Polyoxometalate-Surfactant Hybrids Directed Assembly of Ni3S2 into Hollow Microsphere as Pt-Comparable Electrocatalyst for Hydrogen Evolution Reaction in Alkaline Medium

Host-Guest Structure Enabling Electrocatalytic Hydrogen Evolution Performance by POM@TM-BDC Composites

Developing economical, efficient, and earth-rich electrocatalysts for hydrogen evolution reaction (HER) is quite challenging and ideal. We propose that [P2W18O62]6- as the guest, due to its excellent reversible 18 electron-transfer capacity and redox properties, and then TM-BDC (TM = Ni, Co, Fe, BDC = 1,4-benzene-dicarboxylate) as the host make [P2W18O62]6- packaged and not escape due to its porous structure. Benefiting from strong redox-competent interactions between [P2W18O62]6- and porous structures of TM-BDC and full exposure of abundant active sites, three {P2W18}@TM-BDC composites exhibited excellent HER activity, with {P2W18}@Ni-BDC requiring 198 mV (overpotentials) and 104 mV/dec (Tafel slope) for HER. More importantly, three {P2W18}@TM-BDC composites show excellent stability, with the voltage remaining nearly constant for 24 h. Meanwhile, the linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) curves of three {P2W18}@TM-BDC overlap well with the initial curve after the stability test. Our work offers a promising strategy for synthesizing high-performance electrocatalysts and broadens the scope of nonprecious metal composite material preparation.

Polyoxometalate-Structured Materials: Molecular Fundamentals and Electrocatalytic Roles in Energy Conversion

Polyoxometalates (POMs), a kind of molecular metal oxide cluster with unique physical-chemical properties, have made essential contributions to creating efficient and robust electrocatalysts in renewable energy systems. Due to the fundamental advantages of POMs, such as the diversity of molecular structures and large numbers of redox active sites, numerous efforts have been devoted to extending their application areas. Up to now, various strategies of assembling POM molecules into superstructures, supporting POMs on heterogeneous substrates, and POMs-derived metal compounds have been developed for synthesizing electrocatalysts. From a multidisciplinary perspective, the latest advances in creating POM-structured materials with a unique focus on their molecular fundamentals, electrocatalytic roles, and the recent breakthroughs of POMs and POM-derived electrocatalysts, are systematically summarized. Notably, this paper focuses on exposing the current states, essences, and mechanisms of how POM-structured materials influence their electrocatalytic activities and discloses the critical requirements for future developments. The future challenges, objectives, comparisons, and perspectives for creating POM-structured materials are also systematically discussed. It is anticipated that this review will offer a substantial impact on stimulating interdisciplinary efforts for the prosperities and widespread utilizations of POM-structured materials in electrocatalysis.

One-Step Synthesis of Hollow CoS2 Spheres Derived from Polyoxometalate-Based Metal-Organic Frameworks with Peroxidase-like Activity

In this work, hollow CoS2 particles were prepared by a one-step sulfurization strategy using polyoxometalate-based metal-organic frameworks as the precursor. The morphology and structure of CoS2 have been monitored by scanning electron microscopy, X-ray photoelectron spectroscopy, and X-ray powder diffraction. The mechanism for the formation of CoS2 is discussed. The reaction time and sulfur content are found to be important factors that affect the morphology and pure phase formation of CoS2, and a hollow semioctahedral morphology of CoS2 with open voids was obtained when the sulfur source was twice as large as the precursor and the reaction time was 24 h. The CoS2 (24 h) particles show an excellent peroxidase-like activity for the oxidation of colorless 3,3',5,5'-tetramethylbenzidine (TMB) to blue oxidized (oxTMB) by hydrogen peroxide. The polyoxometalate used as a precursor helps to stabilize oxTMB during catalytic oxidation, forming a stable curve platform for at least 8 min. Additionally, the colorimetric detection of hydroquinone is developed with a low detection limit of 0.42 μM. This research provides a new strategy to design hollow materials with high peroxidase-mimicking activity.

Electronic Structure Engineering of Highly-Scalable Earth-Abundant Multi-Synergized Electrocatalyst for Exceptional Overall Water Splitting in Neutral Medium

Efficient neutral water splitting may represent in future a sustainable solution to unconstrained energy requirements, but yet necessitates the development of innovative avenues for achieving the currently unmet required performances. Herein, a novel paradigm based on the combination of electronic structure engineering and surface morphology tuning of earth-abundant 3D-hierarchical binder-free electrocatalysts is demonstrated, via a scalable single-step thermal transformation of nickel substrates under sulfur environment. A temporal-evolution of the resulting 3D-nanostructured substrates is performed for the intentional enhancement of non-abundant highly-catalytic Ni3+ and pSn 2- species on the catalyst surface, concomitantly accompanied with densification of the hierarchical catalyst morphology. Remarkably, the finely engineered NiSx catalyst synthesized via thermal-evolution for 24 h (NiSx -24 h) exhibits an exceptionally low cell voltage of 1.59 V (lower than Pt/C-IrO2  catalytic couple) for neutral water splitting, which represents the lowest value ever reported. The enhanced performance of NiSx -24 h is a multi-synergized consequence of the simultaneous enrichment of oxygen and hydrogen evolution reaction catalyzing species, accompanied by an optimum electrocatalytic surface area and intrinsic high conductivity. Overall, this innovative work opens a route to engineering the active material's electronic structure/morphology, demonstrating novel Ni3+ /pSn 2- -enriched NiSx catalysts which surpass state-of-the-art materials for neutral water splitting.

