Anthropogenic climate change necessitates a shift towards net-zero carbon emissions by 2050. Renewable hydrogen, produced via water electrolysis powered by renewable energy sources, is pivotal in decarbonizing hard-to-abate sectors like steel manufacturing, long-haul transport, and aviation. It also offers potential for seasonal energy storage and chemical feedstock applications. However, the levelized cost of green hydrogen (LCOH) currently surpasses that of fossil fuels, primarily due to the high capital expenditure (CAPEX) and operational expenditure (OPEX) of existing water electrolyzers. OPEX, dominated by energy efficiency and renewable electricity costs, represents the larger component of LCOH. State-of-the-art commercial electrolyzers typically require ~53 kWh of electricity to produce 1 kg of hydrogen (containing 39.4 kWh of energy). The International Renewable Energy Agency (IRENA) aims to reduce this to <42 kWh/kg by 2050. This research presents a novel capillary-fed electrolysis (CFE) cell design that promises significant reductions in both CAPEX and OPEX, enhancing the cost-competitiveness of renewable hydrogen.

The historical evolution of water electrolysis cells has progressed from classic designs to asymmetric polymer electrolyte membrane (PEM) cells that directly produce one gas in a collection chamber. This work builds upon these advancements, developing a CFE cell concept that directly produces both hydrogen and oxygen in separate chambers. Previous research has explored bubble-free electrolysis using breathable electrode structures, aiming to improve efficiency by preventing bubble blockage of active catalytic sites and reducing mass transport limitations associated with multiphase flows. The CFE cell's design seeks to eliminate these inefficiencies, thereby achieving superior energy efficiency compared to conventional water electrolyzers.

The research involved developing a model for capillary-induced liquid transport through porous materials, based on the Hagen-Poiseuille equation modified for tortuosity. This model was used to predict the flow rate of 27 wt% aqueous KOH electrolyte through commercially available porous polyether sulfone (PES) filtration membranes with varying pore sizes (0.45 µm, 1.2 µm, 5 µm, and 8 µm). The membrane with 8 µm pores exhibited the highest flow rate and was selected for cell fabrication. The porosity and ionic resistance of the selected PES separator were characterized. A CFE cell was constructed using the PES separator, a NiFeOOH oxygen evolution electrocatalyst on the anode, and a Pt/C hydrogen evolution electrocatalyst on the cathode. The cell's performance was evaluated at 80-85 °C using 27 wt% KOH electrolyte, and compared to conventional bubbled cells and commercial alkaline and PEM electrolyzers. Electrochemical measurements (linear sweep voltammetry, galvanostatic electrochemical impedance spectroscopy) were conducted to determine cell voltage, current density, energy efficiency, Faradaic efficiency, and gas crossover. The effects of adding polytetrafluoroethylene (PTFE) to the anode were also investigated. The balance-of-plant complexities were compared between the CFE cell and conventional systems. Detailed descriptions of materials, porosity and flow measurements, ionic resistance determination, electrode preparation, and electrochemical measurement techniques are included in the supplementary information.

The capillary-induced flow of electrolyte in the PES separator was successfully modeled and experimentally verified, demonstrating its capacity to sustain water electrolysis at high current densities and temperatures. The CFE cell, utilizing the 8 µm pore PES separator, significantly outperformed conventional bubbled cells and commercial electrolyzers. At 85 °C and 0.5 A cm⁻², the CFE cell achieved a cell voltage of 1.506 V, corresponding to 98% energy efficiency (HHV) and an energy consumption of 40.4 kWh/kg H₂. This surpasses the IRENA 2050 target and represents a substantial improvement over commercial cells (~47.5 kWh/kg H₂). At 1.47 V (100% energy efficiency), the cell produced a current density of ~0.3 A cm⁻². Faradaic efficiencies approached 100%, with low hydrogen crossover (0.04-0.14 vol% at 0.1-1.0 A cm⁻²), significantly lower than in conventional systems. Incorporation of PTFE in the anode further enhanced performance, potentially by increasing the electrochemically active surface area and facilitating gas removal. The CFE cell demonstrated stable long-term performance (1 to 30 days). The absence of gas bubbles led to a substantial reduction in voltage fluctuations compared to conventional bubbled cells. The CFE cell design allows for a significantly simplified balance-of-plant, eliminating the need for liquid circulation pumps, gas-liquid separators, and potentially water-cooled chillers. This simplifies manufacturing and reduces operational costs. The reduced water volume needed for the CFE stack compared to conventional systems was also observed.

The exceptional performance of the CFE cell stems from several factors: the high activity of the NiFeOOH and Pt/C electrocatalysts, the low ionic resistance of the PES separator, bubble-free operation, avoidance of counter-current multiphase flows, and the beneficial effect of PTFE on the anode. The bubble-free operation, confirmed by voltage stability analysis, prevents masking of active catalytic sites and minimizes mass transport limitations. The avoidance of counter-current multiphase flows improves overall efficiency. PTFE addition enhanced anode performance by increasing the electrochemically active surface area and likely facilitated gas transport. The observed low hydrogen crossover is attributed to the absence of advective crossover mechanisms and the low solubility and diffusion coefficients of gases in the high-molarity alkaline electrolyte. The simplified balance-of-plant translates into lower CAPEX and OPEX, making the CFE cell a promising technology for cost-effective renewable hydrogen production.

The capillary-fed electrolysis cell represents a significant advancement in water electrolysis technology, achieving superior energy efficiency and enabling a simplified balance-of-plant. The exceptional performance, exceeding current state-of-the-art electrolyzers and surpassing the IRENA 2050 target, brings cost-competitive green hydrogen production closer to reality. Future research could focus on optimizing catalyst materials, further refining the cell design, and exploring large-scale implementation and integration into existing renewable energy systems.

The study focused on a relatively small-scale CFE cell. Scaling up the technology to industrial levels requires further investigation. The long-term durability of the CFE cell under continuous operation needs comprehensive assessment. The model for capillary-induced flow assumes ideal conditions. Deviations in real-world applications could affect performance. The precise contribution of each factor (e.g., catalyst activity, separator properties, PTFE effect, bubble-free operation) to the overall improvement is difficult to quantify exactly.