode Large-area, transparent photocathode consisting of electrodeposited Cu2O thin film, Figure 18. (a) A transparent large-scale AZO overlayered Cu2O photocathode with Cu grids. (b) A transparent large-scale Ga2O3 overlayered Cu2O photocathode with an active area of 5 x 10 cm 2. (c) Linear sweep voltammetry scans in the ph 5 electrolyte under 1 sun illumination from LED light source of large-scale Cu2O photocathode with AZO overlayer (black), AZO overlayer/cu grids (blue) and Ga2O3 overlayer (red). atomic-layer deposited AZO/TiO2 over-layers and photo-deposited RuOx catalysts were deposited on up to 5 x 10 cm 2 area. For enhanced charge collection we applied metal grids and contacts on the Cu2O photocathode. By optimizing metal grids and contacts on edges, we demonstrated that sputtered Cu grids and Paste-based Ag contacts on edges are effective to improve the charge collection on the large-scale Cu2O photocathode. Especially, Ag contacts on edges improved the resistive PEC performance, while Cu grids assisted to enhance photocurrent density. We could get a photocurrent density of 3.7 ma cm -2 at 0V vs RHE, corresponding to a STH efficiency of 4.6 % using Cu grids and Ag contacts in ph 5 electrolyte under 1 sun illumination from LED light source (Figure 18 a, c). We finally introduced the Ga2O3 overlayer to the 15

19 large-scale Cu2O photocathode instead of AZO overlayer. Consequently, the optimized Ga2O3 film by atomic layer deposition was homogeneous on the largescale, resulting in an anodic shift of the onset potential up to 0.9 V vs RHE. (Figure 18 b, c). Large-scale W:BiVO4 Photo-anodes Large-scale photo-anodes consisting of TEC 15 TM FTO substrate with a electrodeposited Ni grid, a spray pyrolysis deposited thick 1% Tungsten doped BiVO4 (W:BiVO4) absorber (~ 200 nm), and photo-deposited CoPi catalyst were fabricated on up to 7 x 12 cm 2 area, with an active area of 5 x 10 cm 2. In order to limit the ohmic losses, 200 nm thick and 2 mm wide Ni lines were electrodeposited onto treated FTO substrates spaced 9 mm apart prior W:BiVO4 deposition, and Paste-based Ag contacts and Cu tape were placed along the edges. As with the large scale Cu2O photocathodes, the Cu contacts on edges improved the resistive PEC performance, while Ni grids assisted to enhance photocurrent density. We finally introduced a rapid annealing step of the W:BiVO4 photoanodes in 2% H2 Ar 98% atmosphere at 320 o C for 10 mins which shifted the photocurrent onset potential from ~0.4 V to 0.3 V vs RHE and a maximum photo current density of 1.8 macm -2 at 1.25 V vs RHE can was achieved with W:BiVO4 without CiPi in ph M KPi, 0.5M Na2SO3 (hole scavenger) electrolyte Figure 19. W:BiVO4 photoanode deposited on to TEC 15 TM FTO with Ni gridlines b) without Ni gridlines, with the edge coated with Paste-based Ag contacts and Cu tape, protected with Kapton tape. c) Photocurrent densities against potential vs. RHE, in a 3 electrode configuration (WE, CE and Ref.) under 1 sun, in hole scavenger electrolyte (1.0 M KPi, 0.5M Na2SO3). Presented is a comparison of the performance of the W:BiVO4 photo-anodes for a small scale sample (black), a 50 cm2 sample without gridlines (blue), with gridlines (green) and with gridlines + hydrogen annealing (red). (Figure 19). For the optimised large area W:BiVO4 photoanodes with CoPi catalyst and an active area of 5 x 10 cm 2 we could achieve a photocurrent density of 1.5 macm 2 at 1.23 V vs RHE corresponding to a STH efficiency of 1.85 %, using TEC 15 FTO, Ni gridlines, and Cu contacts in a ph 7, 2.0 M KPi electrolyte, with a 3 electrode setup, under 1 sun illumination from a quartz tungsten halogen lamp (Figure 19). 16

20 Large area integrated PV-PEC devices were fabricated consisting of a 2 x HIT Silicon module with an area of 5 x 10 cm 2 and one large scale CoPi coated W:BiVO4 photoanodes as the front window of the electrochemical cell. The integrated PV-PEC device achieved an initial photocurrent density of 1.20 macm -2 at a potential of 1V vs RHE, corresponding to a STH efficiency of 1.48 % (Figure 20). Although lifetime studies for a range of samples showed that over 24 hrs the photocurrent density of the integrated cell decreased and reached a plateau of ~0.6 macm -2 at a potential of 1.1 V corresponding to a STH efficiency of 0.74 %. It is proposed that the loss in photocurrent is due to a combination of, the degradation and loss of CoPi catalyst and, the partial degradation W:BiVO4, which visibly remained on the photoanodes after weeks of testing. Figure 20. a) The voltage and current overtime for the integrated PV-PEC device under 1 sun whilst the 2.0 M KPi ph: 7 electrolyte is