摘要
summary

本文采用强酸预处理和水热处理，成功在石墨毡电极上负载了氮掺杂镍钴氧化物纳米针状阵列。此改性显著增大了电极的BET比表面积，并优化了介孔结构，增强了传质能力。NiCoO2的引入和氮掺杂共同提升了电极的电导率和电催化性能，促进了电荷转移和氧化还原反应。改性后的电极展现出低阻抗、高动力学性能的特点，有助于减少极化并加速反应进程。最后，通过COMSOL模拟验证了改性效果，结果显示与实验数据一致，证实了改性策略的有效性。这一创新性的电极改性方法，为提升电化学性能提供了新的思路和途径。
In this paper, nitrogen-doped nickel-cobalt oxide nanoneedle arrays were successfully loaded on graphite felt electrodes by strong acid pretreatment and hydrothermal treatment. This modification significantly increases the BET specific surface area of the electrode, optimizes the mesoporous structure, and enhances the mass transfer ability. The introduction of NiCoO2 and nitrogen doping together improved the conductivity and electrocatalytic performance of the electrode, and promoted the charge transfer and redox reactions. The modified electrode exhibits low impedance and high dynamics, helping to reduce polarization and accelerate the reaction process. Finally, the COMSOL simulation is used to verify the modification effect, and the results show that they are consistent with the experimental data, confirming the effectiveness of the modification strategy. This innovative electrode modification method provides a new idea and way to improve electrochemical performance.

本文采用了一种创新的改性方法，首先对石墨毡电极进行强酸预处理，以清除表面杂质并增加其活性位点。随后，通过水热处理，在电极表面均匀负载镍钴氧化物前体，为后续的反应奠定基础。接着，在氨气保护下进行退火处理，实现了氮原子的有效掺杂，并促使镍钴氧化物以纳米针状阵列的形式在石墨毡表面生长。
In this paper, an innovative modification method is adopted, in which the graphite felt electrode is first pretreated with strong acid to remove surface impurities and increase its active site. Subsequently, nickel-cobalt oxide precursors are uniformly loaded on the electrode surface by hydrothermal treatment, laying the foundation for the subsequent reaction. Then, annealing treatment under ammonia protection was carried out to achieve effective doping of nitrogen atoms and promote the growth of nickel-cobalt oxide on the surface of graphite felt in the form of nanoneedle-like arrays.

这种改性策略显著提升了电极的比表面积，从而增加了活性物质的负载量，并优化了介孔结构，使得电极的传质效率大幅提升。同时，NiCoO2作为三元金属氧化物，其固有的高活性和可逆性为电极提供了优异的电化学性能。而氮原子的掺杂则进一步提高了电极的电导率，加速了电荷转移过程，并增强了其电催化活性。
This modification strategy significantly increases the specific surface area of the electrode, thereby increasing the loading of active material, and optimizes the mesoporous structure, which greatly improves the mass transfer efficiency of the electrode. At the same time, NiCoO2, as a ternary metal oxide, provides excellent electrochemical performance for the electrode due to its inherent high activity and reversibility. The doping of nitrogen atoms further improves the conductivity of the electrode, accelerates the charge transfer process, and enhances its electrocatalytic activity.

最终，改性后的石墨毡电极在电化学反应中表现出更高的动力学性能，减少了极化现象，促进了氧化还原反应的快速进行。此外，通过COMSOL Multiphysics的模拟验证，我们进一步确认了改性电极在纳米尺度下的优越性能，为其在电化学领域的应用提供了坚实的理论基础和实验支持。
Finally, the modified graphite felt electrode exhibited higher kinetic performance in the electrochemical reaction, reduced the polarization phenomenon, and promoted the rapid progress of the redox reaction. In addition, the COMSOL Multiphysics simulation has further confirmed the superior performance of the modified electrode at the nanoscale, which provides a solid theoretical foundation and experimental support for its application in the field of electrochemistry.

3.3.1扫描电子显微镜分析
3.3.1 Scanning electron microscopy analysis

图 3-1 为 GF、GFs、NiCoO2-GFs 和 N-NiCoO2-GFs 的扫描电子显微镜图，展示不同样品石墨毡纤维表面形貌。对比图 3-1（a）和（b），GF 表面光滑、刻蚀浅，预处理后的 GFs 因王水的强氧化性和腐蚀性，表面刻蚀深，有较多纵向沟壑，引入结构缺陷和含氧官能团，利于金属氧化物晶体生长，提升电极电化学性能。经简单水热处理和退火处理，合成 NiCoO2 修饰（图 3-1（c）（d））和氮掺杂 NiCoO2 修饰的石墨毡电极（图 3-1（e）（f）），可见石墨毡表面被由纳米线交织而成的 NiCoO2 纳米片覆盖，此纳米晶体结构可增氧化还原反应表面积，改善传质，促钒离子渗透反应。400°C 保温 1h 后，NiCoO2-GFs 与 N-NiCoO2-GFs 形貌相似，表明 400°C 氨气处理不影响样品形貌。
Figure 3-1 shows scanning electron microscopy of GF, GFs, NiCoO2-GFs, and N-NiCoO2-GFs, showing the surface morphology of graphite felt fibers in different samples. Comparing Fig. 3-1(a) and (b), GFs have a smooth surface and shallow etching, while the pretreated GFs have deep surface etching and more longitudinal gullies due to the strong oxidation and corrosiveness of aqua regia, which introduces structural defects and oxygen-containing functional groups, which is conducive to the growth of metal oxide crystals and improves the electrochemical performance of the electrode. Graphite felt electrodes modified with NiCoO2 (Fig. 3-1(c)(d)) and nitrogen-doped NiCoO2 were synthesized by simple hydrothermal treatment and annealing (Fig. 3-1(e)(f)). After holding at 400°C for 1 h, the morphology of NiCoO2-GFs was similar to that of N-NiCoO2-GFs, indicating that 400°C ammonia treatment did not affect the morphology of the samples.

