物 理 化 学 学 报
Acta Phys. -Chim. Sin. 2023, 39 (8), 2210039 (1 of 8)
Received: October 27, 2022; Revised: November 12, 2022; Accepted: November 14, 2022; Published online: November 21, 2022. †
These authors contributed equally to this work. *
Correspondingauthors.Emails:**********************(P.D.);********************(Z.M.);******************(X.T.);Tel.:+86-188********(X.T.).
The project was supported by the Hainan Provincial Natural Science Foundation of China (222RC548), the National Natural Science Foundation of China (22109034, 22109035, 52164028, 62105083), the Postdoctoral Science Foundation of Hainan Province (RZ2100007123), the Foundation of State Key Laboratory of Marine Resource Utilization in South China Sea (Hainan University, MRUKF2021029), the Start-up Research Foundation of Hainan University (KYQD(ZR)-20008, 20082, 20083, 20084, 21065, 21124, 21125), and the specific research fund of The Innovation Platform for Academicians of Hainan Province.
海南省自然科学基金(222RC548), 国家自然科学基金(22109034, 22109035, 52164028, 62105083), 海南省博士后科学基金(RZ2100007123), 南海海洋资源利用国家重点实验室基金(海南大学, MRUKF2021029), 海南大学科研启动基金(KYQD(ZR)-20008, 20082, 20083, 20084, 21065, 21124, 21125)和海南省院士创新平台资金资助项目
© Editorial office of Acta Physico-Chimica Sinica
[Article] doi: 10.3866/PKU.WHXB202210039 www.whxb.pku.edu
Atomic Co Clusters for Efficient Oxygen Reduction Reaction
Yanhui Yu †, Peng Rao †, Suyang Feng, Min Chen, Peilin Deng *, Jing Li, Zhengpei Miao *, Zhenye Kang, Yijun Shen, Xinlong Tian *
State Key Laboratory of Marine Resource Utilization in South China Sea, Hainan Provincial Key Lab of Fine Chemistry,  School of Chemical Engineering and Technology, Hainan University, Haikou 570228, China.
Abstract:  Environment-friendly energy storage and conversion technologies, such as metal–air batteries and fuel cells, are considered promising approaches to address growing environmental concerns. The oxygen reduction reaction (ORR) is the core of renewable energy conversion technology and plays an irreplaceable role in this fundamental issue. However, the complex multi-reaction process of the ORR presents a bottleneck that limits efforts to accelerate its kinetics. Traditionally, Pt and Pt-based catalysts are regarded as a good choice to improve the sluggish kinetics of the ORR. However, because Pt-based catalysts are expensive and have low durability, their use to resolve the energy crisis and current environmental challenges is impractical. Hence, exploring low-co
st, highly active, and durable ORR catalysts as potential alternatives to commercial Pt/C is an urgent undertaking. Atomic cluster
catalysts (ACCs) may be suitable alternatives to commercial Pt/C catalysts owing to their ultra-high atomic utilization efficiency, unique electronic structure, and stable nanostructures. However, despite the significant progress achieved in recent years, ACCs remain unusable for practical applications. In this study, a facile plasma bombing method combined with an acid washing strategy is proposed to fabricate an atomic Co cluster-decorated porous carbon supports catalyst (Co AC /NC) showing improved ORR performance. The typical atomic cluster features of the resultant Co AC /NC catalyst are confirmed using comprehensive characterization techniques. The Co AC /NC catalyst exhibits considerable ORR activity with a half-wave potential of as high as 0.887 V (versus  a reversible hydrogen electrode (RHE)), which is much higher than that of a commercial Pt/C catalyst. More importantly, the Co AC /NC catalyst displays excellent battery performance when applied to a Zn-air battery, showing a peak power density of 181.5 mW·cm −2 and long discharge ability (over 67 h at a discharge current density of 5 mA·cm −2). The desirable ORR performance of the fabricated Co AC /NC catalyst could be mainly attributed to the high atom utilization efficiency and stable active sites endowed by the unique Co atomic clusters, as well as synergistic effects between the neighboring Co
atoms of these clusters. Moreover, the high specific surface area and wide pore distribution of the catalyst offer abundant accessible active sites for the ORR. This work not only provides an outstanding alternative to commercial Pt catalysts for the ORR but also offers new insights into the rational design and practical application of ACCs.
