A new study published in the journal Ionics by Zhenyun Zhao, Naihui Hou, and colleagues at Zhejiang Sci-Tech University, working with Yuwen Cheng of Anhui University of Technology, tackles that question head-on. The team’s central insight is that the counter anions in the electrolyte, long treated as passive spectators because they do not carry charge between electrodes, actually play a decisive role at the electrode-electrolyte interface. By combining electrochemical measurements with density functional theory simulations and spectroscopic evidence, the researchers show that ammonium ions and electrolyte anions form interfacial ion pairs that reshape the local electric field, alter charge-transfer barriers, and ultimately govern both capacitance and rate performance. The work provides one of the most systematic pictures to date of how anion identity controls interfacial ammonium-ion storage.

To isolate interfacial effects from the confounding variables of complex electrode architectures, the team needed a structurally well-defined model electrode. They chose TatDha-COF, a fully conjugated covalent organic framework synthesized through the condensation of 4,4′,4″-(1,3,5-triazine-2,4,6-triyl)trianiline with 2,3-dihydroxyterephthalaldehyde in a sealed glass tube heated at 120 degrees Celsius for 72 hours, yielding red powders at 60% efficiency. The framework features a 3.2-nanometer pore size, a hollow microtubular morphology visible in electron microscopy, and a crystalline hexagonal lattice confirmed by powder X-ray diffraction peaks at 2.8 and 26.38 degrees. X-ray photoelectron spectroscopy verified the expected carbon, nitrogen, and oxygen chemistry, including imine C=N groups, C-N linkages, and hydroxyl C-OH functionalities. Critically, the one-dimensional channel architecture and highly exposed surface groups promote pseudocapacitive behavior, making the material an ideal platform for studying what happens at the interface rather than optimizing a commercial device.

The electrochemical comparison across three representative ammonium electrolytes produced a result that initially looks like a paradox. In 1 M ammonium acetate, TatDha-COF delivered the highest specific capacitance of the three salts tested, yet it suffered the poorest rate capability, with capacitance falling off more steeply as scan rates increased from 1 to 5 millivolts per second than in either 0.5 M ammonium sulfate or 1 M ammonium chloride. In ammonium sulfate, the opposite held true: excellent rate performance came at the expense of specific capacitance. Cyclic voltammetry revealed three pairs of redox peaks in the sulfate and chloride electrolytes but only two pairs in ammonium acetate across a stable window of -0.9 to 0.9 volts versus Ag/AgCl. A two-electrode coin cell in ammonium acetate retained 61% of its initial capacitance after 100 cycles at 1 ampere per gram while maintaining a Coulombic efficiency near 100%, indicating highly reversible charge-discharge processes despite gradual decay.

Kinetic analysis using the well-known b-value method, in which the slope of log peak current versus log scan rate distinguishes diffusion-controlled from adsorption-controlled behavior, added an important layer of nuance. Values approaching 1 indicate adsorption-controlled, capacitive processes, while values near 0.5 indicate diffusion control. For the dominant redox pair between -0.3 and 0 volts, the b values were 1.03 and 0.85 in ammonium sulfate, 0.82 and 0.70 in ammonium chloride, and 0.84 and 0.74 in ammonium acetate. This shows that the redox reaction was predominantly adsorption-controlled in all three electrolytes, but with a stronger adsorption contribution in ammonium sulfate. Taken together with the capacitance and rate data, the results suggest that ammonium acetate and ammonium chloride favor pronounced pseudocapacitive behavior, whereas ammonium sulfate leans toward electric-double-layer characteristics.

The theoretical heart of the paper lies in the density functional theory calculations, performed with the Vienna ab initio simulation package using the PBE functional, projected augmented wave method, and Grimme’s DFT-D3 dispersion correction. The team first identified the preferential adsorption site for bare NH4+ on the framework by comparing two distinct imine nitrogen environments, labeled N1 and N2. Adsorption at N1 proved more thermodynamically favorable because the ammonium ion can simultaneously form two hydrogen bonds, an N-H…N bond with an adjacent C=N group and an N-H…O bond with a neighboring hydroxyl, anchoring it firmly to the surface. All subsequent simulations therefore centered on the N1 site, providing a consistent atomic-scale stage on which to test how different anions modify the adsorption picture.

