Oxyanion Engineering Suppressed Iron Segregation in Nickel-Iron Catalysts Toward Stable Water Oxidation

WO3-x as an activation medium to prompt overall water splitting of NiFe-based electrocatalyst

The efficient oxygen evolution reaction (OER) is crucial for various electrochemical processes, especially for overall water splitting (OWS). In this study, we focus on the utilization of WO3-x as an activation medium to enhance the OER performance of NiFe-based electrocatalysts. Firstly, we synthesize WO3-x nanowires supported on nickel foam (NF) and then incorporate NiFe on WO3-x nanowires by a simple hydrothermal method. The WO3-x self-supported NiFe (Oxy)hydroxide (denoted as NiFe-W-O/NF) shows a three-dimensional stereostructure composed of ultrathin nanosheets (∼ 4.0 nm). This unique structure provides a large open surface for fuller diffusion of the electrolyte while exposing more active sites. The electronic interaction of tri-centers of NiFeW accelerates the surface reconstruction process of γ-NiOOH and FeOOH, which are converted into the main active species in a short time. The electrochemical measurements confirm that the NiFe-W-O/NF has low OER overpotentials (233 mV at 10 mA cm-2, 298 mV at 100 mA cm-2) and excellent stability (100 h in total) in 1 M KOH electrolyte. In addition, the NiFe-W-O/NF || NiFe-W-O/NF battery also exhibits a low cell voltage (1.52 V at 10 mA cm-2) with a stable lifetime (50 h) under alkaline conditions. These results highlight the great potential of NiFe-W-O/NF for practical applications.

Confined growth of Ultrathin, nanometer-sized FeOOH/CoP heterojunction nanosheet arrays as efficient self-supported electrode for oxygen evolution reaction

Self-supported electrodes, featuring abundant active species and rapid mass transfer, are promising for practical applications in water electrolysis. However, constructing efficient self-supported electrodes with a strong affinity between the catalytic components and the substrate is of great challenge. In this study, by combining the ideas of in-situ construction and space-confined growth, we designed a novel self-supported FeOOH/cobalt phosphide (CoP) heterojunctions grown on a carefully modified commercial Ni foam (NF) with three-dimensional (3D) hierarchically porous Ni skeleton (FeOOH/CoP/3D NF). The specific porous structure of 3D NF directs the confined growth of FeOOH/CoP catalyst into ultra-thin and small-sized nanosheet arrays with abundant edge active sites. The active FeOOH/CoP component is stably anchored on the rough pore wall of 3D NF support, leading to superior stability and improved conductivity. These structural advantages contributed to a highly facilitated oxygen evolution reaction (OER) activity and enhanced durability of the FeOOH/CoP/3D NF electrode. Herein, the FeOOH/CoP/3D NF electrode afforded a low overpotential of 234 mV at 10 mA cm-2 (41 mV smaller than FeOOH/CoP grown on unmodified Ni foam) and high stability for over 90 h, which is among the top reported OER catalysts. Our study provides an effective idea and technique for the construction of active and robust self-supported electrodes for water electrolysis.

Enhanced stability of boron modified NiFe hydroxide for oxygen evolution reaction

The introduction of non-metal elements including boron has been identified as a significant means to enhance oxygen evolution reaction (OER) performance in NiFe-based catalysts. To understand the catalytic activity and stability, recent attention has widened toward the Fe species as a potential contributor, prompting exploration from various perspectives. Here, boron incorporation in NiFe hydroxide achieves significantly enhanced activity and stability compared to the boron-free NiFe hydroxide. The boron inclusion in NiFe hydroxide is found to show exceptionally improved stability from 12 to 100 hours at a high current density (200 mA cm-2). It facilitates the production and redeposition of OER-active, high-valent Fe species in NiFe hydroxide based on the operando Raman, UV-vis, and X-ray absorption spectroscopy analysis. It is proposed that preserving a homogenous distribution of Fe across the boron-containing catalyst surface enhances OER stability, unlike the bare NiFe hydroxide electrocatalyst, which exhibits uneven Fe dissolution, confirmed through elementary mapping analysis. These findings shed light on the potential of anionic regulation to augment the activity of iron, an aspect not previously explored in depth, and thus are expected to aid in designing practical OER electrocatalysts.

