Electrocatalysis of nitrogen pollution: transforming nitrogen waste into high-value chemicals.

On 16 June 2023, the United Nations Environment Programme highlighted the severity of nitrogen pollution faced by humans and called for joint action for sustainable nitrogen use. Excess nitrogenous waste (NW: NO, NO2, NO2-, NO3-, etc.) mainly arises from the use of synthetic fertilisers, wastewater discharge, and fossil fuel combustion. Although the amount of NW produced can be minimised by reducing the use of nitrogen fertilisers and fossil fuels, the necessity to feed seven billion people on Earth limits the utility of this approach. Compared to current industrial processes, electrocatalytic NW reduction or CO2-NW co-reduction offers a potentially greener alternative for recycling NW and producing high-value chemicals. However, upgrading this technology to connect upstream and downstream industrial chains is challenging. This viewpoint focuses on electrocatalytic NW reduction, a cutting-edge technology, and highlights the challenges in its practical application. It also discusses future directions to meet the requirements of upstream and downstream industries by optimising production processes, including the pretreatment and supply of nitrogenous raw materials (e.g. flue gas and sewage), design and macroscopic preparation of electrocatalysts, and upscaling of reactors and other auxiliary equipment.

Strongly-Interacted NiSe2/NiFe2O4 Architectures Built Through Selective Atomic Migration as Catalysts for the Oxygen Evolution Reaction.

The interactions between the catalyst and support are widely used in many important catalytic reactions but the construction of strong interaction with definite microenvironments to understand the structure-activity relationship is still challenging. Here, strongly-interacted composites are prepared via selective exsolution of active NiSe2 from the host matrix of NiFe2O4 (S-NiSe2/NiFe2O4) taking advantage of the differences of migration energy, in which the NiSe2 possessed both high dispersion and small size. The characteristics of spatially resolved scanning transmission X-ray microscopy (STXM) coupled with analytical Mössbauer spectra for the surface and bulk electronic structures unveiled that this strongly interacted composite triggered more charge transfers from the NiSe2 to the host of NiFe2O4 while stabilizing the inherent atomic coordination of NiFe2O4. The obtained S-NiSe2/NiFe2O4 exhibits overpotentials of 290 mV at 10 mA cm-2 for oxygen evolution reaction (OER). This strategy is general and can be extended to other supported catalysts, providing a powerful tool for modulating the catalytic performance of strongly-interacted composites.

Electron Accumulation Induced by Electron Injection-Incomplete Discharge on NiFe LDH for Enhanced Oxygen Evolution Reaction.

Optimizing the local electronic structure of electrocatalysts can effectively lower the energy barrier of electrochemical reactions, thus enhancing the electrocatalytic activity. However, the intrinsic contribution of the electronic effect is still experimentally unclear. In this work, the electron injection-incomplete discharge approach to achieve the electron accumulation (EA) degree on the nickel-iron layered double hydroxide (NiFe LDH) is proposed, to reveal the intrinsic contribution of EA toward oxygen evolution reaction (OER). Such NiFe LDH with EA effect results in only 262 mV overpotential to reach 50 mA cm-2, which is 51 mV-lower compared with pristine NiFe LDH (313 mV), and reduced Tafel slope of 54.8 mV dec-1 than NiFe LDH (107.5 mV dec-1). Spectroscopy characterizations combined with theoretical calculations confirm that the EA near concomitant Vo can induce a narrower energy gap and lower thermodynamic barrier to enhance OER performance. This study clarifies the mechanism of the EA effect on OER activity, providing a direct electronic structure modulation guideline for effective electrocatalyst design.

Defects in Carbon-Based Materials for Electrocatalysis: Synthesis, Recognition, and Advances.

