Electrodeposited MnO₂ films for energy storage and catalysis: a review

Abstract

Manganese dioxide (MnO₂) has demonstrated significant potential in electrochemical energy storage and catalytic applications due to its low cost, environmental friendliness, and polymorphic structures. Electrodeposition is an efficient and controllable technique that enables direct deposition of uniform MnO₂ thin films on conductive substrates; their morphology and performance can be tuned by adjusting parameters such as electrolyte composition and current density. This review systematically summarizes the principles of anodic and cathodic deposition of MnO₂, compares the advantages and limitations of potentiostatic, galvanostatic, pulsed, and cyclic voltammetric electrodeposition methods, and explores its applications in batteries, supercapacitors, metal electrowinning anodes, and electrocatalysis. MnO₂ film electrodes exhibit outstanding performance in enhancing battery capacity and stability, improving supercapacitor-specific capacitance, reducing anode overpotential, and boosting catalytic activity. However, challenges such as low conductivity, insufficient structural stability, and the need for scalable fabrication optimization remain. Further advancements in process engineering are essential to accelerate industrial applications.

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1. Introduction

MnO₂ is a highly promising material for electrochemical energy storage (e.g., battery materials, supercapacitors) and electrocatalysis due to its low cost, environmental benignity, and diverse crystal structures,¹ including α, β, γ, δ, ε, and λ phases (Fig. 1). The rapid development of the new energy sector has recently driven significant progress in MnO₂ research and application.

Fig. 1 Schematic diagram of the crystal structure of MnO₂. (a) α-MnO₂, (b) β-MnO₂, (c) γ-MnO₂, (d) ε-MnO₂, (e) δ-MnO₂, (f) λ-MnO₂. Reproduced with permission from ref. 1 Copyright 2018, John Wiley and Sons.

The application of MnO₂ is closely related to its crystal structure. In battery materials, α-MnO₂, with its stable tunnel structure and efficient ion diffusion, is used in aqueous zinc-ion battery (AZIB) cathodes.^2–4 Its framework can adsorb Zn²⁺ stably, which enhances the battery's charge/discharge efficiency. γ-MnO₂, with its expanded interlayer spacing, accommodates more H⁺ and Zn²⁺, achieving higher energy density.^5,6δ-MnO₂, which features large interlayer spacing, provides abundant active sites and ion transport channels that significantly improve the battery's rate performance and cycle life.^7,8 In supercapacitor applications, MnO₂ offers a high theoretical specific capacitance of ∼1370 F g⁻¹, based on one-electron redox reactions involving Mn³⁺/Mn⁴⁺ transitions. However, practical systems typically reach only 200–600 F g⁻¹ due to limitations in electronic conductivity, ion diffusion kinetics, and active site utilization. These shortcomings highlight the need for strategies to unlock the material's full charge storage potential. Research has demonstrated that α-MnO₂, with its large specific surface area and abundant active sites, exhibits high specific capacitance and excellent cycling stability.^9,10γ-MnO₂'s layered structure facilitates ion adsorption/desorption, showing high activity and reversibility, thereby enhancing capacitor performance.^9,11δ-MnO₂'s layered structure stores more ions, and its large interlayer spacing and excellent ion exchange ability further improve capacitance and charge/discharge speed.^10,12 In metal electrowinning anodes, γ-MnO₂ exhibits excellent structural stability, particularly in acidic electrolytes, effectively suppressing anode corrosion and dissolution, extending service life, and improving zinc electrowinning efficiency and quality.^13–20 In electrocatalysis, MnO₂ serves as a cost-effective alternative to precious-metal catalysts for reactions such as the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in metal–air batteries and fuel cells.⁸ Its mixed valence states (Mn³⁺/Mn⁴⁺) and tunable oxygen vacancy content facilitate the adsorption and desorption of oxygen intermediates, enabling efficient reaction kinetics. For instance, birnessite-type MnO₂ exhibits promising ORR activity in alkaline media, with half-wave potentials approaching those of commercial Pt/C.²¹ Indeed, the distinct crystal polymorphs of MnO₂ each confer unique advantages.²²α-MnO₂, abundant in oxygen vacancies and Mn³⁺ sites, accelerates electron transfer and the surface turnover of intermediates in both ORR and OER,^23–26 whereas the layered δ-MnO₂, with its large interlayer spacing, promotes reactant adsorption and diffusion, further boosting catalytic activity and selectivity.²⁷

To date, a wide range of synthesis methods have been developed to prepare MnO₂, including hydrothermal, sol–gel, chemical precipitation, thermal decomposition, and electrodeposition.^12,28–30 Each method offers unique advantages and limitations. Hydrothermal synthesis enables good control over crystal phase and morphology under relatively mild conditions.¹⁰ However, it typically requires high pressure, long reaction times, and post-treatment steps such as filtration and drying, which complicate scalability and increase production costs.³¹ Sol–gel methods allow for homogeneous mixing at the molecular level and can produce high-purity materials.^32,33 Yet, they often involve expensive precursors, organic solvents, and complex processing steps, limiting their cost-effectiveness and environmental sustainability. Chemical precipitation is simple and scalable but offers limited control over morphology and crystallinity. The resulting products often suffer from agglomeration and poor adhesion to substrates, which hinders their direct integration into devices.³⁴

In contrast, electrodeposition is a particularly attractive alternative due to its simplicity, scalability, low cost, and precise control over film thickness, morphology, and crystal structure. It enables the direct growth of MnO₂ thin films on conductive substrates, eliminating the need for binders or conductive additives and facilitating direct device integration. Moreover, electrodeposition can be conducted under ambient conditions with minimal waste generation, aligning with green chemistry principles.

By adjusting deposition parameters, such as electrolyte composition, pH, temperature, applied potential or current, and deposition time, the nucleation and growth processes can be tailored to obtain MnO₂ films with desired phases (e.g., α-, γ-, or δ-MnO₂),^35–37 nanostructures (e.g., nanosheets, nanorods, nanoflowers),^37–39 and porosities. This level of control is difficult to achieve with many other synthesis routes. Despite its advantages, electrodeposition faces challenges, including the inherently low conductivity of MnO₂, which can impede charge transfer during deposition, and inconsistent film adhesion under specific conditions. Furthermore, scaling up the process while maintaining uniformity and reproducibility remains a significant hurdle. Nonetheless, recent advances in pulse electrodeposition, template-assisted growth, and the development of hybrid composites have substantially enhanced the effectiveness and applicability of this technique.^16,40

This review provides a comprehensive and critical assessment of the electrodeposition of MnO₂ thin films, focusing on fundamental principles, recent advances, and application potential in energy storage and catalysis. By systematically comparing electrodeposition with other synthesis routes, we highlight its unique advantages and prospects as a scalable and tunable fabrication platform for next-generation MnO₂-based functional materials.

2. Electrodeposition principles of MnO₂

2.1 Anodic deposition

The anodic electrodeposition of MnO₂ involves three interconnected stages: ion migration, charge transfer, and electrocrystallization (lattice formation).⁴¹ Initially, Mn²⁺ ions in the electrolyte migrate to the anode surface. Subsequently, Mn²⁺ undergoes oxidation upon losing electrons at the electrode surface, forming Mn³⁺ or MnOOH intermediates.⁴² Finally, these oxidized intermediates either diffuse to stable lattice sites or aggregate to form nuclei, which ultimately grow into an ordered MnO₂ layer.

MnO₂ electrodeposition primarily occurs in acidic systems, beginning with Mn²⁺ oxidation to Mn³⁺ at the anode surface:

Mn²⁺(aq) → Mn³⁺(aq) + e⁻E° = −1.542 V vs. SHE (1)

The stability of Mn³⁺ depends on the electrolyte's acidity. In strongly acidic conditions, Mn³⁺ tends to hydrolyze to form MnOOH, as shown in Fig. 2a. MnOOH, as an intermediate phase, forms nano-crystalline nuclei on the electrode surface,⁴³ which then convert to MnO₂via solid-phase oxidation:

Mn³⁺(aq) + 2H₂O(l) → MnOOH(s) + 3H⁺(aq) (2)

MnOOH(s) → MnO₂(s) + H⁺(aq) + e⁻ E° = +0.98 V vs. SHE (3)

In weakly acidic conditions, Mn³⁺ stability is low, leading to disproportionation reactions. Subsequently,^43,44 Mn⁴⁺ hydrolysis generates MnO₂:

2Mn³⁺(aq) → Mn²⁺(aq) + Mn⁴⁺(aq) (4)

Mn⁴⁺(aq) + 2H₂O(l) → MnO₂(s) + 4H⁺(aq) (5)

Fig. 2 Schematic diagram of the reaction path for electro-deposition of MnO₂: (a) anodic electro-deposition, (b) cathodic electro-deposition.

2.2 Cathodic deposition

MnO₂ can also be deposited via cathodic reduction, as illustrated in Fig. 2b. Cathodic deposition includes indirect and direct mechanisms, with differing reaction pathways and product characteristics. Compared to anodic deposition, cathodic deposition avoids anode corrosion and dissolution of electrode materials.

(1) Indirect method: in Mn²⁺ solutions containing nitrate, hydroxide ions (OH⁻) generated by nitrate reduction at the cathode surface increase local pH. The elevated OH⁻ concentration promotes Mn(OH)₂ precipitation:⁴⁵

NO⁻₃(aq) + H₂O(l) + 2e⁻ → NO⁻₂(aq) + 2OH⁻(aq) E° = +0.01 V vs. SHE (6)

NO⁻₃(aq) + 7H₂O(l) + 28e⁻ → NH⁺₄(aq) + 10OH⁻(aq) E° = −0.12 V vs. SHE (7)

Mn²⁺(aq) + 2OH⁻(aq) → Mn(OH)₂(s) (8)

Mn(OH)₂ can be transformed into MnO₂ through heat treatment at 300 °C:⁴⁶

(2) Direct method: in acidic solutions containing MnO₄⁻, MnO₂ can be directly reduced at the cathode:⁴⁷

MnO⁻₄(aq) + 2H₂O(l) + 3e⁻ → MnO₂(s) + 4OH⁻(aq) E° = +0.595 V vs. SHE (10)

3. Primary techniques of electrodeposition for MnO₂ preparation

As a precise material preparation technique, electrodeposition is crucial for preparing high-performance MnO₂ electrode materials.⁴⁸ In a three-electrode system (Fig. 3a), controlling potential, current, pulses, and potential scans enables the preparation of MnO₂ films with specific structures and properties.⁴⁹ This technique is compatible with different deposition systems (e.g., three- and two-electrode systems, Fig. 3b), can be performed at low temperatures, and allows for precise and rapid control of film growth. It primarily includes constant potential deposition, constant current deposition, pulse electrodeposition, and cyclic voltammetric deposition (Table 1).

Fig. 3 Schematic diagram of the implementation of preparing MnO₂ membrane electrodes by electrodeposition: (a) three-electrode system (CE: counter electrode, RE: reference electrode, WE: working electrode); (b) electrochemical half-cell and full-cell settings; (c) current–time graphs in pulsing and (d) DC modes; (e) periodic triangular potential excitation signal potential diagram of cyclic voltammetric deposition.

