Aqueous zinc-ion batteries use water-based electrolytes to transport charged particles between their electrodes. Unlike many lithium-based systems that rely on flammable organic solvents, water-based batteries carry a considerably lower fire risk and can potentially be manufactured from relatively abundant and inexpensive materials. These advantages have made them attractive for stationary energy-storage systems that collect electricity from renewable sources and release it when needed. Yet their practical performance has been limited by a fundamental trade-off: electrodes must accommodate enough ions to deliver high capacity while also allowing those ions to move rapidly during charging and discharging.

Zinc ions are appealing because each Zn²⁺ ion carries two units of positive charge, potentially contributing substantially to energy storage. However, their relatively large size and slower movement within an electrode can make it difficult to combine high capacity with fast operation. Protons, by comparison, are exceptionally small and can migrate through materials much more quickly. Their speed makes them attractive for rapid charge transfer, but protons have traditionally created serious problems in aqueous zinc-ion batteries. When they enter an electrode too early or in excessive numbers, they can trigger parasitic reactions and generate byproducts on the electrode surface. Those deposits can block pathways, disrupt zinc-ion transport, and gradually reduce battery performance.

Rather than suppressing proton activity, the KAIST team designed an electrode that controls when protons are allowed to participate. The material is a two-dimensional conductive metal–organic framework, or MOF, called Cu₃(HHTATP)₂. MOFs are porous crystalline structures assembled from metal centers and organic molecules, and their nanoscale channels can be chemically modified to influence how ions enter, move through, and bind within the material. In this case, the researchers introduced amine functional groups into the framework’s micropores. These chemical groups were designed to react with protons only after the electrode reached a specific voltage, effectively giving the material a built-in switching mechanism for sequential ion storage.

The result resembles filling a container with objects of different sizes. The electrode first stores the comparatively larger zinc ions while the battery operates in a higher-voltage range. As the voltage decreases, the amine groups become active and enable additional proton storage in the spaces and chemical environments that remain available. This order is crucial. If protons were admitted at the beginning of the process, they could generate surface byproducts before zinc ions had completed their storage step. By delaying proton participation, the electrode reduces competition between the two charge carriers and uses its porous architecture more efficiently.

Electrochemical measurements showed that zinc ions and protons contributed to energy storage in distinct voltage ranges. The research team used multiple X-ray analysis techniques to track the structural and chemical changes taking place inside the electrode, confirming that zinc ions were stored first and that protons entered afterward. The analyses also indicated that proton storage and release could be repeated, an important requirement for a rechargeable battery. Together, the results suggest that ion-storage sequence is not merely a side effect of electrode chemistry but can be deliberately engineered through molecular-level design.

The electrode achieved a storage capacity of 368.7 milliampere-hours per gram at a current density of 0.5 amperes per gram. Capacity expressed in milliampere-hours per gram indicates how much electrical charge can be stored per unit mass of active electrode material; a higher value means that a smaller quantity of material can hold more charge. The result is notable because aqueous zinc-ion batteries have often struggled to approach the combination of energy capacity and rapid ion transport associated with more established battery chemistries. By combining the charge contribution of zinc ions with the fast mobility of protons, the new material takes advantage of two different storage mechanisms within one electrode.

The material also retained 46.9 percent of its initial capacity when the charging and discharging rate was increased sixteenfold. Batteries commonly lose a significant portion of their storage capacity when operated rapidly because ions cannot move through the electrode quickly enough, and structural or chemical bottlenecks become more pronounced. Maintaining nearly half of the original capacity under such an accelerated condition suggests that the conductive two-dimensional framework and its controlled proton chemistry can support comparatively rapid electrochemical reactions. The electrode further maintained stable operation for more than 500 rapid charge–discharge cycles, indicating that the sequential process did not immediately destroy the material or permanently consume its proton-storage sites.

The study presents a broader design principle for battery research: ions that appear harmful under uncontrolled conditions may become useful when their reactions are separated in time and regulated by the molecular structure of an electrode. Professor Sarah S. Park, who led the KAIST research team, said that protons previously regarded as “troublemakers” could instead be used to store more energy, and that the principle might be applied to other electrode materials. The approach could be particularly relevant to large-scale energy-storage systems, where aqueous batteries offer safety and cost advantages but need higher energy density and faster response. The researchers now point toward the possibility of adapting sequential multi-ion storage to other porous conductive frameworks, potentially creating a new class of water-based batteries that combine rapid charging with greater energy storage.

