What are Gas Diffusion Electrodes (GDEs)?

What are Gas Diffusion Electrodes?

Gas diffusion electrodes are composed of a porous catalyst layer which has been applied on carrier material. The catalyst layer, which conducts electrons, catalyses an electrochemical reaction between the liquid and the gaseous phase. Thus, the gas, electrolyte and the catalyst must be contacted. This is referred to as the so-called three-phase boundary, where three aggregate states (solid, liquid, gaseous) are contacted.
The electrode material as a solid phase is the electron conductor and at the same time the catalyst that enables the actual reaction.
The electrolyte, which is usually liquid, is the ion conductor and reaction medium, providing soluble starting materials such as protons or hydroxide ions. At the same time, reaction products such as hydrogen peroxide or formate can also dissolve in the electrolyte. The charge-balancing ion transport takes place via the electrolyte.
The gaseous reaction partners involved in the reaction, such as hydrogen, air, oxygen, or carbon dioxide, are in the gas phase. The gases and the electrolyte must be brought into contact at the catalyst grains. Therefore, the electrode must have a porous structure. This requires a very large active electrode area with a small geometric surface. The starting materials must enter the pore system, and the reaction products must be transported out of the electrode. This also affects the water balance of the electrode. The higher the temperature, the faster these diffusion processes take place. Temperatures of 80-90°C are optimal. Gas diffusion electrodes are then efficient and allow very high reaction rates.

Gas diffusion electrodes are used in different electrochemical reactions in batteries, fuel cells and electrolysis cells:

• HOR  Hydrogen Oxidation Reaction - oxidation of hydrogen to protons or water (hydrogen consumption)
• HER  Hydrogen Evolution Reaction (HRR  Hydrogen Reduction Reaction) - reduction of protons to hydrogen (hydrogen evolution)
• OER  Oxygen Evolution Reaction (OOR  Oxygen Oxidation Reaction) - the oxidation of water or hydroxide ions to oxygen (oxygen evolution)
• ORR  Oxygen Reduction Reaction - reduction of oxygen molecules to water or hydrogen peroxide (Oxygen consumption)
• CO2RR  CO₂  Reduction Reaction - reduction of carbon dioxide to different products possible