Keryn Lian

Knitted and screen printed carbon-fiber supercapacitors for applications in wearable electronics

The field of energy textiles is growing but continues to face two main challenges: (1) flexible energy storage does not yet exist in a form that is directly comparable with everyday fabrics including their feel, drape and thickness, and (2) in order to produce an “energy textile” as part of a garment, it must be fabricated in a systematic manner allowing for multiple components of e-textiles to be integrated simultaneously. To help address these issues, we have developed textile supercapacitors based on knitted carbon fibers and activated carbon ink. We show capacitances as high as 0.51 F cm−2 per device at 10 mV s−1, which is directly comparable with those of standard activated carbon film electrodes tested under the same conditions. We also demonstrate the performance of the device when bent at 90°, 135°, 180° and when stretched. This is the first report on knitting as a fabrication technique for integrated energy storage devices.

High capacitive performance of exfoliated biochar nanosheets from biomass waste corn cob

A high performance exfoliated biochar carbon with a layered nanosheet structure was prepared from a low cost agricultural residue (corn cob) via a novel synthesis strategy involving biomass pre-treatment, nitrogen pyrolysis, and a high temperature thermal–chemical flash exfoliation. The exfoliation strategy resulted in porous carbon nanosheets with BET specific surface area of 543.7 m2 g−1, far higher than the 7.9 m2 g−1 of the natural biochar produced without any pre- or post-treatment modifications. The exfoliated material also showed increased oxygen functionality in the form of electrochemically active quinone and pyrone surface groups. This combination of high specific surface area and highly active surface functional groups resulted in very promising capacitive performance, demonstrating a high capacitance of 221 F g−1, over 100 times greater than the natural biochar. The exfoliated biochar electrodes fabricated without any conductive or organic additives showed outstanding high rate capability retaining 78% of their low rate capacitance at a fast 40 A g−1 discharge. This combination of high capacitance and fast charge–discharge capability distinguishes this material from most other high surface area activated carbons; in fact, the electrochemical behaviour more closely resembles that of designer nanomaterials such as graphene and carbon nanotubes. The biochar electrodes were also extremely durable showing only a 3% reduction in capacitance after 5000 successive potential cycles. The exfoliation strategy developed here could provide a novel route for the low cost production of high performance energy storage materials from a variety of waste biomass feedstocks.

Electrocatalytic behaviour of Ni-base amorphous alloys

Abstract Amorphous metals may be important electrode materials for use in alkaline fuel cells and electrolyser applications. The electrocatalytic activity of amorphous and crystalline Ni-base alloys with respect to the Hydrogen Evolution Reaction (HER) has been studied in the present work. The Tafel parameters have been measured, and the effect of Ni/Co ratio, metalloids and crystallinity on the catalytic activity of both amorphous and crystalline alloys has been examined and compared. The amorphous alloys exhibit lower activities than that of their crystalline counterparts in the as-polished state. However, an enhanced electrocatalytic activity was observed on amorphous alloys after chemical treatments, which was not found on crystalline alloys after similar chemical treatments. Amorphous alloys are more electrocatalytically active than crystalline alloys after chemical treatment, especially in the high overpotential regime.

Electrochemical and surface characterization of electrocatalytically active amorphous NiCo alloys

The electrochemical behaviour of electrocatalytically active amorphous NiCo alloys together with their crystalline counterparts have been studied by means of cyclic voltammetry (cv) in conjunction with X-ray photoelectron spectroscopy (XPS). The cv profiles of these alloys are quite different when tested after steady state polarization compared with potential cycling. The electrochemical behaviour of NiCo alloys shifted substantially from characteristics of Co to those of Ni. XPS analysis was used to identify the changes in surface oxidation species caused by electrochemical treatments. The oxides formed on amorphous alloys are different from those on crystalline alloys, in terms of their oxidation states, the amount of hydroxyl ions and solvent, film thickness, and ratio of components in the film. Furthermore, a hydrous oxide film, consisting of a micro-porous structure filled with solvent and hydroxyl ions, which is electrocatalytically active for the oxygen evolution reaction, was formed on potential cycled amorphous NiCo alloys.

The electrocatalytic activity of amorphous and crystalline NiCo alloys on the oxygen evolution reaction

Abstract The electrocatalytic behaviour of amorphous and crystalline NiCo alloys on the oxygen evolution reaction (OER) in 1 M KOH solution has been assessed in this work. The peroxidized amorphous NiCo alloys showed Tafel behaviour characteristic of Co oxides. However, after potential cycling, the electrocatalytic activity of amorphous and crystalline NiCo alloys increased drastically and their Tafel behaviour showed characteristics similar to pure Ni electrodes. Nickel-rich amorphous alloys showed particularly high activity towards the OER in comparison with their crystalline counterparts and the Co-rich amorphous alloy. In addition, Ni and the amorphous NiCo alloys showed good stability as electrocatalysis while crystalline NiCo alloys were not stable upon potential cycling.