Key Words:  Atomic cluster catalyst;  Electrocatalyst;  Plasma bombing;  Co cluster;  Oxygen reduction
reaction
钴原子团簇用于高效氧还原反应
于彦会†，饶鹏†，封苏阳，陈民，邓培林*，李静，苗政培*，康振烨，沈义俊，田新龙*
海南大学南海海洋资源利用国家重点实验室，化学工程与技术学院，海南精细化工重点实验室，海口 570228
摘要：探索非贵金属材料作为高效氧还原反应催化剂是迫切需要的，但具有一定的挑战性。本文采用等离子体轰击和酸洗相结合的策略合成了Co原子团簇修饰的多孔碳载体催化剂(Co AC/NC)。通过多种表征手段证实了的原子团簇特征。所得到的Co AC/NC催化剂在三电极体系和锌-空电池方面都表现出优异的氧还原反应活性。该催化剂的氧还原反应半波电位为0.887 V，显著优于商业Pt/C催化剂，且表现出优
异的稳定性。此外，该催化剂组装的锌-空电池的峰值功率密度为181.5 mW·cm−2，同样远高于Pt/C催化剂。这项工作不仅合成了一种高效的氧还原反应催化剂，而且为原子团簇催化剂的理性设计和实际应用提供了新的见解。
关键词：原子团簇催化剂；电催化剂；等离子轰击；钴团簇；氧还原反应
中图分类号：O646
1  Introduction
Oxygen reduction reaction (ORR) is the key reaction in various clean energy conversions and storage systems 1–6. Plenty of Pt metals are usually needed to overcome the sluggish kinetics of the ORR to ensure satisfactory performance of the above-mentioned instruments 7–13. However, Pt-based ORR catalysts are suffering from low reserve and high costs, seriously hindering the practical application 14–17. Hence, it is urgently and desirable to explore robust and unexpansive ORR catalysts. Recently, single-atom catalysts (SACs) have revealed considerable activity and selectively in ORR, and have been considered as powerful alternatives to the Pt-based catalysts 18–21. The isolated metal sites are believed as the main contributors to the good activity 22–25. However, some updated works have revealed that the SACs undergo a dynamic evolution from single atoms to clusters during the dynamic
reactions process, which is observed by the advanced in situ characterization methods 26–29. The final obtained metal clusters are considered as the true active sites of the prepared catalysts to catalyze the reactions. Therefore, the actual activity contributors of the SACs may be the dynamic prepared clusters, not the traditional isolated metal atoms. In addition, the size of the atomic clusters is in the intermediate states between the nanoparticles and SACs, which may provide more stable propriety than the SACs owing to the lower surface energy, and higher atom utilization than the nanoparticles due to the higher active site’s exposure 30. In addition, the presence of the potential metal-metal bonds would trigger synergistic effects between the atoms of the atomic clusters, may further improve the electrocatalytic activity and selectivity of the atomic cluster catalysts (ACCs) 31. Hence, rational constructing ACCs is a reliable idea to explore high-efficient alternatives to Pt-based ORR catalysts, while few works have been reported.
In this work, a Co atomic clusters decorated carbon supports catalyst is successfully synthesized by the proposed “plasma bombing combined with the acid washing” strategy. Some typical characterization methods have been applied to confirm the features of the Co atomic clusters in the prepared catalyst. The Co AC/NC catalyst delivers a good ORR activity and stability. Moreover, Co AC/NC catalyst also efficiently drives a Zinc-air battery, achieving a considerable discharge ability and
stability. The desirable ORR performance of the prepared Co AC/NC catalyst could be attributed to the Co atomic clusters efficiently modulate the adsorption of oxygen-containing intermediates during the ORR process.