Table 1 Comparison of different electrodeposition methods for MnO₂ preparation

Method	Principle	Advantages	Limitations	Impact of deposition parameters
CPD	Drives Mn^2+ reduction and deposition by maintaining constant electrode potential	Strong nucleation controllability, uniform film layers, adjustable crystal structure	Low deposition rate, high requirement for potential stability	Electrolyte pH and concentration affect crystal structure: weakly acidic → α-MnO2 cross-needle structure; strongly acidic → γ-MnO2 nanorod structure
CCD	Drives Mn^2+ reduction and deposition by maintaining constant current density	Fast deposition rate, simple equipment, low cost	Uneven film thickness distribution, poor crystal controllability	Current density affects morphology: low current → micrometer-sized block structures; high current → nanoparticle structures. Electrolyte acidity correlates with crystal structure
PD	Applies periodic pulse current to optimize mass transfer during intervals	Suppresses grain coarsening, forms nano-porous structures with high specific surface area and low energy consumption	Requires dedicated pulse power supply, complex parameter optimization	Pulse duty cycle and average current density adjust porosity and grain size. High current density during on-time promotes nucleation; off-time allows ion concentration recovery
CVD	Utilizes periodic triangular potential scan signals for oxidation–reduction reactions	Suitable for preparing high-quality nanofilms; scan path optimizes crystal selectivity	Complex process, low deposition efficiency, difficult large-scale production	Scan rate affects morphology: low rate (<20 mV s^−1) → nanosheet structures; high rate (>50 mV s^−1) → nanoparticles or dendrites. Wide potential window favors α-MnO2

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3.1 Constant potential deposition

Constant potential deposition (CPD) involves maintaining a constant electrode potential during electrodeposition, driving metal ions to reduce and deposit on the electrode surface. By adjusting the deposition potential, the crystal structure and composition of MnO₂ can be controlled. The deposition kinetics are influenced by the double-layer characteristics at the electrode/solution interface. This method offers controllable nucleation and uniform film layers but suffers from low deposition rates and demands stringent potential control.⁵⁰ The crystal structure of MnO₂ deposited by this method is affected by electrolyte pH and ion concentration. Weakly acidic conditions favor the formation of α-MnO₂ cross-needle nanostructures,³⁵ while strongly acidic conditions promote γ-MnO₂ nanorod films.^51,52 Crystal structure differences directly impact electrochemical performance: α-MnO₂'s (2 × 2) tunnel structure facilitates rapid ion insertion/desorption, suitable for high-rate energy storage devices,⁵³ while γ-MnO₂'s (1 × 1) tunnel structure exhibits good catalytic activity and cycle stability. For example, Sasaki et al.⁵⁴ leveraged this potential control to electrodeposit a [Co(en)₃]-intercalated layered MnO₂ film on a bamboo charcoal-carbon nanotube (b-CNT) composite/ITO substrate by applying +0.8 V vs. Ag/AgCl; the fixed potential produced an ordered layered structure with 1.013 nm interlayer spacing that facilitated [Co(en)₃]³⁺ insertion and greatly enhanced the film's electrocatalytic activity toward the hydrogen evolution reaction. Thus, CPD is a key technique for tailoring MnO₂ film properties for targeted applications through precise potential control.

3.2 Constant current deposition

Constant current deposition (CCD) maintains a constant current density during electrodeposition (Fig. 3c), driving metal ions to reduce, nucleate, and grow on the electrode surface.⁵⁵ By controlling current density and deposition time, nucleation rate and crystal growth can be directly regulated. This method offers fast deposition rates but suffers from uneven film thickness distribution and poor crystal controllability. The deposition kinetics are influenced by the ion concentration gradient at the electrode/solution interface. High current densities accelerate ion migration, increasing nucleation site density, which favors the formation of nanoparticles; while low current densities favor micrometer-sized block structures under ion diffusion control. Electrolyte pH and current density influence MnO₂'s crystal structure: weakly acidic electrolytes and low current densities favor α-MnO₂ nanosheets,^52,56,57 while strongly acidic conditions and high current densities promote γ-MnO₂ as the main phase.^{13–15,17–20} For instance, Zhang et al.⁵⁸ employed a one-step CCD protocol at 0.0045 A in a weakly acidic bath to deposit δ-MnO₂ nanosheets on carbon cloth. This process simultaneously reduced and intercalated Ag⁰/Ag⁺ species. The mild current density preserved the δ-layered framework, while the in situ Ag incorporation overcame the intrinsically poor conductivity and cyclability of pristine MnO₂. Briefly, CCD is instrumental in linking operational parameters to morphological outcomes, enabling the fabrication of MnO₂ films with desired microstructures for enhanced performance.

3.3 Pulse deposition

Pulse deposition (PD) applies periodic non-steady current pulses (Fig. 3d), utilizing the concentration recovery effect during pulse intervals to optimize mass transfer at the electrode/solution interface.⁵⁹ This enables control over MnO₂ grain size and pore structure. Compared to constant current and potential depositions, this method adjusts duty cycle and average current density to suppress concentration polarization and enhance deposition layer uniformity. It allows precise control over coating thickness and composition but requires dedicated pulse power supplies and complex parameter optimization.⁶⁰ The nucleation-growth behavior is jointly influenced by pulse parameters and electrolyte composition. During the on-time (t_on), high current densities (J_p = 10–50 mA cm⁻²) promote Mn²⁺ reduction and high-density nano-nucleation. During the off-time (t_off), Mn²⁺ in the electrolyte diffuses to the electrode surface, preventing local concentration depletion and dendrite growth. For example, Xie et al.⁴⁰ employed PD to grow an α-MnO₂ film on a lead-alloy anode; the repetitive high-density nucleation/relaxation cycles produced a compact, low-Mn³⁺ coating that lowered the anode potential by 60 mV versus direct-current deposition and markedly improved corrosion resistance. This cyclic process is key to forming nanoporous structures with high specific surface area and improved electrochemical activity.

3.4 Cyclic voltammetric deposition

Cyclic voltammetric deposition (CVD) applies periodic linearly varying potential scan signals (Fig. 3e), causing metal ions to oxidize and reduce at the electrode surface.⁶¹ By adjusting scan rate (ν), potential window (ΔE), and cycle number (N), deposition rate and thickness can be controlled, making it suitable for preparing high-quality nanofilms.⁶² For instance, Khayyam Nekouei et al.³⁸ employed CVD to regenerate electrolytic MnO₂ (EMD) predominantly comprising the γ-MnO₂ phase from a precursor solution of MnSO₄–H₂SO₄ (pH = 2) derived from spent Zn–MnO₂ batteries at 95 °C. The regenerated EMD exhibited a higher discharge capacity (213 mA h g⁻¹) and superior cycling reversibility compared to its commercial counterpart (175 mA h g⁻¹) in Zn–MnO₂ cells. Compared to traditional constant potential electrodeposition, it offers higher stability but suffers from complex processes, low deposition efficiency, and difficulties in large-scale production.⁶³

The crystal selectivity of MnO₂ deposited by cyclic voltammetry can be adjusted by scan rate, pH, and potential window. At low scan rates (ν < 20 mV s⁻¹), Mn²⁺ reduction is diffusion-controlled, favoring uniform nanosheet structures. At high scan rates (ν > 50 mV s⁻¹), the reaction is charge-transfer dominated, tending to form nanoparticles or dendrites.⁶⁴ In acidic electrolytes, a wide potential window generates γ-MnO₂ nanorods, while in neutral conditions (pH = 6.0), a narrow window favors α-MnO₂ nanosheets.^37,38 Increased cycle numbers induce internal stress in the deposition layer, raising crack rates. CVD is instrumental in fabricating complex nanostructures by leveraging dynamic potential control.

4. Characterization of electrodeposited MnO₂ film electrodes

The performance of electrodeposited MnO₂ film electrodes in energy storage and catalytic applications is intrinsically governed by their physicochemical properties, including film thickness, crystallographic phase, surface morphology, defect chemistry, and manganese oxidation state. However, unlike their bulk or powder counterparts, electrodeposited MnO₂ films present unique characterization challenges: they are often ultrathin, exhibit low crystallinity, and are strongly adherent to conductive substrate.²⁹ Therefore, a multi-modal, surface-sensitive, and operando-compatible characterization strategy is essential. This section provides a multi-technique approach to elucidating the structure–property relationships in electrodeposited MnO₂ films, with a focus on addressing methodological limitations, minimizing measurement artifacts, and interpreting data within the film's operational electrochemical environment.

4.1 Probing film morphology, thickness, and elementary composition

The morphology and microstructure of electrodeposited MnO₂ films are not merely surface features but also determine ion-transport kinetics, electron-transfer efficiency, and catalytic-site accessibility. However, characterizing these films is technically non-trivial due to their nanoscale thickness, gradients in amorphous-to-crystalline transitions, and substrate-dependent adhesion variability.⁶⁵

Scanning electron microscopy (SEM) is the primary imaging tool, but its limitation to two-dimensional projection necessitates cross-sectional imaging for accurate thickness measurement.⁶⁶ This is particularly critical for films below 200 nm, where focused ion beam-SEM (FIB-SEM) becomes essential.⁶⁷ Energy-dispersive X-ray spectroscopy (EDS), while convenient, suffers from low spatial resolution (approximately 1 µm),⁶⁸ often requiring wavelength-dispersive spectroscopy (WDS) or EDS spectrum deconvolution to validate trace dopants.^69,70 Atomic force microscopy (AFM) offers sub-nanometer vertical resolution, but tip convolution artifacts can distort the apparent porosity or surface roughness.⁷¹ Modes such as PeakForce AFM or phase-imaging AFM can differentiate mechanical heterogeneity (e.g., MnOOH vs. MnO₂ domains),⁷² which is critical for identifying resistive phases that SEM cannot resolve.

Transmission electron microscopy (TEM), particularly high-resolution TEM (HRTEM), is indispensable for lattice fringe imaging and selected area electron diffraction (SAED) to distinguish α-, γ-, or δ-MnO₂ polymorphs at the nanometer scale. However, beam-induced phase transformation (e.g., δ-MnO₂ → Mn₃O₄) is a well-documented artifact, necessitating the use of low-dose TEM.^73,74 Scanning TEM-electron energy loss spectroscopy (STEM-EELS) can map Mn oxidation-state gradients across the film–substrate interface,⁶⁴ revealing Mn²⁺ enrichment—a signature of poor adhesion or under-oxidation—that represents a key failure mode in energy storage devices.

4.2 Determining crystallographic structure

The electrochemical behavior of MnO₂ is governed by its polymorphism. However, electrodeposited films often exhibit (i) nanocrystallinity,^9,51 (ii) stacking faults,⁷⁵ and (iii) intergrowth phases,^14,40 all of which broaden X-ray diffraction (XRD) peaks and obscure phase identification. Conventional XRD should be complemented by synchrotron grazing-incidence XRD (GI-XRD) to enhance surface sensitivity, with a penetration depth typically below 50 nm.⁷⁶ This is especially important for ultrathin films where the substrate signal dominates bulk XRD.

Raman spectroscopy is highly phase-sensitive, yet its reliability hinges on laser power.⁷⁷ At low laser powers (≤0.1 mW), conventional Raman or tip-enhanced Raman spectroscopy (TERS) can resolve local phase heterogeneities, such as Mn³⁺-rich domains, which are averaged out in XRD.⁷⁸ Furthermore, polarized Raman spectroscopy probes the softening of Mn–O–Mn bonds, a phenomenon directly linked to the evolution of spin angles between Mn⁴⁺ ions during the helimagnetic transition.⁷⁹ These low-power, polarization-resolved signatures provide spectral markers for phase changes and capacity fade in electrodeposited MnO₂ films.

4.3 Analyzing surface chemistry, oxidation state, and defects

The surface redox chemistry of MnO₂ is dictated by Mn³⁺/Mn⁴⁺ ratio, oxygen vacancy (V_o) concentration, and adsorbed hydroxyl groups, all of which are dynamic under electrochemical polarization.⁸⁰ Electron paramagnetic resonance (EPR) directly quantifies V_o in the electrodeposited MnO₂ film.⁸¹ However, conventional ex situ characterization techniques frequently fail to capture these operando changes, necessitating correlation across multiple surface-sensitive methods.

