Hierarchical CoFe@N‐Doped Carbon Decorated Wood Carbon as Bifunctional Cathode in Wearable Zn‐Air Battery

Rechargeable Zn‐air batteries (ZAB) have drawn extensive attention due to their eco‐friendliness and safety. However, the lack of high‐performance and low‐cost oxygen redox reactions (OER and ORR) catalysts has become one of the main stumbling blocks in their development. Herein, we successfully fabricate a CoFe nanobubble encapsulated in nitrogen‐doped carbon nanocage on wood carbon support (CoFe@NC/WC) via pyrolysis of a novel Prussian blue analog (PBA)/spruce precursor. The hierarchical CoFe@NC/WC catalyst exhibits an excellent potential difference of 0.74 V between the OER potential at 10 mA cm−2 and half‐wave potential of ORR in 0.1 M KOH, comparable to recently reported preeminent electrocatalysts. Further, CoFe@NC/WC shows outstanding electrochemical performance in liquid ZAB, with a peak power density of 138.9 mW cm−2 and a specific capacity of 763.5 mAh g−1. More importantly, a bacterial cellulose nanofiber reinforced polyacrylic acid (BC‐PAA) hydrogel electrolyte shows ultrahigh tensile‐breaking stress of 1.58 MPa. In conjunction with the as‐prepared CoFe@NC/WC catalyst, BC‐PAA‐based wearable ZAB displays impressive rechargeability and foldability, and can power portable electronics, such as electronic timer and mobile phone, in bent states. This work provides a new approach toward high‐activity and low‐cost catalysts for ZAB.


Introduction
In the energy revolution, gradually replacing fossil energy with renewable energy is the primary goal.As a clean energy technology, rechargeable Zn-air batteries (ZAB) have received widespread attention owing to their low cost, inherent safety, and high theoretical energy density (1086 Wh kg −1 ). [1]Since the electrocatalytic kinetics of oxygen reduction and evolution reactions (ORR/OER) are relatively slow, the development of high-performance ORR and OER catalysts remains a challenge. [2]Up to now, Pt-based and Ru/Ir-based materials are still the state-of-the-art catalysts for ORR and OER, respectively. [3]However, these noblemetal-based materials suffer from high cost, limited durability, and poor bifunctional properties.Therefore, high-efficiency ZAB involving ORR and OER urgently need stable noblemetal-free catalysts with high performance and at the lowest possible cost. [4]uring the last decade, much progress has been made in noble-metal-free bifunctional ORR/OER electrocatalysts. [5,6]Among those, transition metal compounds (TMC) and carbon-based materials are two "prosperous" classes, providing many candidates for integrating ORR-OER active sites.The complementary characteristics of TMC and carbon provide a potential synergistic effect.Accordingly, rational selection of active sites and their effective integration are of great significance."Selection" contributes to the lower limit of ORR-OER catalytic activity, while "integration" determines the synergic effects toward enhanced bifunctionality.
Prussian blue analogues (PBA) with a general formula of A 2 M[M 0 (CN) 6 ] (A = Na or K, M/M 0 = Fe, Co, Mn, Ni, etc.), can be modified via chemical or thermal methods. [7]Since PBA consist of transition metal ions and cyanide ion (CN − ) in their crystal structures, it is highly possible to integrate TMC and nitrogen-doped carbon (NC) simultaneously.Moreover, as the first modern synthetic pigment, Prussian blue has been widely used in dyeing processes and paints due to its good affinity to polymers, such as cellulose.This inspired us to combine PBA with biomass cellulose to improve the dispersion of active materials and further reduce the production cost of catalysts.
