Advances in Carbon Nitride-Based Materials and Their Electrocatalytic Applications

Cutting-Edge PCN-ZnO Nanocomposites with Experimental and DFT Insights into Enhanced Hydrogen Evolution Reaction

Polymeric carbon nitride (PCN) and PCN-ZnO nanocomposites are promising candidates for catalysis, particularly for hydrogen evolution reactions (HER). However, their catalytic efficiency requires enhancement to fully realize their potential. This study aims to improve the HER performance of PCN by synthesizing PCN-ZnO nanocomposites using melamine as a precursor. Two synthesis methods were employed: thermal condensation (Method 1) and liquid exfoliation (Method 2). Method 1 resulted in a composite with a 2.44 eV energy gap and reduced particle size, with significantly enhanced performance as a bifunctional electrocatalyst for simultaneous hydrogen and oxygen production. In contrast, Method 2 produced a nanocomposite with an enhanced surface area and a minor alteration in the band gap. In alkaline electrolytes, the PCN-ZnO0.4 nanocomposite synthesized with Method 1 exhibited high HER performance with an overpotential of 281 mV, outperforming pristine PCN (382 mV) and ZnO (302 mV), along with improved oxygen evolution reaction (OER) activity. Further analysis in a two-electrode alkaline electrolyzer using PCN-ZnO0.4 nanocomposite as both the anode and cathode demonstrated its promise as a bifunctional electrocatalyst. Density functional theory (DFT) calculations explained the enhanced catalytic activity of the PCN-ZnO nanocomposite, confirming that hydrogen evolution occurs through the Heyrovsky process, consistent with experimental results. Notably, the solar-to-hydrogen (STH) efficiency of the PCN-ZnO nanocomposite was four times greater, at 21.7% compared to 5.2% for the PCN monolayer, underscoring its potential for efficient solar-driven hydrogen production. This work paves the way for future advancements in the design of high-performance electrocatalysts for sustainable energy applications.

Fabrication of microfibrillated cellulose from biomass by use of carbon nitride with high nitrogen/carbon ratio

Microfibrillated cellulose (MFC) as an attractive green bio-based material has attracted widespread attention in recent years due to its non-toxicity, degradability, excellent performance, and high aspect ratio. In this study, the g-C3N5 with a high nitrogen/carbon ratio was prepared as a catalyst through the self-polymerization of a nitrogen-rich precursor. The triazole groups at the edges of g-C3N5 were proven to exhibit strong adsorption to biomass and strong alkalinity. In a low-acidic aqueous system with g-C3N5, MFC with diameters of 100-200 nm and lengths up to 100 μm was fabricated from various biomasses within 5 min under microwave radiation. The ultimate yield of the MFC produced from viscose reached 90 %. Young's modulus of the MFC reaches 3.7 GPa. This work provides a particular method with high efficiency to prepare MFC with excellent properties from biomass by chemical method.

Optimizing Energy Solutions: Mott-Schottky Engineered 1D/3D CoWO4(OH)2·H2O/MoS2 Heterostructure for Advanced Energy Storage and Conversion Application

Heterostructure engineering offers a powerful approach to creating innovative electrocatalysts. By combining different materials, it can achieve synergistic effects that enhance both charge storage and electrocatalytic activity. In this work, it is capitalized on this concept by designing a 1D/3D CoWO4(OH)2·H2O/molybdenum disulfide (CTH/MoS2) heterostructure. It is achieved this by in situ depositing 3D MoS2 nanoflowers on 1D CTH nanorods. To explore the impact of precursor choice, various sulfur (S) sources is investigated. Interestingly, the S precursor influenced the dimensionality of the MoS2 component. For example, L-cysteine (L-cys), and glutathione (GSH) resulted in 0D morphologies, thiourea (TU) led to a 2D structure, and thioacetamide (TAA) yielded a desirable 3D architecture. Notably, the 1D/3D CTH/MoS2-TAA heterostructure exhibited exceptional performance in both supercapacitors (SCs) and quantum dot-sensitized solar cells (QDSSCs). This achievement can be attributed to several factors: the synergetic effect between 1D CTH and 3D MoS2, improved accessibility due to the multi-dimensional structure, and a tailored electronic structure facilitated by the Mott-Schottky (M-S) interaction arising from the different material Fermi levels. This interaction further enhances conductivity, ultimately leading to the observed high specific capacity in SCs (154.44 mAh g-1 at 3 mA cm-2) and remarkable photovoltaic efficiency in QDSSCs (6.48%).

