Dual-ion pre-intercalation boosts MnO2 ammonium-ion storage performance

Manganese dioxide has long been one of the most attractive candidates for electrode materials in aqueous supercapacitors, combining low cost, environmental friendliness, and a high theoretical capacitance that, on paper, rivals far more expensive transition metal oxides. In practice, however, the material has consistently fallen short of its promise. Its intrinsically poor electrical conductivity, modest practical capacitance, and tendency to degrade during repeated charge–discharge cycles have kept MnO2 from making the leap from laboratory curiosity to commercial component. Now, a team of researchers in China reports a strategy that tackles all three problems at once, using a dual-ion pre-intercalation approach that modifies the material both within its atomic layers and between them.

The study, published in the journal Ionics, was led by Ting Xiao and Xinyu Tan of China Three Gorges University, together with colleagues at Three Gorges University’s College of Electrical Engineering & New Energy and Jingchu University of Technology. Rather than treating the conductivity and stability problems separately, the team designed a synergistic optimization scheme built on two complementary interventions: molybdenum doping inside the MnO2 layers, and ammonium ion pre-intercalation between them, achieved through a process of in-situ electrochemical activation.

The first half of the strategy addresses the electronic limitations of the material. Molybdenum atoms were introduced into the intralayer sites of the MnO2 crystal structure, where they substitute for manganese and fundamentally alter the material’s microstructure and crystal chemistry. This substitution does more than simply dilute the host lattice. According to the researchers, Mo-doping modulates the crystal structure in ways that increase the number of electrochemically active sites available for charge storage, while simultaneously enhancing electrical conductivity through the introduction of oxygen vacancies. These vacancies, which form as the lattice accommodates the foreign cation, act as defects that improve charge transport and provide additional pathways for ion movement. The approach draws on a growing body of evidence that doping engineering can dramatically reshape the electrochemical behavior of manganese oxides, which have been studied extensively in contexts ranging from zinc-ion and lithium-ion batteries to electrocatalysis.

The second half of the strategy addresses the structural fragility that has plagued MnO2 electrodes during cycling. The team employed in-situ electrochemical activation to pre-intercalate ammonium ions into the interlayer and tunnel sites of the doped material. This electrochemical activation serves a dual purpose. First, the process elevates the concentration of oxygen vacancies even further, compounding the conductive benefits introduced by molybdenum doping. Second, and perhaps more importantly, the resident ammonium ions act as structural pillars within the host lattice. By occupying interlayer sites before the electrode ever sees service, the NH4+ ions cushion the mechanical stress of repeated ion insertion and extraction, significantly improving the structural stability of the material over thousands of cycles. Ammonium ion storage chemistry has attracted growing interest in recent years precisely because NH4+ is a lightweight, earth-abundant charge carrier whose tetrahedral geometry allows it to engage in hydrogen-bonding interactions with host lattices, and the new work leverages these properties to stabilize an otherwise fragile oxide.

The electrochemical results reported for the optimized electrode are striking. The Mo-MnO2/AC electrode delivered a high specific capacitance of 668.5 F g⁻¹ at a current density of 2 mA cm⁻², a figure that places it among the better-performing manganese dioxide electrodes reported for aqueous systems. More remarkably, the electrode retained 97.92 percent of its initial capacity after 10,000 charge–discharge cycles at 25 mA cm⁻². Capacity retention approaching 98 percent over ten thousand cycles represents the kind of cycling durability that has historically been elusive for MnO2-based electrodes, which often suffer from dissolution, phase transformations, and mechanical failure as ions shuttle in and out of the lattice.

To demonstrate the practical potential of the material, the researchers assembled an asymmetric supercapacitor pairing the optimized Mo-MnO2/AC positive electrode with activated carbon cloth as the negative electrode. This full-cell device achieved an areal energy density of 1.52 mWh cm⁻² at a power density of 2 mW cm⁻², performance metrics that underscore the viability of the dual-pre-intercalation strategy beyond simple three-electrode measurements. Asymmetric configurations of this kind are a standard route to widening the operating voltage window of aqueous supercapacitors, since the two electrodes operate in complementary potential ranges, and the reported energy density suggests the approach could translate into devices relevant for energy harvesting and storage applications where safety and cost matter more than ultimate energy density.