stirred. d) Photo of the Large scale PV-PEC cell consisting of 2HIT silicon PV s, two platinum coated counter electrode and a CoPi coated W:BiVO4 photoanode with an active area of 50 cm Work Package 4 Within PECDEMO, WP4 was focused on the design, optimization and assessment of a 50 cm 2 PEC cell for efficient water splitting and on evaluating its potential use under concentrated solar radiation. To accomplish such goal four main tasks were initially identified: the development of both angled and vertical PEC devices (Tasks 4.1 and 4.2); their adaptation for use under concentrated solar radiation conditions (Task 4.3); and, by the end of the project, the selection of the optimal device design (Task 4.4). The key challenge of designing a 50 cm 2 PEC Figure 21. Angled (left) and vertical (right) PEC devices developed within task 4.1 and task 4.2, respectively. device is to significantly reduce ionic and electronic resistances keeping the cell/unit price competitive. These resistances depend on the geometry and volume of the cell, which were optimized to have the lowest ohmic losses. As mentioned, WP4 strategy relied on the development of two different designs: i) the so-called angled PEC cell comprising an innovative system to separate the evolved gases, with exemption of the vertical diaphragm and where the photoelectrode may be back or front illuminated; and ii) the so-called vertical PEC cell with a vertical diaphragm, and where a zero gap distance between electrodes was pursued. Figure 21 displays the angled and vertical PEC devices developed within tasks 4.1 and 4.2, respectively. 17

21 Figure 22. Angled PEC cell: a) disassembled in 3D project, b) final device. 1 acrylic body; 2 Teflon membrane; 3 acrylic cap with separated chambers for oxygen and hydrogen collection; 4 front side black metallic frame; 5 electrolyte inlet; 6 right side electrolyte outlet; 7 one group of acrylic plates. Angled PEC Cell Design The developed angled PEC cell took into account a set of important requirements in its design and construction: i) ionic and electronic resistances were minimized to attain a maximum voltage drop of 50 mv; ii) the cell weight and size were optimized; and iii) its capability to operate under different working conditions (temperature, irradiances and tilted positions) assured. Based on previous assumptions, the PEC cell detailed in Figure 22 was designed and built. The cell body is made of transparent acrylic assuring resistance to corrosive electrolytes and a very acceptable range of operating temperatures 20. Two metallic frames (front and back) screwed to the acrylic embodiment against an O-ring assure the proper sealing of the electrolyte container. The one placed in the front side is lacquered in black allowing an illumination area of 5 10 cm 2 through a transparent glass or synthetic quartz window Figure 22-a4. The interior side of this front window is spin-coated with a thin film of TiO2, developed at UPorto, which allows the evolved gases bubbles to easily slip over the cell s window increasing the amount of light reaching to the photoelectrode up to 9 %. 21 Another important feature of this cell is that the photoelectrode works simultaneously as the cell back window, allowing a narrower construction. Thus, a a) b) conductive path Figure 23. Scaled-up photoelectrode assembly for the angled PEC between both sides of cell: a) photo of the in-house printed Ag metal frame on the front the TCO substrate was and back sides of the TCO glass substrate; b) 5 10 cm 2 bare developed and hematite photoelectrode ready to be assembled in the back side of implemented without the angled PEC cell. changing the dimensions of the housing area. To fulfill this requirement a conductive silver frame was printed in both sides of the substrate and in its lateral area. Thus, the back metallic frame that holds the photoelectrode works as external contact. Such strategy was never reported before Figure 23. By placing the metallic counter-electrodes side-by-side to the photoelectrode it is possible to have an open path for the sunlight to reach the PV cell in the backside of the photoelectrode in a tandem configuration. The cell is sealed with a transparent acrylic cap screwed on the top. This cap has three separated compartments, two for hydrogen collection (internally connected) and one for oxygen collection Figure 22-a3. External tubes can be directly connected to these cambers. Between the top cap and the transparent body a Teflon diaphragm is placed to avoid liquid passage to the gas-collecting chambers due to 18

