Of the many suggested applications of graphene, energy storage is one that often attracts much attention with potential improvements in energy, power and lifetime of such devices often postulated. One such energy storage device that is frequently associated with graphene application is the Supercapacitor. The electrodes found in supercapacitor devices are characterized by a high electronic conductivity and a high surface area, usually in excess of 1000 m2
. Graphene as a pristine material is known to have high electrical conductivity and a theoretical surface area of around 2630 m2
but pristine graphene of such high conductivity is not available at scales needed for making such devices or at a cost that is practical. Furthermore, the actual specific surface areas of graphene powders currently available are considerably lower than the theoretical value. It is often reported in the literature that small research cells using graphene as active material offer excellent performance but the performance of cells at a larger prototype scale are rarely encountered. This is due to availability and cost of the graphene material, problems in its processing and availability of necessary equipment. The lack of information regarding larger format graphene cells presents issues when comparing performances with more established materials such as activated carbon, the mainstay of commercial supercapacitor devices. Whilst some metrics associated with small scale graphene cells can be reliably scaled to commercial devices, others cannot and rely on a number of assumptions that often do not hold true for such a material.
In the current work three commercially available graphene nanoplatelets have been selected based on their availability (in multiple Kg scales), their cost and their specific surface areas (between 750 and 300 m2 g-1). The development of these materials into electrodes and their inclusion in A5 sized pouch cells of >300 F capacitance is demonstrated, with their performance compared in cells at the coin and pouch scale, and to cells of the same size and format using activated carbon as the active material. A number of issues encountered in the scale up and development of the inks and electrodes required are highlighted with their solutions, including the higher than expected ESR of electrodes and the need for conductive additives, viscosity effects and gelling not observed at the small scale but hugely problematic at the pilot scale, and “islanding” electrodes after coating and drying at scale.
The production of A5 sized pouch cells from ca. 60 m of each electrode type is demonstrated and extensive testing presented including EIS, float testing for extended periods, galvanostatic, and constant power testing at currents up to 200 A. The results of the testing is discussed in light of the surface areas of starting graphenes as well as electrodes and comparisons made between the values at pouch cell and those extrapolated from coin cell level, and also to those of comparable pouch cells using activated carbons as active material.