X-ray photoelectron spectroscopy (XPS) is a standard technique, but the energy separation of the Mn 2p_3/2 peaks (ΔE ≈ 11.6 eV) is often insufficient to distinguish Mn³⁺ from Mn⁴⁺ reliably. The Mn 3s multiplet splitting value (ΔE_s) serves as a reliable probe for determining the surface average oxidation state (AOS) of Mn, which can be calculated using the formula: AOS = 8.956 − 1.126ΔE_s.⁸² However, surface charging (especially on insulating or carbon substrates) can significantly shift binding energies, requiring charge referencing to adventitious C 1s at 284.8 eV or flood-gun neutralization.⁸³ Angle-resolved XPS (AR-XPS) can depth-profile Mn oxidation state gradients.⁸⁴ For instance, Mn²⁺ surface enrichment is a signature of disproportionation-driven dissolution, a key degradation mode in Zn-ion batteries.⁸⁵

Complementary synchrotron-based pair distribution function (PDF) analysis and X-ray absorption spectroscopy (XAS) enable quantitative structural characterization of γ-MnO₂ thin films.⁸⁶ PDF fitting distinguishes edge- and corner-sharing MnO₆ octahedra. Extended X-ray absorption fine structure (EXAFS) can validate these values through Mn–Mn coordination number analysis, while X-ray absorption near edge structure (XANES) enables monitoring of the corresponding evolution of the Mn oxidation state. In addition, Operando XAS has directly captured the reversible dissolution and deposition of Mn²⁺ species in aqueous Zn/MnO₂ batteries, providing real-time structural evidence for the dominant redox mechanism across multiple mildly acidic electrolytes.⁸⁷

In summary, a comprehensive understanding of electrodeposited MnO₂ film electrodes cannot be achieved by a single characterization technique. Instead, it requires the synergistic integration of data from a suite of complementary methods. By adopting a stage-wise, data-rich characterization framework, researchers can systematically deconvolute the roles of phase, defects, and interfaces in MnO₂ film electrodes. This approach not only deepens fundamental understanding but also provides a clear pathway for translating laboratory innovations into scalable, industrially viable energy storage and catalytic devices.

5. Application research progress of MnO₂ film electrodes

5.1 Battery materials

MnO₂ prepared via electrodeposition has garnered significant attention as an electrode material for zinc-manganese batteries, aqueous zinc-ion batteries (AZIBs), and flexible energy storage devices, as listed in Table 2. By enabling the design of various MnO₂ morphologies such as nanowires, nanorods, and nanoflowers, electrodeposition effectively enhances electrochemical performance.³⁹ For instance, α-MnO₂ nanosheet arrays deposited on carbon nanotubes via CPD (Fig. 4a),³⁵ combined with the high conductivity of the substrate and the fast ion diffusion characteristics of the α-phase tunnel structure, enable flexible electrodes to achieve a specific capacity of 292.7 mA h g⁻¹ at 0.2 mA cm⁻². The quasi-solid-state Zn–MnO₂ battery assembled with this electrode exhibits an open-circuit voltage of 1.445 V, which remains highly stable over 10 000 seconds (Fig. 4b). After 1000 cycles, the capacity retention rate reaches 85.3% (Fig. 4c), applicable in power design for wearable electronic devices. For battery recycling, electrodeposition technology regenerates thermal chemical MnO₂ precursors into γ-MnO₂ nanorod structures,³⁸ which exhibit significantly better electrochemical energy storage performance than the original EMD in KOH and LiOH electrolytes, providing a technical pathway for efficient retired battery recycling.

Table 2 Electrodeposition of MnO₂ film electrodes for battery materials

Deposition system [ref.]	Substrate	Temperature	Deposition method	Crystal phase	Structural characteristics	Performance
0.1 M Mn(CH3COO)2 (ref. 35)	Carbon nanotube	—	CPD (0.7 V vs. Ag/AgCl, 600 s)	α-MnO2	Nanosheet structure	Specific capacity: 105.6 mA h g^−1, 1000 cycles, discharge capacity retention > 100%, coulombic efficiency: 99%
1 M MnSO4–H2SO4 (pH = 2)^38	Ti	95 °C	CVD (1.4–1.6 V vs. RHE, 6 h)	γ-MnO2 (major)	Nanorod structure	Specific capacity: 213 mA h g^−1, coulombic efficiency: 80%, high-rate stability
				α-, β-MnO2 (minor)
0.3 M MnSO4 (ref. 88)	Carbon paper	RT	CCD (0.3 mA cm^−2, 45 min)	—	Porous structure	0.5 mA cm^−2: Specific capacity 0.35 mA h cm^−2; 1.5 mA cm^−2: 1000 cycles coulombic efficiency: 100%
0.01 M MnSO4–0.1 M Na2SO4–0.001 M ZnSO4 (ref. 52)	Ni	RT	CPD (−1.8 V vs. Ag/AgCl, 15 min)	γ-MnO2	3D porous nanosheets	5 A g^−1: Reversible capacity 1545.4 mA h g^−1, 1500 cycles, coulombic efficiency: 99.7%, capacity retention: 92.2%
0.01 M Mn(NO3)2–0.1 M NaNO3–sodium dodecyl sulfate−0.5 mM C6H5K3O7 (ref. 89)	Carbon cloth	80 °C	CPD (1.2 V vs. Ag/AgCl, 40 min)	—	Dense porous lamellar	0.1 A g^−1: Reversible capacity 311 mA h g^−1, energy density 370 Wh kg^−1, 450 cycles, capacity retention: 202 mA h g^−1
0.06 mol L^−1 Mn(CH3COO)2–0.06 mol L^−1 Na2SO4 (ref. 90)	Ni	RT	CCD (7 mA cm^−2, 200 s)	—	Nanorods	1 A g^−1: Capacitance 415.4 F g; 200 cycles: capacity 818.1 mA h g^−1, coulombic efficiency > 97%
0.06 mol L^−1 Mn(CH3COO)2–0.06 mol L^−1 Na2SO4 (ref. 91)	Ni	50 °C	CPD (0.6 V vs. Ag/AgCl, 5 min)	β-MnO2	Interlaced/parallel nanosheets; nanoflower	0.1C: Specific capacity 1476.7 mA h g^−1, 200 cycles, capacity retention: 625 mA h g^−1
0.66 M MnSO4–0.34 M H2SO4 (ref. 92)	Carbon fiber	85 °C	CCD (0.01 A dm^−2, 2 h)	β-MnO2	Fibrous structure	High working potential, slow potential decay, high energy output efficiency, and stability

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Fig. 4 (a) SEM image of CNT@MnO₂. (b) Open-circuit voltage variation of quasi-solid-state Zn–MnO₂ batteries during bending. (c) Long-term cycling performance of CNT@MnO₂ at 3 mA cm⁻². Reproduced with permission from ref. 35 Copyright 2020, Elsevier. (d) Electrodes prepared by electrodeposition and their application in button-type AZIB as the cathode. (e) Mechanism diagram of the conversion of Mn²⁺ ions to MnO₂ on carbon paper substrates through two pathways during electrodeposition. Reproduced with permission from ref. 88 Copyright 2022, Elsevier. (f) Schematic diagram of the deposition of MnO₂ polymorphs varying with temperature. Voltage curves of (g) ε-MnO₂, γ-MnO₂ and β-MnO₂ during the discharge of MN-H₂ cells. Reproduced with permission from ref. 93 Copyright 2023, John Wiley and Sons.

In AZIBs, electrodeposition can adjust MnO₂ electrode morphology by changing deposition conditions. Porous MnO₂ electrodes prepared via one-step electrodeposition can be directly used as the cathode in button-type AZIB (Fig. 4d). In the electrode reaction, Mn²⁺ is first oxidized at the surface to Mn³⁺, which then forms MnO₂via two pathways,⁸⁸ as shown in Fig. 4e. The hierarchical porous structure provides efficient ion transport channels for Zn²⁺/H⁺, shortens electron diffusion paths, and improves electrochemical reaction kinetics, effectively alleviating volume expansion during Zn²⁺ insertion/desorption. Fibrous MnO₂/carbon fiber composite electrodes, optimized via interface engineering, reduce charge transfer resistance by 40%,⁹² increase operating potential to 1.45 V, and exhibit less than 5% capacity decay after 500 bending cycles, demonstrating excellent mechanical flexibility and electrochemical stability.

Electrodeposition temperature significantly affects MnO₂'s crystal structure. By adjusting deposition temperature, MnO₂'s crystal structure can transition from low-conductive ε-phase to high-conductive γ-phase (Fig. 4f),⁹³ accompanied by microstructure evolution. At 25 °C, ε-MnO₂ forms circular particle stacks, which gradually transform into highly oriented γ-MnO₂ nanorod arrays with increasing temperature. Different crystal phases exhibit significant discharge performance differences. γ-MnO₂ and β-MnO₂, with higher conductivity, achieve specific capacities of 597.4 mA h g⁻¹ and 616.0 mA h g⁻¹, respectively (Fig. 4g), while ε-MnO₂'s specific capacity is only 217.3 mA h g⁻¹. Temperature-controlled crystal phase transformation enables electrodes to achieve ultra-high area capacity of 33 mA h cm⁻² at 75 °C, with a capacity retention rate of 87% after 200 cycles, surpassing traditional ε-phase electrode performance limits. This provides a new pathway for developing high-energy-density aqueous batteries.

The performance of electrodeposited MnO₂ remains inferior to that of other state-of-the-art electrode materials. For instance, nickel-based oxides such as NiCo₂O₄ are recognized for their high electrical conductivity and excellent rate capability, yet they suffer from faster capacity degradation in aqueous zinc-ion batteries compared to the structurally more stable MnO₂ polymorphs.^8,94 Likewise, cobalt oxides (e.g., Co₃O₄) exhibit superior catalytic activity but are hampered by high cost and environmental toxicity.⁹⁵ In contrast, room-temperature electrodeposition offers a compelling advantage for fabricating low-cost, flexible devices, though bridging the conductivity gap with these alternatives through composite engineering remains a priority.

Enhancing MnO₂ electrode conductivity, reversible discharge capacity, ion/electron diffusion kinetics, and cycle stability are critical bottlenecks for its application.⁹⁶ Controllable electrodeposition techniques can prepare MnO₂-based composite materials with nanostructures,³⁸ significantly expanding the effective contact area between the electrode/electrolyte interface and shortening Zn²⁺ and electron transport paths. This optimizes ion/electron transport kinetics, improves MnO₂'s electronic conductivity, and enhances overall electrochemical performance.