3.3.2氮气吸脱附实验
3.3.2 Nitrogen adsorption and desorption experiments

使用 BET 法测试，以酸处理石墨毡 GFs 为对照，测试二者的 C 值均大于零且线性相关性优于 0.999，表明结果可靠，如图 3-2 所示。观察 GFs 等温吸脱附曲线，在 P/P0 约 0.3 处交叉，可能因其比表面积小、氮气吸附量小，出现波动导致吸脱附曲线交叉。N-NiCoO2-GFs 等温吸附曲线为典型Ⅳ型，中压区（0.6 - 0.89）回滞环属 H3 型，表明内部有大量介孔。计算得 GFs 的 BET 比表面积仅 0.4763 m²/g，N-NiCoO2-GFs 达 7.3525 m²/g，提高约 16 倍，其原因在于改性石墨毡表面生成金属氧化物针状阵列，该结构提升了电极比表面积，改善了反应动力学。从 BJH 法的孔径分布图看，GFs 和 N-NiCoO2-GFs 介孔大小分布集中，GFs 介孔平均孔径 18.0678 nm，N-NiCoO2-GFs 为 12.2910 nm，丰富介孔促电解液快速渗透，提高传质能力。
Using the BET method, acid-treated graphite felt GFs were used as a control, and the C values of both were greater than zero and the linear correlation was better than 0.999, indicating that the results were reliable, as shown in Figure 3-2. The isothermal adsorption-desorption curves of GFs were observed to cross at about 0.3 at P/P0, which may be due to the small specific surface area and small nitrogen adsorption capacity, which may lead to the crossover of the adsorption-desorption curves。 The isothermal adsorption curve of N-NiCoO2-GFs is typical type IV., and the hysteresis loop in the medium pressure region (0.6 - 0.89) belongs to the H3 type, indicating that there are a large number of mesopores inside. The calculated BET specific surface area of GFs is only 0.4763 m²/g, and the N-NiCoO2-GFs is 7.3525 m²/g, which is about 16 times higher, due to the formation of metal oxide needle-like arrays on the surface of the modified graphite felt, which improves the structureThe specific surface area of the electrode improves the reaction kinetics. According to the pore size distribution of BJH method, the mesoporous size distribution of GFs and N-NiCoO2-GFs is concentrated, with the average mesoporous size of GFs being 18.0678 nm and N-NiCoO2-GFs being 12.2910 nm.

3.3.2X射线衍射分析
3.3.2 X-ray diffraction analysis

本文对NiCoO2-GFs 和 N-NiCoO2-GFs 进行X射线衍射分析，两者的 XRD 图如图 3-3 所示。NiCoO2-GFs 和 N-NiCoO2-GFs样品均有明显的 NiCoO2 晶体特征峰（JCPDS NO.10-0188）。在 36.8°、42.8°、61.8°、73.9°和78.0°处的衍射峰分别对应 NiCoO2 的晶面（111）、（200）、（220）、（311）和（222）[51]。而石墨材料的对应着两个明显的衍射峰，分为位于 26.1°和 44.3°，这与石墨材料的（002）和（101）晶面一致[59]，其中 26.1°处的峰为图中强度最大的峰。结果表明 NiCoO2 晶体成功在石墨毡表面合成，而 N 原子掺杂对于 NiCoO2 的结构影响不大。
In this paper, X-ray diffraction analysis of NiCoO2-GFs and N-NiCoO2-GFs is performed The XRD plot is shown in Figure 3-3. Both NiCoO2-GFs and N-NiCoO2-GFs samples had obvious characteristic peaks of NiCoO2 crystals (JCPDS NO.10-0188). The diffraction peaks at 36.8°, 42.8°, 61.8°, 73.9°, and 78.0° correspond to the crystal planes (111), (200), (220), (311), and (222) of NiCoO2, respectively [51]. The graphite material corresponds to two obvious diffraction peaks, located at 26.1° and 44.3°, which are consistent with the (002) and (101) crystal planes of the graphite material [59], of which the peak at 26.1° is the peak with the highest intensity in the figure. The results showed that NiCoO2 crystals were successfully synthesized on the surface of graphite felt, while N atom doping had little effect on the structure of NiCoO2.

3.3.3X射线光电子能谱分析
3.3.3 X-ray photoelectron spectroscopy

为了探究 N 掺杂 NiCoO2 纳米片阵列的化学键和元素相对含量，以及 N 掺杂改性的方法是如何增加镍钴氧化物修饰石墨毡的电化学活性，本文使用 X 射线光电子能谱进行分析，得到 NiCoO2-GFs 和 N-NiCoO2-GFs 的元素全谱图，如图 3-4。从图中可以看出，除了最强的 C 峰之外，NiCoO2-GFs 和 N-NiCoO2-GFs 中都出现了 Ni、Co 和 O 三种元素的峰，说明石墨毡表面金属氧化物的存在。而相对于 NiCoO2-GFs，N-NiCoO2-GFs 全谱图中出现了明显的 N 元素峰，证明了 N 元素被成功掺杂。
In order to explore the chemical bonds and relative elemental content of N-doped NiCoO2 nanosheet arrays, and how the N-doping modification method can increase the electrochemical activity of nickel-cobalt oxide-modified graphite felts, X-ray photoelectron spectroscopy was used to obtain the full elemental spectrum of NiCoO2-GFs and N-NiCoO2-GFs, as shown in Figure 3-4. As can be seen from the figure, in addition to the strongest C peak, peaks of Ni, Co and O appear in both NiCoO2-GFs and N-NiCoO2-GFs, indicating the presence of metal oxides on the surface of graphite felt. Compared with NiCoO2-GFs, there is a significant N peak in the full spectrum of N-NiCoO2-GFs, which proves that N is successfully doped.

