Advanced Energy Materials:揭示镍修饰氮掺杂碳催化剂在电还原CO2制CO中的真正活性位

1、研究背景

化石燃料的过度使用导致了严重的环境问题和气候问题。在常温常压条件下电化学还原CO2为高附加值燃料和化学品是缓解温室效应和能源危机的有效方法。在不同的电还原CO2路径中,CO2还原为CO(CO2 + 2H+ + 2e → CO + H2O,-0.11 V vs RHE,双电子转移过程)是最有望先实现工业化的路径之一,因为它具有较低的过电位和高的可实现的法拉第效率(能够接近100%)。此外,气态产物CO可以很容易地收集,与H2混合用于进一步的化学过程,如甲醇生产和费托合成。

氮掺杂碳基过渡金属单原子催化剂已被广泛地证明能够高效电还原CO2为CO。其中,Ni单原子氮掺杂碳催化剂尤其表现出了较高的对CO的选择性(多数>95%)。然而近期的一些工作报道氮掺杂碳部分包覆/包裹的镍纳米颗粒(NPs)具有与Ni单原子催化剂相似的活性,这些研究中往往忽略了Ni单原子的存在,将优异的电催化还原CO2(CO2RR)的活性归因于Ni NPs的作用。然而,在高温碳化过程中,很难在不形成Ni-Nx位点的情况下得到纯的氮掺杂碳部分包覆/包裹的Ni NPs,因为在高温下Ni原子易与 N原子配位形成单个Ni-Nx位点,及Ni单原子位点。与此同时,Javier Pérez-Ramírez教授团队也在最近的一篇论文中提到:“一些研究声称,氮掺杂碳包裹的镍纳米颗粒可以表现出CO2RR活性,但是不能从所提供的数据中排除原子分散镍的存在”(ACS Catal. 2020, 10, 3444-3454)。因此,在缺乏系统对比性实验论证和不存在Ni单原子的确凿证据的情况下,氮掺杂碳部分包覆/包裹的Ni NPs是否真的对电还原CO2具有活性仍然存在疑问。同时,目前尚不清楚碳层厚度对封装的Ni NPs的CO2RR性能有何影响。

2、文章概述

近日,北京林业大学环境科学与工程学院王强教授与中国科学院大连化学物理研究所刘岳峰领导的科研团队联合报道了镍修饰氮掺杂碳催化剂在电还原CO2产CO中的真正活性位的研究。本文通过系统的验证性实验及理论计算,发现氮掺杂碳(N-C)部分包覆/包裹Ni纳米颗粒/单原子体系中电化学CO2还原制CO的真实活性中心是Ni单原子,指出氮掺杂碳部分包覆/包裹的Ni纳米颗粒不具备还原CO2活性。为后续的研究电还原CO2制CO的M-N-C催化剂提供了一个活性位辨识方向的参考。

3、图文导读

A. 材料合成及表征

我们首先采用一步热解法合成了一系列同时含有Ni单原子和氮掺杂碳部分包覆/包裹的Ni NPs的Ni/氮掺杂石墨碳 (Ni-NG) 催化剂。根据表征结果,在三种不同Ni含量的催化剂中单原子Ni和Ni NPs共存,并且分别在不同Ni含量的 Ni-NG 催化剂中占主导地位。随着 Ni-NG 催化剂中Ni负载量的减少,Ni物种更倾向于分散而较少的聚集,即原子分散的Ni更多而Ni NPs更少。

Figure 1. Morphology characterizations of different Ni-NG catalysts. a-c, SEM images showing abundant carbon encapsulated Ni NPs, scale bar: 1 μm. d-f, HAADF-STEM images and the corresponding EDS elemental mapping of C, N, Ni, scale bar: 200 nm, 50 nm and 50 nm for d, e, f, respectively. g, High-resolution HAADF-STEM images of Ni1.11-NG with local FFTs inset in (g), scale bar: 20 nm. h-i, High-resolution HAADF-STEM images of Ni0.37-NG and Ni0.037-NG showing the presence of single Ni atoms, scale bar: 1 nm.

