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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)2 p_{3 / 2} XPS spectra of Ni NPs@S/N-CMF and Ni SAs@S/N-CMF. c) k 2 χ ( k ) k 2 χ ( k ) k^(2)chi(k)k^{2} \chi(k) oscillation curves, d) FT curves, and e) WT contour plots of Ni K-edge EXAFS spectra of Ni NPs@S/N-CMF, Ni SAs@S/N-CMF, and reference materials.
图 4： a) XRD 图样；b) Ni NPs@S/N-CMF 和 Ni SAs@S/N-CMF 的高分辨率 Ni 2 p 3 / 2 2 p 3 / 2 2p_(3//2)2 p_{3 / 2} XPS 光谱；c) Ni NPs@S/N-CMF、Ni SAs@S/N-CMF 和参考材料的 k 2 χ ( k ) k 2 χ ( k ) k^(2)chi(k)k^{2} \chi(k) 振荡曲线；d) FT 曲线；e) Ni K 边 EXAFS 光谱的 WT 等值线图。
resultant Ni − N Ni − N Ni-N\mathrm{Ni}-\mathrm{N} bond is about 4 with an average bond length of 1.87 Å 1.87 Å 1.87"Å"1.87 \AAÅ (Figure S16 and Table S2, Supporting Information). In addition, the energy position of the absorption edge of the Ni SAs@S/N-CMF is situated between those of Ni foil and Ni 2 O 3 Ni 2 O 3 Ni_(2)O_(3)\mathrm{Ni}_{2} \mathrm{O}_{3}, further confirming the intermediate valence state nature (Figure S17, Supporting Information). It is in good agreement with the XPS results discussed above. The detailed wavelet transform (WT) analyses provide a higher resolution in both radial space and K K KK space. As shown in Figure 4e, compared with the WT contour plots of Ni NPs@S/N-CMF ( Ni − Ni Ni − Ni Ni-Ni\mathrm{Ni}-\mathrm{Ni} at about 7.3 Å − 1 7.3 Å − 1 7.3"Å"^(-1)7.3 \AA^{-1}Å ) and Ni 2 O 3 ( Ni − Ni Ni 2 O 3 Ni − Ni Ni_(2)O_(3)(Ni-Ni:}\mathrm{Ni}_{2} \mathrm{O}_{3}\left(\mathrm{Ni}-\mathrm{Ni}\right. at about 6.6 Å − 1 6.6 Å − 1 6.6"Å"^(-1)6.6 \AA^{-1}Å and Ni − O Ni − O Ni-O\mathrm{Ni}-\mathrm{O} at about 4.5 Å − 1 4.5 Å − 1 4.5"Å"^(-1)4.5 \AA^{-1}Å ), only the Ni − N Ni − N Ni-N\mathrm{Ni}-\mathrm{N} scattering path located at about 5 Å − 1 5 Å − 1 5"Å"^(-1)5 \AA^{-1}Å is observed for Ni SAs@S/N-CMF, again verifying that the transformation from Ni NPs to single-atomic Ni sites has indeed taken place.