ConspectusOwing to climate change and over-reliance on fossil fuels, the study and development of sustainable energy is of essential importance in the next few decades. In recent years, rapid advances have been witnessed in various power to gas electrocatalysis technologies including oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER) for realizing the target of blue planet with carbon neutrality. Nevertheless, practical applications with superior performance and affordable cost are largely limited by the electrode materials because the reactions are regularly driven by precious metals such as platinum (Pt) or iridium (Ir) based catalysts. Therefore, it is of significance to develop novel electrocatalysts with high electroactivity and limited cost for boosting the commercialization of green hydrogen technology.Since nitrogen-doped carbon nanotubes were first reported for enhanced ORR performance in 2009, the exploitation of carbon-based metal-free catalysts (CMFCs) as potential replacements for the precious metal electrocatalysts has become an attractive research field. To date, great progress has been made in developing new dopant strategies for CMFCs; however, the details of the catalytic mechanism and identification of active sites remain unclear, owing to the complexity in controlling the dopants and their homogeneity in carbon-based materials. To tackle this issue, our group has presented a series of works on defects catalyzing electrochemical reactions and proposed a defect catalysis mechanism since 2015. This theory is now widely accepted by the research community and has become a very important area in electrocatalysis worldwide.In this Account, we first present the defect theory for the reasonable design of defective carbon-based materials (DCMs) and subsequently summarize our previous works on the state-of-the-art defect engineering strategies to design DCMs possessing high activity, with the particular emphasis on the conjunction between defect structures and electrochemical performances. We also categorize recent defect modulation approaches on active sites in DCMs as well as showcase the advanced characterization techniques to confirm the types and densities of defects in DCMs. Finally, several perspectives on the challenges and future research opportunities of this exciting field are proposed. Remarkably, rapid advances of DCMs possessing both high electrochemical activities and low cost as a new generation of electrode materials may greatly facilitate the deployment of sustainable energy infrastructures.

Facile and Scalable Mechanochemical Synthesis of Defective MoS2 with Ru Single Atoms Toward High-Current-Density Hydrogen Evolution.

Designing a facile strategy to prepare catalysts with highly active sites are challenging for large-scale implementation of electrochemical hydrogen production. Herein, a straightforward and eco-friendly method by high-energy mechanochemical ball milling for mass production of atomic Ru dispersive in defective MoS2 catalysts (Ru1 @D-MoS2 ) is developed. It is found that single atomic Ru doping induces the generation of S vacancies, which can break the electronic neutrality around Ru atoms, leading to an asymmetrical distribution of electrons. It is also demonstrated that the Ru1 @D-MoS2 exhibits superb alkaline hydrogen evolution enhancement, possibly attributing to this electronic asymmetry. The overpotential required to deliver a current density of 10 mA cm-2 is as low as 107 mV, which is much lower than that of commercial MoS2 (C-MoS2 , 364 mV). Further density functional theory (DFT) calculations also support that the vacancy-coupled single Ru enables much higher electronic distribution asymmetry degree, which could regulate the adsorption energy of intermediates, favoring the water dissociation and the adsorption/desorption of H*. Besides, the long-term stability test under 500 mA cm-2 further confirms the robust performance of Ru1 @D-MoS2 . Our strategy provides a promising and practical way towards large-scale preparation of advanced HER catalysts for commercial applications.

Single Carbon Vacancy Traps Atomic Platinum for Hydrogen Evolution Catalysis.

The coordinated configuration of atomic platinum (Pt) has always been identified as an active site with high intrinsic activity for hydrogen evolution reaction (HER). Herein, we purposely synthesize single vacancies in a carbon matrix (defective graphene) that can trap atomic Pt to form the Pt-C3 configuration, which gives exceptionally high reactivity for HER in both acidic and alkaline solutions. The intrinsic activity of Pt-C3 site is valued with a turnover frequency (TOF) of 26.41 s-1 and mass activity of 26.05 A g-1 at 100 mV, respectively, which are both nearly 18 times higher than those of commercial 20 wt % Pt/C. It is revealed that the optimal coordination Pt-C3 has a stronger electron-capture ability and lower Gibbs free energy difference (ΔG), resulting in promoting the reduction of adsorbed H+ and the acceleration of H2 desorption, thus exhibiting the extraordinary HER activity. This work provides a new insight on the unique coordinated configuration of dispersive atomic Pt in defective C matrix for superior HER performance.

Controlled Asymmetric Charge Distribution of Active Centers in Conjugated Polymers for Oxygen Reduction.

Active center reconstruction is essential for high performance oxygen reduction reaction (ORR) electrocatalysts. Usually, the ORR activity stems from the electronic environment of active sites by charge redistribution. We introduce an asymmetry strategy to adjust the charge distribution of active centers by designing conjugated polymer (CP) catalysts with different degrees of asymmetry. We synthesized asymmetric backbone CP (asy-PB) by modifying B←N coordination bonds and asymmetric sidechain CP (asy-PB-A) with different alkyl chain lengths. Both CPs with backbone and sidechain asymmetry exhibit superior ORR performance to their symmetric counterparts (sy-P and sy-PB). The asy-PB with greater asymmetry shows higher catalytic activity than asy-PB-A with relatively smaller asymmetry. DFT calculations reveal that the increased dipole moment and non-uniform charge distribution caused by asymmetric structure endows the center carbon atom of bipyridine with efficient catalytic activity.

