Materials approaches to achieving a net zero carbon economy

Stephen Skinner, Professor of Materials Chemistry at Imperial College London, discusses the impact of net zero carbon policies and the challenges these present for materials and systems developers

Our progress towards a net zero carbon economy is accelerating rapidly with frequent new announcements and targets, most recently with the International Energy Agency suggesting in their report to COP26 that new natural gas boilers should be prohibited from 2025(1).

With these policy announcements it is essential to consider the practical impact of the policy, and the challenges this presents to materials and systems developers. The clear question that arises from such announcements is what is the envisaged alternative?

In terms of replacing domestic boilers a leading contender would be the hydrogen fuel cell, which could be the polymer electrolyte based cell (PEM) or solid oxide cell (SOC)(2). For domestic heat and power, and taking a lead from the experience of fuel cell deployment in Japan(3), it can be argued that SOCs are best placed to provide combined heat and power (CHP) in this setting. These devices offer high total and electrical efficiencies, and have fuel flexibility, with immediate operation on reformed natural gas, leading to direct operation on hydrogen, assuming a hydrogen feedstock is available. It is evident that technologies exist to effectively utilise a hydrogen fuel, but the key consideration is where, or how, is the hydrogen produced.

Innovation in the materials sector

Developing an effective route to hydrogen production requires innovation in the materials sector, and there are several possible routes envisaged. The use of solid oxide cells, which are ceramic devices operating at temperatures in range of 500-900oC, offers a high efficiency route to electrolyse steam which will produce an output of H2(g) but this may need further processing to separate hydrogen from residual steam. In an alternative mode of operation a proton conducting ceramic device utilises proton transport through a ceramic membrane to produce a dry high purity hydrogen stream. Whilst this is an attractive option, the current materials performance and durability are areas of concern for developers, and as such there are significant research efforts devoted towards discovery and optimisation of new high performance ion conducting materials, including in membranes and electrodes.

Ceramic proton conductors were initially identified by Iwahara et al in the 1980’s(4) with the fast protonic transport observed in the A(Zr,Ce)O3 (A = Sr, Ba) family of materials. Since that discovery numerous studies have been undertaken to optimise these phases, as the stability of the phases in CO2 and H2O containing atmospheres was unacceptable. Using mixed oxides with substitution of the metal cations produced the BaCe1-x-yYxYbyO3-d composition, with improved performance. The hydrogen permeation (transport) of these dense ceramics is highly attractive, and has led to significant interest in developing compatible fuel and air electrode materials.

It is then of interest to consider what the requirements of the electrode materials will be, with several researchers considering the advent of ‘triple conductors’ as a potential  solution – that is a single material that conducts protons, oxide ions and electrons. These materials have been proposed as an explanation of the effectiveness of oxides as proton-conducting electrodes, but there is a significant challenge in categorically determining the three contributions to the total conductivity.

To effectively and unambiguously determine the nature of the mobile ions requires use of isotopic labels, including D2O, combined with mass spectrometry. This technique enables the diffusion rate of protons to be directly observed as a function of temperature of operation.

Understanding the ion transport of materials in the solid state is essential in further developing technologies that will enable the transition to a net zero carbon economy, and this is one of the aims of the HERMES project(5), where hydrogen transport and compression in ceramic devices is a key aspect of the research programme. In addition to developing new devices based on established materials it is also essential to pioneer new materials developments and architectures.

Through international collaboration, new approaches using thin film technologies to lower operating temperatures of solid oxide cells, are being developed as part of the EU Epistore(6) programme. The programme also encompasses the development of new characterisation tools to probe materials interfaces and processes at the atomic scale under operating conditions, which can critically limit performance. These collaborative transnational partnerships are essential if we are to advance towards our net zero carbon targets.

New materials discoveries

New materials discoveries including using the latest computational screening tools, continue apace, with remarkable improvements in properties. Recent discoveries have included the development of new oxide ion conductors that have also exhibited fast proton transport(7-9), opening new avenues of research, whilst the development of exsolved metal nanoparticles from oxide surfaces(10) has presented new routes to effective catalysis in fuel cell anodes. Each of these materials discoveries have resulted from international collaboration and the ability of researchers to work on jointly funded programmes, whether through short exchanges as provide by the Royal Society, or larger programmes funded by the EU and EPSRC/JSPS.

Our work at Imperial College has benefitted enormously from this internationalisation. Understanding proton uptake and transport in the new BaNdInO4(11) family of oxides resulted from work in the UK, Japan and Argentina, with multiple exchange visits between these partners.  Without this level of international cooperation our target of net zero carbon is a much greater challenge.

The EpiStore and Hermes projects received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement numbers 101017709 and 952184 respectively.

References

4. https://www.iea.org/reports/net-zero-by-2050, Accessed 21/05/2021.
5. I. Staffell et al, Energy Environ. Sci., 2019 12 463.
6. https://www.eu-japan.eu/sites/default/files/publications/docs/hydrogen_and_fuel_cells_in_japan.pdf, Accessed 21/05/2021 .
7. H. Iwahara et al, Solid State Ionics, 1983 9-10 1021.
8. https://hermesproject.eu/, Accessed 21/05/2021.
9. https://www.epistore.eu/, Accessed 21/05/2021.
10. S. Fop et al, Nature Mater. 2020 19 752.
11. A. Gilane et al, J. Mater. Chem. A, 2020 8 16506.
12. C. Fuller et al, Chem. Mater., 2020 32 4347.
13. D. Neagu et al, Nature. Chem, 2013 5 916.
14. Y. Zhou et al, Chem. Mater., 2021 33 2139.

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© 2019. This work is licensed under CC-BY-NC-ND.