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作者和单位：第一作者和单位：Bing Tang, Yanan Zhou, Qianqian Ji.；中科大国家同步辐射实验室、宁波大学原文链接：https://doi.org/10.1038/s44160-024-00545-1关键词：双原子催化剂、氧还原反应、析氧反应、轨道杂化、Zn-空气电池为了最大限度地发挥双原子催化剂（DACs）在多步催化反应中的潜力，一个重要但很难实现的策略是为两种金属定制不同的阴离子配位点来构建Janus双金属位点，从而通过多种协同效应使其功能最大化。然而，由于缺乏有效的合成方法以及难以确切区分活性位点的非晶体结构，具有明确的多种配位阴离子的 Janus 型 DACs很少被报道。本文通过热解-缺陷制造-再热解的方式，成功构筑了具有Janus双金属位点的FeCo-N3O3@C催化剂。FeCo-N3O3@C在氧还原反应（ORR）中的半波电位达到了0.936 V，同时在10 mA/cm2电流密度下析氧反应（OER）的电位为1.528 V，由其组装的Zn-空气电池表现出143 mW/cm2的功率密度，并能稳定运行200小时以上。光谱和理论计算结果证明FeCo-N3O3@C催化剂中FeN3和CoO3分别为ORR和OER的活性位点，其优异催化活性来源于Fe、Co位点上3d轨道电子填充的优化。
双原子催化剂（DACs）作为单原子催化剂（SACs）的一种扩展类型，不仅继承了SACs高原子利用率的优点，同时， DACs具有更加丰富的表面原子排列方式以及独特的电子结构，这些特点使得DACs能够更好的吸附活化一些反应的中间体。此外，DACs在作为反应机理研究的平台上也表现出潜在的优势。然而目前制备DACs的方法主要依赖于限域或分子锚定，这种缺乏精确性的合成方式通常会为DACs带来结构不确定性，即通常会产生单原子和非目标双原子，从而在后续的催化过程中产生副反应；此外，金属中心的性质和几何结构对反应速率和机理会产生深远的影响，在DACs中的配位阴离子可以起到稳定中间体、促进反应发生的作用。在可充电的金属-空气电池中，ORR和OER的发生通常需要不同的反应位点（Fe-N位点催化ORR，Co-O位点催化OER），但在目前的报道中DACs通常只具有一种配位阴离子。因此构建具有多种配位阴离子的Janus型DACs可能有利于在多步催化过程中的应用。
通过结合分步热解和缺陷制造的方式，制备了一种Janus型FeCo双原子位点的DACs（FeCo-N3O3@C），通过像差校正高角环形暗场扫描透射电子显微镜（HAADF-STEM）、X-射线吸收光谱（XAS）、X射线发射光谱等技术解析了DACs的结构。文章进一步测试了FeCo-N3O3@C的ORR、OER性能，并在此基础上组装了Zn-空气电池。作者结合原位光谱和理论计算证实FeCo-N3O3@C出色的催化性能来源于Fe、Co原子3d轨道上电子填充结构的优化。Figure 1. A survey of atomically dispersed metal catalysts based on carbon substrates. The geometric (top) and electronic (bottom) structures of singleatom, homologous dual-atom, heterogeneous dual-atom and Janus dual-atom materials. In the ball-and-stick diagrams, the grey, blue, red, yellow and cyan balls represent carbon, nitrogen, oxygen, iron and cobalt atoms, respectively. The molecular orbital diagrams depict the orbital interactions between various metal centres (M) and ligands (L).在图1中，作者首先给出了目前几种常见的原子分散于碳基底上的催化剂模型，相比于单原子催化剂、双同原子催化剂以及双异原子催化剂，本文设计制备的Janus型双原子催化剂能够同时实现金属位点之间的协同、金属位点与配位阴离子可调、强电子耦合效应、明确的双功能活性位点以及更高的催化活性和选择性等优势。 ![]()
Figure 2. Synthetic scheme and morphological characterization of the FeCo-N3O3@C catalyst.a, Schematic illustration of the synthesis of FeCo-N3O3@C. b,c, TEM (b) and HRTEM (c) images of the FeN4@C precursor. Inset in c: SAED pattern of FeN4@C. d, Aberration-corrected HAADF-STEM image of the FeN4@C precursor. The dotted circles show single atoms (red) and dual-atom pairs (yellow). e, Three-dimensional atom-overlapping Gaussian-function fitting map of the isolated atom A1 in d. f, Statistical distribution of the single atoms and dual-atom pairs in d. g, HAADF-STEM (top left) image of FeCo-N3O3@C and corresponding energy-dispersive X-ray spectroscopy (EDS) mapping images for C, N, O, Fe and Co. h,i, Aberration-corrected HAADF-STEM image of FeCo-N3O3@C (h) and a magnification of the area highlighted in h (i). The dotted circles show single atoms (red) and dual-atom pairs (yellow). j, Three-dimensional atom-overlapping Gaussian-function fitting map of the atom pair A1-B1 highlighted in the rectangular dashed box in i. k, Statistical distribution of the single atoms and dual-atom pairs in i. l-o, FT k3-weighted Fe (l,m) and Co (n,o) K-edge EXAFS spectra and the corresponding fitting curves for FeN4@C (l), CoO4@C (n) and FeCo-N3O3@C (m,o). k3χ(k) refers to the wave function in wavevector space (k-space), and the vertical bars || represent absolute value. Insets: the corresponding optimized structures (the grey, blue, red, yellow and cyan balls represent carbon, nitrogen, oxygen, iron and cobalt atoms, respectively). 