For nearly a decade, battery researchers have called a certain particle a "troublemaker." That particle is the proton () that infiltrates the electrodes of aqueous zinc-ion batteries. Protons are extremely small and can move quickly through electrodes—at first glance, an ideal charge carrier. But in practice, when protons rush to the electrode surface, byproducts such as basic zinc sulfate precipitate out, blocking the pathways for zinc ions. Capacity drops, and the battery degrades. So researchers have competed to develop ways to keep protons out of the electrode entirely.
In July 2026, a research team led by Professor Sarah S. Park in the Department of Chemistry at KAIST turned this conventional wisdom on its head. Rather than excluding protons, the team showed that if the molecular-level sequence is controlled so that protons enter "after" zinc ions, protons too can become a leading player in energy storage. The team's newly developed two-dimensional conductive MOF electrode, , recorded a high capacity of 368.7 mAh/g at a current density of 0.5 A/g, and retained 46.9% of its initial capacity even when the charge/discharge rate was increased 16-fold. Performance held up even after more than 500 rapid charge/discharge cycles. The paper was published in the peer-reviewed international journal Chem (DOI: 10.1016/j.chempr.2026.103129).
Safe and cheap, but "slow and low-capacity": the double wall facing aqueous zinc-ion batteries
Aqueous zinc-ion batteries use an electrolyte solution with water as the solvent instead of organic solvents. They are non-flammable and carry none of the fire risk associated with lithium-ion batteries. Zinc is abundant in the Earth's crust and free of the supply concerns that plague cobalt or nickel. As a candidate for large-scale grid-connected energy storage systems (grid-scale ESS), this field has seen accelerating research since the mid-2010s.
However, two walls have blocked practical adoption. First, insufficient capacity. Second, slow charging. These two issues are inherently in tension. Zinc ions () carry a divalent charge and experience strong electrostatic interactions as they move through an electrode's crystal lattice, making their diffusion slow and poorly suited to fast charge/discharge. Protons, by contrast, have a small radius and move quickly, making them advantageous for rapid response. But as noted above, when too many protons enter the electrode, side reactions occur.
Prior research has addressed this dilemma along two main lines. One approach accelerates zinc-ion diffusion through nanostructuring or doping of electrode materials. The other suppresses proton reactions through electrolyte additives or coatings. As organized in a 2025 review published in Chemical Society Reviews (DOI: 10.1039/D4CS00810C), proton involvement has been understood as playing an ambivalent role: it boosts both capacity and rate, while also causing degradation through byproduct formation. Whether to "use" or "suppress" protons has been the field's implicit binary choice.
Professor Park's team questioned this binary framing itself. Perhaps the problem isn't whether protons enter, but when they enter.
Amine groups act as voltage-gated "gatekeepers" that manage proton entry
The material the team chose was the two-dimensional conductive MOF . Metal-organic frameworks (MOFs) are a general class of crystalline materials in which metal ions and organic molecules bond in regular, repeating patterns to form fine pores—a class of material that became widely known to the public in 2025 when Susumu Kitagawa (Kyoto University), Richard Robson (University of Melbourne), and Omar M. Yaghi (UC Berkeley) received the Nobel Prize in Chemistry for their work on it. has a structure in which copper ions and the organic ligand HHTATP (2,3,6,7,10,11-hexahydroxy-1,5,9-triaminotriphenylene) connect in a planar arrangement.
The key to this material lies in the three amine groups () built into the ligand. Amine groups are basic sites capable of accepting protons to become , and they react with protons only below a certain voltage. The amine groups behave like "gates that open only when the voltage drops."
Tracing the charge/discharge process reveals the following sequence. First, at higher voltage, ions insert into the pores of the MOF. At this stage, because the voltage is still high, the amine groups do not react with protons, and protons remain outside the electrode. Once the zinc ions have settled into their positions, the voltage drops and the amine groups activate, allowing protons to enter the remaining space within the pores. Because the zinc ions already occupy the structure, the protons have no room left to trigger surface side reactions.
