Phase and crystallinity regulations of Ni(OH)2 by vanadium doping boost electrocatalytic urea oxidation reaction

Simultaneously Boosting Direct and Indirect Urea Oxidation of Nickel Hydroxide via Strategic Yttrium Doping

Urea electrolysis can address pressing environmental concerns caused by urea-containing wastewater while realizing energy-saving hydrogen production. Highly efficient and affordable electrocatalysts are indispensable for realizing the great potential of this emerging technology. Among the numerous candidates, α-Ni(OH)2 has the merits of good electrocatalytic activity, adjustable heteroelement doping, and low cost; consequently, it has received tremendous attention in the electrolytic fields. Herein, a Y3+-doping strategy is developed to effectively enhance the catalytic performance of nickel hydroxide in the urea oxidation reaction (UOR). Our results show that Y3+ incorporation successfully modulates the electronic structure of α-Ni(OH)2 by inducing Ni3+ formation in the crystal lattice to initiate direct UOR, facilitates the Ni3+/Ni2+ redox transition with higher current responses to promote indirect UOR, and maintains the structural stability of YNi-10 (Ni2+/Y3+ molar ratio = 1:0.1) during long-term UOR operation. Owing to these features, the obtained YNi-10 sample exhibits a higher current density (127 vs 79 mA cm-2 at 1.5 V), a lower Tafel slope (48 vs 75 mV dec-1), a larger potential difference between the UOR and oxygen evolution reaction (OER, 0.26 vs 0.22 V at 80 mA cm-2), a higher reaction rate constant (1.1 × 105 vs 3.1 × 103 cm3 mol-1 s-1), and a reduced activation energy of UOR (2.9 vs 14.8 kJ mol-1) compared with the Y-free counterpart (YNi-0). This study presents a promising strategy to simultaneously boost direct and indirect UORs, providing new insights for further developing high-performance electrocatalysts.

Nickel Hydroxide-Based Electrocatalysts for Promising Electrochemical Oxidation Reactions: Beyond Water Oxidation

Transition metal hydroxides have attracted significant research interest for their energy storage and conversion technique applications. In particular, nickel hydroxide (Ni(OH)2), with increasing significance, is extensively used in material science and engineering. The past decades have witnessed the flourishing of Ni(OH)2-based materials as efficient electrocatalysts for water oxidation, which is a critical catalytic reaction for sustainable technologies, such as water electrolysis, fuel cells, CO2 reduction, and metal-air batteries. Coupling the electrochemical oxidation of small molecules to replace water oxidation at the anode is confirmed as an effective and promising strategy for realizing the energy-saving production. The physicochemical properties of Ni(OH)2 related to conventional water oxidation are first presented in this review. Then, recent progress based on Ni(OH)2 materials for these promising electrochemical reactions is symmetrically categorized and reviewed. Significant emphasis is placed on establishing the structure-activity relationship and disclosing the reaction mechanism. Emerging material design strategies for novel electrocatalysts are also highlighted. Finally, the existing challenges and future research directions are presented.

Structural Design of Nickel Hydroxide for Efficient Urea Electrooxidation

Urea stands as a ubiquitous environmental contaminant. However, not only does urea oxidation reaction technology facilitate energy conversion, but it also significantly contributes to treating wastewater rich in urea. Furthermore, urea electrolysis has a significantly lower theoretical potential (0.37 V) compared to water electrolysis (1.23 V). As an electrochemical reaction, the catalytic efficacy of urea oxidation is largely contingent upon the catalyst employed. Among the plethora of urea oxidation electrocatalysts, nickel-based compounds emerge as the preeminent transition metal due to their cost-effectiveness and heightened activity in urea oxidation. Ni(OH)2 is endowed with manifold advantages, including structural versatility, facile synthesis, and stability in alkaline environments. This review delineates the recent advancements in Ni(OH)2 catalysts for electrocatalytic urea oxidation reaction, encapsulating pivotal research findings in morphology, dopant incorporation, defect engineering, and heterogeneous architectures. Additionally, we have proposed personal insights into the challenges encountered in the research on nickel hydroxide for urea oxidation, aiming to promote efficient urea conversion and facilitate its practical applications.

