Operando Identification of Active Species and Intermediates on Sulfide Interfaced by Fe₃O₄ for Ultrastable Alkaline Oxygen Evolution at Large Current Density

Abstract

Transition-metal sulfides are investigated as promising electrocatalysts for oxygen evolution reaction (OER) in alkaline media; however, the real active species remain elusive and the development of oxyhydroxides reconstructed from sulfides delivering stable large current density at low applied potentials is a great challenge. Here, we report a synergistic hybrid catalyst, composed of nanoscale heterostructures of Co9S8 and Fe3O4, that exhibits only a low potential of 350 mV and record stability of 120 h at the 500 mA cm–2 in 1.0 M KOH. Voltage-dependent soft X-ray absorption spectroscopy (XAS) and Operando Raman spectroscopy demonstrate that the initial Co9S8@Fe3O4 is reconstructed into CoOOH/CoO x @Fe3O4 and further to complete CoOOH@Fe3O4. Operando XAS and electron microscopy imaging analyses reveal that the completely reconstructed CoOOH acts as active species and Fe3O4 components prevent the aggregation of CoOOH. Operando infrared spectroscopy indicates cobalt superoxide species (*OOH) as the active intermediates during the OER process. Density functional theory calculations demonstrate the formation of *OOH as the rate-determining step of OER and CoOOH@Fe3O4 exhibits a lower energy barrier for OER. Our results provide an in-depth understanding of the dynamic surface structure evolutions of sulfide electrocatalysts for alkaline OER and insights into the design of excellent nanocatalysts for stable large current density.

Improving the Electrocatalytic Activity of a Nickel‐Organic Framework toward the Oxygen Evolution Reaction through Vanadium Doping

“…Meanwhile, the Tafel slope of V 0.09 À Ni 0.91 MOF/NF is the smallest among the three catalysts with different Ni/V ratios (Figure S5b), confirming V 0.09 À Ni 0.91 MOF/NF has the superior electrocatalytic activity and most effective catalytic kinetics for OER. [24] In addition, the η 10 value and Tafel slope of V 0.09 À Ni 0.91 MOF/NF are also much lower than other phosphoruscontaining OER catalysts recently reported in alkaline media (Figure 5c and Table S1). The electrochemical active surface areas of a series of catalysts were calculated according to double layer capacitance (C dl ) in the non-Faradaic region to evaluate their catalytic activity (Figure S6).…”

“…In Figure 5b, the Tafel slope of V 0.09 −Ni 0.91 MOF (107 mV dec −1 ) is the smallest in compared with NiV‐OH/NF (133 mV dec −1 ), NiV‐MOF/NF (154 mV dec −1 ), Ni‐MOF/NF (115 mV dec −1 ), RuO 2 /NF (78 mV dec −1 ), and bare NF (145 mV dec −1 ). Meanwhile, the Tafel slope of V 0.09 −Ni 0.91 MOF/NF is the smallest among the three catalysts with different Ni/V ratios (Figure S5b), confirming V 0.09 −Ni 0.91 MOF/NF has the superior electrocatalytic activity and most effective catalytic kinetics for OER [24] . In addition, the η 10 value and Tafel slope of V 0.09 −Ni 0.91 MOF/NF are also much lower than other phosphorus‐containing OER catalysts recently reported in alkaline media (Figure 5c and Table S1).…”

Reactive template-engaged synthesis of Ni-doped Co3S4 hollow and porous nanospheres with optimal electronic modulation toward high-efficiency electrochemical oxygen evolution

“…5c, the Tafel slopes of Ni-Co 3 S 4 -1, Ni-Co 3 S 4 -2, Ni-Co 3 S 4 -3, Co 3 S 4 and commercial RuO 2 are determined to be 96.2, 90.5, 95.1, 98.2 and 99.8 mV dec −1 , respectively, reflecting a faster reaction kinetics proceeding on the Ni-Co 3 S 4 -2 electrode. 50 Given the small overpotential and low Tafel slope value, the OER performance of the synthesized Ni-Co 3 S 4 -2 hollow and porous nanospheres in an alkaline medium surpasses those of most of the previously reported TMS-based electrocatalysts, as briefly summarized in Table S1 †…”

“…5c, the Tafel slopes of Ni-Co 3 S 4 -1, Ni-Co 3 S 4 -2, Ni-Co 3 S 4 -3, Co 3 S 4 and commercial RuO 2 are determined to be 96.2, 90.5, 95.1, 98.2 and 99.8 mV dec −1 , respectively, reflecting a faster reaction kinetics proceeding on the Ni-Co 3 S 4 -2 electrode. 50 Given the small overpotential and low Tafel slope value, the OER performance of the synthesized Ni-Co 3 S 4 -2 hollow and porous nanospheres in an alkaline medium surpasses those of most of the previously reported TMS-based electrocatalysts, as briefly summarized in Table S1. † Since the electrochemically active surface area (ECSA) of a given electrocatalyst is linearly proportional to the electrochemical double-layer capacitance (C dl ), the C dl values of all the samples were estimated by measuring non-faradaic capacitive current from the cyclic voltammetry (CV) curves (Fig.…”

Different nanostructured CoP microcubes derived from metal formate frameworks with enhanced oxygen evolution reaction performance

“…Based on these advantages, MOFs are considered to be an ideal precursor for the preparation of micro-nano materials with high electrochemical performance. 19,20 Due to their high conductivity, good stability and strong catalytic activity, cobalt phosphides are widely used in energy conversion/ storage systems. 21,22 Lin et al prepared a mesoporous CoP catalyst by hard template synthesis, which exhibited excellent OER activity and required only an overpotential of 300 mV at a current density of 10 mA cm −2 .…”