Superlight Ambient Temperature Reversible Hydrogen Storage Media Based on Alkali and Alkali Earth Metal-Functionalized 2D Square-Octagon BCN Monolayer

Energy has always been the engine of human beings; however, because of the environmental pollution caused by traditional fossil fuels, the development of more sustainable energy resources is urgently needed. The hydrogen storage medium is essential for its realization. Here, we introduce a hydrogen storage material, a two-dimensional (2D) square-octagon BCN (SO-BCN) monolayer, composed of lightweight elements (B, C, and N) and strategically functionalized with alkali and alkali earth metal atoms (Li, Na, Mg, and K). Notably, Li@SO-BCN and Mg@SO-BCN exhibit exceptional reversible hydrogen storage capabilities, surpassing the DOE standard of 5.5 wt %, with gravimetric capacities of 7.129 wt % and an impressive value of 11.656 wt %, alongside low adsorption energies of -0.21 eV/H2 and -0.268 eV/H2 at room temperature, respectively. Our investigation, which combines analysis of the atomic structure, electronic structure, and hydrogen process, reveals distinct mechanisms at play: Li activates the substrate, creating additional adsorption sites on the SO-BCN monolayer. Compared with Li, the functionalization of Mg atoms not only activates SO-BCN via charge transfer but also allows Mg2+ to act as a potent attractor for H2 molecule adsorption. This work provides both promising candidates for future hydrogen storage media based on 2D monolayers and a design paradigm for next-generation hydrogen storage materials, emphasizing the synergy of electron-donating species, substrate activation, and hierarchical porous structures.

Semimetallic state transition in Two-Dimensional carbon nitride covalent networks and enhanced electrocatalytic activity for nitrate to ammonia conversion

Single-atom catalysts (SACs), due to their maximum atomic utilization rate, show tremendous potential for application in the electrocatalytic synthesis of ammonia from nitrate. Yet, the development of superior supports that preserve the high selectivity, activity, and stability of SACs remains an imperative challenge. In this work, based on first-principles calculations and tight-binding (TB) model analysis, a new two-dimensional (2D) carbon nitride monolayer, C7N6, is proposed. The C7N6 structure exhibits a strong covalent network, with dynamical, thermal, and mechanical stability. Surprisingly, the structural transition from C9N4 to C7N6 corresponds to a semimetallic state transition. Further symmetry analysis unveils that the Dirac states in C7N6 are protected by space-time inversion symmetry, and the physical origin of the Dirac cone was confirmed using the TB model. Additionally, a non-zero Z2 invariant and significant topological edge states demonstrate its topologically nontrivial nature. Considering the excellent structural and topological properties of C7N6, a three-step screening strategy is designed to identify eligible SACs for electrochemical nitrate reduction reaction (NO3RR), and Ti@C7N6 is identified as possessing the best activity, with the last proton-electron coupling step *NH2→*NH3 being the potential-determining step (PDS), for which the limiting potential is 0.48 V. Moreover, a free energy diagram shows that the *NOH reaction pathway is energetically preferred on Ti@C7N6, and ab initio molecular dynamics (AIMD) calculations at 500 K confirm its good thermal stability. Our study not only provides excellent CN-based support material but also offers theoretical guidance for constructing highly active and selective SACs for nitrate reduction.

