Citation

Shuo Yao, Taizhong Huang, Hengyi Fang, Jiemei Yu, Mayilvel Dinesh Meganathan, Zhaoxin Cui, Xianxia Yuan. Cobalt sulfides as efficient catalyst towards oxygen reduction reactions[J]. Chin. Chem. Lett., 2020, 31(2): 530-534  

Shuo Yao^a,b, Taizhong Huang^a,*, Hengyi Fang^a, Jiemei Yu^a, Mayilvel Dinesh Meganathan^a, Zhaoxin Cui^a, Xianxia Yuan^b,*     

With the increasing requirement for high efficiency new energy, great efforts have been conducted to develop fuel cells owing to their advantages of non-pollutant and superior energy conversion efficiency [1-3]. Oxygen reduction reaction (ORR) is one of the major factors determining the performance of fuel cells. In general, it is expected that the ORR can proceed via a 4-electron process, which fully utilizes the potential of oxygen [4-6]. Pt/C is consented to be the most efficient ORR catalyst judging from its high onset potential and 4-electron-reaction pathway [7, 8]. However, high cost and scarcity of Pt have inhibited large-scale commercial applications of fuel cells [9-11]. Exploring non-noblemetal based catalysts with low cost and excellent catalytic performance is a feasible way to fasten the development of fuel cells [12, 13].

Fe, Co and Ni based transition metals as well as their compounds have been paid much attention as the substitute of Pt/C with superior catalytic activities [14-17]. As a typical member of them, cobalt sulfides have been widely investigated [18-20]. Cobalt and sulfur can form a variety of compounds with different stoichiometric states, such as CoS, CoS₂, Co₃S₄, Co₄S₃, Co₉S₈, Co_1-xS [4, 18], making the synthesis of a stable compound with good crystallinity rather difficult [21]. Furthermore, the accumulation and stacking of cobalt sulfides nanoparticles also inhibit the catalytic performance. Many works on CoS₂ as ORR catalysts have been reported, but the activity and durability are still dissatisfactory. For example, Jakub et al. [22] found the agglomeration of large CoS₂ particles reduces catalytic activity of CoS₂ as ORR catalysts. Some researches fabricated Co₂S particles with beautiful morphology, but the active sites were blocked and the activity was reduced [21]. Using conductive carbon as a support is a generally adopted strategy to enhance the performance of catalysts [23, 24]. For example, Liang et al. [25] reported the improved ORR activity of Co₃O₄ when supported on the surface of N-doped graphene, better ORR performance of CoSe could be achieved by loading on carbon substrate [26], Cao [27] and his coworkers fabricated a Co₉S₈/nitrogen-doped carbon nanocomposites as a bifunctional catalyst and applied in oxygen reduction/evolution reactions with high catalytic activity. Reduced graphene oxide (rGO) is a typical material in carbon group, its excellent conductivity and high specific surface area makes it a potential catalyst support. When a catalyst is uniformly distributed on its surface, the conductivity as well as the catalytic performance is expected to be promoted.

Herein, both flower-like CoS and octahedral CoS₂ materials were synthesized through a facile one-pot hydrothermal method without any adjunction of surfactants or follow-up thermolysis, their catalytic performance towards ORR were comparatively investigated. Then, the rGO supported catalyst was synthesized to further improve the performance. Thus obtained CoS₂/rGO showed great promise for ORR in alkaline fuel cells.

XRD patterns were captured to detect phase compositions in the as-prepared cobalt sulfides. As shown in Fig. 1a, the XRD pattern of CoS matches well with PDF card with a JCPDS No. 65-3418, implying the obtained CoS is with a hexagonal crystal structure (Fig. 1b). All the diffraction peaks of CoS₂ could be well indexed to the standard PDF #65-3322, indicating successful synthesis of CoS₂ with a cubic crystal structure (Fig. 1c). Moreover, no additional characteristic peaks could be observed in both cobalt sulfide materials, confirming the pure phase structure in the asprepared CoS and CoS₂.

Figure 1

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Fig. 1. XRD patterns of CoS, CoS2 and CoS2/rGO (a), crystal structure of CoS (b) and CoS2 (c).

In the morphology images, the flower-like CoS particles are constituted with 2D flake-like structure unit interwoven with each other (Fig. 2a), while CoS₂ is configured with small octahedral granules (Fig. 2b). Micro-structure of the as-prepared cobalt sulfides was also observed with TEM, flower-like CoS assembled with flake-like unit (Fig. 2d) and octahedral CoS₂ (Fig. 2e) are clearly displayed, agreeing well with the SEM images. In the HRTEM images, the d-spacing of 0.292 nm for CoS (Fig. 2g) matches the (100) facet of hexagonal CoS (PDF #65-3418), while the spacing of 0.321 nm (Fig. 2h) is corresponding to (111) facet of cubic CoS₂ (PDF #65-3322). Moreover, the SAED patterns inserted in Figs. 2g and h were also employed to have a deeper insight into the crystallographic structure of the as-prepared cobalt sulfides. The concentric rings for CoS could be assigned to the (100), (101), (102) and (110) planes of hexagonal CoS (PDF #65-3418), and the spots for CoS₂ agree with the (332), (222), (210) and (321) facets of cubic CoS₂ (PDF #65-3322).

