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[Display omitted] Heterojunction can effectively improve oxygen evolution reaction (OER) activity by regulating the interfacial electronic structure of catalysts. However, p-p type heterojunction OER catalysts have obtained less attention, and the corresponding catalytic mechanisms are unclear either. Herein, the self-supported CoP-CoO p-p type heterojunction arrays are fabricated on carbon cloth substrate (CoP-CoO/CC). Band structure analysis shows that the formation of p-p heterojunction can drive the electrons from CoO to flow into CoP. This electronic modulation contributes to positively charged regions on the CoO and enhances the OH− adsorption during OER, proven by X-ray photoelectron spectroscopy and methanol molecular detection, respectively. As a result, the CoP-CoO/CC electrode only needs 210 mV overpotential to drive a current density of 10 mA cm−2 in an alkaline medium, superior to the most reported OER catalysts. Additionally, the CoP-CoO/CC also exhibits an ideal hydrogen evolution reaction response, and a water splitting system has been successfully constructed which can drive a 10 mA cm−2 within 1.65 V. This study supplies insight for catalytic origins p-p type heterojunctions OER catalyst, which provides a reference value for the efficient and reasonable design of heterojunction catalysts. [ABSTRACT FROM AUTHOR]

Developing efficient and durable oxygen evolution reaction (OER) electrocatalysts with non-noble elements is particularly urgent for electrochemical water splitting. Herein, we report heterostructural Ni3S2–Fe5Ni4S8 hybrids directly grown on redox-NiFe foams (denoted as NFS/rNF) as excellent pre-catalysts for OER in 1 M KOH media. The electrochemical performance can be tuned by amount of sulfur source, and the optimized electrode only needs low overpotentials of 199 mV and 240 mV to attain 10 mA cm−2 and 100 mA cm−2, respectively. Moreover, the overpotential is further reduced to 231 mV to deliver 100 mA cm−2 after multi-cyclic voltammogram scans, due to the exposure of abundant in situ generated Ni(Fe)OOH species. Excellent stability with continuous 40 h operation is also demonstrated. Our study provides a new strategy for direct synthesis of self-standing integrated hybrid electrodes for efficient and durable water oxidation. [ABSTRACT FROM AUTHOR]

Interfacial engineering is an effective strategy for the design of active oxygen evolution electrode. Yet the intrinsic mechanism of heterogeneous interfaces remains largely unclear for the lack of direct evidence of real active phases. Herein, operando ultraviolet-visible (UV–vis) and Raman spectroscopies were used to probe the surface reconstruction behaviors of FeOOH@NiFe layered double hydroxides (LDH) hybrid catalysts, which revealed that FeOOH@NiFe LDH was converted into highly active FeOOH@ β-Ni(Fe)OOH phases during the OER process. The FeOOH-NiFe LDH interface can promote interfacial charge transfer, delay the Ni oxidation and induce the NiFe LDH phases to highly reactive β-Ni(Fe)OOH. The FeOOH@ β-Ni(Fe)OOH exhibited excellent OER performance (252 mV at 100 mA cm−2) and a 1.6 times increase (at overpotential of 300 mV) in turnover frequency (TOF) with respect to the active phase γ-Ni(Fe)OOH of NiFe LDH. This work clarifies that ration interface engineering can regulate the generation of highly active phases in the OER process. [Display omitted] • Reconstruction of FeOOH@NiFe LDH was study by operando UV–vis and Raman spectroscopies. • FeOOH induced the phase transformation of NiFe LDH to β-Ni(Fe)OOH at the interface. • The interfacial electric field of NiFe LDH and FeOOH promoted the charge transfer. • As-formed FeOOH@ β-Ni(Fe)OOH showed superior OER activity to γ-Ni(Fe)OOH. [ABSTRACT FROM AUTHOR]

Oxygen evolution reaction (OER) is a key step for electrochemical water splitting and understanding the surface reconstruction of OER pre-catalysts is of vital importance. Herein, hybrid Ni 5 P 4 @FeP nanosheet arrays were evaluated as promising OER pre-catalysts. The dynamic surface evolution was probed by in situ Raman spectroscopy, which revealed that Ni 5 P 4 @FeP was rapidly reconstructed to NiFe 2 O 4 during the anodic scan. The structural instability of amorphous NiFe 2 O 4 led to partial reconstitution to Ni/FeOOH at high oxidation potentials. As-formed Ni/FeOOH@NiFe 2 O 4 hybrid with high structural reversibility was established as a truly active species, which exhibited excellent alkaline OER performance with a low overpotential of 205 and 242 mV under current densities of 10 and 100 mA cm−2, respectively. This work provides a facile strategy to in situ construct an amorphous spinel/oxyhydroxide hybrid structure using electrochemical activation that holds strong promise for potential application in electrochemical water splitting and related energy devices. [Display omitted] • Surface reconstruction of Ni 5 P 4 @FeP was probed by in situ Raman spectroscopy. • Ni 5 P 4 @FeP was oxidized to NiFe 2 O 4 and further to Ni/FeOOH at high potentials. • The real active intermediate was identified as Ni/FeOOH and NiFe 2 O 4 hybrid. • High structural reversibility between NiFe 2 O 4 and Ni/FeOOH was found. • As-formed amorphous NiOOH and NiFe 2 O 4 hybrid showed superior OER activity. [ABSTRACT FROM AUTHOR]