22 its high hydrophobicity Figure 22-a2. This membrane imposes, however, a pressure drop to permeate the evolved gases (between ca. 5.5 mbar and 10 mbar) that the feeding electrolyte has to compensate; this membrane allows the continuous supply of electrolyte to the cell. The electrolyte inlet is located in the bottom of the cell to force the upward movement of produced gas bubbles - Figure 22-a5. Vertical PEC Cell Design a) b) Figure 24. The vertical PEC cell: a) disassembled in 3D project, b) final prototype. 1 acrylic body; 2 diaphragm holder; 3 acrylic cap with separated chambers for oxygen and hydrogen collection; 4 front and back side black metallic frames; 5 Photoactive electrode; 6 back window; 7 metallic counter-electrode; 8 left side electrolyte inlet; 9 - left side electrolyte outlet. 9 8 The vertical PEC cell design was based in a PEC device previously developed by UPorto the PortoCell. 22 Taking advantage of some important features from the PortoCell, it comprises a reservoir holding the electrolyte wherein the two electrodes are immersed and physically separated by a diaphragm Figure 24. The main advantage of this cell is the existence of a vertical diaphragm that separates the oxygen and hydrogen evolution compartments. This membrane allows placing both electrodes very close to each other near zero gap configuration, minimizing ionic resistances. The dimensions of the photoelectrode were set at cm 2 by the PECDEMO consortium in accordance with the the original proposal of having a 50 cm 2 PEC cell. In this PEC device the photoelectrode also works as cell window taking advantage of the new strategy implemented in the angled PEC cell. Depending on the diaphragm transparency and the counter-electrode shape different arrangements could be exploited to maximize the light reaching the photoelectrode and the PV cell in a tandem configuration as detailed in D4.2 report. Still, this design does not allow the existence of an open path for the sunlight to reach the PV cell, which is a major drawback in comparison to the angled PEC cell. Similarly, to the angled PEC cell, this cell is made of transparent acrylic and it has a cap that allows placing a Teflon diaphragm to collect the evolved gases separated from the electrolyte. Alternatively, without the Teflon membrane, this cap allows collecting the evolved gases together with electrolyte. The cell was engineered considering continuous electrolyte feeding and gas collection in separate chambers. Concentrated Solar Radiation Different concentrator concepts for use with PEC-PV devices were developed and assessed in the scope of Task 4.3. One of the favourite concepts is the Two-axis tracking Linear Fresnel Reflector realised before in DLR s test platform SoCRatus (Solar Concentrator with a Rectangular Flat Focus ). 23 Thus, it was decided to employ the SoCRatus, which provides homogeneously concentrated sunlight with a concentration ratio of about 17.5, as the solar concentrator for the final prototype. In order to prepare the development of the final cell design, the angled and vertical PEC cells were tested on the SoCRatus. Their general behavior under varying 19

23 irradiances and tilt angles was assessed with respect to gas mixing, electrolyte and cell temperature, electrolyte flow stabilization, and gas separation. The experiments resulted in the identification of cell specific advantages and drawbacks under practical conditions. The tested devices mounted in the focal plane of the SoCRatus are shown in Figure 25. Figure 25. The vertical PEC cell (left) and the angled PEC cell (right) mounted in the focal plane of the SoCRatus. Final Design The knowledge and experience gained during the initial tasks of WP4 allowed designing and building the final prototype that best fulfills the targets of the project, and which is a combination of both cell designs 24. A demonstration module comprising four identical PEC cells of 50 cm 2, combining key features of the "angled" and "vertical" PEC cells and going beyond, is presented in Figure 26. Each cell has an open path for the sunlight, from the front to the back window, allowing the use of a tandem PEC/PV arrangement in which the PV-cell is placed in the back of the photoelectrode. Raytracing simulations Figure 26. Single PEC cell identical to the 4 units of the sub-module prototype used in the field tests. 1 Photoelectrode (back window); 2 stainless steel frame with an in-built screw for electrical contact; 3 platinized-ti meshes (CE) fixed against the ionic exchange membrane; 4 screws for electrical contact with the CE; 5 electrolyte inlets (Ø = 10 mm); 6 electrolyte outlets (Ø= 3 mm). 20 confirmed the applicability of the prototype design for operation with the SoCRatus concerning concentration profiles on the relevant cell surfaces, i.e. on photoelectrodes and PV modules. The counter-electrodes (CE) are placed side-by-side to the working-electrode (WE), but physically separated by an anion exchange membrane to avoid gas mixture. The modular prototype embodies an acrylic skeleton in which the components detailed in Figure 26 are assembled. In this arrangement, the active area of each photoelectrode is 5 10 cm 2 based on the project target ensuring minimum values of