Figure 2. Structure characterizations of different Ni-NG catalysts. a, XRD patterns. The JCPDS profiles of graphite (dark green) and metallic nickel (purple) are exhibited as reference. b, Raman spectraof Ni1.11-NG, Ni0.37-NG, Ni0.037-NG, NC catalysts.Two peaks at 1355 cm−1 (D band) and 1585 cm−1 (G band) were observed for all samples. c, High resolution XPS Ni 2p spectra, the binding energies of Ni 2p in Ni0.037-NG and Ni0.37-NG stay in a ionic Niδ+ (0 <δ<2) state, lower than +2 but higher than 0, while Ni1.11-NG stay in metallic Ni0 state. d, Ni content of different Ni-NG catalysts, obtained from XPS and ICP-OES measurements. e, High resolution XPS N 1s spectra of Ni1.11-NG, Ni0.37-NG, Ni0.037-NG and NC catalysts, which is deconvoluted into four peaks using the Shirley background.

B. 电催化还原CO2性能测试

根据线性扫描伏安曲线、不同电位下电还原CO2产物选择性、及电流密度等结果,不同Ni含量的Ni/氮掺杂石墨碳 (Ni-NG) 催化剂电还原CO2 产CO的活性与其单原子Ni的含量成正相关,并且与动力学数据分析结果(Tafel slope,EIS)相吻合。单原子Ni占主导的Ni0.037-NG催化剂电还原CO2的电荷转移过程最快,表现出了最优的CO选择性及活性。

Figure 3. Performance of CO2 electroreduction. a, The Linear sweep voltammetric (LSV) curves in CO2-saturated 0.5 M KHCO3 electrolyte on glassy carbo electrode at a scan rate of 20 mV s-1. b-c, The CO FEs and CO partial current densities of catalysts loaded on glassy carbon electrode (GCE) at independent potentials from -0.5 to -1.3 V (versus RHE). The data were averaged over three repeated measurements for three Ni-NG catalysts. d, Mass activities for CO formation corresponding to Ni. e, Tafel plots of polarization overpotential (η) versus CO partial current density of different catalysts. f, The Nyquist plots of catalysts by applying an AC voltage with 5 mV amplitude.

C. Ni-NG催化剂酸洗前后电催化还原CO2性能对比

为了进一步确定活性位点是氮掺杂碳部分包覆/包裹的Ni NPs还是Ni单原子,或者是两者兼有,我们对三种Ni-NG催化剂进行了长时间的强酸处理,表征结果显示大部分Ni NPs被去除,仅有少数被多层碳层包裹的Ni NPs保留。通过长时间的酸浸处理去除大部分Ni NPs,所得的三种Ni-NG-H催化剂CO2RR活性并没有降低,甚至CO选择性略有增加,表明去除的Ni NPs不能促进CO2RR。展现出了最优性能的Ni0.037-NG-H催化剂,其中Ni主要以原子分散形式存在,在-0.8 V vs RHE电解条件下表现出了出色的稳定性,在64 h的电解过程中CO选择性及电流密度较稳定,CO平均选择性达到95%,平均电流密度约为15 mA cm-2

Figure 4. Activity comparison of Ni-NG and Ni-NG-H catalysts and stability test of Ni0.037-NG-H catalyst. a-b, The CO FEs and CO partial current densities comparison of Ni-NG and Ni-NG-H catalysts loaded on carbon paper (CP) at independent potentials from -0.5 to -1.2 V (versus RHE). c, FE and current density as a function of time during long-term reaction of 64 h for Ni0.037-NG-H at -0.8 V (vs RHE) in CO2-saturated 0.5 M KHCO3.

