Ampere-level current density ammonia electrochemical synthesis using CuCo nanosheets simulating nitrite reductase bifunctional nature

Picture a polluted river on one side and a fertilizer plant on the other. One is overloaded with nitrate, while the other burns fossil fuels to make ammonia. Fang, Zheng, Lou, Sun, and colleagues look at that split screen and ask a beautifully simple question: could the electrons from green electricity pull nitrate out of water and, in the same breath, turn it into ammonia? It's a two-for-one—clean up the nitrate and decarbonize a chunk of ammonia production that still relies on the Haber–Bosch process. Their bet is to copy nature's division of labor. In copper-type nitrite reductase, an enzyme that moves nitrogen down the ladder to ammonia, one copper site acts as the electron shuttle and the other serves as the catalytic workbench. Electrons slide from the first to the second; a nearby amino acid hands over protons; and the nitrogen-oxygen bond gives way step by step. Translate that to a piece of metal, and you want one site that grabs and activates nitrate and its cousins, with another that keeps a steady trickle of hydrogen on hand to finish the job. In their design, copper plays the "grab and activate" role, cobalt acts as the "electron and proton supplier," and the two are fused into ultrathin nanosheets so the handoff is fast and constant. How do you actually build that? They co-electrodeposited copper and cobalt onto a three-dimensional nickel foam, running a steady current of 50 milliamps per square centimeter for 300 seconds. That quick burst lays down a micro-pine scaffold draped in nanosheets roughly 10 nanometers thick. The sweet spot composition—half copper and half cobalt—looks and behaves like a true alloy. You can see it in the crystallography: the main copper lattice plane tightens from 0.212 to 0.208 nanometers when cobalt slips in, a classic sign of lattice contraction. You see it in the imaging, where sheets coat the foam uniformly. And you see it in the electronics: X-ray spectroscopy shows electron density shifted toward copper, and charge analysis flags copper as more electron-rich than cobalt. All that says the two metals are interacting, not sitting in separate islands. The architecture matters as much as the chemistry. Nickel foam acts as a highway for electrons and a sponge for liquid, so ions and products don't bottleneck. Before testing, they flip the surface to its working state with a brief electroreduction at minus 0.2 volts, clearing away passive oxides. The nickel itself essentially acts as a bystander for nitrate reduction, which means the numbers you're about to hear come from the copper-cobalt skin, not the scaffold underneath. Now the headline. In alkaline nitrate solution, the copper fifty and cobalt fifty nanosheets run at ampere-scale current with almost no penalty in selectivity. At minus 0.2 volts versus the reversible hydrogen electrode, they achieve a Faradaic efficiency for ammonia of 100 percent within measurement error and a geometric current density of 1,035 milliamps per square centimeter. That translates to 4.8 millimoles of ammonia per square centimeter per hour—about 960 millimoles per gram of catalyst per hour when you account for the tiny mass of active material. For an electrochemical reaction that often encounters side paths like hydrogen evolution, that's a striking combination: significant current and clean product. The team is careful to show the ammonia really comes from nitrate. They quantify it with Nessler's reagent, the classic color test, and they cross-check with nuclear magnetic resonance. Most telling, they swap in nitrogen-15 labeled nitrate and, after an hour at that same potential, see the distinct twin peaks of nitrogen-15 ammonia. The labeled nitrogen walks into the product. No hand-waving needed. Performance isn't just a single sweet spot. The alloy maintains its balance across a broad swath of potentials. From minus 0.1 to minus 0.4 volts, ammonia Faradaic efficiency stays above 90 percent. Even at exactly zero volts—so, no overpotential relative to the hydrogen couple—the catalyst pushes a nitrate-to-ammonia current of 347 milliamps per square centimeter at 88 percent selectivity. That kind of headroom gives engineers options: you can tune for voltage, current, or selectivity depending on the constraints of a future system. And they don't just run this in a beaker for a few minutes and declare victory. In a higher-concentration batch test that starts around 6,200 parts per million nitrate, the reactor pulls nitrate down to 31 parts per million after 10 hours. That beats World Health Organization guidelines for drinking water. The conversion stays focused too: the ammonia Faradaic efficiency is 96 percent in that longer run, so you're not swapping one pollutant for another; you're making a valuable product and leaving little else behind. If you're wondering whether this is just more copper plus more cobalt equals more activity, the answer appears to be no. Under the same conditions and at the same potential, copper alone manages an ammonia current density of 34 milliamps per square centimeter at 32 percent selectivity. Cobalt alone achieves 21 at 84 percent. The alloy, at that point, reaches 347 at 88 percent. That's not additive; that's synergy—copper's stickiness for nitrogen species combined with cobalt's ability to supply surface hydrogen, without either one choking the other. The device maintains its footing at speed. Ten one-hour electrolysis cycles at a fixed potential hold Faradaic efficiency above 90 