5.2 Supercapacitors

The MnO₂ synthesized via electrodeposition exhibits advantages such as high theoretical specific capacitance, wide operating potential window, environmental friendliness, and low cost, which is widely used in pseudocapacitive supercapacitor electrode materials,^56,97 as listed in Table 3. Its energy storage mechanism primarily occurs on the surface, providing high theoretical pseudocapacitance (>1000 F g⁻¹) through electrolyte ion adsorption/desorption and surface Faraday reactions.^37,56

Table 3 Electrodeposition of MnO₂ film electrodes for supercapacitors

Deposition system [ref.]	Substrate	Temperature	Deposition method	Crystal phase	Structural characteristics	Performance
Mn(CH3COO)2·4H2O–Na2SO4 (ref. 51)	Cu nano leaves	—	CPD (0.5 V vs. SCE, 350 s)	γ-MnO2	Nanoscale structure	Specific capacitance: 486 F g^−1 (250 s deposition: 376.7 F g^−1)
0.1 M MnSO4–0.1 M Na2SO4 (ref. 102)	AISI 304	RT	CPD (−1.5 V vs. SCE, TAC −0.55C cm^−2)	Amorphous/poor crystallinity	Porous wrinkled nanosheets	1 A g^−1: Specific capacitance 305 F g; 10 A g^−1: rate performance 61%
0.01 M Mn(NO3)2·4H2O–0.24 M KNO3–20 mg/50 mL (ref. 100)	Ni	25 °C	CPD (1.2 V vs. SCE, 30 min)	—	Nanosheet structure	1 A g^−1: Specific capacitance 665 F g^−1, 30 000 cycles, capacity retention: 92.13%
0.1 M KMnO4 (ref. 75)	AISI 304	—	CPD (0.2 V vs. Ag/AgCl)	ε-MnO2	Sparse porous crystalline structure	Specific capacitance: 259.4 F g^−1
			CCD (1 mA cm^−2)	—	Dense fine-grained stack	Specific capacitance: 180.3 F g^−1
			CVD (0.0–0.3 V vs. Ag/AgCl)	—	Compact fine grains	Specific capacitance: 187.1 F g^−1
0.1 M Mn(CH3COO)2–0.1 M Na2SO4 (ref. 56)	Composite carbon	RT	CCD (5 mA cm^−2, 20 h)	α-MnO2	3D porous network	5 mA cm^−2: Specific capacitance 3553.74 mF cm^−2
100 mM KMnO4 (ref. 37)	Graphite felt	RT	CVD (2 V to −4 V vs. SCE)	γ-, δ-MnO2	Uniform dense coating	1.4 mA cm^−2: Specific capacitance 832 mF cm^−2 (226 F g^−1); 14 mA cm^−2: 9000 cycles, capacity retention 98%, capacitance retention 118%
0.5 M Mn(CH3COO)2–0.1 M Na2SO4 (ref. 103)	Carbon cloth	RT	CPD (0.92 V vs. SCE, 120 s)	—	Nanoneedle multilayers	0.2 A g^−1: Specific capacitance 325 F g; 5.0 A g^−1: capacitance retention 70%
0.1 M Mn(CH3COO)2–0.1 M Na2SO4 (ref. 104)	Reduced graphene oxide	RT	CPD (0.8 V vs. Ag/AgCl, 60 s)	—	3D porous chain/ring structure	1 A g^−1: Specific capacitance 942.6 F g; 30 A g^−1: 10 000 cycles, capacitance retention 96.1%
0.07 M Mn(CH3COO)2–0.07 M Na2SO4 (ref. 105)	Ni	—	CPD (0.6 V vs. Ag/AgCl, 50 s)	—	3D continuous porous nanodendrites	5 A g^−1: Specific capacitance 358 F g; 2500 cycles, capacitance retention 83.9%
0.1 M MnSO4–0.1 M H2SO4 (ref. 106)	rGO-CNTs	RT	PD (32 mA cm^−2, 0.1 s on/1 s off)	—	Fine nanoparticle structure	Specific capacitance: 209 F g; 3000 cycles, capacitance retention 96%
0.1 M Mn(CH3COO)2–0.05 M Na2SO4 (ref. 57)	Graphite	RT	CCD (25 mA cm^−2, 20 s)	α-MnO2	Porous nanospheres	5 mA cm^−2: Specific capacitance 0.59 F cm^−2; 10 mA cm^−2: 1000 cycles, stability 93.2%
0.1 M KMnO4 (ref. 107)	Ni (coated with g-C3N4)	—	CPD (10 V vs. Ag/AgCl, 1 min)	—	3D heterogeneous nanoparticles	0.5 A g^−1: Specific capacitance 87.6 F g^−1
0.1 M Mn(CH3CO2)2·(H2O)n^9	Multiwalled carbon nanotubes (CNTs)	60 °C	CPD (1.3 V vs. Ag/AgCl, 40 min)	α-, γ-MnO2	Nanoparticles-on-nanosheets	1 mA cm^−2: Specific capacitance 3.54 F cm^−2
0.02 M Mn(NO3)2 (ref. 108)	Ni foam pre-coated with NiCo2O4	RT	CPD (−1.0 V vs. SCE, 10 min)		Nanowire–nanosheet core–shell	1 A g^−1: Specific capacitance 1186 F g^−1
0.1 M Mn(CH3COO)2·4H2O–0.1 M Na2SO4 (ref. 109)	Al/Au	RT	CPD (+0.8 V vs. SCE, 400 s)		Nanowire structure	0.2 mA cm^−2: Specific capacitance 222.13 mF cm^−2; 2000 cycles, capacitance retention 86.3%
0.01 M MnSO4–0.1 M Na2SO4 (ref. 110)	Carbon nanofibers	RT	CCD (40 µm cm^−2, 4 h)	α-MnO2	Clustered nanofibers	Specific capacitance: 630 F g; 2000 cycles, capacitance retention 81.8%

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MnO₂'s value proposition is clarified through comparison with other pseudocapacitive materials. Ruthenium dioxide (RuO₂) serves as the performance benchmark but is limited by its prohibitively high cost.⁹⁸ While advanced materials such as perovskite oxides (e.g., SrRuO₃) exhibit outstanding power density, they require complex synthesis processes at high temperatures.⁹⁹ In contrast, electrodeposited MnO₂ occupies a strategic position by offering a favorable balance of moderate cost, good capacitance, and the distinct advantage of simple, room-temperature fabrication of binder-free electrodes on a wide range of substrates.

Electrodeposition precisely controls temperature, deposition potential/current, and other key parameters to effectively regulate MnO₂'s crystal structure and microstructure, thereby significantly enhancing electrochemical performance. Electrodeposition temperature is particularly critical in influencing MnO₂'s phase composition and structure. By optimizing electrodeposition temperature (25–80 °C), MnO₂ on multi-walled carbon nanotubes (MWNTs) evolves from single α-phase to α/γ coexisting phases.⁹ At 60 °C, a composite structure of primary α-MnO₂ nanosheets and secondary γ-MnO₂ nanoparticles forms (Fig. 5a). This dual-phase interfacial synergistic effect not only creates multidimensional ion transport channels but also increases active site exposure. Consequently, the electrode exhibits a high specific capacitance of 3.54 F cm⁻² at 1 mA cm⁻² (Fig. 5b). Even after 10 000 charge–discharge cycles at a scan rate of 50 mV s⁻¹, it achieves a remarkable capacity retention rate exceeding 98.9% (Fig. 5c), far outperforming traditional single-phase MnO₂ electrodes. Additionally, the carbon-based substrate's high strength and uniform anchoring of MnO₂ nanostructures enable stable charge storage under severe conditions, such as 1000 cycles of 135° bending (Fig. 5d), showing great potential in wearable electronic devices. Introducing cetyltrimethylammonium bromide (CTAB) as an additive to control electrode material surface morphology yields high-conductivity MnO₂ layered nanosheet arrays with high specific surface area and open porous structures.¹⁰⁰ Compared to unoptimized MnO₂, it demonstrates superior rate performance, retaining 92.1% capacity after 30 000 cycles.

Fig. 5 (a) Overview schematic diagram of the preparation process of yarn supercapacitors. (b) Constant current discharge curves of MnO₂/CNTs-25, MnO₂/CNTs-40, MnO₂/CNTs-60 and MnO₂/CNTs-80 gauze electrodes measured at 1 mA cm⁻². (c) The variation of cycling stability of MnO₂/CNTs-60 yarn electrode with the number of cycles at a scanning rate of 50 mV s⁻¹ and (d) CV curves and optical images at different bending angles.⁹ (e) Phase transition during the charging/discharging process of MnO₂. (f) Expansion of nanostructured MnO₂ particles during discharge and their local contraction during proton deintercalation. (g) Statistical chart of expansion rate caused by discharge of more than 200 grains. Reproduced with permission from ref. 101 Copyright 2019, John Wiley and Sons.

MnO₂'s pseudocapacitive behavior is intrinsically linked to its crystal phase transformation and ion intercalation/deintercalation processes.¹⁰¹In situ observations confirm that the (de)intercalation of charge carriers (e.g., H⁺, Na⁺) during cycling triggers the phase transformation between MnO₂ and Mn₃O₄ (Fig. 5e), accompanied by local lattice contraction and overall expansion (Fig. 5f). This volumetric change is statistically demonstrated to be inhomogeneous (Fig. 5g). α/γ heterostructured MnO₂, through interfacial stress regulation, effectively suppresses structural degradation (Jahn–Teller distortion). This allows the electrode to maintain a 59.6% capacity retention rate at a high current density of 15 mA cm⁻², showcasing excellent rate performance. Composite electrode design combined with interfacial optimization can further enhance supercapacitor performance. For example, constructing a core–shell structure of MnO₂ and conductive metal oxides (e.g., NiCo₂O₄) on a three-dimensional porous substrate utilizes the substrate's high conductivity and MnO₂'s high pseudocapacitance, increasing the composite electrode's specific capacitance by over 40% compared to single materials.¹⁰⁸ When preparing Al/Au/MnO₂ electrodes, pre-treating aluminum foil substrates with acid etching and Au coating, followed by electrodeposition, allows nano-porous skeletons to support inward/outward autonomous growth of MnO₂.¹⁰⁹ The ultrathin Au layer, as an intermediate layer, significantly improves the current collector's overall conductivity (0.35 Ω m) and adhesion to MnO₂, thereby enhancing the electrode's electrochemical performance.

MnO₂, as a supercapacitor electrode material, still suffers from low conductivity (10⁻⁵ to 10⁻³ S cm⁻¹) and insufficient cycle stability.³⁷ By constructing heterostructures, synergistically optimizing conductive substrates, and implementing defect engineering, the specific surface area can be increased while significantly enhancing electron and ion transport efficiency, effectively prolonging cycle life.¹¹¹ Future research could focus on developing low-temperature ionic liquid-based electrolytes and flexible interfacial electrodeposition processes, which will help advance the practical application of MnO₂-based electrodes in wearable energy storage devices. Beyond morphology and phase tuning, composite engineering has emerged as a powerful lever to boost the power and energy density of MnO₂-based supercapacitors. A representative example is the electrochemical deposition of MnO₂ onto laser-induced graphene (LIG) directly written on cellulose paper, yielding binder-free electrodes with 7.3 µWh cm⁻² energy density and robust mechanical resilience.¹¹² Such carbon-MnO₂ hybrids simultaneously provide 3-D conductive highways, mechanical reinforcement, and sustainable substrates, directly addressing the long-standing conductivity and scalability challenges.

5.3 Metal electrowinning anodes

In zinc electrowinning, Pb–Ag anodes are commonly used but suffer from low oxygen evolution activity and high-potential corrosion,¹¹³ leading to lead dissolution and harmful anode mud formation, which may cause voltage increases and short-circuit risks.^114,115 Pre-depositing MnO₂ films can effectively suppress corrosion and dissolution of Pb–Ag anodes while significantly enhancing oxygen evolution reaction (OER) catalytic activity and reducing oxygen overpotential.¹⁹ In actual zinc electrowinning processes, the system contains Mn²⁺ ions (2–7 g L⁻¹), where Pb–Ag anodes undergo three competing reactions: OER, manganese oxidation reaction (MOR), and lead corrosion reaction (LCR) (Fig. 6a).¹⁴ Electrodeposition technology can enhance the electrochemical performance and service life of Pb–Ag anodes through phase control (e.g., transforming α-MnO₂ to γ-MnO₂), heterostructure engineering, and metal doping (e.g., Fe–MnO₂, Co–MnO₂),^{14,16,19,20,116} as listed in Table 4.

Fig. 6 (a) Three different competitive reactions occur simultaneously on the surface of the Pb–Ag anode.¹⁴ (b) The bicrystalline phase Pb–Ag/MnO₂ anode obtained by pulsed deposition. (c) Polarization curves of Pb–Ag/MnO₂-3 (pulsed deposition β/γ phase), Pb–Ag/MnO₂-2 (stirred deposition β phase), Pb–Ag/MnO₂-1 (constant current deposition β phase), and Pb–Ag anode. Reproduced with permission from ref. 16 Copyright 2025, Elsevier. (d) Phase control of MnO₂ on lead-based anodes. Reproduced with permission from ref. 14 Copyright 2023, Elsevier. (e) The regulatory mechanism of impurity ions on electrodeposition. Reproduced with permission from ref. 116 Copyright 2025, Elsevier. (f) Generation of harmful sludge from different anode samples. Reproduced with permission from ref. 20 Copyright 2024, Elsevier.

