Current Status and Perspectives of Dual-Atom Catalysts Towards Sustainable Energy Utilization

The exploration of sustainable energy utilization requires the imple-mentation of advanced electrochemical devices for efficient energy conversion and storage,which are enabled by the usage of cost-effective,high-performance electro-catalysts.Currently,heterogeneous atomically dispersed catalysts are considered as potential candidates for a wide range of applications.Compared to conventional cata-lysts,atomically dispersed metal atoms in carbon-based catalysts have more unsatu-rated coordination sites,quantum size effect,and strong metal-support interactions,resulting in exceptional catalytic activity.Of these,dual-atomic catalysts(DACs)have attracted extensive attention due to the additional synergistic effect between two adja-cent metal atoms.DACs have the advantages of full active site exposure,high selectiv-ity,theoretical 100%atom utilization,and the ability to break the scaling relationship of adsorption free energy on active sites.In this review,we summarize recent research advancement of DACs,which includes(1)the comprehensive understanding of the synergy between atomic pairs;(2)the synthesis of DACs;(3)characterization meth-ods,especially aberration-corrected scanning transmission electron microscopy and synchrotron spectroscopy;and(4)electrochemical energy-related applications.The last part focuses on great potential for the electrochemical catalysis of energy-related small molecules,such as oxygen reduction reaction,CO_(2) reduction reaction,hydrogen evolution reaction,and N_(2) reduction reaction.The future research challenges and opportunities are also raised in prospective section.

Porous LaFeO3 nanofiber with oxygen vacancies as an efficient electrocatalyst for N2 conversion to NH3 under ambient conditions

Electrocatalytic N2 reduction to NH3 under ambient conditions is an eco-friendly and sustainable alternative to the traditional Haber-Bosch process. However, inhibited by the high activation barrier of N2, this process needs efficient electrocatalysts to adsorb and activate the N2, enabling the N2 reduction reaction(NRR). Herein, we report that porous LaFeO3 nanofiber with oxygen vacancies acts as an efficient NRR electrocatalyst with abundant active sites to enhance the adsorption and activation of N2. When tested in 0.1 M HCl, such electrocatalyst achieves a high Faradaic efficiency of 8.77% and a large NH3 yield rate of 18.59 μg h–1 mgcat–1.at-0.55 V versus reversible hydrogen electrode. This catalyst also shows high long-term electrochemical stability and excellent selectivity for NH3 formation. Density functional theory calculations reveal that, by introducing oxygen vacancy on LaFeO3, the subsurface metallic ions are exposed with newly localized electronic states near the Fermi level, which facilitates the adsorption and activation of N2 molecules as well as the subsequent hydrogenation reactions.

Bismuth-Based Free-Standing Electrodes for Ambient-Condition Ammonia Production in Neutral Media

Electrocatalytic nitrogen reduction reaction is a carbon-free and energy-saving strategy for efficient synthesis of ammonia under ambient conditions.Here,we report the synthesis of nanosized Bi2O3 particles grown on functionalized exfoliated graphene(Bi2O3/FEG)via a facile electrochemical deposition method.The obtained free-standing Bi2O3/FEG achieves a high Faradaic efficiency of 11.2%and a large NH3 yield of 4.21±0.14μgNH3 h^-1 cm^-2 at-0.5 V versus reversible hydrogen electrode in 0.1 M Na2SO4,better than that in the strong acidic and basic media.Benefiting from its strong interaction of Bi 6p band with the N2p orbitals,binder-free characteristic,and facile electron transfer,Bi2O3/FEG achieves superior catalytic performance and excellent long-term stability as compared with most of the previous reported catalysts.This study is significant to design low-cost,high-efficient Bi-based electrocatalysts for electrochemical ammonia synthesis.

Reduced graphene oxide-based materials for electrochemical energy conversion reactions

There have been ever-growing demands to develop advanced electrocatalysts for renewable energy conversion over the past decade.As a promising platform for advanced electrocatalysts,reduced graphene oxide(rGO)has attracted substantial research interests in a variety of electrochemical energy conversion reactions.Its versatile utility is mainly attributed to unique physical and chemical properties,such as high specific surface area,tunable electronic structure,and the feasibility of structural modification and functionalization.Here,a comprehensive discussion is provided upon recent advances in the material preparation,characterization,and the catalytic activity of rGO-based electrocatalysts for various electrochemical energy conversion reactions(water splitting,CO2 reduction reaction,N2 reduction reaction,and O2 reduction reaction).Major advantages of rGO and the related challenges for enhancing their catalytic performance are addressed.

