Numerical and experimental investigation of slurry electrodes

Using non-renewable energy resources like fossil fuels has far-reaching effects on humans, the environment and eco-systems and it is the main reason for global warming. To overcome this issue, different types of renewable energy resources have been developed. Due to intermittency of the most abundant sources (like solar and wind), there is a need to store the obtained energy. Various energy storage systems like batteries and super capacitors have been developed and recently flowable electrochemical systems are introduced. Slurry electrodes, a suspension of particles in an electrolyte, are used in different electrochemical systems offering several advantages compared to solid electrodes. Slurry electrodes can be used in electrochemical hydrogen storage in order to store hydrogen within porous conductive particles, particularly carbon particles, to avoid issues associated with high-pressure gas or ultra-low temperature liquid hydrogen. Because of the many variables associated with slurry electrodes such as particle size, shape, conductivity and concentration, their properties are not completely understood for system design considerations. In flowable slurry electrodes, charge transfer coincides with particle/particle and particle/current collector interactions, and it is challenging to predict/control how carbon particles suspended in slurry electrodes participate in the charge transfer process. For this purpose, in this research slurry electrodes are simulated by utilising a novel coupled Computational Fluid Dynamic and Discrete Element Method (CFD-DEM) technique to analyse the dynamic and hydrodynamic forces resulting from exchanging momentum between the particles and electrolyte. For the first time, the effects of Brownian motion on charge transport are simulated. Additionally, by introducing a flow dependent charge transfer efficiency coefficient into the model, the impacts of added electrical resistances due to the electrolyte and particle contacts are simulated. The obtained results show that, with a parallel plate electrode configuration, in the creeping flow regime, the electric conductivity of the slurry electrode decreases with increased velocity. It is shown how increasing the particle concentration increases the electric conductivity of the electrode. Our study demonstrates that the particle-wall interactions dominate the resistance compared to the particle-particle conductivity. To increase the accuracy of CFD-DEM simulations, the flow-dependant charge transfer efficiency coefficient is introduced to the model, and the effects of current collector shapes and configurations on the percolation threshold and electric conductivity of slurry electrodes are investigated. The results showed that contact and electrolyte resistances play a vital role on efficiency of slurry electrodes. It is also demonstrated that with a perpendicular current collector configuration the conductivity of slurry electrodes doubles as the velocity increases almost 3 orders of magnitude, while with a traditional parallel configuration it decreases by approximately 50% over the same velocity range. Additionally, it is shown that the charge percolation network associated with rapid increases in conductivity forms after 12 vol.% with a perpendicular current collector, and after 20 vol.% with a parallel configuration. Hexadecyltrimethylammonium bromide (HTAB) surfactant (5 mM) is used in slurries to avoid agglomeration and sedimentation of the carbon particles in the channel. It is proven that after adding HTAB surfactant, electrical conductivity of slurries is increased by roughly 10% while the viscosity is reduced by 30%. Furthermore, ideal slurry electrodes which have high electrical (both electronic and proton) conductivity to minimise the electric resistance and ohmic power loss, and low viscosity to minimise parasitic pumping power, while utilising porous particles with high surface areas for hydrogen storage are introduced. It is shown that although carbon black (CB) particles have higher electronic conductivity than activated carbon (aC) particles, their proton conductivity is significantly lower, and they cannot be used for hydrogen storage. The electronic conductivity of aC slurries is increased by adding CB particles. It is demonstrated that the addition of a 1:10 ratio by weight of CB to aC particles reduces the electric resistance and ohmic power loss by 50%, while parasitic pumping power increases by only 15% compared to slurries with no CB particles. It is concluded that at low Reynolds numbers, for 5 and 20 wt.% aC slurries with different particle sizes, slurries containing 20 wt.% spherical aC particles smaller than 50 µm mixed with 2 wt.% CB particles provide the highest electronic and proton conductivity, while not significantly increasing parasitic pumping power. Finally, a microfluidic proton flow reactor (MPFR) as a small-scale PFR is designed and fabricated to enable in-situ visualisation of the key processes during PFR operations. The PFR is a hybrid system of a reversible PEM fuel cell combined with a hydrogen-storage electrode which stores energy in the form of hydrogen in porous carbon particles in slurry electrodes. However, the hydrogen storage mechanisms and the reactions in this system are not well understood. In this study, the behaviour of slurries and water in both hydrogen and oxygen sides is observed with emphasis on processes in the vicinity of membranes. The fluorescence microscopy with quinine is used to visualize hydronium transporting from the oxygen to the hydrogen side. Furthermore, in-situ Raman spectroscopy is employed to analyse surface structural changes in carbon particles before and after charging. The formation of hydronium ions on the oxygen side and their subsequent migration to the hydrogen side is demonstrated by fluorescence microscopy, proving that the oxygen evolution reaction occurs on the oxygen side. Raman heat maps prove the formation of carbon–hydrogen bonding in particles after charging with PFR. Although the MPFR is operated at non-optimal slurry concentrations to allow optical access, it is demonstrated that it provides maximum hydrogen storage capacity of 0.64 wt.%. This research contributes to a better understanding of charge transfer phenomena in slurry electrodes and their application in electrochemical energy storage. The findings provide valuable insights into system design considerations, optimizing the properties of slurry electrodes, and improving the efficiency of electrochemical hydrogen storage systems.