Solar fuels from sustainable feedstock using Earth-abundant catalysts: Can light drive affordable electrocatalysts for fuel production?

The proposed research targets new technology to synthesise fuels from sustainable feedstocks and renewable energy. This is important work since three fifths of all global energy usage is in the form of fuel burning for transportation and heating. While renewable electricity generation from solar and wind amongst others is on track to reduce the carbon emissions of electricity generation, only two fifths of end use energy is in the form of electricity. It is globally critical to find sustainable and cost effective ways to decarbonise transport and heating.

Synthesising fuels from CO2 feedstocks using the only infinite source of renewable energy - solar - would be an ideal solution. Yet no viable technology have been developed.

Why not?
Traditional catalysts for conversion of CO2 are Nobel-metal based, expensive, and not suitable for mass deployment. Rhenium, which is the basis of the most broadly used catalysts for CO2 reduction, is extremely expensive and rare. Its analog, Manganese, is 1.3 million times more abundant, constituting 0.1% of the Earth's crust. In 2011, researchers showed that Mn-complexes with diimine ligands and carbonyls could be even more active then their Re analogs in reducing CO2. These catalysts are used now used in electrochemical reduction, where the electrons flow from the "mains" to the electrode, then to the catalyst, and finally to CO2. In 2016, we developed a new class of versatile Mn-based catalysts which can be attached to surfaces.

Can we use renewable energy to activate these cheap, versatile, Earth-abundant catalysts?
The major obstacle so far has been that these catalysts are light-sensitive, and we can not use sunlight to activate them directly. We propose to combine the cheap catalysts (Mn-based) with available feedstock (CO2) and renewable energy (solar) in a device which uses sunlight indirectly. We will build on recent (2016) progress in light-absorbing semiconductors and investigate an integrated technology that could provide the sought after breakthrough.

The overall vision is a plate based technology (much like a solar photovoltaic panel) that can be manufactured cheaply in high volumes, that absorbs sunlight and transfers the solar energy to a catalyst that is anchored on the light absorbing surface. The catalyst is fed CO2 in a water based electrolyte and the energy from the sunlight reduces the CO2 to CO, a reactive intermediate from which further, well-known, reactions can make fuels.

Our plan is to use a particular light absorbing electrode (Cu2O/AlZnO protected by TiO2) that has been shown to be highly effective in combination with scarce rhenium based catalysts. We will substitute Rhenium for highly abundant Manganese catalysts and measure how effective they are. The catalyst needs to be anchored to the electrode and must not be directly exposed to sunlight. Our research will overcome these constraints using chemical modification of the catalyst to attach it to the light-absorbing semiconductor electrode, and by illuminating the absorbing electrode from the back of the structure.

In addition, we will build a prototype industrial process scheme from which we will investigate the energy economic performance and carbon emissions of the proposed device. This will allow us to evaluate the likely impact of the technology in terms of mitigation of climate change, and in providing cost effective access to fuels.

We have a team of researchers with expertise spanning chemistry, physics, materials and devices, and techno-economic analysis - the cross-section that is vital for such research to succeed.

Overall, finding a way to solar-power these cheap, versatile catalysts, will make a huge step forward towards clean, renewable ways of producing fuels, and energy, for all. It will invigorate research in cheaper catalysts, materials and devices, improve quality of life - and help the Planet.

The ultimate potential impact of this research is vast from both economic and societal points of view.

In the scenario whereby a viable, cost effective, net energy positive and carbon neutral solar fuel technology is developed, solar energy could be captured and stored in an easily transportable form. Such a fuel would be compatible with existing combustion processes for heating, electricity generation and transportation. The consequences of such a technology would be vast.

The new technology would generate a global industry of the scale of the current oil and gas industries. However, the new industries would not be geographically limited to the locations of fossil fuel reserves but spread across the sunbelt regions of the globe. The technology would unlock the potential for substantial economic development in Africa, South Asia, and the Americas.

In terms of climate change mitigation, the technology would be limited by the ability to recapture and store CO2 from the as synthesised fuel. Understanding the potential for closed cycle uses of synthetic fuel is an important current and future area of research. However it is clear that application in power generation could be more or less closed cycle and therefore solar fuels could become the solution (through dispatchable thermal generation) to inter-seasonal renewable energy storage for substantial electrification of heating and transportation. Solar fuels have the possibility of addressing the biggest issue in global decarbonisation strategy - low carbon energy for transport and heating.

In the short term, impact will be achieved by enhancing cross disciplinary knowledge of energy issues in both the research networks that we are members of but also in the wider dissemination of energy issues to policy makers, schools and technology industries. The UK is regarded as a leading nation in carbon dioxide utilisation and is strong in emerging PV materials science and in photocatalysis. This multidisciplinary project brings together novel technologies and will greatly contribute to retaining, and enhancing, the UK's leading position in this areas.

Industry are key stakeholders of this project and we will concentrate on providing "market intelligence" for approaches to solar fuels that is engaging and understandable by these communities. All three investigators have expertise in this regard, in particular Buckley who has 8 years of industrial technology development and manufacturing experience and a wide range of connections in the UK PV industry. We will produce an industry facing report and will organise a one-day workshop with relevant industrial stakeholders invited, along with academia. We will be aided in this by our links with the CO2Chem Network, Directed Assembly Network, and the Energy KTN.

We will develop an engagement activity to aid public awareness of the potential of solar fuels. The Department of Chemistry has a dedicated RSC-funded School Lab, where school children from the region, as well as the wider UK, come to perform experiments. We will make a prototype device for the school children to work with to aid their understanding and interest in solar fuels. These activities will also be used in our outreach and engagement events, for example Festival of the Mind and Engineering Imagination. Rothman has extensive media training and as part of her role as Faculty Director for Women in Engineering, leads numerous outreach events and engagement activities. The PDRA will also be actively involved in public engagement, including the dedicated School Lab in Chemistry; Researchers Night; and National Science Week.

Finally the PDRA will develop an interdisciplinary skill set, and will receive extensive training, across chemistry, physics, material sciences, engineering devices, and will be exposed to techno-economic analysis, thereby equipping them to be a highly skilled research leader with a broad knowledge base.