Engineering advanced noble-metal-free electrocatalysts for energy-saving hydrogen production from alkaline water via urea electrolysis

When the anodic oxygen evolution reaction (OER) of water splitting is replaced by the urea oxidation reaction (UOR), the electrolyzer can fulfill hydrogen generation in an energy-economic manner for urea electrolysis as well as sewage purification. However, owing to the sluggish kinetics from a six-electron process for UOR, it is in great demand to design and fabricate high-performance and affordable electrocatalysts. Over the past years, numerous non-precious materials (especially nickel-involved samples) have offered huge potential as catalysts for urea electrolysis under alkaline conditions, even in comparison with frequently used noble-metal ones. In this review, recent efforts and progress in these high-efficiency noble-metal-free electrocatalysts are comprehensively summarized. The fundamentals and principles of UOR are first described, followed by highlighting UOR mechanism progress, and then some discussion about density functional theory (DFT) calculations and operando investigations is given to disclose the real reaction mechanism. Afterward, aiming to improve or optimize UOR electrocatalytic properties, various noble-metal-free catalytic materials are introduced in detail and classified into different classes, highlighting the underlying activity-structure relationships. Furthermore, new design trends are also discussed, including targetedly designing nanostructured materials, manipulating anodic products, combining theory and in situ experiments, and constructing bifunctional catalysts. Ultimately, we point out the outlook and explore the possible future opportunities by analyzing the remaining challenges in this booming field.

Urea catalytic oxidation for energy and environmental applications

Urea is one of the most essential reactive nitrogen species in the nitrogen cycle and plays an indispensable role in the water-energy-food nexus. However, untreated urea or urine wastewater causes severe environmental pollution and threatens human health. Electrocatalytic and photo(electro)catalytic urea oxidation technologies under mild conditions have become promising methods for energy recovery and environmental remediation. An in-depth understanding of the reaction mechanisms of the urea oxidation reaction (UOR) is important to design efficient electrocatalysts/photo(electro)catalysts for these technologies. This review provides a critical appraisal of the recent advances in the UOR by means of both electrocatalysis and photo(electro)catalysis, aiming to comprehensively assess this emerging field from fundamentals and materials, to practical applications. The emphasis of this review is on the design and development strategies for electrocatalysts/photo(electro)catalysts based on reaction pathways. Meanwhile, the UOR in natural urine is discussed, focusing on the influence of impurity ions. A particular emphasis is placed on the application of the UOR in energy and environmental fields, such as hydrogen production by urea electrolysis, urea fuel cells, and urea/urine wastewater remediation. Finally, future directions, prospects, and remaining challenges are discussed for this emerging research field. This critical review significantly increases the understanding of current progress in urea conversion and the development of a sustainable nitrogen economy.

Synergistic engineering of heteroatom doping and heterointerface construction in V-doped Ni(OH)2/FeOOH to boost both oxygen evolution and urea oxidation reactions

The exploitation of cost-effective and abundant non-noble metal electrocatalysts holds great significances in enhancing the efficiency of oxygen evolution reaction (OER) and/or urea oxidation reaction (UOR). Herein, we report an electrocatalyst with co-existing V-dopants and Ni(OH)2/FeOOH interfaces (referred to as A-NiFeV/NF, with "A" indicating "activated"). The electron coupling between Ni, Fe and V, analyzed through X-ray photoelectron spectroscopy, indicates that Ni and Fe both receive electrons from the V. Additionally, the Fe can also lead to a bias toward a lower valence of the Ni centers in Ni(OH)2. Further in situ Raman spectroscopy reveals that Ni2+(OH)2 inevitably undergoes transformation into amorphous Ni3+OOH during the activation process, however, the synergistic effects of V-dopants and Ni(OH)2/FeOOH interfaces keep the Ni centers mostly in a lower oxidation state of +2 even at high potential ranges. These low-valence Ni centers are proposed to be positively correlated with the optimized OER activity of the Ni-based electrocatalysts. As a result, the designed A-NiFeV/NF electrocatalyst exhibits low overpotentials of 234 and 313 mV to propel current densities of 10 and 100 mA/cm2, and a small Tafel slope of 37.8 mV/dec for OER in 1.0 M KOH. The catalyst demonstrates a stable OER activity for over 100 h at 100 mA/cm2. Additionally, it can be integrated with a solar cell to construct a solar-driven electrolytic OER device without additional electric input. Similarly, for the small molecule oxidation, UOR, only ∼1.33 and ∼1.39 V vs. RHE (RHE: reversible hydrogen electrode) are required to achieve 10 and 100 mA/cm2, respectively, in an electrolyte composed of 1.0 M KOH with 0.33 M urea.

