Catalyst degradation mechanisms in Proton Exchange Membrane Electrolysers

Hydrogen is recognised as a clean energy carrier that could play a key role in reducing global carbon emissions. In Proton Exchange Membrane Water Electrolysers (PEM WE), the hydrogen evolution reaction (HER) takes place at the cathode, where protons from the acidic electrolyte combine with electrons to form hydrogen gas. Catalysts are essential to this process, as they reduce the activation energy required for the reaction. Noble metals such as platinum are the benchmark materials for HER catalysts due to their exceptional activity and stability. However, their scarcity and high cost limit large-scale adoption. Non-noble catalysts, including transition metal-based materials such as nickel, molybdenum, and cobalt compounds, are attractive alternatives due to their lower cost and abundance. Despite these advantages, non-noble catalysts are more susceptible to degradation under the acidic conditions of PEM WE.

 

The degradation of non-noble metal catalysts can arise from several processes, which often are interdependent. While the exact mechanism of the catalyst’s degradation is still unknown, the most common processes include:

 

Electrochemical dissolution: Non-noble metals are thermodynamically unstable in the highly acidic environment of PEMWE, and even at negative potentials, some dissolution can occur. The dissolved metal ions leached from the catalyst surface into the electrolyte lead to the gradual loss of active material and can also cause the membrane degradation. This process is particularly accelerated when there are localized pH changes, mechanical stress, or small voltage fluctuations during operation.

 

Hydrogen embrittlement: Under cathodic polarization, hydrogen atoms can diffuse into the non-noble metal lattice, leading to embrittlement. This occurs when hydrogen penetrates interstitial spaces in the catalyst structure, reducing mechanical strength and causing micro-cracks. Over time, this mechanical degradation results in the detachment of catalyst particles and loss of performance.

 

Surface reconstruction: During long-term operation in acidic environments, under continuous gas evolution, non-noble catalysts can undergo surface reconstruction, where the catalyst surface evolves into a less active state. Although passivation layers can temporarily protect the catalyst, they also block active HER sites, reducing efficiency.

 

Mechanical degradation and detachment: The acidic electrolyte and high current densities in PEM WE induce significant mechanical stress on the catalyst layer. Non-noble catalysts, often less robust than noble metals, can suffer from particle agglomeration, detachment, or cracking, particularly if they are supported on unstable substrates.

 

Impurity Effects: Impurities in the water feed or electrolyte, such as chloride ions, sulfur species, or transition metal contaminants, can trigger localized corrosion and pitting of non-noble catalysts. Chloride, for example, facilitates aggressive corrosion through the formation of soluble complexes, destabilizing the catalyst surface.

 

To make green hydrogen production economically viable, the stability of non-noble catalysts must be improved without compromising their activity. A deeper understanding of the fundamental mechanisms of catalyst degradation is crucial. NICKEFFECT combines experimental studies with computational modeling to provide insights into optimizing catalyst compositions and structures. These advances will enable PEM WE to operate efficiently over extended lifetimes, while significantly reducing the reliance on noble metal use in  energy sector.