Self-supported bimetallic Ni-Co compound electrodes for urea- and neutralization energy-assisted electrolytic hydrogen production

CTAB-Assisted Synthesis of FeNi Alloy Nanoparticles: Effective and Stable Electrocatalysts for Urea Oxidation Reactions

Development of highly efficient electrocatalysts for treating urea-rich wastewater is an important problem in environmental management and energy production. In this work, an iron-nickel alloy (Fe-Ni alloy) was synthesized via soft-template cetyltrimethylammonium bromide (CTAB)-assisted precipitation using low-temperature calcination. The as-synthesized nanoalloy was characterized by X-ray diffraction (XRD), which revealed the formation of a face-centered cubic (FCC) structure of the Fe-Ni alloy; field emission-scanning electron microscopic (FE-SEM) analysis revealed the spherical shape of the Fe-Ni alloy; high-resolution transmission electron microscopy (HR-TEM) revealed the average size to be ∼33.09 nm; and X-ray photoelectron spectroscopy (XPS) showed the presence of Fe, Ni, C, and O components and their chemical composition and valence states in the Fe-Ni alloy. The electrochemical urea oxidation reaction (UOR) was investigated by conducting linear sweep voltammetry (LSV) tests on the synthesized electrocatalysts with different Ni/Fe ratios in alkaline electrolytes with urea. The potential required to reach a current density of 10 mA cm-2 is 1.27 V vs RHE, which demonstrates the higher electrochemical activity of the Fe-Ni alloy compared to other individual compounds. This could be due to CTAB which improved the structural stability and synergetic and electronic effects in the nanoscale. This study will further contribute to renewable energy generation technology with long-term energy sustainability and also opens up great potential for reducing water pollution.

A universal approach to dual-metal-atom catalytic sites confined in carbon dots for various target reactions

Here, a molecular-design and carbon dot-confinement coupling strategy through the pyrolysis of bimetallic complex of diethylenetriamine pentaacetic acid under low-temperature is proposed as a universal approach to dual-metal-atom sites in carbon dots (DMASs-CDs). CDs as the "carbon islands" could block the migration of DMASs across "islands" to achieve dynamic stability. More than twenty DMASs-CDs with specific compositions of DMASs (pairwise combinations among Fe, Co, Ni, Mn, Zn, Cu, and Mo) have been synthesized successfully. Thereafter, high intrinsic activity is observed for the probe reaction of urea oxidation on NiMn-CDs. In situ and ex situ spectroscopic characterization and first-principle calculations unveil that the synergistic effect in NiMn-DMASs could stretch the urea molecule and weaken the N-H bond, endowing NiMn-CDs with a low energy barrier for urea dehydrogenation. Moreover, DMASs-CDs for various target electrochemical reactions, including but not limited to urea oxidation, are realized by optimizing the specific DMAS combination in CDs.

High-entropy NiFeCoV disulfides for enhanced alkaline water/seawater electrolysis

Creating electrocatalysts with high activity and stability to meet the needs of highly effective seawater splitting is of great importance to achieve the goal of hydrogen production from abundant seawater source, which however is still challenging owing to sluggish oxygen evolution reaction (OER) dynamics and the existed competitive chloride evolution reaction. Herein, high-entropy (NiFeCoV)S₂ porous nanosheets are uniformly fabricated on Ni foam via a hydrothermal reaction process with a sequential sulfurization step for alkaline water/seawater electrolysis. The obtained rough and porous nanosheets provide large active surface area and exposed more active sites, which can facilitate mass transfer and are conducive to the improvement of the catalytic performance. Combined with the strong synergistic electron modulation effect of multi elements in (NiFeCoV)S₂, the as-fabricated catalyst exhibits low OER overpotentials of 220 and 299 mV at 100 mA cm^-2 in alkaline water and natural seawater, respectively. Besides, the catalyst can withstand a long-term durability test for more than 50 h without hypochlorite evolution, showing excellent corrosion resistance and OER selectivity. By employing the (NiFeCoV)S₂ as the electrocatalyst for both anode and cathode to construct an overall water/seawater splitting electrolyzer, the required cell voltages are only 1.69 and 1.77 V to reach 100 mA cm^-2 in alkaline water and natural seawater, respectively, showing a promising prospect towards the practical application for efficient water/seawater electrolysis.