The US Department of Energy has defined the major characteristics of biomass as versatile, valuable, economically beneficial, and very abundant.As the most abundant biomass, wood materials have been applied in energy storage and conversion, signaling/sensing, and water treatment, because of their intrinsically hierarchical porous structure offering an appealing platform for multiscale ion transportation. [8]specially, natural wood with its porous structure and ultrahigh content of cellulose, hemicellulose and lignin is an ideal precursor for low-cost carbon electrodes. [9]11][12] Herein, for the first time, we successfully in-situ decorate wood channels with CoFe-PBA.Due to the good ionic affinity of wood cellulose, PBA grows evenly and firmly on the lumen of wood.The PBA/wood precursor can be conveniently converted into a carbon-based composite via one-step calcination.More specifically, CoFe alloy encapsulated in NC grown on WC matrix (CoFe@NC/WC) is obtained and serves as a bifunctional catalyst for oxygen redox reactions in rechargeable ZAB.The as-designed CoFe@NC/WC has hierarchical hollow structures, leading to high mass transfer of gas and ionic liquid and thus accelerating the reaction kinetics and providing more reactive sites. [2]xperimentally, CoFe@NC/WC-850 exhibits powerful bifunctional oxygen reversibility with a potential gap (ΔE) of 0.74 V between the half-wave potential of ORR (E 1/2 ) and the OER potential at 10 mA cm −2 (E j = 10 ), which is close to that of RuO 2 /WC¦Pt/C (0.68 V) and outperforming most of the TMC-carbon composite catalysts reported (Table S2, Supporting Information).Liquid ZAB based on CoFe@NC/WC-850 generates a peak power density of 138.9 mW cm −2 with a specific capacity of 763.5 mAh g −1 .The liquid ZAB demonstrates good stability over 240 continuous cycles, while the Pt/C + RuO 2 battery fails after 100 cycles.In addition, we also design a bacterial cellulose nanofiber-reinforced polyacrylic acid (BC-PAA) hydrogel toward wearable ZAB (WZAB).BC-PAA hydrogel reaches ultrahigh tensile stress with acceptable breaking strain.The hydrogelbased WZAB achieves an open-circuit voltage (OCV) of ~1.405 V and a peak power density of 73.9 mW cm −2 and can power electronics such as electronic timer and mobile phone.Moreover, density functional theory (DFT) calculations show that the adsorption free energy (ΔG *OH ) of *OH intermediate on CoFe@NC is more favorable than on CoFe alloy and NC alone.The moderate ΔG *OH facilitates efficient generation and reduction processes of *OH on the CoFe(110)/NC interface and therefore improves its catalytic activity toward oxygen.

Results and Discussion
The fabrication process of CoFe@NC/WC is displayed in Scheme 1. First, the spruce log was sawed into chips of 20 × 20 × 2 mm 3 (radial × tangential × longitudinal, R × T × L).Subsequently, spruce chips were dyed with CoFe-PBA at room temperature (RT).Lastly, the CoFe-PBA/spruce underwent reduction carbonization at different temperatures, during which the cellulose in wood, CN − and Co/Fe ions in PBA were transformed into WC, NC and metallic CoFe, respectively.Meanwhile, the K ions remained unchanged in the form of soluble potassium salt, which can be easily removed by washing with water.
From the cross-sectional scanning electronic microscope (SEM) images of pristine spruce (Figure S1a, Supporting Information), the size of the main channels (namely lumen) is around 25 μm, and the lumen wall consists of many pits with a diameter of ~5 μm (Figure S1b, Supporting Information).The X-ray diffraction (XRD) pattern of spruce (Figure S1c, Supporting Information) exhibits a relatively intense peak at 22.8°, two small peaks at 15.0°and 16.5°, corresponding to (002), (101), and (10-1) planes of cellulose (PDF#50-2241), respectively. [13]The SEM image (Figure S2a, Supporting Information) shows that the as-synthesized CoFe-PBA exhibits a cube morphology with a size ranging from 200 to 300 nm.XRD pattern of CoFe-PBA precursor (Figure S2b, Supporting Information) is in agreement with K 2 CoFe(CN) 6 (PDF#75-0038). [14]he SEM images in Figure 1a,b reveal that the morphology of lumen and pit in wood is well preserved after carbonization.Similarly, from the cross-sectional (Figure 1c) and internal SEM images of CoFe@NC/WC-850 (Figure 1d-f), the channels and small pits on the lumen remain intact.During pyrolysis, the spruce experienced an evident areal reduction (from 20 × 20 mm 2 to 15 × 15 mm 2 ) due to the transformation of cellulose into carbon (Figure S3, Supporting Information).At high magnification, cubes with a size of 200-300 nm deposited on the channel wall (Figure 1c, inset; Figure 1f) can be observed.It is worth noting that CoFe@NC/WC-850 without water washing presents many irregular crystals of soluble potassium salt (Figure S4, Supporting Information).This verifies the necessity of posttreatment washing with plenty of water.The structure of the calcinated nanocubes was further studied by transmission electron microscope (TEM).Figure 1g indicates the cavity feature of the nanocubes and therefore it is denoted CoFe@NC nanocage.Magnified TEM observation (Figure 1h,i) suggests in-situ formation of CoFe nanobubbles with a diameter of ~30 nm in the nanocage wall.Figure 1j,l show HRTEM images of the boundary between the carbon matrix and CoFe nanobubble, where lattice spacings of 3.40 and 2.02 Å correspond to NC layers (002) plane [1,15] and CoFe alloy (110) plane, [16,17] respectively.Figure 1k illustrates the crystal structure of the formed metallic CoFe and Figure 1m shows the energy dispersive X-ray spectroscope (EDS) Scheme 1. Schematic illustration of the formation process of CoFe@NC/WC.Energy Environ.Mater.2024, 7, e12499 mappings of a CoFe@NC nanocage, which suggests the homogeneous distribution of C, N, O, Co, and Fe in the nanocage.