24 Figure 27. The optimized design for the electrolyte container of the individual PEC cell that is part of the modular prototype: a) front, side and tilted views; b) sectional plans considered for the CFD results. 1 CE compartments; 2 WE compartment; 3 main electrolyte inlet ( = 10 mm); 4 electrolyte outlets ( = 3 mm); 5 inlets of the counter-electrode compartments ( = 8 mm); 6 inlets of the WE compartment close to the back window ( = 6 mm); 7 inlets of the WE compartment close to the front window ( = 4 mm). ionic and electronic resistances. The electrolyte flowpath inside each compartment of the cell was optimized in a CFD-based simulator to improve heat dissipation and to allow efficient collection of gases at the top, preventing its accumulation inside the reservoir. The final and optimized geometry in terms of fluid pattern is presented in Figure 27. Two main inlets located at the bottom of the cell force the upward movement of produced gas bubbles Figure 27. Inside the cell body there are two inlet manifolds: i) 10 inlets located in the bottom of the WE compartment (5 close to the back window and 5 close to the front window Figure 27-7 and Figure 27-8, respectively); and ii) 2 inlets located at the bottom of each CE compartment ( = 8 mm, Figure 27-5). The electrolyte flow pattern created by the inlets located close to both windows is important to assure the bubbles detachment. In this optimized design the top Teflon Figure 28 CFD simulations of the PEC cell: a) Non-optimized membrane was not considered and b) optimized design in terms of temperature profile to avoid the drawbacks [electrolyte: water; total flow rate: 500 ml min -1 ; external reported on deliverables D4.1, temperature: 25 C under 10-SUN irradiance] and D4.2 and D4.3. Alternatively, electrolyte flow distribution [electrolyte: water; total flow rate: 500 ml min -1 ; external temperature: 25 C]. without the Teflon membrane, the evolved gases are collected together with electrolyte through the four outlets located in the cell cap, two on top of the WE 21

25 compartment and one on top of each CE compartment ( = 3 mm, Figure 27-4). A transparent acrylic plate (2 mm thick) was placed in the middle of the WE compartment to enhance the fluid flow towards the outlets. This arrangement of inlets and outlets allowed creating a uniform upward flow with an efficient collection of the evolved gases. Figure 28 shows the differences between a non-optimized and optimized design in terms of temperature profile and electrolyte distribution. Following the same strategy applied to the angled and vertical PEC cells, photoelectrodes work simultaneously as front or back windows. In both configurations the photoelectrode must have an electrical collecting frame at the semiconductor side, connected to the back of the substrate where the electrical cables are connected or the PV cell is installed. The ultimate goal of WP4 was successfully accomplished; a sub-modular prototype composed by four individual PEC cells, each with an active area of 50 cm 2 was designed, optimized, built and tested under artificial solar conditions and concentrated solar radiation in field tests. 25 Complementary results are presented hereafter within the framework of WP6, modular prototype and field tests Work Package 5 The fifth work package addresses design of process, pilot plant and infrastructure following by inventory analysis and component sizing. Finally, Life-Cycle-Analysis (LCA), cost estimation for hydrogen production using PEC-PV technology as well as benchmarking with common H2 production technologies like steam methane reforming, coal and biomass gasification, wind and PV electrolysis were performed and evaluated. Task 5.1. Design of process, pilot plant and infrastructure The generation of hydrogen with photoelectrochemical-photovoltaic (PEC-PV) tandem devices via water splitting finally has to be economically viable and industrially applicable. The PEC-PV system has to be embedded in suitable processes and plants. Three hydrogen production and application scenarios were considered: a single home application (SHA), a hydrogen refuelling station (HRS), and an industrial