Construction of Nitrogen-Doped Biphasic Transition-Metal Sulfide Nanosheet Electrode for Energy-Efficient Hydrogen Production via Urea Electrolysis

Urea-assisted hybrid water splitting is a promising technology for hydrogen (H₂ ) production, but the lack of cost-effective electrocatalysts hinders its extensive application. Herein, it is reported that Nitrogen-doped Co₉ S₈ /Ni₃ S₂ hybrid nanosheet arrays on nickel foam (N-Co₉ S₈ /Ni₃ S₂ /NF) can act as an active and robust bifunctional catalyst for both urea oxidation reaction (UOR) and hydrogen evolution reaction (HER), which could drive an ultrahigh current density of 400 mA cm^-2 at a low working potential of 1.47 V versus RHE for UOR, and gives a low overpotential of 111 mV to reach 10 mA cm^-2 toward HER. Further, a hybrid water electrolysis cell utilizing the synthesized N-Co₉ S₈ /Ni₃ S₂ /NF electrode as both the cathode and anode displays a low cell voltage of 1.40 V to reach 10 mA cm^-2 , which can be powered by an AA battery with a nominal voltage of 1.5 V. The density functional theory (DFT) calculations decipher that N-doped heterointerfaces can synergistically optimize Gibbs free energy of hydrogen and urea, thus accelerating the catalytic kinetics of HER and UOR. This work significantly advances the development of the promising cobalt-nickel-based sulfide as a bifunctional electrocatalyst for energy-saving electrolytic H₂ production and urea-rich innocent wastewater treatment.

Self-propelled nanojets an interfacial Schottky junctions modulated oxygen vacancies enriched for enhanced photo-Fenton degradation of organic contaminant: Improving H2O2 generation, Fe3+/Fe2+ cycle and enhancing plant metabolism

The study reports an innovative approach on sunlit driven heterostructure photocatalytic generation of H₂O₂ and removal of cefixime. In the present work, we have fabricated Mn/Mg doped CoFe₂O₄ modified CaCr₂O₄ decorated by Ag₃PO₄ quantum dots (Ag₃PO₄ QDs), a p-n-p nano heterojunction. The study promotes the photocatalytic production of H₂O₂ and self-Fenton photocatalytic degradation of cefixime. Egg white-assisted synthesis of Mn-doped CoFe₂O₄ causes the lattice oxygen defect, which enhances the photocatalytic activity. Lattice oxygen defect enable the adsorption of O₂, which enable the conversion of •O² in the valence band of CoFe₂O₄ for the endogenous production of H₂O₂. The higher in the surface area enhance the photocatalytic activity under visible light irradiation. Mn-CoFe₂O₄-CaCr₂O₄-Ag₃PO₄ QDs enables the complete photocatalytic degradation of cefixime (99.9%) and the complete removal was determined by total organic carbon (TOC) removal and it was around 99.4%. Meanwhile the photocatalytic degradation pathway of cefixime was determined by LC-MS/MS. Reusability of the nano heterojunction was determined by six cycle test, and the reusability of the nano heterojunction was 99.8%. Further, the toxicity of the nanomaterial was studied in maize plant and the results shows that the nanoheterojunction enhances the maize growth. The study systematically reveals the robust activity of nano heterojunction for sustainable water treatment.

Surface Spin Enhanced High Stable NiCo2 S4 for Energy-Saving Production of H2 from Water/Methanol Coelectrolysis at High Current Density

Nickel based materials are promising electrocatalysts to produce hydrogen from water in alkaline media. However, the stability is of great challenge, limiting its practical material functions. Herein, a new technique for electro-deposition flower-like NiCo₂ S₄ nanosheets on carbon-cloth (CC@NiCo₂ S₄ ) is proposed for energy-saving production of H₂ from water/methanol coelectrolysis at high current density by constructing array architectures and regulating surface magnetism. The optimized and fine-tuned magnetism on the surface of the electrochemical in situ grown CC@NiCo₂ S₄ nanosheet array result in (0 1 -1) surface universally exposed, high catalytic activity for methanol electrooxidation, and long-term stability at high current density. X-ray photoelectron spectroscopy in combination of density functional theory calculations confirm the valence electron states and spin of d electrons for the surface of NiCo₂ S₄ , which enhance the surface stability of catalysts. This technology may be utilized to alter the surface magnetism and increase the stability of Ni-based electrocatalytic materials in general.

