Lukas Fieber1 Chun Huang1 Patrick Grant1

1, University of Oxford, Oxford, , United Kingdom

Advances in additive manufacture (AM) processes such as fused deposition modelling (FDM) and direct ink writing (DIW) may offer a flexible, cost-effective approach to address conventional manufacturing limitations, such as time-consuming, high work-in-progress, multi-step assembly. In principle, AM can also allow more novel geometric or even bespoke designs of structural and functional products. However, in terms of energy storage devices, such as batteries and supercapacitors, the benefits of AM have yet to be exploited fully and current manufacturing approaches remain concerned only with planar large area electrode assembly, through many stages, into simple rolled or planar cell configurations.

In this paper, we first assume increased design flexibility from additive manufacture for all key sub-components or materials of an electrochemical double-layer capacitor (EDLC, supercapacitor) in order to propose a range of novel supercapacitor materials arrangements and overall device geometries (form-factor). We then use mean-field local-density approximations based on dilute-solution theory to model the non-linear induced charge electro-kinetics and the overall supercapacitor energy storage behaviour. We predict performance in various novel arrangements of the constituent materials and their spatial arrangement at the electrode length-scale, for various unusual geometric form-factors at the packaged device scale.

To explore the model predictions we have designed, built and commissioned a novel, modular AM system, extending typical AM functionality by combining FDM and DIW techniques simultaneously, together with various in-situ monitoring tools. The system automatically deposits supercapacitor elements including an encapsulating housing (PLA/ABS/PMMA), binder-less electrodes (activated carbon, surfactant), aqueous electrolyte (dilute sodium sulfate), and current collectors (silver based) in a single non-stop operation, allowing for finished cell manufacturing times of less than 20 minutes. The manufactured EDLCs demonstrate high aerial capacitance of 1240 mFcm-2 at 50mVs-1 and good cycling stability of 94% over 1000 cycles. The design factors affecting gravimetric performance were also revealed.

We show that single-step manufacture of encapsulated supercapacitors removes the need for the traditional discrete and time wasting steps of cell assembly, suggesting supercapacitors and other energy storage devices can be directly integrated into 3D-printed systems and components. At a more fundamental level, the flexibility AM of supercapacitor sub-elements provided an opportunity, not available from conventional manufacture, to validate designs for optimised ion-diffusion pathways based on simulation. AM also showed how changes in the overall cell geometry can be used to generate unusual combinations of energy and power in terms of areal, gravimetric and volumetric performance.