Synthesis and characterization of Co SA-NDGs. The synthetic procedure of Co SA-NDGs was described in Supplementary Fig. 1, including electrostatic adsorption, hydrothermal treatment, subsequent pyrolysis and acid leaching. Scanning electron microscopy (SEM, Supplementary Fig. 2) image shows that Co SA-NDGs possesses a 3D interconnected porous architecture with a highly wrinkled surface, delivering a specific surface area of 396.2 m2 g−1. The small mesopore centered at about 2~4 nm is conductive to balance the electrolyte permeation and the ion diffusion27. X-ray diffraction (XRD) pattern and Raman spectra suggest a lower graphitization degree of Co SA-NDGs (Supplementary Fig. 3 and 4, respectively)9,16. Meanwhile, a strong electron paramagnetic resonance (EPR) signal affirms the presence of plenty of defects (e.g., dangling bonds and vacant atom sites) (Supplementary Fig. 5). Transmission electron microscopy (TEM) image in Fig. 1a shows that the graphene nanosheet is highly curved. The high-curvature carbon lattices and the wrinkled nanosheets are clearly observed for Co SA-NDGs in high-resolution TEM images (Fig. 1b and Supplementary Fig. 6a-c). The ring-like selected area electron diffraction (SAED) pattern signifies the poor crystallinity. The nanosheet structure is further confirmed by atomic force microscopy (AFM) analysis with an average thickness of 3.24 nm (Supplementary Fig. 7). Aberration-corrected high angle annular dark-field scanning transmission electron microscope (AC HAADF-STEM) image distinctly detects the monodispersed bright dots (marked by yellow cycles) at atomic level (Fig. 1c), corresponding to the atomically dispersed Co atoms. The elemental mapping images of Co SA-NDGs (Supplementary Fig. 6d) verify the uniform distribution of Co, N, O and C species.
The electronic structure and local atomic configuration of Co SA-NDGs were studied by X-ray photoelectron spectroscopy (XPS) in Supplementary Fig. 8. The N 1s spectrum (Fig. 1d) is divided into pyridinic-N (398.2 eV), Co-Nx (399.1 eV), pyrrolic-N (399.9 eV), graphitic-N (400.1 eV), oxidized-N (402.1 eV), respectively. The abundant defects and nanopore make more N atoms occupy the edges, resulting in the formation of high percentage of pyridinic-N28. For the N-doped graphenes (NDGs), the positive shift (ཞ0.2 eV) of C-N bond in C 1s spectrum (Fig. 1e) of Co SA-NDGs is associated with a strong charge transfer effect between single Co atoms and curved graphene supports. The decreased electron density of C atoms and the atomic Co-Nx-C interface effectively facilitate the adsorption of intermediates and tailor the reaction energy barrier29. The loading content of Co is determined as 0.14 wt.% by inductively coupled plasma optical emission spectrometry (ICP-OES). Furthermore, the Co K-edge of Co SA-NDGs in X-ray absorption near-edge structure (XANES) exhibits an increased white-line intensity compared to that of Co foil (Supplementary Fig. 9), suggesting the valence state of Co atoms is positive. The extended X-ray absorption fine structure (EXAFS) spectra (Fig. 1f) display two peaks located at ~1.4 Å and 1.9 Å respectively, which can be assigned to Co-N and Co-C scattering paths30,31. The coordination number of Co-N is ཞ3.8 (Supplementary Table 1), suggesting the dominant Co-N coordination in Co SA-NDGs is likely to be Co-N4 structure. Besides, no peak of the metallic Co-Co bond or other high-shell peaks for Co SA-NDGs is detected compared with Co foil, confirming the Co atoms are atomically dispersed.
To deeply understand the evolution process of atomically dispersed Co atoms during pyrolysis, in-situ temperature-dependent TEM investigations and molecular dynamics (MD) simulations were performed. The structural evolution was timely monitored in the temperature window from 25 to 600 oC (Fig. 1g). In our case, the reduced graphene oxide (rGO) becomes less crystallization as the temperature rises to 600°C, which may be ascribed to the heteroatom doping effect32,33. More interestingly, the related Co-based nanoparticles or clusters do not appear. Furthermore, based on our previous model34, we place Co dimers on tetrapyridine N-doped carbon sites that act synergistically with vacancy defects to examine possible changes at 800 oC. The results show that two Co single atoms do not form a stable Co dimer (Fig. 1h). The Co-Co bond of the dimer gradually becomes longer until it is broken. The Co connected to the tetrapyridine N automatically forms a typical planar Co-N4 configuration, while other Co atom will become more and more away from the Co-N4 coordination. This result fully proves that stable Co dimers are not easy to form, but due to the synergistic effect of charge transfer and vacancy defects, the typical planar four-coordinate configuration of Co-N4 is formed. Simultaneously, the escaped Co atom may be captured by the doped N species or graphene defects until it is stabilized. The entire evolution process with more sets of images is supplemented (Supplementary Movie S1). These experimental observations and MD simulations provide solid evidences of the evolution process for the atomically dispersed Co-N4 structure during synthesis.
