Boosting electrocatalytic performance via electronic structure regulation for acidic oxygen evolution.

High-purity hydrogen produced by water electrolysis has become a sustainable energy carrier. Due to the corrosive environments and strong oxidizing working conditions, the main challenge faced by acidic water oxidation is the decrease in the activity and stability of anodic electrocatalysts. To address this issue, efficient strategies have been developed to design electrocatalysts toward acidic OER with excellent intrinsic performance. Electronic structure modification achieved through defect engineering, doping, alloying, atomic arrangement, surface reconstruction, and constructing metal-support interactions provides an effective means to boost OER. Based on introducing OER mechanism commonly present in acidic environments, this review comprehensively summarizes the effective strategies for regulating the electronic structure to boost the activity and stability of catalytic materials. Finally, several promising research directions are discussed to inspire the design and synthesis of high-performance acidic OER electrocatalysts.

Boosting Alkaline Seawater Oxidation of CoFe-layered Double Hydroxide Nanosheet Array by Cr Doping.

The pursuit of stable and efficient electrocatalysts toward seawater oxidation is of great interest, yet it poses considerable challenges. Herein, the utilization of Cr-doped CoFe-layered double hydroxide nanosheet array is reported on nickel-foam (Cr-CoFe-LDH/NF) as an efficient electrocatalyst for oxygen evolution reaction in alkaline seawater. The Cr-CoFe-LDH/NF catalyst can achieve current densities of 500 and 1000 mA cm -2 with remarkably low overpotentials of only 334 and 369 mV, respectively. Furthermore, it maintains at least 100 h stability when operated at 500 mA cm-2.

Fe3O4 nanoparticle-decorated 3D pinewood-derived carbon for high-efficiency electrochemical nitrate reduction to ammonia.

Electrochemical nitrate (NO3-) reduction is a sustainable pathway for ambient ammonia (NH3) synthesis while eliminating NO3- pollutants in water. However, the NO3- reduction reaction (NO3-RR) involves a complicated eight-electron transfer process, which needs highly selective and efficient electrocatalysts. This work describes the synthesis of Fe3O4 nanoparticle-decorated 3D pinewood-derived carbon (Fe3O4/PC) as a high-efficiency catalyst for the electroreduction of NO3- to NH3 at ambient reaction conditions. When tested in 0.1 M NaOH containing 0.1 M NO3-, the Fe3O4/PC obtains a large NH3 yield of 394.8 µmol h-1 cm-2 and high faradaic efficiency (FE) of 91.6% at -0.4 V. Significantly, Fe3O4/PC also delivers high stability.

3D cauliflower-like Ni foam: a high-efficiency electrocatalyst for ammonia production via nitrite reduction.

A 3D cauliflower-like Ni foam on titanium plate (Ni foam/TP) shows high electrocatalytic performance for ambient ammonia (NH3) synthesis via nitrite (NO2-) reduction. In 0.1 M phosphate-buffered saline solution with 0.1 M NO2-, such Ni foam/TP attains a high NH3 Faradaic efficiency (FE) of 95.9% and a large NH3 yield of 742.7 µmol h-1 cm-2 at -0.8 V. Its Zn-NO2- battery offers a high power density of 6.2 mW cm-2 and an NH3 FE of 90.1%.

High-Performance Electrocatalytic Reduction of Nitrite to Ammonia under Ambient Conditions on a FeP@TiO2 Nanoribbon Array.

Electrochemical nitrite (NO2-) reduction is recognized as a promising strategy for synthesizing valuable ammonia (NH3) and degrading NO2- pollutants in wastewater. The six-electron process for the NO2- reduction reaction is complex and necessitates a highly selective and stable electrocatalyst for efficient conversion of NO2- to NH3. Herein, a FeP nanoparticle-decorated TiO2 nanoribbon array on a titanium plate (FeP@TiO2/TP) is proposed as an efficient catalyst for NH3 production under ambient conditions. In 0.1 M NaOH with 0.1 M NO2-, such a FeP@TiO2/TP affords a large NH3 yield of 346.6 µmol h-1 cm-2 and a high Faradaic efficiency of 97.1%. Additionally, it demonstrates excellent stability and durability during long-term cycling tests and electrolysis experiments.