If this sequence were reversed, protons would react first at the electrode surface, forming byproducts that would then obstruct the movement of zinc ions arriving afterward. In a KAIST press release, Professor Park stated, "We showed that protons—long viewed as a 'troublemaker' that degrades battery performance—can actually be used to store more energy. Applying this principle to a variety of electrode materials could lead to the development of batteries that can store more energy rapidly."
Using analytical techniques such as X-ray diffraction and X-ray absorption spectroscopy, the research team experimentally confirmed a two-stage storage process in which zinc ions are stored first and protons afterward. They also demonstrated that proton insertion and extraction can be repeated over multiple cycles.
Capacity rose from 228 to 368.7 at 10 times the current density
To gauge the significance of this result, it helps to compare it with earlier research on the same family of materials. A study published in 2019 in Nature Communications reported a similar MOF lacking amine groups, (HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene), as a cathode for aqueous zinc batteries. Its capacity was 228 mAh/g at 50 mA/g (0.05 A/g).
The new achieves 368.7 mAh/g at 0.5 A/g—ten times that current density. Given that increasing current density typically lowers capacity in battery materials, the fact that capacity increased by more than 60% under these harsher conditions suggests that proton storage enabled by the amine groups makes a substantial contribution.
| Metric | (2019, Nat. Commun.) | (2026, Chem) |
|---|---|---|
| Capacity | 228 mAh/g (0.05 A/g) | 368.7 mAh/g (0.5 A/g) |
| High-rate retention | 57.9% (80x current density, 4 A/g) | 46.9% (16x current density, 8 A/g) |
| High-rate cycle life | 500 cycles (75% of initial capacity retained at 4 A/g) | Over 500 cycles (stable operation confirmed) |
| Proton utilization | No intentional control | Sequence managed via voltage-gated amine groups |
It's also worth comparing against other cathode material systems. Manganese dioxide () is theoretically capped at roughly 308 mAh/g (single-electron reaction), and in practice suffers from manganese dissolution and structural collapse. Vanadium-based oxides have been reported to reach 400–500 mAh/g at 0.2–0.3 A/g, but vanadium's toxicity and cost remain barriers to practical use. , a non-toxic material composed of copper, carbon, nitrogen, hydrogen, and oxygen, reaches 368.7 mAh/g—giving it an edge on the axis of material sustainability.
One more figure is worth noting. At 16 times the charge/discharge rate (8 A/g), the capacity corresponds to roughly 368.7 × 0.469 ≈ 173 mAh/g. While this falls short of the 228 mAh/g that the 2019 achieved at low rate (0.05 A/g), it represents a high level among aqueous zinc battery cathode materials when compared under high-rate conditions.
The distance remaining to the grid
The direct target application of this achievement is not electric vehicles but stationary power storage. Aqueous zinc-ion batteries fall short of lithium-ion batteries in energy density per unit weight, but their safety and cost profile suit grid-scale ESS applications. For absorbing fluctuations in renewable energy output, what matters more than energy density per cell is the overall system cost, lifespan, and safety.
That said, the distance to practical deployment remains long. First, while stable operation over more than 500 cycles was confirmed, grid-scale energy storage demands lifespans of thousands to tens of thousands of cycles. This paper does not report long-term evaluation in a full cell (a complete battery combining cathode and anode). There is no guarantee that half-cell results will be reproduced in a full cell, and issues such as dendrite growth and hydrogen evolution on the zinc anode side will need to be addressed separately.
Second, whether this design principle of "controlling ion storage sequence" holds for material systems other than remains unverified. Professor Park refers to "applications to a variety of electrode materials," but at this stage that remains a hypothesis. Whether a similar voltage-dependent gate can be achieved with functional groups other than amines, and whether the sequence-control approach can be extended to ions other than protons—such as or —are questions for future research.
The fact that co-first authors Geunchan Park (PhD candidate, POSTECH) and Gyuwon Lee (master's student) published this as a peer-reviewed paper in Chem indicates that the results have undergone expert scrutiny. Even so, until replication by other laboratories and confirmation under more demanding conditions (high temperature, high current density, long-term storage) accumulate, it is reasonable to regard this as one promising design guideline rather than a settled conclusion.
Rather than shutting the "troublemaker" out, managing its timing of entry is a shift in thinking that is reshaping the very design language of aqueous batteries.