Brief introduction of the results
Recently,Academician Huang Wei and Professor Han Yunhu of Northwestern Polytechnical University (corresponding author).Our research group prepared a Cl-doped metal-free porous electrocatalyst (MF-PCLNC) by physical grinding and pyrolysis. The electrocatalyst exhibits good oxygen reduction reaction (ORR) activity in the alkaline electrolyte (E12 = 0.).91 v vs.RHE) and high stability, surpassing most reported transition metal carbon-based electrocatalysts and comparable to commercial PT C electrocatalysts. In addition, the electrocatalyst exhibits best-in-class ORR activity and stability in acidic electrolytes. Experimental and theoretical calculations suggest that the better ORR activity may be due to the introduction of Cl to promote the increase of Sp3 hybrid carbon, and the combination of Sp3 hybrid carbon and Cl changes the electronic structure of N-adjacent carbon as active sites, while NaCl molten salt etching provides a rich path for electron proton transport.
In addition, when the MF-PCLNC electrocatalyst is used as the cathode of the liquid-rechargeable ZAB, it exhibits excellent performance with a peak power density of 27688 mw·cm-2。In this paper, the flexible quasi-solid-state rechargeable ZAB constructed by MF-PCLNC electrocatalyst showed satisfactory performance at low temperature, high temperature and room temperature. This study provides a highly efficient oxygen reaction electrocatalyst for ZAB batteries and demonstrates its excellent performance under different conditions.
Background:
Zinc-air batteries (ZABS) are considered to be one of the most promising next-generation energy storage conversion technologies due to their safety, zero pollution, high energy density and low cost. The flexible quasi-solid-state ZABS developed on the basis of traditional liquid ZABS has the advantages of high safety and convenient preparation, and provides energy guarantee for the promotion and development of portable, wearable and flexible electronic devices such as smart clothing, smart bracelets, and scroll screens. However, due to the slow kinetic process of the air cathode oxygen reaction, the actual efficiency, discharge power, and cycling stability of ZABS are limited, hindering its wide range of commercial applications. In addition, electrocatalysts with high-efficiency oxygen reactions are still dominated by high-efficiency oxygen reactions, and their scarcity, grid, and limited stability make it difficult to be widely used in rechargeable ZABS. Although many non-based electrocatalysts have been developed to exhibit high catalytic activity and good stability, their activity and stability are not as expected due to the easy shedding of their metal active centers. Therefore, the design and development of efficient, stable, and low-cost cathode oxygen reaction electrocatalysts are essential to advance the further commercial application of rechargeable ZABS.
**Reading guide
Figure 1Catalyst preparation and morphology characterization
Specifically, ZIF-8 is physically mixed and ground with NaCl, then calcined at high temperatures and washed to remove residual NaCl. During the calcination process, in addition to the accompanying volatilization of Zn atoms and the embedding of Cl atoms, molten NaCl also etched the carbonized Zif-8 backbone during this period, resulting in a large number of defects and pores. Transmission electron microscopy (TEM) images show that the MF-PCLNC electrocatalyst has a defect morphology, which almost maintains the dodecahedral structure of ZIF-8 with an average size of 300-500 nm and uniform size distribution. However, the TEM image of the MF-NC electrocatalyst only shows the microporous dodecahedral structure of ZIF-8 after carbonization. Based on the powder X-ray diffraction (PXRD) results, the authors observed no other diffraction peaks, indicating the absence of metal nanoparticles, which is consistent with the phenomenon observed by transmission electron microscopy. Energy Spectrum (EDS) elemental mapping measurements show that the C, N, and Cl elements are evenly distributed. Inductively coupled plasma mass spectrometry (ICP-MS) was used to determine the Zn content of 156%。
Figure 2Catalyst structure characterization
Compared with MF-NC electrocatalysts, MF-PCLNC electrocatalysts had a higher D3 g value (D3 G=1.).01), which means that intrinsic defect densities similar to sp3 hybrid carbon are higher. The high-resolution spectra of C1S can be fitted to three peaks with binding energies of 2848 ev、285.6 ev and 2893 EV, which is attributed to SP2 C, SP3 C, and C-N Cl, respectively. In addition, pyridine N, pyrrole N, and graphite N in the MF-PCLNC electrocatalyst were in the range of 3983 ev、4000.5 ev and 403There are three high-resolution spectral peaks at 2 EV. The results showed that the pyridine N content of MF-PCLNC was higher than that of MF-NC electrocatalyst. The k-margin spectrum of c shows the presence of four characteristic peaks of *c-c, c-n-c, c-cl antibond orbital, and *c-c. 284.The characteristic peak at 5 EV can be interpreted as a defect in the carbon lattice, which can reflect the conformation of the sp3 hybrid carbon. 284.The decrease in peak intensity at 5 eV indicates that the integrity of the aromatic ring *c=c state is broken, 309The increase in peak intensity at 2 EV is attributed to the increase in sp3 hybrid carbon.
