Decoupled Electrolysis: A Novel Method for Water Splitting Using WO3 Auxiliary Electrode

Abstract

More emphasis must be placed on the use of clean and renewable energy sources. Production of electricity using solar, wind geothermal etc is becoming more widespread however these types of energy production suffer from intermittence in production and a mismatch of production and consumption cycles. These problems result in a need for an energy storage medium. A hydrogen is a viable option because of its high gravimetric heat capacity (143 MJ/kg) and low flashpoint (-231°C) [1]. For now, most of the produced hydrogen is from hydrocarbons and only 4% is being produced using electrolysis. Hydrogen production via electrolysis is an efficient way how to store surplus energy from renewable energy sources but electrolysis must be made more achievable. Commonly used water electrolysis is limited by the need for complicated gas distribution systems to avoid mixing of H2 and O2 which can lead to a serious safety hazard. Membranes and diaphragms can solve gas mixing problems but they significantly reduce efficiency and increase hydrogen production costs.Here we demonstrate the principle of decoupled electrolysis where oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is separated in space with the use of a WO3 redox mediator. WO3 was produced by the solvothermal method. In accordance with SEM (Fei, NanoSEM 650), XRD (Rigaku, Ultima+) and XPS (Thermo Scientific, Escalab Xi+) data, monoclinic WO3 nanosheet assemblies with a cluster size of 10-20 μm and plate thickness of 50-100 nm were produced without lower oxidation states. Electrodes were produced using carbon felt which was saturated with ink suspension containing active material (WO3), carbon black and polyvinyl difluoride (PVDF) in a mass ratio of 60-30-10. DMF was used as a solvent. Electrolysis was achieved in a two-step process. In the first step, an OER reaction occurs and the H ion is intercalated into WO3 forming tungsten bronze HxWO3. OER reaction occurs on platinum and O2 is released with gas purity above 99.9 %. In the second step, H ion deintercalation occurs and H2 is released at the Pt electrode with a purity greater than 99.9%. Gas purity was determined using a gas mass spectrometer. Electrochemical measurements were performed using Metohm AutoLAB potentiostat with Nova software. The electrode was analysed using cyclic voltammetry (CVP), impedance spectroscopy (EIS) and chronopotentiometry. 0.5 M H2SO4 was used as an electrolyte solution.In accordance with the CVP capacity of the WO3 electrode reaches 325 F/g at a scan rate of 2 mV/s and decreases with an increase in scan speed. A decrease in capacity due to an increase in scanning speed can be associated with limited charge diffusion [2]. The shift of the maximum in the anodic direction associated with the transition from W5+ to W6+ with an increase in scan speed is polynomial and can be associated with the pseudocapacitive behaviour of electrode material. The Nyquist plot from EIS measurements shows the formation of an incomplete arc in the high-frequency interval and linear correlation in lower frequencies. In chronopotentiometry with a current density of 100 mA/g faradaic efficiency reached 90.2% and increased to 100% when 1000 mA/g were used, however taking into account both OER and HER reactions, cycle efficiency decreased from 65 to 43 % with the same current densities. If only HER reaction is analysed when hydrogen is being released and deintercalation happens, cycle efficiency reaches 341 %. High efficiency can be explained by extra energy obtained from the building potential of the WO3 electrode that lowers the standard potential for water electrolysis. Long-term tests showed good stability and no degradation was observed in accordance with SEM and XPS studies. At high intercalation-deintercalation cycle times, a non-Faradaic process can be observed which reduced HER cycle time. A spontaneous deintercalation can be observed. One of the future goals is to prevent this process by the use of different auxiliary electrode materials.[1]-Tashie-Lewis BC, Nnabuife SG. Hydrogen production, distribution, storage and power conversion in a hydrogen economy - a technology review. Chemical Engineering Journal Advances 2021;8:100172.[2]- Das AK, Karan SK, Khatua BB. High energy density ternary composite electrode material based on polyaniline (PANI), molybdenum trioxide (MoO3) and graphene nanoplatelets (GNP) prepared by sonochemical method and their synergistic contributions in superior supercapacitive performance. Electrochim Acta 2015;180:1e15. Figure 1