由此产生的 Ni − N Ni − N Ni-N\mathrm{Ni}-\mathrm{N} 键约为 4，平均键长为 1.87 Å 1.87 Å 1.87"Å"1.87 \AAÅ （图 S16 和表 S2，佐证资料）。此外，Ni SAs@S/N-CMF 吸收边的能量位置介于 Ni 箔和 Ni 2 O 3 Ni 2 O 3 Ni_(2)O_(3)\mathrm{Ni}_{2} \mathrm{O}_{3} 之间，进一步证实了其中间价态性质（图 S17，佐证资料）。这与上文讨论的 XPS 结果十分吻合。详细的小波变换 (WT) 分析提供了更高的径向空间和 K K KK 空间分辨率。如图 4e 所示，与 Ni NPs@S/N-CMF 的 WT 等值线图（ Ni − Ni Ni − Ni Ni-Ni\mathrm{Ni}-\mathrm{Ni} 处约为 7.3 Å − 1 7.3 Å − 1 7.3"Å"^(-1)7.3 \AA^{-1}Å ）和 Ni 2 O 3 ( Ni − Ni Ni 2 O 3 Ni − Ni Ni_(2)O_(3)(Ni-Ni:}\mathrm{Ni}_{2} \mathrm{O}_{3}\left(\mathrm{Ni}-\mathrm{Ni}\right. 处约为 6.6 Å − 1 6.6 Å − 1 6.6"Å"^(-1)6.6 \AA^{-1}Å 和 Ni − O Ni − O Ni-O\mathrm{Ni}-\mathrm{O} 处约为 4.5 Å − 1 4.5 Å − 1 4.5"Å"^(-1)4.5 \AA^{-1}Å ）相比、对于 Ni SAs@S/N-CMF，只观察到位于约 5 Å − 1 5 Å − 1 5"Å"^(-1)5 \AA^{-1}Å 处的 Ni − N Ni − N Ni-N\mathrm{Ni}-\mathrm{N} 散射路径，这再次验证了从 Ni NPs 到单原子 Ni 位点的转变确实发生了。
Electron paramagnetic resonance is an efficient technique for detecting unpaired electrons in materials. [ 27 , 28 ] [ 27 , 28 ] ^([27,28]){ }^{[27,28]} The intensity at the g g gg value of 2.003 for Ni SAs@S/N-CMF is higher than that of Ni NPs@S/N-CMF, suggesting that abundant C and N defects emerge on the skeleton of Ni SAs@S/N-CMF during the atomization process (Figure S18, Supporting Information). [ 29 − 31 ] [ 29 − 31 ] ^([29-31]){ }^{[29-31]} Besides, the N K-edge X-ray absorption near edge structure spectrum (Figure S19, Supporting Information) reveals that three peaks can be attributed to the π κ π κ pi^(kappa)\pi^{\kappa}-transition of pyridinic- N (peak a) and graphitic- N (peak b), and the σ ⋆ σ ⋆ sigma^(***)\sigma^{\star} transition of C − N C − N C-N\mathrm{C}-\mathrm{N} bonds (peak c). It is worthwhile to note that the pyridinic- N is divided into double peaks (a1 and a2), manifesting that a portion of pyridinic- N is bonded to the Ni atoms. [ 32 , 33 ] [ 32 , 33 ] ^([32,33]){ }^{[32,33]} These pyridinic- N sites could share one p-electron to the adjacent p-conjugated system, which would further boost the chemical stabilization of single Ni sites. [ 34 , 35 ] [ 34 , 35 ] ^([34,35]){ }^{[34,35]}
电子顺磁共振是检测材料中未成对电子的一种有效技术。 [ 27 , 28 ] [ 27 , 28 ] ^([27,28]){ }^{[27,28]} Ni SAs@S/N-CMF在 g g gg 值为2.003时的强度高于Ni NPs@S/N-CMF，表明在雾化过程中，Ni SAs@S/N-CMF的骨架上出现了丰富的C和N缺陷（图S18，Supping Information）。 [ 29 − 31 ] [ 29 − 31 ] ^([29-31]){ }^{[29-31]} 此外，N K 边 X 射线吸收近边缘结构光谱（图 S19，佐证资料）显示，有三个峰可归因于吡啶- N 的 π κ π κ pi^(kappa)\pi^{\kappa} 转变（峰 a）和石墨- N 的 π κ π κ pi^(kappa)\pi^{\kappa} 转变（峰 b），以及 C − N C − N C-N\mathrm{C}-\mathrm{N} 键的 σ ⋆ σ ⋆ sigma^(***)\sigma^{\star} 转变（峰 c）。值得注意的是，吡啶- N 分为双峰（a1 和 a2），表明部分吡啶- N 与镍原子成键。 [ 32 , 33 ] [ 32 , 33 ] ^([32,33]){ }^{[32,33]} 这些吡啶- N位点可与相邻的对共轭体系共享一个对电子，这将进一步提高单个镍位点的化学稳定性。 [ 34 , 35 ] [ 34 , 35 ] ^([34,35]){ }^{[34,35]}
