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Exposing Single Ni Atoms in Hollow S/N-Doped Carbon Macroporous Fibers for Highly Efficient Electrochemical Oxygen Evolution
在中空 S/N 掺杂碳大孔纤维中暴露单个镍原子，实现高效电化学氧进化

Yafei Zhao, Yan Guo, Xue Feng Lu, Deyan Luan, Xiaojun Gu,** and Xiong Wen (David) Lou*
赵亚非、郭艳、卢雪峰、栾德艳、顾晓军**和楼雄文（大卫）*

Abstract  摘要

The development of efficient and cost-effective electrocatalysts toward the oxygen evolution reaction (OER) is highly desirable for clean energy and fuel conversion. Herein, the facile preparation of Ni single atoms embedded hollow S/N-doped carbon macroporous fibers (Ni SAs@S/N-CMF) as efficient catalysts for OER through pyrolysis of designed CdS − NiS x / CdS − NiS x / CdS-NiS_(x)//\mathrm{CdS}-\mathrm{NiS}_{x} / polyacrylonitrile composite fibers is reported. Specifically, CdS provides the sulfur source for the doping of polyacrylonitrile-derived carbon matrix and simultaneously creates the hollow macroporous structure, while NiS x NiS x NiS_(x)\mathrm{NiS}_{x} is first reduced to nanoparticles and finally evolves into single Ni atoms through the atom migration-trapping strategy. Benefiting from the abundantly exposed single Ni atoms and hollow macroporous structure, the resultant Ni SAs@S/N-CMF electrocatalysts deliver outstanding activity and stability for OER. Specifically, it needs an overpotential of 285 mV to achieve the benchmark current density of 10 mA cm − 2 10 mA cm − 2 10mAcm^(-2)10 \mathrm{~mA} \mathrm{~cm}{ }^{-2} with a small Tafel slope of 50.8 mV dec − 1 50.8 mV dec − 1 50.8mVdec^(-1)50.8 \mathrm{mV} \mathrm{dec}^{-1}.
开发高效、经济的氧进化反应（OER）电催化剂，是清洁能源和燃料转化的迫切需要。本文报道了通过热解设计的 CdS − NiS x / CdS − NiS x / CdS-NiS_(x)//\mathrm{CdS}-\mathrm{NiS}_{x} / 聚丙烯腈复合纤维，简便地制备出嵌入镍单原子的中空 S/N 掺杂碳大孔纤维（Ni SAs@S/N-CMF），作为氧进化反应的高效催化剂。具体来说，CdS 为聚丙烯腈衍生碳基质的掺杂提供了硫源，并同时形成了中空的大孔结构，而 NiS x NiS x NiS_(x)\mathrm{NiS}_{x} 首先被还原成纳米颗粒，最后通过原子迁移捕获策略演变成单个 Ni 原子。得益于大量裸露的单个镍原子和中空的大孔结构，制备出的镍SAs@S/N-CMF电催化剂在OER方面具有出色的活性和稳定性。具体来说，它需要285 mV的过电位才能达到 10 mA cm − 2 10 mA cm − 2 10mAcm^(-2)10 \mathrm{~mA} \mathrm{~cm}{ }^{-2} 的基准电流密度和 50.8 mV dec − 1 50.8 mV dec − 1 50.8mVdec^(-1)50.8 \mathrm{mV} \mathrm{dec}^{-1} 的小塔菲尔斜率。

1. Introduction  1.导言

The electrochemical oxygen evolution reaction (OER) is a vital reaction and usually a limiting step in many electrochemical devices that holds great potential for clean energy storage and conversion, including rechargeable metal-air batteries and reversible fuel cells. [ 1 − 5 ] [ 1 − 5 ] ^([1-5]){ }^{[1-5]} However, the multistep proton-coupled electron transfer processes impede its kinetics because of the high energy barriers, leading to a high overpotential to achieve the desired current density. [ 6 − 8 ] [ 6 − 8 ] ^([6-8]){ }^{[6-8]} Up to now, Ru/Ir-based oxides have been proven to be the benchmark OER electrocatalysts, which yet greatly suffer from high cost and poor stability. [ 9 − 12 ] [ 9 − 12 ] ^([9-12]){ }^{[9-12]}
电化学氧进化反应（OER）是一种重要反应，通常是许多电化学装置中的限制步骤，在清洁能源储存和转换方面具有巨大潜力，包括可充电金属空气电池和可逆燃料电池。 [ 1 − 5 ] [ 1 − 5 ] ^([1-5]){ }^{[1-5]} 然而，多步质子耦合电子转移过程因能量障碍高而阻碍了其动力学，导致过电位高，难以达到所需的电流密度。 [ 6 − 8 ] [ 6 − 8 ] ^([6-8]){ }^{[6-8]} 迄今为止，Ru/Ir 基氧化物已被证明是基准 OER 电催化剂，但却存在成本高和稳定性差的问题。 [ 9 − 12 ] [ 9 − 12 ] ^([9-12]){ }^{[9-12]}
The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.202203442.