Single-Step Solid-State Scalable Transformation of Ni-Based Substrates to High-Oxidation State Nickel Sulfide Nanoplate Arrays as Exceptional Bifunctional Electrocatalyst for Overall Water Splitting

Hydrogen, undoubtedly the next-generation fuel for supplying the world's energy demands, needs economically scalable bifunctional electrocatalysts for its sustainable production. Non-noble transition metal-based electrocatalysts are considered an economic solution for water splitting applications. A single-step solid-state approach for the economically scalable transformation of Ni-based substrates into single-crystalline nickel sulfide nanoplate arrays is developed. X-ray diffraction and transmission electron microscopy measurements reveal the influence of the transformation temperature on the crystal growth direction, which in turn can manipulate the chemical state at the catalyst surface. Ni-based sulfide formed at 450 °C exhibits an enhanced concentration of electrocatalytically-active Ni³⁺ at their surface and a reduced electron density around sulfur atoms, optimal for efficient H₂ production. The Ni-based sulfide electrocatalysts display exceptional electrocatalytic performance for both oxygen and hydrogen evolution, with overpotentials of 170 and 90 mV respectively. Remarkably, the two-electrode cell for overall electrolysis of alkaline water demonstrates an ultra-low cell potential of 1.46 V at 10 mA cm^-2 and 1.69 V at 100 mA cm^-2 . In addition to the exceptionally low water-splitting cell voltage, this self-standing electrocatalyst is of binderfree nature, with the electrode preparation being a low-cost and single-step process, easily scalable to industrial scales.

Surface Atom Regulation on Polyoxometalate Electrocatalyst for Simultaneous Low-Voltage H2 Production and Phenol Degradation

The electrocatalytic hydrogen evolution reaction is an ideal method for H₂ production. To improve the performance of polyoxometalate-based electrocatalyst in the hydrogen evolution reaction, one O^2- in polyoxometalate is replaced by S^2-. This weakens the binding of polyoxometalate to H*, facilitates its desorption, and improves the H₂ generation property. Vulcanized polyoxometalate only requires 55 mV to achieve 10 mA·cm^-2 current in the hydrogen evolution reaction. This electrocatalyst also exhibits promising performance in phenol degradation reaction, which is an ideal substitute for high-energy-consuming oxygen evolution reaction in H₂ production due to low voltage to drive. To acquire 100 and 200 mA·cm^-2 in the phenol degradation reaction, this vulcanized polyoxometalate only consumes 1.38 and 1.41 V. With this electrocatalyst working as a cathode and an anode simultaneously, an electrolyzer is constructed by employing phenol-containing KOH as an electrolyte. To obtain 100 and 200 mA·cm^-2 current, the electrolyzer only requires 1.54 and 1.57 V. Because energy-efficient phenol degradation reaction occurs, these values are obviously lower than the oxygen evolution reaction involved in the overall water-splitting H₂ production. This work provides a universal method to enhance the hydrogen evolution reaction (HER) activity of polyoxometalates. Furthermore, a new method is explored, which achieves energy conservation and phenol degradation simultaneously in H₂ production.

Surface selectivity of Ni3S2 toward hydrogen evolution reaction: a first-principles study

Exploring materials with high catalytic performance toward hydrogen evolution reaction (HER) is of importance for the development of clean hydrogen energy, and their surface structure is essential for this function. In this study, using density functional theory (DFT), we reported a comprehensive study on the phase stability, surface structures, electronic properties and HER catalytic properties of the low-index surfaces of Ni3S2, including the (0001), (101[combining macron]0), (101[combining macron]1), (112[combining macron]0) and (112[combining macron]1) planes with different terminations. Our calculated results demonstrate that S-rich surfaces and several stoichiometric surfaces of Ni3S2 are thermodynamically stable, including (0001)A, (101[combining macron]0)A, (112[combining macron]0)C, (101[combining macron]0)C, (101[combining macron]0)B and (112[combining macron]1)A surfaces. Among the six stable surface structures, the (0001)A, (101[combining macron]0)B and (101[combining macron]0)C surfaces of Ni3S2 are indispensable for high HER performance because of their high catalytic activity, suitable potential and high thermodynamic stability. The calculated changes of Gibbs free energy (ΔGH*) of the Top S2 site on (0001)A, Hollow Ni2S3S4 site on (101[combining macron]0)C, and Bridge Ni1Ni3 site and Hollow Ni2S1S2 site on (101[combining macron]0)B are -0.143, 0.122, 0.012, and -0.112 eV, respectively, comparable with or even better than those of Pt(111) (-0.07 eV). In addition, the possible Volmer-Heyrovsky and Volmer-Tafel processes on the considered surfaces are also investigated. When the overpotential is in the range of 0 to 300 mV, the density of active sites on the (101[combining macron]0)B surface of Ni3S2 is found to be the highest. This work provides significant insights on the surface selectivity of Ni3S2 toward HER and provides a route to optimize the performance of Ni3S2 with exposed surfaces.