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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - PECSYS (Technology demonstration of large-scale photo-electrochemical system for solar hydrogen production)

Teaser

Summary

Solar driven hydrogen generation via water electrolysis is an archetypal use of renewable energy that ensures a sustainable energy supply while minimising green-house emissions. The challenges for adaptation by society are the high cost of the technology and maintaining the reliability of supply compared to conventional energy supply from fossil fuels. Thus the specific objectives of the project are:
â€¢ To study and develop devices for integrated photo-electrochemical concepts and scale viable concepts to prototype size > 100 cmÂ².
â€¢ To use socio-techno-economic analysis to predict and select concepts with levelised cost of hydrogen production below â‚¬ 5/kg.
â€¢ To scale the prototypes of the less mature but promising technologies to a demonstrator with active area > 10 mÂ².
â€¢ To achieve a hydrogen production of 16 gH2/h from the demonstrator resulting from a solar to hydrogen conversion efficiency (Î·STH) of at least 6 %.
â€¢ To ensure that the initial solar to hydrogen conversion efficiency of the demonstrator does not reduce by more than 10 % relative after six months of continuous operation.

Work performed

A Î·STH value of 7.5 % was achieved for a laboratory scale integrated device consisting of a 2.25 cmÂ² area a-Si:H/a-Si:H/Î¼c-Si:H triple junction solar cell with an optimized side-by-side arrangement of transition metal based HER (NiMo) and OER (NiFeOx) catalysts, each 1.92 cmÂ².
For the Cu(InGa)Se2 (CIGS) approach, a Î·STH of 12.8 % was obtained using CIGS (0.27 cm2)-NiO/NiMoW4 (0.25 cm2). Also a 20 cm2 PEM cell using Nafion 212 membranes has been developed with low catalyst loading (0.4 mgIr/cm2 and 0.1 mgPt/cm2) that can operate at 25 Â°C at 1.73 V with 1.2 A/cm2 (2 W/cm2). No significant effects of sub-zero temperature were observed and the voltage loss at 1 A/cm2, after frosting at -5Â°C, was only 100 mV. The concept is therefore suitable for outdoor installation.

Further, a cassette approach consisting of an electrolyser wired to a PV minimodule was used for reference purposes and to aid design of the balance of plant system. Under outdoor solar irradiation ranging from 0.09 to 0.11 W/cm2, a cassette system using 2 electrolysis cells with zero gap and Ni foam with Pt nano- coating at the cathode, and a 3-cell TF-Si module of area 19.8 cm2 achieved a Î·STH value of 7.5 %. Under similar irradiation with an ambient temperature of 30 Â°C, a 4-cell SHJ module (active area 230 cm2) connected to an 8 cmÂ² alkaline cell achieved an operating current of 1.4 A which, assuming a 100 % faradaic efficiency, corresponds to Î·STH of 7.5 %.

A semi empirical model, was developed using experimental data for all PV solutions and that of a PEM EC solution to simulate energy conversion efficiency under different weather and climatic conditions. The results indicate that it is possible to obtain a Î·STH of up to 80 % of the conversion efficiency of the solar cells. For a best solar cell efficiency of 17 %, a 14 % Î·STH estimated in terms of the electrical operating point, is possible.

Procedures for qualifying corrosion protection layers in the integrated PV-EC device and seals for electrolysers were suggested. A literature review on the state of the art for electrolysers and redox flow batteries revealed that PTFE as the optimum edge sealant material, because of its high compressibility, and long term chemical stability to highly alkaline and acidic electrolytes above 90 Â°C. Nevertheless, in order to reduce costs, metal gasket seals may be a viable replacement provided that a protective metal oxide based coating can be developed in the project to prevent degradation by chemical attack from the electrolyte. Titanium oxide (TiOx) and tin oxide (SnOx) films grown by atomic layer deposition have been developed as monolithic anticorrosion protection layers. Both oxides appear to provide sufficient short term protection to a molybdenum coated glass on exposure to stagnant 1M KOH at room temperature.

Lastly, a concept for the test field has been developed and also the required hardware has been selected, in preparation for the demonstrator.

Final results

The highest Î·STH value, for a laboratory scale device a PV integrated EC device, of 16.2 % was reported by NREL (USA) in 2017 [1]. However it consisted of a PV cell using GaInAs and GaInP grown by epitaxy, a costly deposition method, and platinum group catalysts for the EC. The initial Î·STH value reduced by 15 %, relative, after 1 hour operation in 3M sulphuric acid. Also, the highest Î·STH, using crystalline silicon and earth abundant electrocatalysts for a laboratory scale device, of 15.1 % was reported in 2017 by KU Leuven (Belgium) [2]. Compared to the 7.8 % reported here for TF-Si PV with earth abundant catalysts, that cell design did not include physical and thermal contact between the PV and EC components thus the overall device design was simpler as it is not affected by possible corrosion of the PV part.
FZJ, Germany reported an integrated device comprising of a thin film silicon PV mini-module with a solar collection area of 5.3 cmÂ² and nickel electrocatalysts for which the Î·STH remained stable at 3% even after the PV cell was exposed to 1 M KOH for 40 h [3]. So far for the thin film silicon approach with thermal contact between the PV and EC components, in PECSYS, a Î·STH value of 7.6 % persisted for 100 minutes in 1 M KOH. A higher Î·STH of 12.8% has also been reached using CIGS (PV) and nickel transition metal alloy catalysts (EC), with electrocatalyst stability retained when operating for 100h as a standalone electrolyser, while stability tests for the full device are pending and planned for the next period.
Lastly, in PECSYS, a 20 cm2 PEM electrolyser using Nafion 212 membranes has been developed with low catalyst loading (0.4 mgIr/cm2 and 0.1 mgPt/cm2) that can operate at 25 Â°C at 1.73 V with 1.2 A/cm2 (2 W/cm2 @ 80 Â°C). These values are comparable to those arising from a reduction in total PGM catalyst loading from the conventional 2 mgâ€¢cmâˆ’2 to 0.5 mgâ€¢cmâˆ’2 for Ir0.7Ru0.3Ox and Pt/C catalysts reported by the H2020 funded HPEM2GAS project in 2017 [4]. The PECSYS project, extends progress in the knowledge of the feasibility of electrolysers with reduced PGM catalyst loading since the stability after exposure to sub-zero temperatures is demonstrated.
If the project can successfully upscale the demonstrated laboratory devices using less than 0.8 g of rare materials to a 10 mÂ² demonstrator with high efficiency and long term durability, then this would provide progress in the feasibility and deployment of hydrogen to serve as a greenhouse gas free storage medium for renewable energies. Moreover, the reduction or complete elimination of critical raw materials in the device would also promote faster deployment of the technology on a large scale by reducing the cost of hydrogen. Electric power generation using photovoltaics is a worldwide fast growing business and is expected to give a substantial contribution to the transition towards a clean and sustainable energy supply. Finding cost-efficient technological solutions to generate a zero-carbon fuel such as hydrogen for later use in stationary applications or in the transportation sector, would enable an even stronger growth and significantly increase the flexibility for utilizing the solar energy in the society.

[1] Nature Energy (2017)3, 17028.
[2] Sustainable Energy Fuels (2017), 1, 2061.
[3] Nature Communications (2016) 7, 12681.
[4] Applied Energy (2017) 192, 477.