Figure S5, Supporting Information, shows XRD patterns of spruce pyrolyzed at different temperatures.Two broad diffraction peaks at ~24°and 43°correspond to the (002) and (100) diffraction modes of graphite, respectively. [18]With increasing temperature, the (002) diffraction peak is red-shifted with a slightly increased intensity, which indicates a decrease in the interplanar space of the (002) planes. [19]The XRD patterns of CoFe@NC/WC are displayed in Figure 2a, where all diffractions are perfectly consistent with the CoFe phase (PDF#49-1567). [16]Figure 2b shows the Raman spectra of the samples.The ratio of D band to G band (I D /I G ) of CoFe@NC/WC is found to decrease with increasing temperature, indicating that high temperature is vital for the growth of graphitic carbon.The highest I D /I G ratio in WC Energy Environ.Mater.2024, 7, e12499 reveals the importance of CoFe in forming more graphitized carbon. [20]he characteristic 2D band at ~2691 cm −1 of the nanostructured carbon (partially graphene) is observed in CoFe@NC/WC-850 and CoFe@NC/WC-950, indicating higher graphitization and conductivity. [3,21]igure 2c presents the X-ray photoelectron spectroscopy (XPS) survey spectra of CoFe@NC/WC, which reveal the presence of Co, Fe, O, N and C, being consistent with the EDS results (Figure S7, Supporting Information).The N1s spectra of CoFe@NC/WC in Figure 2d can be deconvoluted into three main peaks of pyridinic N (398.10eV), pyrrolic N (399.04eV), and graphitic N (400.40eV).[22] As the pyrolysis temperature reaches 950 °C, a strong signal at 401.90 eV associated with oxidized N (N-O) in CoFe@NC/WC-950 is observed.This can be explained by partial oxidation of pyridinic-N into pyridine N-O at such high temperature (950 °C), as shown in Figure S8, Supporting Information.The percentage of pyridinic-N in each sample is provided in Figure S9, Supporting Information, where CoFe@NC/WC-850 shows the highest pyridinic-N content (21.04%).While the presence of pyridinic N is believed to improve oxygen adsorption and reduction processes through electronic modulation of adjacent carbon atoms, graphitic N is reported to increase conductivity, thereby enhancing electron transport.[3,23,24] Compared to bare WC, CoFe@NC/WC shows C-N bonds at 285.50 eV in the C1s spectra, which verifies N doping (Figure 2e). Figure 2f shows the Co2p spectra of CoFe@NC/WC.The split peaks at ~781.70 and 796.75 eV correspond to 2p 3/2 and 2p 1/2 , respectively, while the peak at 779.79 eV binding energy is assigned to zero-valent Co in CoFe alloy.[25] Analogously, the Fe2p spectra can be deconvoluted into three-pair peaks (Figure 2g).The peaks located at ~711.42 and 724.38 eV can be ascribed to 2p 3/2 and 2p 1/2 , respectively.[26] The peaks at ~707.56 and 719.78 eV originate from metallic Fe. [27] The ratios of Co 0 /Co δ+ and Fe 0 /Fe δ+ in their 2p3/2 regions for CoFe@NC/WC are provided in Table S1, Supporting Information, where CoFe@NC/WC-750 shows the smallest ratio (0.125) with CoFe@NC/WC-850 and CoFe@NC/WC-950 showing close ratios (0.589 and 0.676), indicating higher crystallinity of CoFe alloy at higher pyrolysis temperature.The content of CoFe in CoFe@NC/WC-850 as measured by thermogravimetric analysis (TGA) is ~2.25   3b) and electrochemical impedance spectroscopy (EIS). In Fgure 3c, CoFe@NC/WC-850 shows the smallest radius illustrating the fastest charge transfer among all samples, confirming its superior electrocatalytic performance.Koutecky-Levich (K-L) plots (Figure 3d) were used to study the electron transfer number (n) during ORR.Inset of Figure 3d shows a quasi-four-electron transfer which manifests a nearly complete conversion from O 2 to OH − . [28]By contrast, the electron transfer number for WC is only 2.64-3.08,which is far below CoFe@NC nanocages (Figure S11, Supporting Information).Figure 3h shows ORR stability evaluation of CoFe@NC/WC-850 and Pt/C, where CoFe@NC/WC-850 demonstrates superior stability to Pt/C with 82.0% current retention after 29 000 s of continuous chronoamperometry (CA) at a high potential of 0.79 V versus reversible hydrogen electrode (RHE).Accordingly, CoFe@NC/WC-850 exhibits a smaller E 1/2 loss (14 mV) than Pt/C (17 mV), confirming its superior longterm ORR stability (Figure S12, Supporting Information).
