N‐, O‐, and S‐Tridoped Carbon‐Encapsulated Co₉S₈ Nanomaterials: Efficient Bifunctional Electrocatalysts for Overall Water Splitting

Abstract

The development of highly active and stable earth-abundant catalysts to reduce or eliminate the reliance on noble-metal based ones in green and sustainable (electro)chemical processes is nowadays of great interest. Here, N-, O-, and S-tridoped carbon-encapsulated Co 9 S 8 (Co 9 S 8 @NOSC) nanomaterials are synthesized via simple pyrolysis of S-and Co(II)-containing polypyrrole solid precursors, and the materials are proven to serve as noble metal-free bifunctional electrocatalysts for water splitting in alkaline medium. The nanomaterials exhibit remarkable catalytic performances for oxygen evolution reaction in basic electrolyte, with small overpotentials, high anodic current densities, low Tafel slopes as well as very high (nearly 100%) Faradic efficiencies. Moreover, the materials are found to efficiently electrocatalyze hydrogen evolution reaction in acidic as well as basic solutions, showing high activity in both cases and maintaining good stability in alkaline medium. A two-electrode electrolyzer assembled using the material synthesized at 900 °C (Co 9 S 8 @NOSC-900) as an electrocatalyst at both electrodes gives current densities of 10 and 20 mA cm −2 at potentials of 1.60 and 1.74 V, respectively. The excellent electrocatalytic activity exhibited by the materials is proposed to be mainly due to the synergistic effects between the Co 9 S 8 nanoparticles cores and the heteroatom-doped carbon shells in the materials.

Hollow TiO2@Co9S8 Core–Branch Arrays as Bifunctional Electrocatalysts for Efficient Oxygen/Hydrogen Production

“…The present overpotential is also substantially lower than recently reported Co 9 S 8 -based catalysts at the same current density, such as Co 9 S 8 @N and S codoped porous carbon tube (310 mV), [27] hollow Co 9 S 8 microplates (273 mV), [26] and Fe 3 O 4 @Co 9 S 8 / rGO (340 mV). [27] This indicates the construction of hollow core-branch architecture is favorable for the reinforcement of OER, further proven by the Tafel slope analysis (Figure 3b). The TiO 2 @Co 9 S 8 electrode exhibits the lowest of Tafel slope of 55 mV dec −1 , much better than other counterparts, suggesting its fastest OER process (Table S1, Supporting Information).…”

“…Figure 5a shows the overall water splitting activity of two-electrode system with the TiO 2 @Co 9 S 8 electrocatalysts as both cathode and anode in 1 m KOH solution (denoted as TiO 2 @Co 9 S 8 || TiO 2 @Co 9 S 8 ). Impressively, a significantly low cell voltage of 1.56 V is obtained at the current density of 10 mA cm −2 (Figure 5a), substantially lower than the Co 9 S 8 || Co 9 S 8 catalyzer cell (1.71 V) and other reported bifunctional electrocatalysts, [6,16,27,[38][39][40][41][42][43][44][45][46] (Figure 5b), and even close to the Pt/C || IrO 2 (1.54 V) catalyzer cell. [44] Figure 5c compares the chronopotentiometry curves of the TiO 2 @Co 9 S 8 || TiO 2 @Co 9 S 8 and Co 9 S 8 || Co 9 S 8 catalyzer cells collected at 10 mA cm −2 .…”

“…In comparison to bulk Co 9 S 8 , nanostructured Co 9 S 8 and its composites could afford more active sites and faster transfer rate of ions/electrons during the electrocatalytic reaction, and thus usually exhibiting enhanced HER and OER activities. [20][21][22][23][24] Currently, several Co 9 S 8 nanostructures (e.g., nanoparticles [25] and nanospheres [26] ) and their composites with carbons (Co 9 S 8 /reduced graphene oxides (RGO), [22] Co 9 S 8 /(N, S, P)-doped carbons, [27] Co 9 S 8 /Fe 3 O 4 / RGO, [23] and Co 9 S 8 /MoS 2 /Carbon fibres [25] ) have been reported. For example, Co 9 S 8 /N,P-carbon powder nanocomposites prepared by molten-salt calcination method at 900 °C exhibited an HER overpotential of 261 mV at 10 mA cm −2 in alkaline medium.…”

A Surfactant‐Free and Scalable General Strategy for Synthesizing Ultrathin Two‐Dimensional Metal–Organic Framework Nanosheets for the Oxygen Evolution Reaction

“…To summarize,as eries of ultrathin MOF-74 nanosheets with abundant coordinatively unsaturated metal sites are synthesized through coordination assisted 2dOSA from their corresponding M-ONS,w hich cannot be obtained from the [39] a-CoSe/ Ti, [40] Co 9 S 8 /NOSC, [41] CoFePO, [42] CoP/NC, [43] CoNiP, [44] Ni-NiP, [44] Ni@NC-800/NF, [45] NiFeO x /CNF, [46] NiMo, [47] NiS/NF, [48] NiSe/NF, [49] N-Ni 3 S 2 -NF, [50] and Ni 5 P 4 . [51] delamination of their bulk counterparts or the conventional solvothermal method.…”

The Design of a New Cobalt Sulfide Nanoparticle Implanted Porous Organic Polymer Nanohybrid as a Smart and Durable Water‐Splitting Photoelectrocatalyst

“…It can be evidently observedt hat most of the CoS x -NPs are tightly wrapped by the N-doped porouso rganic polymer framework shell, which facilitates in preventing the CoS x -NPs from undergoing furthero xidation in the presence of oxygen and moisture (Figure 5b). [36] This intimatec ontact between the Co 9 S 8 nanoparticles andt he POP framework is more helpfuli ni nhibiting the catalytic active NPs from undergoing surfaceo xidation in the presence of either oxygen or moisture. [36] This intimatec ontact between the Co 9 S 8 nanoparticles andt he POP framework is more helpfuli ni nhibiting the catalytic active NPs from undergoing surfaceo xidation in the presence of either oxygen or moisture.…”