The effect of UV ozone treatment on poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)

The interface between ultraviolet (UV) ozone treated poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and N,N′-diphenyl-N,N′-bis-(1-naphthyl)-1-1′-biphenyl-4,4′-diamine (α-NPD) was investigated using single carrier hole-only devices and in situ ultraviolet and x-ray photoelectron spectroscopy to elucidate the implications for device applications. It is found that although the work function of PEDOT:PSS is increased by UV ozone treatment, the injection barrier to α-NPD is in fact increased, resulting in lower current density in devices. The apparent increase in work function is attributed to a metastable surface dipole as a result of UV ozone treatment, which does not significantly influence the energy-level alignment.

Advanced proton conducting membrane for ultra-high rate solid flexible electrochemical capacitors

Polymer electrolytes comprised of silicotungstic acid (SiWA) in cross-linked polyvinyl alcohol have been developed and characterized for ultra-high rate electrochemical capacitors (ECs). The solid polymer electrolytes outperformed liquid H2SO4 electrolytes in a similar cell setup. The polymer electrolyte enabled thin flexible solid double layer ECs demonstrated an exceptionally high rate capability of 50 V s−1 in cyclic voltammograms and a 10 ms time constant in impedance analyses. Enhanced stability and water uptake were observed for the cross-linked polymer electrolytes compared to their non-cross-linked counterparts. Thermal analysis revealed an increase of water content with the percentage of cross-linking. Structural analyses suggested that SiWA in the cross-linked polymer electrolyte was more hydrated than SiWA in the non-cross-linked polymer electrolyte. In addition, a stronger hydrogen bonding interaction was observed due to an increase in the crystallized water content in hydrated SiWA suggesting a more stable proton-hopping network that can be leveraged for high conductivity.

Ionic Liquid-Derived Imidazolium Cation Linkers for the Layer-by-Layer Assembly of Polyoxometalate-MWCNT Composite Electrodes with High Power Capability.

Imidazolium cations derived from ionic liquids were demonstrated as effective linker molecules for the layer-by-layer (LbL) deposition of polyoxometalates (POMs) to increase the charge storage of multi-walled carbon nanotube (MWCNT) electrodes. MWCNTs modified with GeMo12O40(4-) (GeMo12) via an imidazolium cation linker demonstrated highly reversible redox reactions and a capacitance of 84 F cm(-3), close to 4 times larger than bare CNT. Compared to CNT-GeMo12 composites fabricated with a conventional polyelectrolyte linker poly(diallyldimethylammonium chloride), (PDDA), the imidazolium cations resulted in lower POM loading, but higher conductivity and in turn superior performance at fast charge-discharge conditions. A polymerized imidazolium linker (PIL) was also synthesized based on the ethyl-vinyl-imidazolium monomer. CNT-GeMo12 composites fabricated with this PIL achieved high POM loading comparable to PDDA, while still maintaining the good conductivity and high rate capabilities shown by the monomer imidazolium units. The high conductivity imparted by the PIL is especially valuable for the fabrication of multilayer POM composites. Dual-layer GeMo12 O40(4-)-SiMo12O40(4-) (GeMo12-SiMo12) electrodes built with this PIL demonstrated a combined contribution of the individual POMs resulting in a capacitance of 191 F cm(-3), over nine times larger than bare MWCNT. The PIL dual layer composites also maintained 72% of this capacitance at a fast rate of 2 V s(-1), compared to just over 50% retention for similar electrodes fabricated with PDDA.

Polyoxometalate modified pine cone biochar carbon for supercapacitor electrodes

Pine cones were used as a biomass template for the synthesis of activated carbons with high specific surface area (up to 2450 m2 g−1) and a pore structure optimized for the adsorption of redox active polyoxometalate (POM) clusters. We have found that POM adsorption is highly favored within a carbon matrix possessing pore diameters in the 1–2 nm range. These large micropores are big enough to accommodate the large POM cluster, while still being small enough to effectively trap and hold the molecule. Pine cone activated carbon with this optimal pore arrangement demonstrated ultra-high loading of the PMo12O403− (PMo12) molecule resulting in carbon–POM hybrid materials consisting of over 55 wt% PMo12. This large POM loading imparted tremendous redox activity to the already large double layer capacity of the carbon substrate, leading to a high areal capacitance of 1.19 F cm−2 for the hybrid material, close to 2.5 times larger than for unmodified carbon. We have also demonstrated that a mixed molecular modifier combining multiple POM chemistries can be adsorbed onto the activated carbon substrate to create a more ideally capacitive charge storage profile. These results demonstrate a promising method for the design of high performance yet cost effective hybrid energy storage electrodes.

Heteropoly Acid Electrolytes for Double-Layer Capacitors and Pseudocapacitors

Two heteropoly acids, phosphotungstic acid (PWA) and silicotungstic acid (SiWA), were studied as aqueous electrolytes for applications on electrochemical storage devices. Their performance was compared to the H 2 SO 4 benchmark with respect to both double-layer and pseudocapacitance using cyclic voltammetry, electrochemical impedance spectroscopy, and self-discharge measurements. Both heteropoly acids showed a very good proton conductivity in aqueous solutions, with overall performance comparable to H 2 SO 4 . Therefore, in spite of their electrochemical active nature, PWA and SiWA are feasible as electrolytes provided the operation window is chosen appropriately.