We have further evaluated the OER capabilities of CoFe@NC/WC in comparison with commercial RuO 2 on WC in 0.1 M KOH.While RuO 2 /WC shows the lowest onset potential, its catalytic kinetics is slow with a limited current density of ~50 mA cm −2 at the end of the polarization profile (Figure 3e).On the other hand, CoFe@NC/WC-850 presents the lowest overpotential of 315 mV at 10 mA cm −2 and Tafel slope of 57.6 mV dec −1 along with superior kinetics (Figure 3f).The Tafel slope of RuO 2 /WC reaches 245.0 mV dec −1 due to binderinduced resistance, which in turn proves the merits of the in-situ formation of CoFe@NC in WC.The electrochemical active surface area (ECSA) of catalysts is calculated based on double layer capacitance (C dl ) as shown in Figure 3g and Figure S13, Supporting Information. [29]ncouragingly, WC exhibits a high C dl of 17.1 mF cm −2 , resulting from the high surface area of wood and laying the foundation for high ECSA of RuO 2 /WC and CoFe@NC/WC.Compared to RuO 2 /WC (39.0 mF cm −2 ), CoFe@NC/WC-950 (27.1 mF cm −2 ) and CoFe@NC/WC-750 (22.7 mF cm −2 ), CoFe@NC/WC-850 manifests the highest ECSA (40.9 mF cm −2 ), implying more accessible reactive sites. [30]The specific activity curves normalized by ECSA confirmed the much higher intrinsic activity of CoFe@NC/WC-850 (Figure S14, Supporting Information).Chronopotentiometry (CP) response of CoFe@NC/WC-850 displays negligible increase while maintaining the lowest potential throughout 24 h of operation among all CoFe@NC/ WC samples (Figure 3i).The LSV curve of CoFe@NC/WC-850 shows Energy Environ.Mater.2024, 7, e12499 a small change after the CP test (Figure S15a, Supporting Information).Further, the XRD pattern after the OER durability test (Figure S15b, Supporting Information) shows no change in crystallinity, indicating the endurance of CoFe@NC/WC-850 as an OER electrocatalyst.The E 1/2 , E j = 10 , and ΔE of as-designed catalysts are summarized in Figure 3j.CoFe@NC/ WC-850 displays a ΔE of 0.74 V in 0.1 KOH, which is superior to recently reported preeminent electrocatalysts (Table S2, Supporting Information), reflecting superior bifunctional oxygen reversibility and promising potential in rechargeable ZAB.In general, the distinguished bifunctional oxygen catalysis of CoFe@NC/ WC-850 is owed to i) high graphitization of WC substrate and crystallinity of CoFe alloy rendering good electrical conductivity, ii) intrinsic high surface area of WC and hierarchical hollow structures of CoFe@NC enhancing the number of active catalytic centers, and iii) synergetic action between NC and CoFe improving the activity of the reactive sites. [31]ensity functional theory calculations were conducted to further understand the ORR catalytic origin of the CoFe@NC catalyst.34] The calculation starts with *OH adsorption energy (ΔE *OH ) on CoFe(110) surface at six different sites (Figure S16, Supporting Information).From Table S3, Supporting Information, it is found that the most preferable site is OH adsorbed on site b* (Figure 4a and Figure S16b 1 , Supporting Information) with a ΔE *OH of −4.87 eV.This value is comparable with that predicted for Fe(100) (−4.13 eV) [35] but noticeably higher than the values obtained previously for Au(111) (−2 eV) and Au(100) (−2.4 eV), [36] and Pt(111) (−3.3 eV). [37]Finally, ΔG *OH after all corrections was estimated to be −3.77eV (see S1.3.1,Supporting Information).Such a strong adsorption of CoFe(110) can be alleviated by surface modification, therefore ΔE *OH of carbon (C 54 H 18 , pristine graphene) and NC (C 53 NH 18 , pyridinic-N doped graphene) was calculated, where NC has five potential adsorption centers as shown in Figures S17 and S18, Supporting Information.As it can be seen from the results in Table S4, Supporting Information, the preferable