Table 4 Electrodeposition of MnO₂ Film Electrodes for Metal Electrowinning Anodes

Deposition system [ref.]	Substrate	Temperature	Electrodeposition method	Crystal phase	Structural characteristics	Performance
40 g L^−1 Mn^2+–40 g L^−1 H2SO4 (ref. 14)	Pb–Ag	90 °C	CCD (5 mA cm^−2, 1 h)	γ-, ε-MnO2	Dense nanoneedles grains	OER overpotential: 0.656 V (50 mA cm^−2); Pb dissolution current density reduced by 86.8%, Pb sludge decreased by 92.5%
30 g L^−1 Mn^2+–120 g L^−1 (NH4)2SO4–30 mg L^−1 SeO2 (pH = 7)^40	Pb–Ag	—	CCD (35 mA cm^−2, 30 min)	α-, β-, ε-MnO2	Low crystallinity, defective rough porous	PD lowers anode potential by 60 mV, increases current efficiency by 3.11–3.77%, reduces energy consumption by 5.30–8.17%
			PD (35 mA cm^−2, 50% DC@500 Hz)	α-MnO2	Uniform dense structure with low roughness
38–44 g L^−1 Mn^2+–38–44 g L^−1 H2SO4–0.15 g L^−1 Fe^2+ (ref. 19)	Pb–Ag	90–96 °C	CCD (5 mA cm^−2, 2 h)	γ-MnO2	Tunnel-like cross	OER overpotential: 1.385 V (50 mA cm^−2, 23 mV lower than undoped), Tafel slope: 10^1 to 10^7 mV dec^−1
MnSO4·H2O–H2SO4 (ref. 20)	Pb–Ag	90 °C	CCD (5 mA cm^−2, 1 h)	γ-MnO2	Urchin-like (undoped); litchi-like (co-doped)	OER over potential: 595.2 mV (50 mA cm^−2), Tafel slope: 116.4 mV dec^−1
40 g L^−1 H2SO4–40 g L^−1 Mn^2+ (ref. 18)	Pb	—	CCD (5 mA cm^−2)	γ-MnO2	Sphere@γ-MnO2 nanoneedles	OER overpotential 620–641 mV, Tafel slope: 116.4 mV dec^−1
40 g L^−1 Mn^2+–40 g L^−1 H2SO4 (ref. 13)	Pb–Ag	94–95 °C	CCD (5 mA cm^−2, 2 h)	γ-MnO2	Loose coral-like (microporous/tunneled)	Anode sludge reduced by 95%, Zn product Pb content decreased by 81%, cell voltage stabilized at 3.0 V
40 g L^−1 Mn^2+–40 g L^−1 H2SO4 (ref. 16)	Pb–Ag	—	PD (5 mA cm^−2, 1 h, 40% DC@1 Hz)	β-, γ-MnO2	Hierarchical nanorod architecture	OER potential: 424 mV (10 mA cm^−2), 685 mV (50 mA cm^−2); Tafel slope: 217.1 mV dec^−1
40 g L^−1 Mn^2+–40 g L^−1 H2SO4 (ref. 15)	Pb–Ag	—	CCD (5 mA cm^−2,2 h)	γ-, ε-MnO2	Nanoneedles	Zn product Pb content reduced by 86%, total anode sludge decreased by 94%, cell voltage stabilized at 2.8–2.9 V
38–44 g L^−1 Mn^2+–38–44 g L^−1 H2SO4 (ref. 17)	Pb–Ag (0.8%)	90–96 °C	CCD (5 mA cm^−2, 2 h)	γ-MnO2	Burr spherical structure	24 day cycle: anode sludge reduced by 91%, Zn product Pb content ≤ 0.0008% (89% reduction), cell voltage stabilized at 3.3 V

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Electrodeposition, by regulating MnO₂'s crystal phase composition, can significantly enhance the catalytic activity and structural stability of Pb–Ag anodes. For example, pulse deposition yielded MnO₂ films featuring a three-dimensional spherical hydrophilic structure composed of composite β/γ phases (Fig. 6b),¹⁶ which not only enhances the coating's hydrophilicity but also increases the content of active Mn³⁺ and O_abs, thereby improving the inherent OER activity of MnO₂ anodes. Compared to single-phase β-MnO₂-coated anodes and pure Pb–Ag substrates, this anode exhibits a more positive corrosion potential and a lower corrosion current density, indicating significantly improved corrosion resistance (Fig. 6c). Depositing γ/ε-MnO₂ composite phase films on Pb–Ag anodes forms a dense protective layer of nanoneedle-like particle structures, suppressing lead substrate chemical corrosion.¹⁸ The synergistic effect of the composite phase enhances interfacial charge transfer capability, stabilizing cell voltage at 3.26 V and saving 4.7% energy. By altering Mn²⁺ ion oxidation pathways,¹⁴ MnO₂ films (MnO₂-PC) are prepared on lead-based anodes (Pb–Ag/MnO₂-PC) via phase-controlled synthesis (Fig. 6d). This method forms γ-MnO₂ and ε-MnO₂ with specific crystal plane exposure and significantly increased oxygen vacancy concentration under low voltage and high Mn²⁺ concentration conditions. This electrode material exhibits excellent OER performance. During long-term electrolysis, the generation of harmful lead-containing MnO₂ sludge is reduced by 92.5%, and the energy-saving effect of Pb–Ag/MnO₂-PC anodes remains stable.

Metal ion doping can effectively regulate MnO₂'s electronic structure and surface-active site density. Research by Xu reveals the regulatory mechanism of impurity ions on electrodeposition.^19,20,116 Doping γ-MnO₂ with impurity ions (Fe²⁺, Co²⁺, Ni²⁺, and Cu²⁺) introduces more Mn³⁺ sites with oxygen vacancies to promote oxygen release (Fig. 6f).¹¹⁶ Due to the adsorption of impurity ions from the electrolyte onto the anode surface, their electro-oxidation preferentially consumes the newly deposited MnO₂ sludge rather than the pre-coated γ-MnO₂ film. Subsequently, the dissolved high-valence Fe³⁺, Co³⁺, Ni³⁺, and Cu²⁺ ions are reduced at the cathode surface and participate again in the MnO₂ deposition-dissolution chemistry in their lower valence states. This cyclic process effectively suppresses anode corrosion. For instance, Fe²⁺ doping induces a 1.6-fold increase in oxygen vacancy concentration in γ-MnO₂,¹⁹ enhancing interfacial charge transfer efficiency and reducing harmful anode mud generation by 69.1–90%. Co³⁺ doping forms lychee-like hierarchical nanostructured balls, exposing more (101) active crystal planes.²⁰ This significantly improves the OER activity of Pb/Co–MnO₂ anodes, reducing lead dissolution by 72.3% (Fig. 6f) and minimizing lead contamination in zinc products.

In industrial zinc electrowinning conditions, MnO₂ films encounter significant challenges regarding stability and interfacial bonding strength when exposed to high oxidation potentials and strongly acidic environments. Studies show that MnO₂ film layers may undergo phase transformation (γ → α), leading to increased porosity and local corrosion perforation. Future research could focus on developing gradient film structures (e.g., outer layer of high catalytic activity γ-MnO₂ and inner layer of dense α-MnO₂ barrier layer), combining in situ doping and interfacial alloying treatments to enhance anode catalytic activity and corrosion resistance.

5.4 Electrocatalysis

MnO₂, with its multiple valence states (Mn²⁺/Mn³⁺/Mn⁴⁺) and tunable crystal structures, has been widely employed as an electrodeposited catalyst in various electrochemical reactions, such as OER, hydrogen evolution reaction (HER), and oxygen reduction reaction (ORR),^46,117–124 as listed in Table 5. Electrodeposition technology, by regulating MnO₂'s microstructure, crystal phase composition, and composite structures, can significantly enhance its catalytic activity and stability. Compared to chemically synthesized powder materials, electrodeposition-prepared electrodes eliminate the need for binders (e.g., Nafion/PTFE), reducing interfacial impedance and ion transport hindrance.⁴⁶

Table 5 Electrodeposition of MnO₂ Film Electrodes for Electrocatalysis

Deposition system [ref.]	Substrate	Temperature	Electrodeposition method	Crystal phase	Structural characteristics	Performance
0.1 M MnCl2–Na2SO4 (ref. 118)	Ni	27 °C	CVD (0–0.6 V vs. Hg/HgO, 75 cycles)	Amorphous	Interconnected nanowires	OER overpotential: 260 mV (10 mA cm^−2), Tafel slope: 47 mV dec^−1; 1000 cycles with negligible decay, 10 h stable operation
0.1 M Mn(CH3COO)2·4H2O–0.1 M Ni(CH3COO)2·4H2O–1.0 M Na2SO4 (ref. 119)	AISI 316L	—	CVD (0–1.4 V vs. SCE, 10 cycles)	Poor crystallinity	Nanosheet structure	OER overpotential: 379 mV (10 mA cm^−2), Tafel slope: 47.84 mV dec^−1; stable for 28 800 s, 100 cycles without decay
1 M Mn(CH3COO)2 (ref. 120)	Ni	—	CPD (1.4 V vs. SCE, 50 s)	Amorphous/poor crystallinity	Uniform protruding crystals	OER overpotential: 270 mV (10 mA cm^−2), Tafel slope: 118.62 mV dec^−1; stable for 42 h
1.69 g MnSO4·H2O–1.42 g Na2SO4 (ref. 121)	Carbon cloth	—	CCD (12 mA cm^−2, 15 min)	ε-MnO2	Nanoflower structure	OER overpotential: 410 mV (50 mA cm^−2), Tafel slope: 165 mV dec^−1; 12 h stability at 10 mA cm^−2
1 mM MnSO4–50 mM TBACl^122	Fluorine-doped Tin oxide	—	CPD (−1.044 V vs. Ag/AgCl)	TBA^+-intercalated layered MnO2	Buserite-type, wrinkled & folded nanosheets	Mass activity: 63.5 A gCo^−1 (overpotential(η): 0.4 V), 100 h stability (overpotential increase: 0.073 V), 100 cycles without decay
MnSO4·H2O–Co(NO3)3 (ref. 125)	Nickel foam	—	CPD (1.8 V, 15 min)	Hexagonal MnO2	Wrinkled nanoflower-like nanosheets	HER overpotential: 102 mV (10 mA cm^−2), Tafel slope: 102.24 mV dec^−1; OER overpotential: 225 mV (10 mA cm^−2), Tafel slope: 44.81 mV dec^−1; 3000 cycles with minimal LSV decay
0.16 M MnSO4·H2O, 0.16 M Na2SO4 (ref. 126)	Stainless steel	RT	CVD (0–1.2 V, 500 cycles)	α-MnO2	3D porous nanosheets	HER overpotential: 439.7 mV (10 mA cm^−2), Tafel slope: 161.7 mV dec^−1; OER overpotential: 381.2 mV (10 mA cm^−2), Tafel slope: 59.4 mV dec^−1; 12 h stability
0.18 M MnSO4·H2O–0.90 M (NH4)2SO4–90 mg L^−1 SeO2 (ref. 127)	Cu	18 °C	CCD (500 mA cm^−2, 80 s)	α-MnO2	3D porous nanosheets	ORR: Half-wave potential 0.86 V, current density 2.32 mA cm^−2 (0.88 V), Tafel slope: 49.65 mV dec^−1; 30 h stability (>97% current retention)
10 mM KMnO4–0.01 M H2SO4 (ref. 128)	SnO2@NC	25 °C	CVD (0.3–0.5 V vs. Ag/AgCl, 5 cycles)	Bimessite type MnO2	Uniform nanoparticles	ORR: half-wave potential −0.878 V, limiting current density −4.14 mA cm^−2; 10 000 s stability (80% current retention)
0.15 M Mn(CH3COO)2–0.15 M Na2SO4 (ref. 129)	Ti	—	CPD (2–8 V, 15 min)	Bimessite type MnO2	Birnessite-type layered porous nanoflowers	ORR onset potential: −0.12 V, half-wave potential: −0.21 V; diffusion-limited current density increases with rotation rate

