Urea Electro-oxidation Research Articles

Vertical Cross-Alignments of 2D Semiconductors with Steered Internal Electric Field for Urea Electrooxidation via Balancing Intermediates Adsorption.

Single-component electrocatalysts generally lead to unbalanced adsorption of OH- and urea during urea oxidation reaction (UOR), thus obtaining low activity and selectivity especially when oxygen evolution reaction (OER) competes at high potentials (>1.5 V). Herein, a cross-alignment strategy of in situ vertically growing Ni(OH)2 nanosheets on 2D semiconductor g-C3N4 is reported to form a hetero-structured electrocatalyst. Various spectroscopy measurements including in situ experiments indicate the existence of enhanced internal electric field at the interfaces of vertical Ni(OH)2 and g-C3N4 nanosheets, favorable for balancing adsorption of reaction intermediates. This heterojunction electrocatalyst shows high-selectivity UOR compared to pure Ni(OH)2, even at high potentials (>1.5 V) and large current density. The computational results show the vertical heterojunction could steer the internal electric field to increase the adsorption of urea, thus efficiently avoiding poisoning of strongly adsorbed OH- on active sites. A membrane electrode assembly (MEA)-based electrolyzer with the heterojunction anode could operate at an industrial-level current density of 200 mA cm-2. This work paves an avenue for designing high-performance electrocatalysts by vertical cross-alignments of active components.

Ni-tethered MoS2: In-situ fast reduction synthesis as an ultra-durable and highly active electrocatalyst for water splitting and urea oxidation

Developing an extraordinarily robust and efficient green electrocatalyst derived from earth-abundant elements is becoming an important part of green hydrogen generation. We present here a simple single-step in-situ fast reduction synthesis of Ni-tethered MoS2 (NiMoS2), which exhibits excellent activity for the Oxygen Evolution Reaction (OER), Hydrogen Evolution Reaction (HER), and Urea Oxidation Reaction (UOR). To supply 10 mA cm−2 current density, the active catalyst NiMoS2 exhibits minor overpotentials of 141 mV vs RHE (reversible hydrogen electrode) for HER, and 250 mV vs RHE for OER in 1.0 M KOH. NiMoS2 displays ultra-durability (over 90 h) with a current loss of <4.6% for OER and <3.6% for HER, respectively. The enhanced trifunctional catalyst NiMoS2/NF enables the construction of a stable behavior water electrolyzer that delivers 10 mA cm−2 at 1.52 V. With a <4.8% current loss, the NiMoS2/NF//NiMoS2/NF exhibits an ultra-stable behavior (over 150 h). The water electrolysis at 1.64 V with solar assistance demonstrates the enhanced efficacy of the synthesized catalyst. The fabricated electrode, NiMoS2 exhibits efficient catalytic activity toward the electro-oxidation of urea (UOR) (1 M KOH +0.33 M urea) at 1.39 V.

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.

Engineering active sites on binary metal selenide heterointerface catalyst to boost urea electrooxidation

Electrocatalytic urea oxidation reaction (UOR) with a low thermodynamic potential is a perfect substitute for anodic oxygen evolution process (OER) in the effective generation of hydrogen. However, because of the slow kinetics of UOR, its potential application for commercial use remains untapped. Nickel-based materials may be an option for urea oxidation reaction but the heavily filled d orbital prevents enhanced adsorption and activity. Here, taking into account the adsorption-energy scaling limitations, Co3+ with partially filled d orbital is introduced into nickel selenide to form a heterointerface catalyst (2D CoSe2/Ni0.85Se) that enhances UOR. As anticipated, the 2D CoSe2/Ni0.85Se electrode displays a low potential of 1.33 V to achieve 100 mA cm−2 for UOR while retaining strong durability for 300 h. Further, 2D CoSe2/Ni0.85Se catalyst is used as an anode in an anion exchange membrane flow electrolyzer, which achieved an industrial-level voltage of 1.91 V at 1 A cm−2 and robust durability.