2 Experimental and computational section
2.1  Preparation of NC
A methanol solution (30 mL) containing Zn(NO3)2·6H2O (3.6 g) was mixed with methanol solution (70 mL) containing 2-methylimidazole (13 g) under vigorous stirring for 24 h. The prepared solid was centrifuged and washed with methanol for five times, then dried for 12 h at 60 °C in a vacuum oven (obtained materials were named ZIF-8). Then, the prepared ZIF-8 was heated at 950 °C for 2 h under Ar flowing, the final products were denoted as NC.
2.2  Synthesis of Co AC/NC
The synthesis process took place in plasma enhanced chemical vapor deposition (PECVD) machine. Firstly, CoCl2·6H2O (30 mg) and NC (40 mg) were ground and moved to the PECVD, then open the PECVD machine with the setting parameters, the atmosphere was N2, the tube pressure was 50 Pa, tr
eating time was 40 min, the final temperature was 600 °C, the radio frequency power was 500 W. The final obtained solid material was Co AC + CoO/NC catalyst. Then the obtained Co AC + CoO/NC was washed with 10% hydrochloric acid for 24 h with constant vibration and stirring during the period. After the reaction, it was collected by centrifugation and then pyrolysis at 900 °C for 2 h under Ar flowing to obtain Co AC/NC.
To further explore the properties of the clusters, we adjusted
the ratio of CoCl 2·6H 2O and NC. Adjusting the mass ratios of NC and CoCl 2·6H 2O were 1 : 1, 1 : 0.75, and 1 : 0.5, respectively, with other conditions remaining unchanged. 2.3  Material characterizations
X-ray diffraction, transmission electron microscopy, aberration-corrected high-angle annular dark field scanning transmission electron microscopy, and X-ray photoelectron spectroscopy are used to test the structure, morphology, and chemical composition of the prepared catalyst. N 2 adsorption/desorption isotherm was used to identify the specific area of the prepared catalyst. More detailed information was shown in Supporting Information. 2.4  Electrochemical measurements
The electrochemical measurements were measured on Multi autolab M240 with a typical three-electro
de system. The working electrode, reference electrode, counter electrode, and electrolyte were glassy carbon electrode, Hg/HgO, graphite rod, and O 2-saturated 0.1 mol·L −1 KOH, respectively. The catalysts slurry was obtained by dispersing 4 mg catalyst in 800 μL ethanol solution (containing 0.25% (w , mass fraction) nafion). 7 μL of the prepared catalysts slurry were dropped onto the GCE to prepare the working electrode.
Linear sweep voltammetry curves were tested with the specific parameters, scan rate is 5 mV·s −1, rotation speed is 1600 r·min −1. Accelerated durability test was conducted to test the
durability, cycling the potential from 0.6 to 1.0 V for 5000 cycles at 100 mV·s −1. During the test, the oxygen flowing was kept above the electrolyte to ensure O 2 saturation. 2.5  Zinc-air battery (ZAB) test
The anode was polished Zinc plate, cathode was carbon paper loaded catalysts, and the electrolyte was 6 mol·L −1 KOH. The polarization curves were recorded by the Multi autolab M240 electrochemical workstation. The LANHE (CT2001A) station was applied to test the discharge polarization curve of the assembled ZAB.
3  Results and discussion
3.1  Synthesis and characterizations of Co AC /NC
catalyst
The synthesis process of the prepared Co AC /NC catalyst is shown in Fig. 1a. Briefly, the CoCl 2·6H 2O and NC (the carbon supports) were firstly uniformly mixed and ground, and transferred to the PECVD machine. Then, driven by the high energy of plasma bombing treatment, Co atoms are excited and stripped from the CoCl 2·6H 2O, and subsequently, the Co atoms groups are captured and anchored on the carbon supports. After that, subsequent acid washing and annealing treatments remove unstable elements, resulting in the Co atomic clusters decorated carbon supports catalyst (Co AC /NC).
