Fig. 1. (A) Application potential of SACs in portable ZABs, PEMFCs [2], and sensors [19,20]. Reprinted with permission from Ref. [2], Copyright 2022, Elsevier. Reprinted with permission from Ref. [19]. Copyright 2023, Springer Nature. Reprinted with permission from Ref. [20]. Copyright 2024, Springer Nature. (B) Data were retrieved from Scopus for the period 2011-2025 using the keywords “single-atom catalysts”, “zinc-air batteries”, “proton exchange membrane fuel cells”, and “portable sensors” (accessed on July 4, 2025).

Fig. 2. (A) Schematic illustration of the structure of a rechargeable ZABs using an aqueous electrolyte. (B) Polarization curves of the zinc anode and air cathode in the rechargeable ZABs.

Fig. 3. (A) Schematic illustration of a flexible quasi-solid-state ZAB. (B) Stability evaluation of Fe1/d-CN and Pt/C + RuO2 catalysts as air cathodes in alkaline and neutral flexible quasi-solid-state ZABs. (C) Cycling stability of flexible quasi-solid-state ZABs under different bending angles in both alkaline and neutral electrolytes. (D) Demonstration of three flexible quasi-solid-state ZABs powering an light emitting diode (LED) [61]. Reprinted with permission from Ref. [61]. Copyright 2021, Royal Society of Chemistry. (E) Schematic diagram of the flexible solid-state ZAB configuration. (F) Discharge and power density curves of ZAB-Fe3Co7-NC at various temperatures. (G) Cycling performance of ZAB-Fe3Co7-NC under current densities of 2 and 5 mA cm-2 at different temperatures [62]. Reprinted with permission from Ref. [62]. Copyright 2022, John Wiley and Sons. (H) Schematic illustration of the low-temperature flexible rechargeable ZAB based on FeMn-N/S-C-1000. (I) Comparison of discharge profiles and power density. (J) Discharge curves of flexible rechargeable ZABs at different current densities and temperatures. (K) Charge-discharge cycling performance at -40?°C under a constant current density of 2?mA?cm-2 [63]. Reprinted with permission from Ref. [63]. Copyright 2024, Elsevier.

Fig. 4. (A) Illustration of sandwich-type FZABs. (B) Liquid retention capacity comparison among PAM/EG, PAM, and PVA. (C) Cycling stability of FeSA/FeAC@PPy/CC-based FZABs under a current density of 2 mA cm-2 in different bending states and under hammering [71]. Reprinted with permission from Ref. [71]. Copyright 2025, Royal Society of Chemistry. (D) Schematic illustration of the fabrication process of IE-M/HPLIG. (E) Discharge profiles and power density performance of rechargeable flexible quasi-solid-state ZABs assembled with IE-NiFeCo/HPLIG as the air cathode. (F) Charge-discharge cycling performance at 5 mA cm-2 under various bending conditions. (G) Practical demonstration of three serially connected batteries [72]. Reprinted with permission from Ref. [72]. Copyright 2022, John Wiley and Sons. (H) Schematic illustration of the electrospinning process for the fabrication of Co SA/NCFs. (I) Galvanostatic cycling performance of Co SA/NCFs and 20% Pt/C + IrO2 cathodes at a current density of 2 mA cm-2. (J) Cycling performance of Co SA/NCFs-based batteries under different bending angles at a current density of 1 mA cm-2 [69]. Reprinted with permission from Ref. [69]. Copyright 2022, American Chemical Society.