Sn-based electrocatalysts for electrochemical CO2 reduction.

Electrochemical CO2 reduction into value-added chemicals represents an attractive and promising approach to capitalize on the abundant CO2 present in the atmosphere. This reaction, however, is hampered by low energy efficiency and selectivity owing to competition from hydrogen evolution reaction and multiple-electron transfer processes. Therefore, there is a pressing need to develop efficient yet cost-effective electrocatalysts to facilitate practical applications. Sn-based electrocatalysts have gained increasing attention in this active field due to their outstanding merits such as abundance, non-toxicity, and environmental friendliness. This review provides a comprehensive overview of recent advances in Sn-based catalysts for the CO2 reduction reaction (CO2RR), beginning with a brief introduction to the CO2RR mechanism. Subsequently, the CO2RR performance of various Sn-based catalysts with different structures is discussed. The article concludes by addressing the existing challenges and offering personal perspectives on the future prospects in this exciting research area.

High-Efficiency Electroreduction of Nitrite to Ammonia on Ni Nanoparticles Strutted 3D Honeycomb-Like Porous Carbon Framework.

Electroreduction of nitrite (NO2 - ) to ammonia (NH3 ) provides a sustainable approach to yield NH3 , whilst eliminating NO2 - contaminants. In this study, Ni nanoparticles strutted 3D honeycomb-like porous carbon framework (Ni@HPCF) is fabricated as a high-efficiency electrocatalyst for selective reduction of NO2 - to NH3 . In 0.1 M NaOH with NO2 - , such Ni@HPCF electrode obtains a significant NH3 yield of 12.04 mg h-1 mgcat. -1 and a Faradaic efficiency of 95.1 %. Furthermore, it exhibits good long-term electrolysis stability.

Ni nanoparticle-decorated biomass carbon for efficient electrocatalytic nitrite reduction to ammonia.

Electrocatalytic nitrite (NO2-) reduction to ammonia (NH3) can not only synthesize value-added NH3, but also remove NO2- pollutants from the environment. However, the low efficiency of NO2--to-NH3 conversion hinders its applications. Here, Ni nanoparticle-decorated juncus-derived biomass carbon prepared at 800 °C (Ni@JBC-800) serves as an efficient catalyst for NH3 synthesis by selective electroreduction of NO2-. This catalyst shows a remarkable NH3 yield of 4117.3 µg h-1 mgcat.-1 and a large faradaic efficiency of 83.4% in an alkaline electrolyte. The catalytic mechanism is further investigated by theoretical calculations.

High-Performance Electrochemical Nitrate Reduction to Ammonia under Ambient Conditions Using a FeOOH Nanorod Catalyst.

Electrocatalytic nitrate reduction is promising as an environmentally friendly process to produce high value-added ammonia with simultaneous removal of nitrate, a widespread nitrogen pollutant, for water treatment; however, efficient electrocatalysts with high selectivity are required for ammonia formation. In this work, FeOOH nanorod with intrinsic oxygen vacancy supported on carbon paper (FeOOH/CP) is proposed as a high-performance electrocatalyst for converting nitrate to ammonia at room temperature. When operated in a 0.1 M phosphate-buffered saline (PBS) solution with 0.1 M NaNO3, FeOOH/CP is able to obtain a large NH3 yield of 2419 µg h-1 cm-2 and a surprisingly high Faradic efficiency of 92% with excellent stability. Density functional theory calculation demonstrates that the potential-determining step for nitrate reduction over FeOOH (200) is *NO2H + H+ + e- → *NO + H2O.

Recent advances in MoS2-based materials for electrocatalysis.