本文作者的 ORCID 识别码可在 https://doi.org/10.1002/adma.202203442 下找到。
DOI: 10.1002/adma. 202203442
DOI: 10.1002/adma.
Hence, there is an urgent need to develop efficient and low-cost transition metalbased electrocatalysts as promising alternatives to Ru / Ir Ru / Ir Ru//Ir\mathrm{Ru} / \mathrm{Ir}-based electrocatalysts, to significantly upgrade the present energyrelated devices for achieving large-scale commercialization.
因此，迫切需要开发高效、低成本的过渡金属基电催化剂，作为 Ru / Ir Ru / Ir Ru//Ir\mathrm{Ru} / \mathrm{Ir} 基电催化剂的理想替代品，以显著提升现有能源相关设备的性能，实现大规模商业化。
Recently, single-atom catalysts (SACs), featuring maximized atom-utilization efficiency, uniform metal active sites, and adjustable coordination environments, have become a promising candidate for OER. [ 13 − 18 ] [ 13 − 18 ] ^([13-18]){ }^{[13-18]} Although gratifying progress has been made, SACs still face many challenges, including low atom loading, limited exposure sites, and poor molecular accessibility. [ 19 ] [ 19 ] ^([19]){ }^{[19]} Specifically, these reported SACs are usually synthesized through a bottom-up method, in which limited metal cations are first adsorbed in a defectrich solid substrate and then reduced into isolated metal sites spreading over the entire matrix. [ 20 ] [ 20 ] ^([20]){ }^{[20]} Moreover, these metal sites are mostly stabilized and trapped in confined spaces or inaccessible regions, which generally do not participate in catalytic reactions, thus resulting in unsatisfactory OER performance.
单原子催化剂（SAC）具有原子利用效率最大化、均匀的金属活性位点和可调节的配位环境等特点，最近已成为 OER 的理想候选催化剂。 [ 13 − 18 ] [ 13 − 18 ] ^([13-18]){ }^{[13-18]} 虽然已经取得了可喜的进展，但 SAC 仍面临许多挑战，包括原子负载量低、暴露位点有限以及分子可及性差。 [ 19 ] [ 19 ] ^([19]){ }^{[19]} 具体来说，这些已报道的 SAC 通常是通过自下而上的方法合成的，其中有限的金属阳离子首先吸附在富含缺陷的固体基底中，然后还原成遍布整个基体的孤立金属位点。 [ 20 ] [ 20 ] ^([20]){ }^{[20]} 此外，这些金属位点大多被稳定并被困在密闭空间或无法进入的区域，这些区域一般不参与催化反应，因此导致 OER 性能不尽人意。
To address these problems, a favorable architecture, for instance, hollow nanostructures, might greatly improve the catalytic performance by dramatically improving the surface area-to-volume ratio and maximally exposing the metal sites. These merits will eventually boost the intrinsic activity of SACs. Some advanced carbon-based materials decorated with single-atoms have been prepared for efficient OER. For instance, nitrogendoped holey graphene frameworks loaded with single Ni sites have been fabricated and shown to exhibit enhanced OER performance. [ 21 ] [ 21 ] ^([21]){ }^{[21]} These works highlight the vital role of spatial structure on the performance of SACs. Therefore, exploring novel strategies toward advanced architectures is highly desired for maximizing the activity of SACs but remains a challenge.
