Zero-gap Electrode Configurations

The zero-gap design is also utiUzed for processes in which gases are consumed at the electrodes. The most noteworthy example is in fuel cells where H2 and O2 react at the electrode surfaces to form H2O and release electrical energy. 

Porous electrodes must be carefiiUy designed to ensure that four processes can occur  

Ideally four media, a hydrophobic pore connected to a gas manifold, a protrusion of the electrode, a protrusion of the membrane, and a hydrophilic pore connected to a solvent manifold, would converge at a point that is geometrically optimized so that the resistance to transfer is the same in all four media. Further, there would need to be an abundance of these convergence points packed into a small volume with very short transport distances to minimize resistances. Practically it is very difficult to achieve the convergence of four separate media and maintain their cormectivity with short transport distances. Therefore, it is usually necessary for one or more of the media to perform double or triple duty. 

In the conventional design where a solution flows between the membrane and the electrode surface, the solution must perform three of the functions Hsted above. That solution must contain an electrolyte so that ions can transport the current through the gap. The formation and growth of gas bubbles blocks the current flow, so the bubbles must be swept away by swift solution flow. 

In zero-gap membrane-electrode assemblies the membrane performs the double duty of ion removal and delivery of solvent molecules to the electrode 

---