The electrochemical OER performance of Ni SAs@S/N-CMF was performed in a 1.0 m KOH solution with commercial RuO 2 RuO 2 RuO_(2)\mathrm{RuO}_{2},
Ni SAs@S/N-CMF 的电化学 OER 性能是在含有商用 RuO 2 RuO 2 RuO_(2)\mathrm{RuO}_{2} 的 1.0 m KOH 溶液中进行的、
Ni-free S/N-CMF (Figures S20 and S21, Supporting Information), and Ni NPs@S/N-CMF as references. Figure 5a displays the linear sweep voltammetry (LSV) plots of the four samples. The Ni SAs@S/N-CMF catalyst requires only a small overpotential ( η 10 ) η 10 (eta_(10))\left(\eta_{10}\right) of 285 mV to deliver a current density of 10 mA cm − 2 10 mA cm − 2 10mAcm^(-2)10 \mathrm{~mA} \mathrm{~cm}{ }^{-2} (Figure 5b), clearly outperforming Ni NPs@S/N-CMF (391 mV), Ni-free S / N − CMF S / N − CMF S//N-CMF\mathrm{S} / \mathrm{N}-\mathrm{CMF} ( 471 mV ), and RuO 2 ( 366 mV RuO 2 ( 366 mV RuO_(2)(366mV\mathrm{RuO}_{2}(366 \mathrm{mV} ). Benefiting from the hollow macroporous structure which facilitates mass transfer during OER, an impressive Tafel slope of 50.8 mV dec − 1 dec − 1 dec^(-1)\mathrm{dec}^{-1} is obtained for the Ni SAs@S/N-CMF catalyst (Figure 5c), which is superior to that of Ni NPs@S/N-CMF ( 81.9 mV dec − 1 81.9 mV dec − 1 81.9mVdec^(-1)81.9 \mathrm{mV} \mathrm{dec}^{-1} ), Ni-free S/N-CMF ( 141 mV dec − 1 141 mV dec − 1 141mVdec^(-1)141 \mathrm{mV} \mathrm{dec}{ }^{-1} ), and RuO 2 ( 100.9 mV dec − 1 RuO 2 100.9 mV dec − 1 RuO_(2)(100.9mVdec^(-1):}\mathrm{RuO}_{2}\left(100.9 \mathrm{mV} \mathrm{dec}{ }^{-1}\right. ). Additionally, a high turnover frequency (TOF) value of 0.16 s − 1 0.16 s − 1 0.16s^(-1)0.16 \mathrm{~s}^{-1} is obtained for the Ni SAs@S/N-CMF catalyst at the overpotential of 300 mV (Figure 5d), which is much higher than that of Ni NPs@S/N-CMF ( 0.006 s − 1 0.006 s − 1 0.006s^(-1)0.006 \mathrm{~s}^{-1} ), demonstrating the high utilization of Ni atoms in the catalytic processes. Based on the above results, the Ni SAs@S/N-CMF catalyst displays enhanced OER activity and kinetics compared to Ni NPs@S/N-CMF and Ni-free S/N-CMF catalysts. Importantly, the OER activity and kinetics of this Ni SAs@S/N-CMF catalyst are outstanding, and comparable with the most active transition-metal-based OER electrocatalysts reported (Table S3, Supporting Information).