In Situ, Rapid Synthesis of Carbon-Loaded High Density and Ultrasmall High Entropy Oxide Nanoparticles as Efficient Electrocatalysts

High entropy materials offer almost unlimited catalytic possibilities due to their variable composition, unique structure, and excellent electrocatalytic performance. However, due to the strong tendency of nanoparticles to coarsen and agglomerate, it is still a challenge to synthesize nanoparticles using simple methods to precisely control the morphology and size of the nanoparticles in large quantities, and their large-scale application is limited by high costs and low yields. Herein, a series of high-entropy oxides (HEOs) nanoparticles with high-density and ultrasmall size (<5 nm) loaded on carbon nanosheets with large quantities are prepared by Joule-heating treatment of gel precursors in a short period of time (≈60 s). Among them, the prepared (FeCoNiRuMn)3O4-x catalyst shows the best electrocatalytic activity for oxygen evolution reaction, with low overpotentials (230 mV @10 mA cm-2, 270 mV @100 mA cm-2), small Tafel slope (39.4 mV dec-1), and excellent stability without significant decay at 100 mA cm-2 after 100 h. The excellent performance of (FeCoNiRuMn)3O4-x can be attributed to the synergistic effect of multiple elements and the inherent structural stability of high entropy systems. This study provides a more comprehensive design idea for the preparation of efficient and stable high entropy catalysts.

Interface Enables Faster Surface Reconstruction in a Heterostructured CuSey/NiSex Electrocatalyst for Realizing Urea Oxidation

Creating affordable electrocatalysts and understanding the real-time catalytic process of the urea oxidation reaction (UOR) are crucial for advancing urea-based technologies. Herein, a Cu-Ni based selenide electrocatalyst (CuSey/NiSex/NF) was created using a hydrothermal technique and selenization treatment, featuring a heterogeneous interface rich in Cu2-xSe, Cu3Se2, Ni3Se4, and NiSe2. This catalyst demonstrated outstanding urea electrooxidation performance, achieving 10 mA cm-2 with just 1.31 V and sustaining stability for 96 h. Through in-situ Raman spectroscopy and ex-situ characterizations, it is discovered that NiOOH is formed through surface reconstruction in the UOR process, with high-valence Ni serving as the key site for effective urea oxidation. Moreover, the electrochemical analysis revealed that CuSey had dual effects. An analysis of XPS and electrochemical tests revealed that electron transfer from CuSey to NiSex within the CuSey/NiSex/NF heterostructure enhanced the UOR kinetics of the catalyst. Additionally, according to the in-situ Raman spectroscopy findings, the existence of CuSey facilitates a easier and faster surface reconstruction of NiSex, leading to the creation of additional active sites for urea oxidation. More significantly, this work provides an excellent "precatalyst" for highly efficient UOR, along with an in-depth understanding of the mechanism behind the structural changes in electrocatalysts and the discovery of their true active sites.

Amorphous/Crystalline Phases Mixed Nanosheets Array Rich in Oxygen Vacancies Boost Oxygen Evolution Reaction of Spinel Oxides in Alkaline Media

As promising oxygen evolution reaction (OER) catalysts, spinel-type oxides face the bottleneck of weak adsorption for oxygen-containing intermediates, so it is challenging to make a further breakthrough in remarkably lowering the OER overpotential. In this study, a novel strategy is proposed to substantially enhance the OER activity of spinel oxides based on amorphous/crystalline phases mixed spinel FeNi2O4 nanosheets array, enriched with oxygen vacancies, in situ grown on a nickel foam (NF). This unique architecture is achieved through a one-step millisecond laser direct writing method. The presence of amorphous phases with abundant oxygen vacancies significantly enhances the adsorption of oxygen-containing intermediates and changes the rate-determining step from OH*→O* to O*→OOH*, which greatly reduces the thermodynamic energy barrier. Moreover, the crystalline phase interweaving with amorphous domains serves as a conductive shortcut to facilitate rapid electron transfer from active sites in the amorphous domain to NF, guaranteeing fast OER kinetics. Such an anodic electrode exhibits a nearly ten fold enhancement in OER intrinsic activity compared to the pristine counterpart. Remarkably, it demonstrates record-low overpotentials of 246 and 315 mV at 50 and 500 mA cm-2 in 1 m KOH with superior long-term stability, outperforming other NiFe-based spinel oxides catalysts.

