Electrocatalyst based on Ni2P nanoparticles and NiCoP nanosheets for efficient hydrogen evolution from urea wastewater

Well-defined ternary metal phosphide nanowires with stabilized Pt nanoclusters to boost alkaline hydrogen evolution reaction

Designing low-content and high-activity Pt-based catalysts with the high durability for the electrochemical hydrogen production remains a challenge. In this study, a ternary metal phosphide (NiCoP) with 1D nanowire (NW) and 2D nanosheet (NS) morphologies incorporating Pt clusters (denoted as Ptcluster-NiCoP@NF NWs and Ptcluster-NiCoP@NF NSs, respectively) was prepared using a hydrothermal-phosphorization-electrodeposition method. Based on the "tip effect" of NWs and a high electrochemical surface area, the as-prepared Ptcluster-NiCoP@NF NWs display better hydrogen evolution reaction (HER) performance, with a low overpotential of 65 mV at a high current density of 100 mA cm-2 and a low Tafel slope of 38.86 mV dec-1, than the Ptcluster-NiCoP@NF NSs, with an overportential of 95 mV at 42.53 mV dec-1. This indicates that the NiCoP NW-based support exhibits faster HER kinetics. The mass activity (11.47 A mgPt-1) of the Ptcluster-NiCoP@NF NWs is higher than that of commercial Pt/C catalysts. Significantly, the Ptcluster-NiCoP@NF NWs display excellent cyclic stability with negligible losses for 5000 cycles and 30-h tests at a high current of 500 mA cm-2.

Phosphide-Based Electrocatalysts for Urea Electrolysis: Recent Trends and Progress

Electrolysis is a trend in producing hydrogen as a fuel for renewable energy development, and urea electrolysis is considered as one of the advanced electrolysis processes, where efficient materials still need to be explored. Notably, urea electrolysis came into existence to counter-part the electrode reactions in water electrolysis, which has hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Among those reactions, OER is sluggish and limits water splitting. Hence, urea electrolysis emerged with urea oxidation reaction (UOR) and HER as their reactions to tackle the water electrolysis. Among the explored materials, noble-metal catalysts are efficient, but their cost and scarcity limit the scaling-up of the Urea electrolysis. Hence, current challenges must be addressed, and novel efficient electrocatalysts are to be implemented to commercialize urea electrolysis technology. Phosphides, as an efficient UOR electrocatalyst, have gained huge attention due to their exceptional lattice structure geometry. The phosphide group benefits the water molecule adsorption and water dissociation, and facilitates the oxyhydrate of the metal site. This review summarizes recent trends in phosphide-based electrocatalysts for urea electrolysis, discusses synthesis strategies and crystal structure relationship with catalytic activity, and presents the challenges of phosphide electrocatalysts in urea electrolysis.

Tailoring competitive adsorption sites of hydroxide ion to enhance urea oxidation-assisted hydrogen production

Alloys with bimetallic electron modulation effect are promising catalysts for the electrooxidation of urea. However, the side reaction oxygen evolution reaction (OER) originating from the competitive adsorption of OH- and urea severely limited the urea oxidation reaction (UOR) activity on the alloy catalysts. This work successfully constructs the defect-rich NiCo alloy with lattice strain (PMo-NiCo/NF) by rapid pyrolysis and co-doping. By taking advantage of the compressive strain, the d-band center of NiCo is shifted downward, inhibiting OH- from adsorbing on the NiCo site and avoiding the detrimental OER. Meanwhile, the oxygenophilic P/Mo tailored specific adsorption sites to adsorb OH- preferentially, which further released the NiCo sites to ensure the enriched adsorption of urea, thus improving the UOR efficiency. As a result, PMo-NiCo/NF only requires 1.27 V and -57 mV to drive a current density of ±10 mA cm-2 for UOR and hydrogen evolution reaction (HER), respectively. With the guidance of this work, reactant competing adsorption sites could be tailored for effective electrocatalytic performance.

Construction of bimetallic phosphide nanostructures with in situ growth, reduction, and phosphidation of ultra-thin graphene layers as highly efficient catalysts towards the OER

We present a novel approach for the in situ growth of bimetallic silicate onto ultrathin graphene, followed by in situ reduction and phosphorization to obtain uniformly dispersed bimetallic phosphides (rGO@FeNiP/rGO@FeCoP) on graphene layers. Unlike the traditional simple composites of single-metallic phosphides and carbon materials, the bimetallic synergy of rGO@FeNiP/rGO@FeCoP obtained through in situ growth, reduction, phosphorization, and alkaline treatment exhibits a large surface area, more nanopores and defects, and more active sites, facilitates electrolyte diffusion and gas release, accelerates electron transfer and enhances electrocatalytic oxygen evolution reaction (OER) performance. Furthermore, the continuous carbon layer architecture surrounding FeNiP/FeCoP provides structural support, improving catalyst stability. We have investigated the effect of different proportions of bimetals on electrocatalytic performance, providing a rational design and synthesis strategy for carbon-based bimetallic phosphides as a promising electrocatalyst for the OER.

