Nature Energy: Major Breakthrough in Proton Exchange Membrane Electrolyzers—Acidic Oxide Modification Enables Hydrogen Production via Non-Pure Water Electrolysis

A recent breakthrough published in Nature Energy by research teams from Tianjin University, the University of New Mexico, Sun Yat-sen University, and other institutions has successfully overcome the critical challenge of proton exchange membrane (PEM) electrolyzers requiring ultrapure water. By introducing Brønsted acidic oxides into the cathode catalyst layer of PEM electrolyzers, they constructed a locally strongly acidic microenvironment, laying a key foundation for low-cost and wide-scenario applications of green hydrogen.

Research Background: Dependence on Ultrapure Water Becomes a Bottleneck for Technology Promotion

PEM electrolyzers are a core technology for green hydrogen production, but they impose extremely stringent water quality requirements, necessitating the use of ultrapure water with a resistivity of 18 MΩ·cm. Cation impurities such as Ca²⁺, Mg²⁺, and Fe³⁺ in the feedwater can trigger a series of issues: on one hand, they penetrate the proton exchange membrane and replace the H⁺ ions within the membrane, causing the local pH at the cathode to surge to strongly alkaline levels, which inhibits hydrogen evolution reaction kinetics and leads to hydroxide precipitation, deactivating the catalyst; on the other hand, ions like Fe³⁺ can initiate Fenton reactions, generating reactive oxygen species that accelerate proton membrane degradation, ultimately resulting in the retirement of the electrolyzer. This not only increases water pretreatment costs but also limits deployment in water-scarce regions.

Core Strategy: Constructing a Locally Strongly Acidic Microenvironment

Using scanning electrochemical microscopy combined with pH ultramicroelectrode technology, the research team conducted in situ monitoring and found that when using impure water, the local pH at the cathode surged to 10.4 at a current density of 0.5 A·cm⁻². To address this, the team proposed a "local pH regulation" mechanism and selected molybdenum trioxide (MoO₃), which exhibits the strongest Brønsted acidity, to be loaded onto Pt/C catalysts. The working mechanism is as follows: driven by the cathode potential, MoO₃ promotes the dissociation of surface water molecules and becomes protonated to form Mo-O-H structures, subsequently releasing H⁺ ions into the electric double layer at the electrode/electrolyte interface. Through a cycle of "water dissociation–protonation–deprotonation," a strongly acidic microenvironment is maintained in situ around the catalyst.

Core Finding: Outstanding Performance in Non-Pure Water Operation

The new electrolyzer equipped with MoO₃-Pt/C catalysts maintained a stable local cathode pH in the strongly acidic range of 2.5–3.5 even in water containing various cation impurities. Polarization curves showed that its performance was nearly identical to that of standard electrolyzers operating in pure water, and it remained stable for over 100 hours at a current density of 1.0 A·cm⁻². Long-term operation tests confirmed that the strongly acidic environment effectively suppressed hydroxide precipitation and metal deposition, preserving catalyst activity.

Key Advantage: Breakthrough in Membrane Protection and Water Quality Tolerance

Comparative tests revealed that after operating for 100 hours in water containing Fe³⁺, standard electrolyzers exhibited extremely high levels of reactive oxygen species in the effluent and fluoride ions from membrane degradation, with damaged membrane structures and Fe deposition on the cathode. In contrast, the relevant indicators for the new electrolyzer were almost identical to those observed during operation in pure water, with no significant membrane degradation or Fe deposition. More stringent tap water testing revealed that standard electrolyzers failed after less than 100 hours of operation in tap water, while the new electrolyzer could operate stably for over 3,000 hours in tap water at an industrial current density of 1.0 A·cm⁻² with almost no performance degradation. In the "pure water–tap water" switching test, the voltage of the new electrolyzer only increased slightly from 1.71 V to 1.73 V, demonstrating strong insensitivity to water quality.

Conclusion and Outlook: Cost Reduction and Efficiency Improvement Drive Green Hydrogen Adoption

By incorporating Brønsted acidic oxides, this technology achieved three effects: overcoming pH increase, inhibiting precipitation, and protecting the proton exchange membrane, freeing PEM electrolyzers from reliance on ultrapure water systems. Techno-economic analysis shows that, in non-pure water supply scenarios, it can reduce the levelized cost of hydrogen by 0.30%–8.19%, saving tens of thousands to hundreds of thousands of US dollars annually. This breakthrough significantly reduces the infrastructure investment and operation and maintenance costs of PEM hydrogen production, providing a viable solution for the large-scale application of green hydrogen technology in water-scarce regions.