The transmission electron microscopy (TEM) images of the
Fig. 1  (a) Synthesis process of the prepared Co AC /NC catalyst, (b) HRTEM, (c) EDS mapping and (d) profile images of the Co AC /NC catalyst,
(e, f) AC HAADF STEM images of the Co AC /NC catalyst, and (g) XRD patterns of the Co AC
/NC and NC.
Co AC /NC catalyst deliver a typical disorder nanostructure, and no metal nanoparticles are observed on the surface of the prepared catalysts (Fig. 1b and Fig. S1 (Supporting Information)). Energy-dispersive X-ray spectrometry (EDX) mapping and the related EDX profile images of the Co AC /NC are displayed in Fig. 1c,d, the Co and N species are homogeneously dispersed on the carbon supports, suggesting Co species are successfully striped from the CoCl 2·6H 2O precursors and captured as well as anchored on the carbon supports. EDX mapping and profile images reveal the existence of the Co species, but not any Co nanoparticles are observed on the surface of carbon supports from the TEM images, suggesting that the Co species may be existing around the carbon supports at the atomic level 32,33.
Aberration-corrected high-angle annular dark-field scanning TEM (AC HAADF-STEM) images of the prepared Co AC /NC catalyst demonstrate that the Co species are dispersed at the atomic level on the
carbon supports, and Co atomic clusters are domination in the all atomic Co species of the prepared catalyst (Fig. 1e,f, some of the atomic clusters are highlighted by the red circles). The atomic distances of the neighbored metal atoms of Co atomic clusters are measured and shown in Fig. S2 and Table S1 (Supporting Information), the dominant atomic spacing is confirmed at around 0.25 nm, which allows the generation of the synergistic interactions from neighbored Co atoms. In addition, the mentioned synergistic effects have been confirmed to be beneficial for realizing impressive electrocatalytic performance. Moreover, some isolated Co metal atoms are also observed around the Co atomic clusters (Fig. 1e,f). Electron interactions may be established between Co atomic clusters and the around isolated Co atoms due to short interacting distances and unblocked electron transfer
pathways, which could optimize the absorption and desorption of the oxygen-containing intermediates during the ORR process, resulting in improved ORR performance. Fig. S3 and Fig. 1g show the XRD patterns of Co AC /NC before acid washing, Co AC /NC and pure NC, the Co AC /NC shows similar XRD singles with the pure NC, indicating that the CoO species were removed from the acid washing. All the physical characterization data confirm the typical Co atomic clusters features of the prepared Co AC /NC catalyst 34,35. Moreover, the actual metal loading of the prepared Co AC /NC catalyst is tested by the inductively coupled plasma mass spectrometry (ICP-MS), and Co metal loading is measured at 2.67% (w ) (Table S2).
To deeply understand the state and chemical composition of the Co AC /NC catalyst, X-ray photoelectron spectroscopy (XPS) was carried out on Co AC /NC and NC. The XPS survey spectra of the Co AC /NC and NC are shown in Fig. 2a. Compared with the pure NC, a mild peak appears at about 779.8 eV , it should be assigned to the Co 2p , suggesting the existence of Co species in the prepared Co AC /NC catalyst (Fig. 2a) 36–38. The high-resolution N 1s  XPS spectra of NC and Co AC /NC could be further deconvoluted into four peaks, corresponding to Pyridinic N (398.2 eV), Pyrrolic N (399.6 eV), Graphite N (401.3 eV), and Oxidized N (402.8 eV) (Fig. 2b) 39. The high-resolution Co 2p  XPS spectrum is deconvoluted into three types of peaks, which are attributed to the Co 2+, Co 3+, and Sat., the absence of the Co(0) also confirms the Co atomic clusters are the domination in the prepared Co species (Fig. 2c) 40.