center (Figure 4b) shows the highest ΔG *OH of −1.34 eV.As for CoFe@NC, its adsorption orientation remains on the outer layer NC and thus ΔG *OH should be corrected to −0.7 eV due to the alloy-graphene plane separation and C-O bond distance (Table S4, Supporting Information).The reaction activation barriers (ΔE a ) correlated with ΔG *OH can be deduced using the Brønsted relation or Marcus formula as shown in Table S5, Supporting Information.Figure 4c shows the volcano plot for various catalysts as function of *OH binding free energy in alkaline condition.On the left side of the volcano, since CoFe(110) binds oxygen-containing species too strongly, the desorption of *OH to form an OH − anion (the fourth step) is difficult, limiting the rate of ORR. [38]On the right side, incorporating carbon weakens the oxygen-binding strengths of the catalysts to a great extent, and hence the dissociation of O 2 to form *OH (the  Energy Environ.Mater.2024, 7, e12499 third step) controls the reaction pace. [39]This statement rests on the ORR mechanism recently suggested. [36]Note that for the sake of simplicity we did not take into account the effect of orbital overlap [40] on the ΔE a when considering the last ORR step (Eq.S4, Supporting information).For a chemisorbed molecule the orbital overlap decreases the energy barrier and leads, therefore to a less steep slope of the left part of volcano plot (Figure 4c).Among all catalysts, CoFe(110)@NC shows the smallest ΔE a for ORR, which benefits from the incorporated NC (pyridinic N) that optimizes the ΔG *OH of CoFe(110).The moderate ΔG *OH favors efficient forming and removing of intermediate *OH on the CoFe(110)@NC surface and therefore enhances its catalytic activity toward ORR.It should also be noted that CoFe(110)@C and CoFe (110)@NC interfaces can result in an additional enhancement of outersphere electron transfer (i.e., the first step, see Eq. S1, Supporting information) as compared with the CoFe(110) surface. [41]iven its optimal ORR and OER performance, a CoFe@NC/WC-850-based liquid rechargeable ZAB was fabricated (Figure 5a).The battery OCV of 1.430 V is close to the commercial Pt/C + RuO 2 reference of 1.475 V (Figure 5b).Two tandem CoFe@NC/WC-850 batteries with an OCV of 2.871 V can power a board with 77 LEDs (Figure 5b, inset; Figure S19, Supporting Information).Figure 5c compares the charge and discharge polarization profiles of CoFe@NC/WC-850 with Pt/C + RuO 2 .While Pt/C + RuO 2 exhibits a narrower voltage gap at the very beginning, it is predominantly larger than CoFe@NC/WC-850.Consequently, CoFe@NC/WC-850 gives a higher power density of 138.9 mW cm −2 (Figure 5d), and high specific capacity (763.5 mAh g −1 ) and energy density (867.4[44] Galvanostatic discharge curves of the CoFe@NC/ WC-850-based ZAB at various current densities in Figure 5e shows stable and reproduceable voltage plateaus, suggesting excellent discharge stability and rate performance. [45]Further, the cycling stability was evaluated via charging/discharging at 5 mA cm −2 .The CoFe@NC/WC-850-based battery showed higher stability compared to the Pt/C + RuO 2 counterpart (Figure 5f).The initial round-trip efficiency of CoFe@NC/WC-850 is 60.8% (Figure 5g).After 120 cycles, the CoFe@NC/WC-850 maintained a high efficiency of 55.0%, whereas Pt/C + RuO 2 failed at the 100th cycle.At the 240th cycle, the efficiency of CoFe@NC/WC-850 shows a slight decline to 53.9%.The ex-situ XRD pattern of CoFe@NC/WC-850-loaded carbon paper electrode after 240-cycle of galvanostatic discharge-charge (GDC) test was recorded.The result in Figure S21, Supporting Information, shows a clear peak at 44.9°, which is identified as the (110) plane of CoFe (PDF#49-1567), [46] further confirming the electrochemical stability of CoFe@NC/WC-850.