Breaking the linear correlations for enhanced electrochemical nitrogen reduction by carbon-encapsulated mixed-valence Fe_(7)(PO_(4))_(6)

Electrochemical nitrogen reduction(NRR)is deemed as a consummate answer for the traditional Haber–Bosch technology.Breaking the linear correlations between adsorption and transition-state energies of intermediates is vital to improve the kinetics of ammonia synthesis and obtain a less energy-intensive process.Herein,carbon-encapsulated mixed-valence Fe_(7)(PO_(4))_(6) was prepared and applied as an electrocatalyst for high-efficiency NRR.A dramatic faradaic efficiency(FE)of 36.93%and an NH_(3) production rate of 13.1μg h^(-1) mg_(cat)^(-1) were obtained at-0.3 V versus RHE,superior to nearly all Fe-based catalysts.Experiments and DFT calculations revealed that the superior performance was ascribed to the synergistic effect of mixed-valence iron pair,which braked the linear correlations to improve the kinetics of ammonia from collaborative hydrogenation and*NH_(3) separation.This work proves the feasibility of mixedvalence catalysts for nitrogen reduction and thus opening a new avenue towards artificial nitrogenfixation catalysts.

Main group metal elements for ambient-condition electrochemical nitrogen reduction

Electrocatalytic N_(2) reduction under ambient-condition is considered to be the most appealing strategy to the conventional Haber-Bosch process for synthetic ammonia to alleviate greenhouse emissions and reduce environmental pollution, mainly powered by renewable energy. Recent years, rapid advances have been gained in this attractive research field, and numerous electrocatalysts have been exploited. However, its conversion efficiency is still far behind the requirement of industrial applications owing to the breakage of the N≡N triple bond, which is an energetically challenging kinetically complex multistep reaction and the strong competing reaction of hydrogen evolution reaction. Recently, main group metal-based catalysts have been demonstrated promising application prospect for ammonia production, significantly boosting their further application in this field. However, a comprehensive review of main group metal-based catalysts towards electrochemical ammonia production applications is still lacking. In this review, the fundamentals of N_(2) reduction, such as the reaction pathways, the reaction potential and the challenges of N_(2) reduction have been comprehensively discussed. And then, the role, mechanism, and effect of each main group element-based catalysts used for N_(2) reduction (Li, K, Al, Ga, Sn, Sb, Bi, and their compounds) are systematically summarized. Finally, several state-of-the-art strategies to promote their NRR catalytic performance, as well as the existing problems and prospects are put forward. This review is expected to guide the design and establishment of more efficient electrocatalytic N_(2) reduction systems based on main group metal elements in the future.

Effect of nickel oxide morphology on the nitrogen electrochemical reduction reaction

In order to study the effect of catalysts’morphology on the electrochemical reduction of nitrogen gas,sample catalysts of NiO with four different morphologies(hollow spherical,sea urchin-shape,cubic block,and rod-like)were prepared.Characterization of the NiO catalysts was carried out using SEM,BET,XRD and electrochemical investigation techniques.The results indicated that the nitrogen reduction reaction(NRR)is strictly dependent on the morphology of the NiO catalysts,as the hollow spherical NiO showed the best electrochemical NRR performance of NH3 yield rate(3.21μg h^-1 mg^-1 cat.,4.1910^-11 mol cm^-2 s^-1)and Faradaic efficiency(1.37%),which was higher than the yields and efficiencies of the rod-NiO(1.8μg h^-1 mg^-1 cat.,3.2410^-11 mol cm^-2 s^-1,1.17%),sea urchin-NiO(1.66μg h^-1 mg^-1 cat.,2.4410^-11 mol cm^-2 s^-1,1.08%)and cubic block-NiO(1.32μg h^-1 mg^-1 cat.,2.1410^-11 mol cm^-2 s^-1,0.81%),respectively.These results match the order of the specific surface area of the NiO samples,with hollow spherical(113.91 m^2 g^-1)>rod-NiO(55.12 m^2 g^-1)sea urchin-NiO(55.29 m^2 g^-1)>cubic block-NiO(38.57 m^2 g^-1).This correlation can be attributed to the fact that large specific surface areas can provide more active sites for electrocatalysis.This work demonstrates the effect of the morphology of the NiO catalysts on its electrochemical NRR properties,which could offer some opportunity for the preparation of new electrode materials with improved electrocatalytic properties.

