Fig. 1: Overview of the value chain proposed in the Sun-To-X project<\/figcaption><\/figure>\nSolar hydrogen production<\/h3>\n Solar hydrogen can be produced from a variety of technologies, including the combination of photovoltaic panels and electrolysers (PV-E), which are already commercialised at a small scale. However, challenges in the production cost of solar hydrogen have driven the development of alternative technologies, such as photoelectrochemical approaches. Photoelectrochemical technologies combine the functionalities of light absorption and electrodes into a single component: semiconductor photoelectrode. The realisation of these more integrated systems could result in a lower cost of future solar hydrogen production (Shaner et al<\/em>., Energy Environmental Science<\/em>, 2016). Our targets aim towards developing a 10% solar-to-hydrogen efficiency device.<\/p>\nMost of the research into photoelectrochemical technologies has focused on the use of liquid water as a water feedstock. The use of ambient humidity as an alternative is an increasingly researched option to expand the geographical applicability of the device, to solve technical issues such as bubble formation (which can scatter light and block catalytic sites) and reflection of light from the water\u2019s surface. The key component difference between utilising a liquid and gas phase water source is the use of porous photoelectrodes to allow humidity to enter into the device, whereas a thin film photoelectrode can be used in the liquid phase case. Additionally, the use of a water-absorbing solid electrolyte such as Nafion is required for the gas phase reaction, to bring the humidity in contact with the photoelectrode.<\/p>\nFig. 2: Device structure for photoelectrochemical hydrogen production<\/figcaption><\/figure>\nIdeally, the photoelectrodes would be positioned in a so-called tandem configuration, where the photoanode and photocathode each absorb a different portion of the solar spectrum (i.e., blue and red light), with our target device structure shown in Fig. 2, to maximise the solar to hydrogen efficiency. This is challenging when using the gas phase configuration, as to transfer charge effectively, the photoelectrode must be deposited on a charge-conducting support. In the case of flat photoelectrodes, fluorine-doped tin oxide (FTO) coated glass panels can be used, which is both conductive and transparent \u2013 allowing light to pass through to the second photoelectrode. However, the porous, conducting supports, or gas diffusion layers, are typically prepared from carbon or metals (such as titanium) which are completely opaque. Currently, challenges in the scalability and stability of photoelectrochemical systems means that the technology readiness level (TRL) of these tandem systems is currently three (functional in laboratory set-up). During the scope of the project, the target is to increase the TRL to five (demonstration in a relevant environment).<\/p>\nFig. 3: Transparent gas diffusion wafer: a. Photograph of quartz fibre wafer; b. Photograph of FTO-coated quartz fibre pellet; c. Scanning electron microscopy image of FTO-coated quartz fibre wafer with inset (zoomed in), figures reproduced from Adv. Mater. 2023, 2208740, \u00a9 2023 The Authors<\/figcaption><\/figure>\nOvercoming this challenge is one of the key results of the Sun-To-X project, namely the development of transparent gas diffusion layers by a scalable preparation technique, as filed recently as a patent and published in Advanced Materials<\/em> (M Caretti et al<\/em>., 2023). The first quartz fibres were formed by blending a commercial quartz wool. The resulting fibres were pressed into a wafer with a porosity of around 90% and annealed to fuse the quartz fibres. A coating of FTO was applied through a chemical vapour deposition process. The resulting gas diffusion electrode had a conductivity similar to commercially available gas diffusion layers and a transmittance of >30% (Fig. 3). This allowed us to move to the following phase of the project \u2013 preparation of semiconductor deposition methods onto porous supports.<\/p>\nSince thin film deposition techniques would not have allowed us to coat the entirety of the porous structure, we had to focus on techniques that would allow a homogeneous deposition inside the pores. We have developed a variety of techniques for various semiconductor materials for both the photoanode (BiVO4<\/sub>\u00a0by electrodeposition and Fe2<\/sub>O3<\/sub>\u00a0by chemical bath deposition) and photocathode (Cu2<\/sub>O by electrodeposition and conjugated polymers by dip coating). The semiconductor deposition process was followed by electrocatalyst deposition of an inorganic protection layer (if needed), to accelerate the kinetics of the electrochemical reaction and finally the deposition of the water-absorbing layer: Nafion.<\/p>\nOur first results for gas phase H2<\/sub>\u00a0production were demonstrated with a conjugated polymer semiconductor photocathode membrane assembly that resulted in a photocurrent density on the order of 1 mA cm-2<\/sup>\u00a0(consistent with around 1.3% solar to hydrogen efficiency). In comparison, the same photocathode achieved around 5 mA cm-2<\/sup>\u00a0when measured in the liquid phase. Therefore, we are now focusing on how to improve water transport within the photoelectrode through experimental and simulation studies.<\/p>\nLiquid energy carrier: HydroSil<\/h3>\n HydroSil is a silicon-hydride-based chemical that has been developed by HySiLabs \u2013 a project partner. Silicon hydride molecules are an appealing solution for energy storage through Si-H bonds, due to their high energy storage capacity. Depending on their molecular structure, silicon hydrides can be in the gas, liquid, or solid state at ambient temperature and pressure, however, the liquid form is the most interesting for transportation and energy storage, as it can take advantage of existing infrastructures.<\/p>\nFig. 4: HydroSil properties overview<\/figcaption><\/figure>\nSeveral liquid silicon hydride molecules have been investigated for energy storage. However, of the known molecules, tetrasilylmethane, phenylsilance, and methylhydrosiloxane decompose with CO2<\/sub>\u00a0emissions and pentasilane is pyrophoric (ignites spontaneously in the air), raising safety issues. In comparison, HydroSil releases only hydrogen during the energy release process, simplifying the recycling process, and is as safe as conventional liquid fuels. The chemical reaction to release hydrogen from HydroSil is simple, rapid, and requires only water and a catalyst.<\/p>\nWhilst the hydrogen release process from HydroSil is now well-established, the focus of Sun-To-X has been on the development of an efficient and cost-effective charging process. During the course of the project, over 400 chemical reactions were benchmarked in terms of energy efficiency and a three-step process has been designed to form HydroSil to minimise cost and energy input. A solar receiver has been designed which uses concentrated solar irradiation to heat air, which is used to heat the process reactors. HySiLabs is currently looking at ways to optimise the overall yield of the HydroSil synthesis process.<\/p>\n
