Ir-Free, Membrane-Free, Added-Electrolyte-Free Pure Water Splitting via Overlapping Electric Double Layers:

Splitting water molecules into green hydrogen is a critical technology for the sustainability of mankind. However, the low energy efficiency and high material costs of the electrochemical water splitting remain major obstacles to its large-scale deployment. Despite the significant progresses that have been achieved in electrocatalysts design and optimization, via material informatics, atomic-level engineering, etc., existing variants of the electrochemical water splitting technologies all fall short in competing with the incumbent gas-reforming technology.

Here, we will report a novel idea of electrochemical water splitting which uses an intense electric field as a “virtual catalyst” [1]. By narrowing the distance between the anode and cathode from well-above micrometers in conventional water electrolyzers down to nanometers, an overlap of electric double layers (EDLs) is formed. This overlapping electric double layers (OEDLs) eliminates the bulk region and creates a pervasive strong electric field in a nanoconfined region between two electrodes, which acts as a “virtual catalyst” enabling efficient pure water splitting. Unlike traditional catalytic strategies that act locally at EEI, this "virtual catalyst” globally accelerates the process via dramatically enhancing the ion migration between electrodes, yielding high ionic strength and conductivity in the original bulk region filled with pure water. Equally important, this “virtual catalyst” generates in situ a favorable acidic and basic environment at the cathode and anode, respectively, opening the possibility of using non-precious metal catalysts like nickel for oxygen evolution reaction (Fig.1 a). Leveraging these effects, we demonstrate an Ir-free, membrane-free, added-electrolyte-free pure water splitting with a current density of 2.8 A/cm2 at 1.7 V (Fig.1 b), breaking the conventional trade-off between high energy efficiency and low material cost that having beleaguered the conventional water electrolyzers.

Despite this breakthrough, this membrane-free pure water splitting technology faces formidable challenges upon scaling up [2]. One issue is that, while exhibiting high intrinsic current density, the reliance on flat electrodes precludes the use of porous structures to enhance the electrochemical active surface area (ECSA), thereby ECSA can only be enlarged through electrode geometric area expansion. Another issue is that the proximity of two cathode and anode exacerbates gas crossover, creating a spuriously high current with a low Faraday efficiency. To address these challenges, we deconstruct the electrolysis nanoreactor into three key components: electrodes, electrode support and water supply, and reconfigured them into three model reactors. Using these model reactors, we systematically analyze how these components affect OEDLs scalability. It was found that forced water supply within nanoconfined flow channels creates an intrinsic trade-off between current efficiency and flow pressure drop which constrains scalability (Fig.1 c). In addition, the silicon-based support within the OEDLs region forms a conductive surface layer, induces parasitic current, and jeopardizes scalability (Fig.1 d). Guided by these findings, we designed a new reactor architecture featuring an open-flow channel for water supply to resolve the trade-off between efficiency and pressure drop, alongside an interdigitated configuration of two electrodes and support to suppress parasitic current. The new design achieved electrode area expansion and gas crossover mitigation, resulting in a two-order-of-magnitude amplification of electrolytic current (Fig.1 e).

Finally, challenges and future directions in pure water splitting will be discussed, including modeling, in-situ characterization, and our latest attempt to further reduce production costs by adding organics.

Reference:

[1] Xu, H.; Zhang, J.; Eikerling, M.; Huang, J. Am. Chem. Soc. 2024, 146 (29), 19720–19727.

[2] Shi, W.; Meng, J.; Xu, H.; Huang, J.; Zhang, J. ChemRxiv. 2025.