无 Ni 的 S/N-CMF（图 S20 和 S21，佐证资料）以及 Ni NPs@S/N-CMF 作为参考。图 5a 显示了四种样品的线性扫描伏安 (LSV) 图。Ni SAs@S/N-CMF 催化剂只需要 ( η 10 ) η 10 (eta_(10))\left(\eta_{10}\right) 285 mV 的小过电位就能产生 10 mA cm − 2 10 mA cm − 2 10mAcm^(-2)10 \mathrm{~mA} \mathrm{~cm}{ }^{-2} 的电流密度（图 5b），明显优于 Ni NPs@S/N-CMF （391 mV）、无 Ni S / N − CMF S / N − CMF S//N-CMF\mathrm{S} / \mathrm{N}-\mathrm{CMF} （471 mV ）和 RuO 2 ( 366 mV RuO 2 ( 366 mV RuO_(2)(366mV\mathrm{RuO}_{2}(366 \mathrm{mV} 。）Ni SAs@S/N-CMF 催化剂的中空大孔结构有利于 OER 期间的传质，因此获得了令人印象深刻的 50.8 mV dec − 1 dec − 1 dec^(-1)\mathrm{dec}^{-1} 的塔菲尔斜率（图 5c），优于 Ni NPs@S/N-CMF （ 81.9 mV dec − 1 81.9 mV dec − 1 81.9mVdec^(-1)81.9 \mathrm{mV} \mathrm{dec}^{-1} ）、无 Ni S/N-CMF （ 141 mV dec − 1 141 mV dec − 1 141mVdec^(-1)141 \mathrm{mV} \mathrm{dec}{ }^{-1} ）和 RuO 2 ( 100.9 mV dec − 1 RuO 2 100.9 mV dec − 1 RuO_(2)(100.9mVdec^(-1):}\mathrm{RuO}_{2}\left(100.9 \mathrm{mV} \mathrm{dec}{ }^{-1}\right. ）。此外，在过电位为 300 mV 时，Ni SAs@S/N-CMF 催化剂获得了 0.16 s − 1 0.16 s − 1 0.16s^(-1)0.16 \mathrm{~s}^{-1} 的高翻转频率 (TOF) 值（图 5d），远高于 Ni NPs@S/N-CMF ( 0.006 s − 1 0.006 s − 1 0.006s^(-1)0.006 \mathrm{~s}^{-1} )，这表明催化过程中 Ni 原子的利用率很高。基于上述结果，与 Ni NPs@S/N-CMF 和无 Ni 的 S/N-CMF 催化剂相比，Ni SAs@S/N-CMF 催化剂显示出更高的 OER 活性和动力学性能。重要的是，这种 Ni SAs@S/N-CMF 催化剂的 OER 活性和动力学都非常出色，可与已报道的活性最高的过渡金属基 OER 电催化剂相媲美（表 S3，佐证资料）。
We further assess the electrochemically active surface area of Ni SAs@S/N-CMF and Ni NPs@S/N-CMF by measuring the electrochemical double-layer capacitance ( C dl ) . [ 36 , 37 ] C dl . [ 36 , 37 ] (C_(dl)).^([36,37])\left(C_{\mathrm{dl}}\right) .^{[36,37]} As expected, the Ni SAs@S/N-CMF catalyst possesses more surface exposed active sites with a C dl C dl C_(dl)C_{\mathrm{dl}} value of 3.04 mF cm − 2 3.04 mF cm − 2 3.04mFcm^(-2)3.04 \mathrm{mF} \mathrm{cm}^{-2} (Figure 5e and Figure S22, Supporting Information), which is about 11.7 times that of Ni NPs@S/N-CMF ( 0.26 mF cm − 2 0.26 mF cm − 2 0.26mFcm^(-2)0.26 \mathrm{mF} \mathrm{cm}{ }^{-2} ). Electrochemical impedance spectroscopy investigation is further carried out. The Ni SAs@S/N-CMF catalyst displays a smaller charge transfer resistance than Ni NPs@S/N-CMF and Ni-free S/N-CMF,
我们通过测量电化学双层电容 ( C dl ) . [ 36 , 37 ] C dl . [ 36 , 37 ] (C_(dl)).^([36,37])\left(C_{\mathrm{dl}}\right) .^{[36,37]} 进一步评估了 Ni SAs@S/N-CMF 和 Ni NPs@S/N-CMF 的电化学活性表面积。正如预期的那样，Ni SAs@S/N-CMF 催化剂具有更多的表面暴露活性位点，其 C dl C dl C_(dl)C_{\mathrm{dl}} 值为 3.04 mF cm − 2 3.04 mF cm − 2 3.04mFcm^(-2)3.04 \mathrm{mF} \mathrm{cm}^{-2} （图 5e 和图 S22，佐证资料），约为 Ni NPs@S/N-CMF 的 11.7 倍（ 0.26 mF cm − 2 0.26 mF cm − 2 0.26mFcm^(-2)0.26 \mathrm{mF} \mathrm{cm}{ }^{-2} ）。进一步进行了电化学阻抗谱研究。与 Ni NPs@S/N-CMF 和无 Ni 的 S/N-CMF 相比，Ni SAs@S/N-CMF 催化剂显示出较小的电荷转移电阻、
