Replacing Oxygen Evolution with Hydrazine Oxidation at the Anode for Energy-Saving Electrolytic Hydrogen Production

Nitrate-coordinated FeNi(OH)2 for hydrazine oxidation assisted seawater splitting at the industrial-level current density

In this study, we developed a nitrate-coordinated iron-nickel hydroxide [NC-FeNi(OH)2] catalyst for hydrazine oxidation-assisted seawater splitting. Replacement of O2 evolution by hydrazine oxidation in a two-electrode setup resulted in a cell voltage of 1.20 V at 100 mA cm-2. This represents a voltage reduction of 470 mV compared to conventional seawater splitting. Additionally, NC-FeNi(OH)2 demonstrated remarkable stability over a period of 60 hours.

Hydrogen Production via Electrolysis of Wastewater

The high energy consumption of traditional water splitting to produce hydrogen is mainly due to complex oxygen evolution reaction (OER), where low-economic-value O2 gas is generated. Meanwhile, cogeneration of H2 and O2 may result in the formation of an explosive H2/O2 gas mixture due to gas crossover. Considering these factors, a favorable anodic oxidation reaction is employed to replace OER, which not only reduces the voltage for H2 production at the cathode and avoids H2/O2 gas mixture but also generates value-added products at the anode. In recent years, this innovative strategy that combines anodic oxidation for H2 production has received intensive attention in the field of electrocatalysis. In this review, the latest research progress of a coupled hydrogen production system with pollutant degradation/upgrading is systematically introduced. Firstly, wastewater purification via anodic reaction, which produces free radicals instead of OER for pollutant degradation, is systematically presented. Then, the coupled system that allows for pollutant refining into high-value-added products combined with hydrogen production is displayed. Thirdly, the photoelectrical system for pollutant degradation and upgrade are briefly introduced. Finally, this review also discusses the challenges and future perspectives of this coupled system.

Adsorption Site Regulations of [W-O]-Doped CoP Boosting the Hydrazine Oxidation-Coupled Hydrogen Evolution at Elevated Current Density

Hydrazine oxidation reaction (HzOR) assisted hydrogen evolution reaction (HER) offers a feasible path for low power consumption to hydrogen production. Unfortunately however, the total electrooxidation of hydrazine in anode and the dissociation kinetics of water in cathode are critically depend on the interaction between the reaction intermediates and surface of catalysts, which are still challenging due to the totally different catalytic mechanisms. Herein, the [W-O] group with strong adsorption capacity is introduced into CoP nanoflakes to fabricate bifunctional catalyst, which possesses excellent catalytic performances towards both HER (185.60 mV at 1000 mA cm-2) and HzOR (78.99 mV at 10,00 mA cm-2) with the overall electrolyzer potential of 1.634 V lower than that of the water splitting system at 100 mA cm-2. The introduction of [W-O] groups, working as the adsorption sites for H2O dissociation and N2H4 dehydrogenation, leads to the formation of porous structure on CoP nanoflakes and regulates the electronic structure of Co through the linked O in [W-O] group as well, resultantly boosting the hydrogen production and HzOR. Moreover, a proof-of-concept direct hydrazine fuel cell-powered H2 production system has been assembled, realizing H2 evolution at a rate of 3.53 mmol cm-2 h-1 at room temperature without external electricity supply.

Electro-Synthesis of Organic Compounds with Heterogeneous Catalysis

Electro-organic synthesis has attracted a lot of attention in pharmaceutical science, medicinal chemistry, and future industrial applications in energy storage and conversion. To date, there has not been a detailed review on electro-organic synthesis with the strategy of heterogeneous catalysis. In this review, the most recent advances in synthesizing value-added chemicals by heterogeneous catalysis are summarized. An overview of electrocatalytic oxidation and reduction processes as well as paired electrocatalysis is provided, and the anodic oxidation of alcohols (monohydric and polyhydric), aldehydes, and amines are discussed. This review also provides in-depth insight into the cathodic reduction of carboxylates, carbon dioxide, CC, C≡C, and reductive coupling reactions. Moreover, the electrocatalytic paired electro-synthesis methods, including parallel paired, sequential divergent paired, and convergent paired electrolysis, are summarized. Additionally, the strategies developed to achieve high electrosynthesis efficiency and the associated challenges are also addressed. It is believed that electro-organic synthesis is a promising direction of organic electrochemistry, offering numerous opportunities to develop new organic reaction methods.

Double Active Sites in Co-Nx-C@Co Electrocatalysts for Simultaneous Production of Hydrogen and Carbon Monoxide

The hydrogen evolution reaction (HER) by electrocatalytic water splitting is a prospective and economical route. However, the approach is severely hindered by the sluggish anodic OER, poor reactivity of electrocatalysts, and low-value-added byproducts at the anode. Herein, formaldehyde was added as an anode sacrificial agent, and a bifunctional Co-N_x-C@Co catalyst containing abundant Co-N₄ sites and Co nanoparticles was successfully fabricated and evaluated as both a cathodic and an anodic material for the HER and formaldehyde selective oxidation reaction (FSOR), respectively. Co-N_x-C@Co displayed a remarkable electrocatalytic performance simultaneously for both HER and FSOR with high hydrogen (H₂) and carbon monoxide (CO) selectivity. Density functional theory calculations combined with experiments identified that Co-N₄ and Co nanoparticles were dominating active sites for CO and H₂ generation, respectively. The coupling tactic of FSOR at the anode not only expedites the reaction rate of HER but also offers a high-efficiency and energy-saving means for the generation of valuable H₂/CO syngas.

