The production of hydrogen (and oxygen) via water electrolysis is a non-spontaneous process. This means that an external source of energy is needed to create a potential difference between the electrodes and drive the electrons flow. Theoretically, the minimum potential difference needed between the electrodes is ΔE = 1,23 V at standard conditions. However, the theoretical potential to drive the water electrolysis has never been experimentally achieved without an additional source of heat to have both splitting of water and having the phase change from liquid into gaseous (Gibbs-Helmholtz equation). Hence, the minimum required potential is ΔE = 1,23 V + η, where η is the so-called overpotential. The overpotential of a redox reaction is the potential needed to overcome the energetic barriers of the half-reactions, determined essentially by the activation energy of the reaction, and it is intrinsic to the materials used as electrodes. If the voltage exceeds the thermoneutral potential, which is 1,48 V for water splitting under standard conditions, the temperature of the system will increase finally dissipating into heat.
High overpotential values leading to potentials larger than 1.48V are translated to high power consumption when producing H2, as shown by the definition P = ΔE · I. Therefore, materials with higher overpotential will require more power consumption to produce the same amount of H2 than materials with lower overpotentials, at constant current density and temperature. Evaluating the material overpotential is, hence, a crucial factor when selecting the electrodes for an efficient larger scale production.
In order to assess performance of the electrode materials, either as anodes or as cathodes, they have to be measured in a “three electrodes setup”. In this setup, besides the known anode and cathode, there is an additional electrode, so-called reference electrode, which possess a stable potential. This allows the potential difference of the desired electrode material to be measured versus the reference electrode, and therefore to be evaluated individually. This individual evaluation cannot be done at a full stack nor in a single cell, making the laboratory scale essentially necessary for such assessment.
On the same way than the overpotential, the degradation of the material performance is another key factor to be considered. Changes in stack potential over time are, besides the factors affecting the internal components of the stack, due to mechanical stress of the electrode materials, loss of coating, or loss of active sites (e.g. due to anode oxidation).
Accelerated Stress Tests (AST) are defined to simulate the long-term performance of the electrodes, providing the necessary data to assess the changes of the materials performance along their lifetime. Performing ASTs at an early stage, such as in laboratory conditions, can save both time and costs compared to later testing.
In conclusion, selecting the right electrode materials and predicting their long-term performance is crucial for an efficient large H2 production. For that reason, at McPhy, besides our stack testing facilities to evaluate electrodes in real conditions, we focus on developing a state-of-the-art laboratory, which allows us to identify at an early stage the electrodes performance and reliability.
REFERENNCES:
Leitner, W., Quadrelli, E. A., Schlögl, R. Harvesting renewable energy with chemistry (2017)
Carmo, M., & Stolten, D. Energy storage using hydrogen produced from excess renewable electricity (2019)
Tsotridis, G., & Pilenga, A. EU harmonized protocols for testing of low temperature water electrolysis (2021)