为了解决这些问题，采用有利的结构（如中空纳米结构）可能会大大改善催化性能，因为它能显著提高表面积与体积比，并最大限度地暴露金属位点。这些优点最终将提高 SAC 的内在活性。一些用单原子装饰的先进碳基材料已被制备出来，用于实现高效的 OER。例如，已制备出负载单个镍位点的硝基掺杂孔状石墨烯框架，并证明其具有更强的 OER 性能。 [ 21 ] [ 21 ] ^([21]){ }^{[21]} 这些工作突出了空间结构对 SAC 性能的重要作用。因此，要想最大限度地提高 SAC 的活性，探索先进结构的新策略是非常必要的，但仍然是一项挑战。
Herein, we report the preparation of novel Ni single atoms embedded hollow S / N S / N S//N\mathrm{S} / \mathrm{N}-doped carbon macroporous fibers (designated as Ni SAs@S/N-CMF) as promising catalysts for OER through pyrolysis of rationally designed CdS-NiS x / x / _(x)//{ }_{x} / polyacrylonitrile composite fibers. Briefly, starting from CdS solid nanoparticles (NPs), the NiS x NiS x NiS_(x)\mathrm{NiS}_{x} species are then deposited on the surface of the CdS NPs to obtain the CdS-NiS x x _(x){ }_{x} NPs. Afterward, the CdS-NiS x x x x x_(x)x_{x} NPs are electrospun with polyacrylonitrile
在此，我们报告了通过热解合理设计的 CdS-NiS x / x / _(x)//{ }_{x} / 聚丙烯腈复合纤维制备新型镍单原子嵌入式中空 S / N S / N S//N\mathrm{S} / \mathrm{N} 掺杂碳大孔纤维（命名为镍 SAs@S/N-CMF）的情况，该纤维是很有前景的 OER 催化剂。简而言之，从 CdS 固体纳米粒子（NPs）开始，在 CdS NPs 表面沉积 NiS x NiS x NiS_(x)\mathrm{NiS}_{x} 物种，得到 CdS-NiS x x _(x){ }_{x} NPs。然后，用聚丙烯腈对 CdS-NiS x x x x x_(x)x_{x} NPs 进行电纺
(PAN) to yield the CdS-NiS / / /// /PAN composite fibers (denoted as CdS-NiS x x @ P A N ) x x @ P A N {:x_(x)@PAN)\left.x_{x} @ P A N\right). Following a carbonization treatment, the resultant Ni SAs@S/N-CMF catalysts are obtained. Thanks to the advantages of the abundantly exposed single Ni atoms and hollow macroporous structure, the resultant Ni SAs@S/N-CMF electrocatalysts deliver superior electrocatalytic performance for OER with an overpotential of 285 mV at 10 mA cm − 2 10 mA cm − 2 10mAcm^(-2)10 \mathrm{~mA} \mathrm{~cm}^{-2}, a small Tafel slope of 50.8 mV dec − 1 50.8 mV dec − 1 50.8mVdec^(-1)50.8 \mathrm{mV} \mathrm{dec}^{-1}, and good stability.
(PAN) 产生 CdS-NiS / / /// /PAN 复合纤维（表示为 CdS-NiS x x @ P A N ) x x @ P A N {:x_(x)@PAN)\left.x_{x} @ P A N\right) 。经过碳化处理后，得到 Ni SAs@S/N-CMF 催化剂。得益于大量暴露的单个镍原子和中空大孔结构的优势，得到的 Ni SAs@S/N-CMF 电催化剂具有优异的 OER 电催化性能，在 10 mA cm − 2 10 mA cm − 2 10mAcm^(-2)10 \mathrm{~mA} \mathrm{~cm}^{-2} 处的过电位为 285 mV， 50.8 mV dec − 1 50.8 mV dec − 1 50.8mVdec^(-1)50.8 \mathrm{mV} \mathrm{dec}^{-1} 的塔菲尔斜率小，稳定性好。

2. Results and Discussion
2.结果与讨论

Solid CdS NPs are used as the starting material. A panoramic field emission scanning electron microscopy (FESEM) image shows that these CdS NPs are highly uniform with an average diameter of about 210 nm (Figure S1a,b, Supporting Information). Transmission electron microscopy (TEM) image demonstrates their solid nature (Figure S1c, Supporting Information). The energy-dispersive X-ray (EDX) analysis of the CdS NPs demonstrates that the atomic ratio of Cd and S is ≈ 1 : 1 ≈ 1 : 1 ~~1:1\approx 1: 1 (Figure S1d, Supporting Information). After depositing NiS x NiS x NiS_(x)\mathrm{NiS}_{x} on the surface of as-prepared CdS NPs through a facile hydrothermal method, [ 22 ] [ 22 ] ^([22]){ }^{[22]} the resultant CdS − NiS x CdS − NiS x CdS-NiS_(x)\mathrm{CdS}-\mathrm{NiS}_{x} NPs well inherit the original morphology of CdS with a similar uniform size distribution (Figure S2a-c, Supporting Information). EDX spectrum of obtained CdS-NiS x x _(x){ }_{x} NPs reveals that the atomic percentage of the Ni atoms is about 0.63 % 0.63 % 0.63%0.63 \% (Figure S2d, Supporting Information). The X-ray diffraction (XRD) pattern of CdS-NiS x x x x x_(x)x_{x} (Figure S3, Supporting Information) shows that all