percent, and there's no detectable nitrogen gas, which would be a sign of the pathway veering off to nitrogen gas. After those cycles, the nanosheets look a little more clumped under the microscope—a hint at one reason for minor decay—but the core structure, composition, and oxidation states stay essentially the same when probed by diffraction and photoelectron spectroscopy. In other words, the catalyst doesn't dissolve or transform into something else under load. So why does this specific pairing work so well? The mechanistic picture is satisfyingly simple and backed by a lot of observations on the surface. In situ infrared spectroscopy captures the relay in motion: as the potential moves into the nitrate-reduction window, nitrate bands fade, nitrite signals rise and fall, and a clear signature of hydroxylamine—an on-path intermediate to ammonia—appears before giving way to product. Surface-enhanced Raman measurements add another layer, showing a hydrogen-on-the-surface vibration on cobalt-containing electrodes that's absent on copper alone. That's the hydrogen reservoir you need if you want to drive stepwise hydrogenation rather than just bubble hydrogen away. An old electrochemistry trick rounds out the picture. Using a spin-trapping molecule that latches onto hydrogen radicals, they see a strong trapped-hydrogen signal on the alloy. Add nitrate, and that signal drops. The obvious read: active hydrogen on the surface is being consumed by nitrate reduction, not wasted in side reactions. It's the kind of control experiment that turns a nice story into a solid one. Computation ties the threads together with numbers that map directly onto the spectroscopy. On a model copper-cobalt alloy surface, the free energy cost to adsorb the first nitrate is just 0.17 electronvolts. Do the same calculation on pure copper and it jumps to 0.61. On pure cobalt, it reaches 0.34. That matters because adsorption is the on-ramp; miss it and nothing else follows. The alloy also tempers hydrogen binding compared to cobalt alone, which avoids clogging the surface with hydrogen and starving it of nitrate. From there, the modeled pathway looks like what the spectroscopies see: nitrate to adsorbed nitrate to nitrite, down to hydroxylamine, and finally to ammonia as hydrogen adds in the right places at the right times. You can even watch the electron flow that makes this choreography possible. X-ray absorption shows the copper edge shifted to lower energy in the alloy—copper is a bit richer in electrons than it would be by itself. Photoelectron spectra move in the same direction: copper's core levels step down, cobalt's step up. A Bader charge analysis, which is a way of parceling out electron density in a calculation, also marks copper as more electron-rich than cobalt. In plain language, cobalt leans into the role of donor, while copper acts as acceptor and activator. That fits the enzyme analogy and provides a microscopic reason for the macroscopic selectivity. Selectivity shows up in another simple ratio. As they tune the cobalt content, the amount of nitrite left over relative to ammonia plunges—from about seven down to roughly 0.6. The alloy at fifty-fifty hits a sweet balance where nitrate sticks, is quickly hydrogenated, and leaves almost nothing but ammonia behind. You achieve both a high yield and a clean stream. Behind the scenes, they do the standard kinetics homework too. Rotating the electrode and applying the Koutecký-Levich framework separates mass-transport limits from charge-transfer limits, and Tafel analysis maps how current scales with overpotential. The punchline, consistent with everything else, is that the alloy lowers the barrier right where you'd hope: the early steps that bring nitrate onto the surface and start breaking nitrogen-oxygen bonds. Move past that, and the high currents follow without dragging hydrogen evolution along for the ride. It's worth zooming back out to the piece of river we started with. In a lab cell, this nanosheet catalyst can consume high-nitrate water, drop the contaminant below drinking-water limits, and convert the nitrogen into ammonia at ampere-scale rates with essentially complete Faradaic efficiency at the best point. That's a compelling hint that valorization—turning waste into value—isn't just a slogan here. It's a process you can observe in real time, with a product stream and a remediation outcome you can measure. There are real-world steps between this and a plant on the riverbank. The authors are clear about that. What they have is a fast, selective surface on a conductive scaffold in alkaline media, proven in a three-electrode cell and in batch runs. What remains is engineering: long-term stability in continuous flow, integration with renewable power profiles, and, crucially, scaling electrode area while maintaining the same balance of adsorption and hydrogenation. But that's a welcome kind of problem. It's not about whether the chemistry works; it's about how to package it. The broader idea here—borrowing the division of labor from enzymes and building it into alloys—feels durable. If you can separately dial in "grab the molecule" and "feed it hydrogen," and if your scaffold allows ions and electrons to move without friction, you can shift reactions that used to be finicky into high-rate, high-selectivity regimes. As Fang and colleagues showed in Nature Communications, the transformation from nitrate to ammonia is a poster child for that strategy. And if you're the kind of person who appreciates a two-for-one, cleaning water while making a key chemical is a hard combination to beat.