图2首先介绍了FeCo-N3O3@C的合成方式，作者以FeZn双金属二维MOF作为原始材料，在900℃下碳化的同时去除Zn2+，得到FeN4@C前驱物，再以Ar气等离子体刻蚀，得到具有缺陷的d-FeN3@C，在吸附乙酰丙酮钴之后在低温下再次热解，最终得到Janus型FeCo-N3O3@C催化剂。通过HAADF-STEM对比在引入Co原子位点后，催化剂中大量出现双金属原子对，其间距约为0.252 nm，表明金属原子对之间可能存在键合。作者通过对催化剂中Fe、Co的扩展边X-射线精细谱进行拟合，得到Janus型FeCo-N3O3@C催化剂中Fe、Co的配位结构，同时，扩展边的结果也证实Fe、Co原子位点之间存在键合。Figure 3. Structural characterization of the FeCo-N3O3@C catalyst.a-c, C (a), N (b) and O (c) XANES spectra of FeCo-N3O3@C and reference samples. d,e, Fe (d) and Co (e) valence-to-core XES spectra for FeCo-N3O3@C, FeN4@C and CoO4@C. f,g, Fe (f) and Co (g) L2,3-edge XANES spectra of FeCo-N3O3@C and reference samples. h,i, Experimental Fe (h) and Co (i) K-edge XANES spectra of FeCo-N3O3@C and the corresponding theoretical spectra. Insets: structures used to simulate the XANES spectra (the grey, blue, red, yellow and cyan balls represent carbon, nitrogen, oxygen, iron and cobalt atoms, respectively).为了进一步区分配位阴离子，紧接着作者对FeCo-N3O3@C中C、N、O的存在方式进行了表征，C、N、O的K边X-射线吸收光谱分别证明了C作为基底不直接参与金属位点的配位、N主要参与Fe的配位以及O主要参与Co的配位；进一步Fe、Co的XES谱也证实Fe、Co也会分别与O、N配位。这样的结果也与EXAFS的拟合结果一致。Fe、Co的L2,3-edge的XANES则证明Fe的3dz2轨道存在未被电子占据而Co的3dz2轨道被更多的电子占据的情况，进一步证明Fe、Co之间存在成键的趋势。![]()
Figure 4. Catalytic performance of FeCo-N3O3@C in the ORR and OER.a, LSV curves for FeCo-N3O3@C, FeN4@C, CoO4@C, Pt/C, and CNO in the ORR in O2-saturated 0.1 M KOH solution at 1,600 r.p.m. b, Half-wave potential (E1/2) and kinetic current density (Jk) at 0.90 V for the different catalyst samples. c, Tafel plots derived from the LSV curves in a. The vertical bars || represent the absolute values of current density before taking the logarithm in the x-axis label. d, Normalized chronoamperometry curves for FeCo-N3O3@C and Pt/C at a constant potential of 0.7 V. e, Tolerance of FeCo-N3O3@C and Pt/C towards methanol. f, Radar plot comparing the ORR performance of FeCo-N3O3@C and the reference samples. The numbers below the parameters in the radar represent the range of the scale. g, LSV curves of FeCo-N3O3@C, FeN4@C, CoO4@C, RuO2 and CNO in the OER in 0.1 M KOH. h, Mass activity and TOF of the samples in the OER at 1.6 V. i,j, Comparison of the bifunctional catalytic performance of FeCo-N3O3@C and the reference samples (i) and some representative DACs (j) in the ORR and OER. DSAC, dual single-atom catalyst; NCAG, N-doped carbon aerogel; NC-1000, N-doped carbon-1000; GHS, graphene hollow nanospheres; SA, single atom; NC-50, N-doped porous carbon-50; CS, carbon nanospheres. 作者分别测试了FeCo-N3O3@C在0.1 M KOH电解液中的ORR和OER性能，FeCo-N3O3@C表现出0.936 V的半波电位，具有最优的扩散电流和四电子ORR性能，并且具有出色的抗甲醇中毒能力，其ORR性能优于Pt@C催化剂；FeCo-N3O3@C表现出较低的10 mA/cm2的电位（1.528 V），其OER性能由于商业RuO2催化剂。FeCo-N3O3@C的OER与ORR电位差仅为0.592 V，表明FeCo-N3O3@C具有应用于金属-空气电池的前景。 