固体 CdS NPs 被用作起始材料。全景场发射扫描电子显微镜（FESEM）图像显示，这些 CdS NPs 高度均匀，平均直径约为 210 nm（图 S1a、b，佐证资料）。透射电子显微镜（TEM）图像显示了它们的固体性质（图 S1c，佐证资料）。CdS NPs 的能量色散 X 射线（EDX）分析表明，Cd 和 S 的原子比为 ≈ 1 : 1 ≈ 1 : 1 ~~1:1\approx 1: 1 （图 S1d，佐证资料）。通过简单的水热法在制备好的 CdS NPs 表面沉积 NiS x NiS x NiS_(x)\mathrm{NiS}_{x} 后， [ 22 ] [ 22 ] ^([22]){ }^{[22]} 得到的 CdS − NiS x CdS − NiS x CdS-NiS_(x)\mathrm{CdS}-\mathrm{NiS}_{x} NPs 很好地继承了 CdS 的原始形貌，具有相似的均匀尺寸分布（图 S2a-c，佐证资料）。获得的 CdS-NiS x x _(x){ }_{x} NPs 的 EDX 光谱显示，镍原子的原子百分比约为 0.63 % 0.63 % 0.63%0.63 \% （图 S2d，佐证资料）。CdS-NiS x x x x x_(x)x_{x} 的 X 射线衍射 (XRD) 图样（图 S3，佐证资料）显示，所有的 CdS-NiS x x x x x_(x)x_{x} NPs 都含有镍原子。
the diffraction peaks can be matched to a well-crystallized CdS (JCPDS card no. 41-1049). The absence of the diffraction peaks for NiS x NiS x NiS_(x)\mathrm{NiS}_{x} indicates its low content and/or poor crystallinity. The CdS-NiS x x x x x_(x)x_{x} NPs are compactly assembled with PAN through the electrospinning process, forming hierarchical solid fibers (Figure 1a-c). TEM images verify that the CdS-NiS x x _(x){ }_{x} NPs are well dispersed within the whole fiber (Figure 1d,e). Additionally, the thickness of the PAN layer is about 10.5 nm (Figure 1f). The high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image and the corresponding elemental mapping images of an individual CdS-NiS x x x x x_(x)x_{x} PAN fiber demonstrate the homogenous distribution of C, N, S, Cd, and Ni elements over the whole fiber (Figure 1g).
NiS x NiS x NiS_(x)\mathrm{NiS}_{x} 的衍射峰可与结晶良好的 CdS（JCPDS 证号 41-1049）相匹配。 NiS x NiS x NiS_(x)\mathrm{NiS}_{x} 没有衍射峰表明其含量低和/或结晶度差。通过电纺工艺，CdS-NiS x x x x x_(x)x_{x} NPs 与 PAN 紧密结合，形成分层固体纤维（图 1a-c）。TEM 图像证实，CdS-NiS x x _(x){ }_{x} NPs 在整个纤维中分散良好（图 1d、e）。此外，PAN 层的厚度约为 10.5 nm（图 1f）。单根 CdS-NiS x x x x x_(x)x_{x} PAN 纤维的高角度环形暗场扫描透射电子显微镜（HAADF-STEM）图像和相应的元素图谱图像显示，C、N、S、Cd 和 Ni 元素在整根纤维中均匀分布（图 1g）。
After the carbonization treatment under a nitrogen atmosphere at 950 ∘ C 950 ∘ C 950^(@)C950{ }^{\circ} \mathrm{C}, the solid CdS-NiS @ @ @@ PAN fibers are transformed into hollow macroporous fibers (Figure S4a, Supporting Information). The hollow structure of the asobtained Ni SAs@S/N-CMF can be confirmed by FESEM images (Figure 2a,b). The magnified FESEM image reveals that the hollow Ni SAs@S/N-CMF possesses a rough surface (Figure 2c). In addition, HAADF-STEM images also demonstrate that the Ni SAs@S/N-CMF samples are composed of interconnected hollow carbon spheres packed in a space-efficient arrangement (Figure 2d,e). [ 23 ] [ 23 ] ^([23]){ }^{[23]} Moreover, no obvious Ni clusters or NPs are observed (Figure 2f and Figure S5, Supporting Information). To elucidate the atomic form of these invisible Ni species, we further utilized aberration-corrected HAADF-STEM measurements to directly discern these Ni species. As shown in Figure 2g, the atomically dispersed Ni sites
在 950 ∘ C 950 ∘ C 950^(@)C950{ }^{\circ} \mathrm{C} 氮气环境下进行碳化处理后，固态 CdS-NiS @ @ @@ PAN 纤维转变为中空大孔纤维（图 S4a，佐证资料）。获得的 Ni SAs@S/N-CMF 的中空结构可以通过 FESEM 图像得到证实（图 2a、b）。放大的 FESEM 图像显示，中空的 Ni SAs@S/N-CMF 具有粗糙的表面（图 2c）。此外，HAADF-STEM 图像还表明，Ni SAs@S/N-CMF 样品由相互连接的空心碳球组成，这些碳球以空间效率高的方式排列（图 2d、e）。 [ 23 ] [ 23 ] ^([23]){ }^{[23]} 此外，没有观察到明显的镍团簇或 NPs（图 2f 和图 S5，佐证资料）。为了阐明这些看不见的镍物种的原子形式，我们进一步利用像差校正 HAADF-STEM 测量来直接分辨这些镍物种。如图 2g 所示，原子分散的镍位点
Figure 1. a-c) FESEM, d-f) TEM, and g) HAADF-STEM and elemental mapping images of CdS − NiS x @ P A N CdS − NiS x @ P A N CdS-NiS_(x)@PAN\mathrm{CdS}-\mathrm{NiS}_{x} @ P A N fibers.