process (IP). The SHA refers to a decentralised approach of hydrogen production rated at 1 6 bar and subsequent use in fuel cells to provide electrical power needed in small buildings. The HRS offers hydrogen at a nominal production rate of bar to fuel vehicles such as cars and buses, which carry a pressurised hydrogen tank, whereas the introduced IP features a nominal production rate of 4, bar and addresses utilisation of hydrogen as a feedstock for diverse processes. Numerous criteria are relevant for the choice of a suitable location for the hydrogen production plants (weather conditions, politics, terrain, infrastructure, etc.). However, the most important criterion for the provision of hydrogen at reasonable costs is a high level of global irradiance. Thus, Seville (Spain) and Negev (Israel) were identified as promising locations. Appropriate plant designs for the three scenarios were elaborated, which comprise beside the PEC-PV system major components such as pumps, heat exchangers, compressors, tanks, and blowers. The latter component refers to an air cooling system that controls the temperature of the electrolyte, which flows through the PEC-PV system, in order to avoid critical temperatures with respect to efficiency and stability. According to the project targets a solar-to-hydrogen efficiency (STH) of 8% based on the higher heating value of hydrogen was considered. Collector sizes between 89.1 m 2 22

26 and 378,139 m 2 were calculated depending on the location and the scenario. Active power management (APM) 18 allows in-situ generation of excess electricity and was implemented in the plant models with 5% solar-to-electricity efficiency. Passive cooling due to convective and radiative heat transfer from the PEC-PV system and the electrolyte piping to the ambience is relevant and was taken into account. Moderate concentration ratios up to 30 were considered. The solar plant should be operated at temperatures as high as reasonable in terms of stability and efficiency, since higher temperatures clearly decrease the needed active cooling capacity because of enhanced passive cooling. Task 5.2. Inventory analysis and component sizing Mass and energy flows regarding the main plant components were estimated for 60 C maximum temperature of the PEC-PV system and 8 K temperature increase between inlet and outlet of the PEC-PV system. Average operation conditions as well as severest operation conditions concerning ambient temperature and solar input were considered in order to assess a) representative mean operating modes of the solar plants and their mean demands with respect to electricity and water and b) the required maximum operating capacity of the main components of the solar plants. In both cases negligible influence of wind was assumed. Cooling is a crucial issue and the implemented blower of the air cooling system dominates the electricity demand under severest conditions. However, even under severest conditions and concentrated sunlight APM completely or to a large extend covers the electricity demand of the entire plant. Since the electricity demand of the plant increases only moderately for a concentration ratio of 30 compared to 10 or 20 a concentration ratio of 30 was chosen for further investigation related to the HRS and the IP. Task 5.3. Life-cycle-analysis (LCA) To quantify the environmental impact associated with all the stages during the life of the product, i.e. from the raw material extraction until disposal or recycling (so-called cradle-to-gate cycle), the LCA was performed in accordance with ISO using GaBi 7.0 software 27. The focus of LCA was set on the global worming potential (GWP), which is a measure of the amount of heat trapped by a certain mass of the gas in question to the amount of heat trapped by a similar mass of carbon dioxide expressed as kg CO2 eq per kg of produced hydrogen. It was found that if grid electricity from local sources is used to meet the electricity demand of plant components, in all analysed scenarios the GWP impact of the PEC- PV technology is higher than the best in class wind electrolysis technology (1.0 kg CO2 eq kg -1 H2). However, if no external electricity is needed due to APM, the GWP impact of PEC-PV technology can be lowered up to 1.4 kg CO2 eq kg -1 H2 assuming implementation of 1 m 2 PEC-PV cells. Moreover, if solar concentration (C = 30) is combined with APM a new state of the art technology with lowest reported to date GWP impact of 0.4 kg CO2 eq kg -1 H2 could be obtained. Task 5.4. Cost estimation Economic analysis was performed using H2A Hydrogen Production model (version 3.1) provided by the US Department of Energy (DoE) on its web page 28 and levelised costs of hydrogen production (LCHP) were estimated for three different hydrogen production scenarios. The H2A Hydrogen Production model is based on process design assumptions, which were verified by an international H2A team. Required input parameters to the H2A models include capital and operating costs, efficiencies of 23