Metallic nonstoichiometric WO3–x-decorated NiFe-layered double hydroxide for boosted urea electro-oxidation reaction: the facilitated dynamics and increasing active sites

Metallic nonstoichiometric WO3–x-decorated NiFe-layered double hydroxide for boosted urea electro-oxidation reaction: the facilitated dynamics and increasing active sites

Oxalate-mediated synthesis of hybrid nickel cobalt-based nanostructures for boosting water and urea electrooxidation efficiency

The utilization of urea during electrolysis presents an appealing option for anodic oxidation, offering environmental benefits by converting waste into energy. The present work focuses on synthesizing mixed transition metal oxide (MMO) nanostructures composed of nickel rich nickel cobaltite electrode materials. Interesting, cobalt rich nickel cobaltite have been widely studied; however, the nickel-rich nickel cobaltite remains relatively unexplored. For stabilization of nickel rich nickel cobaltite, we have designed a process for synthesizing three-dimensional (3D) Ni-Co oxalate as a precursor via the coprecipitation method using diammonium oxalate monohydrate as a precipitating agent. The as obtained precursor was further subjected to thermal decomposition which results in the formation of Ni2CoO4. Microstructural analysis of Ni-Co oxalate indicates that oxalate-assisted synthesis leads to anisotropic ultrafine nanoparticle growth, forming hierarchically arranged 3D microflowers. Interestingly, the use of NaOH as a precipitating agent induces a significant change in the morphology of Ni2CoO4, (nanoparticles). Furthermore, the effect of morphology changes over the electrochemical performance was investigated. Ni2CoO4 microflower demonstrates an overpotential of 330 mV@25 mA/cm2 for O2 and −190 mV@10 mA/cm2 for H2 production during water oxidation in 1 M KOH, exhibiting excellent stability for up to 24 h. During urea oxidation, the potential required by Ni2CoO4 microflowers was 1.31 V@10 mA/cm2. Ni2CoO4 nanoparticles' performance is significantly poor during water and urea oxidation compared to the microflower counterpart. Therefore, this work provides detailed insights into designing MMOs with high porosity, increased exposed active sites, and unique structural features that enhance electrocatalytic activity.

Mechanistic Analysis of Urea Electrooxidation Pathways: Key to Rational Catalyst Design.

Urea electrolysis is an emerging approach to treating urea-enriched wastewater and an attractive alternative anodic process to the oxygen evolution reaction (OER) in electrochemical clean energy conversion and storage technologies (e. g., hydrogen production and CO2 electroreduction). While the thermodynamic potential for urea oxidation to dinitrogen is quite low compared to that of the OER, the catalysts reported to date require high overpotentials that far exceed those for the OER. Consequently, there is much room for improvement and rational catalyst design for the urea oxidation reaction (UOR). At the same time, due to the urea molecule having a more complex structure than water, UOR can lead to the formation of various products beyond the commonly assumed N2 and CO2. This concept article will critically assess recent efforts of the research community to decipher the formation mechanisms of UOR products focusing on the systematic analysis of the reaction selectivity. This work aims to analyze the current state of the art and identify existing gaps, providing an outlook for the future design of UOR catalysts with superior activity and selectivity by applying the knowledge of the molecular transformation mechanisms.

Superhydrophilic Dendritic FeP/Cu3P Electrocatalyst for Urea Splitting via the Intramolecular Mechanism.

The electrocatalytic overall urea splitting can achieve the dual goals of urea treatment and hydrogen energy acquisition. Herein, we exploited the principle of precipitation dissolution equilibrium to obtain bimetallic phosphide FeP/Cu3P/CF for the simultaneous oxidation of urea and reduction of water and comprehensively reveal the inherent molecular thermodynamic mechanisms on the surface of catalysts. The excellent electrochemical performance can be derived from the super water affinity and synergistic effect. Especially, the theoretical calculation unveils that the synergistic effect between FeP and Cu3P can lower the activation energy required for urea electrooxidation, thereby promoting urea splitting. In situ differential electrochemical mass spectrometry (in situ DEMS) measurements further demonstrated that urea oxidation on FeP/Cu3P/CF proceeded according to the intramolecular mechanism. This work has laid the foundation for constructing highly efficient superhydrophilic bifunctional electrocatalysts.

