Fe doping and interface engineering-induced dual electronic regulation of CoSe₂/Co₉S₈ nanorod arrays for enhanced electrochemical oxygen evolution

Abstract

Accurately engineering the electronic configuration of electrocatalysts to enhance their electrocatalytic performance is of importance in water splitting applications. Herein, a dual electronic regulation concept through constructing Fe-incorporated CoSe₂ and Fe-incorporated Co₉S₈ heterostructure nanorod arrays directly grown on a carbon cloth substrate (abbreviated as Fe-(CoSe₂/Co₉S₈)@CC hereafter) is demonstrated. Owing to the synergistic effect of Fe doping and CoSe₂/Co₉S₈ heterointerfaces, the resultant Fe-(CoSe₂/Co₉S₈)@CC electrode exhibits an accelerated charge transfer rate, improved electrical conductivity and sufficient active sites, thereby exhibiting excellent oxygen evolution reaction (OER) performance. Specifically, the engineered Fe-(CoSe₂/Co₉S₈)@CC electrode only requires low overpotentials of 243 and 337 mV to afford current densities of 10 and 300 mA cm⁻², respectively, and shows a low Tafel slope of 44.5 mV dec⁻¹, an ideal faradaic efficiency of nearly 100%, and prominent long-term durability in an alkaline medium. This adopted double optimization strategy provides valuable guidance for the development of low-cost and high-performance transition metal-based electrocatalysts in the energy conversion field.

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Introduction

In the face of the energy dilemma and environmental pollution issues, hydrogen has been commonly considered as a potential energy substitute to supersede fossil fuel thanks to its high atom utilization and extreme energy density.^1–4 At present, overall water electrolysis involving the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) represents an accessible and energy-efficient technology for generating green and high-purity hydrogen.^5–7 Nevertheless, compared with the straightforward two-electron HER, the OER undergoes a complicated proton-coupled electron transfer process for the breakage of the O–H bond and the generation of a rigid O–O double bond, thus being the bottleneck of water splitting.^8–10 As a consequence, high-powered electrocatalysts are indispensable for conquering the sluggish kinetics and ameliorating the reaction rate.^11,12 Currently, commercial RuO₂/IrO₂ catalysts are generally recognized as OER benchmark electrocatalysts, but their expensive price and scarce reserves extremely confine their extensive application.^13,14 Therefore, it is of great concern to consider more economical and effective non-precious metal catalysts as substitutes for the electro-catalysis of the OER.

In the recent few years, transition metal sulfides,¹⁵ phosphides,¹⁶ selenides,¹⁷ nitrides,¹⁸ oxides,¹⁹ hydroxides,²⁰ perovskites,²¹ and so on have been extensively employed as potential non-noble electrocatalysts for the OER. Among the various alternatives, Co-based chalcogenides (e.g., Co_xS_y and Co_xSe_y) have attracted fascinating attention in electrochemical water oxidation due to their tunable electronic structure and abundant reserves.^22,23 However, their electrochemical activity and long-range stability still require further promotion for satisfactory practical applications. To confront the above challenges, tremendous endeavors have been made to modify the electronic structure to improve the intrinsic activities of catalysts, including but not limited to element incorporation and heterointerface engineering.^24–27 Numerous studies have primely demonstrated that Fe atom doping into the lattice of Co-based electrocatalysts could generate a strong “Fe effect” to enhance OER performance through regulating the local electronic configuration of active centers, suppressing electrochemical oxidation of host metals and heightening electrical conductivity.^28,29 In addition, integrating two glorious active components into a heterostructure at the atomic scale not only allows it to inherit the electrochemical ability of each species, but also creates plentiful heterointerfaces.^30–33 The strong interfacial electron interaction could dramatically tailor the electronic distribution at the junction, thereby accelerating charge transfer efficiency and modulating the adsorption state of reaction intermediates.^34–36 However, few reports are devoted to rationally considering dual electronic regulation strategies into one feasible approach to construct a metal-incorporated heterostructure for a high-performance electrochemical OER.