为进一步分析 N 掺杂改性对石墨毡表面镍钴氧化物的影响，实验绘制了 NiCoO2-GFs 和 N-NiCoO2-GFs 的 Ni 2p、Co 2p 和 O 1s 的高分辨谱图，以及 N-NiCoO2-GFs 的 N 1s 高分辨谱图（如图 3-5）。在图 3-5(a)中，Ni 2p 能谱可拟合为两个自旋轨道峰（Ni 2p1/2 和 Ni 2p3/2）和两个卫星峰，自旋轨道峰均分为 Ni2+（854.02 和 871.58 eV）和 Ni3+（855.66 和 873.30 eV），两个振激卫星峰（标为“Sat”）位于 861.21 和 879.23 eV。图 3-5(b)中，Co 2p 能谱也分两个自旋轨道峰（Co 2p1/2 和 Co 2p3/2）和两个卫星峰，价态 Co2+（779.58 和 794.71 eV）和 Co3+（780.9 和 796.51 eV），卫星峰在 785.20 和 802.70 eV。N 掺杂后，Ni 和 Co 峰位无明显变化，但 Ni2+和 Ni3+、Co2+和 Co3+比值变小，可能因 N 电负性强于氧降低金属离子价态，且氨气的还原性将部分 Co3+和 Ni3+还原为 Co2+和 Ni2+，其与 N 结合形成 N 掺杂的 NiCoO2。图 3-5(c)中，N 1s 高分辨谱图可拟合为取代 N 掺杂（397.8 eV）和间隙 N 掺杂（399.4 eV），表明 N 被掺杂到结构中，间隙 N 原子与 O 原子相互作用产生更多氧空位增强催化性能。此外，通过分析 O 1s 的高分辨谱图得到相似结论，图 3-5(c)，O 1s 高分辨谱图在 529.9、531.3 和 532.7 eV 处有对应金属氧键、氧空位缺陷及表面化学吸附水分子的三个特征峰。N 掺杂后氧缺陷峰比例显著增加，通过拟合计算，氧空位比从 30.0%提高到 42.4%，O 缺陷增量浓度为 12.4%高于 N 元素掺杂浓度，所以 N-NiCoO2-GFs 性能提升可能主要源于 O 缺陷浓度增加。
In order to further analyze the effect of N-doping modification on nickel-cobalt oxides on the surface of graphite mats, the high-resolution spectra of Ni 2p, Co 2p and O 1s of NiCoO2-GFs and N-NiCoO2-GFs, and the high-resolution spectra of N 1s of N-NiCoO2-GFs were plotted experimentally (Fig. 3-5). In Figure 3-5(a), the Ni 2p spectra can be fitted to two spin-orbit peaks (Ni 2p1/2 and Ni 2p3/2) and two satellite peaks, with the spin orbit peaks being Ni2+ (854.02 and 871.58 eV) and Ni3+ (855.66 and 873.30 eV) with two oscillationsSatellite peaks (labeled "Sat") are located at 861.21 and 879.23 eV. In Figure 3-5(b), the Co2p spectrum is also divided into two spin-orbit peaks (Co 2p1/2 and Co 2p3/2) and two satellite peaks, valence Co2+ (779.58 and 794.71 eV) and Co3+ (780.9 and 796.51 eV), and satellite peaks at 785.20 and 802.70 eV. After N doping, there was no significant change in the peak positions of Ni and Co, but the ratios of Ni2+ and Ni3+, Co2+ and Co3+ became smaller, which may reduce the valence state of metal ions because N electronegativity is stronger than that of oxygen, and the reduction of ammonia reduces part of Co3+ and Ni3+ to Co2+ and Ni2+, which combine with N to form N-doped NiCoO2. In Figure 3-5(c), the N1s high-resolution spectra can be fitted as substituted N-doped (397.8 eV) and interstitial N-doped (399.4 eV), indicating that N is doped into the structure, and that interstitial N atoms interact with O atoms to generate more oxygen vacancies to enhance catalytic performance. In Fig. 3-5(c), the high-resolution spectra of O 1s have three characteristic peaks corresponding to metal oxygen bonds, oxygen vacancy defects, and surface chemisorption of water molecules at 529.9, 531.3, and 532.7 eV. The proportion of oxygen defect peaks increased significantly after N doping, and the oxygen vacancy ratio increased from 30.0% to 42.4% by fitting, and the incremental concentration of O defects was 12.4% higher than that of N element doping, so the performance improvement of N-NiCoO2-GFs may be mainly due to the increase of O defect concentration.

3.3.4循环伏安分析
3.3.4 Cyclic voltammetry analysis

通过上述分析，为了评估改性后石墨毡的电化学性能，使用电化学工作站并采用三电极体系对 GF、GFs、NiCoO2-GFs 和 N-NiCoO2-GFs 四种样品进行循环伏安测试，分析相关的电化学数据。电解液采用 0.1molꞏL-1 VO2++3.0molꞏL-1 H2SO4、测试温度为 25℃、扫描速率为5mVꞏs-1,电压窗口为 0.0V-0.9V。得到的循环伏安曲线和其电化学数据如图 3-6 和表 3-1。
Based on the above analysis, in order to evaluate the electrochemical performance of the modified graphite felt, four samples of GF, GFs, NiCoO2-GFs and N-NiCoO2-GFs were tested by cyclic voltammetry using an electrochemical workstation and a three-electrode system, and the relevant electrochemical data were analyzed. The electrolyte uses 0.1molꞏL-1 VO2++3.0molꞏL-1 H2SO4, the test temperature is 25°C, the scan rate is 5mVꞏs-1, and the voltage window is 0.0V-0.9V. The obtained cyclic voltammetry curves and their electrochemical data are shown in Figure 3-6 and Table 3-1.