Metal-Free Thiophene-Sulfur Covalent Organic Frameworks: Precise and Controllable Synthesis of Catalytic Active Sites for Oxygen Reduction.

Defective or heteroatom-doped metal-free carbon materials (MFCMs) have been regarded as efficient oxygen reduction reaction (ORR) catalysts in the past decade. However, the active centers for ORR in MFCMs are hard to confirm precisely and synthesize controllably through common methods such as high-temperature pyrolysis or heteroatom doping. To verify the precise structure acting as the active center for the ORR, we first report two crystalline metal-free thiophene-sulfur covalent organic frameworks (MFTS-COFs) as ORR catalysts. The MFTS-COFs show more positive catalytic capability than the thiophene-free COF, indicating that pentacyclic thiophene-sulfur building blocks act as active centers to induce ORR catalytic activity. MFTS-COFs with higher contents of thiophene-sulfur exhibit better ORR performance. The experimental identification is supported by density functional theory calculations. These results thus demonstrate that rational design and precise synthesis of metal-free crystalline organic materials can promote the development of new ORR catalysts.

Sulfur-Modified Oxygen Vacancies in Iron-Cobalt Oxide Nanosheets: Enabling Extremely High Activity of the Oxygen Evolution Reaction to Achieve the Industrial Water Splitting Benchmark.

The oxygen vacancies of defective iron-cobalt oxide (FeCoOx -Vo) nanosheets are modified by the homogeneously distributed sulfur (S) atoms. S atoms can not only effectively stabilize oxygen vacancies (Vo), but also form the Co-S coordination with Co active site in the Vo, which can modulate the electronic structure of the active site, enabling FeCoOx -Vo-S to exhibit much superior OER activity. FeCoOx -Vo-S exhibits a mass activity of 2440.0 A g-1 at 1.5 V vs. RHE in 1.0 m KOH, 25.4 times higher than that of RuO2 . The Tafel slope is as low as 21.0 mV dec-1 , indicative of its excellent charge transfer rate. When FeCoOx -Vo-S (anode catalyst) is paired with the defective CoP3 /Ni2 P (cathode catalyst) for overall water splitting, current densities of as high as 249.0 mA cm-2 and 406.0 mA cm-2 at a cell voltage of 2.0 V and 2.3 V, respectively, can be achieved.

Understanding the Activity of Co-N4-x Cx in Atomic Metal Catalysts for Oxygen Reduction Catalysis.

Atomic metal catalysis (AMC) provides an effective way to enhance activity for the oxygen reduction reaction (ORR). Cobalt anchored on nitrogen-doped carbon materials have been extensively reported. The carbon-hosted Co-N4 structure was widely considered as the active site; however, it is very rare to investigate the activity of Co partially coordinated with N, for example, Co-N4-x Cx . Herein, the activity of Co-N4-x Cx with tunable coordination environment is investigated as the active sites for ORR catalysis. The defect (di-vacancies) on carbon is essential for the formation of Co-N4-x Cx . N species play two important roles in promoting the intrinsic activity of atomic metal catalyst: N coordinated with Co to manipulate the reactivity by modification of electronic distribution and N helped to trap more Co to increase the number of active sites.

Defects on carbons for electrocatalytic oxygen reduction.

The exploration of highly active and durable cathodic oxygen reduction reaction (ORR) catalysts with economical production costs is still the bottleneck to realize the large-scale commercialization of fuel cells. In recent years, remarkable progress has been achieved in fabricating effective non-precious metal based ORR catalysts. In particular, modified carbon materials have aroused extensive research interest because of their excellent performance and low cost. In this review, we present an overview on recent advancements in developing defective carbon based materials for catalyzing the ORR. In particular, three general kinds of defective carbon electrocatalysts will be summarized. They are non-metal induced defective carbons (modified by heteroatoms), intrinsic defective carbons (defects created by a physical or chemical method), and atomic metal species induced/coordinated defective carbons (metal-macrocycle complexes with different coordination environments). The common configurations of various defective carbons will be discussed, with typical examples on recently developed both metal-free and precious/non-precious metal species coordinated carbons. Finally, the future research directions of the defective carbon materials are proposed. The newly established defect promoted catalysis mechanism will be beneficial for the design and fabrication of highly effective electrocatalysts for practical energy storage and conversion applications.

A Surfactant-Free and Scalable General Strategy for Synthesizing Ultrathin Two-Dimensional Metal-Organic Framework Nanosheets for the Oxygen Evolution Reaction.

