The Sun-To-X project focuses on using solar energy to produce a carbon-free, non-toxic, energy-dense, liquid fuel for use in transport and energy storage.
Significant progress has been made in recent years toward the production of electricity from renewable energy. However, the production of chemical fuels, which currently make up around 80% of the energy we consume (IEA, World Energy Balances, 2020), from renewable energy is more challenging. The development of green production methods of chemical fuels, where renewable energy is stored in chemical bonds, such as hydrocarbons and hydrogen, is critical as a way to buffer the intermittent energies (such as wind and solar), key in supporting the national or international transport of energy, and useful in providing energy to remote or decentralised locations.
To tackle this challenge, Sun-To-X has been funded by the European Union’s Horizon 2020 programme and is a consortium of nine partners comprised of Research and Technology Organisations (RTOs), industry, and Small-Medium Enterprises (SMEs). The project started in September 2020 and is planned to conclude in February 2024.
A decarbonisation of the fuel chain
The Sun-To-X project aims to explore a new value chain for chemical energy storage (Fig. 1). As a first step, solar energy is used to produce hydrogen from ambient humidity or rain, as a water feedstock. This hydrogen is then reacted through a thermochemical process with a recyclable silicon-oxide-based precursor to form HydroSil – a carbon-free, non-toxic, energy-dense liquid fuel, which can be directly applicable in the transport and energy sectors.
The HydroSil molecule is stable for more than one year, making it suitable for long-term storage of renewable energy. We then explore another use for HydroSil in the reductive depolymerisation of waste plastic towards the development of a circular economy. For all of the processes in this value chain, the consortium has focused on the use of abundant materials to minimise its environmental impact.
The project has the following key technical objectives:
- Building an efficient prototype device for solar hydrogen production;
- Storing this hydrogen in the form of HydroSil through a thermochemical process; and
- Demonstrating the use of HydroSil for reductive depolymerisation of waste plastics.
These objectives contribute to the European Union and Mission Innovation goals for economic development, and the enhancement of energy security through the construction of a sustainable energy system.
Solar hydrogen production
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., Energy Environmental Science, 2016). Our targets aim towards developing a 10% solar-to-hydrogen efficiency device.
Most 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’s 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.
Ideally, 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 – 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).
© 2023 The Authors
Overcoming 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 (M Caretti et al., 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 – preparation of semiconductor deposition methods onto porous supports.
Since 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 by electrodeposition and Fe2O3 by chemical bath deposition) and photocathode (Cu2O 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.
Our first results for gas phase H2 production were demonstrated with a conjugated polymer semiconductor photocathode membrane assembly that resulted in a photocurrent density on the order of 1 mA cm-2 (consistent with around 1.3% solar to hydrogen efficiency). In comparison, the same photocathode achieved around 5 mA cm-2 when measured in the liquid phase. Therefore, we are now focusing on how to improve water transport within the photoelectrode through experimental and simulation studies.
Liquid energy carrier: HydroSil
HydroSil is a silicon-hydride-based chemical that has been developed by HySiLabs – 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.
Several liquid silicon hydride molecules have been investigated for energy storage. However, of the known molecules, tetrasilylmethane, phenylsilance, and methylhydrosiloxane decompose with CO2 emissions 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.
Whilst 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.
Depolymerisation of waste plastics
Driven by consumer demand, global plastic production reached 365 million tonnes in 2020 (Plastics – 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.
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 ‘down-cycling process’, where plastic use is limited to certain applications, such as outdoor furniture.
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 – particularly the reductive depolymerisation of PCL to hexane (Fig.5).
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.
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°C 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.
Mission Innovation
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.
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 – a workshop on global Mission Innovation projects to identify collaboration opportunities and discuss regional roadmaps – 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.
Conclusions and future perspectives
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.
Acknowledgements
This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement 883264.
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