Fig. 5. (A) Copper plating without electrolyte on spandex fabric through a pre-stretching-loading-recovery strategy, followed by dynamic stretching for zinc/nickel electroplating, and assembly into a ZAB based on in-situ crosslinked hydrogel electrolyte. (B) Photographs of the zinc-coated fabric before and after 100% stretching. (C) SEM images of the zinc-coated fabric before and after stretching. (D) Discharge polarization curve of large-area ZABs [74]. Reprinted with permission from Ref. [74]. Copyright 2022, John Wiley & Sons Australia. (E) Schematic of a solid-state rechargeable zinc-air battery utilizing a functionalized nanocellulose membrane. (F) Schematic of an FZAB device integrated with a band-aid, demonstrating the flexible device wrapped around the index finger in a bent state, successfully powering a red LED. Also provided is the power density versus current density curve of a single flexible battery, along with the Nyquist plot measured at various bending angles [75]. Reprinted with permission from Ref. [75]. Copyright 2016, Royal Society of Chemistry. (G) Biomass-based ICNF/WCNF SSEs fabricated by weaving ICNF and WCNF into a mesh structure. (H) Wet biomass ICNF/WCNF SSEs. (I) Schematic of a sandwich-structured FZAB. (J) Comparison of the cycling life and discharge voltage with previously reported biomass-based SSE FZABs in the literature [76]. Reprinted with permission from Ref. [76]. Copyright 2024, John Wiley and Sons.

Fig. 6. (A) Configuration of cable-type flexible ZABs utilizing PVAA-GO GPE. (B) Galvanostatic discharge and charge curves of the cable-type flexible ZAB under various conditions. (C) Three cable-type ZABs connected in series to power a commercial smartwatch [89]. Reprinted with permission from Ref. [89]. Copyright 2020, John Wiley and Sons. (D) Schematic of the one-dimensional FZAB structure. (E) Continuous power supply to a smartwatch achieved by integrating FZABs into a custom-woven wristband. (F) Illustration of the jumping motion and corresponding EMG signals. (G) Schematic diagram of serially connected FZABs powering a sensor patch. Photographs of the dumbbell-lifting process (H) and EMG signal variations recorded during the lifting and lowering movements (I) [90]. Reprinted with permission from Ref. [90]. Copyright 2024, American Chemical Society.

Fig. 7. (A) Fabrication process of the PAR-ZAB. (B) Polarization and power density curves of a single device, along with the performance of serially integrated devices. (C) Polarization and power density curves of the integrated device connected in parallel. (D) Three PAR-ZAB integrated units connected in series exhibit mechanical flexibility and are capable of powering commercial LED lights. (E) A self-designed smartwatch powered by a PAR-ZAB wristband with customized dimensions and voltage output, demonstrating safe and direct skin contact (as shown in the inset). (F) Schematic illustration of a runner wearing a smartwatch powered by an integrated PAR-ZAB wristband [93]. Reprinted with permission from Ref. [93]. Copyright 2019, Royal Society of Chemistry. (G) Optical image, microscopic view, and actual geometric parameters of a single mZAB active unit. (H) Charge-discharge polarization curves of the mZAB based on NdDCF-OIM/Co-800 and Pt/C + RuO2 [94]. Reprinted with permission from Ref. [94]. Copyright 2021, Royal Society of Chemistry.

Fig. 8. (A) HAADF-STEM and EDS mapping images of FCN-TM/NC. B) Bifunctional LSV curves of FCN-TM/NC. (C) Graphical depiction of the solid-state ZABs. (D) Discharge polarization and power density curves of the flexible solid-state ZAB. (E) Charge-discharge cycling test of the FSS-ZAB at -60 °C under a current density of 1 mA cm-2. (F) Performance comparison of the FSS-ZAB with previously reported studies. (G) Smartwatch and smartphone powered by the FSS-ZAB under bending conditions [96]. Reprinted with permission from Ref. [96]. Copyright 2025, John Wiley and Sons.

Fig. 9. (A) Structural model diagram of a PEMFC. (B) Schematic diagram of the ORR mechanism in acidic media [108]. Reprinted with permission from Ref. [108]. Copyright 2016, Advancement of Science.

Fig. 10. (A) Fabrication process of the novel and commercial GDL. (B) MEA prepared using commercial GDL and carbon paper after hot pressing. (C) MEA prepared with the novel GDL. (D) Flexible air-breathing PEMFC based on the novel GDL and its structural schematic (inset). (E) Polarization curves of PEMFCs assembled with commercial and novel GDLs. [113]. Reprinted with permission from Ref. [113]. Copyright 2020, Royal Society of Chemistry. (F) Schematic diagram of flexible PEMFCs. (G) Polarization curves of flexible PEMFCs with different GDLs under bending conditions [114]. Reprinted with permission from Ref. [114]. Copyright 2023, Springer Nature. (H) Application demonstrations of flexible PEMFC stacks [115]. Reprinted with permission from Ref. [115]. Copyright 2017, American Chemical Society.