Figure 2

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Fig. 2. SEM images of CoS (a), CoS2 (b) and CoS2/rGO (c); TEM images of CoS (d), CoS2 (e) and CoS2/rGO (f); HRTEM images of CoS (g), CoS2 (h) and CoS2/rGO (i), the insets in g-i are the corresponding SAED images.

Electrochemical performance of CoS and CoS₂ were comparatively investigated in oxygen-saturated 0.1 mol/L KOH solution. In CV curves shown Fig. 3a, the obvious peaks in the cathodic direction imply both CoS and CoS₂ are capable of catalyzing ORR, but careful comparison reveals higher peak potential and higher peak current density of CoS₂ than CoS, indicating better catalytic performance of CoS₂. Linear sweeping voltammetry (LSV) curves for both materials were recorded with RDE at a rotating rate of 1600 rpm and the results are displayed as Fig. 3b. In the whole studied potential range, CoS₂ delivers evidently higher current density than CoS, it also demonstrates clearly higher ORR onset potential and the half-wave potential. The lower Tafel slope in Fig. 3c confirms again the superior performance of CoS₂.

Figure 3

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Fig. 3. CV curves (a) of CoS, CoS2 and CoS2/rGO in O2-saturated 0.1 mol/L KOH electrolyte at a potential sweeping rate of 5 mV/s, RDE curves (b) of CoS, CoS2 and CoS2/rGO in O2-saturated 0.1 mol/L KOH electrolyte at 1600 rpm at a sweeping rate of 5 mV/s, Tafel plots (c) of CoS, CoS2 and CoS2/rGO, long-term stability tests (d) of CoS2 and CoS2/rGO.

To understand the different catalytic performance of CoS₂ and CoS towards ORR, RDE and RRDE experiments were conducted. Figs. 4a and c depicts the RDE curves of both materials in O₂-saturated 0.1 mol/L KOH electrolyte at various electrode rotating speeds from 900 rpm to 2025 rpm. For each catalyst, the current density increases with the increase in electrode rotating rate, owing to the enhanced oxygen diffusion [28]. Generally, the electron transfer number (n) during ORR can be calculated with RDE data and K-L plot according to equations described in Supporting information [29]. When J^-1 is plotted against ω^-1/2, slope of the obtained line could be used to calculate the value of n. Thus obtained K–L plots are displayed in Figs. 4b and d using the current densities at 0.45, 0.50, 0.55 and 0.60 V, respectively, in the RDE data. The calculated n values for of CoS and CoS₂ are 3.5 and 3.9, respectively, meaning that the ORR catalyzed by CoS₂ is almost through a 4-electron transfer pathway, while the reaction by CoS proceeds by parallel 4-electron and 2-electron processes [11]. These results declare CoS₂ has faster ORR kinetics than CoS.

Figure 4

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Fig. 4. RDE curves of CoS (a), CoS2 (c) and CoS2/rGO (e) in O2-saturated 0.1 mol/L KOH electrolyte at a potential sweeping rate of 5 mV/s and various electrode rotating rates, and the corresponding K-L plots of CoS (b), CoS2 (d) and CoS2/rGO (f) at various potentials.

The catalytic mechanism of CoS and CoS₂ for ORR was also investigated with RRDE and the results are shown in Fig. S2a (Supporting information). With these data, the value of n and the H₂O₂ yield (H₂O₂%) could be calculated using the equations of 3 and 4 displayed in Supporting information [30]. The obtained potential dependence of n and H₂O₂% is displayed as Fig. S2b (Supporting information). Similar to the results above from RDE, the electron-transfer number of CoS₂ is clearly larger than that of CoS, while its H₂O₂ yield is evidently smaller by about 10%, verifying again the superior performance of cobalt disulfide including higher ORR activity and faster kinetics. This is probably due to the fact that the higher electron density around S-S bond of S₂^2- in CoS₂ crystal structure than that around S^2- in CoS promotes the adsorption of oxygen on the catalyst surface and facilitates the breakage of O-O bond in oxygen leading to direct 4-electron transfer ORR [31, 32].

