Recent Advances in Energy Storage and Conversion

1. Introduction

The global transition towards carbon neutrality and sustainable energy systems has spurred intensive research into advanced energy storage and conversion technologies [1,2]. Electrochemical energy storage devices (e.g., supercapacitors, lithium-ion batteries, zinc-ion batteries, solid-state capacitors) and functional materials (e.g., ion-selective electrodes, proton-conducting composites, mineral-derived electrode materials) are at the core of this transition, as they determine the efficiency, stability, cost, and environmental impact of energy systems. To showcase the latest progress and address critical challenges in this field, the Special Issue of Inorganics titled “Recent Advances in Energy Storage and Conversion” has compiled ten high-quality research articles covering novel material design, synthesis optimization, mechanism elucidation, and performance evaluation for energy storage and conversion applications. This Editorial provides a comprehensive overview of the contributions published in this Special Issue, highlighting their key findings, innovations, and potential implications for the development of next-generation energy storage and conversion technologies.

2. Overview of the Contributions

2.1. Fe-Based MOF Nanosheet Arrays for High-Performance Hybrid Supercapacitors

Zhao et al. [3] addressed the long-standing challenge of balancing cathode/anode capacity and mitigating volume expansion in Fe-based electrode materials for hybrid supercapacitors. A facile one-pot solvothermal strategy was developed to synthesize Fe(BPDC) nanosheet arrays on nickel foam as binder-free cathodes, with solvothermal time (3 h, 6 h, 12 h, 18 h) tuned to optimize the material morphology. The optimized Fe(BPDC)-3 (12 h reaction time) exhibited a uniform, interconnected nanosheet structure, delivering an ultrahigh specific capacitance of 17.54 F/cm² at 1 mV/s and exceptional cycling stability (129% capacitance retention after 10,000 cycles)—a significant improvement over conventional Fe-based electrodes. The assembled Fe(BPDC)//activated carbon (AC) hybrid supercapacitor achieved an energy density of 45.64 Wh/kg at a power density of 4919.6 W/kg, with 87.05% capacitance retention after 10,000 cycles, and demonstrated excellent temperature adaptability across −10 °C to 100 °C. This work validates Fe-based metal–organic framework (MOF) nanosheet arrays as a promising cathode material for high-performance hybrid supercapacitors, offering a scalable and cost-effective alternative to conventional inorganic cathode materials.

2.2. One-Dimensional Fe₂O₃@3D Graphene Composites as Anodes for Lithium-Ion Batteries

Zhu et al. [4] reported a temperature-driven synthesis approach to fabricate Fe₂O₃-anchored three-dimensional graphene (3DG) composites with tailored nanostructures (nanocubes at 120 °C, 1D nanorods at 150 °C, ellipsoids at 180 °C). The composite synthesized at 150 °C featured 1D Fe₂O₃ nanorods uniformly embedded in 3DG, forming an interpenetrating 1D-3D structure that enhanced electrochemical stability and accelerated Li⁺ diffusion. As an anode material for lithium-ion batteries (LIBs), this composite exhibited a reversible specific capacity of 1041 mAh/g at 0.1 A/g and maintained 775 mAh/g after 200 cycles—outperforming most reported Fe₂O₃/graphene composites. The superior performance was attributed to the stable interpenetrating structure (which buffers volume changes during lithiation/delithiation), improved electrical conductivity of 3DG, and graded porous features that provide abundant Li⁺ storage sites. This study provides a new paradigm for the structural design of transition metal oxide/graphene composites for advanced LIB anodes.

2.3. Cycling Stability Mechanism of Anthraquinone Cathodes for Aqueous Zinc-Ion Batteries

Chen et al. [5] investigated the cycling stability of anthraquinone (AQ) cathodes for aqueous zinc-ion batteries (ZIBs), a low-cost and safe alternative to LIBs. Experimental results revealed a stark contrast in cycling performance: AQ exhibited a capacity fading rate of only 0.03% per cycle at 4000 mA/g (1000 cycles), while the fading rate increased to 0.08% per cycle at 200 mA/g. Density functional theory (DFT) calculations uncovered the underlying mechanism: at low current densities, Zn²⁺ ions insert into the bulk of AQ crystals, causing significant structural deformation (5.46% volume change and 21.99% intermolecular distance variation) and irreversible diffusion (energy barrier of 3.185 eV). In contrast, at high current densities, Zn²⁺ ions preferentially adsorb and diffuse on the AQ surface (energy barrier of 0.171 eV) without bulk insertion or structural damage, leading to high cycling stability. This work clarifies the structure–performance relationship of organic cathodes for aqueous ZIBs and provides guidance for optimizing their cycling stability through rational material design.