Fig. 11. (A) Digital camera images of the foldable PEMFC in flat, 90° bent, and 180° folded states. (B) Polarization curves (solid symbols, left axis) and power density curves (hollow symbols, right axis) of the foldable fuel cell in flat, bent, and folded states. (C) Demonstration of the foldable fuel cell powering an electric motor; the motor remains inoperative without hydrogen supply. (D) Polarization and power density curves of the foldable fuel cell after 100, 200, and 300 folding cycles [119]. Reprinted with permission from Ref. [119]. Copyright 2024, Elsevier. (E) Schematic diagram of the bent PEMFC structure, showing the reaction zone and process under bending. (F) 3D-printed flexible flow-field plates. (G) Performance comparison of the bent PEMFC under different bending states. (H) Finite element analysis showing the compressive stress generated during the bending process of the bent PEMFC [2]. Reprinted with permission from Ref. [2]. Copyright 2022, Elsevier.

Fig. 12. (A) Digital image showing the weight and thickness of the air-breathing RUFC. (B) Photographs of RUFCs in different configurations (rolled and S-shaped). (C) Portable 10-cell RUFC stack used for smartphone charging and LED fan operation, including outdoor application with a hydrogen storage metal hydride cartridge. (D) Polarization curves of the stacked RUFCs [121]. Reprinted with permission from Ref. [121]. Copyright 2017, Springer Nature. (E) Integrated micro fuel cell with three cells connected in series. (F) Micro fuel cell powering an LED in a bent configuration. (G) Design features of PEM micro fuel cells: Design 1 with serpentine anode flow field and Design 2 with parallel flow channels [122]. Reprinted with permission from Ref. [122]. Copyright 2004, Elsevier. (H) Schematic illustration of the tubular PEMFC fabrication process. (I) Optical image of a two-cell stacked flexible t-PEMFC. (J) Polarization curves of the two-cell stacked flexible t-PEMFC [123]. Reprinted with permission from Ref. [123]. Copyright 2021, American Chemical Society.

Fig. 13. (A) HAADF-STEM image of Pt1-MoS2-def. (B,C) Dynamic response curves of MoS2, Pt1-MoS2, and Pt1-MoS2-def at SO2 concentrations of 0.5-5 ppm and 20-40 ppm, respectively. (D) Comparison of response values for the three materials under different SO2 concentrations. (E) Gas selectivity results for MoS2, Pt1-MoS2, and Pt1-MoS2-def. (F) Schematic of real-time SO2 concentration monitoring in a greenhouse environment. (G) Modular block diagram of the detection system. (H) Presentation of measured SO2 concentration data [138]. Reprinted with permission from Ref. [138]. Copyright 2024, Springer Nature. (I) Schematic of the sensor device structure. (J), (K) Dynamic gas sensing performance of rGO, N@rGO, Co-rGO, and M SACs-N@rGO (M = Co, Ni, Pt) under 0.1-20 ppm NO2 [139]. Reprinted with permission from Ref. [139]. Copyright 2023, American Chemical Society. (L) Transient response curves of five sensors under 5 ppm NH3 exposure at room temperature. (M) Response comparison of the MNPE-Ni-N-C/Ti3C2Tx sensor to 5 ppm NH3 under varying relative humidity conditions (room temperature). (N) Optical image of the MNPE-Ni-N-C/Ti3C2Tx flexible sensor under bending. (O) Resistance response curves of the MNPE-Ni-N-C/ Ti3C2TxTx sensor to 5 ppm NH3 over different weeks [140]. Reprinted with permission from Ref. [140]. Copyright 2024, Springer Nature.