Enlightened by the abovementioned designing mentality, we herein report a “two in one” electronic modulation concept for synthesizing novel Fe-doped CoSe₂/Fe-doped Co₉S₈ heterojunction nanorod arrays in situ grown on conductive carbon cloth (denoted as Fe-(CoSe₂/Co₉S₈)@CC hereafter) through a simple topological conversion strategy using CC loaded Co(CO₃)_0.5OH·0.11H₂O nanorod arrays (denoted as Fe-CoCH@CC hereafter) as reactive precursors. The synergistic effect of Fe doping and well-defined CoSe₂/Co₉S₈ heterointerfaces generates a dual electronic modification function. On the one hand, the Fe incorporation could modulate the electronic structure of active species and improve the electrical conductivity. On the other hand, the construction of CoSe₂/Co₉S₈ heterointerfaces could also regulate the electronic configuration at the heterojunction and expose numerous active sites. In addition, the in situ formation of the nanorod array structure on the CC substrate could generate numerous void channels for rapid mass transfer and favorable electrolyte penetration and provide robust mechanical strength. Benefiting from the valid multiple regulation, the resultant Fe-(CoSe₂/Co₉S₈)@CC electrode demonstrates prominent OER performance, with small overpotentials of 243 and 337 mV to reach 10 and 300 mA cm⁻², respectively, a low Tafel slope of 44.5 mV dec⁻¹, and long-term stability for at least 28 h. This adopted “two in one” regulation method for electronic modification would open up a new avenue for the construction of transition metal-based electrocatalysts for water electrolysis.

Results and discussion

As schematically described in Fig. 1, the Fe-(CoSe₂/Co₉S₈)@CC was fabricated through thermal transformation of the pre-prepared Fe-CoCH@CC via a synchronous “selenation–sulfuration” strategy. First, Fe-CoCH@CC was synthesized through a typical hydrothermal reaction. The X-ray diffraction (XRD) pattern (Fig. S1a†) manifests that the Fe-CoCH@CC exhibits the orthorhombic Co(CO₃)_0.5OH·0.11H₂O phase. As observed from scanning electron microscopy (SEM) images (Fig. S1b–d†), numerous nanorod arrays with smooth surfaces are vertically arranged on the CC substrate. During the following low-temperature selenation–sulfuration process, Fe-CoCH nanorods could react with the chosen sulfur and selenium powders to form Fe-(CoSe₂/Co₉S₈) heterostructure nanorods, in which Fe atoms are incorporated into the lattices of both CoSe₂ and Co₉S₈. For comparison, Fe-doped CoSe₂, Fe-doped Co₉S₈ and CoSe₂/Co₉S₈ heterostructure decorated CC samples with single electronic modification were also fabricated on the basis of an identical preparation procedure apart from tuning the reactant species. As shown in Fig. S2,† under identical synthetic parameters apart from the absence of sulfur powder, the selenium could react with Fe-CoCH to form Fe-doped cubic-phased CoSe₂ nanorod arrays, which are homogeneously loaded on CC (denoted as Fe-CoSe₂@CC hereafter). Analogously, the absence of selenium leads to the formation of Fe-doped Co₉S₈ with cubic phase nanorod arrays grown on the CC (denoted as Fe-Co₉S₈@CC hereafter, Fig. S3†). In addition, when employing CoCH@CC as the reaction precursor, upon “selenation–sulfuration” treatment, the final product shows the CoSe₂/Co₉S₈ heterostructure nanorod arrays aligned on the CC substrate (abbreviated as CoSe₂/Co₉S₈@CC hereafter, Fig. S4†). The abovementioned controlled experiments unquestionably confirm that the simultaneous introduction of S, Se and Fe is essential for the generation of CoSe₂/Co₉S₈ heterostructure nanorod arrays.

Fig. 1 Schematic illustration of the preparation of Fe-(CoSe₂/Co₉S₈)@CC.

As shown in the XRD pattern (Fig. 2a), the diffraction peaks of the resultant Fe-(CoSe₂/Co₉S₈)@CC sample are assigned to cubic-phased CoSe₂ and cubic-phased Co₉S₈, suggesting the generation of a heterojunction. The morphological and structural features of the Fe-(CoSe₂/Co₉S₈)@CC sample were first evaluated using SEM images. As shown in Fig. 2b and c, the obtained Fe-(CoSe₂/Co₉S₈)@CC sample inherits the nanorod array structure of parent Fe-CoCH@CC after mild selenation–sulfuration reactions. However, the magnified SEM image (Fig. 2d) demonstrates that the surface of these nanorods becomes rough, which may be due to the dehydration and decomposition of Fe-CoCH precursors during the selenation–sulfuration process.