Exploration of metal-free 2D electrocatalysts toward the oxygen electroreduction

The advancement of economical and readily available electrocatalysts for the oxygen reduction reaction (ORR) holds paramount importance in the advancement of fuel cells and metal-air batteries. Recently, 2D non-metallic materials have obtained substantial attention as viable alternatives for ORR catalysts due to their manifold advantages, encompassing low cost, ample availability, substantial surface-to-volume ratio, high conductivity, exceptional durability, and competitive activity. The augmented ORR performances observed in metal-free 2D materials typically arise from heteroatom doping, defects, or the formation of heterostructures. Here, the authors delve into the realm of electrocatalysts for the ORR, pivoting around metal-free 2D materials. Initially, the merits of metal-free 2D materials are explored and the reaction mechanism of the ORR is dissected. Subsequently, a comprehensive survey of diverse metal-free 2D materials is presented, tracing their evolutionary journey from fundamental concepts to pragmatic applications in the context of ORR. Substantial importance is given on the exploration of various strategies for enhancing metal-free 2D materials and assessing their impact on inherent material performance, including electronic properties. Finally, the challenges and future prospects that lie ahead for metal-free 2D materials are underscored, as they aspire to serve as efficient ORR electrocatalysts.

Highly Crystalline Poly(heptazine imide)-Based Carbonaceous Anodes for Ultralong Lifespan and Low-Temperature Sodium-Ion Batteries

Carbon nitrides with layered structures and scalable syntheses have emerged as potential anode choices for the commercialization of sodium-ion batteries. However, the low crystallinity of materials synthesized through traditional thermal condensation leads to insufficient conductivity and poor cycling stability, which significantly hamper their practical applications. Herein, a facile salt-covering method was proposed for the synthesis of highly ordered crystalline C3N4-based all-carbon nanocomposites. The sealing environment created by this strategy leads to the formation of poly(heptazine imide) (PHI), the crystalline phase of C3N4, with extended π-conjugation and a fully condensed nanosheet structure. Meanwhile, theoretical calculations reveal the high crystallinity of C3N4 significantly reduces the energy barrier for electron transition and enables the generation of efficient charge transfer channels at the heterogeneous interface between carbon and C3N4. Accordingly, such nanocomposites present ultrastable cycling performances over 5000 cycles, with a high reversible capacity of 245.1 mAh g-1 at 2 A g-1 delivered. More importantly, they also exhibit an outstanding low-temperature capacity of 196.6 mAh g-1 at -20 °C. This work offers opportunities for the energy storage use of C3N4 and provides some clues for developing long-life and high-capacity anodes operated under extreme conditions.

Covalent organic framework-derived fluorine, nitrogen dual-doped carbon as metal-free bifunctional oxygen electrocatalysts

The construction of heteroatom-doped metal-free carbon catalysts with bifunctional catalytic activity for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is highly desired for Zn-air batteries, but remains a great challenge owing to the sluggish kinetics of OER and ORR. Herein, a self-sacrificing template engineering strategy was employed to fabricate fluorine (F), nitrogen (N) co-doped porous carbon (F-NPC) catalyst by direct pyrolysis of F, N containing covalent organic framework (F-COF). The predesigned F and N elements were integrated into the skeletons of COF precursor, thus achieving uniformly distributed heteroatom active sites. The introduction of F is beneficial for the formation of edge-defects, contributing to the enhancement of the electrocatalytic activity. Attributing to the porous feature, abundant defect sites induced by F doping, as well as the strong synergistic effect between N and F atoms to afford a high intrinsic catalytic activity, the resulting F-NPC catalyst exhibits excellent bifunctional catalytic activities for both ORR and OER in alkaline mediums. Furthermore, the assembled Zn-air battery with F-NPC catalyst shows a high peak power density of 206.3 mW cm-2 and great stability, surpassing the commercial Pt/C + RuO2 catalysts.