从图 3-6 可见，所有电极都有代表 VO2+氧化成 VO2+的阳极峰和还原成 VO2+的阴极峰，其中 N-NiCoO2-GFs 的氧化峰电流和还原峰电流最高，峰电位差最小，性能最佳，GF 最差。具体数据见表 3-1，包含不同样品的峰电流(Ipa 和 Ipc)、峰电位（Epa 和 Epc）、峰电流比值(-Ipa/Ipc)和峰电位差值（ΔEp），峰电流等表示电极的电化学活性和可逆性，峰电流越大、峰电流比值越接近 1 且峰电位差值越小，电化学可逆性越好。GF 的氧化和还原峰值电流分别为 61.0 和 40.2mA，GFs 分别为 72.4 和 48.9mA，GFs 峰电位差值 524mV 小于 GF 的 608 mV，峰电流比值也更接近 1，GFs 性能优于 GF，其原因在于酸处理引入了缺陷结构，提高了石墨毡的亲水性，这有助于改善电解质可及性。NiCoO2-GFs 在 GFs 基础上改性，从 GFs 到 NiCoO2-GFs 各方面性能提升最多，如还原反应增加 35.1%，大于从 GF 到 GFs 的 14.9%和从 N-NiCoO2-GFs 到 NiCoO2-GFs 的 12.1%，说明 NiCoO2 修饰改善电化学活性和可逆性。氮原子掺杂后，N-NiCoO2-GFs 峰电流更高，峰电位差值下降到 0.412V 为最低，峰电流比值也最接近 1，有良好电化学可逆性。综上，不同电极电化学性能依次为 GF < GFs < NiCoO2-GFs < N-NiCoO2-GFs，推断三元金属氧化物 NiCoO2 改性石墨毡显著提高电极活性和可逆性，氮原子掺杂提高金属氧化物电导率，进一步提升电极性能。
As can be seen from Figure 3-6, all electrodes have anode peaks representing the oxidation of VO2+ to VO2+ and cathode peaks to VO2+, among which N-NiCoO2-GFs have the highest oxidation peak current and reduction peak current, the smallest peak potential difference, the best performance, and the worst GF. The specific data are shown in Table 3-1, including the peak current (Ipa and Ipc), peak potential (Epa and Epc), peak current ratio (-Ipa/Ipc) and peak potential difference (ΔEp) of different samples, the peak current indicates the electrochemical activity and reversibility of the electrode, the larger the peak current, the closer the peak current ratio is to 1 and the smaller the peak potential difference, the better the electrochemical reversibility. The oxidation and reduction peak currents of GF are 61.0 and 40.2 mA, respectively, the GFs are 72.4 and 48.9 mA, the GFs peak potential difference of 524 mV is less than that of GF, and the peak current ratio is also closer to 1, GFs perform better than GF due to the defect structure introduced by acid treatment, The hydrophilicity of graphite felt is improved, which helps to improve electrolyte accessibility. NiCoO2-GFs were modified on the basis of GFs, and the performance of NiCoO2-GFs from GFs to NiCoO2-GFs was improved the most, such as the reduction reaction increased by 35.1%, which was greater than that of GF to GFs (14.9%) and from N-NiCoO2-GFs to NiCoO2-GFs (12.1%), indicating that NiCoO2 modification improved electrochemical activity and reversibility. After nitrogen atom doping, the peak current of N-NiCoO2-GFs was higher, the peak potential difference decreased to 0.412V as the lowest, and the peak-current ratio was also closest to 1, which had good electrochemical reversibility. In summary, the electrochemical performance of different electrodes was GF < GFs < NiCoO2-GFs < N-NiCoO2-GFs, and it was inferred that ternary metal oxide NiCoO2 modified graphite felt significantly improved the electrode activity and reversibility, and nitrogen atom doping improved the metal oxide conductivity and further improved the electrode performance.

接着，在相同条件下，通过分析四种样品在不同扫速（1、2、5 和 10 mVꞏs-1）下的 CV曲线，得到图 3-7 峰电位差与扫描速率的曲线图以及峰电流与扫描速率平方根的曲线图，进一步研究电极的氧化还原反应动力学。
Then, under the same conditions, the CV curves of the four samples at different scan speeds (1, 2, 5, and 10 mVꞏs-1) were analyzed, and the plots of the peak potential difference and scan rate and the square root of the peak current and scan rate were obtained in Figure 3-7 to further study the oxidation-reduction kinetics of the electrodes.