图 1： CdS − NiS x @ P A N CdS − NiS x @ P A N CdS-NiS_(x)@PAN\mathrm{CdS}-\mathrm{NiS}_{x} @ P A N 纤维的 a-c) FESEM、d-f) TEM 和 g) HAADF-STEM 和元素图谱图像。
Figure 2. a-c) FESEM, d-f) dark-field TEM, and g) aberration-corrected HAADF-STEM images of Ni SAs@S/N-CMF. h) HAADF-STEM image and the corresponding elemental mapping images of Ni SAs@S/N-CMF.
图 2： a-c) Ni SAs@S/N-CMF的 FESEM、d-f) 暗场 TEM 和 g) 经像差校正的 HAADF-STEM 图像；h) Ni SAs@S/N-CMF 的 HAADF-STEM 图像和相应的元素图谱图像。
can be observed on the wall of the carbon sheath. The HAADFSTEM and corresponding elemental mapping images reveal the uniform distribution of C, N, S, and Ni elements in a single Ni SAs@S/N-CMF (Figure 2h). It is interesting to find that the signals of Cd are not detected in the as-prepared Ni SAs@S/NCMF, indicating that the Cd species are completely volatilized during the pyrolysis. More importantly, the Cd metal vapor can be easily deposited and recycled on the homemade collectors (Figure S6, Supporting Information), indicating the synthesis is environmentally friendly. The Ni content in Ni SAs@S/N-CMF is about 2.58 wt % 2.58 wt % 2.58wt%2.58 \mathrm{wt} \% according to the inductively coupled plasma optical emission spectroscopy result (Table S1, Supporting Information).
可以在碳鞘壁上观察到。HAADFSTEM 和相应的元素图谱图像显示，C、N、S 和 Ni 元素在单个 Ni SAs@S/N-CMF 中均匀分布（图 2h）。有趣的是，在制备的 Ni SAs@S/NCMF 中没有检测到镉的信号，这表明镉在热解过程中完全挥发掉了。更重要的是，镉金属蒸气可以很容易地沉积在自制的收集器上并回收利用（图 S6，佐证资料），这表明该合成方法是环保的。根据电感耦合等离子体光发射光谱的结果，Ni SAs@S/N-CMF 中的 Ni 含量约为 2.58 wt % 2.58 wt % 2.58wt%2.58 \mathrm{wt} \% （表 S1，佐证资料）。
Morphological observations are further conducted to study the structural evolution process at different temperatures. As the annealing temperature plays a vital role in CdS − NiS x CdS − NiS x CdS-NiS_(x)\mathrm{CdS}-\mathrm{NiS}_{x} volatilization, we control the annealing temperature to capture the
我们进一步进行了形态学观察，以研究不同温度下的结构演变过程。由于退火温度在 CdS − NiS x CdS − NiS x CdS-NiS_(x)\mathrm{CdS}-\mathrm{NiS}_{x} 挥发过程中起着至关重要的作用，我们控制退火温度以捕捉 CdS − NiS x CdS − NiS x CdS-NiS_(x)\mathrm{CdS}-\mathrm{NiS}_{x} 的挥发过程。
corresponding intermediate products. Representative FESEM and TEM images show that the CdS-NiS x x @ x x @ x_(x)@x_{x} @ PAN fibers could remain unchanged even when the annealing temperature reaches 650 ∘ C 650 ∘ C 650^(@)C650^{\circ} \mathrm{C} (Figure 3a,e and Figure S7, Supporting Information). When the annealing temperature is elevated to 750 ∘ C 750 ∘ C 750^(@)C750{ }^{\circ} \mathrm{C}, some CdS-NiS x x _(x){ }_{x} NPs evaporate from the supports, leaving PANderived carbon voids (Figure 3b,f and Figure S8, Supporting Information). The CdS-NiS x x x x x_(x)x_{x} NPs completely volatilize when the annealing temperature reaches 850 ∘ C 850 ∘ C 850^(@)C850{ }^{\circ} \mathrm{C} (Figure 3c,g and Figure S9, Supporting Information). Moreover, the highresolution TEM images reveal that some ultra-small Ni NPs are formed in the skeleton of the as-obtained hollow fibers (Figures S10 and S11, Supporting Information). Further elevating the annealing temperature to 950 ∘ C 950 ∘ C 950^(@)C950{ }^{\circ} \mathrm{C}, these Ni NPs vanish fully. The thermogravimetric analysis reveals that about 95.6 % 95.6 % 95.6%95.6 \% of the mass loss occurs during the annealing process from room temperature to 950 ∘ C 950 ∘ C 950^(@)C950{ }^{\circ} \mathrm{C} (Figure S12, Supporting