Aminosilanized Interface Promotes Electrochemically Stable Carbon Nitride Films with Fewer Trap States on FTO for (Photo)electrochemical Systems

We have demonstrated the direct growth of a CNx layer on a plasma-cleaned and aminosilanized F-doped SnO2 (FTO) electrode to study the CNx|FTO interface that is critical for (photo)electrocatalytic systems. The (3-aminopropyl)triethoxysilane (APTES) was chosen as a bifunctional organosilane, with the amino end incorporating into CNx and the silane end connecting to the hydroxylated FTO surface. Plasma cleaning and aminosilanization resulted in a highly hydrophilic surface, which leads to better contact of melted thiourea to the aminosilanized FTO (p-FTONH2) during CNx polymer condensation, thus generating a thinner and more compact CNx layer. The modification at the interface was shown to influence the CNx growth on length scales of tens of micrometers. We grew CNx thin films on p-FTONH2 (CNx/p-FTONH2) and nonaminosilanized p-FTO (CNx/p-FTO). CNx/p-FTONH2 had a smaller density of trap states and passed 2.4 times the charges before failure compared to CNx/p-FTO. Additionally, a 40% decrease in interfacial charge transfer resistance at the CNx|electrolyte interface was measured for CNx/p-FTONH2 compared to CNx/p-FTO under -0.5 V vs RHE in 0.1 M Na2SO4. Furthermore, with the CNx surface coated with a Pt cocatalyst, Pt/CNx/p-FTONH2 exhibited faster hydrogen evolution rates and larger current densities than Pt/CNx/p-FTO. The highest Faraday efficiency toward electrochemical hydrogen evolution (FEH2) in 0.1 M Na2SO4 (pH = 7) was 46.1%, 37.3%, 57.7%, and 70.5% for Pt/CNx/p-FTONH2, Pt/CNx/p-FTO, CNx/p-FTONH2, and CNx/p-FTO, respectively. The increase in hydrogen evolution rate did not follow the magnitude of the current density change, indicating electrochemical processes other than proton reduction. Overall, we have carefully investigated the CNx|FTO interface and suggested potential solutions to make CNx films better (photo)electrodes for (photo)electrochemical systems.

Well-Defined Iron Sites in Crystalline Carbon Nitride

Carbon nitride materials can be hosts for transition metal sites, but Mössbauer studies on iron complexes in carbon nitrides have always shown a mixture of environments and oxidation states. Here we describe the synthesis and characterization of a crystalline carbon nitride with stoichiometric iron sites that all have the same environment. The material (formula C6N9H2Fe0.4Li1.2Cl, abbreviated PTI/FeCl2) is derived from reacting poly(triazine imide)·LiCl (PTI/LiCl) with a low-melting FeCl2/KCl flux, followed by anaerobic rinsing with methanol. X-ray diffraction, X-ray absorption and Mössbauer spectroscopies, and SQUID magnetometry indicate that there are tetrahedral high-spin iron(II) sites throughout the material, all having the same geometry. The material is active for electrocatalytic nitrate reduction to ammonia, with a production rate of ca. 0.1 mmol cm-2 h-1 and Faradaic efficiency of ca. 80% at -0.80 V vs RHE.

A simple and efficient process for the synthesis of 2D carbon nitrides and related materials

We describe here a new process for the synthesis of very high quality 2D Covalent Organic Frameworks (COFs), such a C2N and CN carbon nitrides. This process relies on the use of a metallic surface as both a reagent and a support for the coupling of small halogenated building blocks. The conditions of the assembly reaction are chosen so as to leave the inorganic salts by-products on the surface, to further confine the assembly reaction on the surface and increase the quality of the 2D layers. We found that under these conditions, the process directly returns few layers material. The structure/quality of these materials is demonstrated by extensive cross-characterizations at different scales, combining optical microscopy, Scanning Electron Microscopy (SEM)/Transmission Electron Microscopy (TEM) and Energy Dispersive Spectroscopy (EDS). The availability of such very large, high-quality layers of these materials opens interesting perspectives, for example in photochemistry and electronics (intrinsic transport properties, high gap substrate for graphene, etc...).

