Architecting Electrospun Carbon Fibres for Redox Flow Batteries and The Influence of Fibre Size and Alignment

As one of the promising uprising technologies in the field of stationary energy storage, redox flow batteries are in focus for the future storage of sustainable energy harvesting. The carbon fibre-based electrodes in these batteries are a crucial component in achieving high efficiency and performance, which is significantly influenced by the composition and morphology of the utilized carbon fibres. Our work is based on the fabrication of carbon fibres by using electrospinning, a potential alternative to replace commercial carbon felts with higher performance based on the outstandingly high surface-to-volume ratio. This also allows the effective fabrication of doped and decorated fibres by an in situ doping of the fabricated fibre with catalytically active metal nanoparticles or through heteroatom-based doping of the fibre material itself.<br/>Electrospinning has been proven to be a highly versatile technique to fabricate submicron carbon fibres and can be performed with alternative polymeric materials to synthesize carbon fibres, including biopolymers from renewable sources, such as lignin, opening the possibility of a fully sustainable future of energy storage, without any dependence on petrol-derived resources. In recent years, different approaches to implementing bioderived carbon into the electrode have been reported, most commonly by using an additive method to enhance the commercial carbon felt with biomass-derived carbon particles, or by using given fibrous biomaterials from fibrous structure.<br/>The versatility of the electrospinning process allows the fabrication of different fibre architectures. By modifying the collector of the electrospinning process to a rotating drum, the fibres can be collected in alignment with the rotation which can then be installed parallel or perpendicular to the flow field of a redox flow battery. The orientation can lower the pressure within the system and significantly influence the current density during the operation of the battery. To optimize the balance between pressure and performance, the degree of orientation of the fibres was gradually controlled through the rotation speed of the drum.<br/>Another important factor that influences the performance of a redox flow battery is the fibre diameter, which can be controlled by changing the parameters of the electrospinning process. While electrospun carbon fibres are generally used due to their electrochemical active surface area compared to commercial felts, the process itself has the potential to form fibres in different sizes, in this case in the magnitude starting from 220 nm and up to 850 nm.<br/>Changing the size of the fibres is a trade-off between the available performance through the higher surface area of the smaller fibres and lowering the pressure of the flow and favouring the mass transport of the larger fibres.