The increasing energy demand and related environmental issues have drawn great attention worldwide, thus necessitating the development of sustainable technologies to preserve the ecosystems for future generations. Electrocatalysts for energy-conversion reactions such as the hydrogen evolution reaction (HER), nitrogen reduction reaction (NRR), and carbon dioxide reduction reaction (CO2RR) are at the heart of these renewable energy technologies, but they suffer from sluggish kinetics due to the multistep electron and mass transfer. State-of-the-art catalysts are thus highly desired to boost the conversion efficiencies, which are still inadequate. Recently, as a typical transition metal dichalcogenide, molybdenum disulfide (MoS2) with unique physicochemical properties has been verified as a promising material for catalyzing key electrochemical reactions (i.e., HER, NRR, and CO2RR), presenting excellent performances. Therefore, in this review, we give insight into the structure and synthetic strategies of MoS2. Recent advances in MoS2-based materials for the three key electrochemical reactions are briefly summarized. Open challenges and perspectives of MoS2-based electrocatalysts toward HER, NRR, and CO2RR are also outlined.

N-doped carbon nanotubes supported CoSe2 nanoparticles: A highly efficient and stable catalyst for H2O2 electrosynthesis in acidic media.

Electrocatalytic oxygen reduction reaction (ORR) provides an attractive alternative to anthraquinone process for H2O2 synthesis. Rational design of earth-abundant electrocatalysts for H2O2 synthesis via a two-electron ORR process in acids is attractive but still very challenging. In this work, we report that nitrogen-doped carbon nanotubes as a multi-functional support for CoSe2 nanoparticles not only keep CoSe2 nanoparticles well dispersed but alter the crystal structure, which in turn improves the overall catalytic behaviors and thereby renders high O2-to-H2O2 conversion efficiency. In 0.1 M HClO4, such CoSe2@NCNTs hybrid delivers a high H2O2 selectivity of 93.2% and a large H2O2 yield rate of 172 ppm·h-1 with excellent durability up to 24 h. Moreover, CoSe2@NCNTs performs effectively for organic dye degradation via electro-Fenton process. Electronic Supplementary Material: Supplementary material (SEM images, EDX mapping images, XPS spectrum, XRD patterns, RRDE voltammogram, Tafel plots, cyclic voltammograms, UV-Vis spectra, and Tables S1) is available in the online version of this article at 10.1007/s12274-021-3474-0.

Ni2P nanosheet array for high-efficiency electrohydrogenation of nitrite to ammonia at ambient conditions.

Ammonia (NH3) plays an important role in agriculture and industry. The industry-scale production mainly depends on the Haber-Bosch process suffering from issues of environment pollution and energy consumption. Electrochemical reduction can degrade nitrite (NO2-) pollutants in the environment and convert it into more valuable NH3. Here, Ni2P nanosheet array on nickel foam is proposed as a 3D electrocatalyst for high-efficiency electrohydrogenation of NO2- to NH3 under ambient reaction conditions. When tested in 0.1 M phosphate buffer saline with 200 ppm NO2-, such Ni2P/NF is able to obtain a large NH3 yield rate of 2692.2 ± 92.1 µg h-1 cm-2 (3282.9 ± 112.3 µg h-1 mgcat.-1), a high Faradic efficiency of 90.2 ± 3.0%, and selectivity of 87.0 ± 1.7% at -0.3 V versus a reversible hydrogen electrode. After 10 h of electrocatalytic reduction, the conversion rate of NO2- achieves near 100%. The catalytic mechanism is further investigated by density functional theory calculations.

High-performance NH3 production via NO electroreduction over a NiO nanosheet array.

Electrocatalytic NO reduction controls NO emission and produces NH3 under ambient conditions. Herein, a NiO nanosheet array on titanium mesh is proposed as a highly active and selective electrocatalyst for NO reduction, attaining a faradaic efficiency of up to 90% with a NH3 yield of 2130 µg h-1 cm-2. Its aqueous Zn-NO battery can generate electricity with a power density of 0.88 mW cm-2 and simultaneously offer an NH3 yield of 228 µg h-1 cm-2. The NO electroreduction mechanism on NiO is revealed using theoretical calculations.