Electrocatalytic O 2 performance and DFT calculation. The O2 electrocatalytic activities of Co SA-NDGs, NDGs and commercial Pt/C are comprehensively evaluated in 0.1M KOH solution. Linear sweep voltammetry (LSV) curves (Fig. 2a) exhibit that Co SA-NDGs has an excellent ORR activity with the onset potential (Eonset) of 1.02 (V vs. RHE), superior to that of Pt/C (0.96 V). At the potential of 0.85 V, Co SA-NDGs reaches a kinetic current density (Jk) of 11.7 mA cm−2 (Fig. 2b), ~ 2.18 times higher than that of Pt/C (5.36 mA cm−2). The rapid ORR kinetic of Co SA-NDGs is supported by a small Tafel slope of 54 mV dec−1 (Supplementary Fig. 10b). The fitted Koutecky-Levich (K-L) plots calculated from the LSV curves show a good linearity (Supplementary Fig. 11), signifying that the Co SA-NDGs mainly follow 4e− ORR pathway35,36. The accelerated degradation test of Co SA-NDGs was performed by cycling catalyst between 0.6 and 1.0 V over 5000 continuous cycles (Supplementary Fig. 12a), showing little negative shift in E1/2 (∼11 mV). After injecting methanol into the electrolyte, almost no disturbance in current response is found for Co SA-NDGs, whereas a sharp decline is observed for Pt/C (Supplementary Fig. 12b). The Co species are still well distributed after durability test, evidenced by TEM and mapping results (Supplementary Fig. 13). These results confirm the remarkable ORR stability and methanol tolerance of Co SA-NDGs.
Then, the electrocatalytic OER behavior of Co SA-NDGs was also assessed. An overpotential of 350 mV for Co SA-NDGs is required to reach current density of 10 mA cm−2 (Fig. 2c), which is comparable to that of Ir/C (345 mV) and RuO2 (338 mV) but lower than that of NDGs (430 mV). Moreover, the rapid OER kinetics are elucidated by Tafel slope and electrochemical impedance spectroscopy (EIS) (Supplementary Fig. 14). Specifically, the activation energy (Ea), which is obtained from the slope of the plotted log (j) vs. 1/T, is a vital criterion for scaling the difficulty of catalytic reaction37. The experimentally measured activation energy (Ea) of Co SA-NDGs and NDGs are 28.6 and 44.0 KJ mol−1 (Fig. 2d and Supplementary Fig. 15), respectively, suggesting the favorable thermodynamic OER behavior of Co SA-NDGs. The electrochemical active surface area (ECSA) can be estimated from double layer capacitance (Cdl) because of a linear relationship34,36. The measured Cdl vale are 26.3 and 20.6 mF cm−2 for Co SA-NDGs and NDGs (Supplementary Fig. 16), corresponding to the ECSA of 657.5 and 515.0 cm−2, respectively. The high ECSA of the Co SA-NDGs represents the sufficient exposure of active sites. Generally, the bifunctional activity is evaluated by ΔE (ΔE=EOER:j=10-EORR:1/2). We use E1/2 and E10 index as the horizontal and vertical axis for better comparison, where the point located at the upper right demonstrates a better bifunctional capability38,39. The ΔE of Co SA-NDGs is only 0.71 V, ranking the top level among non-precious metal SACs and metal nanoparticle-based bifunctional catalysts (Fig. 2e and Supplementary Table S2).
The crucial role of curvature on atomic Co-N4-C system were studied using first-principles calculations based on DFT40 (Fig. 2f). Fig. 2g gives the calculated differential charge densities for the planar Co-N4 and curved Co-N4 model to show the charge transfer difference. The curved Co-N4 shows more localized charge densities and a larger charge gradient than the planar Co-N4, which is believed to facilitate the subsequent O2 activation41. Density of states (DOS) analysis (Fig. 2h) show that the d-band center of curved Co-N4 is downshifted compared to that of planar Co-N4, indicating that the adsorption of oxygenated intermediates is lowered16. Especially, the ORR activity of bifunctional electrocatalyst determines the energy conversion efficiency to a great extent. The free energy pathways of two models are both downhill on the four-electron ORR processes at U = 0 V, revealing that each elementary step can be carried out spontaneously (Supplementary Fig. 17). At U = 1.23 V, the rate-determining step is the first protonation of the adsorbed OOH* species. The ORR overpotential of curved Co-N4 site under alkaline condition is calculated to be 0.758 eV, which is lower than that of planar Co-N4 site (0.90 eV), suggesting the beneficial effect of graphene curvature for optimizing the ORR activity. Meanwhile, the stable adsorption configuration of intermediates is provided (Supplementary Fig. 18).