MXene-mediated reconfiguration induces robust nickel-iron catalysts for industrial-grade water oxidation

Nickel-iron oxy/hydroxides (NiFeOxHy) emerge as an attractive type of electrocatalysts for alkaline water oxidation reaction (WOR), but which encounter a huge challenge in stability, especially at industrial-grade large current density due to uncontrollable Fe leakage. Here, we tailor the Fe coordination by a MXene-mediated reconfiguration strategy for the resultant NiFeOxHy catalyst to alleviate Fe leakage and thus reinforce the WOR stability. The introduction of ultrafine MXene with surface dangling bonds in the electrochemical reconfiguration over Ni-Fe Prussian blue analogue induces the covalent hybridization of NiFeOxHy/MXene, which not only accelerates WOR kinetics but also improves Fe oxidation resistance against segregation. As a result, the NiFeOxHy coupled with MXene exhibits an extraordinary durability at ampere-level current density over 1,000 h for alkaline WOR with an ultralow overpotential of only 307 mV. This work provides a broad avenue and mechanistic insights for the development of nickel-iron catalysts toward industrial applications.

Interface Charge Distribution Engineering of Pd-CeO2 /C for Efficient Carbohydrazide Oxidation Reaction

Carbohydrazide electrooxidation reaction (COR) is a potential alternative to oxygen evolution reaction in water splitting process. However, the sluggish kinetics process impels to develop efficient catalysts with the aim of the widespread use of such catalytic system. Since COR concerns the adsorption/desorption of reactive species on catalysts, the electronic structure of electrocatalyst can affect the catalytic activity. Interface charge distribution engineering can be considered to be an efficient strategy for improving catalytic performance, which facilitates the cleavage of chemical bond. Herein, highly dispersed Pd nanoparticles on CeO2 /C catalyst are prepared and the COR catalytic performance is investigated. The self-driven charge transfer between Pd and CeO2 can form the local nucleophilic and electrophilic region, promoting to the adsorption of electron-withdrawing and electron-donating group in carbohydrazide molecule, which facilitates the cleavage of C-N bond and the carbohydrazide oxidation. Due to the local charge distribution, the Pd-CeO2 /C exhibits superior COR catalytic activity with a potential of 0.27 V to attain 10 mA cm-2 . When this catalyst is used for energy-efficient electrolytic hydrogen production, the carbohydrazide electrolysis configuration exhibits a low cell voltage (0.6 V at 10 mA cm-2 ). This interface charge distribution engineering can provide a novel strategy for improving COR catalytic activity.

Deep Reconstruction of the NiFeSP Nanosheet Catalyst for Large Current-Density Water Oxidation

Evidence shows that the crucial factor in achieving efficient water electrolysis for hydrogen production is the design and synthesis of electrocatalysts that exhibit both high performance and cost-effectiveness for the oxygen evolution reaction (OER). Herein, the NiFeSP nanosheets were facilely prepared on a Ni foam (NF) substrate through a cost-effective electrodeposition method. The electrode structure composed of nanosheets offers a high density of active sites and superior electrical conductivity, thereby enhancing the efficiency of the OER. In addition, the NiFeSP/NF-600 nanosheets exhibit superhydrophilic and superaerophobic characteristics, which effectively enhance the mass-transfer process by facilitating the penetration of electrolytes and enabling rapid release of gas bubbles. Consequently, NiFeSP/NF-600 demonstrates superior electrocatalytic efficacy for OER, exhibiting an overpotential of 292 mV at a high current density of 500 mA cm-2 as well as an exceptional long-term durability of 100 h. More importantly, the rapid reconstruction of FeOOH and NiOOH species from NiFeSP/NF-600 may be the true active species for OER, which is revealed utilizing in situ Raman spectroscopy in conjunction with ex situ characterization. This study not only offers an ideal "pre-catalyst" for an extremely effective OER but also offers a thorough understanding of the mechanism underlying the structural evolution of electrocatalysts and the identification of their actual active sites.