随着扫描速率增大，氧化峰朝横坐标正向移动，还原峰朝横坐标负向移动，致使峰电位差值增大，这是极化增强使较大扫描速率下需更高过电位所致[69]。图 3-7(a)展示了不同样品峰电位差与扫描速率的曲线，可见 N-NiCoO2-GFs 在所有扫描速率下峰电位差最低，表明氮掺杂三元镍钴氧化物改性石墨毡的方法能有效降低电极极化。此外，依据 Randles-Sevcik 方程绘制了峰值电流密度和扫描速率平方根的关系，如图 3-7(b)所示。所有电极的氧化还原峰电流均近似与扫描速率平方根呈线性变化，证明电极电化学行为受扩散控制[70]。其中，斜率体现了氧化和还原反应的扩散系数。经斜率拟合对比，不同样品氧化和还原反应斜率绝对值大小顺序均为 GF < GFs < NiCoO2-GFs < N-NiCoO2-GFs，说明 N-NiCoO2-GFs 活性物质传递速率更快，能很好地促进电化学反应发生。总之，在降低极化和推动氧化还原反应扩散过程上，N-NiCoO2-GFs 电极动力学性能最优。
As the scan rate increases, the oxidation peak moves positively toward the abscissa and the reduction peak moves negatively toward the abscissa, resulting in an increase in the peak potential difference, which is due to the need for higher overpotential at larger scan rates due to the increased polarization [69]. Figure 3-7(a) shows the curves of peak potential difference and scan rate of different samples, and it can be seen that N-NiCoO2-GFs has the lowest peak potential difference at all scan rates, indicating that the nitrogen-doped ternary nickel-cobalt oxide modified graphite felt method can effectively reduce the electrode polarization. In addition, the Randles-Sevcik equation is plotted as a function of the peak current density and the square root of the scan rate, as shown in Figure 3-7(b). The redox peak currents of all electrodes are approximately linear with the square root of the scan rate, demonstrating that the electrochemical behavior of the electrodes is controlled by diffusion [70]. Among them, the slope reflects the diffusion coefficient of the oxidation and reduction reactions. The absolute values of the slopes of the oxidation and reduction reactions of different samples were GF < GFs < NiCoO2-GFs < N-NiCoO2-GFs after slope fitting comparison, indicating that the transfer rate of the active species of N-NiCoO2-GFs was faster and could promote the electrochemical reaction. In conclusion, the N-NiCoO2-GFs electrode had the best kinetic performance in reducing polarization and promoting the diffusion process of redox reaction.

3.3.5电化学阻抗谱分析
3.3.5 Electrochemical impedance spectroscopy

实验对 GF、GFs、NiCoO2-GFs 和 N-NiCoO2-GFs 四种样品进行了 EIS 测试，进一步研究改性石墨毡电化学性能，实验体系同循环伏安测试，测试电压为开路电压，频率范围为 10-2Hz-106Hz，扰动电压为 20mV。测试结果如图 3-8 所示。
Four samples of GF, GFs, NiCoO2-GFs and N-NiCoO2-GFs were tested by EIS to further study the electrochemical properties of modified graphite mats, and the experimental system was tested with cyclic voltammetry, the test voltage was open-circuit voltage, the frequency range was 10-2Hz-106Hz, and the disturbance voltage was 20mV. The test results are shown in Figure 3-8.

通过观察可知，所有样品的 Nyquist 图由高频区和中频区的两个半圆形曲线以及低频区的线性部分组成[71, 72]，同时，对样品进行了等效电路拟合[73]，结果如表 3-2 所示。在拟合的等效电路图中，Rs 表示电解液中电荷转移电阻，CPE1 和 Rc 分别表示石墨纤维间的电容和接触电阻，对应高频区半圆形曲线；CPE2 和 Rct 分别是双电层电容的常相位角元件和电荷转移电阻，对应中频区半圆形曲线[74]；CPE3 为扩散电容，对应低频区直线，其中 Y0 为前置因子，其增加意味着扩散阻抗降低[75, 76]。在浓度相同的电解液中测试，不同样品的 Rs 差距小。Rc 方面，GF 和 GFs 阻值大于 NiCoO2-GFs 和 N-NiCoO2-GFs，这是由于未进行金属氧化物修饰的石墨纤维之间所形成的接触面积较小所致。对比不同样品 Rct ，N-NiCoO2-GFs 最低，能有效降低电池欧姆极化，GFs 最高，因其酸处理破坏了石墨毡纤维表面结构致电阻增大。但循环伏安曲线显示 GFs 电化学性能大于 GF，可能是酸处理引入大量缺陷结构促进活性物质转移降低浓差极化，含氧官能团强化电极催化能力降低活化极化。观察前置因子 Y0 ，GFs 的 Y0 值大于 GF，传质性能强，能降低浓差极化，N-NiCoO2-GFs 的 Y0 值最高，扩散阻抗最低，传质能力最强。综上，N-NiCoO2-GFs 电化学活性和传质能力最佳，能有效降低电极极化。
Observations show that the Nyquist plot for all samples consists of two semicircular curves in the high- and mid-frequency regions and a linear portion in the low-frequency region [71, 72].An equivalent circuit fitting was performed [73], and the results are shown in Table 3-2. In the fitted equivalent circuit diagram, Rs represents the charge transfer resistance in the electrolyte, and CPE1 and Rc represent the capacitance and contact resistance between the graphite fibers, respectively, corresponding to a semicircular curve in the high-frequency region. CPE2 and RCT are the constant-phase elements and charge transfer resistors of electric double-layer capacitors, respectively, corresponding to semicircular curves in the mid-frequency region [74]; CPE3 is the diffusion capacitance, which corresponds to a straight line in the low-frequency region, where Y0 is the prefactor, and its increase implies a decrease in the diffusion impedance [75, 76]. Tested in an electrolyte of the same concentration, the Rs difference between different samples is small. In terms of Rc, the resistance values of GF and GFs are greater than those of NiCoO2-GFs and N-NiCoO2-GFs, due to the smaller contact area formed between the graphite fibers that have not been modified with metal oxides. Compared with the RCT of different samples, N-NiCoO2-GFs were the lowest, which could effectively reduce the ohmic polarization of the battery, and GFs was the highest, because the surface structure of graphite felt fibers was damaged by acid treatment, resulting in an increase in resistance. However, the cyclic voltammetry curve showed that the electrochemical performance of GFs was greater than that of GF, which may be due to the introduction of a large number of defect structures by acid treatment, which promoted the transfer of active species and reduced concentration polarization, and the oxygen-containing functional groups strengthened the catalytic ability of the electrode and reduced the activation polarization. The Y0 value of GFs was greater than that of GF, and the mass transfer performance was strong, which could reduce the concentration polarization, and the Y0 value of N-NiCoO2-GFs was the highest, the diffusion impedance was the lowest, and the mass transfer ability was the strongest. In conclusion, N-NiCoO2-GFs had the best electrochemical activity and mass transfer ability, which could effectively reduce electrode polarization.

