The Effect of Electrolyte and Additive Concentration on Zinc-Nickel Flow Cell Performance

This is the author accepted manuscript. The final version is available from Elsevier via the DOI in this recordThis work aims to identify a suitable electrolyte composition for the operation of a zinc-nickel flow cell at ambient temperature. The effect of varying electrolyte composition containing KOH, ZnO, tetraethylammonium hydroxide (TEAH) or tetrabutylammonium bromide (TEAB) electrolyte additives are investigated. A 15 mM concentration of TEAH is found to provide smooth and compact zinc depositions. Increasing concentrations of KOH are found to be detrimental to voltaic efficiency, with coulombic efficiency peaking in 6 M KOH. The coulombic efficiencies of both zinc and nickel electrodes improve with ZnO concentration. Galvanostatic zinc-nickel flow cell cycling yields the highest efficiencies in an electrolyte of 6 M KOH with 0.5 M ZnO and 15 mM TEAH, with coulombic, voltaic and energy efficiencies of 98 %, 88 % and 86 % respectively over 70 stable charge/discharge cycles.Engineering and Physical Sciences Research Council (EPSRC)University of Exete

Enhanced Surface Area Carbon Cathodes for the Hydrogen–Bromine Redox Flow Battery

This is the final version. Available on open access from MDPI via the DOI in this recordData Availability Statement: Data supporting the results can be found within the MELODY project community on the Zenodo repository: https://zenodo.org/communities/melody (accessed on 30 November 2022).The hydrogen–bromine redox flow battery is a promising energy storage technology with the potential for capital costs as low as 220 $ kWh−1 and high operational power densities in excess of 1.4 W cm−2. In this work, enhanced surface area bromine electrodes incorporating carbon black (CB) and graphene nanoplatelets (GnPs) on carbon paper and carbon cloth substrates were investigated, and the effect of electrolyte concentration on performance of the electrodes was studied. Carbon-black modified electrodes are found to possess the largest electrochemically active surface areas, i.e., up to 11 times that of unmodified materials, while GnP electrodes are shown to have superior kinetic activity towards the bromine electrode reaction. In terms of performance, lower electrolyte concentrations are found to favour the improved kinetic parameters associated with graphene nanoplatelet electrodes, while highly concentrated electrolytes favour the larger electrochemically active surface area of carbon black electrodes. The optimal performance was achieved on a carbon-black-modified carbon cloth electrode in a 6 M HBr/2 M Br2 electrolyte concentration, with polarisation current densities approaching 1.6 A cm−2 at overpotentials of ±400 mV, and mean overpotentials of 364 mV during oxidation and 343 mV during reduction, resulting from bromine oxidation/reduction cycling tests at ±1.5 A cm−2.European Union Horizon 2020Engineering and Physical Sciences Research Council (EPSRC

Electrochemical testing of carbon materials as bromine electrodes for the hydrogen-bromine redox flow battery

This is the final version. Available on open access from MDPI via the DOI in this recordHydrogen-bromine (H2-Br2) redox flow batteries (RFBs) have gained a lot of interest due to their advantages in mitigating the performance shortcomings of conventional zinc-bromine and vanadium flow batteries. Various carbon materials have been tested in H2-Br2 RFBs as bromine electrodes. However, a comparative study between different carbon materials has not been reported in literature. This work reports for the first time an evaluation of carbon papers, felt and cloth in a three-electrode half-cell setup as potential bromine electrodes, in pristine and thermally treated state. A systematic evaluation was performed by comparing the surface morphologies, kinetic parameters, polarisation curves and stability tests of different carbon electrodes. Thermally treated graphite felt electrode demonstrated the best electrochemical performance as bromine electrode owing to its improved surface area, hydrophilicity and intrinsic activity. Further in-depth studies will shed important insights which will help understand the electrode characteristics for future bromine battery design. The current study will assist in evaluating the performance of upcoming novel electrode materials in three electrode assembly.European Union Horizon 2020Engineering and Physical Sciences Research Council (EPSRC)Royal Academy of Engineering (RAE

Selection of oxygen reduction catalysts for secondary tri-electrode zinc-air batteries