Aqueous ZABs with Co SA-NDGs
Then the aqueous ZABs with Co SA-NDGs and Pt/C as air cathode were assembled (Supplementary Fig. 19), in which the ambient air acted as the cathode active agent42. The open circuit voltage (OCV) and maximum power density of Co SA-NDGs are 1.53 V and 251.4 mW cm−2 (Supplementary Fig. 20a and b), respectively, which are significantly higher than these of benchmark Pt/C-based aqueous ZABs (1.48 V and 177.0 mW cm−2). The hierarchically macro/mesoporous structure is believed to facilitate rapid ion transfer and electron transport at high voltage region, thus boosting discharge ability (Supplementary Fig. 21). The Co SA-NDGs cathode delivers a high specific capacity of 757.4 mAh g−1 and an energy density of 956 Wh kg−1 at 10 mA cm−2 in Supplementary Fig. 20c, outperforming that of Pt/C-based counterpart (630.5 mAh g−1, 788 Wh kg−1). The superior rate capability is further assessed by comparing the discharge voltage platform at series of current densities (Supplementary Fig. 20d). The Co SA-NDGs cathode enables high voltage platform especially at high discharge depth over 50 mA cm−2. Notably, the ZABs with Co SA-NDGs could discharge more than 240 h at 100 mA cm−2 (Supplementary Fig. 20e). The instable interface induced by the generated Zn dendrites may account for the voltage loss at initial stage (Supplementary Fig. 20f). Unlike the Pt/C+RuO2-based ZABs with significant voltage decay, a robust discharging/charging stability (300 h, every cycle of 22 min) for Co SA-NDGs-based ZABs is achieved (Supplementary Fig. 22). Conclusively, the superior performance (e.g., maximum power density, specific capacity and cycling time) which is among the top level reported so far (Supplementary Table 3), mainly originate from atomically dispersed Co-N4 sites with curved nanostructure and the hierarchically interconnected structure of graphene architecture.
Quasi-solid-state ZABs with Co SA-NDGs
Here, we firstly synthesize and examine the physicochemical properties of PAM hydrogel electrolyte (Supplementary Fig. 23). The quasi-solid-state ZABs with Co SA-NDGs using PAM hydrogel electrolyte show a maximum power density of 219.9 mW cm−2 and a stable discharge voltage platform when varying discharging current density (2, 5 and 10 mA cm−2). The discharging voltage in rate performance is slightly higher than that of cycling measurement, which may be attributed to the interface difference43,44. However, the high-rate performance is still challenging because of the formation of dense Zn dendrites and the destruction of Zn/hydrogel electrolyte interface (Supplementary Fig. 24 and 25). Therefore, modulating the intrinsic properties of hydrogel electrolyte is particularly vital to alleviate these issues for achieving high-rate capability at ambient condition.
As known, DMSO is a favorable H-bond acceptor to form strong H-bond network with water molecules (Fig. 3a), reconstructing the solvation Zn2+ solvation sheath structure. On the one hand, the side reaction activity in aqueous electrolytes is obviously suppressed (Supplementary Fig. 26)26, 45–47. In situ optical visualization observation reveals a smooth interface of Zn/electrolyte (Fig. 3b and c) when adding DMSO into electrolyte, whereas the severe Zn dendrites appeared in DMSO-free electrolyte48. The PAM organhydrogel electrolyte synthesized in DMSO/H2O binary solvent systems retains a good stretchability (Supplementary Fig. 27). SEM images (Fig. 3d) reveal an interconnected porous structure, facilitating the electrolyte trapping and fast migration of Zn2+ ion diffusion during electrochemical reactions. In Fig. 3e, the ionic conductivities of the PAM organhydrogel electrolyte shows a high conductivity at room temperature (0.26 S cm−1) and even at -40, -60 and 60°C (0.04, 0.0087 and 47.56 S m−1, respectively), illustrating an efficient broad temperature adaptability. In order to investigate the compatibility between Zn anode and organhydrogel electrolyte, Zn/Zn symmetric batteries were constructed. The Zn/Zn symmetric batteries with PAM organhydrogel electrolyte displays a more durable stripping/plating cycles over 200 h in contrast to the PAM hydrogel electrolyte (Fig. 3f), suggesting a stable electrochemical interface at room temperature. Meanwhile, ex XRD patterns (Supplementary Fig. 28) display the relatively weak intensity of formed Zn dendrites (ZnO) when using PAM organhydrogel electrolyte. These results indicate that the introduction of DMSO effectively alleviates Zn dendrites and improves Zn/gel electrolyte interface stability via modulating the H-bond network of organhydrogel electrolyte. Notably, the quasi-solid-state ZABs with Co SA-NDGs using PAM organhydrogel electrolyte present a robust charging/discharging cycle at 50 and 100 mA cm−2 (Fig. 3g). A statistical big data analysis (Fig. 3h and Supplementary Table 4) highlights this groundbreaking result at high cycling rates, implying the promising application.
