Intensified electrochemical processes
Process intensification is in principle a strategy of making dramatic reductions in the capital cost of a production system, improving intrinsic safety, reducing environmental impact and energy consumption. An electrochemical cell is a specific example of a multiphase system that should be capable of being operated very intensively within a centrifugal acceleration field. This thesis presents the first systematical research work in this field. It demonstrates that many electrochemical processes can be enhanced considerably by using a high-acceleration field, which takes full advantage of the improvement in catalytic activity and optimal electrode structure. Six electrochemical processes have been investigated. Namely, the electro-reduction of oxygen, electro-oxidations of hydrogen and methanol, chlorine evolution, water electrolysis, and methanol fuel cell reaction. Several catalysed electrodes including Ti mesh or carbon cloth were prepared and used for the above processes. A number of catalyst deposition routes were explored for the preparation of the technical electrodes, including Pt, Ag, Pt-Ru and RuO₂ electrodes. The action of a centrifugal field is two fold: first, it acts as an mass transfer promotor; second, it accelerates gas bubble disengagement For chlorine evolution, water electrolysis, and methanol oxidation, centrifugal fields play a very significant role in and lead to significant reductions in polarisation and mass transport resistance. The high acceleration fields reduce the cell resistance drastically through effectively disengaging gas bubbles from the electrode surface. It also overcomes mass transport limitations in these systems through generation of powerful interphase buoyancy force. The processes were therefore greatly intensified in centrifugal fields and approached a maximum efficiency. Increase in the operating temperature and the concentration also benefit the processes. Interestingly, both effects were intensified in centrifugal fields, i.e., the current density increases more rapidly in centrifugal fields than in stationary cell at a constant potential corresponding to the same increment in temperature and concentration. For the gas consuming systems, e.g., oxygen reduction and hydrogen oxidation, centrifugal fields also produce positive results. The degree of intensification for these systems was limited to a relatively low level. Centrifugal fields have little benefit for methanol fuel cell operation. The results were disappointing and unexpected. It was realised that the cathode process, an oxygen gas consuming reaction, is difficult to be intensified by using centrifugal means due to the type of electrode used at the cathode, although the anode process was intensified in centrifugal fields. A model for a centrifugal cell with gas evolving electrodes was proposed and tested. In the model the cell voltage and cell voltage reduction are obtained from the hydrodynamic electrochemical, and electrochemical engineering theories.