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The catalytic OER activity of MnO₂ depends on its crystal structure, with the activity order being α-MnO₂ > γ-MnO₂ > β-MnO₂ > δ-MnO₂.¹²³ MnO₂'s catalytic activity is often limited by its low inherent electrical conductivity and limited active sites. Unordered δ-MnO₂, rich in Mn³⁺ active sites and oxygen vacancies, exhibits higher activity than ordered δ-MnO₂ films prepared under constant potential.^130,131 Electrochemical activation of constant potential electrodeposited MnO₂ by CV cycling or multipotential treatments significantly enhances its acidic oxygen evolution activity (Fig. 7a). It has been found that applying anodic-cathodic pulses can enhance the OER activity of MnO_x films prepared under constant anodic potential. Electrochemical activation induces in situ phase transformation of native δ-MnO₂ interfaces (δ-MnO₂ → α-Mn₃O₄), ultimately transforming into disordered δ-MnO₂ with higher OER activity.¹³⁰

Fig. 7 (a) Schematic diagram of the electrodepositation-activated MnO₂ film layer. Reproduced with permission from ref. 130 Copyright 2015, American Chemical Society (b) Comparison of alkaline oxygen evolution activities between Co/MnO₂ and K/MnO₂ electrodes. Reproduced with permission from ref. 122 Copyright 2018, American Chemical Society (c) PL spectra of Mn₃O₄, Na–MnO₂ and Na–MnO_2−x thin films. Reproduced with permission from ref. 126 Copyright 2024, Elsevier. (d) Schematic illustration of the multipotential deposition process for activating MnO_x catalyst films. (e) ORR EIS spectra of P-NS-MnO₂@Mn and commercial Pt/C catalyst. (f) ORR electrochemical polarization curves for different catalysts. (g) ORR I–t timing current curves for P-NS-MnO₂@Mn and commercial Pt/C catalyst. Reproduced with permission from ref. 127 Copyright 2023, American Chemical Society.

Composite modification and structural regulation can significantly improve MnO₂'s catalytic hydrogen evolution activity. For example, flower-like MnO₂/Co₃(PO₄)₂ composite materials prepared via constant potential deposition on nickel foam (NF) substrates¹²⁵ exhibit superior catalytic hydrogen evolution performance compared to single MnO₂ electrodes. Co³⁺ doping optimizes MnO₂'s electronic structure, effectively reducing hydrogen adsorption free energy and accelerating reaction kinetics. Introducing specific metal cations (e.g., Co²⁺, K⁺) during electrodeposition allows cations to intercalate into MnO₂'s crystal lattice.¹²² thereby regulating its electrocatalytic performance. Compared to K⁺, Co²⁺ introduction significantly enhances catalytic oxygen evolution activity (Fig. 7b). Na⁺ intercalation during CPD introduces abundant oxygen vacancies into MnO₂ (Na–MnO_2−x) films and their MXene composites,¹²⁶ and the resulting vacancy boost is verified by the intensified 466–489 nm emission in the PL spectrum (Fig. 7c). In alkaline media, they achieve synergistic catalysis for HER and OER. By designing Janus structures, NiFe layered double hydroxide (LDH) and MnO₂ are constructed on both sides of NF,¹³² enabling efficient synergistic catalysis of OER/ORR in zinc–air batteries.

ORR is the reverse reaction of OER, and the intermediates of the two reactions are the same. Birnessite-type MnO₂ nanosheets prepared by constant potential deposition in acidic medium exhibit excellent ORR performance.¹²⁹ The 3D porous Mn-based catalyst (P-NS-MnO₂@Mn) fabricated by rotating ring-disk electrode electro-deposition shows performance close to commercial Pt/C (Fig. 7d).¹²⁷ The porous structure constructed by electro-deposition efficiently utilizes active sites and optimizes the electron transfer path, effectively increasing the contact area between reactants and the catalyst and lowering the ORR charge transfer resistance (Fig. 7e). Its multi-level channels and ultrathin nanosheets synergistically enhance the local electric field intensity, promoting electron transfer and intermediate adsorption, achieving high activity (half-wave potential E_1/2 = 0.86 V) in ORR, which is superior to commercial Pt/C catalyst (0.83 V) (Fig. 7f). At an applied potential of 0.85 V vs. RHE, the current density reaches 2.32 mA cm⁻² (twice that of Pt/C), and the activity decay is only 2.9% after a 10 000 second of chronoamperometry test (Fig. 7g).

Placing MnO₂ within the broader electrocatalytic landscape reveals its unique application potential. Nickel–iron layered double hydroxide (NiFe-LDH) is established as highly active OER catalysts in alkaline media, often achieving lower overpotentials than MnO₂.¹³³ However, MnO₂ demonstrates comparative advantages in stability across a wider pH range and superior ORR activity, making it a promising bifunctional catalyst for metal–air batteries.¹³⁴ Recent comparative studies highlight that through rational doping and nanostructure engineering, MnO₂-based electrodes can achieve a compelling balance of activity, durability, and cost, positioning them as attractive candidates for scalable sustainable energy applications.¹³⁵ Currently, long-term stability and large-scale preparation processes remain key bottlenecks for practical applications. Future research should focus on reinforcing interfacial stability, precisely controlling active sites, and developing low-cost deposition technologies to promote the large-scale application of MnO₂-based catalysts in hydrogen energy conversion and energy storage systems.

6. Conclusions and outlook

MnO₂, with its unique physicochemical properties and diverse crystal structures, holds broad application prospects in electrochemical energy storage and catalysis. Electrodeposition is a highly attractive fabrication route due to its intrinsic merits, including simplicity, ambient operating conditions, and exceptional capability for tuning material properties through precise control of electrochemical parameters. This synthesis strategy, leveraging the rich electrochemistry and structural diversity of MnO₂, has driven significant progress in developing high-performance electrodes for energy storage and catalytic systems. Furthermore, sophisticated characterization protocols, as discussed in this review, have been instrumental in decoding the intricate relationships between deposition conditions, material characteristics (e.g., morphology, crystal structure, oxidation state), and electrochemical performance.

Notwithstanding considerable academic achievements, the widespread industrial adoption of electrodeposited MnO₂ electrodes requires critical assessment of their scalability, economic viability, and engineering integration. Therefore, future research and development should pivot to address the following key challenges and opportunities.

6.1 Advancing towards scalable manufacturing and cost-effectiveness

The transition from laboratory-scale, batch-type electrodeposition to continuous, high-throughput manufacturing processes is a paramount challenge for industrial scale-up. Techniques such as roll-to-roll (R2R) electrodeposition and flow-cell systems present compelling pathways for the uniform coating of large-area, flexible substrates, which is a prerequisite for applications like wearable energy storage and industrial electrocatalysis. A thorough techno-economic analysis (TEA) is urgently required to evaluate the true cost competitiveness of this technology. This analysis must encompass the entire ecosystem, including the price volatility of manganese precursors, the energy consumption of the deposition process, the efficiency of electrolyte utilization, and the costs of waste stream management. Innovations aimed at increasing deposition rates, employing low-cost electrolyte formulations, and implementing electrolyte recycling will be crucial in driving down the overall manufacturing expense.

6.2 Enhancing performance through material and interface engineering

The intrinsic limitations of MnO₂, namely its modest electrical conductivity and propensity for structural evolution during operation, directly impact device performance across all applications. The choice of current collector or substrate is not merely a passive support but an active determinant of the film's adhesion, conductivity, and mechanical integrity. Exploring low-cost, lightweight, and flexible substrates such as carbon-coated polymers, metallized textiles, and porous 3D carbon architectures (e.g., foams, felts) is essential. A key focus should be on optimizing the interface between the substrate and the deposited MnO₂ layer. In battery applications, where volume changes during ion insertion/extraction can cause mechanical degradation, strategies such as the construction of composites with conductive polymers or carbon networks and the implementation of crystal phase engineering are vital for enhancing structural integrity and electronic wiring. Strategies such as chemical functionalization or the application of conductive seed layers can significantly enhance adhesion and minimize contact resistance, thereby improving the power output and cycle life of the final device. The compatibility of these substrates with high-speed, scalable deposition processes must be a central consideration.

6.3 Enhancing performance durability under realistic conditions

For long-term industrial deployment—for instance, in metal electrowinning anodes operating in strongly acidic and oxidizing environments—the structural and electrochemical stability of MnO₂ films must be rigorously evaluated under harsh operational conditions. This includes testing against mechanical stress, extreme temperatures, and prolonged operation at high current densities. A particular challenge lies in mitigating the performance degradation associated with Mn³⁺ disproportionation and phase transformations during cycling. Research should focus on interface engineering through methods like in situ doping, the creation of compositionally graded films, and interfacial alloying to bolster corrosion resistance and interfacial bonding strength, thereby extending service life. The synergy between advanced computational modeling and high-throughput experimentation can accelerate the discovery of more durable MnO₂-based material systems.

6.4 Advancing electrocatalysis and supercapacitors via structural precision

The pursuit of high performance in electrocatalysis and supercapacitors demands precise control over the active sites and ion transport pathways. In Electrocatalysis, overcoming the bottlenecks of long-term stability and the scalable fabrication of highly active catalysts requires reinforcement of interfacial stability and the precise creation of defective or doped structures to optimize the activity for reactions like the oxygen evolution reaction. Scaling up these optimized structures using the low-cost deposition technologies is a key challenge.

For Supercapacitors, while electrodeposition can produce high-performance films, the focus must shift to optimizing processes for mass production without compromising the high specific capacitance. This involves engineering porous nanostructures that maximize the accessible surface area while ensuring mechanical robustness.

6.5 Accelerating research progress with in-situ insights and data-driven design

To accelerate progress, a paradigm shift in research methodology is essential. The integration of in situ/operando characterization techniques (e.g., XRD, Raman, XAS) is critical for uncovering the dynamic structural evolution of MnO₂ during deposition and electrochemical operation. This approach moves beyond post-mortem analysis, enabling the understanding of degradation mechanisms in real-time. Furthermore, the power of machine learning should be leveraged to navigate the vast multi-dimensional parameter space of electrodeposition, predicting optimal combinations of parameters for desired properties and accelerating the discovery of new composite materials. This will help establish a systematic, closed-loop research framework from material design to process optimization and device integration.

In conclusion, while the scientific foundation of MnO₂ electrodeposition is established, its journey from a laboratory curiosity to an industrial mainstay is just beginning. By strategically focusing on the pillars of scalable production, cost reduction, substrate innovation, and durability enhancement, the research community can unlock the full potential of electrodeposited MnO₂ films, paving the way for their impactful contribution to sustainable energy and catalytic technologies. This endeavor will not only advance the frontiers of energy storage and conversion but also contribute pivotal technological solutions towards achieving a sustainable and carbon-neutral future.

Author contributions

Jiajun Lin: writing – original draft, methodology. Ze Zhang: methodology, writing – review & editing. Mengwei Guo: methodology, writing – review & editing. Hangrui Zhang: methodology, writing – review & editing. Mingyuan Gao: methodology, writing – review & editing. Rongrong Deng: methodology, writing – review & editing. Cunying Xu: methodology, writing – review & editing. Qibo Zhang: project administration, funding acquisition, supervision, resources, conceptualization, writing – review & editing.

Conflicts of interest

The authors declare no conflicts of interest.