Fig. 2 Compositional and morphological features of the as-synthesized Fe-(CoSe₂/Co₉S₈)@CC. (a) XRD pattern, (b–d) SEM images, (e) contact angle test, (f and g) TEM images, and (h and i) HRTEM images.

As shown in Fig. 2e, the water contact angle (CA) of Fe-(CoSe₂/Co₉S₈)@CC is determined to be 51.05°, reflecting its superior hydrophilicity. This favorable hydrophilic feature is conducive to wettability and gas bubble release on the surface of the electrocatalyst. The transmission electron microscopy (TEM) image (Fig. 2f) further confirms the rod-like morphology of Fe-(CoSe₂/Co₉S₈). The magnified TEM image (Fig. 2g) indicates the presence of nanoholes in the nanorods. It is extensively recognized that such nanorod arrays with rough and porous structures could effectively expand the accessible area for electrolyte infiltration, enrich active sites and promote the electron transport efficiency.^37,38 As shown in the high-resolution TEM (HRTEM) images (Fig. 2h and i), the well-resolved lattice fringes with an interplanar spacing of 0.259 nm belong to the (210) plane of cubic CoSe₂, while the distinct interplanar spacings of 0.286 and 0.244 nm can be attributed to be (222) and (400) planes of cubic Co₉S₈, respectively. Notably, distinct interfaces between Co₉S₈ and CoSe₂ could be noticed from the HRTEM images. The energy-dispersive X-ray (EDX) mapping images (Fig. S5a†) and compositional line profiles (Fig. S5b†) convincingly testify to the homogeneous dispersion of Co, Fe, S and Se elements throughout the whole Fe-(CoSe₂/Co₉S₈) nanorods.

X-ray photoelectron spectroscopy (XPS) was employed to analyze the surface chemistry of the electrocatalyst. The XPS survey spectrum (Fig. S6†) discloses the coexistence of Co, Fe, S and Se elements in the harvested product. In the high-resolution Co 2p spectrum of Fe-(CoSe₂/Co₉S₈)@CC (Fig. 3a), the characteristic peaks centered at 778.51 and 793.45 eV are ascribed to the metallic Co 2p_3/2 and Co 2p_1/2 peaks, respectively, while two broad peaks at 781.6 and 797.18 eV are assigned to the Co²⁺ 2p_3/2 and Co²⁺ 2p_1/2 peaks, respectively.^39,40 Furthermore, as shown in Fig. 3b, the remarkable thing is that the metallic Co 2p_3/2 peak in Fe-(CoSe₂/Co₉S₈)@CC slightly positively shifts to a higher binding energy region in comparison with those of Fe-Co₉S₈@CC and CoSe₂/Co₉S₈@CC, but negatively shifts to a lower binding energy region compared with that of Fe-CoSe₂@CC. This result unambiguously manifests that both Fe doping and the CoSe₂/Co₉S₈ heterostructure play important roles in the electronic modulation of active sites.⁴¹ The high-resolution Fe 2p spectrum (Fig. 3c) can be deconvoluted into three peaks, namely the Fe²⁺ 2p_3/2 peak (710.54 eV), Fe³⁺ 2p_3/2 peak (713.96 eV) and satellite peak (718.99 eV).⁴² In the high-resolution S 2p spectrum (Fig. 3d), two peaks located at 161.82 and 162.88 eV can correspond to S 2p_3/2 and S 2p_1/2 peaks, respectively.⁴³ A weak peak at 168.6 eV belongs to the sulfate/sulfite species on account of the surface oxidation of sulfides.⁴⁴ In addition, the other two peaks positioned at 160.6 and 165.4 eV belong to the Se 3p_3/2 and Se 3p_1/2 peaks, respectively.⁴⁵ The high-resolution Se 3d spectrum (Fig. 3e) demonstrates three predominant peaks of Se 3d_5/2 (54.23 eV), Se 3d_3/2 (55.03 eV) and selenate/selenite (59.45 eV), respectively.^46,47 As shown in the electron spin resonance (ESR) spectra (Fig. 3f), the resultant Fe-(CoSe₂/Co₉S₈)@CC sample exerts the most strong signal intensity at a g value of 2.003 compared to Fe-Co₉S₈@CC, Fe-CoSe₂@CC and CoSe₂/Co₉S₈@CC samples, reflecting the generation of more oxygen vacancies.