Fig. 14. (A) Synthesis of Pt-loaded Fe2O3 nanosheets (Pt-Fe2O3-Vo). (B) Dynamic response curves of the sensor under various H2 concentrations. (C) Response performance of the sensor to 50 ppm H2 at different temperatures. (D) Selectivity test results of the sensor against various gases. (E) Long-term stability of the sensor under 50 ppm H2 [143]. Reprinted with permission from Ref. [143]. Copyright 2024, American Chemical Society. (F) Schematic illustration of the synthesis of Fe2O3, Pt1-Fe2O3, and Pt1-Fe2O3-ox. (G) Comparison of the response of the three materials to 100 ppm C2H5OH across a temperature range of 200-320 °C. (H) Response performance of Pt1-Fe2O3-ox to 10-200 ppm C2H5OH at 280 °C. (I) Response and recovery time curve of Pt1-Fe2O3-ox under 100 ppm C2H5OH at 280 °C. (J) Selectivity comparison of Fe2O3, Pt1-Fe2O3, and Pt1-Fe2O3-ox to various gases (100 ppm) [144]. Reprinted with permission from Ref. [144]. Copyright 2020, Springer Nature.

Fig. 15. (A) Schematic illustration of the experimental setup for humidity sensing. (B) Frequency shifts of four modified QCM-based humidity sensors under different RH conditions. (C) Sensitivity curves of four samples within the RH range of 33% to 97%. (D) Comparison of response and recovery times at 97% RH. (E) Analysis of humidity hysteresis characteristics [151]. Reprinted with permission from Ref. [151]. Copyright 2022, American Chemical Society. (F) Resistance variation and linear fitting results of the ZDC800-AL sensor in the RH range of 11%-98%. (G) High-resolution linear fitting curve of the ZDC800-AL sensor in the RH range of 50%-60%. (H) Dynamic response and recovery behavior of the ZDC800-AL sensor in the RH range of 0.3%-90%. (I) Schematic diagram of the humidity sensing mechanism for ZDC-AL-based sensors. (J) Diagram of six-digit binary password recognition and sensor operation workflow. (K) Real-time images of a fingertip approaching and moving away from the humidity sensor array. (L) Dynamic response curves of the humidity sensor array to the approaching fingertip. (M) Real-time humidity sensing signals recorded on the back and palm of a human subject's hand. (N) Sensor responses before and after applying moisturizer on the back of the hand [154]. Reprinted with permission from Ref. [154]. Copyright 2025, Elsevier.

Fig. 16. (A) Schematic illustration of inflammation-free in vivo electrochemical sensing enabled by Fe1/NC-900 catalyst. (B) Histological comparison of NAc brain tissue from untreated rats and those implanted with bare or Fe1/NC-900-modified carbon fiber electrodes (CFEs) for 8 h (n = 3 per group), with astrocytes (green), microglia (magenta), and nuclei (blue) fluorescently labeled (scale bar: 50 μm). The fluorescence intensity variation of astrocytes is also shown. (C) Quantitative ELISA analysis of IL-6, TNF-α, and IL-1β levels in brain tissues from untreated rats (blue columns) and rats implanted with bare (gray) or Fe1/NC-900-modified (red) CFEs (n = 3 for each group). Data are presented as mean ± SEM, and p-values are indicated in the figures via one-way ANOVA. Source data are provided with the article [20]. Reprinted with permission from Ref. [20]. Copyright 2024, Springer Nature. (D) Schematic illustration of the Fe-SAs configuration. (E) CPT curves in standard buffer solutions of different pH. (F) Correlation between open-circuit potential (OCP) and pH (n = 3). (G) Stability test of the pH sensor (n = 3) [155]. Reprinted with permission from Ref. [155]. Copyright 2023, American Chemical Society. (H) Relative resistance variation of Ru-ALD engineered DM-V2CTx MXene-based temperature sensor with respect to temperature. (I) Durability testing of the temperature sensor under repeated heating and water droplet cooling cycles. (J) Continuous response performance under external temperature variations of 30-50 °C. (K) Real-time human skin temperature monitoring and corresponding infrared thermal imaging [156]. Reprinted with permission from Ref. [156]. Copyright 2023, John Wiley and Sons.

Fig. 17. Advantages and disadvantages of single-atom catalysts in portable ZABs, PEMFCs, and sensor systems.

Fig. 18. Opportunities and challenges associated with single-atom catalysts and their integration into portable ZABs, PEMFCs, and sensors across materials, structural design, packaging, and system integration.