The significance of the work lies in its dual-front design philosophy. Pre-intercalation strategies have been explored before in the battery and supercapacitor literature, with ions such as Zn2+, Na+, and various metal cations inserted into host structures to pre-expand tunnels, stabilize phases, or boost conductivity. Similarly, molybdenum doping of manganese oxides has been studied for applications including lithium-ion batteries, zinc batteries, and electrocatalytic nitrogen fixation. What distinguishes the new study is the deliberate combination of an intralayer dopant with an interlayer guest ion, orchestrated so that each modification amplifies the benefits of the other. The Mo-doping prepares a more conductive, defect-rich framework; the electrochemically driven ammonium insertion then exploits and reinforces that framework, raising the oxygen vacancy concentration further while locking the structure into a configuration that resists degradation.

The choice of ammonium as the intercalating species is also notable in the broader context of energy storage research. Aqueous ammonium-ion batteries and supercapacitors have emerged as a family of technologies that avoid lithium, cobalt, and other supply-constrained elements, relying instead on nitrogen and hydrogen, two of the most abundant elements available. Because NH4+ is small, mobile in aqueous electrolytes, and capable of forming hydrogen bonds with lattice oxygen, it can navigate host structures with unusual facility, and several recent studies have documented exceptionally long cycling lives for ammonium-ion cells based on Prussian blue analogs, covalent organic frameworks, and molybdenum oxides. The present work extends that toolkit to manganese dioxide, arguably the most commercially accessible oxide electrode material available, and demonstrates that ammonium pre-intercalation can serve as a stabilization mechanism rather than merely a charge carrier role.

The experimental program behind the results combined structural characterization with electrochemical testing. The team, which included Yiwen Zhang, Can Tang, Yuting Mei, Xiuru Li, Yulong Qiao, Lihua Jiang, Shibing Ni, and Yequan Xiao, examined how Mo-doping altered the crystal structure and microstructure of the MnO2, and tracked the changes in oxygen vacancy concentration that accompanied both the doping step and the subsequent electrochemical activation. The authors acknowledge support from the National Natural Science Foundation of China and the 111 Project, and note that the work builds on their earlier investigations of NH4+ pre-intercalation and surface coordination effects on MnO2, as well as interface engineering strategies for co-intercalation of protons and ammonium ions in molybdenum oxides.

For the field of aqueous energy storage, the study offers a template that could be adapted to other host materials. The central insight, that intralayer doping and interlayer ion pre-intercalation can be choreographed to act synergistically rather than independently, suggests a path forward for electrode materials whose theoretical promise has been undermined by practical limitations. As demand grows for safe, inexpensive, and sustainable storage technologies to complement lithium-ion batteries in grid buffering, wearable electronics, and transient power applications, strategies of this kind that extract maximum performance from abundant materials are likely to attract increasing attention. The demonstration that a simple electrochemical activation step can simultaneously load a host lattice with stabilizing ions and generate beneficial defects is particularly appealing from a manufacturing standpoint, since it requires no exotic reagents or high-temperature post-treatments.

The researchers suggest that the synergistic pre-intercalation strategy provides a general framework for developing high-performance electrode materials for ammonium-ion energy storage, and the numbers reported in the study lend considerable weight to that claim. With a specific capacitance approaching 670 F g⁻¹, near-perfect retention over ten thousand cycles, and a working asymmetric device achieving over 1.5 mWh cm⁻² of areal energy, the Mo-MnO2/AC electrode stands as one of the more compelling demonstrations yet that manganese dioxide, properly engineered, can finally begin to deliver on the theoretical potential that has made it such an enduring object of study.