[Display omitted] • Fe-Ni-S catalysts undergo surface self-reconstruction during alkaline electrocatalysis. • γ-NiOOH and metallic Ni act as reactive species for OER and HER, respectively. • Fe-Ni-S pre-catalysts show low overpotentials in OER and HER bifunctional activity. • An electrolyzer exhibits a cell potential of 1.55 V at 10 mA cm−2. Earth-abundant catalysts for both Oxygen evolution reaction (OER) and Hydrogen evolution reaction (HER) are highly required for electrochemical water splitting. Transition metal sulfides have been considered as efficient catalysts for both OER and HER, but their real active species in operation condition are still controversial. Herein, self-supported Ni 3 S 2 /FeNi 2 S 4 nanosheets (Ni-Fe-S) are synthesized and in-situ Raman measurement is conducted to monitor the phase transformation under realistic conditions. Combined with a series of ex-situ technologies, it can be concluded that, Ni-Fe-S nanosheets function as pre-catalysts which undergo dynamically structural reconstruction during electrolysis. The active species are determined to be γ-NiOOH for OER and metallic Ni0 for HER in 1 M KOH media, respectively. As-converted products only need overpotentials of 50 mV for HER and 201 mV for OER to achieve the current density of 10 mA cm-2, respectively. A two-electrode electrolyzer can afford a current density of 10 mA cm-2 at a low cell voltage of 1.55 V. This work provides new perspectives to recognize the catalytic evolution process of nickel-based sulfides for water splitting and sheds light on the rational design of bifunctional materials in energy devices. [ABSTRACT FROM AUTHOR]

Developing efficient and nonprecious electrocatalysts for oxygen evolution reaction (OER) is crucial for the hydrogen generation via water splitting. Here we report the direct growth of heterostructural Ni 2 P–Fe 2 P hybrid electrocatalysts on NiFe foams. The redox engineered NiFe foam (rNF) significantly enhances the OER performance. Among prepared catalysts, Ni 2 P–Fe 2 P/rNF performs the highest OER activity, driving the current densities of 10 and 100 mA cm−2 at low overpotentials of 218 and 270 mV in 1 M KOH, respectively. Meanwhile, it can maintain 100 mA cm−2 for 24 h without obvious degradation. The morphology and composition of NFP/rNF after OER test were carefully studied. This work suggests that self-supported bimetal phosphides of NFP/rNF is an efficient and durable electrode material for water oxidation. Heterostructural Ni2P-Fe2P hybrid electrocatalysts were direct grown on NiFe foams, which show excellent oxygen evolution activity and stability. Image 1 • The formation of micro-sized pores on the skeleton of NiFe foam (NF) substrate can enlarge surface area. • The heterogeneous Ni2P-Fe2P structure provides plentiful active sites for OER. • The Ni2P-Fe2P/ rNF foam exhibits an extraordinary electro catalytic performance. [ABSTRACT FROM AUTHOR]

Developing high-efficient and Earth-abundant electrocatalysts toward hydrogen evolution reaction (HER) is strongly desired for electrochemical water splitting but remains a considerable challenge. Herein, we propose a 3D hierarchical Fe(PO 3) 2 @Cu 3 P heterostructure nanotube arrays on Cu foams for promising HER catalysts. The HER performance can be tuned by changing FeCl 3 concentrations and the optimized cathode exhibits excellent HER activity in 1 M KOH with a low overpotentials of 108 mV @ 10 mA cm−2 and a small Tafel slope of 84 mV dec−1. Furthermore, this electrode delivers high electrochemical durability with stable operation for 24 h. Such high performance can be ascribed to the unique hierarchical heterostructure with large electrochemical surface area, synergistic effect and fast electron transport. Our study can provide new inspiration for rational construction of copper-based hierarchical nanomaterials for HER electrodes and other energy-related applications. Hierarchical Fe(PO 3) 2 @Cu 3 P nanotube hybrids on 3D copper foam were synthesized, which demonstrate excellent activity toward hydrogen evolution in alkaline solution with robust stability. Image 1 • Hierarchical Fe(PO 3) 2 @Cu 3 P nanotube arrays were successfully synthesized on Cu foam. • The hybrid catalysts exhibit excellent activities for HER in alkaline media. • Synergistic effect between Fe(PO 3) 2 and Cu 3 P was found by XPS. • The hybrid catalyst exhibits considerable stability for HER. [ABSTRACT FROM AUTHOR]