Bifunctional single-atomic Mn sites for energy-efficient hydrogen production

The electrocatalytic hydrogen evolution reaction (HER) for H2 production is essential for future renewable and clean energy technology. Screening energy-saving, low-cost, and highly active catalysts efficiently, however, is still a grand challenge due to the sluggish kinetics of the oxygen evolution reaction (OER) in electrolyzing water. Herein, we present a single atomic Mn site anchored on a boron nitrogen co-doped carbon nanotube array (Mn-SA/BNC), which is perfectly combined with the hydrazine electrooxidation reaction (HzOR) boosted water electrolysis concept. The obtained catalyst achieves 51 mV overpotential at the current density of -10 mA cm-2 for the cathodic HER and 132 mV versus the reversible hydrogen electrode for HzOR, respectively. Besides, in a two-electrode overall hydrazine splitting (OHzS) system, the Mn-SA/BNC catalyst only needs a cell voltage of only 0.41 V to output 10 mA cm-1, with strong durability and nearly 100% faradaic efficiency for H2 production. This work highlights a low-cost and high-efficiency energy-saving H2 production pathway.

Vanadium Substitution Steering Reaction Kinetics Acceleration for Ni3N Nanosheets Endows Exceptionally Energy-Saving Hydrogen Evolution Coupled with Hydrazine Oxidation

Designing highly active transition-metal-based electrocatalysts for energy-saving electrochemical hydrogen evolution coupled with hydrazine oxidation possesses more economic prospects. However, the lack of bifunctional electrocatalysts and the absence of intrinsic structure-property relationship research consisting of adsorption configurations and dehydrogenation behavior of N₂H₄ molecules still hinder the development. Now, a V-doped Ni₃N nanosheet self-supported on Ni foam (V-Ni₃N NS) is reported, which presents excellent bifunctional electrocatalytic performance toward both hydrazine oxidation reaction (HzOR) and hydrogen evolution reaction (HER). The resultant V-Ni₃N NS achieves an ultralow working potential of 2 mV and a small overpotential of 70 mV at 10 mA cm^-2 in alkaline solution for HzOR and HER, respectively. Density functional theory calculations reveal that the vanadium substitution could effectively modulate the electronic structure of Ni₃N, therefore facilitating the adsorption/desorption behavior of H* for HER, as well as boosting the dehydrogenation kinetics for HzOR.

Mimicking Hydrazine Dehydrogenase for Efficient Electrocatalytic Oxidation of N2H4 by Fe-NC

Pursuing nonprecious doped carbon with Pt-like electrocatalytic N₂H₄ oxidation activity for hydrazine fuel cells (HzFCs) remains a challenge. Herein, we present a Fe/N-doped carbon (Fe-NC) catalyst with mesopore-rich channel and highly dispersed Fe-N sites incorporated in N-doped carbon, as an analogue of hydrazine dehydrogenase (HDH), showing the structure-dependent activity for electrocatalytic oxidation of N₂H₄. The maximal turnover frequency of the N₂H₄ oxidation reaction (HzOR) over the Fe-N sites (62870 h^-1) is 149-fold that over the pyridinic-N sites of N-doped carbon. The Fe mass activity of HzOR and maximal power density of HzFCs driven by Fe-NC approximately surpass those of Pt/C by 2.3 and 2.2 times, respectively. Theoretical calculation reveals that the Fe-N sites improve the dehydrogenation process of HzOR-related intermediates. One of the roles of the mesoporous structure in Fe-NC resembles that of a substrate channel in HDH for enhancing the transport of N₂H₄ besides exposing Fe-N sites and improving storage capacity of HzOR-related species.

A Metal-Free Electrode: From Biomass-Derived Carbon to Hydrogen

Hydrogen is the emission-free fuel of the future if produced from non-fossil sources. Biomass gasification or electrolysis of water are possible clean routes. For a global application, the material solution for the electrodes must be sustainable, scalable, and relatively inexpensive compared to the current precious metal-based electrodes. A key requirement to sustainable and green energy systems is that all harmful or rare resources utilized in the process must be part of a closed material cycle. Here, a carbon-based electrode for hydrogen production is presented that can be part of a closed material cycle if produced from biomass. Continuous hydrogen production takes place at the cathode through catalytic water splitting during the oxygen evolution reaction (OER), while intentionally allowing the decomposition of the electrode into CO2 analogous to the process of natural biomass decomposition. This strategy of a sacrificial electrode could provide a scalable and low-cost material solution for hydrogen production from renewable energy sources. The theoretical and technical feasibility of using carbon to produce hydrogen is demonstrated, and it is shown that chemical modification can further improve the performance characteristics towards the catalytic process. Combined with renewable energy derived electricity, this idea offers a real option for future energy systems.