Furthermore, considering the practical demand of rechargeable batteries in cold regions, highland, etc., there is an urgent need to develop ultra-low-temperature (<-40 oC) quasi-solid-state ZABs. As far as we know, ZABs worked below -40°C is seldomly reported owing to the pronounced increase in interfacial and charge-transfer resistance when operating temperature drops from 25 to -60°C (Supplementary Fig. 29). The slow ion transport in ultra-low-temperature environment limits the depth of discharge and leads to low critical current density49. Then, the temperature-tolerance abilities of PAM organhydrogel electrolyte are further rationalized by the differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA)50,51. As shown in Fig. 4a, the PAM organhydrogel electrolyte remains transparent at -40 oC and further transforms into an opaque slurry gel at -60 oC. DSC curve shows that the freezing point of the PAM organhydrogel electrolyte is less than -70 oC without the emergence of an exothermic peak. Apart from the strong inter-molecular hydrogen bonds between DMSO and H2O, the binding energy (Eb) between the water and terminal group of PAM organhydrogel electrolytes also contributes to lower the solid-liquid transition point21,52−54. Although the Eb (-0.268 [email protected]) of terminal acylamino group with neighboring water molecules via the dipole-dipole interaction is slightly higher than that of two water molecules (-0.246 [email protected]), the Eb of alkalified acylamino group with water molecule (A-PAM-W) increases substantially to -0.379 eV (Fig. 4b). The stronger Eb represents the lower the freezing point and the better the ion migration rate. The symmetric Zn/Zn battery with the PAM organhydrogel electrolyte exhibits a stable Zn plating/stripping process over 500 h at -60°C (Supplementary Fig. 30). A rough surface and dendrite-free morphology of the cycled Zn plate (Fig. 4c) contributes to cycling stability55. Additionally, the strong inter-molecular H-bonds between DMSO and H2O result in a decrease in the saturated vapor pressure of H2O molecular, preventing the evaporation of H2O at elevated temperatures. The DMA result shows that the glass transition temperature (Tg) of the PAM organhydrogel is 125°C (Supplementary Fig. 31). These results are indicative of anti-freezing and thermally stable properties for the as-synthesized PAM organhydrogel electrolyte.
At -40°C (Supplementary Fig. 32), the critical current density of the assembled quasi-solid-state ZABs is 2 mA cm−2 under steady-state discharging test. The specific capacity still reaches 778.4 mAh g−1 at 2 mA cm−2 at -40°C, corresponding to an energy density of 918.5 Wh kg−1. A comparison between the energy density and operating temperature of the fabricated quasi-solid-state ZABs and other low-temperature batteries previously reported (Fig. 4d and Supplementary Table 5), suggests the intrinsic advantage of ZABs for low-temperature energy storage. With further decrement to -60°C, the discharge voltages at 0.1, 0.5 and 1.0 mA cm−2 are 1.30, 1.25 and 1.18 V (Fig. 4e), respectively. Fig. 4f exhibits the charging/discharging cycles at 0.5 and 1.0 mA cm−2. The long-cycle durability with capacity retention over 90% is achieved at -60°C. To the best of our knowledge, this is the record of lowest operation temperature of ZABs reported so far. Moreover, this quasi-solid-state ZABs also operate well at from 20 to 60°C (Supplementary Fig. 33). At elevated temperature of 60°C, the maximum power density is 285.7 mW cm−2 with the average discharge voltages of 1.26 V @ 10 mA cm−2 and 1.23 V @ 20 mA cm−2, respectively. The remarkable cycling stability is also recorded without significant decay after 60 h.