To meet the growing demand for flexible electronics, wearable ZAB were fabricated based on BC-PAA solid-state hydrogel electrolyte.SEM images of BC nanofiber and BC-PAA hydrogel are displayed in Figure S22a,b, Supporting Information.It was observed that BC exhibits a porous framework with ultrafine nanofibers of 100 nm diameter.After radical polymerization, the pores were partially filled and the diameter of the nanofibers increased, suggesting successful wrapping of BC with PAA (Figure S22b, Supporting Information).The BC-PAA hydrogel shows significantly improved electrolyte retention compared to PAA with less salting out after 64 days (Figure S23, Supporting Information).Figure 6a illustrates the structure of BC-PAA hydrogel electrolyte, where PAA chains and BC fibers intertwine with each other, leading to good mechanical properties. [47]Figure 6b shows Fourier transform infrared spectroscopy (FTIR) spectra of as-synthesized gels.The BC and BC-PAA spectra consist of two bands at 1023 and 1110 cm −1 , associated with C-O stretching in C-OH and C-O-C of the anhydroglucose unit in cellulose chain, respectively. [48,49]For BC-PAA and PAA, the intensive band at 1558 cm −1 , accompanied by a weaker symmetric v(C-O) at 1408 cm −1 , is associated with -COO-, which indicates the presence of carboxylate ions. [50,51]wo common bands at 2938 and 3350 cm −1 are assigned to C-H and O-H (hydroxyl) stretching, respectively. [52,53]While PAA hydrogel presents a high strain of 500%, its tensile stress is 0.084 MPa only, which restricts its use in wearable batteries (Figure 6c, inset).On the other hand, the tensile stress is dramatically reinforced by BC nanofiber, where BC-PAA composite shows ultrahigh tensile stress of 1.58 MPa with a breaking strain of 147% (Figure 6c).The prepared BC-PAA hydrogel electrolyte was sandwiched by CoFe@NC/WC-850 cathode and Zn anode to form a wearable ZAB (Figure 6d).The OCV of 1.405 V of CoFe@NC/ WC-850-WZAB is comparable to Pt/C + RuO 2 (1.432 V).It provides a peak power density of 73.9 mW cm −2 , outperforming the Pt/ C + RuO 2 -WZAB (51.0 mW cm −2 ) (Figure 6e).Furthermore, the CoFe@NC/WC-850 WZAB gives a smaller and more stable GDC voltage gap than Pt/C + RuO 2 during a 16 h cycling test (Figure 6f).Additionally, the mechanical flexibility and stability of CoFe@NC/WC-850 WZAB were studied by bending at different angles at a charging/ discharging current density of 3 mA cm −2 .Figure 6g shows no noticeable change in the charge (~1.98 V) or discharge (~1.16 V) platforms.The performance of CoFe@NC/WC-850 WZAB is comparable to that of wearable/solid ZAB based on most recent catalysts (Table S7, Supporting Information).Figure 6h and Figure S24b, Supporting Information, show that two tandem CoFe@NC/WC-850 WZAB with an OCV of 2.838 V can continuously power an electronic timer clock.Further, as shown in Figure 6i and Figure S24c,d, Supporting Information, a mobile phone can be charged by five batteries in series (with a high OCV of 7.13 V) under both flat and bending states.The results demonstrate the wearability of solid ZAB based on BC-PAA hydrogel and CoFe@NC/WC-850 and its application in clothing.

Conclusion
In conclusion, nitrogen-doped carbon nanocage encapsulating CoFe nanobubble on wood carbon (CoFe@NC/WC) catalyst was synthesized by pyrolyzing a novel PBA/spruce precursor, exhibiting bifunctional oxygen reversibility and excellent stability as ZAB catalyst.DFT calculations confirmed that introducing NC onto CoFe surface tunes the adsorption energy of intermediates (e.g., *OH), contributing to the lower reaction activation barriers and thereby boosting the oxygen catalytic activity.Moreover, toward enhanced ZAB wearability, a casting method was used to enhance the tensile breaking stress of PAA hydrogel, where BC nanofiber acted as the backbone.As a result, the BC-PAA composite reached ultrahigh tensile stress of 1.58 MPa with a breaking strain of 147%.WZAB based on BC-PAA hydrogel and CoFe@NC/WC-850 delivered a high power density of 73.9 mW cm −2 and its 5-unit battery pack can charge a mobile phone even under bending state.This study demonstrates a costeffective strategy toward hierarchical TMCcarbon catalysts derived from biomass and further promotes the deployment of ZAB in wearable energy storage.