相应的中间产物。具有代表性的 FESEM 和 TEM 图像显示，即使退火温度达到 650 ∘ C 650 ∘ C 650^(@)C650^{\circ} \mathrm{C} 时，CdS-NiS x x @ x x @ x_(x)@x_{x} @ PAN 纤维仍能保持不变（图 3a,e 和图 S7，佐证资料）。当退火温度升高到 750 ∘ C 750 ∘ C 750^(@)C750{ }^{\circ} \mathrm{C} 时，一些 CdS-NiS x x _(x){ }_{x} NPs 会从支撑物中蒸发，留下 PAN 衍生的碳空隙（图 3b、f 和图 S8，"支持信息"）。当退火温度达到 850 ∘ C 850 ∘ C 850^(@)C850{ }^{\circ} \mathrm{C} 时，CdS-NiS x x x x x_(x)x_{x} NPs 完全挥发（图 3c、g 和图 S9，《佐证资料》）。此外，高分辨率 TEM 图像显示，在获得的中空纤维的骨架中形成了一些超小的 Ni NPs（图 S10 和 S11，佐证资料）。将退火温度进一步升高到 950 ∘ C 950 ∘ C 950^(@)C950{ }^{\circ} \mathrm{C} 时，这些镍 NP 完全消失。热重分析表明，在从室温到 950 ∘ C 950 ∘ C 950^(@)C950{ }^{\circ} \mathrm{C} 的退火过程中，约有 95.6 % 95.6 % 95.6%95.6 \% 的质量损失（图 S12，佐证资料）。
Figure 3. a-d) FESEM and e-h) TEM images of the products obtained at different annealing temperatures: a,e) 650 ∘ C , b , f ) 750 ∘ C , c , g ) 850 ∘ C 650 ∘ C , b , f 750 ∘ C , c , g 850 ∘ C {: 650^(@)C,b,f)750^(@)C,c,g)850^(@)C\left.\left.650{ }^{\circ} \mathrm{C}, \mathrm{b}, \mathrm{f}\right) 750^{\circ} \mathrm{C}, \mathrm{c}, \mathrm{g}\right) 850^{\circ} \mathrm{C}, d,h) 950 ∘ C 950 ∘ C 950^(@)C950^{\circ} \mathrm{C}. i) Schematic diagram of the structure evolution process.
图 3：a-d) 在不同退火温度下获得的产品的 FESEM 和 e-h) TEM 图像：a,e) 650 ∘ C , b , f ) 750 ∘ C , c , g ) 850 ∘ C 650 ∘ C , b , f 750 ∘ C , c , g 850 ∘ C {: 650^(@)C,b,f)750^(@)C,c,g)850^(@)C\left.\left.650{ }^{\circ} \mathrm{C}, \mathrm{b}, \mathrm{f}\right) 750^{\circ} \mathrm{C}, \mathrm{c}, \mathrm{g}\right) 850^{\circ} \mathrm{C} , d,h) 950 ∘ C 950 ∘ C 950^(@)C950^{\circ} \mathrm{C} . i) 结构演变过程示意图。
Information). We thus propose the evolution process as illustrated in Figure 3i. Specifically, when the annealing temperature is below 650 ∘ C 650 ∘ C 650^(@)C650{ }^{\circ} \mathrm{C}, the CdS-NiS x x _(x){ }_{x} NPs keep stabilized, showing their original solid structure. As the annealing temperature is raised above the boiling point of the metallic Cd ( 765 ∘ C ) 765 ∘ C (765^(@)C)\left(765^{\circ} \mathrm{C}\right), the Cd species are sharply volatilized, thus generating hollow macroporous fibers. [ 24 ] [ 24 ] ^([24]){ }^{[24]} The readily volatile S species (boiling point of 444 ∘ C 444 ∘ C 444^(@)C444^{\circ} \mathrm{C} ) simultaneously sulfurize the PANderived carbon skeletons. These residual Ni species could be efficiently reduced into Ni NPs at 850 ∘ C 850 ∘ C 850^(@)C850^{\circ} \mathrm{C} and further atomized into single Ni atoms at 950 ∘ C 950 ∘ C 950^(@)C950^{\circ} \mathrm{C}, finally yielding the thermodynamically stable Ni SAs@S/N-CMF catalyst.
