Unveiling the Electrooxidation of Urea: Intramolecular Coupling of the N−N Bond

H2O2 treatment boosts activity of NiFe layered double hydroxide for electro-catalytic oxidation of urea

Urea oxidation reaction (UOR) provides a method for hydrogen production besides wastewater treatment, but the current limited catalytic activity has prevented the application. Herein, we develop a novel H₂O₂ treatment strategy for tailoring the surface oxygen ligand of NiFe-layered double hydroxides (NiFe-LDH). The sample after H₂O₂ treatment (NiFeO-LDH) shows significant enhancement on UOR efficiency, with the potential of 1.37 V (RHE) to reach a current density of 10 mA/cm². The boost is attributed to the richness adsorption O ligand on NiFeO-LDH as revealed by XPS and Raman analysis. DFT calculation indicates formation of two possible types of oxygen ligands: adsorbed oxygen on the surface and exposed from hydroxyl group, lowered the desorption energy of CO₂ product, which lead to the lowered onset potential. This strategy is further extended to NiFe-LDH nano sheet on Ni foam to reach a higher current density of 440 mA/cm² of UOR at 1.8 V (RHE). The facile surface O ligand manipulation is also expected to give chance to many other electro-catalytic oxidations.

Phase Segregation in Cu0.5 Ni0.5 Alloy Boosting Urea-Assisted Hydrogen Production in Alkaline Media

Coupling urea oxidation reaction (UOR) and hydrogen evolution reaction (HER) is promising for energy-efficient hydrogen production. However, developing cheap and highly active bifunctional electrocatalysts for overall urea electrolysis remains challenging. In this work, a metastable Cu_0.5 Ni_0.5 alloy is synthesized by a one-step electrodeposition method. It only requires the potentials of 1.33 and -28 mV to obtain the current density of ±10 mA cm^-2 for UOR and HER, respectively. The metastable alloy is considered to be the main reason causing the above excellent performances. In the alkaline medium, the as-prepared Cu_0.5 Ni_0.5 alloy exhibits good stability for HER; and conversely, NiOOH species can be rapidly formed during the UOR due to the phase segregation of Cu_0.5 Ni_0.5 alloy. In particular, for the energy-saving hydrogen generation system coupled with HER and UOR, only 1.38 V of voltage is needed at 10 mA cm^-2 ; and at 100 mA cm^-2 , the voltage decreases by ≈305 mV compared with that of the routine water electrolysis system (HER || OER). Compared with some catalysts reported recently, the Cu_0.5 Ni_0.5 catalyst owns superior electrocatalytic activity and durability. Furthermore, this work provides a simple, mild, and rapid method for designing highly active bifunctional electrocatalysts toward urea-supporting overall water splitting.

Progress in Hydrogen Production Coupled with Electrochemical Oxidation of Small Molecules

The electrochemical oxidation of small molecules to generate value-added products has gained enormous interest in recent years because of the advantages of benign operation conditions, high conversion efficiency and selectivity, the absence of external oxidizing agents, and eco-friendliness. Coupling the electrochemical oxidation of small molecules to replace oxygen evolution reaction (OER) at the anode and the hydrogen evolution reaction (HER) at the cathode in an electrolyzer would simultaneously realize the generation of high-value chemicals or pollutant degradation and the highly efficient production of hydrogen. This Minireview presents an introduction on small-molecule choice and design strategies of electrocatalysts as well as recent breakthroughs achieved in the highly efficient production of hydrogen. Finally, challenges and future orientations are highlighted.

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.

Surface Activation and Ni-S Stabilization in NiO/NiS2 for Efficient Oxygen Evolution Reaction

Manipulating the active species and improving the structural stabilization of sulfur-containing catalysts during the OER process remain a tremendous challenge. Herein, we constructed NiO/NiS₂ and Fe-NiO/NiS₂ as catalyst models to study the effect of Fe doping. As expected, Fe-NiO/NiS₂ exhibits a low overpotential of 270 mV at 10 mA cm^-2 . The accumulation of hydroxyl groups on the surface of materials after Fe doping can promote the formation of highly active NiOOH at a lower OER potential. Moreover, we investigated the level of corrosion of M-S bonds and compared the stability variation of M-S bonds with Fe at different locations. Interestingly, Fe bonded with S in the bulk as the sacrificial agent can alleviate the oxidation corrosion of partial Ni-S bonds and thus endow Fe-NiO/NiS₂ long-term durability. This work could motivate the community to focus more on resolving the corrosion of sulfur-containing materials.

Coupling Glucose-Assisted Cu(I)/Cu(II) Redox with Electrochemical Hydrogen Production

Water electrolysis is a sustainable technology for hydrogen production since this process can utilize the intermittent electricity generated by renewable energy such as solar, wind, and hydro. However, the large-scale application of this process is restricted by the high electricity consumption due to the large potential gap (>1.23 V) between the anodic oxygen evolution reaction and the cathodic hydrogen evolution reaction (HER). Herein, a novel and efficient hydrogen production system is developed for coupling glucose-assisted Cu(I)/Cu(II) redox with HER. The onset potential of the electrooxidation of Cu(I) to Cu(II) is as low as 0.7 V_RHE (vs reversible hydrogen electrode). In situ Raman spectroscopy, ex situ X-ray photoelectron spectroscopy, and density functional theory calculation demonstrates that glucose in the electrolyte can reduce the Cu(II) into Cu(I) instantaneously via a thermocatalysis process, thus completing the cycle of Cu(I)/Cu(II) redox. The assembled electrolyzer only requires a voltage input of 0.92 V to achieve a current density of 100 mA cm^-2 . Consequently, the electricity consumption for per cubic H₂ produced in the system is 2.2 kWh, only half of the value for conventional water electrolysis (4.5 kWh). This work provides a promising strategy for the low-cost, efficient production of high-purity H₂ .

