Brief introduction of the results
Proton batteries are competitive due to their advantages such as high safety, low cost, and fast kinetic speed. However, it is often difficult to achieve high capacity and high stability at the same time, and the research on proton storage mechanism and redox behavior is still in its infancy.
Based on this,Professor Mai Liqiang and researcher Xu Lin (corresponding author) team of Wuhan University of Technology, etcFor the first time, multi-anionic layered copper oxalate was proposed as the anode material for high-capacity proton batteries. Copper oxalate enables reversible insertion of protons through layered structure extraction, and high capacity is achieved by simultaneous redox reactions of Cu2+ and C2O42-.
During discharge, the divalent copper ions are reduced while the C O portion of the oxalate group is converted to C-O. This synchronous behavior results in a charge transfer of two units, causing the proton to be embedded in the (110) crystal plane for two units. As a result, the copper oxalate anode exhibits a high specific capacity of up to 226 mAh g-1 and is able to operate stably for 1000 cycles with a retention rate of up to 98%. This research provides a new idea for the development of double redox electrode materials for high-capacity proton batteries.
Background:
Rechargeable batteries are considered to be an effective way to solve the energy crisis, while aqueous batteries have attracted much attention due to their high safety, low cost, strong ionic conductivity, and environmental protection. In general, the charge carriers in aqueous batteries are limited to Zn2+ or alkali metal ions such as Li+, Na+, and K+.
However, these metal ions are usually present in the electrolyte in the form of hydrates with a large ionic radius, resulting in slower diffusion kinetics. In contrast, protons have a unique grotthuss mechanism that can overcome the slow kinetic problems caused by dissolution and achieve rapid proton transport through a hydrogen bonding network. Recently, protons have been explored as potential charge carriers for novel aqueous batteries due to their abundant availability and minimal ionic radius.
**Reading guide
Figure 1shapeMorphological and structural analysis
SEM and TEM analyses show that CuCox has an excellent cubic morphology with a size of about 1 μm and a relatively flat surface (Figure 1A, B). The element distribution shows a uniform distribution of all elements in Cucox (Figure 1C). The fine crystal structure shown in Figure 1d illustrates the projection model of CUCOX in the (010) direction in the P121 N1 environment. The green part represents the layered stacked cells of Cucox, which are connected by Cu-O bonds. The gray box in Figure 1e represents the crystal structure in which the Cu atom is located in the center of the body and consists of 6 O atoms forming an octahedron.
It is worth noting that in order to facilitate the understanding of the system, the model ignores the frequent lamination faults that occur in the layered structure. In the XRD spectrum, all diffraction peaks are well matched to the monoclinic phase CuCox, and the (110) plane dominates the structure, confirming the layered structure of CuCox (Figure 1F). In the XPS spectrum, 9359 and 955The binding energies of 8 EV correspond to 2P32 and 2P12 for Cu2+, respectively, while the other peaks belong to specific satellite peaks of divalent copper (Figure 1G). This conclusion is further confirmed by Raman spectroscopy, where the characteristic peaks at 1516 cm-1 are from the vibrations of Cu-O, C-C, and C=O, respectively (Figure 1h).
Figure 2Electrochemical performance of a Cucox half-cell in a typical three-electrode system
As shown in Figure 2a, at 01 to 0Cyclic voltammetry (CV) tests were performed on Cucox electrodes at different scan rates of 5 mV S-1 to assess redox potential. The CV curve is at 035/-0.There is a clear pair of redox peaks at 45 V. Judging from the shape of the CV curve, the pseudo-capacitance effect on the surface of the carbon cloth current collector affects the test results. The CV curve of CuCox in the first three cycles shows that an irreversible *** occurs at a lower potential in the first cycle, as can be seen from the equation i = **b, where a and b are parameters, i is the peak current, and v is the scan rate.
The b values of the Cucox electrodes are 041 and 052, indicating that ion diffusion dominates the electrochemical process, rather than capacitive behavior. Figure 2b shows the constant current charge/discharge (GCD) voltage curve at a current density of 1 A G-1, which is in good agreement with the CV curve. Due to the insertion of partially irreversible protons, the first few cycles exhibit significant overdischarge. Subsequently, the overdischarge continued into the 10th cycle, and in the subsequent 60 cycles, there was a slight decrease in capacity due to the poor binding of the Cucox active particles to the carbon cloth current collector, resulting in a small amount of peeling.
As can be seen from Figures 2c and d, the specific capacitance of the Cucox electrode decreases slightly with the increase of current density, while the voltage remains stable. Even at a current density of 3 A g-1, the Cucox electrode provides a reversible capacity of 170 mAh g-1 and exhibits excellent electrochemical reaction kinetics. The authors then investigated the cycling performance of Cucox at a current density of 1 A g-1.
Notably, the reversible capacity of the Cucox electrode stabilized at 226 mAh g-1 after 70 active particle strippings, which remained at about 98% after 1000 cycles (220.).8 mAh g-1), which indicates that the cycling stability of the Cucox electrode is commendable (Figure 2E).
FigProton intercalation mechanism of CuCox electrodes
In this study, in-situ XRD was used to analyze the structural evolution of CuCox during charging and discharging. Due to the limitations of the equipment, the test was carried out using a two-electrode system. As can be seen from the in-situ XRD plot shown in Figure 3b, a significant phase transition occurs periodically over the course of two complete charge-discharge cycles (Figure 3A). Discharge to 1At 2 V, 26The diffraction peak at 1° (110) disappears, 45A new diffraction peak appears at 3° corresponding to a PDF standard card for H2Cuc2O4 (Cuc2O4 binds to two H+s), indicating that the crystal structure of CuCox changes with the insertion of two protons (Figure 3C).
