Rapid Surface Reconstruction of Amorphous Co(OH)2 /WOx with Rich Oxygen Vacancies to Promote Oxygen Evolution

Strong Bonding of Lattice N Activates Metal Ni to Achieve Efficient Water Splitting

Developing efficient and robust free-standing electrocatalysts for overall water splitting is a promising but challenging task. Herein, the N-incorporated Ni nanosheets non-fully encapsulated by N-doped carbon (NC) layer are fabricated (N─Ni©NC). The introduction of N not only regulates the size of nanosheets in N─Ni©NC but also promotes the electrochemical activity of metal Ni. Experimental and theoretical results reveal that strong bonding of the lattice N activates the inert metal Ni by promoting charge transfer between Ni and N. In addition, the upward shift of the d-band center induced by lattice N enhances the adsorption of intermediates, thereby making Ni as a new OER active site together with C. This strategy of generating Ni and C dual active sites by introducing lattice N greatly accelerates oxygen evolution reaction (OER) kinetics, resulting in excellent electrocatalytic performance of N─Ni©NC. At the current density of 10 mA cm-2, the overpotentials of hydrogen evolution reaction (HER) and OER are 27 and 206 mV, respectively, and the cell voltage for overall water splitting only needs 1.47 V. This work offers a unique heteroatom activation approach for designing free-standing electrodes with high activity.

Highly Efficient NiS/Ni(OH)x Heterogeneous Structure Electrocatalyst with Regenerative Oxygen Vacancies for Oxygen Evolution Reaction

Developing low-cost and highly efficient electrocatalysts toward oxygen evolution reaction (OER) is of vital significance for electrochemical water splitting. Herein, we fabricate a heterostructure NiS/Ni(OH)x electrocatalyst (Ni-S-n) with regenerative oxygen vacancies via electro-deposition on nickel foam (NF) followed by a facile NaBH4 reduction. The resulting Ni-S-5 catalyst with appropriate amount of oxygen vacancies (Ovs) exhibits extraordinary activity for alkaline OER with overpotential of 142 mV and 248 mV to reach the current density of 10 mA cm-2 and 100 mA cm-2, respectively. This catalyst also shows remarkable durability with 40 h. After the stability test, the excellent OER performance is well recovered by regenerating the surface oxygen vacancies (Ovs) significantly with additional NaBH4 reduction. The Ni-S-5 catalyst still displays good activity even after repeating it three times (180 h). The surface oxygen vacancies act as vital active sites for OER. A mechanism of Ovs species transformation and regeneration based on the Ni-S-5 catalyst is proposed, which provides a new direction for exploring ultrastable and efficient OER electrocatalysts with renewable active species.

Engineering Water-Lotus-like Iridium-Cobalt Carbonate Hydroxides on Plasma-Treated Carbon Fibers for Enhanced Electrocatalytic Oxygen Evolution

The sluggish kinetics of the oxygen evolution reaction (OER) in alkaline water electrolysis remains a significant challenge for developing high-efficiency electrocatalytic systems. In this study, we present a three-dimensional, micrometer-sized iridium oxide (IrO2)-decorated cobalt carbonate hydroxide (IrO2-P-CoCH) electrocatalyst, which is engineered in situ on a carbon cloth (CC) substrate pretreated with atmospheric-pressure dielectric barrier discharge (DBD) plasma (PCC). The electrocatalyst features petal-like structures composed of nanosized rods, providing abundant reactive areas and sites, including the oxygen vacancy caused by the air-DBD plasma. As a result, the IrO2-P-CoCH/PCC electrocatalyst demonstrates an outstanding OER performance, with overpotentials of only 190 and 300 mV required to achieve current densities of 10 mA cm-2 (j10) and 300 mA cm-2 (j300), respectively, along with a low Tafel slope of 48.1 mV dec-1 in 1.0 M KOH. Remarkably, benefiting from rich active sites exposed on the IrO2-P-CoCH (Ir) heterostructure, the synergistic effect between IrO2 and CoCH enhances the charge delivery rates, and the IrO2-P-CoCH/PCC exhibits a superior electrocatalytic activity at a high current density (300 mV/j300) compared to the commercial benchmarked RuO2/PCC (470 mV/j300). Furthermore, the IrO2-P-CoCH/PCC electrocatalyst shows exceptional OER stability, with a mere 1.3% decrease with a current density of j10 for 100 h testing, surpassing most OER catalysts based on CC substrates. This work introduces a novel approach for designing high-performance OER electrocatalysts on flexible electrode substrates.