信息）。因此，我们提出了如图 3i 所示的演化过程。具体来说，当退火温度低于 650 ∘ C 650 ∘ C 650^(@)C650{ }^{\circ} \mathrm{C} 时，CdS-NiS x x _(x){ }_{x} NPs 保持稳定，呈现出原始的固态结构。当退火温度高于金属镉的沸点 ( 765 ∘ C ) 765 ∘ C (765^(@)C)\left(765^{\circ} \mathrm{C}\right) 时，镉物种急剧挥发，从而产生中空的大孔纤维。 [ 24 ] [ 24 ] ^([24]){ }^{[24]} 易于挥发的 S 物质（沸点为 444 ∘ C 444 ∘ C 444^(@)C444^{\circ} \mathrm{C} ）同时使 PAN 衍生的碳骨架硫化。这些残留的 Ni 物种可在 850 ∘ C 850 ∘ C 850^(@)C850^{\circ} \mathrm{C} 处有效还原成 Ni NPs，并在 950 ∘ C 950 ∘ C 950^(@)C950^{\circ} \mathrm{C} 处进一步原子化为单个 Ni 原子，最终生成热力学稳定的 Ni SAs@S/N-CMF催化剂。
N 2 N 2 N_(2)\mathrm{N}_{2} sorption isotherms (Figure S13, Supporting Information) reveal that the Ni SAs@S/N-CMF catalyst possesses a Brunauer-Emmett-Teller specific surface area of 114 m 2 g − 1 114 m 2 g − 1 114m^(2)g^(-1)114 \mathrm{~m}^{2} \mathrm{~g}^{-1}, which is much higher than that of Ni NPs@S/NCMF ( 34.9 m 2 g − 1 34.9 m 2 g − 1 34.9m^(2)g^(-1)34.9 \mathrm{~m}^{2} \mathrm{~g}^{-1} ). XRD is further carried out to clarify the crystalline structure. As shown in Figure 4a, the Ni NPs@S/NCMF catalyst exhibits an obvious characteristic peak of the (111) plane of the Ni crystals (JCPDS card no. 04-0850). However, no characteristic peaks of Ni are observed in the XRD pattern of the Ni SAs@S/N-CMF catalyst, further indicating the complete transformation of Ni NPs to single atoms. Also, both the Ni NPs@S/N-CMF and Ni SAs@S/N-CMF display similar Raman
N 2 N 2 N_(2)\mathrm{N}_{2} 吸附等温线（图 S13，佐证资料）显示，Ni SAs@S/N-CMF 催化剂的布鲁诺-艾美特-泰勒比表面积为 114 m 2 g − 1 114 m 2 g − 1 114m^(2)g^(-1)114 \mathrm{~m}^{2} \mathrm{~g}^{-1} ，远高于 Ni NPs@S/NCMF 的比表面积（ 34.9 m 2 g − 1 34.9 m 2 g − 1 34.9m^(2)g^(-1)34.9 \mathrm{~m}^{2} \mathrm{~g}^{-1} ）。为明确晶体结构，还进一步进行了 XRD 扫描。如图 4a 所示，Ni NPs@S/NCMF 催化剂显示出明显的 Ni 晶体（111）面特征峰（JCPDS 证号 04-0850）。然而，在 Ni SAs@S/N-CMF 催化剂的 XRD 图谱中没有观察到 Ni 的特征峰，这进一步表明 Ni NPs 已完全转变为单个原子。此外，Ni NPs@S/N-CMF 和 Ni SAs@S/N-CMF 都显示出相似的拉曼图谱。
spectra with typical D and G bands of carbon (Figure S14, Supporting Information). Furthermore, as shown by X-ray photoelectron spectroscopy (XPS) analysis, the binding energy of Ni 2 p 3 / 2 2 p 3 / 2 2p_(3//2)2 p_{3 / 2} peak at 853.9 eV for the Ni SAs@S/N-CMF catalyst is higher than that of Ni NPs@S/N-CMF (853.5 eV) and the reported Ni 0 ( 853.0 eV ) Ni 0 ( 853.0 eV ) Ni^(0)(853.0eV)\mathrm{Ni}^{0}(853.0 \mathrm{eV}), but lower than that of the reported Ni 2 + Ni 2 + Ni^(2+)\mathrm{Ni}^{2+} ions ( 855.1 eV ) ( 855.1 eV ) (855.1eV)(855.1 \mathrm{eV}). This result indicates the ionic Ni δ + ( 0 < δ < 2 ) Ni δ + ( 0 < δ < 2 ) Ni^(delta+)(0 < delta < 2)\mathrm{Ni}^{\delta+}(0<\delta<2) nature of the single Ni sites in Ni SAs@S/N-CMF (Figure 4b and Figure S15, Supporting Information). [ 25 ] [ 25 ] ^([25]){ }^{[25]}