Further, when the battery was charged to 0 V, the XRD pattern returned to its original position, demonstrating the reversibility of the proton intercalation reaction in the CuCox electrode. The Rietveld refinement results of Cucox powder are shown in Figure 3d, with differences attributed to lamination faults and trace lattice defects. Proton insertion in the Cucox electrode The extraction process is shown in Figure 3e. When discharged, at the (110) layer (d = 0.).388 nm) to convert Cucox to H2Cuc2O4.
In the subsequent charge, the protons are extracted, returning the Cucox electrode to its original state. From the in-situ Raman plot (Figure 3f, g), the peak at approximately 750 cm-1 exhibits periodic fluctuations in the cycle, corresponding to the bending vibration of the C-O bonds, confirming that the protons are embedded in the layers connected by the C-O bonds (Figure 3h). Thus, it further demonstrates the reversible intercalation chemistry of protons in CuCox electrodes. The redox mechanism of copper was analyzed by XPS spectroscopy.
For the sake of clarity, only the XPS spectra of the CuCox electrode in its original discharge state are given. As shown in Figure 3i, in the initial state, 9359 and 955The peaks at 8 EV correspond to 2P32 and 2P12 for Cu2+, respectively. After full discharge, only the Cu+ peak (931.) was observed5 ev and 9525 eV), the characteristic satellite peaks of Cu2+ disappeared, indicating that the redox reaction was complete.
Figure 4Redox reaction mechanism of oxalic acid group
To study the redox behavior of oxalic acid groups, high-wavenumber in-situ infrared spectroscopy and ex situ Raman spectroscopy were used for analysis. The in-situ FTIR plot of the CuCox electrode after two full cycles (Figure 4A) is shown in Figure 4B. The infrared transmittance peak belonging to the C-O bond at 1450 1500 cm-1 was observed to show periodic changes, indicating that the concentration of the C-O group changed. During the discharge process, the transmittance of C-O decreases gradually, indicating that the concentration of C-O groups is increasing.
Subsequently, after the charging process, the peak transmittance returns to the initial level, indicating that this change is excellent reversible. In addition, at 3400 3500 cm-1, the strength of the OH group also changed, indicating that the OH group was formed. Figure 4c shows the Raman spectra of the original Cucox powder and the Cucox electrode, where the additional peaks at 1340 and 1600 cm-1 are consistent with the D and G bands of C, especially from the conductive carbon in the electrode. The Raman peak at 1516 cm-1 corresponds to the asymmetric stretching of c=o, while the remaining peaks in the Raman spectrum are blurred due to the adhesive.
In this study, non-situ Raman spectroscopy was further used to observe the changes of c=o in oxalic acid groups. The different states are tested, as shown by the colored dots in Figure 4d, and the Raman plot with wavenumber 900 2000 cm-1 is shown in Figure 4e. It is worth noting that the intensities of all spectra are normalized. As the discharge process deepens, the strength of the c=o bond gradually decreases until it disappears after complete discharge.
However, the peaks did not disappear completely, but were only masked by the higher D-band and G-band on both sides. When the potential returns, the signal in the C=O band is fully restored, indicating a reversible reduction of the C=O concentration during the discharge-charge process. The in-situ FTIR and non-situ Raman spectra of the copper oxalate electrode showed that the C=O content decreased and the C-O content increased during the discharge process. When charged to 0 V, both return to their initial state.
Specifically, similar to organic electrodes, the O heteroatoms of C=O in the C2O42- group act as oxidation centers for proton absorption and removal, thus conferring the ability of C=O to be reversibly coordinated with H+ to C-OH. CV curves and in-situ XRD plots confirm that the redox reaction and proton embedding are done in a single step, indicating that the redox reactions of Cu2+ and C2O42- occur simultaneously.
Thus, the synchronous redox reaction is characterized by the reduction of Cu2+ to Cu+ binding one proton during discharge, while the cross-section C=O group opens up and binds another proton (C-OH) (Figure 4F). In this way, one CuC2O4 molecule can be grafted onto two protons, thus exhibiting extraordinary capacity.
FigElectrochemical properties of the whole Cufe-TBA CuCox cell
To further demonstrate the potential of CuCox electrodes in practice, the authors assembled a full cell with Cufe-TBA as the cathode and CuCox as the negative electrode, 01 M H2SO4 as the electrolyte (Figure 5A). Considering that Cucox and Cufe-TBA are at 0., respectively7 to 0 and 02 to 06 V range, so choose 0 to 0A potential window of 7 V is used as the full cell operating voltage (Figure 5b). Figure 5C shows two pairs of distinct redox peaks in the CV curve, and the GCD curve in Figure 5D validates similar results.
Due to irreversible structural restructuring and distortion, after 100 cycles, the capacity of a fully charged cell stabilized at 40 mAh g-1 (calculated according to the mass of the cathode). The superior rate performance is shown in Figure 5e, f. At a current density of 0At 2 A g-1, the specific capacity of the whole cell is 42 mAh g-1 and 35 mAh g-1 at a current density of 3 A g-1. In addition, the capacity retention rate of the whole cell of Cufe-TBA CuCox was 95% after 2000 cycles, showing excellent cycling stability (Figure 5G), and the above results indicate that the CuCox anode has excellent proton storage compatibility.
Finally, as shown in Figure 5h, Cucox has significant advantages as a proton storage anode compared to the maximum capacity and average potential of commonly used electrode materials.
Bibliographic information
wanxin song, jianyong zhang, cheng wen, haiyan lu, chunhua han, lin xu*, and liqiang mai*, synchronous redox reactions in copper oxalate enable high-capacity anode for proton battery. j. am. chem. soc.