Figure 5. DFT calculations and in situ SR-FTIR and XANES measurements. a-c, Charge density differences for FeCo-N3O3@C (a), FeN4@C (b) and CoO4@C (c). The yellow and cyan areas depict charge accumulation and depletion, respectively. d,e, The PDOS of Fe 3d (d) and Co 3d (e) for FeCo-N3O3@C, FeN4@C and CoO4@C. f,g, In situ SR-FTIR spectra of FeCo-N3O3@C in the ORR (f) and OER (g). OCP, open-circuit potential. h,i, In situ Fe and Co K-edge XANES spectra of FeCo-N3O3@C in the ORR (h) and OER (i). j, Schematic of the fourelectron ORR and OER pathways on the FeCo-N3O3@C catalyst. 为了进一步解释催化活性的来源以及活性位点的划分，作者进行了理论计算。差分电荷图表明在FeCo双原子对上出现了电荷的重新分配，得益于更少电子占据的3dz2轨道，Fe原子能够接收O配位阴离子的电荷，从而有利于含氧中间体的吸附、促进ORR反应，而Co的3dz2轨道存在更多的电子占据，更加有利于OER反应的发生。d带中心的计算发现FeCo-N3O3@C中FeCo双原子位点的d带中心都明显下降，更加有利于含氧中间体的吸附。原位红外和原位XAS证实了Fe位点是ORR活性位点，而Co位点是OER活性位点，且捕捉到的*OOH中间体证明ORR和OER都是四电子转移过程。并计算了Fe、Co位点上ORR、OER各反应步骤的自由能。![]()
Figure 6. Performances of ZABs based on FeCo-N3O3@C and Pt/C + RuO2. a, Schematic configuration of the homemade ZAB. b, Open-circuit voltage of the FeCo-N3O3@C-based ZAB. Inset: photograph showing the open-circuit voltage being measured with a multimeter. c, Discharge polarization and the corresponding power density curves of ZABs based on FeCo-N3O3@C and Pt/C + RuO2 catalysts. d, Zn-mass-normalized specific capacities for the ZABs at a constant current density of 10 mA/cm2. e, Rate capabilities of the ZABs at different current densities. f, Optical image of an LED powered by three FeCo-N3O3@C-based ZABs in series. g, Galvanostatic charge/discharge cycling tests for the ZABs at a current density of 10 mA/cm2. Insets: corresponding voltage efficiency curves during the cycling test. 由于FeCo-N3O3@C具有出色的ORR和OER性能，作者最终将FeCo-N3O3@C组装成Zn-空气电池，具有1.43 V的开路电压，其比容量、功率密度以及循环稳定性相较于Pt/C + RuO2催化剂组装的Zn-空气电池都更加出色，显示了潜在的应用前景。 构筑具有不同的阴离子配位点的Janus双金属位点催化剂对于发挥DACs的协同效应在多步反应中的优势具有重大意义，然而具有不同阴离子配位位点的Janus型DACs的合成具有很大的挑战。该工作的主要创新点在于：（1）巧妙的利用了杂原子掺杂带来的更高的表面能，利用Ar等离子体在FeN4@C前驱体的Fe位点周围构筑缺陷位点，并利用缺陷位点捕捉金属盐，进一步低温热处理得到具有N/O桥连的FeN3-CoO3位点的FeCo-N3O3@C，并通过XAS、XES等多种方式解析证明了这种结构的存在；（2）通过原位光谱和理论计算区分出ORR和OER的活性位点，并通过光谱证实了这种Janus型DACs中存在的电子结构优化是出色催化性能的来源。[1] Zhang, Y.-X. et al. General synthesis of a diatomic catalyst library via a macrocyclic precursor-mediated approach. J. Am. Chem. Soc. 145, 4819-4827 (2023).[2] Chen, S. et al. Dehydrogenation of ammonia borane by platinum-nickel dimers: regulation of heteroatom interspace boosts bifunctional synergetic catalysis. Angew. Chem. Int. Ed. 61, e202211919 (2022).[3] Yuan, K. et al. Boosting oxygen reduction of single iron active sites via geometric and electronic engineering: nitrogen and phosphorus dual coordination. J. Am. Chem. Soc. 142, 2404-2412 (2020).[4] Pei, Z. et al. Highly eficient electrocatalytic oxygen evolution over atomically dispersed synergistic Ni/Co dual sites. Angew. Chem. Int. Ed. 61, e202207537 (2022).[5] Zhang, X. et al. Identifying and tailoring C-N coupling site for eficient urea synthesis over diatomic Fe-Ni catalyst. Nat. Commun. 13, 5337 (2022).![]()
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