27 used process, plant life as well as financial parameters such as the type of financing, discounted cash flow rate and desired internal rate of return. Using H2A Hydrogen Production model levelised costs of hydrogen production were estimated. It was found out that H2 production using 1 m 2 PEC-PV cells will lead to LCHP values of 9 kg -1 for SHA scenario, LCHP of kg -1 for HRS scenario, and LCHP of kg -1 for IP scenario. In the case of HRS and IP scenarios higher LCHP values correspond to the case when electricity from the grid is used and no solar concentration is applied, while lower LCHP values correspond to the case when no electricity from the grid is used due to implementation of APM and solar concentration with C = 30 is used. Higher LCHP values for HRS scenario are caused by high electrical energy consumption due to compression of H2 to 810 bars, while comparably low LCHP value for SHA scenario is mainly caused by rather simple hardware required. The specified conditions of hydrogen at the outlet of the plant have a great influence on the LCHP. An increase of STH from 8% to 12% or even 15% would have a significant influence on the LCHP value. Additionally, selling of generated by PV module excessive electrical energy would generate a revenue and would lead to further decrease of the LCHP value. Task 5.5. Benchmarking Hydrogen production via PEC-PV water splitting was assessed in the context of alternative hydrogen production technologies. Steam reforming of methane, which uses as coal gasification fossil feedstocks and therefore inherently involves the generation of carbon dioxide, is the dominating hydrogen production technology today. Prominent technologies which use renewable feedstocks are biomass gasification and electrolysis powered by electricity produced by wind turbines or PV modules. Respective global warming potentials and H2 production costs were analysed and compared. PEC-PV water splitting could potentially reach lowest reported to date GWP impact of 0.4 kg CO2 eq kg -1 H2 followed by wind electrolysis (1.0 kg CO2 eq kg -1 H2) and PV electrolysis (2.5 kg CO2 eq kg -1 H2), biomass gasification (8.0 kg CO2 eq kg -1 H2), steam methane reforming (14.5 kg CO2 eq kg -1 H2), and coal gasification (23.7 kg CO2 eq kg - 1 H2). Comparison of H2 production costs showed that the hydrogen production costs for all three considered scenarios (9-23 kg -1 H2) are higher than estimated costs for steam reforming of methane ( kg -1 H2), coal ( kg -1 H2) and biomass gasification ( kg -1 H2), as well as wind electrolysis ( kg -1 H2). In the case of PV electrolysis, which shows most similarity to PEC-PV water splitting since it uses the same feedstocks, a rather broad range of kg -1 of costs has been estimated, that shows a wide overlap with cost ranges determined here for PEC-PV hydrogen production technology. Though, solar concentration with C = 30, active power management, and large active area per PEC-PV device are promising approaches to reduce hydrogen production costs and should be pursued, further efforts have to be made to reach economic viability. Cost figures could effectively be enhanced by higher STH maintained at higher operating temperatures (aiming at superseding the active cooling system) and higher concentration ratios Work Package 6 WP6, Modular Prototype and Field Tests, as a demonstration work package was focused on the final assessment of the sub-modular PEC-PV prototype, which 24

28 embraces four identical 50 cm 2 compartments. The optimized cell design with respect to optical path and electrolyte flow characteristics was developed in the scope of WP4 as presented above. The performance of the prototype under practical conditions was evaluated in particular regarding efficiency and stability. The final demonstration phase began in Nov 2016 meeting MS7 Start of field tests of prototype module. 25 Two sets of experiments were simultaneously conducted to test the optimized device design: i) with the 1 x 4 demonstration module array under concentrated solar radiation at DLR and ii) with an individualized 50 cm 2 cell, identical to the four cells comprised in the sub-modular prototype, under non-concentrated artificial sunlight at UPorto. Each set of experiments was divided into two campaigns. Bare hematite photoelectrodes produced by UPorto were used in the first campaign, whereas bismuth vanadate (BiVO4) photoelectrodes with cobalt oxide/phosphate (CoPi) catalyst prepared at HZB within the framework of WP1 were installed in the second campaign. Hematite was the semiconductor selected for the first campaign due to its high stability under continuous operation. 