Single transition metal atoms anchored on a C2N monolayer as efficient catalysts for hydrazine electrooxidation

Searching for highly-efficient and low-cost electrocatalysts for the hydrazine oxidation reaction (HzOR) is a key issue in the development of direct hydrazine fuel cells for hydrogen production, which is a promising energy-efficient conversion technology to replace the sluggish oxygen evolution reaction in water splitting. Herein, the potential of a series of single transition metal atoms anchored on nitrogenated holey graphene (TM@C₂N, TM = Ti, Mn, Fe, Co, Ni, Cu, Mo, Rh, Ru, Pd, Pt, Au, Ag, and W) as catalysts for the HzOR was systematically explored by means of comprehensive density functional theory (DFT) computations. Our results revealed that these TM atoms anchored on a C₂N monolayer exhibit high stability due to their strong interactions with the N atoms on the C₂N monolayer. Furthermore, on the basis of the computed free energy profiles, Ru@C₂N, Mo@C₂N, Ti@C₂N, Co@C₂N, and Fe@C₂N were shown to display high HzOR catalytic activity due to their lower (or comparable) limiting potential compared to the well-established Fe-doped CoS₂ nanosheet. In particular, Ru@C₂N is identified as the best catalyst with the lowest limiting potential of -0.24 V due to its optimum difference between the adsorption strength of N₂H₃* and N₂H₂* species. More interestingly, we found that single Mo and Ti atoms also exhibit excellent catalytic performance for the hydrogen evolution reaction, suggesting their bifunctional activity towards hydrazine splitting for H₂ production. Our findings provide a new avenue to develop an efficient single-atom electrocatalyst for experimental validation to convert hydrazine into hydrogen.

Defective crystalline molybdenum phosphides as bifunctional catalysts for hydrogen evolution and hydrazine oxidation reactions during water splitting

Defect-rich crystalline molybdenum phosphide nanoparticles anchored on reduced graphene oxide serve as an efficient bifunctional electrocatalyst for both hydrogen evolution and hydrazine oxidation reactions.

Electrochemical nitrogen fixation and utilization: theories, advanced catalyst materials and system design

Design and synthesis of advanced nanomaterials towards electrocatalytic nitrogen reduction and transformation are concluded from both structural and compositional aspects.

Nitrogen-Doped CoP Electrocatalysts for Coupled Hydrogen Evolution and Sulfur Generation with Low Energy Consumption

Hydrogen production is the key step for the future hydrogen economy. As a promising H₂ production route, electrolysis of water suffers from high overpotentials and high energy consumption. This study proposes an N-doped CoP as the novel and effective electrocatalyst for hydrogen evolution reaction (HER) and constructs a coupled system for simultaneous hydrogen and sulfur production. Nitrogen doping lowers the d-band of CoP and weakens the H adsorption on the surface of CoP because of the strong electronegativity of nitrogen as compared to phosphorus. The H adsorption that is close to thermos-neutral states enables the effective electrolysis of the HER. Only -42 mV is required to drive a current density of -10 mA cm^-2 for the HER. The oxygen evolution reaction in the anode is replaced by the oxidation reaction of Fe²⁺ , which is regenerated by a coupled H₂ S absorption reaction. The coupled system can significantly reduce the energy consumption of the HER and recover useful sulfur sources.

Electrocatalytic and photocatalytic hydrogen evolution integrated with organic oxidation

We have summarized the recent progress in electrocatalytic and photocatalytic water splitting integrated with organic oxidation for efficient H₂ generation, which features no formation of explosive H₂/O₂ mixtures and reactive oxygen species, higher efficiency compared to conventional water splitting and potential co-production of value-added organic products.

A nickel–borate–phosphate nanoarray for efficient and durable water oxidation under benign conditions

A nickel–borate–phosphate nanoarray (Ni–Bi–Pi/CC) topotactically converted from a nickel phosphide nanoarray (Ni₂P/CC) acts as a durable catalyst electrode for water oxidation needing an overpotential of 440 mV to drive 10 mA cm⁻² in 0.1 M K–Bi (pH: 9.2).

Hydrazine-assisted electrolytic hydrogen production: CoS2nanoarray as a superior bifunctional electrocatalyst

A CoS₂nanoarray on Ti mesh acts as an efficient and durable catalyst for the hydrazine oxidation reaction and it only needs 0.81 V to attain 100 mA cm⁻²in 1.0 M KOH with 100 mM hydrazine for its two-electrode electrolyser.

Integrating natural biomass electro-oxidation and hydrogen evolution: using a porous Fe-doped CoP nanosheet array as a bifunctional catalyst

An Fe-doped CoP nanosheet array acts as an efficient catalyst electrode for aloe extract oxidation reaction (AOR) and hydrogen evolution reaction (HER). Its two-electrode alkaline electrolyzer requires 1.51 V for 20 mA cm⁻².