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

The classic view of weathering as a stabilizing feedback traces back to influential work in the 1980s and 1990s. Robert Berner and colleagues formalized how silicate weathering responds to atmospheric carbon dioxide and temperature, creating a negative feedback loop: more carbon dioxide warms the planet and acidifies rain, which accelerates weathering, which in turn removes carbon dioxide from the atmosphere. Later, Maureen Raymo and William Ruddiman proposed that the uplift of mountain ranges such as the Himalaya may have cooled the Cenozoic Earth by exposing fresh rock to erosion. Francis Macdonald and colleagues later connected arc volcanism and associated weathering to major climatic transitions. These frameworks cemented the idea that mountain building and rock exposure are, in broad strokes, allies of a cool climate.

But as Bufe emphasizes, drawing down carbon dioxide is only one side of the ledger. Weathering and erosion also release carbon. The oxidation of fossil organic carbon buried in sedimentary rocks, and the oxidation of sulfide minerals such as pyrite, both deliver carbon dioxide to the atmosphere, in some landscapes at rates that rival or exceed the silicate weathering sink. Recent work by Bufe, Jeremy Rugenstein and Niels Hovius, published in Science in 2024, compiled global evidence that mountain ranges act not merely as carbon sinks but as carbon sources as well, with the balance between drawdown and release depending on what the rocks are made of, how fast they erode, and how water moves through them. In the Southern Alps of New Zealand, for example, rapid erosion of carbon-rich sedimentary rock can push the net carbon balance toward emission, whereas volcanic arcs dominated by fresh basalt tend to favor drawdown.

The picture grows still more complicated when biology and hydrology enter the equation. Susan Brantley and colleagues showed in 2023 that the depth and structure of the critical zone, the weathered skin of Earth where rock, soil, water and life interact, exert fundamental control on how much mineral surface area is available for reactions. Vegetation, microbial communities and soil organic matter can both accelerate mineral dissolution and shield it from infiltrating water. Meanwhile, organic carbon itself is constantly being eroded from soils, transported by rivers, buried in floodplains and deltas, or oxidized back to carbon dioxide. Studies led by Robert Hilton and A. J. West have mapped this biospheric carbon loop, and work by Guillaume Soulet and colleagues has quantified how efficiently the biosphere exchanges carbon with the atmosphere through erosion, complicating any simple accounting of weathering as a one-way carbon pump. Sue Tank and colleagues have further shown that in high-latitude permafrost landscapes, the fate of weathering-derived carbon in streams and lakes can shift dramatically as climates warm.

Spatial scale matters profoundly. A weathering flux measured on a single hillslope may not scale linearly to a whole catchment, and catchment-scale estimates may not extrapolate to mountain belts or continents. Supply of fresh mineral surface, runoff, temperature and lithology each impose their own dependencies, and these dependencies can switch sign across environments. In hot, wet tropical basaltic terrains, weathering rates can be extraordinarily high and largely supply-limited. In cold, arid or transport-limited settings, the same mineralogy may weather sluggishly. Time introduces its own twists: pulses of tectonic uplift or glacial grinding expose fresh rock that weathers rapidly at first, then slows as easily dissolved minerals are exhausted. Over million-year timescales, these transients mean that today’s weathering fluxes partly reflect climate and tectonics from deep in the past, making it treacherous to read modern weathering rates as a simple response to modern conditions.

Disciplinary boundaries compound the challenge. Geochemists who trace dissolved ions in rivers, geomorphologists who measure erosion and sediment transport, soil scientists who study profile development, ecologists who quantify vegetation dynamics and carbon cycling, and climate modelers who simulate global feedbacks each hold pieces of the puzzle. Bufe argues that only research spanning spatial scales, temporal scales and disciplines can resolve how weathering modulates the carbon cycle in a warming world. As the climate changes, weathering itself will change: rising temperatures and altered hydrology will accelerate some reactions, thawing permafrost will expose new mineral surfaces and release ancient organic carbon, and intensifying storms will mobilize sediment in ways that models are only beginning to capture. Whether the geological thermostat will help blunt anthropogenic warming, and on what timescale, remains an open and consequential question.