A Cr-FeOOH@Ni-P/NF binder-free electrode as an excellent oxygen evolution reaction electrocatalyst.

Refining the size of nanoparticles to exhibit larger specific surface areas and expose much more active sites is of great significance for enhancing the oxygen evolution reaction (OER) activity of the electrocatalyst, but still a tremendous challenge. Herein, a Cr-FeOOH@Ni2P-Ni5P4/NF (Cr-FeOOH@Ni-P/NF) catalyst was constructed by electrodepositing a layer of CrFe oxyhydroxides on the self-grown Ni-P nanoparticles, which exhibits ultrafine nanoparticles and thus superexcellent electrocatalytic OER performance. The final catalyst affords ultra-low overpotentials of 144 mV and 210 mV to achieve current densities of 10 and 50 mA cm-2, respectively. Meanwhile, it demonstrates robust stability for at least 80 hours with no activity decay. This strategy of refining nanoparticles on a three-dimensional electrode has once again been further demonstrated to be feasible and highly effective and opens a new door for the exploration of electrocatalysts with excellent comprehensive properties.

Self-supported Ni3S2@Ni2P/MoS2 heterostructures on nickel foam for an outstanding oxygen evolution reaction and efficient overall water splitting.

Hydrogen production by electrocatalytic water splitting is a pollution-free, energy-saving, and efficient method. The low efficiency of hydrogen production, high overpotentials and expensive noble-metal catalysts have limited the development of hydrogen production from electrocatalytic water splitting. Therefore, the exploration of bifunctional electrocatalysts for water overall splitting to produce hydrogen is of profound significance. Herein, Ni3S2@Ni2P/MoS2 heterostructure electrocatalysts were synthesized on Ni foam through an environmentally friendly hydrothermal method and low-temperature phosphating method. The synergistic effects between different components and the mutual substitution principle between sulfur atoms and phosphorus atoms greatly improve the OER performance of the electrocatalyst. It is also an effective strategy to optimize the adsorption energies of intermediates by the design of heterostructured catalysts composed of multiple substances. Ni3S2@Ni2P/MoS2 only requires a low overpotential (η10) of 175 mV at a current density of 10 mA cm-2 in 1.0 M KOH solution and the stable duration exceeds 40 h. In addition, this heterogeneous structure is assembled into an electrolytic cell for overall water splitting, which exhibits a low cell voltage of 1.61 volts and retains the robust stability over 30 h at 10 mA cm-2. The Ni3S2@Ni2P/MoS2 heterostructure prepared in this research provides a strategy for exploring other heterostructured electrocatalysts with different components.

Greatly Facilitated Two-Electron Electroreduction of Oxygen into Hydrogen Peroxide over TiO2 by Mn Doping.

Ambient electrochemical oxygen reduction into valuable hydrogen peroxide (H2O2) via a selective two-electron (2e-) pathway is regarded as a sustainable alternative to the industrial anthraquinone process, but it requires advanced electrocatalysts with high activity and selectivity. In this study, we report that Mn-doped TiO2 behaves as an efficient electrocatalyst toward highly selective H2O2 synthesis. This catalyst exhibits markedly enhanced 2e- oxygen reduction reaction performance with a low onset potential of 0.78 V and a high H2O2 selectivity of 92.7%, much superior to the pristine TiO2 (0.64 V, 62.2%). Additionally, it demonstrates a much improved H2O2 yield of up to 205 ppm h-1 with good stability during bulk electrolysis in an H-cell device. The significantly boosted catalytic performance is ascribed to the lattice distortion of Mn-doped TiO2 with a large amount of oxygen vacancies and Ti3+. Density functional theory calculations reveal that Mn dopant improves the electrical conductivity and reduces ΔG*OOH of pristine TiO2, thus giving rise to a highly efficient H2O2 production process.

High-efficiency nitrate electroreduction to ammonia on electrodeposited cobalt-phosphorus alloy film.

