图文解析Fig. 1. The fabrication of NH2BPA-Ag with functional ligand layer. 图1为NH2BPA-Ag的合成过程。NH2BPA分子通过膦酸基在Ag表面化学键合。PVP分子的羰基O原子也可以与表面Ag原子结合,与NH2BPA一同在Ag NPs上编织一层薄薄的有机层。另外,丁胺(BNH2)和丁基膦酸(BPA)分子也通过类似的方法在Ag NPs上修饰(记作BNH2-Ag和BPA-Ag)。此外,也制备了未功能化的Ag NPs,表面仅存在PVP(记为Ag)。 Fig. 2. (a) SEM image, (b-d) TEM images and (e) HRTEM image of NH2BPA-Ag. (f) Bright field STEM image and (g) elemental mappings of NH2BPA-Ag. 图2为催化剂的形貌和结构。从图2c-e可以看出,NH2BPA-Ag表面包裹了一层均匀的有机膜,厚度约为2 nm。元素映射显示C、N和P元素在整个催化剂中均匀分布(图2g),表明NH2BPA分子均匀地锚定在Ag NP表面。 Fig. 3. (a) FT-IR spectra, (b) XRD patterns and (c) Ag 3d XPS spectra of catalysts. (d) N 1 s, (e) P 2p and (f) O 1 s XPS spectra of NH2BPA-Ag. 图3为催化剂表面化学组成的研究。XPS分析发现BPA-Ag的Ag 3d5/2峰出现了正位移(图3c),NH2BPA-Ag出现了与BPA-Ag相似的正位移,这表明Ag主要与磷酸基团发生电子相互作用,而配体另一端氨基则向外舒展用于CO2捕获。O 1s XPS(图3f)可以看出,NH2BPA-Ag在530.5 eV处的峰归属于P-O-Ag,这表明膦酸基团可以通过O原子与Ag NP表面共价结合。 Fig. 4. Potential-dependent (a) CO FEs and (b) CO partial current densities of catalysts in 0.5 M aqueous KHCO3 under SFG condition (15% CO2 + 4% O2 + 81% N2). (c) CO and H2 FEs for catalysts operating at -0.7 V vs. RHE under CO2, DCG (15% CO2 + 85% N2) or SFG. (d) LSVs of catalysts under SFG in 0.1 M TBAPF6/MeCN. (e) LSVs of catalysts in O2-saturated 0.5 M aqueous KHCO3 electrolyte. (f) CO2 adsorption isotherms of catalysts at 298 K. 图4为催化剂在H-cell中电催化SFG还原性能。在SFG条件下,NH2BPA-Ag在-0.7 V vs. RHE时达到82.0%的FECO。由于ORR的竞争,Ag和BNH2-Ag的FEs都有明显的损失,而膦酸功能化的催化剂(NH2BPA-Ag和BPA-Ag)的FEs缺失损失较小。在非质子电解质(0.1 M TBAPF6/MeCN)中用LSV对反应中间体进行研究(Fig. 4d),波峰的出现是由CO2•-中间体(一电子还原的CO2)的累积引起的。在SFG气氛下,含有氨基的催化剂表现出更大的电流,说明氨基促进CO2吸附并稳定中间体。通过LSV研究了催化剂的ORR活性(图4e),NH2BPA-Ag和BPA-Ag的ORR峰电位负移,电流密度减小,表明膦酸基团增加了ORR过电位,且减缓了其动力学。 Fig. 5. (a) Schematic illustration of a zero-gap MEA cell using AEM. (b) CO FEs and current densities vs. applied cell voltage on NH2BPA-Ag for humid SFG electrolysis in MEA cell. (c) Spider chart of performance comparison between NH2BPA-Ag and previously published data for flue gas electrolysis. (d) Stability test of humid SFG electrolysis for NH2BPA-Ag in MEA cell at 2.8 V. 图5为催化剂在零间隙MEA电解池中电催化SFG还原性能。NH2BPA-Ag电极,在2.8 V池压下,电流密度达到48 mA cm-2,FECO达到79.6%。NH2BPA-Ag MEA相比于目前最先进的含O2的CO2-to-CO电解池(图5c)获得了更高的电流密度和全电池能量效率(EE,38.1%)。 Fig. 6. (a) The differential charge density diagram of CO2 adsorption on NH2BPA-Ag. Yellow and cyan indicate accumulation and depletion of electrons, respectively. (b) Geometric structures and adsorption energies of *OOH intermediate on NH2BPA-Ag and Ag. (c) Bader charge analysis of NH2BPA-Ag and Ag with adsorbed *OOH. (d) O 1 s XPS for NH2BPA-Ag after SFG electrolysis. (e) Schematic illustrating the proposed mechanism for SFG electrolysis over NH2BPA-Ag. 图6为机理分析。差分电荷显示,氨基向吸附的CO2提供电子(图6a),从而激活CO2并增强CO2RR的活性。∆E*OOH用于评价ORR的活性,当底物由Ag变为NH2BPA-Ag时,∆E*OOH从-1.42 eV变为1.35 eV(图6b),说明Ag表面的膦酸基团不利于*OOH的稳定吸附,阻断ORR。图6c的Bader电荷分析显示,与膦酸基团相邻的Ag原子的电子耗损更多,表明其ORR的化学环境不太有利。
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