Stability of Multivalent Ruthenium on CoWO4 Nanosheets for Improved Electrochemical Water Splitting with Alkaline Electrolyte

Engineering low-cost electrocatalysts with desired features is vital to decrease the energy consumption but challenging for superior water splitting. Herein, we development a facile strategy by the addition of multivalence ruthenium (Ru) into the CoWO4/CC system. During the synthesis process, the most of Ru3+ ions were insinuated into the lattice of CoWO4, while the residual Ru3+ ions were reduced to metallic Ru and further attached to the interface between carbon cloth and CoWO4 sheets. The optimal Ru2(M)-CoWO4/CC exhibited superior performance for the HER with an overpotential of 85 mV@10 mA cm-2, which was much better than most of reported electrocatalysts, regarding OER, a low overpotential of 240 mV@10 mA cm-2 was sufficient. In comparison to Ru2(0)-CoWO4/CC with the same Ru mass loading, multivalence Ru2(M)-CoWO4/CC required a lower overpotential for OER and HER, respectively. The Ru2(M)-CoWO4/CC couple showed excellent overall water splitting performance at a cell voltage of 1.48 V@10 mA cm-2 for used as both anodic and cathodic electrocatalysts. Results of the study showed that the electrocatalytic activity of Ru2(M)-CoWO4/CC was attributed to the in-situ transformation of Ru/Co sites, the multivalent Ru ions and the synergistic effect of different metal species stimulated the intrinsic activity of CoWO4/CC.

NiFe codoping-regulated amorphous/crystalline heterostructured Co-based hydroxides/tungstate with rich oxygen vacancies for efficient water oxidation catalysis

Oxygen evolution reaction (OER) is a crucial half-reaction in water splitting, generating hydrogen for sustainable development, but it is often subject to sluggish kinetics. Abundant transition metal-based OER electrocatalysts have been utilized to expedite the process. However, traditional amorphous catalysts suffer from low conductivity, while the activity of crystalline catalysts is also unsatisfactory. Herein, an amorphous/crystalline heterostructured Co-based hydroxide/tungstate was meticulously constructed and further tailored using a NiFe codoping method (NiFeCoW). Following NiFe codoping, the electronic structure had been modulated, subsequently altering the adsorption toward intermediates. From the electrochemical measurements, the NiFeCoW catalyst demonstrated superior electrocatalytic activity for OER in alkaline media, with a minimal overpotential of 297 mV at 10 mA cm-2 and a cell voltage of 1.57 V for water splitting. This study provides valuable guidance for regulating the amorphous/crystalline heterophase in catalysts through bimetallic modulating engineering.

Defect Enriched Tungsten Oxide Phosphate with Strategic Sulfur Doping for Effective Seawater Oxidation

The intrinsic ability of defects within the electrocatalysts can be judiciously utilized in designing robust electrocatalysts for efficient seawater oxidation. Herein, we have fabricated a novel tungsten oxide phosphate (W12PO38.5) with optimized sulfur doping triggering the insertion of a large number of defect sites. This allows for boosted OER performance in alkaline freshwater as well as seawater, avoiding the unwanted chlorine evolution reaction. The optimized electrocatalyst achieved high current densities of 500 mA cm-2 at an overpotential of just 387 mV in fresh water and 100 mA cm-2 at 380 mV in alkaline seawater for OER. Besides the excellent catalytic performances, the developed electrocatalyst appeared to be a durable catalyst as well. An interesting electrocatalytic activation caused by the generous electronic redistribution led the electrocatalyst to achieve great stability over 100 h at a 100 mA cm-2 current density in alkaline real seawater.

Dopant-Induced Surface Self-Etching of Cobalt Carbonate Hydroxide Boosts Efficient Water Splitting

Herein, vanadium-doped cobalt carbonate hydroxide, V-CoCH, was synthesized as efficient catalyst for water splitting. Vanadium species were partially dissolved in the early stages of the oxygen-evolution reaction (OER), inducing self-etching of the catalyst surface, which is helpful for catalyst surface reconstruction and resulted in a higher number of active sites and oxygen vacancies. The synergy between V-doping and oxygen vacancies improved the catalytic activity: V-CoCH showed an exceptional OER catalytic performance with an overpotential of 183 mV at 10 mA cm^-2 . The water-splitting cell consisting of V-CoCH only required 1.52 V to reach 10 mA cm^-2 . Theoretical calculations revealed that vanadium in V-CoCH played an important role in electron regulation of active sites. The oxygen vacancies had an important effect on improvement of the OER performance through not only the exposure of more active sites but also through modulation of the electronic structure. This work provides an effective strategy for constructing high-performance electrocatalysts.

W exsolution promotes the in situ reconstruction of a NiW electrode with rich active sites for the electrocatalytic oxidation of 5-hydroxymethylfurfural (HMF)

W exsolution induces surface defects, promoting the in situ self-reconstruction and formation of high-valance Ni with superior activity towards HMFOR.