The device, which the authors call a hydroxide exchange membrane carbon capture cell, or HEMCC, belongs to a growing family of electrochemical approaches to direct air capture. Unlike conventional solid sorbent systems that swing between hot and cold temperatures to grab and release CO2, electrochemical cells use renewable electricity to drive the capture chemistry, promising lower energy costs, modular scaling, and compatibility with intermittent power sources. The problem, the researchers note, is that few proposed electrochemical devices have simultaneously delivered low energy cost, high durability, and full purification of the captured CO2, and almost none have addressed the practical realities of scale-up and pressure drop that matter enormously when you are pushing enormous volumes of air through a machine.

The core of the new design is a pair of identical nickel hydroxide electrodes, the same kind of material that has served for decades as the positive electrode in nickel-metal hydride batteries. Nickel hydroxide cycles reversibly between Ni(OH)2 and nickel oxyhydroxide, NiOOH, in alkaline conditions, and it is this pH-swinging battery chemistry that the team exploits. When one electrode is discharged, it consumes hydroxide ions and acidifies its local environment, releasing CO2 from bicarbonate and carbonate; the opposite electrode charges and generates hydroxide, which converts CO2 in the incoming air into carbonate and bicarbonate, effectively capturing it. A hydroxide exchange membrane, an 80-micrometre-thick PiperION membrane supplied by Versogen, separates the two sides while allowing hydroxide ions to shuttle between them.

In operation, ambient air flows past the charging electrode, where its CO2 is stripped out and chemically bound, and the depleted air exits the cell. During the discharge half of the cycle, the roles reverse: the previously charged electrode now releases its stored CO2 as a concentrated stream that can be collected, purified, and sequestered or utilized. Because the cell is symmetric, with identical electrodes on either side of the membrane, the air feed and product collection lines simply switch sides each cycle, allowing continuous operation without any change in hardware. The team’s laboratory-scale device used 25-square-centimetre electrodes made by electrochemically precipitating nickel hydroxide onto nickel foam, a process in which the working electrode gained 0.83 grams of material while counter electrodes sacrificed nickel from a nickel chloride bath.

Before testing, the electrodes underwent a careful break-in protocol in 1 molar potassium hydroxide, cycling at low current densities to stabilize their capacity, which settled at roughly 2.1 to 2.4 milliampere-hours per square centimetre. Cyclic voltammetry revealed the key electrochemical signatures: an oxidation peak at 1.44 volts versus the reversible hydrogen electrode corresponding to the Ni(OH)2 to NiOOH conversion, a reduction peak at 1.28 volts for the reverse reaction, and the onset of parasitic oxygen evolution at 1.52 volts. That narrow window between the useful battery reaction and the wasteful oxygen evolution reaction is central to the cell’s efficiency, and at 2 milliamperes per square centimetre the kinetic overpotential for the battery reaction averaged just 0.09 volts.

The durability results are among the most striking in the study. The 25-square-centimetre laboratory cell ran for 5,000 hours, more than half a year of continuous operation, without catastrophic degradation, addressing one of the most persistent doubts about electrochemical capture devices, which often show rapid performance decay. Transient analysis of the cycling behaviour showed that electron efficiency, the fraction of electrical charge that goes into useful CO2 capture rather than side reactions, peaked at 0.32 during optimal portions of the cycle, with an average of 0.25, and that flux and efficiency were tightly coupled to the phase of the battery cycle. The researchers used these insights to design an operating strategy with dedicated capture and regeneration phases for each electrode.

Perhaps most importantly, the team did not stop at the laboratory bench. Working with RepAir DAC, they built a pilot-scale electrochemical stack of nine cells, each with 300 square centimetres of active area, and ran it for 48 hours. The stack achieved the same headline figures, 132 kilojoules per mole of CO2 and a flux of 0.19 moles of CO2 per square metre per hour, equivalent to 75 kilograms of CO2 captured per square metre per year, while meeting the demanding 300-pascal pressure drop requirement that direct air capture systems must satisfy to keep fan energy manageable. Pressure drop is a frequently ignored constraint in academic capture studies, yet it can dominate the total energy budget of a real-world plant handling vast volumes of air, so demonstrating compliance at pilot scale is a significant step toward commercial credibility.

On the strength of these results, the authors present a techno-economic pathway to capturing CO2 for less than 100 US dollars per tonne, a threshold widely regarded as the price point at which direct air capture becomes viable at climate-relevant scale. Their projection rests on energy technology learning rates, the well-documented tendency of manufacturing costs for electrochemical and energy hardware to fall steadily as production volumes grow, a pattern seen in solar panels, lithium-ion batteries, and fuel cells. Because the HEMCC borrows mature manufacturing concepts from batteries and fuel cells, including membrane electrode assemblies and porous electrode architectures grounded in decades of theory dating back to Newman’s classic analyses, the researchers argue that its cost trajectory could follow those successful technologies rather than the steeper curves typical of bespoke chemical plants.

The work was supported by the US Department of Energy’s National Energy Technology Laboratory and the US Department of Defense Army Research Laboratory, and it arrives at a moment when governments and companies are racing to build gigatonne-scale carbon removal capacity. The Intergovernmental Panel on Climate Change and the International Energy Agency both count direct air capture among the tools likely needed to reach net-zero emissions by mid-century, particularly for offsetting hard-to-abate sectors such as aviation and agriculture. If the Delaware and RepAir teams’ durability and cost projections hold up as the technology scales beyond the pilot stage, the humble nickel hydroxide electrode, a workhorse of twentieth-century batteries, may find itself pressed into service as a cornerstone of twenty-first-century climate repair, quietly scrubbing the sky one reversible charge cycle at a time.

Subject of Research: A nickel hydroxide symmetric battery cell with a hydroxide exchange membrane for electrochemical direct air capture of carbon dioxide.

Article Title: A Ni(OH)2 symmetric battery cell for hydroxide exchange membrane-based direct air capture of CO2

Article References: Buchen, J. R., Wang, T., Geiger, B. K., Gluz, N. Y., Artoul, M., Hiegel, J.-P., Achrai, B., Setzler, B. P., & Yan, Y. (2026). A Ni(OH)2 symmetric battery cell for hydroxide exchange membrane-based direct air capture of CO2. Nature Energy. https://doi.org/10.1038/s41560-026-02129-z

Image Credits: AI Generated

DOI: 10.1038/s41560-026-02129-z

Keywords: direct air capture, carbon dioxide removal, nickel hydroxide, hydroxide exchange membrane, electrochemical carbon capture, battery cell, climate change, Nature Energy, energy cost, carbon capture and storage, electrochemistry, pilot-scale stack