This is the final version. Available on open access from Nature Research via the DOI in this recordOxygen reduction reaction (ORR) electrocatalysts, which are highly efficient, low-cost, yet durable, are important for secondary Zn-air cell applications. ORR activities of single and mixed metal oxide and carbon electrocatalysts were studied using rotating disc electrode (RDE) measurements, Tafel slope and Koutecky-Levich plots. It was found that MnOx combined with XC-72R demonstrated high ORR activity and good stability – up to 100 mA cm-2. The performance of the selected ORR electrode and a previously optimised oxygen evolution reaction (OER) electrode was thereafter tested in a custom-built secondary Zn-air cell in a tri-electrode configuration, and the effects of current density, electrolyte molarity, temperature, and oxygen purity on the performance of the ORR and OER electrode were investigated. Finally, the durability of the secondary Zn-air system was assessed, demonstrating energy efficiencies of 58-61% at 20 mA cm-2 over 40 h in 4 M NaOH + 0.3 M ZnO at 333 K.Engineering and Physical Sciences Research Council (EPSRC)University of ExeterEuropean Regional Development Fund (ERDF)Royal Academy of Engineering (RAE

3D Hierarchically Structured CoS Nanosheets: Li+ Storage Mechanism and Application of the High-Performance Lithium-ion Capacitors

This is the author accepted manuscript. the final version is available from the American Chemical Society via the DOI in this recordLithium-ion capacitors, which possess excellent power and energy densities, can combine both those advantages from supercapacitors and lithium-ion batteries, leading to the novel generation hybrid devices for storing energy. This study synthesized one three-dimensional (3D) hierarchical structure self-assembled from CoS nanosheets, according to a simple and efficient manner, have been used as anode for lithium ion capacitors. This CoS anode, based on a conversion-type Li+ storage mechanism dominated by diffusion controlled, showed a large reversible capacity, together with excellent stability for cycling. The CoS shows a discharge capacity ≈ 434 mA h/g at 0.1 A/g. The hybrid lithium-ion capacitor, which had the CoS anode as well as the biochar cathode, exhibits excellent electrochemical performance with ultra-high energy and power densities of 125.2 Wh/kg and 6400 W/kg, respectively, and an extended cycling life of 81.75% retention after 40000 cycles. The CoS with self-assembled 3D hierarchical structure in combination with a carbon cathode offers a versatile device for future applications in energy storage.National Natural Science Foundation of Chin

The influence of zinc electrode substrate, electrolyte flow rate and current density on zinc-nickel flow cell performance

This is the author accepted manuscript. The final version is available from Elsevier via the DOI in this recordThis work aims to identify a suitable material for use as a zinc electrode substrate material in alkaline media, then employ this to study the effect of electrolyte flow rate and current density on zinc-nickel flow cell performance. Three metallic and four graphite composite materials are investigated, with the coulombic efficiency of zinc electrode charge / discharge cycling found to increase as hydrogen evolution onset potentials become more negative. A graphite / PVDF composite substrate demonstrates the highest coulombic efficiency at 96.7 % and the most negative hydrogen evolution onset potential at -1.595 V vs. Hg/HgO. Using this material, the effect of electrolyte flow rate and current density on a zinc-nickel flow cell is investigated. Zinc morphology and flow cell performance is related to the ratio of applied current density to limiting current density. At values between 0.47 and 1, boulder type zinc morphologies have been shown to occur, with smooth and compact zinc deposits resulting from current density ratios of 0.39 and below. Stable zinc-nickel flow cell performance is achieved over 200 cycles with coulombic, voltaic and energy efficiencies of 98.3, 88.1 and 86.6 % respectively, at a current density of 20 mA cm-2.Engineering and Physical Sciences Research Council (EPSRC

Cu-assisted induced atomic-level bivalent Fe confined on N-doped carbon concave dodecahedrons for acid oxygen reduction electrocatalysis

This is the author accepted manuscript. The final version is available from Elsevier via the DOI in this recordAtomically dispersed transition metals anchored on N-doped carbon have been successfully developed as promising electrocatalysts for acidic oxygen reduction reaction (ORR). Nonetheless, how to introduce and construct single-atomic active sites is still a big challenge. Herein, a novel concave dodecahedron catalyst of N-doped carbon (FeCuNC) with well confined atomically dispersed bivalent Fe sites was facilely developed via a Cu-assisted induced strategy. The obtained catalyst delivered outstanding ORR performance in 0.5 M H2SO4 media with a half-wave potential (E1/2) of 0.82 V (vs reversible hydrogen electrode, RHE), stemming from the highly active bivalent Fe-Nx sites with sufficient exposure and accessibility guaranteed by the high specific surface area and curved surface. This work provides a simple but efficient metal-assisted induced strategy to tune the configurations of atomically dispersed active sites as well as microscopy structures of carbon matrix to develop promising PGM-free catalysts for proton exchange membrane fuel cell (PEMFC) applications.National Natural Science Foundation of ChinaNatural Science Foundation of Jiangsu Provinc