Figure 5. a) LSV plots, b) the overpotentials at 10 mA cm − 2 10 mA cm − 2 10mAcm^(-2)10 \mathrm{~mA} \mathrm{~cm}^{-2}, and c) the corresponding Tafel slopes of RuO 2 , S / N − CMF , Ni NPs @ S / N − CMF RuO 2 , S / N − CMF , Ni NPs @ S / N − CMF RuO_(2),S//N-CMF,NiNPs@S//N-CMF\mathrm{RuO}_{2}, \mathrm{~S} / \mathrm{N}-\mathrm{CMF}, \mathrm{Ni} \mathrm{NPs} @ \mathrm{~S} / \mathrm{N}-\mathrm{CMF}, and Ni SAs@S/N-CMF. d) TOF values, and e) the capacitive current density ( 1 / 2 Δ J 1 / 2 Δ J 1//2Delta J1 / 2 \Delta J ) at 0.95 V versus reversible hydrogen electrode against the scan rate for Ni NPs@S/N-CMF and Ni SAs@S/N-CMF. f) Potential-time curves at 10 mA cm − 2 10 mA cm − 2 10mAcm^(-2)10 \mathrm{~mA} \mathrm{~cm}^{-2} of RuO 2 RuO 2 RuO_(2)\mathrm{RuO}_{2}, Ni NPs@S/N-CMF, and Ni SAs@S/N-CMF.
图 5： a) LSV 图；b) 10 mA cm − 2 10 mA cm − 2 10mAcm^(-2)10 \mathrm{~mA} \mathrm{~cm}^{-2} 时的过电位；c) RuO 2 , S / N − CMF , Ni NPs @ S / N − CMF RuO 2 , S / N − CMF , Ni NPs @ S / N − CMF RuO_(2),S//N-CMF,NiNPs@S//N-CMF\mathrm{RuO}_{2}, \mathrm{~S} / \mathrm{N}-\mathrm{CMF}, \mathrm{Ni} \mathrm{NPs} @ \mathrm{~S} / \mathrm{N}-\mathrm{CMF} 和 Ni SAs@S/N-CMF 的相应 Tafel 斜率；d) TOF 值；e) 0.f) RuO 2 RuO 2 RuO_(2)\mathrm{RuO}_{2} 、Ni NPs@S/N-CMF 和 Ni SAs@S/N-CMF 在 10 mA cm − 2 10 mA cm − 2 10mAcm^(-2)10 \mathrm{~mA} \mathrm{~cm}^{-2} 处的电位-时间曲线。
implying a faster charge-transfer process during the OER process (Figure S23, Supporting Information). The chronoamperometry curve of Ni SAs@S/N-CMF shows long-term OER stability with a negligible increase in potential after a 60 − h 60 − h 60-h60-\mathrm{h} test at 10 mA cm − 2 10 mA cm − 2 10mAcm^(-2)10 \mathrm{~mA} \mathrm{~cm}^{-2} (Figure 5f). By contrast, the Ni NPs@S/N-CMF and commercial RuO 2 RuO 2 RuO_(2)\mathrm{RuO}_{2} exhibit apparently increased overpotential only after 10 h . In addition, the Ni SAs@S/N-CMF catalyst shows a slight activity decay in the LSV curves before and after the stability test (Figure S24, Supporting Information). The hollow structure of Ni SAs@S/N-CMF is maintained after the long-term OER test (Figure S25, Supporting Information). The positive shift of the binding energy implies a slight increase in the valence