Bismuth stabilized by ZIF derivatives for electrochemical ammonia production:Proton donation effect of phosphorus dopants

N2 electroreduction reaction(NRR)offers a feasible and promising alternative for NH_(3)production by using clean energy sources.However,it is still obstructed by the pretty low NH3 yield rate and Faradaic efficiency(FE)primarily due to the undesired competing hydrogen evolution reaction and the extremely stable N≡N bond.Herein,bismuth nanoparticles were successfully embedded in N and P co-doped carbon nanoflakes(Bi/NPC)by high-temperature pyrolyzation of Bi-zeolitic imidazole frameworks(ZIF)followed by phosphorization,and used as a high-efficiency catalyst toward N2 electroreduction to NH3.In 0.1 M KHCO_(3)electrolyte,Bi/NPC exhibits excellent NRR performances,including a high NH3 yield rate of 3.12μg·h^(−1)·cm^(−2)(−0.6 V vs.reversible hydrogen electrode(RHE)),an outstanding FE of 13.58%(−0.4 V vs.RHE),and a remarkable stability up to 36 h under ambient conditions.This outstanding NRR catalytic activity is mainly attributed to the intrinsic electrocatalytic NRR activity combined with the inert hydrogen evolution reaction(HER)activity of Bi,the adsorption and activation of N2 facilitated by N dopants,as well as the superior conductivity and the large specific surface area of the two-dimensional layered carbon matrix.Notably,the hydrogen source provided by P dopant promotes the hydrogenation of the adsorbed N,which further boosts the NRR performance in alkaline electrolyte.The ultralong durability of Bi/NPC is attributed to the highly dispersed bismuth catalytic active centers confined in the skeleton of N and P co-doped carbon nanoflakes,which inhibits the agglomeration of bismuth centers.This work presents a novel avenue for designation and fabrication of high-performance Bi-based electrocatalysts for NRR.

Mn3O4 nanoparticles@reduced graphene oxide composite:An efficient electrocatalyst for artificial N2 fixation to NH3 at ambient conditions

Currently,industrial-scale NH3 production almost relies on energy-intensive Haber-Bosch process from atmospheric N2 with large amount of CO2 emission,while low-cost and high-efficient catalysts are demanded for the N2 reduction reaction (NRR).In this study,Mn3O4 nanoparticles@reduced graphene oxide (Mn3O4@rGO) composite is reported as an efficient NRR electrocatalyst with excellent selectivity for NH3 formation.In 0.1 M Na2SO4 solution,such catalyst obtains a NH3 yield of 17.4 μg·h^-1·mg^-1cat.and a Faradaic efficiency of 3.52% at-0.85 V vs.reversible hydrogen electrode.Notably,it also shows high electrochemical stability during electrolysis process.Density functional theory (DFT) calculations also demonstrate that the (112) planes of Mn3O4 possess superior NRR activity.

Aqueous electrocatalytic N2 reduction under ambient conditions

Recently, the electrochemical N2 reduction reaction （NRR） in aqueous electrolytes at ambient temperature and pressure has demonstrated its unique advantages and potentials. The reactants are directly derived from gaseous N2 and water, which are naturally abundant, and NH3 production is important for fertilizers and other industrial applications. To improve the conversion yield and selectivity （mainly competing with water reduction）, electrocatalysts must be rationally designed to optimize the mass transport, chemisorption, and transduction pathways of protons and electrons. In this review, we summarize recent progress in the electrochemical NRR. Studies of electrocatalyst designs are summarized for different categories, including metal-based catalysts, metal oxide-derived catalysts, and hybrid catalysts. Strategies for enhancing the NRR performance based on the facet orientation, metal oxide interface, crystallinity, and nitrogen vacancies are presented. Additional system designs, such as lithium-nitrogen batteries, and the solvent effect are introduced. Finally, existing challenges and prospects are discussed.

Nitrous oxide reductase gene (nosZ) and N_2O reduction along the littoral gradient of a eutrophic freshwater lake