A template-assisted method for synthesizing TiO2 nanoparticles and Ni/TiO2 nanocomposites for urea electrooxidation

The exploration of electrocatalytic urea-assisted wastewater treatment as a method for sustainable energy generation presents a compelling approach within the domain of green chemistry. In this context, titanium dioxide (TiO2) and nickel-doped TiO2 (Ni@TiO2) nanocomposites were developed through an innovative organic template-assisted synthesis strategy. These were denoted as m-TiO2-org and Ni@m-TiO2-org, respectively, to create cost-effective and high-performance electrodes for urea electrooxidation in urea-based fuel cells. The structural and chemical properties of the fabricated m-TiO2-org and Ni@m-TiO2-org materials were meticulously analyzed using a suite of physicochemical characterization techniques, including Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD), Thermogravimetric Analysis (TGA), and Energy-Dispersive X-ray Spectroscopy (EDX). These investigations revealed that both m-TiO2-org and Ni@m-TiO2-org materials predominantly consisted of irregular monolithic microparticles formed through the aggregation of spherical nanoparticles, with diameters ranging from 8.16 nm to 13.1 nm. Subsequent evaluations of these TiO2-based materials as electrocatalysts for urea electrooxidation demonstrated their efficacy, as evidenced by voltammetric and electrochemical impedance spectroscopy measurements. Notably, the Ni@m-TiO2-org variant exhibited enhanced electrochemical performance, attributed to the presence of Ni2+ sites. These sites facilitated electrochemical activation and the formation of active NiOOH sites, leading to a significant increase in the current density (0.78 mA/cm2 at 1.523 V vs. the Reversible Hydrogen Electrode (RHE)), as a fiftyfold enhancement compared to m-TiO2-org@Glassy Carbon Electrode (GCE) under similar conditions (3 M KOH containing 3 M urea), with a Tafel slope of 33 mV/decade. This research not only elucidates the potential of TiO2-based nanocomposites in the electrooxidation of urea but also heralds the development of innovative electrode materials for commercial electrochemical cells, marking a significant advancement in the field of electrocatalysis and sustainable energy technology.

CO2 laser-induced formation of a nitrogen-doped graphene aerophobic layer on the nickel foam surface for enhanced electro-oxidation of urea

The construction of a superhydrophobic electrode through substructure tuning is an effective strategy to enhance electrochemical performance for gas-involved reactions such as the urea oxidation reaction (UOR). However, determining the specific contributions of changes in surface properties versus increases in electrochemically active sites remains challenging. In this study, we present a CO2 laser-assisted approach to rapidly construct a nitrogen-doped graphene (NDG) aerophobic layer on nickel foam, resulting in an NDG-covered electrode (NDG-NF-0.52 %). The presence of the aerophobic layer significantly reduces gas bubble adhesion force, reducing it to negligible levels compared to NF, which exhibits a force of about 42 μN. Electrochemical characterization reveals that the electrochemically active surface area of NDG-NF-0.52 % is nearly identical to that of NF. NDG-NF-0.52 % exhibits excellent properties for the UOR, requiring only a potential of 1.38 V to achieve a current density of 50 mA cm-2 in a 1 M KOH solution with 0.33 M urea, significantly lower than that required for NF (1.49 V). The impressive electrochemical performance of the electrode can be attributed to the increased aerophobicity, which enhances the mass-transfer capability at the gas-liquid-solid interface, reducing the duration of active site coverage by gas bubbles. This work underscores the crucial role of surface properties in the electrode reaction process and holds significance for the design of high-performance oxygen evolution reaction (OER) and UOR electrodes.