图 S14，佐证资料）。此外，如 X 射线光电子能谱 (XPS) 分析所示，Ni SAs@S/N-CMF 催化剂中 Ni 2 p 3 / 2 2 p 3 / 2 2p_(3//2)2 p_{3 / 2} 峰的结合能为 853.9 eV，高于 Ni NPs@S/N-CMF 的结合能（853.5 eV）和报道的 Ni 0 ( 853.0 eV ) Ni 0 ( 853.0 eV ) Ni^(0)(853.0eV)\mathrm{Ni}^{0}(853.0 \mathrm{eV}) ，但低于报道的 Ni 2 + Ni 2 + Ni^(2+)\mathrm{Ni}^{2+} 离子 ( 855.1 eV ) ( 855.1 eV ) (855.1eV)(855.1 \mathrm{eV}) 。这一结果表明，Ni SAs@S/N-CMF 中的单个 Ni 位点具有离子 Ni δ + ( 0 < δ < 2 ) Ni δ + ( 0 < δ < 2 ) Ni^(delta+)(0 < delta < 2)\mathrm{Ni}^{\delta+}(0<\delta<2) 的性质（图 4b 和图 S15，佐证资料）。 [ 25 ] [ 25 ] ^([25]){ }^{[25]}
X-ray absorption fine structure (XAFS) is another mighty tool for discerning SACs. As shown in Figure 4c, over the whole range of 2.0 to 10.0 Å − 1 10.0 Å − 1 10.0"Å"^(-1)10.0 \AA^{-1}Å, the k 2 χ ( k ) k 2 χ ( k ) k^(2)chi(k)k^{2} \chi(k) oscillation curve of the Ni SAs@S/N-CMF catalyst at the Ni K-edge shows a different trend in shape and oscillating frequency. These results indicate that the coordination configuration of the Ni species in the Ni SAs@S/N-CMF catalyst is quite different from that of Ni NPs@S/N-CMF, Ni 2 O 3 Ni 2 O 3 Ni_(2)O_(3)\mathrm{Ni}_{2} \mathrm{O}_{3}, and Ni foil. The Fourier transform (FT) of the Ni K-edge extended X-ray absorption fine spectrum (EXAFS) of the Ni SAs@S/N-CMF catalyst (Figure 4d) exhibits a dominant Ni − N Ni − N Ni-N\mathrm{Ni}-\mathrm{N} scattering path at around 1.4 Å 1.4 Å 1.4"Å"1.4 \AAÅ, indicating the Ni species are coordinated with N rather than S . [ 26 ] S . [ 26 ] S.^([26])\mathrm{S} .{ }^{[26]} For Ni NPs@S/N-CMF, an obvious Ni − Ni Ni − Ni Ni-Ni\mathrm{Ni}-\mathrm{Ni} scattering path can be observed at around 2.2 Å 2.2 Å 2.2"Å"2.2 \AAÅ. The corresponding EXAFS fitting results demonstrate that the coordination number of the
X 射线吸收精细结构（XAFS）是鉴别 SAC 的另一个有力工具。如图 4c 所示，在 2.0 到 10.0 Å − 1 10.0 Å − 1 10.0"Å"^(-1)10.0 \AA^{-1}Å 的整个范围内，Ni SAs@S/N-CMF 催化剂在 Ni K 边缘的 k 2 χ ( k ) k 2 χ ( k ) k^(2)chi(k)k^{2} \chi(k) 振荡曲线在形状和振荡频率上都呈现出不同的趋势。这些结果表明，Ni SAs@S/N-CMF 催化剂中 Ni 物种的配位构型与 Ni NPs@S/N-CMF、 Ni 2 O 3 Ni 2 O 3 Ni_(2)O_(3)\mathrm{Ni}_{2} \mathrm{O}_{3} 和 Ni 箔有很大不同。Ni SAs@S/N-CMF 催化剂的 Ni K 边扩展 X 射线吸收精细光谱（EXAFS）的傅立叶变换（FT）（图 4d）在 1.4 Å 1.4 Å 1.4"Å"1.4 \AAÅ 附近显示出主要的 Ni − N Ni − N Ni-N\mathrm{Ni}-\mathrm{N} 散射路径，表明 Ni 物种与 N 配位，而不是 S . [ 26 ] S . [ 26 ] S.^([26])\mathrm{S} .{ }^{[26]} 对于 Ni NPs@S/N-CMF，可以在 2.2 Å 2.2 Å 2.2"Å"2.2 \AAÅ 附近观察到明显的 Ni − Ni Ni − Ni Ni-Ni\mathrm{Ni}-\mathrm{Ni} 散射路径。相应的 EXAFS 拟合结果表明，Ni NP@S/N-CMF 的配位数为 S . [ 26 ] S . [ 26 ] S.^([26])\mathrm{S} .{ }^{[26]}。
Figure 4. a) XRD patterns, b) high-resolution Ni 2 p 3 / 2 2 p 3 / 2 2p_(3//2)