The requirements of potential end-users were first translated into requirements for a prototype system capable of validating the photoelectrochemical conversion of solar energy and water into hydrogen and oxygen. Target specifications for the system components, module size, cell area, fluid handling systems and storage, were deduced from process analyses.
Electronic computational screening of promising electrode materials was undertaken, aimed at selecting appropriate candidates (mainly ternary oxides). Regarding device-level modelling, the flat geometry of the photoelectrochemical cell made it possible to simulate its behaviour with a simple one-dimensional semi-analytical model. A continuum multi-physics computational model was also implemented as a holistic tool for the optimization of the photoelectrochemical system.
Attention was paid to the research and development of the cell components. Various non-noble photoelectrodes were selected through theoretical screening and they were experimentally benchmarked. As a result of this work, electrode materials based on hematite and cupric oxide were finally selected and optimized (including co-catalysts). Materials and procedures to optimize thin anion-conducting membranes and ionomer dispersions were explored. The hydrophobic backing substrate to be applied at the cathode for separating water from the produced hydrogen was developed in parallel.
Together with the completion of the materials research, the consortium focused on developing the scaled-up cell concept, having a large effective surface area, a proper water distribution over the cell and an effective method for the removal of hydrogen and oxygen. The techno-economic analysis was completed as well and the life-cycle analysis (LCA) to determine the potential impact of the technology.
Once the design finalized and the components fabricated, the integrated system was assembled. To demonstrate TRL 5, the complete system has been field-tested outdoors for validating the concept, performance, and degradation rate. In parallel, laboratory stress-testing has further elucidated the system operating limits and expected lifetime.
The most relevant results obtained within the project are the following:
(1) At the laboratory cell level, a throughput efficiency over 10% at a bias of 1.23 V has been achieved. At a lower bias (0.6 V), a throughput efficiency and enthalpic efficiencies were over 2% and 3 %, respectively.
(2) The use of a polymer electrolyte has been found to be beneficial for enhancing the stability of cupric oxide, which is critical for increasing the durability of the device. Remarkable results have been obtained by tuning the cupric oxide synthesis procedure: a 10 % decrease of the efficiency of the upscaled unit cell under continuous light stress (light soaking) for 40 h.
(3) Innovative designs of the upscaled unit cells, module, and panel, with appropriate oxygen, hydrogen and water management have been devised. The constructed panel has an area of ca. 1 sqm.
(4) Life cycle and techno-economical assessments have been finalised based on different scenarios. Importantly, the Global Warming Potential (per sqm of panel) is half that of a PV panel. In addition, a hydrogen cost lower than 5 €/kg for realistic scenarios has been demonstrated.
(5) Experiments with the upscaled unit cell reveal that the use of the hydrophobic backing layer allows for obtaining a dry hydrogen output stream, which is further purified using dedicated ancillary equipment.
Project dissemination is being addressed through a dedicated website and a wide range of appropriate communication tools. The most relevant results are being published in scientific journals (a total of seven papers so far) and presented at related international conferences (over 10 contributions). Lectures for university students have been prepared as part of their training. An online final event was held together with an associated webinar. The built-up field-testing site is planned to serve as a pilot solar hydrogen production site also after project completion, including further dissemination activities in the field of hydrogen knowledge.
