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A fantastic journey of electrolytes: What happens to concentration polarization when current flows?
When electric current flows through an electrolyte solution, concentration polarization occurs quietly, which not only affects the electrochemical process, but also profoundly affects the operation of membrane science and technology. This phenomenon is a phenomenon of great significance in the scientific community, especially in electrochemistry and membrane technology. It is essentially caused by the change in electrolyte concentration caused by the passage of current at the interface between the electrode and the solution.
Concentration Polarization in Electrochemistry
In the context of electrochemistry, concentration polarization describes the portion of electrolytic cell polarization that is due to changes in electrolyte concentration when current passes through the electrode/solution interface. In this sense, concentration polarization is equivalent to "concentration overpotential". When certain chemical substances participate in electrochemical reactions, if their supply is insufficient, the concentration of the substance on the electrode surface will decrease. This causes diffusion processes to maintain a balance between the supply and consumption of substances.
Concentration polarization has a profound impact on the rate of electrode reactions and the migration process of ions.
Concentration Polarization in Membrane Science
With the development of membrane technology, the concept of concentration polarization has also been introduced and extended to membrane science. In membrane technology, concentration polarization refers to the concentration gradient that occurs at the membrane/solution interface, which is caused by the selective transfer of certain species under the influence of the driving force across the membrane. This phenomenon is an inherent feature of all membrane separation processes.
For example, in processes such as gas separation, evaporative membrane separation, reverse osmosis, nanofiltration, ultrafiltration, and microfiltration, the solute concentration near the upstream surface of the membrane is often higher than in the uniform fluid away from the membrane surface.
When the membrane has different permeabilities to certain substances, the slope of the peach number and the concentration difference across the membrane surface can lead to the formation of concentration polarization.
Effects of concentration polarization on membrane separation process
Concentration polarization can significantly affect performance in membrane separation processes. First, changes in solution concentration will reduce the driving force inside the membrane, so the effective rate of separation will decrease. In a pressure-driven process, this phenomenon leads to an increase in the osmotic pressure gradient inside the membrane, thereby reducing the net driving pressure. During dialysis, the driving concentration gradient inside the membrane also decreases.
The large concentration gradient will affect the potential difference and the conductivity of the membrane, reducing the separation efficiency.
Improving strategies to address concentration polarization
Generally speaking, in order to reduce concentration polarization, increasing the flow rate of the solution between membranes and spacers that promote turbulence are common methods. This technology enables better solution mixing and reduces the thickness of the diffusion boundary layer. Especially during electrodialysis, higher voltage is applied to promote current-induced convection, further improving the mixing of the solution.The inductive conduction phenomenon is particularly important in dilute solutions, and can significantly increase the current density when exceeding the limiting current density, opening up new ideas for the design of microfluidic devices. At the same time, this also prompts the development of membrane materials to move towards higher performance.
Future Challenges and Opportunities
In the future development of membrane technology and microfluidic devices, the study of concentration polarization still has great challenges and opportunities. By gaining a deeper understanding of the mechanisms of concentration polarization, scientists can develop more efficient separation technologies and further revolutionize water treatment and energy conversion using emerging electrokinetic microfluidics.
Is it possible for us to break through the limitations of concentration polarization in the near future and achieve more efficient resource allocation and utilization?