It is against this backdrop of genuine scientific uncertainty that Bufe levels his sharpest critique. Enhanced rock weathering, the practice of spreading crushed silicate rock such as basalt on farmland to accelerate carbon dioxide drawdown, has surged in popularity as a carbon removal strategy. A growing industry now sells carbon credits based on the assumption that a quantifiable and permanent fraction of the applied rock dissolves, and that the resulting bicarbonate represents durable carbon sequestration. Commercial rankings of the best enhanced weathering credit projects have proliferated, and the market is expanding faster than the underlying science can constrain it. In a companion review in Nature Reviews Earth & Environment, Maximilian Schiedung and colleagues catalogued the substantial uncertainties in measuring, reporting and verifying enhanced weathering outcomes, from dissolution kinetics in real soils to secondary carbonate formation that can return carbon to the atmosphere.

Bufe contends that issuing carbon credits for enhanced weathering at the present stage is premature. The very complexities that make natural weathering hard to quantify, the competing sources and sinks, the dependence on lithology, hydrology, biology and timescale, apply with full force to engineered deployments. Crushed rock applied to one field may behave very differently from the same material in another, and the net climate benefit must account for the carbon cost of mining, grinding and transporting rock, as well as for nitrous oxide and other greenhouse gas effects that soil amendments can trigger. Without robust, context-specific measurement of how much carbon is actually drawn down and for how long, credits risk monetizing an assumption rather than a verified outcome, potentially undermining confidence in carbon removal as a whole.

None of this argues that enhanced weathering should be abandoned, Bufe stresses. Rock weathering remains one of the most promising and scalable negative-emissions pathways, rooted in a process that has regulated Earth’s climate for eons. But realizing that promise responsibly demands investment in the fundamental science: long-term field experiments across climates and soil types, improved tracers of mineral dissolution and carbon fate in rivers and oceans, and models that honor the tangled interplay of tectonics, erosion, hydrology and life. The geological thermostat is real, but it is not a simple dial. Understanding its gears, and the directions in which some of them spin backward, is a prerequisite for turning rock weathering into a trustworthy tool against climate change rather than a cautionary tale about moving faster than the science.

Subject of Research: The complex role of rock weathering in Earth's carbon cycle and the risks of premature carbon crediting for enhanced weathering

Article Title: Rock weathering’s tangled role in Earth’s carbon cycle

Article References: Bufe, A. (2026). Rock weathering’s tangled role in Earth’s carbon cycle. Nature Water. https://doi.org/10.1038/s44221-026-00707-9

Image Credits: AI Generated

DOI: 10.1038/s44221-026-00707-9

Keywords: rock weathering, carbon cycle, enhanced weathering, carbon credits, chemical weathering, silicate minerals, carbon dioxide removal, geomorphology, biogeochemistry, erosion, climate mitigation, Nature Water

The numbers at the heart of the analysis are stark. By synthesizing published data on global gains and losses of seaweed forests, the team estimated that continued declines in kelp forests, the most widespread and productive seaweed ecosystems on Earth, could reduce natural carbon sequestration by somewhere between 1.5 and 30 million tons per year. To grasp the imbalance, consider that this projected loss is between 1,000 and 10,000 times greater than the entire annual carbon dioxide sequestration benefit achieved by all kelp forest restoration projects completed to date. In other words, for every ton of carbon that restoration efforts have managed to capture through replanting, climate-driven habitat loss is erasing one thousand to ten thousand tons of sequestration capacity somewhere else on the planet. No realistic scaling of current restoration activity could close a gap of that magnitude within the timeframe that climate targets demand.

The spatial arithmetic is equally sobering. The authors estimate that approximately 25 million hectares of wild kelp forests will be lost by 2050 as warming oceans, marine heatwaves, and shifting ecological pressures continue to degrade these habitats. That projected loss area is more than 6,000 times the total area of kelp forest that has been successfully restored since 1958, when records of restoration activity in this field essentially began. The comparison illustrates a fundamental asymmetry: destroying a kelp forest takes years at most, while rebuilding one demands sustained labor, favorable conditions, and enormous financial investment, and even then the restored patch may never recover the structural complexity and ecological function of the original.

This asymmetry is not hypothetical. The essay points to large-scale kelp restoration projects in Norway and California as case studies in what genuine restoration actually requires. These flagship efforts consumed enormous investments of time, money, and human effort to restore comparatively modest areas of seabed. Extrapolating from those real-world costs, the researchers calculate that tens of thousands of additional projects of similar scale would be needed merely to compensate for the kelp forest losses projected to occur as a consequence of climate change. The price tag for such a global undertaking would run to tens or even hundreds of billions of United States dollars, a sum that far exceeds the current budgets of all marine restoration programs combined. Even if the funding were somehow secured, the ecological and logistical constraints of growing kelp at that pace and scale remain essentially untested and likely unattainable in the required window.