UV-Visible-Near-Infrared-Driven Photoelectrocatalytic Urea Oxidation and Photocatalytic Urea Fuel Cells Based on Ruddlensden–Popper-Type Perovskite Oxide La2NiO4

Photocatalysis and photoelectrocatalysis, as green and low-cost pollutant treatment technologies, have been widely used to simultaneously degrade pollutants and produce clean energy to solve the problems of environmental pollution and energy crisis. However, the disadvantages of photocatalysts in a narrow absorption range and low utilization rate of solar energy still hinder the practical application. Here we fabricate two-dimensional porous Ruddlensden–Popper type nickel-based perovskite oxide La2NiO4 as a noble metal-free photoanode for photoelectrocatalytic urea oxidation under full spectrum sunlight irradiation. The transient photocurrent density under near infrared (NIR) light (λ > 800 nm) can reach 50 μA cm−2. Urea wastewater was used as the fuel to obtain low-energy hydrogen production, and round-the-clock hydrogen production was achieved with the optimal yield of 22.76 μmol cm−2 h−1. Moreover, a photocatalytic urea fuel cell (PUFC) was constructed with La2NiO4 as the photoanode. The power density under UV-vis-NIR was 0.575 μW cm−2. Surprisingly, the filling factor (FF) under NIR light was 0.477, which was much higher than those under UV-vis-NIR and visible light. The results demonstrated that PUFCs constructed from low-cost nickel-based perovskite oxides have potential applications for low-energy hydrogen production and efficient utilization of sunlight.

Innovative Electrochemical Strategies for Hydrogen Production: From Electricity Input to Electricity Output

Renewable H₂ production by water electrolysis has attracted much attention due to its numerous advantages. However, the energy consumption of conventional water electrolysis is high and mainly driven by the kinetically inert anodic oxygen evolution reaction. An alternative approach is the coupling of different half-cell reactions and the use of redox mediators. In this review, we, therefore, summarize the latest findings on innovative electrochemical strategies for H₂ production. First, we address redox mediators utilized in water splitting, including soluble and insoluble species, and the corresponding cell concepts. Second, we discuss alternative anodic reactions involving organic and inorganic chemical transformations. Then, electrochemical H₂ production at both the cathode and anode, or even H₂ production together with electricity generation, is presented. Finally, the remaining challenges and prospects for the future development of this research field are highlighted.

Energy-saving H2 production from a hybrid acid/alkali electrolyzer assisted by anodic glycerol oxidation

Water electrolysis is a promising technology for efficient hydrogen production, but it has been heavily hindered by the sluggish kinetics and high potential of the anodic oxygen evolution reaction (OER). Replacing the OER with the glycerol oxidation reaction (GOR) at the anode is recognized as a potential strategy to address this issue. In this work, the self-supported electrocatalytic electrode of Cu-Cu₂O nanoclusters on carbon cloth (Cu-Cu₂O/CC) is fabricated for the electrocatalysis of the GOR, which has high activity towards the GOR, reaching 10 mA cm^-2 at an applied voltage of 1.21 V, and shows high selectivity for formate production with a faradaic efficiency (FE) of over 80% in a wide potential range. Moreover, a hybrid acid/alkali electrolyzer is assembled by coupling the Cu-Cu₂O/CC anode for the GOR in an alkaline electrolyte with commercial Pt/C as the cathode for the hydrogen evolution reaction (HER) in an acid electrolyte. The dual-electrolyte electrolytic cell only requires an applied voltage of 0.59 V to reach 10 mA cm^-2 with a FE of ∼100% for H₂ and 97% for formate production. This work provides a facile strategy for the application of glycerol upgradation in energy-saving water electrolysis systems.