Coupling Electrocatalytic Nitric Oxide Oxidation over Carbon Cloth with Hydrogen Evolution Reaction for Nitrate Synthesis

NO is a harmful pollutant to the environment. The traditional removal of NO is hindered by the harsh operating conditions and sacrifice of value-added chemicals. Efficient electrocatalytic oxidation of NO was achieved over plasma-treated commercial carbon cloth, serving as a promising anode substitution reaction to couple with the hydrogen evolution reaction without consumption of hydrogen-containing resources. The introduction of carboxyl groups onto the carbon cloth boosted the electrocatalytic activity via the enhancement of NO chemisorption. Only potentials of 1.39 V and 1.07 V were applied to reach the current density of 10 mA cm^-2 in neutral and acidic conditions, respectively, which is superior to the state-of-the-art electrocatalysts for oxygen evolution. Energy and environmental concerns on fossil-fuel-derived hydrogen production, ammonia manufacture and nitrate synthesis, are greatly alleviated. This work provides an original strategy to realize the resource utilization of NO, the sustainable nitrate synthesis and hydrogen production in a green and economical way.

Platinum Modulates Redox Properties and 5-Hydroxymethylfurfural Adsorption Kinetics of Ni(OH)2 for Biomass Upgrading

Nickel hydroxide (Ni(OH)₂ ) is a promising electrocatalyst for the 5-hydroxymethylfurfural oxidation reaction (HMFOR) and the dehydronated intermediates Ni(OH)O species are proved to be active sites for HMFOR. In this study, Ni(OH)₂ is modified by platinum to adjust the electronic structure and the current density of HMFOR improves 8.2 times at the Pt/Ni(OH)₂ electrode compared with that on Ni(OH)₂ electrode. Operando methods reveal that the introduction of Pt optimized the redox property of Ni(OH)₂ and accelerate the formation of Ni(OH)O during the catalytic process. Theoretical studies demonstrate that the enhanced Ni(OH)O formation kinetics originates from the reduced dehydrogenation energy of Ni(OH)₂ . The product analysis and transition state simulation prove that the Pt also reduces adsorption energy of HMF with optimized adsorption behavior as Pt can act as the adsorption site of HMF. Overall, this work here provides a strategy to design an efficient and universal nickel-based catalyst for HMF electro-oxidation, which can also be extended to other Ni-based catalysts such as Ni(HCO₃ )₂ and NiO.

Deciphering and Suppressing Over-Oxidized Nitrogen in Nickel-Catalyzed Urea Electrolysis

Urea electrolysis is a prospective technology for simultaneous H₂ production and nitrogen suppression in the process of water being used for energy production. Its sustainability is currently founded on innocuous N₂ products; however, we discovered that prevalent nickel-based catalysts could generally over-oxidize urea into NO₂ ^- products with ≈80 % Faradaic efficiencies, posing potential secondary hazards to the environment. Trace amounts of over-oxidized NO₃ ^- and N₂ O were also detected. Using ¹⁵ N isotopes and urea analogues, we derived a nitrogen-fate network involving a NO₂ ^- -formation pathway via OH^- -assisted C-N cleavage and two N₂ -formation pathways via intra- and intermolecular coupling. DFT calculations confirmed that C-N cleavage is energetically more favorable. Inspired by the mechanism, a polyaniline-coating strategy was developed to locally enrich urea for increasing N₂ production by a factor of two. These findings provide complementary insights into the nitrogen fate in water-energy nexus systems.

Accelerating Hydrogen Evolution by Anodic Electrosynthesis of Value-Added Chemicals in Water over Non-Precious Metal Electrocatalysts

Integrating electrolytic hydrogen production from water with thermodynamically more favorable aqueous organic oxidation reactions is highly desired, because it can enhance the energy conversion efficiency in relation to traditional water electrolysis, and produce value-added chemicals instead of oxygen at the anode. In this Minireview, we introduce some key considerations for anodic auxiliary electrosynthesis and outline three types of electrocatalytic organic reactions including biomass derivative, alcohol and amine oxidation reactions, which can boost cathodic hydrogen generation. Furthermore, frequently used noble-metal-free electrocatalysts are classified into nickel-based, cobalt-based, other transition-metal-based and bimetallic electrocatalysts. The preparation methods of these catalysts and their performance towards electrochemical oxidation reactions are also discussed in detail. We specifically highlight the importance of redox active sites on the surface of the electrocatalysts, which act as electron mediators to promote oxidation reactions. Finally, the current challenges and future developments in this emerging field are also discussed.

Understanding the Roles of Electrogenerated Co3+ and Co4+ in Selectivity-Tuned 5-Hydroxymethylfurfural Oxidation

The Co-based electrocatalyst is among the most promising candidates for electrochemical oxidation of 5-hydroxymethylfurfural (HMF). However, the intrinsic active sites and detailed mechanism of this catalyst remains unclear. We combine experimental evidence and a theoretical study to show that electrogenerated Co³⁺ and Co⁴⁺ species act as chemical oxidants but with distinct roles in selective HMF oxidation. It is found that Co³⁺ is only capable of oxidizing formyl group to produce carboxylate while Co⁴⁺ is required for the initial oxidation of hydroxyl group with significantly faster kinetics. As a result, the product distribution shows explicit dependence on the Co oxidation states and selective production of 5-hydroxymethyl-2-furancarboxylic acid (HMFCA) and 2,5-furandicarboxylic acid (FDCA) are achieved by tuning the applied potential. This work offers essential mechanistic insight on Co-catalyzed organic oxidation reactions and might guide the design of more efficient electrocatalysts.