Binary and nanostructured NiMn perovskite fluorides as efficient electrocatalysts for urea oxidation reaction

Urea electrolysis holds tremendous promise to provide green and sustainable energy and environmental solutions, because it can simultaneously remedy urea-containing wastewater and provide energy-saving hydrogen. However, the development of this emerging technology remains challenging mainly due to a dearth of high-performance electrocatalysts for efficient urea oxidation reaction (UOR). Perovskite fluorides have the advantages of intrinsic 3D diffusion pathways, robust architecture, and tunable chemical composition, thus receiving increasing attention in many applications. In this work, the UOR performances of a series of ABF3 samples (A = K; B = Ni/Mn, Ni/Co, Co/Mn) with various compositions are investigated in a systematic fashion for the first time. Among the binary samples, KNMF41 (Ni/Mn atomic ratio = 4:1) is the optimal sample with reduced overpotential (reaching 100 mA cm-2 at 1.43 V), low Tafel slope (40 mV dec-1), enhanced reaction rate constant (6.3 × 105 cm3 mol-1 s-1), and high turnover frequency (TOF, 0.19 s-1 at 1.60 V) toward urea oxidation. By comparing with NiCo and CoMn samples, the binary NiMn design is confirmed to endow the perovskite fluoride with higher electrocatalytic activity, thanks to the directed adsorption of urea molecules on the adjacent NiMn active sites. This work presents a targeted synthetic strategy for obtaining efficient electrocatalysts.

Metal-organic framework (MOF)-derived flower-like Ni-MOF@NiV-layered double hydroxides as peroxidase mimetics for colorimetric detection of hydroquinone

BACKGROUND

Nanozymes are one of the ideal substitutes for natural enzymes because of their excellent chemical stability and simple preparation methods. However, due to the limited catalytic ability of most reported nanozymes, constructing nanomaterials with low cost and high activity is gradually becoming an exploration focus in the field of nanozymes. Heteroatom doping of metal-organic frameworks is one of potential approaches to design nanozymes with high catalytic performance. Due to their multiple valence states properties, V-doped metal-organic framework (MOF)-derived LDH is expected to be a good enzyme-like catalyst. To our knowledge, the V-doped MOF-derived LDH as nanozyme is not explored before.

RESULTS

We report the in-situ synthesis of NiV-layered double hydroxides (LDHs) on nickel-based MOF, i.e. Ni-MOF@NiV-LDHs. The MOF surface is covered by 2D nanosheets. This unique structural design increases the specific surface area of the material, enables more exposure of catalytic active sites to participate in reactions and accelerates the electron transfer rate. The Ni-MOF@NiV-LDHs have high peroxidase-like activity able to catalyze TMB oxidation by H2O2 via the generation of •OH and O2•-. Relative to Ni-MOF, the Ni-MOF@NiV-LDHs shows 47-fold peroxidase-like activity rise. It had good affinity to TMB and H2O2, with the Michaelis-Menten constants of 0.12 mM and 0.007 mM, respectively. The hydroquinone (HQ) consumed the reactive oxygen species generated in the TMB + H2O2+Ni-MOF@NiV-LDHs system to inhibit the TMB oxidation. On this basis, a sensitive and rapid assay for determining HQ was developed, with a linear range of 0.50-70 μM and a LOD of 0.37 μM.

SIGNIFICANCE

This work provided some clues for the further development of novel nanozymes with high catalytic performance via a strategy of heteroatom doping. And the constructed colorimetric analysis method was successfully utilized for the determination of HQ in actual waters, which has the potential for practical application in the analysis of environmental pollutants.