Lake littoral zones are characterized by heterogeneity in the biogeochemistry of nutrient elements. This study aimed to explore the relationship between the nitrous oxide reductase gene （nosZ）-encoding denitrifier community composition/abundance and N2O reduction. Five samples （deep sediment, near-transition sediment, transition site, near-transition land and land soil） were collected along a littoral gradient of eutrophic Baiyangdian Lake, North China. To investigate the relationship between the nosZ-encoding denitrifier community structure and N20 reduction, the nosZ-encoding denitrifier community composition/abundance, potential denitrification rate （DNR） and potential N20 production rate （pN20） were investigated using molecular biological technologies and laboratory incubation experiments. The results showed that the average DNR of sediments was about 25 times higher than that of land soils, reaching 282.5 nmol N/（g dry weight （dw）.hr） and that the average pN20 of sediments was about 3.5 times higher than that of land soils, reaching 15.7 nmol N/（g dw-hr）. In the land area, the nosZ gene abundance showed a negative correlation with the N20/（N20＋N2） ratio, indicating that nosZ gene abundance dominated N20 reduction both in the surface soils of the land area and in the soil core of the transition site. Phylogenetic analysis showed that all the nosZ sequences recovered from sediment clustered closely with the isolates Azospirillum largimobile and Azospirillum irakense affiliated to Rhodospirillaceae in alpha-Proteobacteria, while about 92.3% （12/13） of the nosZ sequences recovered from land soil affiliated to Rhizobiaceae and Bradyrhizobiaceae in a-Proteobacteria. The community composition of nosZ gene-encoding denitrifiers appeared to be coupled with N20 reduction along the littoral gradient.

Co3(hexahydroxytriphenylene)2:A conductive metal-organic framework for ambient electrocatalytic N2 reduction to NH3

As a carbon-neutral alternative to the Haber-Bosch process,electrochemical N2 reduction enables environment-friendly NH3 synthesis at ambient conditions but needs active electrocatalysts for the N2 reduction reaction.Here,we report that conductive metal-organic framework CO3(hexahydroxytriphenylene)2(Co3 HHTP2)nanoparticles act as an efficient catalyst for ambient electrochemical N2-to-NH3 fixation.When tested in 0.5 M LiClO4,such Co3 HHTP2 achieves a large NH3 yield of 22.14μg·h^-1·mg^-1 cat.with a faradaic efficiency of 3.34%at-0.40 V versus the reversible hydrogen electrode.This catalyst also shows high electrochemical stability and excellent selectivity toward NH3 synthesis.

Nitrogen reduction reaction on small iron clusters supported by N-doped graphene:A theoretical study of the atomically precise active-site mechanism

Nonprecious metal catalysts are known of significance for electrochemical N2 reduction reaction(NRR)of which the mechanism has been illustrated by ongoing investigations of single atom catalysis.However,it remains challenging to fully understand the size-dependent synergistic effect of active sites inherited in substantial nanocatalysts.In this work,four types of small iron clusters Fen(n=1–4)supported on nitrogen-doped graphene sheets are constructed to figure out the size dependence and synergistic effect of active sites for NRR catalytic activities.It is revealed that Fe3 and Fe4 clusters on N4G supports exhibit higher NRR activity than single-iron atom and iron dimer clusters,showing lowered limiting potential and restricted hydrogen evolution reaction(HER)which is a competitive reaction channel.In particular,the Fe4-N4G displays outstanding NRR performance for“side-on”adsorption of N2 with a small limiting potential(−0.45 V).Besides the specific structure and strong interface interaction within the Fe4-N4G itself,the high NRR activity is associated with the unique bonding/antibonding orbital interactions of N-N and N-Fe for the adsorptive N2 and NNH intermediates,as well as relatively large charge transfer between N2 and the cluster Fe4-N4G.

FeOOH quantum dots decorated graphene sheet:An efficient electrocatalyst for ambient N2 reduction

Electrochemical N2 reduction offers a promising alternative to the Haber-Bosch process for sustainable NH3 synthesis at ambient conditions,but it needs efficient catalysts for the N2 reduction reaction(NRR).Here,we report that FeOOH quantum dots decorated graphene sheet acts as a superior catalyst toward enhanced electrocatalytic N2 reduction to NH3 under ambient conditions.In 0.1 M LiClO4,this hybrid attains a large NH3 yield rate and a high Faradaic efficiency of 27.3µg·h^−1·mg−1cat.and 14.6%at−0.4 V vs.reversible hydrogen electrode,respectively,rivalling the current efficiency of all Fe-based NRR electrocatalysts in aqueous media.It also shows strong durability during the electrolytic process.

Fe doping promoted electrocatalytic N2 reduction reaction of 2H MoS2

Electrocatalytic N2 reduction to ammonia is a fascinating alternative to Haber-Bosch process and also considered as an energy sto rage method.This work,Fe doped MoS2/carbon cloth(CC) has been studied on the electro-catalysis fix nitrogen indicating the doped Fe can indeed enhance the MoS2 material ability.Compared with MoS2/CC,Fe-Mo-S-3/CC not only increases 10 times in the rate of production ammonia,but also 5 times in Faraday efficiency.