3.4石墨毡表面针状阵列结构建模与模拟
3.4 Modeling and simulation of needle-like array structure on graphite felt surface

利用COMSOL Multiphysics 商用软件进行模拟计算进一步展示针状阵列结构相对于原始石墨毡表面在传质方面的优势，首先建立了相对比较简单的石墨毡电极表面结构模型，使用三次电流分布，同时考虑扩散、对流和电迁移三种物质传递方式，计算模拟两种结构在电解质流动的情况下反应物的浓度分布。电极相关的动力学参数通过 LSV 测试所拟合出的极化曲线得到。
Firstly, a relatively simple model of the surface structure of the graphite felt electrode was established, and the tertiary current distribution was used to consider the three species transport modes of diffusion, convection, and electromigration. The calculation simulates the concentration distribution of reactants in the case of electrolyte flow for both structures. The electrode-related kinetic parameters were obtained from the polarization curves fitted by the LSV test.

3.4.1电极电化学极化测定及动力学参数拟合
3.4.1 Electrochemical polarization determination of electrodes and fitting of kinetic parameters

为了使模拟结果更加接近于真实情况，通过实验测得 N-NiCoO2-GFs 和 GF 在 5mV 扫速下的线性扫描伏安曲线，随后变换坐标拟合得到 Tafel 曲线及相关参数，如图 3-7 所示。具体拟合过程如下：
In order to make the simulation results closer to the real situation, the linear scanning voltammetry curves of N-NiCoO2-GFs and GFs at 5mV sweep speed were experimentally measured, and then the Tafel curves and related parameters were obtained by transforming the coordinates and fitting, as shown in Figure 3-7. The fitting process is as follows:

实验所测的为正极反应，其 Tafel 方程：
The experiment is measured for the positive electrode reaction, and its Tafel equation:

其中，η：活化过电位（η=（E-Eeq））；i0：电极交换电流密度；i：电极电流密度；αc：
Among them, η: activation overpotential (η=(E-Eeq)); i0: electrode exchange current density; i: electrode current density; αc：

正极电荷传递系数。根据塔菲尔曲线取阳极极化曲线中 η 与极化电流密度的对数 logi 呈线性
Positive electrode charge transfer coefficient. According to the Tafel curve, the logarithm logi of the η and the polarization current density in the anodic polarization curve is linear

关系的区域（称为 Tafel 直线区）进行拟合
The region of the relationship, called the Tafel straight line area, is fitted

3.4.2模型三维结构设计及物理场控制方程
3.4.2 Model 3D structure design and physical field governing equations

通过软件内置的几何建模工具，建立 N-NiCoO2-GFs 样品石墨纤维表面针状阵列的简化模型，同时建立原始石墨毡 GF 电极表面模型作为对照。模型求解域为电解液以及电极-电解液交界面，将电极视为等势体，不考虑电极本体区域。模型如图 3-10 所示。
Through the built-in geometric modeling tools of the software, a simplified model of the needle-like array on the surface of graphite fibers of the N-NiCoO2-GFs sample was established, and the original graphite felt GF electrode surface model was established as a control. The solution domain of the model is the electrolyte and the electrode-electrolyte interface, and the electrode is regarded as an equipotential body, and the electrode body region is not considered. The model is shown in Figure 3-10.

该模型通过二次电流分布、稀物质传递以及层流三个物理场之间的耦合，来考虑电极电化学极化和浓度极化。在稳态且不发生均相反应的前提假设下，考虑了扩散、对流和电迁移所带来的质量传递效应，电解质中的总质量平衡可通过以下公式来描述：
The model considers electrochemical polarization and concentration polarization of electrodes through the coupling between the three physics of secondary current distribution, transport of dilute species, and laminar flow. The total mass balance in the electrolyte can be described by the following formula, taking into account the mass transfer effects of diffusion, convection, and electromigration, under the assumption that there is a steady state and no homogeneous reactions:

公式中的 表示的是物质 i 的通量大小 （mol·m2/s），其控制方程表示为下式：
The formula in is the magnitude of the flux of matter i (mol · m2/s), and its governing equation is expressed as the following formula:

式中， 表示的是离子 i 的浓度（mol/m3）、 为化合价、 为扩散率（m2/s）、 为迁移率（mol·m2（s·V·A））、F 表示法拉第常数（As/mol）、 是离子电势以及 u 是速度矢量（m/s）。上述方程包含三项，分别代表扩散、电迁移和对流这三种物质传递机制。而净电流密度可以被描述为：
where is the concentration of ion i (mol/m3), is the valency, is the diffusivity (m2/s), is the mobility (mol · m2 (s · V·A)), F is Faraday's constant (As/mol), is the ionic potential, and u is the velocity vector (m/s). The above equation contains three items, representing the three transport mechanisms of species: diffusion, electromigration, and convection. Whereas, the net current density can be described as:

电流密度守恒：
Conservation of current density:

由此，可以将离子迁移率、化合价、法拉第常数以及浓度常数综合成一个典型的电导率（S/m），进而方程（3-5）可以转变为新的形式：
In this way, the ion mobility, valency, Faraday constant, and concentration constant can be combined into a typical conductivity (S/m), and equations (3-5) can be transformed into new forms:

得到的最终方程就是欧姆定律。
The resulting final equation is Ohm's law.

