锌空电池凭借其高能量密度、可靠的安全性等优点有望成为下一代能量储存装置，但是其空气阴极催化剂多采用贵金属材料，高昂的成本和无法满足生产、生活需要的稳定性阻碍了锌空电池的进一步发展。近年来，纳米材料与技术的发展使得碳材料用作催化剂取得了令人瞩目的成就，以碳材料作为基底，通过引入杂原子、构建特殊的一维/二维/三维结构，将其催化性能大幅提升。不过在催化反应中的活性位点的准确识别是一个难点，此外在实际的工作环境中，锌空电池的功率密度与理论值相比还有较大差异。
为了进一步提高碳材料的催化性能，兰州大学材料与能源学院彭尚龙教授团队利用简单的模板法制备了Co、N共掺杂的三维多孔碳材料（3D Co/N-C），将其用于锌空电池，展示了高的双功能催化活性、令人瞩目的功率密度以及优异的稳定性。并通过巧妙的实验设计和腐蚀方法，辅以DFT理论计算，准确地识别出了ORR和OER中的活性位点。相关的研究成果以“Precise Identification of Active Sites of a High Bifunctional Performance 3D Co/N-C Catalyst in Zinc-air Batteries”发表在《Chemical Engineering Journal》期刊上。彭尚龙教授和马飞副教授为通讯作者。
在这项工作中，采用氯化钠为模板的方法制备出了三维蜂窝状的催化剂3D Co/N-C，通过SEM、TEM、BET、XPS等手段对丰富的分级多孔结构和材料的化学组成进行了表征，将材料制备成电极测试了其ORR和OER性能，并组装了锌空电池测试器件的功率密度和稳定性，3D Co/N-C表现出优异的双功能催化性能、令人满意的功率密度和循环稳定性。最后通过酸腐蚀、硫氰酸根毒化的方法确定了在ORR和OER过程中的活性位点，并通过DFT计算证实了实验结论。
【图文导读】
Scheme 1. Schematic depiction of the synthesizing processes of the 3D Co/N-C catalyst.
Figure 1. Morphology characterization of 3D Co/N-C. (a) SEM, (b) TEM, (c) HRTEM, (d) STEM images and the corresponding elemental mapping.
Figure 2. (a) XRD patterns of 3D Co/N-C and Co/N-C. (b) N2 adsorption-desorption curves, micropore and mesopore size distributions (inset). dV/dD is the differential pore volume distribution, where V represents pore volume and D means pore diameter. (c) Co 2p spectra, (d) N 1s spectra of 3D Co/N-C.
Figure 3. Electrochemical and Zinc-air cell performances. (a) ORR performance of 3D Co/N-C, Co/N-C and commercial 20% Pt/C catalysts. The obtained linear sweep voltammetry (LSV) curves were measured at a potential step of 5 mV. Electrolyte 0.1 M KOH saturated with O2, rotation rate 1600 rpm, room temperature. (b) OER performance of 3D Co/N-C, Co/N-C and commercial IrO2 catalysts. Steady-state polarization plots were measured at a potential step of 5 mV. Electrolyte 1 M KOH, room temperature. (c) Schematic diagram of an aqueous zinc–air battery, as-prepared catalyst was used as cathode, zinc plates were used as anode, the electrolyte was 6 M KOH and 0.2 M zinc acetate. (d) Discharge polarization lines of 3D Co/N-C and 20% Pt/C based air electrodes for the rechargeable ZABs and the correspondent power density curves of ZABs. (e) Galvanostatic discharging plots at a current density of 10 mA cm-2 of 3D Co/N-C and 20% Pt/C+IrO2.
Figure 4. Identification of active sites. (a) Deconvolution of N 1S spectra, (b) ORR performance of 3D Co/0.5N-C, 3D Co/N-C and 3D Co/1.5N-C. (c) XRD spectra, (d) OER performance of 3D Co/N-C and 3D Co-N-C in the electrolyte of 1 M KOH and 0.5 M NaSCN. Figure 5. (a) O adsorption energy on graphitic N, pyridinic N and pyrrolic N, respectively. (b) Reaction paths on pyridinic N and pyrrolic N. (c) The local structural illustrations for demonstrating the ORR (alkaline) process within the 3D Co/N-C system.
【总结】
该工作采用简易、环保的模板法成功合成了三维蜂窝状的3D Co/N-C催化剂，表现出优异的双功能活性（E1/2 = 0.84 V，Ej=10 = 1.56 V），其组装的锌空电池展示了优秀的功率密度（239 mW cm-2）和超过200小时的高稳定性。通过控制不同氮的比例、设计腐蚀和毒化实验，以及DFT理论计算，准确地识别出吡啶氮、钴金属分别为ORR、OER的活性位点，为准确设计高性能双功能催化剂提供了思路。
Precise Identification of Active Sites of a High Bifunctional Performance 3D Co/N-C Catalyst in Zinc-air Batteries, Chemical Engineering Journal, 2022, DOI: 10.1016/j.cej.2022.134500
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