Metal-organic frameworks-derived oxalate ligand modified NiCo hydroxides for enhanced electrochemical glycerol oxidation reaction

Glycerol oxidation reaction can be substituted for oxygen evolution reaction for more efficient hydrogen production due to its lower thermodynamic potential. Herein, a series of NiCo hydroxide nanosheets containing abundant Ni3+ species and surface ligands were synthesized by in-situ structural transformation of bimetallic organic frameworks in alkaline media for efficient glycerol oxidation reaction. It is found that the incorporation of Co ions increases the content of the Ni3+ species, and that the Ni/Co ratio of 1.0 lead to the optimal catalytic performance. The oxalate-modified nickel-cobalt hydroxide with the optimized Ni/Co ratio can deliver a current density of 10 mA cm-2 at 1.26 V vs. RHE (reversible hydrogen electrode), and reaches its maximum selectivity and Faradaic efficiency at 1.30 V vs. RHE. A high selectivity of 82.9% and a Faradaic efficiency of 91.0% are achieved. The high catalytic activity can be mainly attributed to the abundant Ni3+ species and surface carboxyl groups.

Recent Progress on Phase Engineering of Nanomaterials

As a key structural parameter, phase depicts the arrangement of atoms in materials. Normally, a nanomaterial exists in its thermodynamically stable crystal phase. With the development of nanotechnology, nanomaterials with unconventional crystal phases, which rarely exist in their bulk counterparts, or amorphous phase have been prepared using carefully controlled reaction conditions. Together these methods are beginning to enable phase engineering of nanomaterials (PEN), i.e., the synthesis of nanomaterials with unconventional phases and the transformation between different phases, to obtain desired properties and functions. This Review summarizes the research progress in the field of PEN. First, we present representative strategies for the direct synthesis of unconventional phases and modulation of phase transformation in diverse kinds of nanomaterials. We cover the synthesis of nanomaterials ranging from metal nanostructures such as Au, Ag, Cu, Pd, and Ru, and their alloys; metal oxides, borides, and carbides; to transition metal dichalcogenides (TMDs) and 2D layered materials. We review synthesis and growth methods ranging from wet-chemical reduction and seed-mediated epitaxial growth to chemical vapor deposition (CVD), high pressure phase transformation, and electron and ion-beam irradiation. After that, we summarize the significant influence of phase on the various properties of unconventional-phase nanomaterials. We also discuss the potential applications of the developed unconventional-phase nanomaterials in different areas including catalysis, electrochemical energy storage (batteries and supercapacitors), solar cells, optoelectronics, and sensing. Finally, we discuss existing challenges and future research directions in PEN.

Self-assembled c-oriented Ni(OH)2 films for enhanced electrocatalytic activity towards the urea oxidation reaction

This study investigates the electrocatalytic properties of the transparent c-oriented Ni(OH)2 films self-assembled from colloidal 2D Ni(OH)2 nanosheets for urea oxidation. The synthesis process yields highly uniform close-packed superlattices with a dominant c-axis orientation. The self-assembled c-oriented Ni(OH)2 films exhibit advantageous electrocatalytic performance in urea oxidation, presenting significantly lower overpotentials and higher current densities compared to randomly distributed Ni(OH)2 particles. In-depth in situ impedance analysis and Raman spectroscopy demonstrate that the c-oriented Ni(OH)2 films possess a higher propensity for a Ni valence transition from +2 to +3 during the urea oxidation process. This finding provides crucial insights into the catalytic behavior and electronic transformations of c-oriented Ni(OH)2 films, shedding light on their superior electrocatalytic activity for urea oxidation. Overall, this study advances our understanding of urea electrooxidation mechanisms and contributes to the design of efficient urea electrocatalysts.

Superwettable and photothermal all-in-one electrocatalyst for boosting water/urea electrolysis

Developing multifunctional all-in-one electrocatalysts for energy-saving hydrogen generation remains a challenge. In this study, a simple and feasible thermal phosphorization strategy is explored to rationally construct P-doped MoO₂-NiMoO₄ heterostructure on nickel foam (NF). The heterointerfaced P-MoO₂-NiMoO₄/NF can simultaneously realize the integrated all-in-one functionalities, innovatively introducing superwettable surfaces, photothermal conversion capabilities and electrocatalytic functions. The superwettability gives P-MoO₂-NiMoO₄/NF sufficient electrolyte permeation and smooth bubble detachment. The plasmonic MoO₂ with photothermal performance greatly elevates the local surface temperature of in P-MoO₂-NiMoO₄/NF, which is conducive to improve the electrocatalytic efficiency. The favorable in-situ surface reconstruction brings abundant active sites for electrocatalytic reactions. As an advanced multifunctional electrocatalyst, the superwettable and photothermal P-MoO₂-NiMoO₄/NF exhibits significantly improved performances in oxygen evolution reaction (OER) and urea oxidation reaction (UOR). More importantly, the highly efficient and stable overall water-urea electrolysis assisted by photothermal fields can be simply achieved by exposing P-MoO₂-NiMoO₄/NF to near-infrared (NIR) light.