2.4. Plasticizer-Free Lead(II) Ion-Selective Electrodes Based on Dibenzo-18-crown-6 Aldimines

Jackson et al. [6] synthesized three novel dibenzo-18-crown-6 aldimine derivatives and systematically characterized their lead(II) ion-binding behavior using spectroscopic techniques, DFT modeling, and electrochemical methods. These derivatives were electrochemically polymerized on platinum surfaces to form plasticizer-free lead ion-selective membranes (ISMs), characterized by scanning electron microscopy (SEM) and electrochemical impedance spectroscopy (EIS). The resulting ion-selective electrodes (ISEs) enabled selective detection and quantification of lead ions with a limit of detection (LOD) as low as 10 ppm and a linear range of 15–60 ppm, with minimal interference from Hg²⁺ and Al³⁺ ions at a 1:1 analyte–interferent ratio. This work presents a low-cost, rapid, and portable electrochemical sensing platform for lead ion detection, with potential applications in environmental monitoring, food safety, and biological analysis.

2.5. Proton Conduction of Intrinsically Sulfonated COF Composites

Yang et al. [7] focused on developing stable proton-conducting materials for proton exchange membrane fuel cells (PEMFCs), addressing the issue of guest molecule leakage in conventional COF-based proton conductors. An intrinsically sulfonated covalent organic framework (COF), TpPa-SO₃H, was synthesized, exhibiting more stable proton conductivity than TpPa@H₂SO₄ (a COF loaded with sulfuric acid as guest molecules). To further enhance performance, sodium polyacrylate (PANa), a superabsorbent polymer, was coated on TpPa-SO₃H via in situ polymerization to form PANa@TpPa-SO₃H composites. The modified composite achieved an ultrahigh proton conductivity of 2.33 × 10⁻¹ S/cm at 80 °C under 95% relative humidity (RH), with a low swelling rate (11.5%) and excellent long-term durability. This strategy leverages the intrinsic proton conductivity of sulfonated COFs and water retention properties of PANa to create a high-performance proton-conducting platform, offering a viable alternative to commercial Nafion membranes for PEMFCs.

2.6. Surface Modification of Natural Serpentinite Ore for Secondary Batteries

Zhao et al. [8] explored the potential of natural serpentinite ore as a low-cost, eco-friendly electrode material for secondary batteries. Serpentinite-derived magnesium iron silicate ((Mg,Fe)₂SiO₄) was calcined at 900 °C to achieve high crystallinity, and a 20 nm thick sulfidation layer of cubic FeS₂ was formed on its surface via low-vacuum sulfidation. The sulfidated serpentinite electrode exhibited significantly improved discharge capacity, as the FeS₂ layer provided additional ion pathways for Li⁺/Mg²⁺ insertion/extraction and enhanced surface conductivity. The electrode maintained stable performance at elevated temperatures (55–75 °C), with the FeS₂ layer remaining intact and mitigating the poor conductivity of hematite (α-Fe₂O₃) formed during calcination. This study demonstrates the feasibility of utilizing abundant natural mineral resources for sustainable battery electrode materials, reducing reliance on synthetic electrode precursors.

2.7. Interaction Between PEDOT:PSS Dispersions and Aluminum Electrodes for Solid-State Capacitors

Calabia Gascón et al. [9] conducted the first systematic study on the interface interaction between poly(3,4-ethylenedioxythiophene)/polystyrene sulfonate (PEDOT:PSS) dispersions and aluminum electrodes for solid-state electrolytic capacitors (a companion study to [10], further expanding on this topic). Acidic PEDOT:PSS dispersions (pH 1.9) were found to etch the aluminum oxide surface, forming pores that increased the interfacial area and improved the capacitance of the aluminum oxide/PEDOT:PSS system. In contrast, neutral PEDOT:PSS dispersions (pH 4.9 and 5.8) did not damage the aluminum electrode but exhibited poor film-forming properties, leading to inconsistent electrochemical behavior (capacitive to resistive). The study identified a critical balance between material compatibility and electrochemical performance: acidic dispersions enhance capacitance but risk electrode degradation, while neutral dispersions preserve electrode integrity but compromise performance. These findings provide critical guidance for the design of PEDOT:PSS/aluminum systems for solid-state electrolytic capacitors.

2.8. Effect of PEDOT:PSS Impregnation on Porous Aluminum Electrodes for Solid-State Electrolytic Capacitors

Calabia Gascón et al. [10] further investigated the performance of PEDOT:PSS/porous aluminum systems as model solid-state electrolytic capacitors, focusing on the link between PEDOT:PSS impregnation depth and electrical performance. Porous aluminum electrodes with 26 nm/65 nm thick Al₂O₃ dielectric layers were coated with undoped and doped PEDOT:PSS dispersions (dopants: ethylene glycol (EG), dimethyl sulfoxide (DMSO), (3-glycidyloxypropyl)trimethoxysilane (GOPS)). Combining odd random phase electrochemical impedance spectroscopy (ORP-EIS) with surface characterization (SEM-EDX, GDOES), the study found that PEDOT:PSS only penetrated the first 6 μm of the porous aluminum structure (20 μm total depth) for all formulations, with only trace amounts detected in deeper pores. However, doping PEDOT:PSS with EG/DMSO (polar solvents) improved its electrical conductivity, leading to a 5-fold increase in capacitance (from ~5 μF/cm² to ~25 μF/cm²) and a phase angle shift from −83° (non-ideal capacitive behavior) to −89° (near-ideal capacitive behavior). GOPS (a cross-linking agent) did not impact electrical performance despite enhancing PEDOT:PSS stability. This work clarifies that the performance of PEDOT:PSS/aluminum solid-state capacitors depends on the electrical properties of PEDOT:PSS within the pores rather than the impregnation depth, providing critical insights for optimizing the industrial production of solid electrolytic capacitors.