Underlying all of this is a deeper structural problem in how natural climate solutions have been folded into global climate planning. Carbon sequestration by natural ecosystems has historically been so reliable that its continued performance has been implicitly embedded in climate projections and net-zero strategies. Forests, wetlands, seagrasses, and seaweed forests have simply been assumed to keep doing what they have always done: absorbing carbon dioxide and storing it. Climate change is now compromising that assumption from within, degrading the very ecosystems on which those projections depend. When the carbon sink itself is shrinking faster than human intervention can expand it, the accounting that underpins net-zero commitments becomes dangerously optimistic. The authors argue that this dynamic applies with particular force to seaweed, where restoration enthusiasm has outpaced a rigorous assessment of what restoration can realistically deliver.

There is also a subtler risk that the essay identifies: the psychological and political effect of promoting seaweed restoration as a carbon dioxide removal method. If governments, companies, and the public believe that replanting kelp can meaningfully offset emissions, the perceived need for the harder work of eliminating fossil fuel combustion diminishes. The authors warn that this narrative may offer false hope and could delay the phase-out of fossil fuels, locking in further warming that will, in turn, accelerate the loss of the very seaweed forests that restoration schemes claim to rescue. It is a feedback loop of misplaced confidence, in which optimism about technological and ecological fixes erodes the political will required to address the root cause of the problem.

The solution the authors advocate is a shift in emphasis from restoration to protection. Preventing the loss of threatened seaweed habitats, they contend, is a cheaper and more effective climate change mitigation strategy than attempting to rebuild what has already been destroyed. Conservation measures such as reducing local stressors, including pollution, overgrazing by herbivores whose predators have been overfished, and destructive coastal development, can maintain existing carbon sequestration capacity at a fraction of the cost of restoration. A hectare of kelp forest preserved today sequesters carbon immediately and continues supporting fisheries, buffering coastlines, and sheltering biodiversity, whereas a hectare of restored kelp may take years or decades to approach comparable function, if it survives at all. In climate terms, the cheapest ton of carbon is always the one never lost.

None of this means seaweed forests are unworthy of restoration, and the authors are careful to make that distinction. Restoring and conserving these ecosystems remains worthwhile for the countless ecological and economic benefits they provide, from nursery habitat for commercially valuable fish species to coastal protection against storm surge. The argument is about honesty in carbon accounting and realism about scale. As the authors state, conserving and restoring seaweed forests is worthwhile, but society needs to be realistic about what these efforts can achieve in terms of meaningful climate change mitigation through carbon dioxide removal. Climate-driven losses of seaweed forests are currently orders of magnitude greater than gains due to restoration, and restoring seaweed forests cannot deliver meaningful climate change mitigation at the scales and speeds required to offset growing emissions. That statement, grounded in the quantitative analysis of the PLOS Biology essay, should serve as a corrective to a decade of enthusiasm that has sometimes outrun the evidence.

The broader lesson extends well beyond kelp. Around the world, natural climate solutions are being marketed with carbon removal claims that have not been tested against the accelerating losses that climate change itself imposes on the ecosystems in question. Mangroves, seagrass meadows, peatlands, and forests all face the same dynamic: degraded sinks, expensive restoration, and policy frameworks that assume permanence where none is guaranteed. The seaweed analysis offers a template for evaluating these claims honestly, comparing projected losses against achieved gains and asking, bluntly, whether a proposed intervention can operate at the scale the carbon budget requires. For seaweed forests, the answer is currently no. Protecting what remains, cutting emissions at the source, and treating restoration as an ecological priority rather than a climate offset are, according to this analysis, the only strategies that align with both the biology of these ecosystems and the mathematics of the climate crisis. The underwater forests that still stand are worth far more than any forest we might hope to replant.

Subject of Research: Climate-driven declines in kelp and seaweed forests undermining restoration-based carbon dioxide removal

Article Title: Climate-driven losses in seaweed carbon sequestration outpace habitat restoration gains

Article References: Climate-driven losses in seaweed carbon sequestration outpace habitat restoration gains. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: kelp forests, seaweed, blue carbon, carbon sequestration, climate change, habitat restoration, natural climate solutions, carbon dioxide removal, marine ecosystems, PLOS Biology, ocean warming, net-zero