New generation of supercapacitors set to electrify green transportation

Collabrating with University College London Electrochemical Innovation Lab (UCLEIL), we have produced a cheaper, more sustainable and energy-dense electrode material for supercapacitors which could pave the way for wider market penetration of this high-power, quick charging electric vehicle technology.

doi.org/10.1002/advs.202100016

https://www.imperial.ac.uk/news/223353/new-generation-supercapacitors-electrify-green-transportation/

Supercapacitors are increasingly used in short-distance electric transportation due to their long lifetime (≈15 years) and fast charging capability (>10 A g−1). To improve their market penetration, while minimizing onboard weight and maximizing space-efficiency, materials costs must be reduced (<10 $ kg−1) and the volumetric energy-density increased (>8 Wh L−1). Carbon nanofibers display good gravimetric capacitance, yet their marketability is hindered by their low density (0.05–0.1 g cm−3). Here, the authors increase the packing density of low-cost, free-standing carbon nanofiber mats (from 0.1 to 0.6 g cm−3) through uniaxial compression. X-ray computed tomography reveals that densification occurs by reducing the inter-fiber pore size (from 1–5 µm to 0.2–0.5 µm), which are not involved in double-layer capacitance. The improved packing density is directly proportional to the volumetric performances of the device, which reaches a volumetric capacitance of 130 F cm−3 and energy density of 6 Wh L−1 at 0.1 A g−1 using a loading of 3 mg cm−2. The results outperform most commercial and lab-scale porous carbons synthesized from bioresources (50–100 F cm−3, 1–3 Wh L−1 using 10 mg cm−2) and contribute to the scalable design of sustainable electrodes with minimal ‘dead volume’ for efficient supercapacitors.

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Publication: Progress and Perspectives in Photo- and Electrochemical-Oxidation of Biomass for Sustainable Chemicals and Hydrogen Production

Biomass is recognized as an ideal CO2 neutral, abundant, and renewable resource substitute to fossil fuels. The rich proton content in most biomass derived materials, such as ethanol, 5-hydroxymethylfurfural (HMF) and glycerol allows it to be an effective hydrogen carrier. The oxidation derivatives, such as 2,5-difurandicarboxylic acid from HMF, glyceric acid from glycerol are valuable products to be used in biodegradable polymers and pharmaceuticals. Therefore, combining biomass-derived compound oxidation at the anode and hydrogen evolution reaction at the cathode in a biomass electrolysis or photo-reforming reactor would present a promising strategy for coproducing high value chemicals and hydrogen with low energy consumption and CO2 emissions. This review aims to combine fundamental knowledge on photo and electro-assisted catalysis to provide a comprehensive understanding of the general reaction mechanisms of different biomass-derived molecule oxidation. At the same time, catalyst requirements and recent advances for various feedstock compounds are also reviewed in detail. Technoeconomic assessment and life cycle analysis are performed on various feedstocks to assess the relative benefits of various processes, and finally critical prospects are given on the challenges and opportunities for technology development to meet the sustainability requirement of the future global energy economy.

doi.org/10.1002/aenm.202101180

Strategies for High Energy Density Dual‐Ion Batteries Using Carbon‐Based Cathodes

The rapid-growing demands for lithium-ion batteries (LIBs) have raised concerns over lithium's scarcity as well as the scarcity of other materials and components used in LIBs. Tremendous efforts have been dedicated to investigating alternative technologies. Dual-ion batteries (DIBs) represent an emerging battery technology with an attractive future such as high working voltage and a high-power density enabled by a “nonrocking chair” operation. Research in DIBs is still at an early stage. The energy density of DIBs remains a challenge to solve, especially in comparison with LIBs. This review highlights current challenges in the research on DIBs from different aspects, including undesirable graphite exfoliation during ions intercalation, limited choices of cathode materials, unstable electrolytes, battery safety, and discusses potential strategies for addressing these challenges. Perspectives for exploring the next-generation DIBs with high energy density are also provided.

https://doi.org/10.1002/aesr.202100074

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New publication on Investigating the effect of edge and basal plane surface functionalisation of carbonaceous anodes for alkali metal (Li/Na/K) ion batteries

Alkali metal ion batteries are instrumental in the widespread implementation of electric vehicles, portable electronics, and grid energy storage. From experimental characterisation of hard carbons, these carbon anodes were shown to contain a variety of functional groups. Through density functional theory simulations, the effect of functional groups (O, OH, NH2, and COOH) on edges and basal plane surfaces of carbonaceous materials on the adsorption of lithium, sodium, and potassium are investigated. These simulations show that the functionalisation of H-terminated edges and curved surfaces rather than basal planes is more energetically favourable and thus more likely to be present. Comparison of experimental FTIR and computational vibrational frequency analysis confirmed the occurrence of the investigated functional groups (O, OH, NH2, and COOH) in the synthesised hard carbon materials. Metal adsorption on the functionalised models showed that adsorption energies were stronger on the functionalised basal plane in comparison to the functionalised edge sites and contribute to the metal ion immobilization and consequent irreversible capacity loss. The metal adsorption on the curved surface was further improved by the addition of functional groups, benefitting the initial lithiation/sodiation/potassiation of the carbon anode. Hence, the morphology of the functionalised carbon systems plays an important role in the charge/discharge performance of carbonaceous anodes.