state of Ni after the reaction (Figure S26, Supporting Information). After the stability test, the Ni SAs@S/N-CMF catalyst could retain the hollow fiber structure (Figure S27a, Supporting Information), and the single Ni sites in the Ni SAs@S/N-CMF catalyst also keep isolated distribution and good stability (Figure S27b,c, Supporting Information). These results unequivocally indicate that the Ni SAs@S/N-CMF catalyst possesses excellent chemical and structural stability. The low current intensities associated with the low metal loading are not yet able to make our catalyst suitable for industry-level OER requirements (current density > 500 mA cm − 2 500 mA cm − 2 500mAcm^(-2)500 \mathrm{~mA} \mathrm{~cm}{ }^{-2} ) or other related applications, [ 38 ] [ 38 ] ^([38]){ }^{[38]} which still need to be further optimized in the future.
这意味着在 OER 过程中电荷转移的速度更快（图 S23，佐证资料）。Ni SAs@S/N-CMF 的计时器曲线显示出长期的 OER 稳定性，在 10 mA cm − 2 10 mA cm − 2 10mAcm^(-2)10 \mathrm{~mA} \mathrm{~cm}^{-2} 下进行 60 − h 60 − h 60-h60-\mathrm{h} 测试后，电位的增加可以忽略不计（图 5f）。相比之下，Ni NPs@S/N-CMF 和商用 RuO 2 RuO 2 RuO_(2)\mathrm{RuO}_{2} 仅在 10 小时后才表现出明显增加的过电位。此外，Ni SAs@S/N-CMF 催化剂在稳定性测试前后的 LSV 曲线中显示出轻微的活性衰减（图 S24，佐证资料）。Ni SAs@S/N-CMF 的中空结构在长期 OER 测试后保持不变（图 S25，佐证资料）。结合能的正移意味着反应后 Ni 的价态略有上升（图 S26，佐证资料）。经过稳定性测试，Ni SAs@S/N-CMF 催化剂保留了中空纤维结构（图 S27a，佐证资料），Ni SAs@S/N-CMF 催化剂中的单个 Ni 位点也保持了孤立分布和良好的稳定性（图 S27b、c，佐证资料）。这些结果明确表明，Ni SAs@S/N-CMF 催化剂具有优异的化学和结构稳定性。与低金属负载相关的低电流强度还不能使我们的催化剂适用于工业级 OER 要求（电流密度大于 500 mA cm − 2 500 mA cm − 2 500mAcm^(-2)500 \mathrm{~mA} \mathrm{~cm}{ }^{-2} ）或其他相关应用， [ 38 ] [ 38 ] ^([38]){ }^{[38]} 这还需要在未来进一步优化。

3. Conclusion  3.结论

Novel Ni single atoms embedded hollow S/N-doped carbon macroporous fibers (Ni SAs@S/N-CMF) have been prepared
制备了新型镍单原子嵌入中空 S/N 掺杂碳大孔纤维（Ni SAs@S/N-CMF）
as a promising OER electrocatalyst. The synthetic method can readily control the structure and composition of the resultant hollow fibers. Benefiting from the abundant exposed single Ni atoms and hollow macroporous structure, the resultant Ni SAs@S/N-CMF electrocatalyst delivers excellent OER performance with an overpotential of 285 mV at 10 mA cm − 2 10 mA cm − 2 10mAcm^(-2)10 \mathrm{~mA} \mathrm{~cm}^{-2}, a Tafel slope of 50.8 mV dec − 1 50.8 mV dec − 1 50.8mVdec^(-1)50.8 \mathrm{mV} \mathrm{dec}{ }^{-1}, and outstanding durability. This work may offer some inspiration for designing and constructing efficient SACs for various challenging reactions.