Kirkendall effect Strengthened-Superhydrophilic/superaerophobic Co-Ni3N/NF heterostructure as electrode catalyst for High-current hydrogen production

The development of efficient electrocatalysts for large-scale water electrolysis is crucial and challenging. Research efforts towards interface engineering and electronic structure modulation can be leveraged to enhance the electrochemical performance of the developed catalysts. In this work, a surface-engineered Co-Ni₃N/NF heterostructure electrode was prepared based on Kirkendall effect for high-current water electrolysis. In the experiments, the textural feature and intrinsic activity of the Co-Ni₃N/NF heterostructure were tuned through cobalt-doping and the creation of structural defects. As a result, the increased surface energy endowed Co-Ni₃N/NF heterostructure with superhydrophilic and superaerophobic properties. Meanwhile, the contact area of the gas-liquid-solid three phases was optimized. With a large underwater bubble contact angle (CA) of 169°, the electrolyte solution can infiltrate the Co-Ni₃N/NF electrode within 150 ms. Sequentially, the generated gas bubbles were able to detach at high frequency, which ensured the rapid mass exchange. The performance tests showed that the optimal Co-Ni₃N/NF electrode sample reached current densities of 100 mA cm^-2 and 500 mA cm^-2 at the overpotentials of 98 mV and 123 mV, respectively. Benefiting from the reduction of hydrogen embrittlement, the HER performance of the prepared Co-Ni₃N/NF electrode sample decreased slightly after 100 h durability test, but the overall structure remained well. Those results allowed us to conclude that the prepared Co-Ni₃N/NF electrocatalyst holds the promises for large-scale water electrolysis in industries. More specifically, this work provided a new perspective that the efficiency of electrocatalysts for large-scale water electrolysis can be enhanced by constructing a heterostructure with good wettability and gas repellency.

P-induced bottom-up growth of Fe-doped Ni(12)P(5) nanorod arrays for urea oxidation reaction

Synthesis of regular morphology catalysts with self-growing substrates is one of the effective methods to solve the problem of easy shedding of heterogeneous catalysts. In this study, Fe-doped Ni₁₂P₅ nanorods were prepared by depositing 1,1' -bis (diphenylphosphine) ferrocene (DPPF) on N-doped C/NF. The bottom-up growth of the nanorod is ascribed to the preferential adsorption of DPPF with a P site to NF that is surface-doped with the solid-solving C, and the length of nanorods can reach tens of microns and has good robustness. The N-doped carbon-constrained rod-shaped Fe-doped Ni₁₂P₅ catalyst (Fe-Ni₁₂P₅/N_dC/NF-800) that grows on NF has excellent catalytic performance for the urea oxidation reaction. In addition, the current density can be maintained as high as 100 mA cm^-2 and the current attenuation is weak for 12 h, and the rod shape remains good. This work provides a new idea for synthesizing self-growing catalysts with regular morphology to improve the performance of heterogeneous catalysts.

Biomimicry-inspired fish scale-like Ni3N/FeNi3N/NF superhydrophilic/superaerophobic nanoarrays displaying high electrocatalytic performance

The mass transfer efficiency and structural stability of the electrode are critical for industrialized water electrolysis operations. Herein, the biomimicry-inspired design of Ni₃N/FeNi₃N/NF nanoarrays with a fish scale-like structure, which endowed the Ni₃N/FeNi₃N/NF nanoarrays with rapid infiltration of aqueous solution within 60 ms and 169° bubble contact angle, is demonstrated. The optimal Ni₃N/FeNi₃N/NF sample displayed catalytic activity with hydrogen evolution reaction (HER) overpotentials of only 48 mV at 10 mA cm^-2 and 102 mV at 100 mA cm^-2. Similarly, the overpotential of the anodic-coupled urea oxidation reaction (UOR) was only 1.3 V at 10 mA cm^-2 and 1.35 V at 100 mA cm^-2. Besides, the small impact resulting from the rapid bubble extraction within the Ni₃N/FeNi₃N/NF nanoarrays ensured excellent HER cycling stability over 100 h at a current density of 50 mA cm^-2. The further scale-up experiment suggests the industrialization prospects of the prepared Ni₃N/FeNi₃N/NF electrocatalysts.

Nickel-Doped Carbon Dots as an Efficient and Stable Electrocatalyst for Urea Oxidation

Urea is a typical contaminant present in wastewater which may cause severe environmental problems. Electrochemical catalytic oxidation of urea has emerged as an efficient approach to solve this problem. Nevertheless, the current nickel-based catalysts (e.g., nickel hydroxide/sulfides) feature a high metal content. It not only lowers the utilization efficiency of nickel but also causes secondary pollution to the environment. Here, nickel-doped carbon dots (Ni-CDs) with an excellent and stable catalytic activity for the electrocatalytic urea oxidation reaction (UOR) are reported. Specifically, carbon dots (CDs) with abundant functional groups are synthesized by a one-pot hydrothermal method and then Ni-CDs with a very low metal content (1.1 at%) are prepared. The Ni²⁺ sites by coordination with carboxylic groups on the CDs provide excellent electrocatalytic activity and excellent durability for the UOR, as demonstrated by an anodic current density of 100 mA cm^-2 at a potential of 1.38 V (vs RHE) and similar experimental results in practical application. To the best of knowledge, this is the first report of CDs-based materials applied for the UOR, which opens an important new area of applicability for CDs as well as broadens the scope of the materials for electrochemical catalysis of urea.

Hierarchical Ni–Mo–P nanoarrays toward efficient urea oxidation reaction

Ni2P–MoP2 heterostructure with hierarchical structure grown on carbon paper (Ni–Mo–P/CP) exhibits robust activity for electrocatalytic urea oxidation compared to the oxide evolution reaction.