在模型中引入质量传递依赖，涉及阳极上被氧化和还原的物质受到质量传递的制约，导致局部浓度 co 和 cr（mol/m3）对电极动力学产生影响。所以，阳极上的 Butler-Volmer 表达式转变为与浓度相关，即存在浓度依赖的动力学公式：
Mass transport dependence is introduced into the model, and the oxidized and reduced species on the anode are constrained by mass transport, resulting in the influence of local concentrations of CO and CR(mol/m3) on electrode dynamics. So, the Butler-Volmer expression on the anode shifts to a concentration-dependent, i.e., concentration-dependent kinetic formula:

其中，co0 和 cr0（mol/m3）为参考浓度（也即入口浓度）。该模型还引入了一个动量守恒方程来刻画对流现象，在此情形中，假设流态是稳态且不可压缩的层流，并采用 Navier-Stokes Equation（纳维-斯托克斯方程）来描述这一过程：
CO0 and CR0 (mol/m3) are the reference concentrations (i.e., the inlet concentrations). The model also introduces a momentum conservation equation to describe the convective phenomenon, in which case the flow regime is assumed to be a steady-state and incompressible laminar flow, and the Navier-Stokes equation is used to describe this process:

其中，是动力黏度（Ns/m2）、 是密度（kg/m3）以及 p 是压力 （Pa）。电极表面为固体表面，没有物质传递，所以采用壁面无滑移边界条件来描述固体壁面，而在求解域的顶部和两侧使用滑移边界条件来描述求解域中电解液与外部电解液相交。在入口处，设定了一个平均入口速度作为层流流入的边界条件；而在出口处，参考压力被设定为零作为压力条件。
where  is the dynamic viscosity (Ns/m2),  is the density (kg/m3), and p is the pressure (Pa). The electrode surface is a solid surface with no material transport, so the wall no-slip boundary condition is used to describe the solid wall, and the slip boundary condition is used at the top and sides of the solution domain to describe the intersection of the electrolyte and the external electrolyte in the solution domain. At the inlet, an average inlet velocity is set as the boundary condition for laminar inflow. Whereas, at the outlet, the reference pressure is set to zero as the pressure condition.

要想提高网格质量，使模型计算时能够更好的收敛，需要根据模型的不同几何结构选择不同的网格剖分方式：针状结构顶面和底面选择自由三角形网格以适应针状结构的剖分，而无针状结构的几何模型相对简单，顶面和底面选择网格质量最好的自由四边形网格，随后在高度方向通过扫掠的方式对三维几何结构进行网格剖分。由于电极-电解液交界面处反应物浓度变化较大，所以对该处网格进行加密处理。网格剖分结果如图 3-11 所示。
In order to improve the mesh quality and make the model better convergent during calculation, it is necessary to choose different meshing methods according to the different geometric structures of the model: the top and bottom surfaces of the needle-like structure choose free triangle meshes to adapt to the partitioning of the needle-like structure, while the geometric model without needle-like structures is relatively simple, and the free quadrilateral mesh with the best mesh quality is selected for the top and bottom surfaces. The 3D geometry is then meshed by sweeping in the height direction. Due to the large change in the concentration of reactants at the electrode-electrolyte interface, the grid was encrypted. The meshing result is shown in Figure 3-11.

3.4.3计算模型结果分析
3.4.3 Analysis of the results of the calculation model

图 3-12 为两种不同电极表面结构在外部电势为 1.2V 时的局部电流分布图，其中图（a）是 N-NiCoO2-GFs 电极表面结构，图（b）是 GF 电极表面结构。从中可直观看到，两种结构的局部电流分布均相对均匀。尽管针状阵列结构会增大电极表面比表面积，使出口处反应物浓度降低，致出口处局部电流密度小于入口处，但总体局部电流比原始石墨毡电极表面结构高得多。而且因电极表面建模的局限性，实际的针状阵列存在于圆柱体形状的石墨纤维表面，所以实际的局部电流密度分布可能比模拟结果更均匀。
Figure 3-12 shows the local current distribution of two different electrode surface structures with an external potential of 1.2 V, where figure (a) is the surface structure of the N-NiCoO2-GFs electrode and figure (b) is the surface structure of the GF electrode. It can be seen that the local current distribution of both structures is relatively uniform. Although the needle-like array structure increases the specific surface area of the electrode surface, reduces the concentration of reactants at the outlet, and causes the local current density at the outlet to be smaller than that at the inlet, the overall local current is much higher than that of the original graphite felt electrode surface structure. Moreover, due to the limitations of electrode surface modeling, the actual needle-like array exists on the surface of the cylindrical graphite fiber, so the actual local current density distribution may be more uniform than the simulation results.