Experimental Section
Materials: Spruce was sawed into chips of 20 × 20 × 2 mm 3 (R × T × L).Bacterial cellulose (BC) was cultivated in lab and freeze-dried before use.
Synthesis of CoFe@NC/WC: 0.22 g of Co (NO 3 ) 2 Á6H 2 O and 0.33 g of sodium citrate dihydrate were solved in 30 mL deionized (DI) water.Then, wood chips were soaked in the above solution.20 mL solution containing 0.165 g K 3 [Fe(CN) 6 ] was quickly poured into the above solution with stirring for 10 min.After that, the obtained mixture was incubated for 24 h at RT.After washing several times with DI water, the CoFe-PBA/spruce precursor was dried at 60 °C overnight.The precursor samples of CoFe-PBA/spruce were pyrolyzed at 750 °C, 850 °C, and 950 °C for 2 h with a ramp rate of 5 °C min −1 in N 2 , respectively.The asprepared samples were immersed into large volume of DI water for another 2 h and dried at 60 °C overnight to get CoFe@NC/WC-T (T = 750, 850, 950).
For comparison, spruce chips derived WC was synthesized using the same pyrolytic procedures as that of CoFe@NC/WC-850.
Materials characterization: The morphology and microstructure evolution were acquired on SEM (ZEISS EVO MA10) and TEM (Tecnai G2 F30).Elemental mapping images were also collected using the same TEM equipment with an EDS.The crystal structure for all samples was studied by XRD (PANalytical X'pert3 Powder).Raman spectra were collected on a Renishaw RM3000 Raman spectrometer with 532 nm wavelength incident laser light.The surface chemical states were identified using XPS (Thermo Scientific NEXSA).All spectra were calibrated to C 1 s peak to 284.6 eV.The FTIR was conducted on Shimadzu IRAffinity-1 spectrometer.The tensile properties of the BC-PAA hydrogel were measured on an Instron 1185 uniaxial machine.TGA of the sample was examined using Netzsch STA 449 F3 ranging from 30 to 1000 °C at a heating rate of 20 °C min −1 in air followed by keeping at 1000 °C for 20 min.
Electrochemical measurement: All the measurements were performed with a CHI760E electrochemical workstation using a typical three-electrode configuration in 0.1 M KOH solution.The as-obtained catalysts acted as working electrode, while saturated calomel electrode (SCE) as reference electrode and Pt mesh as counter electrode, respectively.For ORR test, the electrolyte was O 2 -purged 0.1 M KOH.Prior to use, glassy carbon rotating disk electrode (RDE, 4 mm in diameter) was polished carefully with alumina powder, rinsed with DI water, and finally sonicated in ethanol.All catalysts were prepared by mixing 5 mg of ground catalysts in 1 mL of solution containing 550 μL of ethanol, 425 μL of DI water and 25 μL of 5% Nafion solution, and then sonicating for 0.5 h to form homogeneous catalysts inks.The obtained catalysts inks were then dropped on the pretreated RDE surface and dried before the electrocatalytic tests, leading to 0.4 mg cm −2 loading for all catalysts.The RDE (working electrode) tests were measured at various rotating speeds from 0.36 to 12.9 × g with a scan rate of 10 mV s −1 .For the ORR at the RDE, the transferred electron number was determined by the K-L equation: where j is the measured current, j k is the kinetic current, j l is the limiting current, and ω is the electrode rotation rate (rpm).The theoretical value of the Levich slope (B) is evaluated from the equation below: where n is the overall transferred electron number in the ORR process, F is the Faradaic constant (96 485 C mol −1 = 96 485 000 mAs mol −1 ), C o is the oxygen concentration in 0.1 M KOH (1.26 × 10 −6 mol cm −3 ), C o is the oxygen diffusion coefficient in 0.1 M KOH (1.93 × 10 −5 cm 2 s −1 ) and v is the kinematic viscosity of the 0.1 M KOH (1.09 × 10 −2 cm 2 s −1 ). [44,54,55]EIS was performed at 0.81 V versus RHE within a frequency range of 0.1-1 M Hz.