Metal-organic framework (MOFs) two-dimensional (2D) nanosheets have many coordinatively unsaturated metal sites that act as active centres for catalysis. To date, limited numbers of 2D MOFs nanosheets can be obtained through top-down or bottom-up synthesis strategies. Herein, we report a 2D oxide sacrifice approach (2dOSA) to facilely synthesize ultrathin MOF-74 and BTC MOF nanosheets with a flexible combination of metal sites, which cannot be obtained through the delamination of their bulk counterparts (top-down) or the conventional solvothermal method (bottom-up). The ultrathin iron-cobalt MOF-74 nanosheets prepared are only 2.6 nm thick. The sample enriched with surface coordinatively unsaturated metal sites, exhibits a significantly higher oxygen evolution reaction reactivity than bulk FeCo MOF-74 particles and the state-of-the-art MOF catalyst. It is believed that this 2dOSA could provide a new and simple way to synthesize various ultrathin MOF nanosheets for wide applications.

Charge Polarization from Atomic Metals on Adjacent Graphitic Layers for Enhancing the Hydrogen Evolution Reaction.

Atomic metal species-based catalysts (AMCs) show remarkable possibilities in various catalytic reactions. The coordination configuration of the metal atoms has been widely recognized as the determining factor to the electronic structure and the catalytic activity. However, the synergistic effect between the adjacent layers of the multilayered AMCs is always neglected. We reported an atomic Co and Pt co-trapped carbon catalyst, which exhibits a ultrahigh activity for HER in the wide range of pH (η10 =27 and 50 mV in acidic and alkaline media, respectively) with ultralow metal loadings (1.72 and 0.16 wt % for Co and Pt, respectively), which is much superior to the commercial Pt/C. Theoretical analysis reveals that the atomic metals on the inner graphitic layers significantly alter the electronic structure of the outmost layer, thus tailoring the HER activity. This finding arouses a re-thinking of the intrinsic activity origins of AMCs and suggests a new avenue in the structure design of AMCs.

Coordination of Atomic Co-Pt Coupling Species at Carbon Defects as Active Sites for Oxygen Reduction Reaction.

Platinum (Pt) is the state-of-the-art catalyst for oxygen reduction reaction (ORR), but its high cost and scarcity limit its large-scale use. However, if the usage of Pt reduces to a sufficiently low level, this critical barrier may be overcome. Atomically dispersed metal catalysts with high activity and high atom efficiency have the possibility to achieve this goal. Herein, we report a locally distributed atomic Pt-Co nitrogen-carbon-based catalyst (denoted as A-CoPt-NC) with high activity and robust durability for ORR (267 times higher than commercial Pt/C in mass activity). The A-CoPt-NC shows a high selectivity for the 4e- pathway in ORR, differing from the reported 2e- pathway characteristic of atomic Pt catalysts. Density functional theory calculations suggest that this high activity originates from the synergistic effect of atomic Pt-Co located on a defected C/N graphene surface. The mechanism is thought to arise from asymmetry in the electron distribution around the Pt/Co metal centers, as well as the metal atoms' coordination with local environments on the carbon surface. This coordination results from N8V4 vacancies (where N8 represents the number of nitrogen atoms and V4 indicates the number of vacant carbon atoms) within the carbon shell, which enhances the oxygen reduction reaction via the so-called synergistic effect.

Activity Origins in Nanocarbons for the Electrocatalytic Hydrogen Evolution Reaction.

Sustainable hydrogen production is an essential prerequisite for realizing the future hydrogen economy. The electrocatalytic hydrogen evolution reaction (HER), as the cornerstone of exploring the mechanism of water electrolysis, has attracted extensive interest in the past decades. Carbon-based materials with significant merits such as abundance, low cost, high conductivity, and tunable molecular structures, are considered as promising candidates for replacing the commercial noble metal electrocatalysts. To date, activity origins of these carbon-based electrocatalysts are mainly attributed to the dopants (e.g., N, B, P or S), whereas the contribution of intrinsic/induced carbon defects has recently been a hot research topic. In this Review, besides the development of heteroatoms doping strategies, the latest studies on defective carbon-based materials for HER electrocatalysis are summarized, especially for various approaches to prepare defective carbons and the detailed introduction regarding the defect catalysis mechanism. Finally, an outlook into the development of future defective carbon-based HER electrocatalysts is presented.

Plasma-Triggered Synergy of Exfoliation, Phase Transformation, and Surface Engineering in Cobalt Diselenide for Enhanced Water Oxidation.