Fig. 2d shows the Raman spectrum of the Co AC /NC catalyst, and the I D  : I G  value was 1.02, suggesting the low graphitization degree of the prepared catalyst. Moreover, Brunner-Emmet-Teller measurement is tested to reveal specific surface area and pore distributions of the Co AC /NC. The N 2 isothermal adsorption
Fig. 2  (a) XPS survey and (b) high-resolution N 1s  XPS peaks of the Co AC /NC and NC, (c) high-resolution Co 2p  XPS peak of the Co AC /NC,
reaction mass
(d) Raman spectrum of the Co
AC /NC, (e) N 2 isothermal adsorption curves and (f) pore-distribution of the Co AC /NC.
curves exhibit a typical IV hysteresis loop, and the specific surface area is calculated as 910.5 m 2·g −1, such the big specific surface area would provide more anchored sites and adsorption area during the synthesis and reaction process (Fig. 2e) 41. Fig. 2f shows the pore distribution curve of the prepared catalyst, the main pore dimeters are located at around 3.6 nm, corresponding to the mesoporous, which is believed to improve the mass transfer ability of the electrocatalysts, and result in improved electrocatalytic performance (Fig. 2f) 42.
3.2  Electrocatalytic ORR activity of Co AC /NC catalyst Given the above interesting finding, the prepared Co AC /NC was tested as an ORR catalyst. The ORR electrocatalytic activity of Co AC /NC was firstly evaluated via  the three-electrode system. Linear sweep voltammetry (LSV) curves of Co AC /NC, Pt/C, and NC are displayed in Fig. 3a. Moreover, the half-wave potential (E 1/2) and kinetics current density (j k ) are calculated based on the LSV curves. The Co AC /NC delivers considerable ORR activity, with an E 1/2 and j k  of 0.887 V and 3.662 mA·cm −2, respectively, which are more positive than those of the pure NC (E 1/2: 0.711 V , j k : 0.112 mA·cm −2) and Pt/C (E 1/2: 0.857 V , j k : 1.759 mA·cm −2) (Fig. 3b). The Co AC /NC catalyst shows a Tafel slope of 82.4 mV·dec −1, which is close to the Tafel slope of the Pt/C (75.9 mV·dec −1), indicating the fast kinetics of the prepared catalyst toward ORR (Fig. 3c).
Besides the considerable activity, stability and durability are other key directors of a good electrocatalyst. The stability and durability of the prepared Co AC /NC catalyst and Pt/C are measured by the chronoamperometry (CA) test and accelerated durability test (ADT). The Co AC /NC catalyst exhibits an impressive ORR durability with a negligible active loss after 5000 cycles ADT test (Fig. 3d). Fig. 3e demonstrates the CA results of the prepared catalyst and Pt/C, the current decay of the Co AC /NC is 6.87% after CA tests. The Pt/C shows a huge activity loss after ADT and CA test with similar measurement parameters of the Co AC /NC catalyst, suggesting the good stability and durability of synthesized Co AC /NC (Fig. 3e and Fig. S4). Moreover, the electronic transfer number and H 2O 2 yield were shown in Fig. S5, the electron transfer number of the prepared Co AC /NC catalyst is around 3.8–3.98 at the potential range of 0.6–0.9 V , indicating the 4-electron pathway is domination during the ORR process of the prepared Co AC /NC catalyst. Fig. 3f and Table S3 summarize the ORR activity of the prepared Co AC /NC and recently reported advanced catalysts. Obviously, compared with the recently reported materials, the prepared Co AC /NC catalyst exhibits competitive ORR performance, demonstrating the advantages of the proposed ACCs.
To explore the effect of different Co contents on the catalytic performance of the prepared catalysts, we have changed the
Fig. 3  (a) LSV curves, (b) comparison of ORR activity, and (c) Tafel slopes of the catalysts. (d) LSV curves of Co AC /NC before and after ADT,
(e) CA results of the Co AC /NC and Pt/C, (f) comparison of the ORR performance of the Co AC /NC and recently reported catalysts,
(g) LSV curves, (h) comparison of  E 1/2 and j k  of the Co AC /NC-X, (i) LSV curves of the Co AC /NC-S and Co AC /NC.

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