Electrooxidation of Urea in Alkaline Solution Using Nickel Hydroxide Activated Carbon Paper Electrodeposited from DMSO Solution

Electrooxidation of urea plays a substantial role in the elimination of urea-containing wastewater and industrial urea. Here, we report the electrodeposition of nickel hydroxide catalyst on commercial carbon paper (CP) electrodes from dimethyl sulphoxide solvent (Ni(OH)2-DMSO/CP) for urea electrooxidation under alkaline conditions. The physicochemical features of Ni(OH)2-DMSO/CP catalysts using scanning electron microscopy and X-ray photoelectron spectroscopy revealed that the Ni(OH)2-DMSO/CP catalyst shows nanoparticle features, with loading of <1 wt%. The cyclic voltammetry and electrochemical impedance spectroscopy revealed that the Ni(OH)2-DMSO/CP electrode has a urea oxidation onset potential of 0.33 V vs. Ag/AgCl and superior electrocatalytic performance, which is a more than 2-fold higher activity in comparison with the counterpart Ni(OH)2 catalyst prepared from the aqueous electrolyte. As expected, the enhancement in electrocatalytic activity towards urea was associated with the superficial enrichment in the electrochemically active surface area of the Ni(OH)2-DMSO/CP electrodes. The results might be a promising way to activate commercial carbon paper with efficient transition metal electrocatalysts, for urea electrooxidation uses in sustainable energy systems, and for relieving water contamination.

Electrochemical neutralization energy: from concept to devices

Electrochemical neutralization energy establishes a bridge for facilitating energy conversion between some chemical energy and electric energy, shedding light on a promising strategy to develop high-performance aqueous electrochemical energy devices.

High-Voltage Rechargeable Alkali-Acid Zn-PbO2 Hybrid Battery

Aqueous rechargeable batteries have attracted attention owning to their advantages of safety, low cost, and sustainability, while the limited electrochemical stability window (1.23 V) of water leads to their failure in competition with organic-based lithium-ion batteries. Herein, we report an alkali-acid Zn-PbO₂ hybrid aqueous battery obtained by coupling an alkaline Zn anode with an acidic PbO₂ cathode. It shows the capability to deliver an impressively high open-circuit voltage (V_oc ) of 3.09 V and an operate voltage of 2.95 V at 5 mA cm^-2 , thanks to the contribution of expanding the voltage window and the electrochemical neutralization energy from the alkali-acid asymmetric-electrolyte hybrid cell. The hybrid battery can potentially deliver a large area capacity over 2 mAh cm^-2 or a high energy density of 252.39 Wh kg^-1 and shows almost no fading in area capacity over 250 charge-discharge cycles.

Efficient nanointerface hybridization in a nickel/cobalt oxide nanorod bundle structure for urea electrolysis

Urea electrolysis has received great attention for the energy-relevant applications, and efficient nanostructured catalysts are required to overcome the sluggish urea oxidation kinetics. Herein, we noticed that the valence state of Ni in the hybrid Ni/Co oxide nanorods can be correlated to the catalytic capability for urea oxidation. Crystal lattice hybridization was found in the interface of Ni/Co oxide nanoparticles that assembled as a nanorod bundle structure. The more or the less of Ni2+/Ni3+ generated lower catalytic ability, and Ni/Co oxide with the optimum content of Ni2+/Ni3+ exhibited the highest catalytic ability for urea oxidation because of the efficient synergism, resulting from the formation of high valence state of Ni species and improved kinetics. A low onset potential of 1.29 V was required for the urea oxidation compared with the high onset potential of 1.52 V for water oxidation; high selectivity for urea oxidation was found in the potential below 1.50 V, and as a promising application for urea-assisted water electrolysis about 190 mV less was required to provide 10 mA cm-2 in the two-electrode system, indicating the energy-efficient nature for hydrogen evolution. The study provides some novel insights into the Ni/Co catalyst design and fabrication with efficient catalytic synergism for electrocatalysis.