11 HIT silicon mini modules manufactured by HZB/PVcomB, connected in series to the photoelectrodes, delivered bias voltage to promote the water splitting reactions. Tests under Concentrated Sunlight The experiments under concentrated sunlight were conducted employing DLR s test facility SoCRatus 23 in Cologne. The prototype was mounted in the rectangular focus of the two-axis tracking solar concentrator and provided with homogeneous, about 17.5-fold concentrated sunlight. The developed PEC-PV device was implemented in the set-up using two fluid cycles of the SoCRatus. They both fed the inlets of the prototype, where the flow was distributed to the hydrogen and oxygen chambers connected to Fluid Cycle 1 and Fluid Cycle 2 respectively. The first experimental campaign was carried out with front illuminated hematite Figure 29. Modular prototype equipped with BiVO4 photoelectrodes irradiated with concentrated sunlight in the focal plane of the SoCRatus with reflective shields to protect sensitive parts of the setup. photoelectrodes, whereas in the second campaign two BiVO4 photoelectrodes equipped with grid lines were installed in each compartment the first one being back illuminated as part of the front window, the second one being front illuminated as part of the back window. HIT silicon mini modules with an active area of 50 cm 2 each were placed behind the back windows realizing a true tandem configuration and delivered bias voltage to the system. In case of hematite partly an additional bias of +325 mv 25

29 a) b) Figure 30. Total irradiation on the prototype (smoothed ± 30 s), average current density (smoothed ± 30 s), and hydrogen flow relative to respective mean values as well as average solar-to-hydrogen efficiencies (STH) of the particular days associated with a) Campaign 1 (hematite photoelectrodes, 1 M KOH, 25 C, 1.9 l min -1, membrane: Fumasep FAA-3-PK-130) and b) Campaign 2 (BiVO4 photoelectrodes, 0.5 M K2SO M K2HPO4/KH2PO4, 30 C, 1.7 l min -1, membrane: Fumasep FAA-3- PK Nafion NE-1110 / Nafion NE-1110 / none / none). was applied to reach a total bias of about 1.6 V. The prototype operating with BiVO4 photoelectrodes under concentrated sunlight in the focal plane of the SoCRatus can be seen in Figure 29. The total irradiation on the prototype, the achieved average current density, and the estimated molar flow of generated hydrogen relative to respective mean values for Campaign 1 and 2 are shown in Figure 30. The hydrogen flow generally follows the current density with a certain delay due to mixing and saturation effects in the fluid cycle. With hematite a total experimental time of about 15 h was reached, thereof more than 8.5 h without additional bias and close to 6.5 h with 325 mv additional bias. A total irradiance of 12.4 kw m -2 on average and of 14.0 kw m -2 in peak time was applied. Current densities of about 0.2 ma cm -2 and 0.5 ma cm -2 as well as maximum hydrogen flows of 924 µmol h -1 and 2,078 µmol h -1 were achieved without and with the additional bias respectively. Efficient product gas separation was obtained. The daily solar-to-hydrogen efficiency (STH) based on the higher heating value of hydrogen reached 0.059% with additional bias. Within the duration of operation the prototype featured stable performance. Campaign 2 with BiVO4 photoelectrodes covered about 48 h. An applied total irradiance of 7.85 kw m -2 on average and of 16.5 kw m -2 in peak time was estimated. The mean current density was calculated to 0.87 ma cm -2 while a maximum value of 1.88 ma cm -2 was obtained on Day 2 at about 13 kw m -2. Hydrogen was produced at rates up to 6,741 µmol h -1. The daily STH reached 0.42% on Day 5 at comparably low levels of irradiance. Though a certain degradation of the photoelectrochemical system could be observed within the duration of the campaign, even after 48 h operation under demanding conditions the BiVO4 system was still active. Since in both campaigns a non-proportional dependency of hydrogen formation on the solar input became apparent, further efforts have to be made in order to allow efficient exploitation of concentrated sunlight. 26