Fig. 3 Surface chemistry of the as-prepared Fe-(CoSe₂/Co₉S₈)@CC. (a and b) Co 2p spectra, (c) Fe 2p spectrum, (d) S 2p spectrum, (e) Se 3d spectrum, and (f) ESR spectra.

To investigate the dual electronic regulation effect of Fe incorporation and the CoSe₂/Co₉S₈ heterostructure, the electrochemical OER performances of all the electrodes were evaluated in an O₂-saturated 1.0 M KOH solution by utilizing a three-electrode system. For comparison, commercial RuO₂ was also estimated as a benchmark. Fig. 4a shows the linear sweep voltammetry (LSV) curves of all the samples. Apparently, the earlier turning point and steep curve indicate that the Fe-(CoSe₂/Co₉S₈)@CC electrode exhibits the most prominent OER activity among all the electrodes. Concretely speaking, as shown in Fig. 4b, to reach a current density of 10 mA cm⁻², the Fe-(CoSe₂/Co₉S₈)@CC electrode only requires a low overpotential of 243 mV, evidently smaller than those of Fe-Co₉S₈@CC (266 mV), Fe-CoSe₂@CC (298 mV), CoSe₂/Co₉S₈@CC (279 mV), Fe-CoCH@CC (309 mV) and commercial RuO₂ (341 mV). Notably, even at a large current density, the Fe-(CoSe₂/Co₉S₈)@CC electrode demonstrates superior electrocatalytic performance. For instance, when the current density is at 300 mA cm⁻² (Fig. 4c), the overpotentials for the Fe-(CoSe₂/Co₉S₈)@CC electrode is 337 mV, whereas Fe-Co₉S₈@CC, Fe-CoSe₂@CC, CoSe₂/Co₉S₈@CC and Fe-CoCH@CC need evidently larger overpotentials of 386, 391, 395 and 407 mV, respectively. Tafel slopes were employed to appraise the reaction kinetics and mechanisms. As displayed in Fig. 4d, the Tafel slope of the Fe-(CoSe₂/Co₉S₈)@CC electrode is identified as 44.5 mV dec⁻¹, visibly smaller than those of Fe-Co₉S₈@CC (63.7 mV dec⁻¹), Fe-CoSe₂@CC (61.4 mV dec⁻¹), CoSe₂/Co₉S₈@CC (64.8 mV dec⁻¹), Fe-CoCH@CC (68.2 mV dec⁻¹) and commercial RuO₂ (95.7 mV dec⁻¹), reflecting a favorable reaction kinetics proceeding on the Fe-(CoSe₂/Co₉S₈)@CC electrode. These above results make clear that the OER performance of Fe-(CoSe₂/Co₉S₈)@CC surpasses the performances of the majority of recently reported transition metal chalcogenide-based electrocatalysts, as briefly summarized in Table S1.† The electrochemical double-layer capacitance (C_dl) was calculated by cyclic voltammetry (CV) with different scan rates in a non-faradaic region (Fig. S7†). As shown in Fig. 4e, the C_dl values of Fe-(CoSe₂/Co₉S₈)@CC, Fe-Co₉S₈@CC, Fe-CoSe₂@CC, CoSe₂/Co₉S₈@CC and Fe-CoCH@CC electrodes are determined to be 351.1, 290.9, 332.5, 286.9 and 112.8 mF cm⁻², respectively. The larger C_dl value demonstrates that the Fe doping and CoSe₂/Co₉S₈ heterostructure exert a positive effect on the generation of active sites, thereby promoting the OER activity. In view of the similar nanorod array structures of all the tested electrodes, the evidently enhanced OER performance of the Fe-(CoSe₂/Co₉S₈)@CC electrode can be reasonably put down to the “two in one” regulation strategy. This phenomenon may be due to the reason that the dual optimization method makes up for the shortfall in single electronic adjustment via heteroatom doping or heterointerface engineering.