What emerged from the ion-pair calculations is the study’s most consequential finding. Although anions do not adsorb directly onto the negatively biased electrode, they can coordinate with the already-adsorbed ammonium ion, sitting on its distal side and forming stable interfacial complexes. The calculated adsorption energy for a [NH4+(SO4^2-)] pair was -4.09 electron volts, while a [NH4+(AC^-)3] complex with three acetate ions reached -3.05 electron volts, both substantially more favorable than bare NH4+ adsorption alone and both far lower than a single [NH4+(AC^-)] pair. This confirms that interfacial ion-pair formation is thermodynamically spontaneous. The stronger association of three acetate ions with one ammonium ion, compared with one sulfate paired with one ammonium, means the acetate complex is harder to rearrange and desorb during charging, which raises the interfacial charge-transfer barrier and slows ion diffusion, exactly the behavior observed in impedance measurements.

Experimental evidence backed the simulations. Ex situ X-ray photoelectron spectroscopy of the discharged electrode revealed a weak but discernible high-binding-energy component between 402 and 405 electron volts in the N 1s spectrum, consistent with chemically adsorbed ammonium, alongside a weakening of C=N peaks and strengthening of C-N and N…H-N features. More tellingly, the O 1s spectrum showed a new component at approximately 531.3 electron volts after discharge, assignable to the carboxylate group of acetate anions retained at the surface through strong association with ammonium. Electrochemical impedance spectroscopy at different voltage states completed the picture: the ammonium acetate electrolyte exhibited a notably larger charge-transfer resistance and a steeper Warburg slope than ammonium sulfate, indicating higher interfacial barriers and slower ion diffusion, in direct agreement with the computed ion-pair energetics.

The final piece of the puzzle came from Bader charge analysis, which quantifies how electrons redistribute across the interface. The total charge transferred to the [NH4+(AC^-)3] pair was 0.38 electrons, nearly eight times the 0.05 electrons transferred to [NH4+(SO4^2-)]. The reason lies in polarizability: acetate is far more polarizable than sulfate, and it strongly perturbs the ammonium ion’s symmetric tetrahedral electronic environment. The charge dispersion among the four hydrogen atoms of adsorbed ammonium grew from roughly 0.04 electrons in the sulfate complex to about 0.11 electrons in the acetate complex, a signature of strong anisotropic polarization. This distortion enhances the local positive charge density on the electrode-proximal side of the ammonium ion and increases the number of electrons transferred at the interface. Since more transferred electrons mean more stored charge, this mechanism neatly explains why ammonium acetate yields higher capacitance than ammonium sulfate.

The study thus resolves the capacitance-rate trade-off into a single coherent picture rooted in interfacial coordination chemistry. Acetate anions, by pairing tightly with adsorbed ammonium, boost the electron transfer number and pseudocapacitive charge storage but simultaneously immobilize the interface, degrading rate performance. Sulfate forms a less stabilizing, more mobile ion pair that sacrifices capacitance for speed. The work, which builds on earlier interface-focused studies of acetate electrolytes with MXene electrodes, establishes that electrolyte anions are not merely charge-balancing bystanders but active architects of the ammonium storage process. For a technology whose appeal rests on safety and cost, that realization could prove pivotal: rational electrolyte design, tuned at the level of the anion, now stands out as a powerful and largely unexplored lever for engineering the next generation of ammonium-ion supercapacitors.

