2022-05-21
研究背景
Li-CO2电池具有能量密度高、环境友好以及应用前景广阔等诸多优点。但是其仍然面临着CO2吸附和还原动力学差、放电产物Li2CO3难以分解等诸多问题。因此科研工作者开发了各种催化剂来解决这些问题。其中，贵金属催化剂尤其是贵金属Ru基催化剂由于具有高CO2RR和CO2ER催化活性以及相较于其他铂族金属相比具有成本优势等优点收到了人们的青睐。然而，目前关于Ru基催化剂的研究也存在一些不足，如Ru催化剂结构单一，缺乏不同Ru金属结构之间协同作用的研究。
因此，本文基于功能化碳载体的设计，通过对Ru单原子和Ru原子团簇活性位点的构建，制备了Ru单原子（RuSA）和Ru原子团簇（RuAC）复合的催化剂（RuAC+SA@NCB）并将其应用于Li-CO2电池，用以提升电池的电化学性能。该工作发表在Advanced Materials上，硕士生林江枫为文章第一作者，王浩志博后、丁佳教授和韩晓鹏副教授为文章共同通讯作者。
主要工作
文章首先利用Ru3+与NH2-CQD中-NH2于NCB中吡啶氮/吡咯氮吸附能力的差异制备了RuSA与RuAC复合的RuAC+SA@NCB。此外，还制备了仅含有Ru单原子的RuSA@NCB和仅含有Ru原子团簇的RuAC@NCB。
Figure 1. a) Schematic illustrating the synthesis procedure for RuAC+SA@NCB. b) TEM image of RuAC+SA@NCB. c) HRTEM image of RuAC+SA@NCB. d) EDX mapping of C, N, and Ru for RuAC+SA@NCB. e,f) HAADF-STEM images of RuAC+SA@NCB at different magnifications. g) HAADF-STEM image of RuSA@NCB.
接下来使用XAS对RuAC+SA@NCB和RuSA@NCB中活性位点的电子结构和原子构型进行分析。XANES图像可以看出两种材料中Ru的价态均位于1~4之间且RuSA@NCB中Ru的价态略高。R空间和小波转换图谱显示出RuAC+SA@NCB样品中同时存在Ru-N键和Ru-Ru键，而RuSA@NCB样品中只存在Ru-N键。随后对两者的结构进行拟合得到如图所示结构。
Figure 2. a) The normalized Ru K-edge XANES curves of RuO2, Ru powder, RuAC+SA@NCB, and RuSA@NCB. b) FT-EXAFS spectra in R space for RuO2, Ru powder, RuAC+SA@NCB, and RuSA@NCB. c) Wavelet transforms for the k3-weighted EXAFS signals of RuO2, Ru powder, RuAC+SA@NCB, and RuSA@NCB. d,e) EXAFS fitting curves at R space for RuAC+SA@NCB and RuSA@NCB sample, respectively. Insets: schematic atom configurations for the fitting of EXAFS spectra.
将所制备的材料应用于Li-CO2电池进行电化学性能测试。使用RuAC+SA@NCB的Li-CO2电池显示出良好的循环性能和倍率性能，此外其过电位与近期文献中发表的Li-CO2电池相比同样显示出巨大优势。
Figure 3. Electrochemical performance of Li–CO2 batteries using 1 m LiTFSI/TEGDME electrolyte. a) Voltage profiles of RuAC+SA@NCB cell at a current density of 300 mA g−1 and capacity limit of 500 mAh g−1. b) Discharge/charge curves of RuAC+SA@NCB cell tested at current densities from 100 to 2000 mA g−1. c,d) Discharge/charge curves of RuAC+SA@NCB, RuSA@NCB, and NCB cells at 500 mA g−1 (c) and 2 A g−1 (d) with capacity limit of 1000 mAh g−1. e) Total overpotentials of RuAC+SA@NCB, RuSA@NCB, and NCB cells at different current densities. The overpotential refers to the voltage difference between the middle points of discharge and charge curves. f) Comparison of overpotentials at 1 A g−1 for RuAC+SA@NCB with the-state-of-the-art catalysts in the literature.
对放电产物进行验证，发现放电产物为Li2CO3，且放电产物在充电后完全消失，说明RuAC+SA@NCB具有良好的可逆性。
Figure 4. a) TEM image of the RuAC+SA@NCB cathode after discharge at current density of 100 mA g−1 with a cut-off voltage of 2.0 V. b) TEM image of the RuAC+SA@NCB cathode after charge back at current density of 100 mA g−1. c) XRD patterns and d) FTIR spectra of RuAC+SA@NCB at pristine, after discharge, and after charge. e,f) XPS spectra of C 1s (e) and Li 1s (f) ffor RuAC+SA@NCB at discharge and charge states.
通过理论计算验证了Ru单原子和Ru原子团簇之间的协同机理。Ru原子团簇的存在调控了Ru单原子活性位点的电子结构，优化了关键中间产物的吸附效应，从而降低了速控步骤的能力，提升了CO2ER/CO2RR动力学。
Figure 5. a) The planar averaged potential and work function of RuAC+SA@NCB and RuSA@NCB. b) Total density of states (TDOS) of RuAC+SA@NCB and RuSA@NCB. c) Projected density of states (PDOS) of Ru atom in the Ru–N4 site in RuAC+SA@NCB and RuSA@NCB, respectively. d) Gibbs free energy diagrams during the CO2ER process. e) The atomic configuration of *Li2C2O4 intermediate adsorption on RuAC+SA@NCB and RuSA@NCB. f) −COHP of the Li-O bonds in the configurations in (e).
结论
本文利用Ru3+与NH2-CQD中-NH2于NCB中吡啶氮/吡咯氮吸附能力的差异制备了RuSA与RuAC复合的RuAC+SA@NCB。DFT计算表明，Ru单原子位点为活性中心，而Ru原子团簇的存在调控了Ru单原子的电子结构，从而优化了关键中间产物的吸附效应，降低了速控步骤的能垒，提升了反应动力学。使用RuAC+SA@NCB的Li-CO2电池在1和2A/g的大电流密度下的过电位仅为1.65和1.86 V。该工作为Li-CO2电池中新型催化剂的设计与制备提供了新的思路。
文章下载地址：https://onlinelibrary.wiley.com/doi/10.1002/adma.202200559
