Energy Materials for a Low Carbon Future: Insights for the next Decade

Article by Leonardo Buizza, University of Oxford

Sept. 27, 2018, 1:39 p.m.

The venue of the Royal Society, just off Pall Mall in central London, was just as impressive as the list of speakers gathered to discuss technologies ranging from solid-state batteries through to thin-film perovskite tandem solar cells. Having run for several years as a meeting hosted by the University of Bath, the ‘Energy Materials for a Low Carbon Future’ meeting won a competition allowing it to be hosted by the Royal Society on the 17th-18th September. I will try and briefly do justice to all of the speakers, but feel free to skip the details and go straight to the final three paragraphs, where I give a broader summary (as a side note – apologies to any battery/fuel cell specialists out there who will inevitably feel I have butchered their scientific field…). I will not be running through the talks chronologically – I figured it would be easier to group them into rough ‘clumps’, which then makes explaining them easier.

The field of next-generation rechargeable batteries is an increasingly important one, with wide adoption of batteries expected for electric vehicles in the next decade. The current state of the art are lithium-ion batteries (LIBs), which involve two solid electrodes that allow the ions to pass into and out of them (intercalation), and which are separated by a liquid electrolyte and a porous separator, allowing the Li ions to diffuse between the electrodes whilst keeping the electrodes separate. Researchers are pushing to develop solid-state batteries (SSBs), which swap the liquid electrolyte for a ‘superionic’ material that still allows for ion diffusion, and which aim to provide higher energy densities and higher operating temperatures. Jürgen Janek gave an excellent talk introducing both LIBs and new SSBs such as LPS or LLZO (lithium in sulphide or oxide structures). He stressed the importance of ensuring that interfaces between the new solid-state electrolytes and the electrodes are stable. The topic of interfaces was a recurring theme across the battery talks, with several speakers highlighting the challenge of preventing formation of dendrites at interfaces. Linked to this was the importance of developing a lithium-metal anode, which allows for a higher energy density, but leads to issues of stability when it interacts with electrolytes. Clare Grey’s group investigates these processes using NMR to resolve oxidation sites within materials, whilst Linda Nazar presented research into nanoscale stabilising layers between electrodes and the solid electrolyte. Jean-Marie Tarascon also presented work on both Li- and Na-ion batteries, highlighting similarities between the two such as the presence of polyanionic compounds in the electrodes.

From a more fundamental perspective, Anton Van der Ven presented research that made use of Density Functional Theory to link the electronic structure of materials to their macroscopic electrochemical properties. These talks can often be dangerously impenetrable, so it was a credit to the speaker that the research was presented in a highly engaging way, discussing how variations in the type of ion, from lithium to sodium or magnesium, would lead to changes in transport properties and thus alter the voltage curves of the materials. Finally, Jennifer Rupp covered a wide range of lithium-based technologies being investigated by her group: new solid-state lithium-ion ceramic electrolytes, Li-garnet structures for CO2 sensing, and even using lithium oxides to create memristors for computation (that last one went over my head, too…).

Fuel cells, which typically combine hydrogen and oxygen continuously to produce electricity, also came up across the series of talks. Their importance with regards to long-term storage was stressed: as we tend towards a more renewables-based intermittent electricity supply, the need for long-term storage across several months grows. This is difficult to provide using large-scale batteries, so the potential of using fuel cells in reverse to produce hydrogen, which can be stored (although this also has issues), is an attractive option. Similar challenges exist for fuel cells and batteries: namely, ensuring good ionic transport and stable interfaces between electrolytes and electrodes. Sossina Haile gave an excellent talk on reversible cells, where her group has been investigating the shift away from oxygen ion-conducting electrolytes to ceramic oxides that act as proton conductors. The issue of interfaces in solid-oxide fuel cells was discussed further by John Irvine, whose group has done substantial work on engineering materials and interfaces to promote either infiltration or exolution of ions at the electrodes.