Depolymerisation of waste plastics<\/h3>\n Driven by consumer demand, global plastic production reached 365 million tonnes in 2020 (Plastics \u2013 the Facts 2020 An Analysis of European Plastics Production, Demand and Waste Data) and, with recycling rates as low as 35% in the European Union (Plastics – the Facts 2021 An Analysis of European Plastics Production, Demand and Waste Data), much of this plastic is finding its way into the environment.<\/p>\n
In addition to this, conventional plastic recycling uses a mechanical recycling process in which the plastic is ground into small pieces and re-moulded. The shortening of the polymer length through the milling process, along with the inability to remove impurities, such as dyes or plasticisers, results in a \u2018down-cycling process\u2019, where plastic use is limited to certain applications, such as outdoor furniture.<\/p>\n
Therefore, chemists have looked towards chemical recycling processes, such as reductive depolymerisation. This method is applicable for plastics, such as polyethylene terephthalate (PET), polycaprolactone (PCL), polylactic acid (PLA), polypropylene carbonate (PPC), and polyurethane (PU), where the monomers are joined by C-O bonds. During the depolymerisation, the C-O bonds between each monomer are broken through a reductive reaction, resulting in a monomer solution from which impurities can be easily removed to re-form a high-value virgin plastic. Alternatively, the reactivity can be tuned to reduce the monomers to other high-value materials, such as hydrocarbons, as we investigate in the Sun-To-X project \u2013 particularly the reductive depolymerisation of PCL to hexane (Fig.5).<\/p>\n
Molecules containing Si-H bonds are particularly promising for reductive depolymerisation of plastics with significant development on the catalysis which can enable these reactions to proceed at ambient or near-ambient temperature and pressure. However, one of the challenges has been the lack of recyclability of the Si-H containing molecules. After they have been depleted of hydrogen, the molecules need to be re-synthesised from new starting materials. HydroSil provides an interesting alternative to the conventional Si-H molecules, as after use, it can be recharged with hydrogen and used again for the same process. The Sun-To-X project proposes this method to utilise Si-H reactivity in a way that is compatible with a circular economy and has the potential to be economically feasible.<\/p>\n
The focus of the project has been to develop catalysts that can catalyse the depolymerisation of PCL to hexane at high yield using HydroSil. Through optimisation of reaction conditions, our studies have shown that using a tris(pentafluorophenyl)borane (BCF) catalyst at ambient temperature results in the formation of hexane at a 68% yield. Further, heating the reaction mixture to 60\u00b0C results in an impressive 85% yield of hexane. We have also been able to demonstrate the conversion of other plastics to hydrocarbons, for example, PLA to propane and PPC to propane and methane using the same BCF catalyst.<\/p>\nFig. 5: HydroSil properties overview<\/figcaption><\/figure>\nMission Innovation<\/h3>\n Mission Innovation was involved in the writing of the call that resulted in the granting of the Sun-To-X project and we have collaborated with them throughout the project. Mission Innovation is a global initiative catalysing a decade of action and investment in research, development, and demonstration to make clean energy affordable, attractive, and accessible for all through encouraging discussion and collaboration between various countries.<\/p>\n
Its aim is to accelerate progress towards the Paris Agreement goals and pathways to net zero. Sun-To-X organised a joint event with Mission Innovation in 2021 \u2013 a workshop on global Mission Innovation projects to identify collaboration opportunities and discuss regional roadmaps \u2013 and plans to organise a follow-up event in 2023. For further details, please look out for updates on our website, LinkedIn, and Twitter pages.<\/p>\n
Conclusions and future perspectives<\/h3>\n The Sun-To-X project is contributing to the development of the energy-efficient synthesis of alternative liquid fuels for use in transport and energy storage. We are still working towards the optimisation of our process efficiency and demonstrating our value chain at the end of the project. The project results are expected to build a sustainable future for the mitigation of climate change.<\/p>\n
Acknowledgements<\/h3>\n This project has received funding from the European Union\u2019s Horizon 2020 research and innovation programme under grant agreement 883264.<\/p>\n
Please note, this article will also appear in the thirteenth edition of our\u00a0quarterly publication<\/a>.<\/em><\/strong><\/p>\n","protected":false},"excerpt":{"rendered":"The Sun-To-X project uses solar energy to produce a carbon-free, energy-dense, liquid fuel for use in transport and energy storage.<\/p>\n","protected":false},"author":18,"featured_media":29776,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"_monsterinsights_skip_tracking":false,"_monsterinsights_sitenote_active":false,"_monsterinsights_sitenote_note":"","_monsterinsights_sitenote_category":0,"footnotes":""},"categories":[24204],"tags":[529,24161,730,24477],"acf":[],"yoast_head":"\n
Sun-To-X: Creating a carbon-free liquid fuel for energy storage<\/title>\n \n \n \n \n \n \n \n \n \n \n \n \n \n\t \n\t \n\t \n \n \n \n \n \n\t \n\t \n\t \n