Value-Added Formate Production from Selective Methanol Oxidation as Anodic Reaction to Enhance Electrochemical Hydrogen Cogeneration

Electrolytic overall water splitting is a promising approach to produce H₂ , but its efficiency is severely limited by the sluggish kinetics of the oxygen evolution reaction (OER) and the low activity of current electrocatalysts. To solve these problems, in addition to the development of efficient precious-metal catalysts, an effective strategy is proposed to replace the OER by the selective methanol oxidation reaction. Ni-Co hydroxide [Ni_x Co_1-x (OH)₂ ] nanoarrays were obtained through a facile hydrothermal treatment as the bifunctional electrocatalysts for the co-electrolysis of methanol/water to produce H₂ and value-added formate simultaneously. The electrocatalyst could catalyze selective methanol oxidation (≈1.32 V) with a significantly lower energy consumption (≈0.2 V less) than OER. Importantly, methanol was transformed exclusively to value-added formate with a high Faradaic efficiency (selectivity) close to 100 %. Specifically, a cell voltage of only approximately 1.5 V was required to generate a current density of 10 mA cm^-2 . Furthermore, the Ni_0.33 Co_0.67 (OH)₂ /Ni foam nanoneedle arrays presented an outstanding stability for overall co-electrolysis.

Designing Advanced Catalysts for Energy Conversion Based on Urea Oxidation Reaction

Urea oxidation reaction (UOR) is the underlying reaction that determines the performance of modern urea-based energy conversion technologies. These technologies include electrocatalytic and photoelectrochemical urea splitting for hydrogen production and direct urea fuel cells as power engines. They have demonstrated great potentials as alternatives to current water splitting and hydrogen fuel cell systems with more favorable operating conditions and cost effectiveness. At the moment, UOR performance is mainly limited by the 6-electron transfer process. In this case, various material design and synthesis strategies have recently been reported to produce highly efficient UOR catalysts. The performance of these advanced catalysts is optimized by the modification of their structural and chemical properties, including porosity development, heterostructure construction, defect engineering, surface functionalization, and electronic structure modulation. Considering the rich progress in this field, the recent advances in the design and synthesis of UOR catalysts for urea electrolysis, photoelectrochemical urea splitting, and direct urea fuel cells are reviewed here. Particular attention is paid to those design concepts, which specifically target the characteristics of urea molecules. Moreover, challenges and prospects for the future development of urea-based energy conversion technologies and corresponding catalysts are also discussed.

A one-pot "shielding-to-etching" strategy to synthesize amorphous MoS2 modified CoS/Co0.85Se heterostructured nanotube arrays for boosted energy-saving H2 generation

In this study, a facile one-pot "shielding-to-etching" strategy has been designed for the synthesis of amorphous MoS2 modified CoS/Co0.85Se heterostructured nanotube arrays (a-MoS2/CoS/Co0.85Se HNTs) as a highly efficient electrocatalyst for boosted energy-saving H2 production, using Co(OH)x(CO3)y nanorods as the template. Interestingly, these multi-composition and hollow structured products were achieved in a controllable manner via only a one-pot synthesis, in which the nanorods with shielding layers were firstly obtained and then converted into nanotubes through further etching reaction. Benefiting from the combined nature of multiple-component hollow nanostructures, the electronic structure of the electrocatalyst is efficiently modulated, the electron transfer and ion diffusion pathways are effectively shortened and abundant active sites are created, which leads to excellent activity for urea oxidation and hydrogen evolution reactions with the as-prepared a-MoS2/CoS/Co0.85Se HNTs being employed as the electrodes. As a result, the whole urea electrolysis cell exhibits a driven voltage of only 1.42 V to achieve a current density of 10 mA cm-2, surpassing most reported transition-metal-based electrocatalysts. Moreover, a battery-assisted urea electrolyzer was assembled as well to demonstrate the feasibility of practical less-energy-intensive H2 generation. The method developed in this work is expected to broaden the way of designing and synthesizing multiple-component hollow nanostructures for various applications.

Recent progress with electrocatalysts for urea electrolysis in alkaline media for energy-saving hydrogen production

Urea electrolysis is a promising energy-saving avenue for hydrogen production owing to the low cell voltage, wastewater remediation and abundant electrocatalysts.