Subject of Research: Sequential storage of zinc ions and protons in a conductive metal–organic framework for aqueous zinc-ion batteries.

Article Title: Sequential Zn²⁺–H⁺ Storage in a 2D Conductive Metal–Organic Framework for Advanced Aqueous Zinc-Ion Battery

Web References: https://doi.org/10.1016/j.chempr.2026.103129

References: Chem, DOI: 10.1016/j.chempr.2026.103129; article publication date: 7 July 2026.

Image Credits: KAIST

Keywords

Aqueous zinc-ion batteries, proton storage, zinc ions, metal–organic frameworks, conductive MOFs, Cu₃(HHTATP)₂, energy storage, fast charging, battery technology, sustainable batteries, large-scale energy storage, electrochemistry

The breakthrough centers on a new electrolyte architecture (MTP) that is designed to behave like a scaffolded transport network rather than a passive water reservoir. Researchers explain that the binding hydrogen-bond networks typically used to stabilize hydrogels can unintentionally trap solvated Zn²⁺, worsening concentration gradients at higher current densities and undermining Coulombic efficiency.

To overcome this paradox, MTP is built through a straightforward one-pot thermal polymerization that integrates tannic-acid–modified MXene nanosheets (MT) into a polyacrylamide (PAM) matrix. Rather than relying on a single mechanism, the design combines mechanical resilience with directed ion migration: PAM provides structural “web threads,” while the MXene/tannic acid components generate dense polar “sticky sites” that interact selectively with water and ions at the electrode interface.

Computational studies—Monte Carlo simulations and density functional theory—support the claimed synergy. The sticky sites are proposed to coordinate with water molecules to suppress interfacial side reactions and hydrogen evolution, while also lowering barriers to Zn²⁺ desolvation. This dual effect helps homogenize the ion flux and reduces the energetic penalties that typically promote nonuniform deposition.

Electrochemically, the MTP electrolyte reaches an ionic conductivity of 27.69 mS cm⁻¹ and a Zn²⁺ transference number of 0.833, values that outperform many traditional aqueous and less structured hydrogel electrolytes. In symmetric Zn//Zn cells, the system sustains an ultralong lifetime of 4600 hours (>6 months) at 0.5 mA cm⁻², and remains stable even when the current density is doubled.

Crucially, the optimized interface chemistry tunes the nucleation overpotential to 68 mV, promoting uniform Zn deposition with a textured (002) growth preference and effectively suppressing dendrite formation. In half-cells, Zn//Cu cycles last beyond 850 rounds with an average Coulombic efficiency of 98.57%, while Zn//Z-VO full cells retain 74.5% capacity after 2000 cycles at 2 A g⁻¹ and demonstrate a remarkable 10,000-cycle durability at 5 A g⁻¹.

When translated to flexible pouch cells, the electrolyte supports a high specific capacity of 234.22 mAh g⁻¹ and maintains 76.16% retention after 1000 cycles at 1 A g⁻¹, showing negligible performance fluctuation across bending angles from 0° to 180°. Together, these results position MTP as a viral, biomimetic pathway for next-generation flexible energy storage that does not force safety, conductivity, and cycling life to compete.

Subject of Research:
Hydrogel electrolyte design for flexible zinc-ion batteries

Article Title:
Bioinspired Hierarchical Hydrogel Electrolyte for Ultralong‑Life Flexible Zinc‑Ion Batteries

News Publication Date:
10-Jun-2026

Web References:
http://dx.doi.org/10.1007/s40820-026-02246-0

References:
Nano-Micro Letters. DOI: 10.1007/s40820-026-02246-0

Image Credits:
Ran Wang, Qian Gao, Runhai Wu, Yongqi Mi, Shaopei Yang, Hongxiao Wang, Ting Wan, Sehrish Gull, Kefeng Xie, Guankui Long, Pengcheng Du*

Keywords

Hydrogels