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New Publication on Engineering the Electrochemical Interface of Oxygen Reduction Electrocatalysts with Ionic Liquids: A Review

Hydrogen fuel cells are a promising technology for the environmentally sustainable production of electricity. However, their commercialization is hindered by the sluggish kinetics of the oxygen reduction reaction and by the high cost of the state‐of‐the‐art platinum catalysts. To address these challenges, research has focused on the enhancement of the activity of platinum and platinum group metal (PGM)‐free electrocatalysts, by modifying their composition and topology. Recently, a new approach has emerged to boost the activity of ORR catalysts, based on engineering the electrochemical interface. Herein, the recent developments in the use of ionic liquids (ILs) to modify the triple‐point interface of ORR catalysts are summarized. In this review, the current understanding in the literature of the effect of IL layers is presented, along with the open questions and remaining challenges. A short perspective on the applicability of this simple and effective modification to other electrochemical reactions is discussed.

https://doi.org/10.1002/aesr.202000062

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Sustainable Batteries—Quo Vadis?

Over the past 20 years, a revolution has been seen in battery research culminating with a much‐awaited Nobel Prize in Chemistry in 2019 for the development of Li‐ion batteries. New Li‐ion battery materials have been developed recently with improvements in performance. New Li battery chemistries have also emerged, exhibiting high energy density such as Li‐S, Li‐O2, Li‐metal with solid state electrolytes as well as zero‐excess Li anode metal batteries. This is tremendous progress and batteries are becoming more efficient and cheaper each year. Yet, most research in batteries is entirely focused on performance while the sustainability of all battery components making up the cell, as well as the battery chemistry itself are much overlooked. In this essay some perspectives are discussed and opinion is provided on the advancement of sustainability in battery research.

https://doi.org/10.1002/aenm.202003700

New Publication on Carbon Composite Anodes with Tunable Microstructures for Potassium‐Ion Batteries

Among the post‐lithium battery technologies, potassium‐ion batteries are promising for cost‐effective large‐scale energy storage, as potash is an abundant resource. However, a major challenge is to understand the structure‐performance relationships of carbon anodes for potassium‐ion storage. In this study, we have designed a variety of carbon composite materials from 100 % graphite to 100 % soft carbon and in between, with tunable structural features to fundamentally understand the roles of different carbon structural features in potassium ion insertion. We have found that the graphite‐soft carbon composites (G‐SC) show a high charge capacity of 280.2 mAh g−1 with an increased initial coulombic efficiency, representing the best reversibility among different carbon composites. Electrochemical impedance spectroscopy, cyclic voltammetry, and ex‐situ structural characterizations have been applied to substantiate that the presence of soft carbon in G‐SC inhibits the solid electrolyte interface layer formation and provides structural protection to the graphitic layers.

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New Publication on Sodium Storage Mechanism Investigations through Structural Changes in Hard Carbons

Hard carbons, due to their relatively low cost and good electrochemical performance, are considered the most promising anode materials for Na-ion batteries. Despite the many reported structures of hard carbon, the practical use of hard carbon anodes is largely limited by low initial Coulombic efficiency (ICE), and the sodium storage mechanism still remains elusive. A better understanding of the sodium-ion behavior in hard carbon anodes is crucial to develop more efficient sodium-ion batteries. Here, a series of hard carbon materials with tailored morphology and surface functionality was synthesized via hydrothermal carbonization and subsequent pyrolysis from 1000 to 1900 °C. Electrochemical results revealed different sodiation-desodiation trends in the galvanostatic potential profiles and varying ICE and were compared with theoretical studies to understand the effect of the varying hard carbon structure on the sodium storage process at different voltages. Furthermore, electrode expansion during cycling was investigated by in situ dilatometry; to the best of our knowledge, this is the first time that the technique has been applied to hard carbons for ion storage mechanism investigation in Na-ion batteries. Combining experimental and theoretical results, we propose a model for sodium storage in our hard carbons that consist of Na-ion storage at defect sites and by intercalation in the high voltage slope region and via pore filling in the low voltage plateau region; these findings are important for the design of future electrode materials with high capacity and efficiency.