0.2 V Electrolysis Voltage-Driven Alkaline Hydrogen Production with Nitrogen-Doped Carbon Nanobowl-Supported Ultrafine Rh Nanoparticles of 1.4 nm

The development of highly effective and low-cost electrocatalysts for energy-saving hydrogen production via water splitting is still a great challenge. Herein, porous nitrogen-doped carbon nanobowls (N-CBs) have been designed and used for the controlled growth of ultrafine rhodium (Rh) nanoparticles. With the aid of interfacial bonding of Rh and N, ultrafine Rh nanoparticles with an average size of 1.4 nm have been successfully immobilized on the N-CBs. This Rh/N-CB electrocatalyst shows superior activity and high stability for the hydrogen evolution reaction (HER) and the hydrazine oxidation reaction (HzOR). More importantly, the Rh/N-CBs exhibit high activity for hydrogen production from water electrolysis, marking with a cell voltage of 0.2 V to achieve a current density of 20 mA cm^-2 when they serve as cathodic electrocatalysts for the HER and anodic electrocatalysts for the HzOR in 1 M KOH with 0.5 M hydrazine. The density functional theory calculations demonstrate that a near-zero hydrogen adsorption free energy produced by the chemical bonding of Rh with the pyrrole-N doped in N-CBs is responsible for the excellent HER activity of Rh/N-CBs electrocatalysts.

Defective NiFe2 O4 Nanoparticles for Efficient Urea Electro-oxidation

Urea is an important organic pollutants in sewage and needs to be removed for environmental protection. Here, we report defective NiFe₂ O₄ (NFO) nanoparticles with excellent performance for urea electro-oxidation. The results show that defects can be effectively implanted at the surface of NFO nanoparticles by a facile and versatile lithium reduction method without affecting its main crystal structure and grain size. The defective NFO-5Li nanoparticles displayed a significantly improved urea electro-oxidation performance compared with NFO-Pristine nanoparticles. Particularly, the NFO-Pristine and NFO-5Li show a potential of 1.398 and 1.361 V at the current density of 10 mA cm^-2 and Tafel slope of 37.3 and 31.4 mV dec^-1 , respectively. In addition, the NFO-5Li nanoparticles also revealed outstanding electrocatalytic stability. The superior performance can be attributed to the designed tunable surface defect engineering. Furthermore, the defect engineering strategy as well as the defective NFO nanoparticles hold great potential for applications in other materials and areas.

[MoS4]2--Intercalated NiCo-Layered Double Hydroxide Nanospikes: An Efficiently Synergized Material for Urine To Direct H2 Generation

Substituting the energy-uphill water oxidation half-cell with readily oxidizable urea-rich urine, a ground-breaking bridge is constructed, combining the energy-efficient hydrogen generation and environmental protection. Hence, designing a robust multifunctional electrocatalyst is desirable for widespread implementation of this waste to fuel technology. In this context, here, we report a simple tuning of the electrocatalytically favorable characteristics of NiCo-layered double hydroxide by introducing [MoS₄]^2- in its interlayer space. The [MoS₄]^2- insertion as well as its effect on the electronic structure tuning is thoroughly studied via X-ray photoelectron spectroscopy in combination with electrochemical analysis. This insertion induces overall electronic structure tuning of the hydroxide layer in such a way that the designed catalyst exhibited favorable kinetics toward all the required reactions of hydrogen generation. This is why our homemade catalyst, when utilized both as a cathode and anode to fabricate a urea electrolyzer, required a mere ∼1.37 V cell potential to generate sufficient H₂ by reaching the benchmark 10 mA cm^-2 in 1 M KOH/0.33 M urea along with long-lasting catalytic efficiency. Other indispensable reason of selecting [MoS₄]^2- is its high-valent nature making the catalyst highly selective and insensitive to common catalyst-poisoning toxins of urine. This is experimentally supported by performing the real urine electrolysis, where the nanospike-covered Ni foam-based catalyst showed a performance similar to that of synthetic urea, offering its industrial value. Other intuition of selecting [MoS₄]^2- was to provide a ligand-based mechanism for hydrogen evolution half-cell [hydrogen evolution reaction (HER)] to preclude the HER-competing oxygen reduction. Another crucial point of our work is its potential to avoid the mixing of two explosive product gases, that is, H₂ and O₂.