Mof-derived NiO@NiS hybrids for electrochemical urea assisted water splitting

Urea electrooxidation holds promise for both the treatment of urea wastewater and the generation of clean energy hydrogen fuel. Herein, a NiO@NiS composite was synthesized using a hydrothermal method inspired by natural urease, which demonstrated remarkable catalytic prowess in electrochemical urea oxidation reaction (UOR). Thorough electrochemical analyses uncovered the rapid kinetics and efficient charge transfer capability inherent in urea oxidation. Our work presents a straightforward method for fabricating a nickel-based composite catalyst structure, contributing to the sustainable advancement of urea oxidation.

In Situ Assembly of a Superaerophobic CoMn/CuNiP Heterostructure as a Trifunctional Electrocatalyst for Ampere-Level Current Density Urea-Assisted Hydrogen Production.

Urea electrolysis is a promising energy-efficient hydrogen production process with environmental benefits, but the lack of efficient and sustainable ampere-level current density electrocatalysts fabricated through simple methods is a major challenge for commercialization. Herein, we present an efficient and stable heterostructure electrocatalyst for full urea and water electrolysis in a convenient and time-efficient preparation manner. Overall, superhydrophilic/superaerophobic CoMn/CuNiP/NF exhibits exceptional performance for the hydrogen evolution reaction (HER) (-33.8, -184.4, and -234.8 mV at -10, -500, and -1000 mA cm-2, respectively), urea electro-oxidation reaction (UOR) [1.28, 1.43, and 1.51 V (vs RHE) at 10, 500, and 1000 mA cm-2, respectively], and oxygen evolution reaction (OER) [1.45, 1.67, and 1.74 V (vs RHE) at 10, 500, and 1000 mA cm-2, respectively]. Moreover, the superaerophobic CoMn/CuNiP/NF demonstrates promising potential in full urea (1.33, 1.57, and 1.60 V at 10, 500, and 1000 mA cm-2, respectively) and water (1.46 V, 1.78, and 1.86 at 10, 500, and 1000 mA cm-2, respectively) electrolysis. Based on X-ray photoelectron spectroscopy results, it was determined that the surface of the CoMn/CuNiP electrode was rich in redox pairs such as Ni2+/Ni3+, Cu+/Cu2+, Co2+/Co3+, and Mn2+/Mn3+, which are crucial for the formation of active sites for the OER and UOR, such as NiOOH, MnOOH, and CoOOH, thereby enhancing the catalytic activity. Besides, the in situ assembled CoMn/CuNiP/NF displayed highly stable performance for HER, OER, and UOR with high Faradaic efficiency for over 500 h. This research offers a simple and efficient method for manufacturing a high-efficiency and stable trifunctional electrocatalyst capable of delivering ampere-level current density in urea-assisted hydrogen production. Our density functional theory calculations reveal the potential of CoMn/CuNiP as an effective catalyst, enhancing the electronic properties and catalytic performance. The near-zero Gibbs free-energy change for HER underscores its promise, while reduced CO2 desorption energies and charge redistribution support efficient UOR. These findings signify CoMn/CuNiP's potential for electrochemical applications.

Design of a novel wastewater reuse system for space station based on urea electrooxidation technology

The replenishment of water resources in a space station is essential for achieving an astronaut's long-term and on-orbit residence. Recycling his or her urine is one of important and major ways of wastewater recovery. However, traditional urine treatment methods have the disadvantages of equipment that takes large space, complex procedures and low atomic utilization. Therefore, this paper designs a novel wastewater reuse system that treats urine based on urea electrooxidation technology, introduces methane fuel cell coupled with the Sabatier reactor and hydrogen-oxygen fuel cell to further utilize urine treatment products and to allocate water resources precisely in the space station through designing grading treatment and dynamic control systems. The wastewater reuse system designed in the paper contributes to saving valuable space and reducing energy consumption during urine treatment, thus improving water recovery rate and optimizing resource allocation.