In order to further improve the ORR performance of CoS₂, rGO with high specific surface area and good conductivity was employed to form the composite of rGO supported CoS₂ (CoS₂/rGO) as catalyst, its XRD pattern is also shown in Fig. 2a along with that of CoS₂ and CoS for comparison. The characteristic peaks closely match that of bare CoS₂, proving the existence and successful synthesis of CoS₂ in this composite. The absence of peaks for rGurves of both materials iO is probably because of the thin layers [25]. In the SEM image displayed as Fig. 2c, wrinkled rGO can be clearly observed with uniformly dispersed CoS₂ particles on its surface, confirming the existence of rGO. Moreover, the size of CoS₂ particles in CoS₂/rGO is greatly smaller than that of the bare CoS₂, it is only one fourth or less of the latter. This could be attributed to the influence of rGO according to Chang et al. [33] and Wang et al. [29], probably because of the interactions between rGO and cobalt sulfides and also the large surface area of rGO hinders the agglomration of CoS₂ particles. The TEM images shown in Figs. 2f and i also confirm the co-existence of CoS₂ and rGO in this composite. In Fig. 2f, both the wrinkled rGO and the smaller CoS₂ particles are evidently observed. In the HRTEM image (Fig. 2i), the irregular-sized crystalline lattice fringes can be assigned to wrinkled rGO support, while the d-spacing of 0.226 nm is corresponding to (211) facet of CoS₂. The inset of SEAD pattern also demonstrates obvious diffraction spot array that could be indexed to (211), (311) and (023) facets in CoS₂.

The electrochemical performance of CoS₂/rGO was evaluated and compared with bare CoS₂. In the CV curves shown in Fig. 3a, the ORR peak potential of CoS₂/rGO is increased by 45 mV from that of CoS₂. The peak current density is also higher by about 200 mA/cm². In the LSV curves shown in Fig. 3b, CoS₂/rGO delivers apparently higher onset potential and strongly larger current density in the whole studied voltage range, indicating the increase of catalytic active area due to decrease of particle size. Its onset potential of 0.93 V is also higher than the reported values for similar rGO supported catalysts of NiCo₂S₄@N/S-rGO (0.86 V), Co₃S₄@N/S-rGO (0.85 V) and Co_1-xS/rGO (0.87 V) in 0.1 mol/L KOH electrolyte [34, 35]. Meanwhile, it can be observed that the halfwave potential of CoS₂/rGO is also higher than that of bare CoS₂, demonstrating the strong synergistic effect between CoS₂ and rGO as support and the improved ORR activity of CoS₂/rGO. The decreased Tafel slope in Fig. 3c further confirms the enhanced ORR performance of CoS₂/rGO from bare CoS₂. The durability of CoS₂/rGO was evaluated with chronoamperometric measurement and compared with that of CoS₂. The current density of CoS₂/rGO reaches stable much earlier than CoS₂ as displayed in Fig. 3d, and its current density is greatly higher than CoS₂ during the whole continuous working for 10, 000 s, indicating a better long-term stability as ORR catalyst. RDE and RRDE were also employed to understand the ORR performance improvement. In Figs. 4e and f, the current density of CoS₂/rGO at each rotating rate is greatly larger than that of CoS₂, and its linearity of K-L plot is evidently better than bare CoS₂, indicating first-order reaction kinetics towards the concentration of dissolved oxygen and the more similar electron transfer number per oxygen molecule within the studied potential range [36, 37]. The calculated higher electron transfer number of nearly four also implies faster electron transport rate between electrode and electrolyte and higher efficiency for oxygen reduction to OH^- by accepting four electrons [38]. In the RRDE data shown in Fig. S2, CoS₂/rGO demonstrates evidently increased onset potential than the bare CoS₂, its current density on the disk electrode is greatly increased while that on the ring electrode is decreased. In Fig. S2b which displays the calculated electron transfer number and H₂O₂ yield as a function of potential, CoS₂/rGO exhibits again clearly larger n and lower H₂O₂%. These results disclose that dispersing CoS₂ on the surface of rGO as support can efficiently enhance the ORR kinetics, promote the catalytic activity and facilitate ORR to happen through 4-electron reaction pathway, resulting in enlarged ORR performance. This improvement could be attributed to two aspects. The first one is the excellent synergistic effect between CoS₂ and rGO as support, similar effects were also observed during the research on MoS₂/gaphene hybrid, which exhibited higher catalytic performance than pure MoS₂ for ORR [7, 39]. Moreover, rGO with large specific surface area can improve the dispersion of CoS₂ to expose more catalytic active sites.

In this work, both flower-like CoS and octahedral CoS₂ have been successfully synthesized through a facile one-pot hydrothermal method without any adjunction of surfactants or follow-up thermolysis. Comparative study disclosed that the as-prepared CoS₂ has superior performance to CoS when employed as catalyst for ORR in alkaline electrolyte, judging from the onset potential, peak potential, half-wave potential, peak current density, Tafel slope as well as the electron transfer number and H₂O₂ yield. When supported on the surface of rGO, the particle size of CoS₂ decreased to one fourth or less of the bare CoS₂, its specific surface area is greatly enlarged and much more active sites towards ORR could be exposed, leading to enhanced ORR kinetics as well as activity.

This project was financially supported National Natural Science Foundation of China (No. 21476138), Shandong Provincial Natural Science Foundation (No. ZR2018MB036), Science Development Project of Shandong Province (Nos. 2017GGX40115 and 2016GGX102038), Project of Shandong Province Higher Educational Science and Technology Program (Nos. J17KA094, J13LD08).

Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.cclet.2019.04.069.