Metal organic framework-assisted in-situ synthesis of β-NiMnOOH nanosheets with abundant NiOOH active sites for efficient electro-oxidation of urea

NiOOH has been considered as the active center for urea oxidation reaction (UOR), but it remains challenging to synthesize high-performance NiOOH-based catalysts. Herein, we realize the synthesis of a high-performance NiOOH-based catalyst through in-situ transformation from the NiMn-based metal-organic framework to NiMnOOH. X-ray photoelectron spectroscopy characterization shows that the Ni3+/Ni2+ ratio in the NiMnOOH is 3.9 times as big as that in the Ni(OH)2, and in-situ Raman characterization further consolidates the presence of the NiOOH species in the NiMnOOH and as well unveils the faciliated Ni2+/Ni3+ redox reaction. The abundant NiOOH species, the markedly facilitated Ni2+/Ni3+ redox reaction and the Ni-Mn synergy contribute to the high intrinsic activity of the NiMnOOH towards UOR. The NiMnOOH exhibits an impressively low onset potential of 1.305 V vs reversible hydrogen electrode (RHE) and requires only a small potential of 1.34 V vs RHE to deliver a current density of 100 mA cm-2 in 1.0 M KOH + 0.33 M urea. In addition, the NiMnOOH catalyst possesses good long-term working stability.

Ni2 Se3 -CuSex Heterostructure as a Highly Efficient Bifunctional Electrocatalyst for Urea-Assisted Hydrogen Generation

Coupling urea oxidation reaction (UOR) with hydrogen evolution reaction (HER) is an attractive alternative anode reaction for electrochemical hydrogen generation with low energy consumption. However, the development of highly efficient bifunctional electrocatalysts is still a challenge. In this work, Ni₂ Se₃ -CuSe_x heterostructure was synthesized on copper foam (Ni₃ Se₂ @CuSe_x /CF) by electrodeposition accompanied by a selenization process. Benefiting from the abundant active sites, faster reaction kinetics, and modulated electronic structure, the self-supporting Ni₃ Se₂ @CuSe_x /CF electrode exhibited superior catalytic performance. Extremely low overpotentials of 120 and 140 mV were achieved at the current density of 100 mA cm^-2 for HER/UOR, respectively. Respectively, in HER||UOR coupled electrolyzer for H₂ generation, the Ni₃ Se₂ @CuSe_x /CF||Ni₃ Se₂ @CuSe_x /CF delivered a low cell voltage of 1.49 V to reach a high current density of 100 mA cm^-2 along with good stability, outperforming most of the other well-developed materials to date. The rational design of coupled heterostructure as bifunctional electrodes is a promising approach for energy-saving H₂ production.

Bifunctional keel flower-like Ni-Co-V multicomponent oxide catalyst with enhanced electron transport for accelerating overall water splitting

Heteroatom doping is considered a typical method for improving the electrochemical properties of composites. In this work, the multi-component oxide catalyst (Ni(VO₃)₂ and Co₂V₂O₇ on Ni foam, referred to as NiCoVO_x@NF) is formed by hydrothermal doping of V element into NiCo-based precursors followed by co-oxidation. In the catalyst NiCoVO_x@NF, all three components of Ni, Co and V are particularly tightly coordinated, exhibiting an integrated structure of keel flower-like arrays. The catalyst NiCoVO_x@NF's contact surface with water is increased thanks to this unusual structure, exposing a high number of active sites. Furthermore, NiCoVO_x@NF owns efficient electronic pathways, which greatly enhances the electron transport ability. To generate a current density of 10 mA cm^-2 for hydrogen evolution reaction, just a 107 mV overpotential is required. The electrode exhibits a low overpotential of 217 mV to deliver 50 mA cm^-2 for oxygen evolution reaction. In addition, the total water splitting performance of NiCoVO_x@NF is also excellent, which could be achieved by only one 1.5 V AA battery. This study provides a feasible heteroatom doping route to design bifunctional catalysts with improved performances.