Various strategies, such as increasing active site numbers and structural and surface engineering, have been used to improve the oxygen evolution reaction (OER) performance of transition-metal dichalcogenides. However, it is challenging to combine these strategies in one system to realize the full catalytic potential. Now, an Ar/O2 plasma method is used to simultaneously induce exfoliation, surface reorganization (formation of an oxidative layer with rich oxygen vacancies), and phase transformation (cubic-to-orthorhombic) on CoSe2 to generate an exceptionally outstanding OER electrocatalysts. The as-made samples require an overpotential of only 251 mV at 10 mA cm-2 , outperforming commercial RuO2 and most reported OER catalysts. The striking catalytic activity originates from the optimized chemical and electronic environment. This work provides valuable insights into the design of promising OER electrocatalysts with high natural abundance via multilevel structural modulation.

Scalable and Cost-Effective Synthesis of Highly Efficient Fe2N-Based Oxygen Reduction Catalyst Derived from Seaweed Biomass.

A simple and scalable synthesis of a 3D Fe2N-based nanoaerogel is reported with superior oxygen reduction reaction activity from waste seaweed biomass, addressed the growing energy scarcity. The merits are due to the synergistic effect of the 3D porous hybrid aerogel support with excellent electrical conductivity, convenient mass transport and O2 adsorption, and core/shell structured Fe2N/N-doped amorphous carbon nanoparticles.

Fluorine-doped porous single-crystal rutile TiO2 nanorods for enhancing photoelectrochemical water splitting.

Fluorine-doped hierarchical porous single-crystal rutile TiO(2) nanorods have been synthesized through a silica template method, in which F(-) ions acts as both n-type dopants and capping agents to make the isotropic growth of the nanorods. The combination of high crystallinity, abundant surface reactive sites, large porosity, and improved electronic conductivity leads to an excellent photoelectrochemical activity. The photoanode made of F-doped porous single crystals displays a remarkably enhanced solar-to-hydrogen conversion efficiency (≈0.35 % at -0.33 V vs. Ag/AgCl) under 100 mW cm(-2) of AM=1.5 solar simulator illumination that is ten times of the pristine solid TiO(2) single crystals.

Density functional theory analysis of structural and electronic properties of orthorhombic perovskite CH3NH3PbI3.

The organic-inorganic hybrid perovskite CH3NH3PbI3 is a novel light harvester, which can greatly improve the solar-conversion efficiency of dye-sensitized solar cells. In this article, a first-principle theoretical study is performed using local, semi-local and non-local exchange-correlation approximations to find a suitable method for this material. Our results, using the non-local optB86b + vdWDF functional, excellently agree with the experimental data. Thus, consideration of weak van der Waals interactions is demonstrated to be important for the accurate description of the properties of this type of organic-inorganic hybrid materials. Further analysis of the electronic properties reveals that I 5p electrons can be photo-excited to Pb 6p empty states. The main interaction between the organic cations and the inorganic framework is through the ionic bonding between CH3 and I ions. Furthermore, I atoms in the Pb-I framework are found to be chemically inequivalent because of their different chemical environments.

Strengthening the Synergy between Oxygen Vacancies in Electrocatalysts for Efficient Glycerol Electrooxidation.

Defect-engineered bimetallic oxides exhibit high potential for the electrolysis of small organic molecules. However, the ambiguity in the relationship between the defect density and electrocatalytic performance makes it challenging to control the final products of multi-step multi-electron reactions in such electrocatalytic systems. In this study, controllable kinetics reduction is used to maximize the oxygen vacancy density of a Cu─Co oxide nanosheet (CuCo2O4 NS), which is used to catalyze the glycerol electrooxidation reaction (GOR). The CuCo2O4-x NS with the highest oxygen-vacancy density (CuCo2O4-x-2) oxidizes C3 molecules to C1 molecules with selectivity of almost 100% and a Faradaic efficiency of ≈99%, showing the best oxidation performance among all the modified catalysts. Systems with multiple oxygen vacancies in close proximity to each other synergistically facilitate the cleavage of C─C bonds. Density functional theory calculations confirm the ability of closely spaced oxygen vacancies to facilitate charge transfer between the catalyst and several key glycolic-acid (GCA) intermediates of the GOR process, thereby facilitating the decomposition of C2 intermediates to C1 molecules. This study reveals qualitatively in tuning the density of oxygen vacancies for altering the reaction pathway of GOR by the synergistic effects of spatial proximity of high-density oxygen vacancies.