2.9. Recovery of Ni-Co-Mn Oxides from End-of-Life Lithium-Ion Batteries for NTC Sensor Applications

Mhin [11] addressed the challenges of high cost and supply uncertainty for Ni/Co/Mn-based materials by recycling these critical metals from end-of-life (EOL) LIB cathode materials (Li(Ni_0.33Co_0.33Mn_0.33)O₂, NCM111). A hydrometallurgical process (leaching + oxalate precipitation) combined with post-heat treatment was developed to recover Ni-Co-Mn oxides, which were further synthesized into spinel-type (Ni_0.6Co_0.4Mn₂)O₄. Structural characterization (XRD, SEM, EDS, XPS) confirmed a cubic spinel structure with homogeneous elemental distribution and mixed valence states (Ni²⁺/Ni³⁺, Mn²⁺/Mn³⁺/Mn⁴⁺, Co²⁺/Co³⁺) in the recovered oxides. Electrical performance tests revealed semiconductive behavior with a resistivity of 300 Ω·m at 25 °C and a thermal sensitivity (B value) of 3376.92 K—within the typical range (3000–5000 K) for negative temperature coefficient (NTC) thermistors. This work demonstrates the feasibility of recycling EOL LIB cathode materials to produce high-performance NTC temperature sensors, offering a sustainable solution to critical metal supply challenges and reducing the environmental impact of battery waste.

2.10. MXene Composites: Structure, Preparation, and Application in Supercapacitors (Review)

Sun et al. [12] presented a comprehensive review of two-dimensional transition metal carbides/nitrides (MXenes) and their composites for supercapacitor applications. The review first summarizes MXene’s core structural characteristics (layered M_n+1X_nT_x structure with tunable surface functional groups) and key properties (high capacitive performance, metallic conductivity (~24,000 S/cm), hydrophilicity, and mechanical flexibility). It then systematically categorizes MXene preparation methods, including HF/fluoride salt etching, alkali-based etching, electrochemical etching, Lewis acid molten salt etching, and direct synthesis (CVD), highlighting the trade-offs between safety, cost, and material quality for each method. The core focus of the review is the design and performance of MXene composites with carbon materials (CNTs, graphene), metal oxides (MnO₂, Co₃O₄), metal hydroxides (NiFe-LDH, NiMnCoOH), conductive polymers (PANI, PPy), and other 2D materials (MoS₂, MoSe₂). The review also analyzes the influence of electrolytes (alkaline, neutral, acidic, organic, solid) on MXene-based supercapacitor performance, and identifies key challenges (agglomeration, oxidation, low energy density) and future directions (scalable synthesis, structural optimization, flexible device design) for MXene research. This review provides a holistic and up-to-date overview of MXene-based supercapacitor materials, serving as a valuable reference for researchers in the field.

3. Conclusions and Outlook

The contributions published in this Special Issue cover a broad spectrum of energy storage and conversion research, from the design of novel electrode materials (Fe-based MOFs, Fe₂O₃/graphene composites, sulfidated serpentinite, recycled Ni-Co-Mn oxides) to the elucidation of reaction mechanisms (AQ cathodes for ZIBs, PEDOT:PSS/aluminum interfaces, MXene charge storage) and the development of functional materials (lead ion-selective electrodes, sulfonated COF proton conductors, MXene composites). These studies address critical challenges in the field, including low capacity, poor cycling stability, material compatibility, high cost, and environmental impact, and provide innovative solutions through rational material design, structural optimization, and mechanism-driven engineering.

Looking forward, future research in energy storage and conversion should focus on (1) the scalable synthesis of low-cost, abundant, and eco-friendly materials (e.g., recycled battery metals, natural minerals, biomass-derived carbons) to promote industrialization; (2) in-depth characterization of reaction mechanisms at the atomic/molecular level (via in situ spectroscopy and DFT calculations) to guide material design; (3) the integration of multifunctional materials to develop integrated energy storage/conversion systems (e.g., supercapacitor–battery hybrids, CO₂RR-fuel cell systems); and (4) the evaluation of long-term stability and recyclability of materials to ensure sustainability. We anticipate that the findings presented in this Special Issue will inspire further research and innovation, accelerating the development of high-performance, low-cost, and sustainable energy storage and conversion technologies.