Fig. 4 Evaluation of the OER performances of the harvested self-supported electrodes. (a) LSV polarization curves, (b) overpotentials at 10 mA cm⁻², (c) overpotentials at 300 mA cm⁻², (d) Tafel slopes, (e) calculated C_dl values, (f) LSV curves of Fe-(CoSe₂/Co₉S₈)@CC before and after a 2000-cycle CV test. The inset of panel (f) presents the chronopotentiometry curve at a constant applied potential of 1.55 V. (g and h) Digital photographs of the electrolyzer before and after the OER, and (i) theoretical and generated volumes of O₂ during the OER process at 20 mA cm⁻² for 1 h, and the corresponding faradaic efficiency.

Besides the electrocatalytic activity, long-term stability is also an essential parameter for the estimation of electrocatalysts in practical applications. The almost coincident LSV curves before and after 2000 cycles of CV measurements (Fig. 4f) testify to the prominent electrochemical stability of the obtained Fe-(CoSe₂/Co₉S₈)@CC electrode. As witnessed from the amperometric i–t curve (inset of Fig. 4f), the current density presents negligible degradation even after 28 h at a constant potential of 1.55 V, further confirming its prominent long-term stability. SEM, TEM and XPS techniques were carried out to investigate the physical and chemical characterization of Fe-(CoSe₂/Co₉S₈)@CC after the stability test. SEM images (Fig. S8a and b†) and TEM images (Fig. S8c and d†) reveal that the Fe-(CoSe₂/Co₉S₈)@CC electrode retains the nanorod array structure after the stability test. The HRTEM image (Fig. S8e†) shows that the surface of the sample is oxidized to CoOOH species in the high potential region, forming a CoOOH/Co₉S₈/CoSe₂ heterostructure, which works as the actual active site for the OER.^48,49 What is more, Co 2p spectra (Fig. S8f†) further reveal the generation of CoOOH during the OER stability test, while there is no obvious change of Fe 2p peaks (Fig. S8g†). The S 2p and Se 3d spectra (Fig. S8h and i†) certify that most of S and Se elements are oxidized to sulfate/sulfite and selenate/selenite, respectively. Moreover, as shown in Fig. 4g and h, the generated O₂ was gathered by the water drainage method. Fig. 4i demonstrates that the volume of harvested O₂ gas coincides well with the theoretical value, which implies almost 100% faradaic efficiency during the OER process. Considering the glorious activity and long-term stability, the synthesized Fe-(CoSe₂/Co₉S₈)@CC electrode holds great promise for widespread application in the water splitting field.

In one word, the impressive OER performance of the Fe-(CoSe₂/Co₉S₈)@CC electrode can be ascribed to the particular compositional and morphological advantages. First, the Fe doping dramatically alters the electronic state of active sites and improves the electrical conductivity. Second, the elaborate CoSe₂/Co₉S₈ heterostructure also powerfully regulates the electronic configuration, thus benefiting the enormous exposure of active sites and modifying the adsorption behaviour of the reaction intermediate. Last, the vertically aligned nanorod array structure with open voids effectively promotes electrolyte permeation, accelerates mass transportation and expedites gas bubble release during the OER process.

Conclusions

All in all, this work adopts a dual modulation strategy for regulating the electronic structure of the CoSe₂/Co₉S₈ heterojunction through Fe doping for enhanced OER performance. Electrochemical results indicate that the double electronic modification through Fe incorporation and CoSe₂/Co₉S₈ heterostructure engineering endows the resultant Fe-(CoSe₂/Co₉S₈)@CC electrode with a rapid charge transfer rate, plentiful active sites and tailored adsorption free energies, thus resulting in excellent electrocatalytic OER performance. Specifically, the Fe-(CoSe₂/Co₉S₈)@CC electrode only requires small overpotentials of 243 and 337 mV to afford current densities of 10 and 300 mA cm⁻², respectively, with a low Tafel slope of 44.5 mV dec⁻¹ and long-term durability for at least 28 h. So the prominent OER property makes the Fe-(CoSe₂/Co₉S₈)@CC a potential and economical alternative for practical water splitting. We have infinite faith in the dual electronic engineering approach in this study which could enlighten the future design of transition metal electrocatalysts in energy conversion applications.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (22302096), the Natural Science Foundation of Jiangsu Higher Education Institutions of China (23KJD150004) and the High-level Talents Project of Jinling Institute of Technology (jit-b-202164).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3qi02017g

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