Progress in Manipulating Dynamic Surface Reconstruction via Anion Modulation for Electrocatalytic Water Oxidation

The development of efficient and economical electrocatalysts for oxygen evolution reaction (OER) is of paramount importance for the sustainable production of renewable fuels and energy storage systems; however, the sluggish OER kinetics involving multistep four proton-coupled electron transfer hampers progress in these systems. Fortunately, surface reconstruction offers promising potential to improve OER catalyst design. Anion modulation plays a crucial role in controlling the extent of surface reconstruction and positively persuading the reconstructed species' performances. This review starts by providing a general explanation of how various types of anions can trigger dynamic surface reconstruction and create different combinations with pre-catalysts. Next, the influences of anion modulation on manipulating the surface dynamic reconstruction process are discussed based on the in situ advanced characterization techniques. Furthermore, various effects of survived anionic groups in reconstructed species on water oxidation activity are further discussed. Finally, the challenges and prospects for the future development directions of anion modulation for redirecting dynamic surface reconstruction to construct highly efficient and practical catalysts for water oxidation are proposed.

A quaternary heterojunction nanoflower for significantly enhanced electrochemical water splitting

Designing highly-efficient, cost-effective, and stable electrocatalysts for water splitting is essential to producing green hydrogen. In this work, a nanoflower quaternary heterostructured Ni(NO3)2(OH)4/Ni(OH)2/Ni3S2/NiFe-LDH electrocatalyst is successfully synthesized by two-step hydrothermal reactions. The sulfur in the electrocatalyst induces higher valence state metal atoms as active sites to accelerate the formation of O2. As expected, benefiting from the unique structural features and solid electronic interactions, Ni(NO3)2(OH)4/Ni(OH)2/Ni3S2/NiFe-LDH exhibits remarkable oxygen evolution reaction performance with a low overpotential of 223 mV at a current density of 100 mA cm-2, a slight Tafel slope of 65.4 mV dec-1, and outstanding stability in alkaline media. Attractively, using Ni(NO3)2(OH)4/Ni(OH)2/Ni3S2/NiFe-LDH as both a cathode and an anode, the alkaline electrolyzer delivers a current density of 10 mA cm-2 only at a cell voltage of 1.67 V, accompanied by superior durability. This work provides a facile method for the rational design of high-performance quaternary electrocatalysts.

Synergistic Trimetallic Nanocomposites as Visible-NIR-Sunlight-Driven Photocatalysts for Efficient Artificial Photosynthesis

Here, we report monodispersed tricomponent MnNSs-SnO2@Pt and MnNFs-SnO2@Pt nanocomposites prepared by simultaneous SnO2 and Pt nanodot coating on Mn nanospheres (MnNSs) and Mn nanoflowers (MnNFs) for highly efficient CO2 photoreduction in visible-NIR-sunlight irradiation. MnNFs-SnO2@Pt showed higher efficiency with a quantum yield of 3.21% and a chemical yield of 5.45% for CO2 conversion under visible light irradiation for HCOOH formation with 94% selectivity. Similarly, MnNFs-SnO2@Pt displayed significant photocatalytic efficiency in NIR and sunlight irradiation for HCOOH yield. MnNFs-SnO2@Pt nanocomposites also showed robust morphology and sustained structural stability with shelf-life for at least 1 year and were utilized for at least 10 reaction cycles without losing significant photocatalytic efficiency. The high surface area (94.98 m2/g), efficient electron-hole separation, and Pt-nanodot support in MnNFs--SnO2@Pt contributed to a higher photocatalytic efficacy toward CO2 reduction.

Fe1-xNix(PO3)2/Ni2P Heterostructure for Boosting Alkaline Oxygen Evolution Reaction in Fresh Water and Real Seawater at High Current Density

Oxygen evolution reaction (OER) is a limiting reaction for highly efficient water electrolysis. Thus, the development of cost-effective and highly efficient OER catalysts is the key to large-scale water electrolysis for hydrogen production. Herein, by using an interfacial engineering strategy, a unique nanoflower-like Fe1-xNix(PO3)2/Ni2P/NF heterostructure with abundant heterogeneous interfaces is successfully fabricated. The catalyst exhibits excellent OER catalytic activity in alkaline fresh water and alkaline natural seawater at high current densities, which only, respectively, requires overpotentials of 318 and 367 mV to drive 1000 mA cm-2 in fresh water and natural seawater both containing 1 M KOH. Furthermore, Fe1-xNix(PO3)2/Ni2P/NF demonstrates excellent durability, which can basically remain stable for 80 h during the electrocatalytic OER processes, respectively, in alkaline fresh water and natural seawater. This work provides a new construction strategy for designing highly efficient electrocatalysts for OER at high current densities both in alkaline fresh water and in natural seawater.