Investigation of Hydrogen and Oxygen Evolution on Cobalt-Nanoparticles-Supported Graphitic Carbon Nitride

This study focuses on fabricating cobalt particles deposited on graphitic carbon nitride (Co/gCN) using annealing, microwave-assisted and hydrothermal syntheses, and their employment in hydrogen and oxygen evolution (HER and OER) reactions. Composition, surface morphology, and structure were examined using inductively coupled plasma optical emission spectroscopy, X-ray photoelectron spectroscopy, and X-ray diffraction. The performance of Co-modified gCN composites for the HER and OER were investigated in an alkaline media (1 M KOH). Compared to the metal-free gCN, the modification of gCN with Co enhances the electrocatalytic activity towards the HER and OER. Additionally, thermal annealing of both Co(NO3)2 and melamine at 520 °C for 4 h results in the preparation of an effective bifunctional Co3O4/gCN catalyst for the HER with the lower Eonset of -0.24 V, a small overpotential of -294.1 mV at 10 mA cm-2, and a low Tafel slope of -29.6 mV dec-1 in a 1.0 M KOH solution and for the OER with the onset overpotential of 286.2 mV and overpotential of 422.3 mV to achieve a current density of 10 mA cm-2 with the Tafel slope of 72.8 mV dec-1.

Dimensionality effects of g-C3N4 from wettability to solar light assisted self-cleaning and electrocatalytic oxygen evolution reaction

Unique interfacial properties of 2D materials make them more functional than their bulk counterparts in a catalytic application. In the present study, bulk and 2D graphitic carbon nitride nanosheet (bulk g-C₃N₄ and 2D-g-C₃N₄ NS) coated cotton fabrics and nickel foam electrode interfaces have been applied for solar light-driven self-cleaning of methyl orange (MO) dye and electrocatalytic oxygen evolution reaction (OER), respectively. Compared to bulk, 2D-g-C₃N₄ coated interfaces show higher surface roughness (1.094 > 0.803) and enhanced hydrophilicity (θ ∼ 32° < 62° for cotton fabric and θ ∼ 25° < 54° for Ni foam substrate) due to oxygen defect induction as confirmed from morphological (HR-TEM and AFM) and interfacial (XPS) characterizations. The self-remediation efficiencies for blank and bulk/2D-g-C₃N₄ coated cotton fabrics are estimated through colorimetric absorbance and average intensity changes. The self-cleaning efficiency for 2D-g-C₃N₄ NS coated cotton fabric is 87%, whereas the blank and bulk-coated fabric show 31% and 52% efficiency. Liquid Chromatography-Mass Spectrometry (LC-MS) analysis determines the reaction intermediates for MO cleaning. 2D-g-C₃N₄ shows lower overpotential (108 mV) and onset potential (1.30 V) vs. RHE for 10 mA cm^-2 OER current density in 0.1 M KOH. Also, the decreased charge transfer resistance (R_CT = 12 Ω) and lower Tafel's slope (24 mV dec^-1) of 2D-g-C₃N₄ make it the most efficient OER catalyst over bulk-g-C₃N₄ and state-of-the-art material RuO₂. The pseudocapacitance behavior of OER governs the kinetics of electrode-electrolyte interaction through the electrical double layer (EDL) mechanism. The 2D electrocatalyst demonstrates long-term stability (retention ∼94%) and efficacy compared to commercial electrocatalysts.

Templated Synthesis of Exfoliated Porous Carbon with Dominant Graphitic Nitrogen

We present here a new approach for the synthesis of nitrogen-doped porous graphitic carbon (g-NC) with a stoichiometry of C6.3H3.6N1.0O1.2, using layered silicate as a hard sacrificial template. Autogenous exfoliation is achieved due to the heterostacking of 2D silicate and nitrogen-doped carbon layers. Micro- and meso-porosity is induced by melamine and cetyltrimethylammonium (C16TMA). Our density functional calculations and X-ray photoelectron spectroscopy (XPS) observations confirm that the most dominant nitrogen configuration in g-CN is graphitic, while pyridinic and pyrrolic nitrogens are thermodynamically less favored. Our large-scale lattice dynamics calculations show that surface termination with H and OH groups at pores accounts for the observed H and O in the composition of the synthesized g-NC. We further evaluate the electrocatalytic and the supercapacitance activities of g-NC. Interestingly, this material exhibits a specific capacitance of ca. 202 F g-1 at 1 A g-1, retaining 90% of its initial capacitance after 10,000 cycles.