Electrocatalytic eight-electron nitrate (NO3-) reduction is a sustainable strategy to degrade NO3- and convert it into high value-added ammonia (NH3) but needs efficient catalysts with high activity and selectivity. Our study shows the use of Ti plate supported cobalt-phosphorus alloy film (Co-P/TP) as a highly active and selective electrocatalyst for ambient NO3--to-NH3 conversion. In 0.2 M Na2SO4 with 200 ppm NO3-, Co-P/TP offers an NH3 yield rate of 416.0 ± 7.2 µg h-1 cm-2 and a high faradaic efficiency of 93.6 ± 3.3% at -0.6 V and -0.3 V vs. reversible hydrogen electrode, respectively, with good durability. Noticeably, a conversion rate of 86.9% is achieved after 10 h bulk electrolysis.

High-Performance Electrochemical NO Reduction into NH3 by MoS2 Nanosheet.

Electrochemical reduction of NO not only offers an attractive alternative to the Haber-Bosch process for ambient NH3 production but mitigates the human-caused unbalance of nitrogen cycle. Herein, we report that MoS2 nanosheet on graphite felt (MoS2 /GF) acts as an efficient and robust 3D electrocatalyst for NO-to-NH3 conversion. In acidic electrolyte, such MoS2 /GF attains a maximal Faradaic efficiency of 76.6 % and a large NH3 yield of up to 99.6 µmol cm-2 h-1 . Using MoS2 nanosheet-loaded carbon paper as the cathode, a proof-of-concept device of Zn-NO battery was assembled to deliver a discharge power density of 1.04 mW cm-2 and an NH3 yield of 411.8 µg h-1 mgcat. -1 . Calculations reveal that the positively charged Mo-edge sites facilitate NO adsorption/activation via an acceptance-donation mechanism and disfavor the binding of protons and the coupling of N-N bond.

Ti2O3 Nanoparticles with Ti3+ Sites toward Efficient NH3 Electrosynthesis under Ambient Conditions.

Electrocatalytic nitrogen reduction reaction (NRR) enabled by introducing Ti3+ defect sites into TiO2 through a doping strategy has recently attracted widespread attention. However, the amount of Ti3+ ions is limited due to the low concentration of dopants. Herein, we propose Ti2O3 nanoparticles as a pure Ti3+ system that performs efficiently toward NH3 electrosynthesis under ambient conditions. This work has suggested that Ti3+ ions, as the main catalytically active sites, significantly increase the NRR activity. In an acidic electrolyte, Ti2O3 achieves extraordinary performance with a high NH3 yield and a Faradaic efficiency of 26.01 µg h-1 mg-1 cat. and 9.16%, respectively, which are superior to most titanium-based NRR catalysts recently reported. Significantly, it also demonstrates a stable NH3 yield in five consecutive cycles. Theoretical calculations uncovered that the enhanced electrocatalytic activity of Ti2O3 originated from Ti3+ active sites and significantly lowered the overpotential of the potential-determining step.

Monodisperse Cu Cluster-Loaded Defective ZrO2 Nanofibers for Ambient N2 Fixation to NH3.

Electrocatalytic nitrogen reduction to ammonia has attracted increasing attention as it is more energy-saving and eco-friendly. For this endeavor, the development of high-efficiency electrocatalysts with excellent selectivity and stability is indispensable to break up the stable covalent triple bond in nitrogen. In this study, we report monodisperse Cu clusters loaded on defective ZrO2 nanofibers for nitrogen reduction under mild conditions. Such an electrocatalyst achieves an NH3 yield rate of 12.13 µg h-1 mgcat.-1 and an optimal Faradaic efficiency of 13.4% at -0.6 V versus the reversible hydrogen electrode in 0.1 M Na2SO4. Density functional theory calculations reveal that the N2 molecule was reduced to NH3 at the Cu active site with an ideal overpotential. Meanwhile, the interaction between bonding and antibonding of the Cu-N bond promotes activation of N2 and maintains a low desorption barrier.