In addition, there is a significant negative shift in the C-K spectrum of the MF-PCLNC electrocatalyst, which is due to the transition of the assigned C1S electrons to the unoccupied * state, indicating the presence of the sp3 hybrid carbon conformation. The results show that the introduction of CL can induce more topological carbon defects. The n-k spectrum shows four peaks corresponding to the pyridine state n (399.) in the MF-PCLNC electrocatalyst3 eV), pyrrole N (4002 EV), graphite state N (4021 ev) and *state (408.)2 ev)。The results showed that the characteristic peak intensity of pyridine N in the MF-PCLNC electrocatalyst increased significantly, indicating that the relative content of pyridine N increased, which was consistent with the results of N 1S high-resolution spectroscopy. Pyrrole N is usually found at the edge of the carbon skeleton or around the pores within the skeleton due to its five-membered ring structure. This means that the content of pyrrole N can be linked to defects in the carbon matrix, and as the amount of pyrrole N increases, so does the amount of sp3 hybrid carbon. The experimental results also showed that the introduction of Cl led to an increase in the hybrid carbon of N ortho-sp3. In addition, the introduction of Cl atoms further reduces the conjugation of bonded N atoms in the N heterocycle, increases the electron density of the adjacent carbon in the active center, and thus promotes the adsorption of ORR substrates and intermediates.
Figure 3Electrocatalytic performance testing
In this paper, the ORR performance of the prepared electrocatalyst in O2-saturated electrolyte was evaluated by rotating disc electrode (RDE) technology. MFPCLNC electrocatalyst at 0The highest starting potential (eonset=1.) was exhibited in a 1 M KOH medium02 v vs.RHE) and half-wave potential (E12=0.).91 v vs.RHE), with the starting potential of a commercial PT C electrocatalyst (eonset=1.).01 v vs.RHE and E1 2 = 086 v vs.RHE) comparable, and surpasses, the reported metal-free electrocatalysts and most non-first-class electrocatalysts. This indicates that the MF-PCLNC electrocatalyst has good ORR performance. In 0At 88 V, the kinetic current density (JK) of the MF-PCLNC electrocatalyst was 2059 mA·cm-2, which is a commercial PT C (44 mA·cm-2). The MF-PCLNC electrocatalyst also has a small electrochemical impedance (43 ohms), which also contributes to high ORR activity.
Compared with MF-PCLNC electrocatalysts, MF-NC electrocatalysts exhibit inferior ORR activity (eonset=0.).81 v vs. rhe, e1/2=0.71 v vs.rhe), the larger Tafel slope (10524 MV-1) and a lower JK (023 ma·cm-2)。In addition, the Tafel slope of the MF-PCLNC electrocatalyst is 5329 mV·dec-1, indicating that the MF-PCLNC electrocatalyst has good ORR kinetics. The Koutecky-Levich (K-L) diagram of the MF-PCLNC electrocatalyst is linear at the same potential, and the fitting lines are nearly parallel. In addition, the current density of MFPCLNC and commercial PT C electrocatalysts increases with increasing speed, indicating that the dependence of O2 diffusion in the ORR process is high. In 0The E1 2 of the MF-PCLNC electrocatalyst has no significant attenuation after a 100 K accelerated degradation test (ADT) cycle in 1 Koh medium, while the commercial PT C has a significant attenuation after a 10 K ADT cycle. In addition, MF-PCLNC electrocatalysts are better tolerant to methanol than commercial PTC.