作为一种有前途的 OER 电催化剂。这种合成方法可以很容易地控制所得中空纤维的结构和组成。得益于大量裸露的单个镍原子和中空的大孔结构，得到的镍 SAs@S/N-CMF 电催化剂具有优异的 OER 性能，在 10 mA cm − 2 10 mA cm − 2 10mAcm^(-2)10 \mathrm{~mA} \mathrm{~cm}^{-2} 时过电位为 285 mV，塔菲尔斜率为 50.8 mV dec − 1 50.8 mV dec − 1 50.8mVdec^(-1)50.8 \mathrm{mV} \mathrm{dec}{ }^{-1} ，并且具有出色的耐久性。这项工作可为设计和构建用于各种挑战性反应的高效 SAC 提供一些启发。

Supporting Information  辅助信息

Supporting Information is available from the Wiley Online Library or from the author.
辅助信息可从 Wiley 在线图书馆或作者处获取。

Acknowledgements  致谢

X.W.L. acknowledges the funding support from the Ministry of Education of Singapore through the Academic Research Fund (AcRF) Tier-2 grant (MOE2019-T2-2-049). X.J.G. acknowledges the funding support from the National Natural Science Foundation of China (22162019), and the Science and Technology Projects of Inner Mongolia Autonomous Region (2021GG0195). The authors thank Dr. Shibo Xi and the X-ray absorption fine structure for catalysis (XAFCA) beamline of the Singapore Synchrotron Light Source (SSLS) for supporting the XAFS measurements.
X.W.L. 感谢新加坡教育部通过学术研究基金 (AcRF) Tier-2 grant (MOE2019-T2-2-049) 提供的资助。X.J.G. 感谢国家自然科学基金（22162019）和内蒙古自治区科技计划项目（2021GG0195）的资助。作者感谢奚世波博士和新加坡同步辐射光源的催化 X 射线吸收精细结构（XAFCA）光束线对 XAFS 测量的支持。

Conflict of Interest  利益冲突

The authors declare no conflict of interest.
作者声明没有利益冲突。

Data Availability Statement
数据可用性声明

The data that support the findings of this study are available from the corresponding author upon reasonable request.
支持本研究结果的数据可向相应作者索取。

Keywords  关键词

macroporous materials, oxygen evolution reaction, S / N S / N S//N\mathrm{S} / \mathrm{N}-doped carbon fibers, single Ni atoms
大孔材料、氧进化反应、 S / N S / N S//N\mathrm{S} / \mathrm{N} 掺杂碳纤维、单个镍原子
Received: April 16, 2022  收到：2022 年 4 月 16 日
Revised: May 22, 2022
修订：2022 年 5 月 22 日
Published online: July 29, 2022
在线发表：2022 年 7 月 29 日在线发表
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  1. Y. Zhao, X. F. Lu, D. Luan, X. W. Lou
    Y.Zhao, X. F. Lu, D. Luan, X. W. Lou
    School of Chemical and Biomedical Engineering
    化学与生物医学工程学院
    Nanyang Technological University
    南洋理工大学
    62 Nanyang Drive, Singapore 637459, Singapore
    E-mail: davidlou88@gmail.com
    电子邮件： davidlou88@gmail.com
    Y. Guo, X. Gu
    School of Chemistry and Chemical Engineering
    化学与化学工程学院
    Inner Mongolia University
    内蒙古大学
    Hohhot 010021, China  中国呼和浩特 010021
    E-mail: xiaojun.gu@imu.edu.cn
    电子邮件： xiaojun.gu@imu.edu.cn