The ORR stability of catalysts was evaluated by CA at 0.79 V versus RHE at 1600 rpm.
For the OER test, the as-obtained catalysts acted as monolithic electrodes directly and the LSV curves were obtained at a sweep rate of 5 mV s −1 (90% iRcompensated).The C dl of the samples was measured using cyclic voltammograms (CVs) in the potential range of 0-0.2 V versus SCE at different sweep rates from 20 to 100 mV s −1 .The OER stability of catalysts was evaluated by CP under 10 mA cm −2 over 24 h.All the potentials from SCE were converted into RHE scale using the Nernst equation: E RHE = E SCE + 0.0592*pH + 0.242.
Assembly of liquid Zn-air battery: The liquid ZAB were tested in a homebuilt electrochemical cell.The homogeneous catalyst ink was loaded on carbon paper with a mass loading of ~1.5 mg cm −2 , as the air cathode, and a polished Zn foil was used as the anode.A 0.2 M Zn(Ac) 2 in 6.0 M KOH aqueous solution was used as the electrolyte.GDC cycling was recorded on LANHE-CT3001A.
Fabrication and measurement of wearable Zn-air battery: Before fabrication of WZAB, BC-PAA hydrogel was synthesized.Typically, a concentrated NaOH solution (2.5 mL, 20 M) was dropwise added into an aqueous solution of acrylic acid (AA) monomer (3.6 mL of AA, 99%, mixed with 5 mL H 2 O) under stirring.2 mg of N,N 0 -methylenebisacrylamide (MBAA, 99%) was used as chemical crosslinker, and 55 mg potassium persulfate (KPS, 99%) was used as the initiator.Both were added to the neutralized solution and stirred for 0.5 h at RT. Subsequently, freeze-dried BC membrane absorbed above mixture moderately and was then degassed to remove dissolved air.Free-radical polymerization was initiated at 60 °C for 2 h.Finally, the obtained BC-PAA was peeled off and then soaked in 0.2 M Zn(Ac) 2 with 6.0 M KOH solution overnight.Afterward, BC-PAA was wipe-dried with paper tissues and used as electrolyte in the following tests.The BC-PAA hydrogel was first attached on Zn foil.Then, carbon cloth loaded with catalyst (~1.5 mg cm −2 ) was attached on the electrolyte side and the "sandwich" was wrapped by 3 M micropore paper tape.All tests were performed on CHI760E at RT.
Computational details: Theoretical calculations of CoFe, NC, and CoFe/NC interfaces are provided in the Supporting Information.

Figure 1 .
Figure 1.a) SEM image and b) magnified SEM image of WC. c) Cross-sectional SEM image (inset is the magnified view) and d-f) internal SEM images of CoFe@NC/WC-850.g) TEM image and h, i) magnified TEM images of CoFe@NC nanocage; j, l) HRTEM images of CoFe nanobubble and k) the crystal structure for the formed CoFe alloy; m) EDS mapping including C, N, O, Co, and Fe elements in CoFe@NC nanocage.

Figure 5 .
Figure 5. a) Schematic of liquid ZAB.b) OCV plots (inset is the photograph of tandem battery powering a display with 77 LEDs), c) charge and discharge polarization curves, and d) power density curves for ZAB based on CoFe@NC/WC-850 or Pt/C + RuO 2 .e) Galvanostatic discharge tests for CoFe@NC/WC-850 battery with various current densities.f) Cycling profiles at 5 mA cm −2 with 20 min per cycle, and g) cycling performances at 1st, 120th, and 240th cycles.

Figure 6 .
Figure 6.a) Schematic illustration of BC-PAA hydrogel electrolyte.b) FTIR spectra of BC, BC-PAA and PAA gels.c) Tensile stress-strain curves of as-synthesized BC-PAA and PAA (inset is the enlarged view of PAA).d) OCV curves (inset is the schematic structure for wearable ZAB), e) charge/discharge polarization profiles and power density plots for ZAB based on CoFe@NC/WC-850 and Pt/C + RuO 2 .f) Cycling profiles at 2 mA cm −2 .g) Galvanostatic charge/discharge cycle tests under different bending states at 3 mA cm −2 with 10 min per cycle.Photographs of h) electronic timer powered by two tandem batteries and i) a mobile phone charged by five arm-wrapped batteries connected in series.