Subject of Research: Role of electrolyte anions in interfacial ammonium-ion storage in supercapacitors

Article Title: Role of electrolyte anions in governing ammonium-ion storage performance

Article References: Zhao, Z., Hou, N., Ming, S., Su, Q., Cheng, Y., Wang, T., Chen, H., & Chen, W. (2026). Role of electrolyte anions in governing ammonium-ion storage performance. Ionics. https://doi.org/10.1007/s11581-026-07539-3

Image Credits: AI Generated

DOI: 10.1007/s11581-026-07539-3

Keywords: ammonium-ion supercapacitors, electrolyte anions, electrode-electrolyte interface, covalent organic framework, interfacial ion pairs, pseudocapacitance, density functional theory, Bader charge analysis, electrochemical impedance spectroscopy, aqueous energy storage, ammonium acetate, charge transfer kinetics

The study, conducted by researchers at Banaras Hindu University in Varanasi and published in the journal Discover Soil, grew out of a growing anxiety about potassium mining in one of the world’s most intensively farmed regions. The rice-wheat cropping system that dominates the Indo-Gangetic plains supplies more than half of the food consumed in India, occupying roughly 27 percent of the country’s agricultural land. Yet fertilizer use in India has become badly skewed: between 1971-72 and 2021-22, nitrogen consumption surged 11.3-fold and phosphorus 14.2-fold, while potassium rose only 5.71-fold. The current national ratio of nitrogen to phosphorus to potassium stands at 6.7:2.4:1, far from the 4:2:1 ratio long recommended by the Fertiliser Association of India. An estimated 72 percent of India’s agricultural land is now considered potassium deficient.

Why does this imbalance matter? Potassium is not a minor player in crop physiology. It governs enzyme activation, water regulation, photosynthesis, and the transport of sugars, all of which feed directly into yield, grain quality, and the plant’s resilience to stress. When farmers skimp on potassium while piling on nitrogen, they quietly draw down the soil’s native reserves, a process scientists call potassium mining. Crop residues complicate the picture further: rice and wheat straw is extraordinarily rich in potassium, containing 80 to 85 percent of the nutrient taken up by the plant, and returning it to the field can substitute for 20 to 50 percent of potassium fertilizer. In conservation agriculture systems, where residues are retained on the soil surface and tillage is minimized, this recycling loop changes the chemistry of potassium availability in ways that conventional soil tests were never designed to capture.

To find out which chemical extractants best reflect reality under these conditions, the team turned to a long-term experiment established in 2012 at the university’s agricultural research farm. The site sits on sandy loam soils of the Typic Ustochrept family in a humid subtropical climate with about 1,110 millimeters of annual rainfall. The experiment followed a split-plot design with four crop establishment methods, ranging from full conventional tillage to full conservation agriculture with residue retention in both seasons, crossed with three nutrient management levels. After eleven complete crop cycles, the researchers sampled soil from all 36 plots at a depth of 0 to 15 centimeters and subjected it to a battery of twelve different potassium extractants.

The extractants spanned four mechanistic families. Distilled water and dilute salt solutions such as calcium chloride, barium chloride, ammonium acetate, sodium acetate, and sodium tetraphenyl boron target the potassium held in soil solution and on exchange sites. Multi-nutrient extractants, including ammonium bicarbonate-DTPA and the Mehlich solutions, pull several nutrients simultaneously. Acidic extractants, including strontium chloride mixtures and boiling nitric acid, attack the mineral lattice itself, releasing the non-exchangeable potassium locked between the layers of clay minerals. The logic is that plants do not simply sip from the soil solution; as roots deplete it, potassium desorbs from exchange sites and, over longer timescales, is released from mineral interlayers. A good soil test should mimic that whole cascade.

The results were striking. Boiling nitric acid extracted the most potassium, between 1,042 and 1,189 milligrams per kilogram of soil, followed by sodium tetraphenyl boron at 407.8 to 495.5 milligrams per kilogram, because both tap the non-exchangeable reservoir. At the other extreme, distilled water pulled out a mere 13.98 to 20.63 milligrams per kilogram. The workhorse ammonium acetate test extracted between 46.33 and 62.86 milligrams per kilogram, a middling figure that belies its global status. Among exchangeable-potassium extractants, barium chloride was the most aggressive, yielding 251.15 to 277.41 milligrams per kilogram, likely because barium is not preferentially adsorbed and does not collapse the expanded clay structures that hold potassium.

