P-011

Mairis Iesalnieks

mairis.iesalnieks@rtu.lv

Mārtiņš Vanags, Andris Šutka

Institute of Physics and Materials, Riga Technical University, Latvia

Bismuth-derived tungsten and molybdenum compounds as an effective and novel redox mediator for decoupled electrolysis.

The transition to green and renewable energy is not without problems. For green transition, we must find reliable and robust energy storage media to mitigate the intermittence of renewable energy sources. For now, a huge emphasis is put on battery technology, but they are bulky, expensive, and hard to scale up. Hydrogen is an excellent alternative to batteries as a cheap and versatile storage media or as a raw material in the chemical industry, but its manufacturing must be made more reliable and cheaper. In this work, we will examine the use of bismuth tungstate and molybdenite as possible redox mediators in alkaline media. Bi2WO6 and Bi2MoO6 were synthesised using the hydrothermal method by dissolving Stochiometric amounts of Bi(NO3)3 and Na2WO4 or Na2MoO4 in double distilled water. The pH of the suspension is adjusted to neutral using an NH4OH solution. Suspension is transferred to Teflon lined autoclave and heat treated at 180 oC for 8h. Produced particles are washed multiple times and vacuum-dried at 60 oC overnight. Particles were analysed using SEM, XRD and XPS, to evaluate particle size and morphology. For electrochemical evaluation, nanoparticles were mixed with Vulcan and PVDF in a mass ratio of 80:10:10 and drop-cast on a glassy carbon tip. Mas loading of the samples was kept constant at 100 µg/cm2. Performance was evaluated based on cyclic voltammetry, electrochemical impedance spectroscopy and chronopotentiometry data in 3 and 2-electrode systems. XRD and XPS revealed pure Bi2WO6 and Bi2MoO6 crystalline phases without any alien compounds or unreacted raw materials. The average particle size in both cases was around 50 nm. The electrochemical performance revealed high specific capacity values for Bi2MoO6 at 402 F/g (@5mV/s) with distinctive redox peaks at -0.6 and -0.7V vs Ag/AgCl, far from OER and HER potential in alkaline media, making them suitable for decoupled electrolysis. Electrodes stability was evaluated using chronopotentiometry in 2 electrode system with current densities of 100, 500 and 1000 mA/g. After 2000 cycles, electrodes showed excellent stability with capacity retention above 90%. Faradaic efficiency reached 95%, with a total efficiency of 60%, similar to lower values of commercial electrolysers. Bismuth compounds have an excellent potential for making decoupled electrolysis more achievable and cheaper, but more tests are needed, and stability still must be addressed in future works.