Aenert. Research Laboratory news
One of the most powerful sources of energy is the sun. Its rays can be captured and converted to create electricity or power processes to produce fuels which offer an alternative to hydrocarbon-derived fuels. One such fuel which can be harnessed with the help of sunlight is hydrogen. To do so, photoelectrochemical methods are often employed. Currently, photo electrochemical technologies have only been tested successfully in small-scale applications, including integrated photovoltaic (PV) plus electrolyser (EC), photoelectrochemical cells (PEC), or thermochemical redox cycles. One of the reasons for this is that there are several challenges that need to be overcome when scaling PEC or integrated PV + EC devices, which pertain to the specific device design, experimental configuration and materials used. If the device configuration is not optimal this can lead to problems with maintaining high efficiency and high production rates as well as stability while keeping production costs low and ensuring high sustainability. Possible solutions to these challenges include increasing the photoabsorber area per device or the number of devices deployed and through solar concentration. Solar concentration in particular has proved to be a viable path towards economically competitive, high-power-density devices as it has the potential to improve the device efficiency while simultaneously co-generating useful heat.

Now (2023), with the help of concentrated solar irradiation scientists were able to successfully scale a thermally integrated photoelectrochemical device to a kW-scale pilot plant which was able to co-generate hydrogen and heat. In this installation a solar-to-hydrogen device-level efficiency of greater than 20% at an H2 production rate of >2.0 kW was achieved.

Image: Kilowatt-scale solar hydrogen production system using a concentrated integrated photoelectrochemical device



Source: Isaac Holmes-Gentle, Saurabh Tembhurne, Clemens Suter & Sophia Haussener/ Kilowatt-scale solar hydrogen production system using a concentrated integrated photoelectrochemical device/ Nature Energy volume 8, pages 586–596 (2023), 10 April 2023/ doi.org/10.1038/s41560-023-01247-2/ Open Access. This is an Open Access article is distributed under the terms of the Creative Commons Attribution 4.0 International (CC BY 4.0)

The solar energy to hydrogen, oxygen and heat co-generation system was able to concentrate solar light by a dual-axis tracking parabolic dish concentrator to a solar reactor made up of a shield, aperture with flux homogenizer and triple-junction III–V PV module, proton exchange membrane EC stack lodged inside the reactor unit and water pump.

The complete system was operated over a period of more than 13 days in August 2020 and February/March 2021. This gave the scientists the opportunity to test the device under different environmental conditions where the ambient temperatures ranged from about 20°C in August to about 8°C in late February. During this time, the overall system fuel efficiency based on the reaction enthalpy and the overall system heat efficiency were found to range between 6.6% ± 0.6% and 35.3%, respectively. The system fuel efficiency was measured at 5.5% ± 0.5%.

Scientists have long tried to harness hydrogen through solar-thermal installations. In 2016, a photovoltaic-electrolysis system having a high STH efficiency was designed. The system was made up of two polymer electrolyte membrane electrolysers in series including one InGaP/GaAs/GaInNAsSb triple-junction solar cell which could produce sufficient voltage to drive both electrolysers with no extra energy input needed. The solar concentration was optimised in such a manner that the maximum power point of the photovoltaic was perfectly in tune with the operating capacity of the electrolysers and thus increased the system efficiency. The system achieved a 48-h average STH efficiency of 30%.

Image: Solar water splitting by photovoltaic-electrolysis with a solar-to-hydrogen efficiency over 30%



Source: Jieyang Jia, Linsey C. Seitz, Jesse D. Benck, Yijie Huo, Yusi Chen, Jia Wei Desmond Ng, Taner Bilir, James S. Harris & Thomas F. Jaramillo/ Solar water splitting by photovoltaic-electrolysis with a solar-to-hydrogen efficiency over 30%/ Nature Communications volume 7, Article number: 13237 (2016), 31 October 2016 / doi.org/10.1038/ncomms13237/ Open Access. This is an Open Access article is distributed under the terms of the Creative Commons Attribution 4.0 International (CC BY 4.0)

In 2019, scientists constructed a large area photoelectrochemical–photovoltaic (PEC–PV) solar water splitting device with a metal oxide-based top absorber. It consisted of cobalt phosphate-coated tungsten-doped BiVO₄ (CoPi/W:BiVO₄) photoanodes connected to series-connected silicon heterojunction (SHJ) solar cells. The challenges encountered during the experiment included (a) the high resistivity of the fluoride-doped tin oxidse (FTO) substrate, (b) non-uniform CoPi deposition, and (c) limited ionic conductivity of the 0.1 M phosphate buffer electrolyte. The former two problems were solved by means of applying Ni lines to the FTO substrate, and the latter by increasing the electrolyte concentration to 2.0 M. The overall performance of the large area photoelectrodes was limited by H+/OH− transport in this near-neutral pH electrolyte. This led to H+/OH− depletion in the centre of the large area electrode and a significant potential drop, which, however, was amended by implementing a cell design with a small electrode-area-to-electrolyte-volume ratio. The improved photoanodes were then integrated into tandem PEC–PV devices in either a single or dual photoanode configuration. The optimised small area (0.24 cm²) PEC–PV devices based on a similar configuration showed a STH efficiency of up to 5.5%.

Image: (a) Colour gradient plots of the simulated potential distribution across a 5 × 10 cm² photoanode (i) without and (ii) with Ni lines for a photocurrent density of 3 mA cm⁻², and (b) shows the graphical plots of the potential drop at different points across the photoanodes (dashed lines in Fig. 3a). (c) Photographs of two 50 cm² area W:BiVO₄ photo-anodes, (i) without and (ii) with Ni lines. (d) J–V curves of the corresponding photoanodes in an electrolyte containing 0.5 M Na₂SO₃ and 0.1 M KPi buffer of pH7, with backside AM1.5G illumination (BSI)



Source: Ibbi Y. Ahmet, ORCID logo ‡a   Yimeng Ma, ORCID logo ‡a   Ji-Wook Jang,a   Tobias Henschel,b   Bernd Stannowski, ORCID logo b   Tânia Lopes, ORCID logo c   António Vilanova, ORCID logo c   Adélio Mendes, ORCID logo c   Fatwa F. Abdi ORCID logo *a  and  Roel van de Krol/ Demonstration of a 50 cm² BiVO₄ tandem photoelectrochemical-photovoltaic water splitting device/ Sustainable Energy & Fuels Issue 9, 2019, 04 Jul 2019/ doi.org/10.1039/C9SE00246D/ Open Access. This is an Open Access article is distributed under the terms of the Creative Commons Attribution 4.0 International (CC BY 4.0)

The improved device has many advantages: The two-pump design was able to decouple the conflicting water flow-rate requirements of the PV and EC. This enabled sufficient heat transfer in the CPV heat exchanger while controlling the overall water temperature increase over the CPV module and stoichiometric water ratio in the EC. Owing to the pilot-scale nature of the system, the solar dish size was not adapted to the reactor size and thus an additional water-cooled shield was required to absorb the excess concentrated light. However, the scientists plan to replace this shield with a larger-area homogenizer/concentrated PV unit and passively cooled shield in the future.

The new solar concentrator design offers a new means to effectively harness the energy of the sun. The scientists are convinced that their system will make a valuable contribution to the energy transition once it is ready to be launched onto the market.

By the Editorial Board