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New publication on hardwood versus softwood Kraft lignin – precursor-product relationships in the manufacture of porous carbon nanofibers for supercapacitors

The process of stabilization is essential in the production of carbon fibers from lignins. During stabilization, the initially thermoplastic lignin polymer is converted to a thermoset polymer allowing for high-temperature treatment without a change in shape. In this work, hardwood (HKL) and softwood (SKL) Kraft lignins were stabilized in air at temperatures between 190 and 340 °C before carbonization at 800 °C in a nitrogen atmosphere. Due to the differences in side-chain linkages, functional groups and molar mass, the lignins exhibit different structural changes upon stabilization and hence develop different porosities upon carbonization. Both lignins undergo major crosslinking reactions in the side chains at low temperatures and degradation reactions at high temperatures during stabilization. Crosslinking gives rise to narrow pore size distributions with mainly (sub-) nanometer pores, whereas degradation reactions lead to a more open pore structure with additional mesoporosity (>2 nm). When both types of reactions take place simultaneously, highly accessible (sub-) nanoporosity can be effectively created, which boosts the performance of supercapacitors operating in 6 M KOH(aq). This effect terminates when the crosslinking reactions cease and mainly degradation reactions take place, which occurs in HKL at 340 °C. SKL shows both a lower degree of crosslinking and degradation and hence develops less specific surface area. The optimum performance in an aqueous alkaline supercapacitor is achieved with HKL stabilized at 310 °C. It shows a specific gravimetric capacitance of 164 F g−1 at 0.1 A g−1 and 119 F g−1 at 250 A g−1 with a capacitance retention of more than 90% after 10 000 cycles.

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New Publication on A revised mechanistic model for sodium insertion in hard carbons

Hard carbons have shown considerable promise as anodes for emerging sodium-ion battery technologies. Current understanding of sodium-storage behaviour in hard carbons attributes capacity to filling of graphitic interlayers and pores, and adsorption at defects, although there is still considerable debate regarding the voltages at which these mechanisms occur. Here, ex situ Na solid-state NMR and total scattering studies on a systematically tuned series of hard carbons revealed the formation of increasingly metallic sodium clusters in direct correlation to the growing pore size, occurring only in samples which exhibited a low voltage plateau. Combining experimental results with DFT calculations, we propose a revised mechanistic model in which sodium ions store first simultaneously and continuously at defects, within interlayers and on pore surfaces. Once these higher energy binding sites are filled, pore filling occurs during the plateau region, where the densely confined sodium takes on a greater degree of metallicity.

Heather Au, Hande Alptekin, Anders C. S. Jensen, Emilia Olsson, Christopher A. O’Keefe, Thomas Smith, Maria Crespo-Ribadeneyra, Thomas F. Headen, Clare P. Grey, Qiong Cai, Alan J. Drew and Maria-Magdalena Titirici*, Energy Environ. Sci., 2020, https://doi.org/10.1039/D0EE01363C

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New Publication on Recent Advances in Hydrothermal Carbonisation: From Tailored Carbon Materials and Biochemicals to Applications and Bioenergy

Introduced in the literature in 1913 by Bergius, who at the time was studying biomass coalification, hydrothermal carbonisation, as many other technologies based on renewables, was forgotten during the “industrial revolution”. It was rediscovered back in 2005, on the one hand, to follow the trend set by Bergius of biomass to coal conversion for decentralised energy generation, and on the other hand as a novel green method to prepare advanced carbon materials and chemicals from biomass in water, at mild temperature, for energy storage and conversion and environmental protection. In this review, we will present an overview on the latest trends in hydrothermal carbonisation including biomass to bioenergy conversion, upgrading of hydrothermal carbons to fuels over heterogeneous catalysts, advanced carbon materials and their applications in batteries, electrocatalysis and heterogeneous catalysis and finally an analysis of the chemicals in the liquid phase as well as a new family of fluorescent nanomaterials formed at the interface between the liquid and solid phases, known as hydrothermal carbon nanodots.

Sabina A. Nicolae,* Heather Au, Pierpaolo Modugno, Hui Luo, Anthony E. Szego, Mo Qiao, Liang Li, Wang Yin, Hero J. Heeres, Nicole Berged and Maria-Magdalena Titirici* Green Chem., 2020,22, 4747-4800. https://doi.org/10.1039/D0GC00998A

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New Publication: Local mobility in electrochemically inactive sodium in hard carbon anodes after the first cycle

Sodium ion batteries are a promising alternative to current lithium ion battery technology, providing relatively high capacity and good cycling stability at low cost. Hard carbons are today the anodes of choice but they suffer from poor rate performance and low initial coulombic efficiency. To improve the understanding of the kinetics of sodium mobility in these materials, muon spin rotation spectroscopy and density functional theory calculations were used to probe the intrinsic diffusion of sodium in a characteristic hard carbon sample. This revealed that atomic diffusion between sites is comparable to that observed in transition metal oxide cathode materials in sodium ion batteries, suggesting that the poor rate performance is not limited by site–site jump diffusion rates. In addition, diffusion was observed in the sodium that is irreversibly stored during the first cycle, suggesting that some of these sodium atoms are not immobilised in the solid electrolyte interface (SEI) layer but are still blocked from long range diffusion, thereby rendering the sodium electrochemically inactive.

Anders C. S. Jensen, Emilia Olsson,  Heather Au,  Hande Alptekin,  Zhengqiang Yang,   Stephen Cottrell,  Koji Yokoyama, Qiong Cai,     Maria-Magdalena Titirici*  and  Alan J. Drew* . (2019). Local mobility in electrochemically inactive sodium in hard carbon anodes after the first cycle. Journal of Materials Chemistry A.

https://doi.org/10.1039/C9TA10113F

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