30 Tests under Non-Concentrated Sunlight To assess the stability of the optimized hybrid PEC-PV device under non-concentrated sunlight an experimental setup was assembled at UPorto applying the sulphur plasma lamp system AS 1300 V 2.0 (Plasma International GmbH) which provides 1,000 W m -2. In this setup the PEC cell operates at a constant bias potential provided by a HIT Si-PV module. The medium term test was performed in two experimental campaigns: i) Figure 31. Polarization curve of the hematite photoelectrode obtained under a constant bias of 1.6 V and simulated solar irradiance. Enlarged view of the polarization between 96 h and 192 h. with bare hematite photoelectrodes and ii) with BiVO4 photoelectrodes. A constant electrolyte feeding of ca. 200 ml min -1 was promoted using a peristaltic pump and a water bath was used to keep the electrolyte temperature constant under operation. Again, hematite was the first photoelectrode tested due to its high stability under continuous operation; in this experiment the water bath was set to operate at ca. 45 C. 20 At the initial instant a photocurrent density of 0.62 ma cm -2 was produced by the hematite photoelectrode at 1.6 V in a 2-electrode configuration. Accordingly, to supply the hematite photoelectrode with the necessary bias potential of ca. 1.6 V, two 50 cm 2 PV modules were connected in series and an Autolab potentiostat was used for continuous monitoring the photocurrent produced by the semiconductor over 1,000 h Figure 31. Photocurrent Density / ma cm h h Applied Potential / V Photocurrent Density / ma cm h h Applied Potential / V a) b) Figure 32. a) J-V characteristics of the 50 cm 2 hematite photoanode prepared by spray pyrolysis, before starting the stability test, 0 h ( ), and after 1,005 h ( ) under simulated sunlight; b) J-V characteristic curves for the two 50 cm 2 Si-PV modules connected in series before starting the stability test, 0 h ( ), and after 1,005 h ( ) under simulated sunlight. The presented results show that the PEC-PV device remained stable over 1,000 h (approximately 42 days) delivering an average photocurrent density of ca ma cm -2. The photocurrent oscillations along the polarization curve are due to periodic interruptions to obtain the J-V curves; a slight decrease on the photocurrent 27

31 density before stabilization was always observed at the initial instants of each testing period. The latter, can be easily seen in the zoom out of the polarization curve in Figure 31. This initial photocurrent decay, before stabilization, should be related to the generation of long-lived holes during the formation of a space charge layer, which can oxidize water on a timescale of around 1 s. 36 Figure 32 plots the J-V curves obtained at the initial instant and at the end of the stability test for the two components that comprise the hybrid PEC PV device. Both components, the hematite photoelectrode and the PVs modules, presented very similar performances before and after 1,000 h of operation with no evidence of corrosion or degradation. BiVO4 photoelectrodes were tested at UPorto in a second experimental campaign; in this case the electrolyte was kept at ca. 25 C. Considering the characteristic performance of BiVO4 (Figure 33-a) a single 50 cm 2 PV module was enough to provide the necessary bias to the PEC cell for promoting water electrolysis. In this test the setup operated at the interception point of the J-V curves of BiVO4 photoelectrode and HIT PV module Figure 33-a). The photocurrent history over 24 h is plotted in Figure 33-b). During this period an average photocurrent density of 0.41 ma cm -2 was recorded at 1.28 V, corresponding to a STH efficiency of 0.61%. Over this time the BiVO4 continuously decreased with a current density loss rate of 2 na cm -2 s -1. Similar to the test with hematite, the individual performance of the BiVO4 photoelectrode and the PV module was assessed at the beginning and at the end of the test; the performance of the PV modules remained unchanged after 24 h of operation. However, from Figure 33-c it can be extracted that the performance of the BiVO4 photoelectrode continuously decreased after operating 24 h. The latter may be explained by the material detachment observed during the stability test. a) b) c) Figure 33. a) J-V characteristics curves: 50 cm 2 BiVO4 photoelectrode ( ) under simulated sunlight and in 0.1 M KPi, obtained in a 2-electrode configuration; single 50 cm 2 Si-PV HIT module ( ) under simulated sunlight; b) Polarization curve of the BiVO4 photoelectrode obtained in a 2-electrode configuration under simulated sunlight; c) J-V characteristics curves of 50 cm 2 BiVO4 photoelectrode, before starting the stability test, 0 h ( ), and after operating 24 h ( ) under simulated sunlight and in 0.1 M KPi, obtained in a 3-electrode configuration. 28