On a completely different note, Mercouri Kanatzidis gave a very engaging talk on thermoelectrics, which have now reached high enough conversion efficiencies and reliability that they were used ahead of solar panels on the Curiosity rover on Mars. The crucial figure of merit for thermoelectrics is their ZT value, which accounts for their thermal and electric conductivities, as well as their ability to convert changes in temperature to voltage. The Kanatzidis group have worked on doping lead telluride with strontium, almost doubling the ZT value of the materials. Another fascinating stand-alone talk was given by Andrew Cooper, where he introduced a novel method to combine modelling and lab-based robotics in order to screen materials for use as gas storage. Computational models allow for a wide family of organic molecules to be screened for particular properties, and then a great video demonstrated how a lab-based robot could carry out a multi-step solution processing and screening experiment, removing a significant amount of tedious work. This demonstration left the audience with a firm grasp of how automation has the potential to boost productivity in research labs, although a few PhD students might have worried about whether the robot would also soon learn to write papers as well...

Perovskite solar cells have been one of the star performers of next-generation thin-film materials for photovoltaics. Henry Snaith gave an engaging talk introducing the family of perovskite materials, ranging from the ubiquitous MAPbI3 through to more complex mixed-cation, mixed-halide perovskites. One key point was the discussion of how to integrate perovskites with existing silicon-based technology by making tandem solar cells – a sector within photovoltaics that is expected to grow quickly within the next few years. Thomas Kirchartz discussed perovskite solar cell devices in more detail, presenting research on how to push open-circuit voltages higher. He also touched upon the competition between radiative and non-radiative processes in perovskites, and how the nature of trap states affects non-radiative recombination rates. Laura Herz went into more detail on the intrinsic nature of charge transport within perovskites. The key point is that a good material should be thick enough to absorb enough light, but thin enough that the diffusion length of charges is greater than the thickness of the film, so they can be extracted as current. The diffusion length is determined by the mobility of charges and their recombination rates, both of which are investigated by the Herz group. Finally, Aron Walsh gave a good overview of the insights that computational modelling can bring to perovskites, notably in better understanding the nature of the dielectric function in these materials, which affects many of the properties of charge carriers. Interestingly, he also touched upon how we are entering a “Third Generation” of computational approaches, where one can apply machine learning to raw chemical data in order to try to predict the composition, structure and properties of materials – a step further than previous approaches that relied on knowledge of a material’s composition.

On an altogether different topic, we were also lucky to hear Jim Skea give an excellent talk on energy policy and innovation. He discussed the upcoming IPCC special report on 1.5 oC, and how this ties into the promises made at the Paris agreement. Enforcement of the agreement will occur via Nationally Determined Contributions, which need to be continuously reassessed in order to ‘ratchet’ down future emissions, eventually reaching net-zero by early in the second half of the 21st century. A key part of this is investment in R&D globally, and one crucial point was how science was currently not matching ambitions in the field of carbon capture and storage. The upcoming few months, with the publication of the special report and the 24th Conference of the Parties at Katowice in December, will be an exciting outline for the future decades of energy policy and how the scientific community can rise to the challenges posed by policymakers.

A concluding discussion session involved the organising panel of Saiful Islam, Jenny Nelson, Richard Catlow and Peter Bruce, as well as Julia Higgins, Professor of Polymer Science at Imperial College and Sir David King, ex-Chief Scientific Advisor to Tony Blair and Gordon Brown. There were a variety of take-home messages from the two days of talks and discussion. From a scientific perspective, highlights included the importance of interfaces in all of the approaches discussed, and how there is a requirement to be able to characterise materials at all length scales, with a notable gap currently in characterising mesoscale properties. On a broader level, Jenny Nelson made the point that researchers must not lose sight of the importance of overall system efficiency, for any technological application – something that can often become blurred by in-depth focus on specific properties or approaches. Sir David King’s concluding remarks highlighted Mission Innovation, an offshoot of the Paris agreement, whose goal is to double R&D spending in the EU and 23 other member countries. Although striking a slightly pessimistic tone, he drew attention to the need for rapid development of all of the technologies discussed at the meeting, whilst arguing that a global carbon pricing mechanism could help both meeting climate goals and promote research into novel technologies.

All told, the conference was a whirlwind tour of the state of the art across a variety of fields. From a personal perspective, it was fascinating to be exposed to fields that I am not involved in, and the broader discussions linking policy with research innovation was very stimulating. 10/10, would recommend.