Correlation analysis revealed the real story. Sodium acetate emerged as the standout performer, correlating strongly with sodium tetraphenyl boron (r = 0.979), with relative grain yield (r = 0.927 to 0.984), with plant potassium uptake (r = 0.930 to 0.974), and with key soil indices such as cation exchange capacity, specific surface area, and organic carbon. The conventional ammonium acetate test performed respectably but less impressively, with correlations of 0.80 to 0.94 against plant indices. The acidic extractants fared worst: strontium chloride at 0.002 molar showed correlations as weak as 0.231 and no significant relationship with yield, while even boiling nitric acid, despite extracting the largest quantity of potassium, correlated poorly with what plants actually achieved. The researchers attribute sodium acetate’s advantage to its ability to pull water-soluble and exchangeable potassium along with a fraction of the non-exchangeable pool, capturing a broader spectrum of the potassium a crop can access during a growing season.

The team also established critical concentration thresholds using the Cate-Nelson statistical procedure, which partitions soil test values into ranges below which crops respond to fertilizer and above which they do not. Most extractants predicted critical levels with high coefficients of determination between 0.63 and 0.76. The critical value for ammonium acetate ranged from 51 to 54 milligrams per kilogram, while sodium acetate fell between 108 and 116 milligrams per kilogram. Boiling nitric acid produced the highest critical concentration, 1,224 milligrams per kilogram, but with the weakest predictive power, a coefficient of determination of only 0.35 to 0.46. Notably, critical values differed between rice and wheat for the same extractant, a reminder that soil test interpretation cannot be divorced from crop identity.

The agronomic findings were equally telling. Plots under full conservation agriculture, with residue retention in both seasons and minimal soil disturbance, outyielded conventionally tilled plots by 8.47 percent in rice and 10.72 percent in wheat, and recorded the highest potassium uptake, reaching 135.7 kilograms per hectare in rice and 112.4 kilograms per hectare in wheat. Residues appear to work through two mechanisms: their decomposition releases organic acids that dissolve potassium-bearing minerals, and the straw itself delivers a substantial potassium input back to the soil. Full conservation agriculture also raised cation exchange capacity by 19.4 percent relative to conventional tillage and boosted easily oxidizable organic carbon by 64.32 percent, changes that increase the number of adsorption sites available to hold and release potassium. Interestingly, additional potassium fertilizer did not significantly raise grain yields in this experiment, underscoring how much the residue-recycling system was already supplying.

The implications ripple outward from a single research farm in eastern Uttar Pradesh. Soil test calibration is inherently local, dependent on soil mineralogy, climate, and cropping system, and no single extractant works universally. But the message from Varanasi is clear: the global default test for potassium, ammonium acetate, captures only part of the story in soils where non-exchangeable potassium feeds crops, and it may systematically misjudge the potassium supplying power of conservation agriculture systems. Sodium acetate and the multi-nutrient extractants AB-DTPA, Mehlich I, and Mehlich III, which tap all or part of the non-exchangeable pool, tracked crop performance more faithfully. For a country where potassium fertilizer is expensive and farmers already under-apply it, a soil test that better reflects what the soil can actually deliver could save money, slow the mining of native reserves, and help sustain the rice-wheat system that feeds hundreds of millions. The researchers caution that their calibration must be validated across wider cropping systems, but the direction is unmistakable: the chemistry of measuring potassium needs to catch up with the agriculture it is meant to serve.

Subject of Research: Evaluation of chemical soil extractants for measuring plant-available potassium in long-term conservation agriculture rice-wheat systems

Article Title: Evaluating suitability of chemical extractants for measuring available soil potassium under long term conservation agriculture

Article References: Deepak, Jha, S., Singh, U. P., & Rakshit, A. (2026). Evaluating suitability of chemical extractants for measuring available soil potassium under long term conservation agriculture. Discover Soil, 3(1), Article 170. https://doi.org/10.1007/s44378-026-00330-w

Image Credits: AI Generated

DOI: 10.1007/s44378-026-00330-w

Keywords: soil potassium, chemical extractants, conservation agriculture, rice-wheat system, ammonium acetate, sodium acetate, soil testing, Indo-Gangetic plains, non-exchangeable potassium, critical concentration, Cate-Nelson, crop residue retention