Data availability

This review article is based on an analysis of previously published research, and no new original data were generated during the current work. All relevant data referenced in this review can be accessed through the original studies cited in the text.

Acknowledgements

The authors gratefully acknowledge the financial support of the Yunnan Major Science and Technology Project (202302AG050008), Joint Special Project for Applied Basic Research of China Copper Industry Co., Ltd. (202501BP070002), Yunnan Fundamental Research Project (202401CF070150, 202301BE070001-063), and Yunnan Ten Thousand Talents Plan Young & Elite Talents Project (YNWR-QNBJ-2018-346).

References

1. B. Liu, Y. Sun, L. Liu, S. Xu and X. Yan, Adv. Funct. Mater., 2018, 28, 1704973.
2. X. Zeng, J. Hao, Z. Wang, J. Mao and Z. Guo, Energy Storage Mater., 2019, 20, 410–437.
3. J. Miao, Y. Du, R. Li, Z. Zhang, N. Zhao, L. Dai, L. Wang and Z. He, Int. J. Miner., Metall. Mater., 2024, 31, 33–47.
4. A. Wu, T. Wang, L. Zhang, C. Chen, Q. Li, X. Qu and Y. Liu, Int. J. Miner., Metall. Mater., 2024, 31, 1752–1765.
5. H. Fujimoto, M. Okada, M. Yoshikawa, K. Shimoda, S. Fujinami, M. Morita, Z. Ogumi and T. Abe, J. Electrochem. Soc., 2024, 171, 020534.
6. Q. Duan, Y. Zheng, Y. Zhou, S. Dong, C. Ku, P. H. L. Sit and D. Y. W. Yu, Small, 2024, 20, 2404368.
7. D. Wang, L. Wang, G. Liang, H. Li, Z. Liu, Z. Tang, J. Liang and C. Zhi, ACS Nano, 2019, 13, 10643–10652.
8. B. Chacko and M. Wuppulluri, Mater. Renew. Sustain. Energy, 2025, 14, 10.
9. J.-H. Jeong, J. W. Park, D. W. Lee, R. H. Baughman and S. J. Kim, Sci. Rep., 2019, 9, 11271.
10. F. N. I. Sari, P.-R. So and J.-M. Ting, J. Am. Ceram. Soc., 2017, 100, 1642–1652.
11. X. Tian, Q. Wang, J. Yang, R. Luo, X. Zhang, S. Xue, Y. Shi, W. Hu and Z. Liu, J. Power Sources, 2025, 632, 236377.
12. A. Xia, W. Yu, J. Yi, G. Tan, H. Ren and C. Liu, J. Electroanal. Chem., 2019, 839, 25–31.
13. W. Ye, F. Xu, L. Jiang, N. Duan, J. Li, Z. Ma, F. Zhang and L. Chen, J. Hazard. Mater., 2021, 408, 124931.
14. S. Zhuang, N. Duan, L. Jiang, F. Zhang, J. Hu, Z. Chen and F. Xu, J. Clean Prod., 2023, 404, 13691.
15. W. Ye, F. Xu, L. Jiang, N. Duan, J. Li, F. Zhang, G. Zhang and L. Chen, J. Clean Prod., 2021, 284, 124767.
16. C. Li, H. Jiang, Z. Yan, H. Jing, Z. Hu, C. Zhao, B. Gao, Q. Zhang, X. Yuan and Y. Yin, Appl. Surf. Sci., 2025, 681, 161615.
17. F. Zhang, J. Zuo, W. Jin, F. Xu, L. Jiang, D. Xi, Y. Wen, J. Li, Z. Yu, Z. Li, R. Xu, G. Zhang, C. Zhou and N. Duan, Chemosphere, 2022, 287, 132457.
18. S. Zhuang, N. Duan and F. Xu, J. Hazard. Mater., 2024, 470, 134119.
19. F. Zhang, N. Duan, J. Zuo, L. Jiang, J. Li, S. Zhuang, Y. Liu and F. Xu, Chem. Eng. J., 2023, 476, 146475.
20. S. Zhuang, F. Xu and N. Duan, J. Hazard. Mater., 2024, 480, 136259.
21. M. S. Hossain and M. M. Islam, in Renewable Polymers and Polymer-Metal Oxide Composites, ed. S. Haider and A. Haider, Elsevier, 2022, pp. 45–77.
22. S. Rojas, Nat. Catal., 2024, 7, 227–228.
23. Y. Chen, S. Yang, T. Wang, S. Li, X. Liu, W. Zhang and R. Cao, ChemSusChem, 2025, 18, e202401553.
24. A. K. Worku, D. W. Ayele, N. G. Habtu and M. D. Ambaw, Heliyon, 2022, 8, e10960.
25. M. Rittiruam, N. Aumnongpho, T. Saelee, P. Khajondetchairit, S. Kheawhom, B. Alling, S. Praserthdam, A. Ektarawong and P. Praserthdam, J. Energy Storage, 2024, 78, 110005.
26. M. F. Fink, J. Eckhardt, P. Khadke, T. Gerdes and C. Roth, ChemElectroChem, 2020, 7, 4822–4836.
27. N. Wang, W. Li, J. Liang, Y. Huang, Q. Cai, M. Hu, Y. Chen and Z. Shi, J. Alloys Compd., 2020, 846, 156396.
28. J. Joseph, J. Nerkar, C. Tang, A. Du, A. P. O'Mullane and K. K. Ostrikov, ChemSusChem, 2019, 12, 3753–3760.
29. W. C. Shao, M. W. Guo, E. Z. Wang and Q. B. Zhang, Fine Chem., 2023, 40, 2321–2335.
30. S. Han, S. Park, S.-H. Yi, W. B. Im and S.-E. Chun, J. Alloys Compd., 2020, 831, 154838.
31. S. Hong, S. Jin, Y. Deng, R. Garcia-Mendez, K.-i. Kim, N. Utomo and L. A. Archer, ACS Energy Lett., 2023, 8, 1744–1751.
32. F. Hashemzadeh, M. Mehdi Kashani Motlagh and A. Maghsoudipour, J. Sol-Gel Sci. Technol., 2009, 51, 169–174.
33. A. Sarkar, A. Kumar Satpati, V. Kumar and S. Kumar, Electrochim. Acta, 2015, 167, 126–131.
34. S. Guan, W. Li, J. Ma, Y. Lei, Y. Zhu, Q. Huang and X. Dou, J. Ind. Eng. Chem., 2018, 66, 126–140.
35. A. Huang, J. Chen, W. Zhou, A. Wang, M. Chen, Q. Tian and J. Xu, J. Electroanal. Chem., 2020, 873, 114392.
36. M. Minakshi, R. Aughterson, P. Sharma, A. P. Sunda, K. Ariga and L. K. Shrestha, ChemNanoMat, 2025, 11, e202500270.
37. A. M. Abdelrahim, M. G. Abd El-Moghny, M. E. El-Shakre and M. S. El-Deab, J. Energy Storage, 2023, 57, 106218.
38. R. Khayyam Nekouei, S. Maroufi, S. S. Mofarah and V. Sahajwalla, J. Energy Storage, 2024, 80, 110159.
39. D. Ma, X. Yin, X. Li, X. Qin and M. Qi, Polymers, 2024, 16, 1856.
40. Z. Xie, Z. Liu, J. Chang, Y. Qin and C. Tao, ACS Sustain. Chem. Eng., 2020, 8, 15044–15054.
41. M. Y. Timmermans, N. Labyedh, F. Mattelaer, S. P. Zankowski, S. Deheryan, C. Detavernier and P. M. Vereecken, J. Electrochem. Soc., 2017, 164, D954–D963.
42. M. F. Dupont, A. J. Gibson, A. Elbourne, M. Forghani, A. D. Cross and S. W. Donne, J. Electrochem. Soc., 2020, 167, 040520.
43. H. Adelkhani and M. Ghaemi, Solid State Ionics, 2008, 179, 2278–2283.
44. M. F. Dupont and S. W. Donne, Electrochim. Acta, 2014, 120, 219–225.
45. I. Katsounaros, Curr. Opin. Electrochem., 2021, 28, 100721.
46. M. Nakayama and W. Yoshida, ChemSusChem, 2025, 18, e202401907.
47. M. Nakayama, M. Nishiyama, M. Shamoto, T. Tanimoto, K. Tomono and R. Inoue, J. Electrochem. Soc., 2012, 159, A1176–A1182.
48. A. Singh, D. Kumar, A. Thakur, N. Gupta, V. Shinde, B. S. Saini and R. Kaur, Ionics, 2021, 27, 2193–2202.
49. G. Zangari, Coatings, 2015, 5, 195–218.
50. A. D. Cross, A. Morel, T. F. Hollenkamp and S. W. Donne, J. Electrochem. Soc., 2011, 158, A1160.
51. K. Jeyasubramanian, T. S. Gokul Raja, S. Purushothaman, M. V. Kumar and I. Sushmitha, Electrochim. Acta, 2017, 227, 401–409.
52. W. Y. Ko, R. S. Sitindaon, A. L. Lubis, Y. R. Yang, H. Y. Wang, S. T. Lin and K. J. Lin, J. Energy Storage, 2022, 54, 105329.
53. A. Joseph, C. K. Sangeetha, D. S. Abraham, M. Bhagiyalakshmi and P. M. Aneesh, MRS Commun., 2024, 14, 1480–1487.
54. R. Sasaki, R. Okiguchi, A. Senuma and K. Tomono, Dalton Trans., 2025, 54, 10961–10972.
55. X. Zhang, J. Cai, W. Liu, B. Huang and S. Lin, J. Appl. Electrochem., 2020, 50, 713–722.
56. S. Zhang, Z. Wang, S. Yang, D. Hao, S. Yu and Q. Wu, Int. J. Biol. Macromol., 2024, 259, 129223.
57. M. Kazazi, Ceram. Int., 2018, 44, 10863–10870.
58. Z. Zhang, Z. Liu, Z. Xu, Z. Yao and W. Wang, Int. J. Hydrogen Energy, 2025, 130, 280–289.
59. A. L. Yeang, Z. Li, S. Grunsfeld, G. R. McAndrews, Y. Cai, C. J. Barile and M. D. McGehee, Cell Rep. Phys. Sci., 2023, 4, 101660.
60. Y. Yang, Y. Li and M. Pritzker, Electrochim. Acta, 2016, 213, 225–235.
61. A. Singh, D. Kumar, A. Thakur, B. S. Saini and R. Kaur, Bull. Mater. Sci., 2020, 43, 165.
62. A. Gasco Owens, D. Veys-Renaux and E. Rocca, Electrochim. Acta, 2022, 435, 141361.
63. W. Zhu, Y. Li, Y. Gao, C. Wang, J. Zhang, H. Bai and T. Huang, Electrochim. Acta, 2020, 349, 136415.
64. J. Duay, S. A. Sherrill, Z. Gui, E. Gillette and S. B. Lee, ACS Nano, 2013, 7, 1200–1214.
65. Y. Zhou, Y. He, H. Ding, L. Chen, W. Lu, X. Yu and Y. Zhao, J. Alloys Compd., 2024, 981, 173780.
66. M. J. Raei, M. Trifkovic, E. P. L. Roberts and G. Natale, ACS Appl. Energy Mater., 2025, 8, 10822–10832.
67. T. Rodenas and G. Prieto, Catal. Today, 2022, 405–406, 2–13.
68. C. R. Thurber, Microsc. Microanal., 2018, 24, 1068–1069.