为更直观展现针状阵列结构的传质优势，选取外部电势为 1.45V 时的稳态分布。图 3-13 中（a）（b）（c）（d）均为外部电势为 1.45V 时的稳态分布，V4 表示 VO2+，V5 表示 VO2+。观察图 3-9（a）和（b）可知，相比电极表面光滑的结构，针状阵列的存在使 VO2+在电极表面充分反应，这得益于电极比表面积增大，为反应物提供更多反应活性位点。同时，针状阵列由三元金属氧化物 NiCoO2 构成且掺氮处理，相比原始石墨毡电极能起很好的电催化作用，进一步促进反应物在电极表面转化。图 3-9（c）和（d）为 VO2+在针状阵列结构中的流线图，流线方向及密度代表 VO2+的通量，颜色代表浓度。在图（c）针状阵列区域，入口流线密、浓度高，出口流线少且浓度低，表明针状阵列能使反应物在电极表面快速反应。图（d）显示，生成的 VO2+在接近出口处物质通量斜向上，一方面是电解质主流方向 VO2+浓度低，生成的 VO2+向低浓度扩散；另一方面是针状阵列内部电解液流速慢，上方流速快，导致生成的 VO2+向电解质主流方向移动。此外，针状阵列的垂直结构形成直线通道，能保证钒离子快速渗入和离开活性位点。
In order to more intuitively demonstrate the mass transfer advantages of the needle array structure, the steady-state distribution at an external potential of 1.45V was selected. In Figure 3-13, (a)(b)(c)(d) are steady-state distributions with an external potential of 1.45V, V4 for VO2+ and V5 for VO2+. Observing Figures 3-9(a) and (b), it can be seen that the presence of a needle-like array allows VO2+ to fully react on the electrode surface compared to the smooth structure of the electrode surface, thanks to the increased specific surface area of the electrode, which provides more reactive sites for the reactants. At the same time, the needle-like array is composed of ternary metal oxide NiCoO2 and doped with nitrogen, which can play a good electrocatalytic role compared with the original graphite felt electrode, and further promote the conversion of reactants on the electrode surface. Figure 3-9(c) and (d) show the streamlines of VO2+ in the needle-like array, where the streamline direction and density represent the flux of VO2+, and the color represents the concentration. In the needle array region of Figure (c), the inlet streamlines are dense and the concentration is high, and the outlet streamlines are few and the concentration is low, indicating that the needle array can make the reactants react quickly on the electrode surface. Figure (d) shows that the generated VO2+ material flux is inclined upwards near the exit, on the one hand, the VO2+ concentration is low in the main electrolyte direction, and the generated VO2+ diffuses to the low concentration; On the other hand, the flow rate of the electrolyte inside the needle array is slow, and the flow rate above is fast, causing the generated VO2+ to move towards the main electrolyte stream. In addition, the vertical structure of the needle-like array forms a straight channel, which ensures that vanadium ions can quickly penetrate into and leave the active site.

通过使用参数化求解器进行求解，并在电极表面对电流进行积分得出两种模型在施加不同外电势的情况下的极化曲线图，即电极表面总电流随施加电势的变化曲线，如图 3-14 所示。从图中可以看出，在考虑电极电化学极化和浓度极化的两种条件下，随着外部电势的增大，N-NiCoO2-GFs 电极表面产生的电流更高，说明改性后带有针状阵列结构的电极能够有效降低电极极化，表现出更好的电化学性能。
By using a parameterization solver and integrating the currents at the electrode surface, the polarization curves of the two models under different external potentials are obtained, i.e., the variation curves of the total current on the electrode surface with the applied potential, as shown in Figure 3-14. As can be seen from the figure, the current generated on the surface of the N-NiCoO2-GFs electrode is higher with the increase of the external potential under the two conditions of electrochemical polarization and concentration polarization, indicating that the modified electrode with needle-like array structure can effectively reduce the electrode polarization and exhibit better electrochemical performance.

3.5 本章小结
3.5 Chapter Summary

本章对原始石墨毡进行一系列处理，得到氮掺杂三元镍钴氧化物针状阵列修饰的石墨毡电极，然后通过物理表征、电化学测试和模拟仿真全面探究电极性能。扫描电子显微镜显示，酸处理使石墨毡表面呈缺陷结构，金属氧化物改性后电极表面为针状阵列结构，能极大增加电极比表面，提供更多活性位点。X 射线衍射和 X 射线光电子能谱表明电极表面晶体为 NiCoO2，氮掺杂不影响晶体结构，还引入大量氧空位，后续电化学测试显示氮掺杂提升电极电导率和电催化活性。循环伏安法和电化学交流阻抗法表明，N-NiCoO2-GFs 在电极活性、可逆性、减少极化和促进氧化还原反应扩散过程方面，均展现出最高的电化学性能。最后的电极表面三维结构模拟仿真显示，针状阵列结构可使活性物质在电极表面快速充分反应，促进钒离子渗入和离开电极表面。
In this chapter, a series of treatments were performed on the original graphite felt to obtain a graphite felt electrode modified by a nitrogen-doped ternary nickel-cobalt oxide needle array, and then the electrode performance was comprehensively explored through physical characterization, electrochemical testing and simulation. Scanning electron microscopy showed that the surface of the graphite felt was defective due to acid treatment, and the surface of the electrode was a needle-like array structure after metal oxide modification, which could greatly increase the specific surface of the electrode and provide more active sites. X-ray diffraction and X-ray photoelectron spectroscopy showed that the crystal on the surface of the electrode was NiCoO2, and nitrogen doping did not affect the crystal structure, and a large number of oxygen vacancies were introduced. Cyclic voltammetry and electrochemical AC impedance assay showed that N-NiCoO2-GFs exhibited the highest electrochemical performance in terms of electrode activity, reversibility, reduced polarization, and promotion of redox reaction diffusion process. Finally, the three-dimensional structure simulation of the electrode surface shows that the needle-like array structure can make the active material react quickly and fully on the electrode surface, and promote the penetration and departure of vanadium ions on the electrode surface.