Figure 4ZN-Air Battery Test
In order to further expand the application potential of the obtained ORR electrocatalysts, the authors investigated the performance of homemade liquid rechargeable ZABs assembled with MF-PCLNC electrocatalysts as air cathode electrodes. Compared with PTC+RuO2C electrocatalysts, mf-PCLNC-based rechargeable ZABs have a higher power density (276.).88 mw·cm-2) and a larger specific capacity (1203.).2 ma·h·g-1)。Impressively, this liquid rechargeable ZAB can work stably for at least more than 200 hours, while the ZAB based on PT C+RUO2 electrocatalyst can only work continuously for 50 hours. Rechargeable ZAB based on MF-PCLNC electrocatalyst has 1The high open-circuit voltage (OCV) of 61 V can be applied to devices operating at low voltages. Therefore, the authors assembled only two rechargeable ZABs based on MF-PCLNC electrocatalysts as power sources, which can light LED lamps with an operating voltage of 3 V. Compared with the previously reported liquid rechargeable ZAB, the rechargeable ZAB based on MF-PCLNC electrocatalyst also has good performance.
Figure 5Performance testing of flexible quasi-solid-state batteries
Inspired by the growing demand for flexible and portable devices, the authors developed a home-made flexible quasi-solid-state rechargeable ZAB consisting of MF-PCLNC electrocatalyst as air cathode, ZN foil as anode, and polyacrylamide (PAM)-KOH electrolyte. According to the charge-discharge polarization curve, the current density is 10 mA·cm-2 (0At 50 V), the charge-discharge gap of the MF-PCLNC electrocatalyst was smaller than that of the flexible quasi-solid ZAB based on the PT C+RuO2 C electrocatalyst (0.73 v)。The peak power density of MF-PCLNC electrocatalyst-based flexible quasi-solid-state rechargeable ZAB can reach 17659 mW·cm-2, which is more than twice that of PT C+RUO2 electrocatalyst-based flexible quasi-solid ZAB. In addition, compared with the flexible quasi-solid ZAB (15 h) based on PT C+RUO2 electrocatalyst, the assembled flexible quasi-solid rechargeable ZAB can work stably for at least 70 h. Similarly, stability is also an important indicator for judging ZAB at low temperatures. Rechargeable ZABs based on MF-PCLNC electrocatalysts can operate at -25 °C for up to 50 hours. The rechargeable ZAB based on the MF-PCLNC electrocatalyst can operate at 60 °C for 14 hours. Flexible, quasi-solid-state rechargeable ZABS based on MF-PCLNC electrocatalysts have a wide operating temperature range.
Figure 6Mechanistic studies
In this study, the adsorption configuration of ORR intermediates on MF-PCLNC electrocatalysts was optimized. The energy barrier and potential determined step size (PDS) of the ORR indicate that the binding energy of the MF-PCLNC electrocatalyst decreases by (-0.) at U=0 V52 EV) is greater than the MF-NC electrocatalyst (-038 ev)。*The adsorption energy barrier of OOH on the MF-PCLNC electrocatalyst is 070 EV, which is significantly lower than the adsorption energy barrier of the MF-NC electrocatalyst by 085 ev。According to Bader charge analysis, after the MF-PCLNC electrocatalyst adsorbs *OOH, electrons are transferred from the surface of the electrocatalyst to *OOH. MF-PCLNC electrocatalysts have the shortest c-O (149) The change in the length of the C-O bond also further indicates that the MF-PCLNC electrocatalyst has stronger adsorption of OOH due to the interaction between SP3 hybrid carbon and Cl, thereby reducing the energy barrier. DFT calculations show that the co-doping of Sp3-hybrid carbon and Cl optimizes the adsorption energy of *OOH, thereby increasing the constitutive catalytic activity of the modified nitrogen-carbon active site on ORR.
Bibliographic information
xueting feng, guanzhen chen, zhibo cui, et al, engineering electronic structure of nitrogen‐carbon sites by sp3‐hybridized carbon and incorporating chlorine to boost oxygen reduction activity. angew. chem. int. ed.