69. D. C. Joy and D. Braski, Microsc. Microanal., 2000, 6, 750–751.
70. X. Li, L. Guo, Y. Li and L. Dong, ACS Sustain. Chem. Eng., 2025, 13, 15383–15390.
71. L. Zhou, M. Cai, T. Tong and H. Wang, Sensors, 2017, 17, 938.
72. B. Hadžić, B. Vasić, B. Matović, I. Kuryliszyn-Kudelska, W. Dobrowolski, M. Romčević and N. Romčević, J. Raman Spectrosc., 2018, 49, 817–821.
73. P. Wu, S. Dai, G. Chen, S. Zhao, Z. Xu, M. Fu, P. Chen, Q. Chen, X. Jin, Y. Qiu, S. Yang and D. Ye, Appl. Catal., B, 2020, 268, 118418.
74. C. Sun, E. Müller, M. Meffert and D. Gerthsen, Microsc. Microanal., 2018, 24, 99–106.
75. R. K. Mishra, C. S. Prajapati, R. R. Shahi, A. K. Kushwaha and P. P. Sahay, Ceram. Int., 2018, 44, 5710–5718.
76. O. Werzer, S. Kowarik, F. Gasser, Z. Jiang, J. Strzalka, C. Nicklin and R. Resel, Nat. Rev. Methods Primers, 2024, 4, 15.
77. M. D. L. Gonçalves, P. B. N. Assis, A. N. da Silva, G. M. Bertoldo, R. d. C. F. Bezerra, A. J. R. Castro, A. C. Oliveira, R. Lang and G. D. Saraiva, Spectrochim. Acta, Part A, 2022, 280, 121526.
78. Y. Xin, H. Cao, C. Liu, J. Chen, P. Liu, Y. Lu and Z. Ling, J. Raman Spectrosc., 2021, 53, 340–355.
79. X. L. Yu, J. Chen, S. X. Wu, Y. J. Liu and S. W. Li, J. Raman Spectrosc., 2008, 39, 1440–1443.
80. Y. H. Zhou, X. X. Lei, J. Y. Zhou, D. L. Yan, B. Deng, Y. D. Liu and W. L. Xu, Catal. Surv. Asia, 2023, 27, 319–331.
81. H. Gu, J. Lan, H. Hu, F. Jia, Z. Ai, L. Zhang and X. Liu, J. Hazard. Mater., 2023, 460, 132499.
82. W. Yang, Y. Zhu, F. You, L. Yan, Y. Ma, C. Lu, P. Gao, Q. Hao and W. Li, Appl. Catal., B, 2018, 233, 184–193.
83. G. Greczynski and L. Hultman, Appl. Surf. Sci., 2022, 606, 154855.
84. E. S. Ilton, J. E. Post, P. J. Heaney, F. T. Ling and S. N. Kerisit, Appl. Surf. Sci., 2016, 366, 475–485.
85. H. Lv, Y. Song, Z. Qin, M. Zhang, D. Yang, Q. Pan, Z. Wang, X. Mu, J. Meng, X. Sun and X.-X. Liu, Chem. Eng. J., 2022, 430, 133064.
86. S. Kong, A. Li, J. Long, K. Adachi, D. Hashizume, Q. Jiang, K. Fushimi, H. Ooka, J. Xiao and R. Nakamura, Nat. Catal., 2024, 7, 252–261.
87. D. Wu, L. M. Housel, S. T. King, Z. R. Mansley, N. Sadique, Y. Zhu, L. Ma, S. N. Ehrlich, H. Zhong, E. S. Takeuchi, A. C. Marschilok, D. C. Bock, L. Wang and K. J. Takeuchi, J. Am. Chem. Soc., 2022, 144, 23405–23420.
88. Q. Dai, L. Li, B. Hu, Y. Jia, T. Tu, T. K. A. Hoang, M. Zhang and L. Song, Mater. Today Commun., 2022, 31, 103578.
89. Y. Song, J. Li, R. Qiao, X. Dai, W. Jing, J. Song, Y. Chen, S. Guo, J. Sun, Q. Tan and Y. Liu, Chem. Eng. J., 2022, 431, 133387.
90. X. Dai, M. Zhang, T. Li, X. Cui, Y. Shi, X. Zhu, P. Wangyang, D. Yang and J. Li, Vacuum, 2022, 195, 110692.
91. H. Zhao, J. Zhao, M. Zhang, D. Chen and D. Yang, Mater. Technol., 2024, 39, 2342737.
92. B. Choi, S. Lee, C. Fushimi and A. Tsutsumi, Electrochim. Acta, 2011, 56, 6696–6701.
93. X. Xiao, Z. Zhang, Y. Wu, J. Xu, X. Gao, R. Xu, W. Huang, Y. Ye, S. T. Oyakhire, P. Zhang, B. Chen, E. Cevik, S. M. Asiri, A. Bozkurt, K. Amine and Y. Cui, Adv. Mater., 2023, 35, e2211555.
94. H. Shi, H. Fu, G. Xue, Y. Lian, J. Zhao and H. Zhang, Coord. Chem. Rev., 2025, 545, 216984.
95. Y. Geng, C.-y. Zhang, W.-k. Zhou, H.-f. Qin, Y.-b. Lian and J.-r. Bai, Int. J. Hydrogen Energy, 2025, 177, 151415.
96. S. Yang and L. Zhang, Chem. Eng. J., 2025, 506, 160086.
97. P. Veerakumar, A. Sangili, S. Manavalan, P. Thanasekaran and K.-C. Lin, Ind. Eng. Chem. Res., 2020, 59, 6347–6374.
98. N. Singh, V. Singh, N. Bisht, P. Negi, A. Dhyani, R. K. Sharma and B. S. Tewari, J. Energy Storage, 2025, 121, 116498.
99. M. Neelakandan, P. Dhandapani, S. Ramasamy, R. Duraisamy, S. J. Lee and S. Angaiah, RSC Adv., 2025, 15, 16766–16791.
100. Z. Wang, H. Wang, D. Pei, S. Wan, Z. Fan, M. Yu and H. Lu, Prog. Nat. Sci., 2023, 33, 881–890.
101. T. Nguyen and M. F. Montemor, Adv. Sci., 2019, 6, 1801797.
102. T. Nguyen, M. Boudard, M. J. Carmezim and M. F. Montemor, Electrochim. Acta, 2016, 202, 166–174.
103. X. Fan, X. Wang, G. Li, A. Yu and Z. Chen, J. Power Sources, 2016, 326, 357–364.
104. A. Xu, Y. Yu, W. Li, Y. Zhang, S. Ye, Z. Zhao and Y. Qin, Electrochim. Acta, 2022, 435, 141378.
105. M. Zhang, Y. Chen, D. Yang and J. Li, J. Energy Storage, 2020, 29, 101363.
106. C. Rogier, G. Pognon, P. Bondavalli, C. Galindo, G. T. M. Nguyen, C. Vancaeyzeele and P.-H. Aubert, Surf. Coat. Technol., 2020, 384, 125310.
107. H. Soltani, H. Bahiraei and S. Ghasemi, J. Alloys Compd., 2022, 904, 163565.
108. A. L. Yan, W. D. Wang, W. Q. Chen, X. C. Wang, F. Liu and J. P. Cheng, Nanomaterials, 2019, 9, 1398.
109. D. Huang, Z. Lu, Q. Xu, X. Liu, W. Yi, J. Gao, Z. Chen, X. Wang and X. Fu, RSC Adv., 2021, 11, 21405–21413.
110. Y. Liu, Z. Zeng, B. Bloom, D. H. Waldeck and J. Wei, Small, 2017, 14, 1703237.
111. T. Wang, H. C. Chen, F. Yu, X. S. Zhao and H. Wang, Energy Storage Mater., 2019, 16, 545–573.
112. T. Kasuga, A. Mizui, H. Koga and M. Nogi, Adv. Sustainable Syst., 2023, 8, 2400254.
113. H. Yuan, J. Luan, J. Liu and C. Zhong, Adv. Funct. Mater., 2024, 34, 2400289.
114. Z. Xiao-Cong, G. U. I. Jun-Feng, Y. U. Xiao-Ying, L. I. U. Fang-Yang, J. Liang-Xing, L. A. I. Yan-Qing, L. I. Jie and L. I. U. Ye-Xiang, Acta Phys.-Chim. Sin., 2014, 30, 492–499.
115. Y. Li, L. X. Jiang, X. J. Lv, Y. Q. Lai, H. L. Zhang, J. Li and Y. X. Liu, Hydrometallurgy, 2011, 109, 252–257.
116. S. Zhuang, F. Xu and N. Duan, Sep. Purif. Technol., 2025, 354, 132642.
117. N. Yuan, Q. Jiang, J. Li and J. Tang, Arab. J. Chem., 2020, 13, 4294–4309.
118. S. D. Raut, H. R. Mane, N. M. Shinde, D. Lee, S. F. Shaikh, K. H. Kim, H.-J. Kim, A. M. Al-Enizi and R. S. Mane, New J. Chem., 2020, 44, 17864–17870.
119. S. P. Mooni, K. K. Kondamareddy, S. Li, X. Zhou, L. Chang, X. Ke, X. Yang, D. Li and Q. Qu, Arab. J. Chem., 2020, 13, 4553–4563.
120. X. Liu, D. Wang, Y. Du, X. Zhang and S. Ye, J. Mater. Sci., 2021, 56, 18174–18187.
121. H. Jia, P. Guan, Y. Zhou, Z. Feng, R. Zhu, M. Li, T. Wan, T. Huang, T. Liang, J. Pan, Z. Ma, Y. Xu, C. Wang and D. Chu, Energy Fuels, 2022, 36, 13808–13816.
122. K. Fujimoto, T. Okada and M. Nakayama, J. Phys. Chem., 2018, 122, 8406–8413.
123. Y. Meng, W. Song, H. Huang, Z. Ren, S. Y. Chen and S. L. Suib, J. Am. Chem. Soc., 2014, 136, 11452–11464.
124. F. Zhou, A. Izgorodin, R. K. Hocking, V. Armel, L. Spiccia and D. R. Macfarlane, ChemSusChem, 2013, 6, 643–651.
125. F. Yang, G. Dong, L. Meng, L. Liu, X. Liu, Z. Zhang, M. Zhao and W. Zhang, Int. J. Hydrogen Energy, 2024, 77, 589–597.
126. B. Thanigai Vetrikarasan, A. R. Nair, S. K. Shinde, D.-Y. Kim, J. M. Kim, R. N. Bulakhe, S. N. Sawant and A. D. Jagadale, J. Energy Storage, 2024, 94, 112457.
127. X. Meng, T. Liu, M. Qin, Z. Liu and W. Wang, ACS Appl. Mater. Interfaces, 2023, 15, 20110–20119.
128. N. Roy, S. Yasmin, A. K. Mohiuddin and S. Jeon, Appl. Surf. Sci. Adv., 2023, 18, 100517.
129. C. Xiong, Q. Yang, W. Dang, M. Li, B. Li, J. Su, Y. Liu, W. Zhao, C. Duan, L. Dai, Y. Xu and Y. Ni, J. Power Sources, 2020, 447, 227387.
130. M. Huynh, C. Shi, S. J. Billinge and D. G. Nocera, J. Am. Chem. Soc., 2015, 137, 14887–14904.
131. Y. Gorlin, B. Lassalle-Kaiser, J. D. Benck, S. Gul, S. M. Webb, V. K. Yachandra, J. Yano and T. F. Jaramillo, J. Am. Chem. Soc., 2013, 135, 8525–8534.
132. P. Wang, Y. Lin, L. Wan and B. Wang, ACS Appl. Mater. Interfaces, 2019, 11, 37701–37707.
133. Y. Liu, H. Hu, K. Wang, J. Huang and D. Wang, Adv. Energy Mater., 2025, e04101.
134. A. Rampf, R. Leiter, S. Fleischmann, G. A. Elia and R. Zeis, J. Energy Chem., 2026, 114, 473–484.
135. Z. Wang, L. Shi, H. Xiao, W. Wen, L. Qiang, S. He, N. Gao, M. Zhao and J. Jia, Adv. Funct. Mater., 2025, e18677.

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