Selectively etching manganese from nickel-manganese hydroxides via thiourea corrosion for urea electro-oxidation

Ni-based hydroxides have been extensively explored as potential catalysts for urea oxidation reaction. Defect engineering and heteroatoms doping are efficient routes to boost the catalytic activity. In this work, a facile thiourea corrosion strategy is employed to simultaneously generate vacancy defects and dope S in Ni-Mn hydroxides through one-pot ethanol solvothermal approach. Owing to the solubility difference between NiS and MnS in ethanol with thiourea, Mn species can be selectively etched from Ni-Mn hydroxides, and the corrosion degree of Ni-Mn hydroxides can be rationally regulated by variation of thiourea corrosion time. This corrosion strategy will change the morphology and structure of Ni-Mn hydroxides. Significantly, more vacancy defects can be created for Ni-Mn hydroxides after urea corrosion. These changes are beneficial for increasing the number of active sites, and the electronic states of Ni and Mn species can be also regulated. Hence, Ni-Mn hydroxides after thiourea corrosion delivers a much higher current density towards urea oxidation compared with Ni-Mn hydroxides without thiourea corrosion. The enhanced anti-poisoning capacity and accelerated charge transfer efficiency should be responsible for the boosted catalytic performance of Ni-Mn hydroxides with thiourea corrosion. This study gives a new insight to design and activate the Ni-based catalysts towards urea oxidation via creating of more vacancy defects using thiourea corrosion strategy.

Theoretical Exploration of Anchoring Type Effects of C2N Nanosheet-Supported Nickel Atoms for Urea Electrooxidation

Theoretical Exploration of Anchoring Type Effects of C₂N Nanosheet-Supported Nickel Atoms for Urea Electrooxidation

Nickel-iron in the second coordination shell boost single-atomic-site iridium catalysts for high-performance urea electrooxidation

Nickel-iron in the second coordination shell boost single-atomic-site iridium catalysts for high-performance urea electrooxidation

Self-supported Pt nanoparticles-NiFeP nanosheets arrays nanohybrid with hydrophilic surface towards urea electrolysis

Urea-assisted water electrolysis is a clean, sustainable, and energy-saving method towards hydrogen production. However, the development of earth-abundant, stable, and highly active bifunctional catalysts towards urea electrolysis is still a significant technical challenge. Herein, the highly dispersed Pt nanoparticles decorated NiFeP nanosheets arrays on nickel foam (Pt-NiFeP/NF) electrode is rationally designed, which combines the advantages of superhydrophilic interface, ultrathin nanostructure and open three-dimensional (3D) architecture. Consequently, Pt-NiFeP/NF displays a remarkable electroactivity towards both hydrogen evolution reaction (HER) and urea electrooxidation reaction (UEOR), which only needs the overpotential of 32 mV to achieve the current density of 10 mA cm−2 towards HER, and drives a current density of 50 mA cm−2 at 1.38 V towards UEOR. More importantly, the urea electrolysis system assembled with Pt-NiFeP/NF only requires a cell voltage of 1.41 V to achieve a current density of 100 mA cm−2, much lower than the traditional water electrolysis (1.94 V). This work promotes the application of Pt nanoparticles decorated bimetallic phosphides as a bifunctional catalyst towards urea-rich wastewater treatment and green electrochemical hydrogen production.

V-Doping induced surface electron modulation and nanostructure design for Ni(OH)2/GO towards efficient urea electro-oxidation.

The V-α-Ni(OH)2/GO nanoarrays prepared by simple coprecipitation show excellent catalytic properties in urea electro-oxidation, ascribed to the dual modulation of d-orbital electron regulation and ultrathin hierarchical nanostructure construction, which is caused by the introduction of V.

Porous nickel sulfide nanorods serve as a multifunctional electrocatalyst for the hydrogen evolution reaction, urea electrooxidation reaction, and nitrate reduction reaction

The proposed hard-template-high-temperature calcination method effectively maintains the morphology of the template, and the prepared NiS electrocatalyst exhibits excellent multifunctional electrocatalytic performance.

Boosted urea electrooxidation activity by dynamic steady blending CoOOH–Ni(OH)2 nanoclusters for H2 production in a pH-asymmetric electrolyzer

The blending CoOOH–Ni(OH)2 nanoclusters exhibited superior electrocatalytic activity for urea oxidation reaction in alkaline environment, achieving low onset potential of 1.24 V (vs. RHE).