Hidden Anion Partners at Electrode Interfaces Steer Ammonium-Ion Supercapacitor Performance

Ammonium-ion supercapacitors have quietly become one of the most intriguing propositions in aqueous energy storage. Instead of shuttling metal cations such as lithium or zinc, they rely on the humble, nonmetallic ammonium ion, NH4+, as the charge carrier. That choice eliminates the dendrite formation and short-circuit risks that plague aqueous zinc-ion batteries, while the earth abundance and low cost of ammonium salts give the technology a distinct economic edge over conventional flow batteries. For grid-scale, industrial, and residential storage scenarios where safety, scalability, and price all matter at once, these devices are increasingly hard to ignore. Yet a fundamental question has lingered beneath the field’s progress: if the ammonium ion is the carrier, why do different ammonium salt electrolytes deliver such strikingly different results?

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