D. 报道中的催化剂验证

为了进一步佐证我们的结论,我们也选取了两种文献报道的用于CO2RR的典型氮掺杂碳包裹的Ni NPs催化剂(Ni-NC_TPA@C和Ni@NCNTs),并通过HAADF-STEM表征证实了Ni单原子的存在。两种催化剂经酸洗处理后,其CO2RR性能同样并未下降,证明其活性实际上应来源于被忽略的Ni单原子。

Figure 5. Morphology characterization of two repeated carbon encapsulated Ni NPs catalysts. a, HRTEM image of Ni-NC_TPA@C showing carbon encapsulated Ni NPs. b, HAADF-STEM image and the corresponding EDX elemental mapping of C, N, Ni for Ni-NC_TPA@C. c, High-resolution HAADF-STEM image of Ni-NC_TPA@C showing the presence of single Ni atoms circled by red circles. d, HRTEM image of Ni@NCNTs showing carbon encapsulated Ni NPs with enlarged image inset. e, HAADF-STEM image and the corresponding EDX of C, N, Ni for Ni@NCNT. f, High-resolution HAADF-STEM image of Ni@NCNTs showing the presence of single Ni atoms circled by red circles.

E. DFT计算

对不同氮掺杂碳层数包裹的Ni NPs的CO2RR路径及析氢反应路径进行了DFT计算。计算结果表明,氮掺杂碳包覆的Ni NPs相较于CO2RR更加倾向于析氢反应。差分电荷密度图还表明,对于3层碳层包覆的Ni NPs结构,内部的Ni NPs不会影响最外层碳层的电子结构。因此,在具有三层以上碳层的N掺杂碳包覆的Ni NPs催化剂中, Ni NPs和表面碳层之间很大可能不会存在协同效应。

Figure 6. DFT calculations of CO2 electroreduction and HER. a, Calculation model and charge density difference graph of NGR@Ni (left: side view; right: top view). b, Calculation model and charge density difference graph of 3NGR@Ni (left: side view; right: top view). Yellow and blue represent the charge accumulation and depletion, respectively. c, Free energy diagrams for CO2RR pathways on NGR, NGR@Ni and 3NGR@Ni at 0 V (C atoms, brown; N atoms, purple; Ni atoms, gray). Inset show the optimized configurations of intermediates with the 3NGR@Ni. d, Limiting potentials for CO2 reduction and H2 evolution, respectively. e, Difference in limiting potentials for CO2 reduction and H2 evolution. Free energy diagrams over Ni-N4, NGR, NGR@Ni and 3NGR@Ni were calculated based on CHE model, while for Ni-N1 it was based on AIMD method.

4、总结与展望

本文设计合成了三种同时含有不同比例的Ni单原子和氮掺杂碳部分包覆/包裹的Ni NPs的Ni/氮掺杂石墨碳 (Ni-NG) 催化剂,并且采用强酸长时间酸洗处理去除了催化剂中大部分Ni NPs,对三种催化剂酸洗前后的CO2RR性能进行了比较。并对不同氮掺杂碳层数包覆的Ni NPs的CO2RR路径及析氢反应路径进行了DFT计算。最终通过系统的实验设计及理论计算,指出氮掺杂碳部分包覆/包裹Ni纳米颗粒对CO2电还原没有活性,实际的催化活性来自于Ni单原子。并提出在今后的镍修饰氮掺杂碳催化剂的设计过程中,应避免形成Ni纳米颗粒结构,最大化形成原子分散的Ni-Nx位点。

论文信息:

Revealing the Real Role of Nickel decorated Nitrogen-doped Carbon Catalysts for Electrochemical Reduction of CO2 to CO

Shuyu Liang, Qian Jiang, Qiang Wang*, Yuefeng Liu*

Advanced Energy Materials

DOI:10.1002/aenm.202101477

原文链接:https://onlinelibrary.wiley.com/doi/abs/10.1002/aenm.202101477