Working-in-tandem mechanism of multi-dopants in enhancing electrocatalytic nitrogen reduction reaction performance of carbon- based materials

Developing carbon-based electrocatalysts with excellent N2 adsorption and activation capability holds the key to achieve highly efficient nitrogen reduction reaction(NRR)for reaching its practical application.Here,we report a highly active electrocatalyst--metal-free pyrrolic-N dominated N,S co-doped carbon(pyrr-NSC)for NRR.Based on theoretical and experimental results,it is confirmed that the N and S-dopants practice a working-in-tandem mechanism on pyrr-NSC,where the N-dopants are utilized to create electropositive C sites for enhancing N2 adsorption and the S-dopants are employed to induce electron backdonation for facilitating N2 activation.The synergistic effect of the pyrrolic-N and S-dopants can also suppress the irritating hydrogen evolution reaction,further boosting the NRR performance.This work gives an indication that the combination of two different dopants on electrocatalyst can enhance NRR performance by working in the two tandem steps-the adsorption and activation of N2 molecules,providing a new strategy for NRR electrocatalyst design.

Low-overpotential electrochemical ammonia synthesis using BiOCl-modified 2D titanium carbide MXene

Electrochemical synthesis of ammonia has the advantages of low energy consumption and promising environmental protection,as compared to the traditional Haber-Bosch process.However,the commercial utilization of this novel system is limited by the low Faradaic efficiency,poor ammonia yield and high overpotential due to the strong NN bond and the dominant competing reaction of hydrogen evolution reaction(HER).Herein,a BiOCl-modified two-dimensional(2D)titanium carbide MXenes nanocomposite(BiOCl@Ti_(3)C_(2)T_(x))is proposed as a promising electrocatalyst for ambient nitrogen(N_(2))reduction reaction with excellent catalytic performance and superior long-term stability at low overpotential.In 0.1 mol/L HCl,this catalyst attains a high Faradic efficiency of 11.98%and a NH_(3)yield of 4.06μg h^(-1)cm^(-2)at-0.10 V(vs.RHE),benefiting from its strong interaction of Bi 6p band with the N 2p orbitals,combined with its large specific surface area and the facile electron transfer.

2020 Roadmap on gas-involved photo-and electro-catalysis

Green reactions not only provide us chemical products without any pollution,but also offer us the viable technology to realize difficult tasks in normal conditions.Photo-,photoelectro-,and electrocatalytic reactions are indeed powerful tools to help us to embrace bright future.Especially,some gas-involved reactions are extremely useful to change our life environments from energy systems to liquid fuels and cost-effective products,such as H2 evolution(H2 production),02 evolution/reduction,CO2 reduction,N2 reduction(or N2 fixation) reactions.We can provide fuel cells clean H2 for electric vehicles from H2 evolution reaction(HER),at the same time,we also need highly efficient 02 reduction reaction(ORR) in fuel cells for improving the reaction kinetics.Moreover,we can get the clean oxidant O2 from water through O2 evolution reaction(OER),and carry out some reactions without posing any pollution to reaction systems.Furthermore,we can translate the greenhouse gas CO2 into useful liquid fuels through CO2 reduction reaction(CRR).Last but not the least,we can get ammonia from N2 reduction reaction(NRR),which can decrease energy input compared to the traditional Hubble process.These reactions,such as HER,ORR,OER,CRR and NRR could be realized through solar-,photoelectro-and electro-assisted ways.For them,the catalysts used play crucial roles in determining the efficiency and kinds of products,so we should consider the efficiency of catalysts.However,the cost,synthetic methods of catalysts should also be considered.Nowadays,significant progress has been achieved,however,many challenges still exist,reaction systems,catalysts underlying mechanisms,and so on.As extremely active fields,we should pay attention to them.Under the background,it has motivated us to contribute with a roadmap on ’GasInvolved Photo-and Electro-Catalysis’.

Recent advances in TiO_(2)-based catalysts for N2 reduction reaction

Nitrogen(N_(2))fixation under mild conditions is a promising approach for green production of ammonia(NH_(3)).In the past decades,various advanced catalysts have been fabricated to achieve this goal through electrocatalytic and photocat-alytic processes.Among them,the TiO_(2)-based catalysts have been recognized as promising candidates due to their high activity,low cost,chemical stabil-ity,and nontoxicity.In this review,recent advances in the fabrication of high-performance TiO_(2)-based materials for N_(2)reduction reaction(NRR)under mild conditions are summarized,including electrocatalytic and photocatalytic NRR.The design principles,synthetic strategies,and corresponding chemical/physical properties of TiO_(2)-based NRR catalysts are described in detail.Moreover,the key challenges and potential opportunities in this field are presented and discussed.