3D Printed Structural Pseudocapacitors - a Multi-Scale X-Ray Tomography Study

Abstract

Pseudocapacitors are energy storage devices which offer energy and power densities greater than supercapacitors and lithium-ion batteries respectively, however their mainstream adoption has been limited by poor lifetime and low areal capacitance. Of the pseudocapacitive materials, manganese oxides have received broad interest due to their relatively high gravimetric capacitances and low cost, however poor electronic conductivity results in low areal capacitance and thus impractical devices. To mitigate this, authors have combined manganese oxides with conductive additives in order to achieve higher mass loadings with high power rate capability. Of the conductive additives, doped polymers such as PEDOT:PSS are attractive as they are not only highly conductive but also exhibit pseudocapacitive behaviour, however the volume expansion observed upon charging/discharging results in limited electrode lifetime due to irreversible microstructural changes. 3D printing is a technology which has yet to be widely adopted within the development of electrochemical devices but could potentially offer new avenues for intelligently designed electrodes. Here we present a novel 3D printed pseudocapacitor, suitable for structural energy storage applications which exhibits improved performance and lifetime over traditional planar electrodes. Using direct metal laser sintering, we create intelligently designed scaffolds of stainless steel onto which we perform a co-deposition of manganese oxides (electrochemical deposition) and doped conducting polymer (electrophoretic deposition) to create a composite electrode with hierarchical porosity. Raman and x-ray photoelectron spectroscopy are used to confirm the presence of the manganese oxides and conducting polymers via the co-deposition process. Using a combination of scanning electron microscopy, multi-scale x-ray computed tomography and electrochemical tests, we then show new insights into how microstructural evolution relates to observed performance increases and decreases over the device lifetime. Through the creation of porous 3D printed scaffolds, we show that this approach can improve durability via mechanical confinement of the active material, minimising the detrimental effects of the volume expansion such as electrode delamination. High resolution x-ray computed tomography was then use to demonstrate how microstructural features such as the lamella like electrode structure and evolution of cracks in the electrode can explain the initial observed performance increase upon cycling due to an increase in accessible surface area. The figure below shows the reconstructed x-ray image of the 3D printed metal scaffold (grey) and co-deposited pseudocapacitive material (purple) with a orthogonal slice shown indicating the distribution of active material. The orthogonal slice of the high-resolution x-ray tomography image shows the lamella like structure, porous under-layer and cracks in the active material which contributes to changes in the electrochemical performance of the device. These novel insights can potentially open new routes for the design of future structural energy storage devices. Figure 1