Progresses in lignin, cellulose, starch, chitosan, chitin, alginate, and gum/carbon nanotube (nano)composites for environmental applications: A review

Water sources are becoming increasingly scarce, and they are contaminated by industrial, residential, and agricultural waste-derived organic and inorganic contaminants. These contaminants may pollute the air, water, and soil in addition to invading the ecosystem. Because carbon nanotubes (CNTs) can undergo surface modification, they can combine with other substances to create nanocomposites (NCs), including biopolymers, metal nanoparticles, proteins, and metal oxides. Furthermore, biopolymers are significant classes of organic materials that are widely used for various applications. They have drawn attention due to their benefits such as environmental friendliness, availability, biocompatibility, safety, etc. As a result, the synthesis of a composite made of CNT and biopolymers can be very effective for a variety of applications, especially those involving the environment. In this review, we reported environmental applications (including removal of dyes, nitro compounds, hazardous materialsو toxic ions, etc.) of composites made of CNT and biopolymers such as lignin, cellulose, starch, chitosan, chitin, alginate, and gum. Also, the effect of different factors such as the medium pH, the pollutant concentration, temperature, and contact time on the adsorption capacity (AC) and the catalytic activity of the composite in the reduction or degradation of various pollutants has been systematically explained.

Chitosan-supported metal nanocatalysts for the reduction of nitroaromatics

The second most abundant natural polymer in the earth's crust is chitosan (CS). The unique physical, chemical, structural, and mechanical features of this natural polymer have led to its increased application in a variety of fields such as medicine, catalysis, removal of pollutants, etc. To eliminate various pollutants, it is preferable to employ natural compounds as their use aids the removal of contaminants from the environment. Consequently, employing CS to eliminate contaminants is a viable choice. For this aim, CS can be applied as a template and support for metal nanoparticles (MNPs) and prevent the accumulation of MNPs as well as a reducing and stabilizing agent for the fabrication of MNPs. Among the pollutants present in nature, nitro compounds are an important and wide category of biological pollutants. 4-Nitrophenol (4-NP) is one of the nitro pollutants. There are different ways for the removal of 4-NP, but the best and most effective method for this purpose is the application of a metallic catalyst and a reducing agent. In this review, we report the recent developments regarding CS-supported metallic (nano)catalysts for the reduction of nitroaromatics such as nitrophenols, nitroaniline compounds, nitrobenzene, etc. in the presence of reducing agents. The metals investigated in this study include Ag, Au, Ni, Cu, Ru, Pt, Pd, etc.

Carbon-Nitride Popcorn-A Novel Catalyst Prepared by Self-Propagating Combustion of Nitrogen-Rich Triazenes

The interest in development of non-graphitic polymeric carbon nitrides (PCNs), with various C-to-N ratios, having tunable electronic, optical, and chemical properties is rapidly increasing. Here the first self-propagating combustion synthesis methodology for the facile preparation of novel porous PCN materials (PCN3-PCN7) using new nitrogen-rich triazene-based precursors is reported. This methodology is found to be highly precursor dependent, where variations in the terminal functional groups in the newly designed precursors (compounds 3-7) lead to different combustion behaviors, and morphologies of the resulted PCNs. The foam-type highly porous PCN5, generated from self-propagating combustion of 5 is comprehensively characterized and shows a C-to-N ratio of 0.67 (C₃ N_4.45 ). Thermal analyses of PCN5 formulations with ammonium perchlorate (AP) reveal that PCN5 has an excellent catalytic activity in the thermal decomposition of AP. This catalytic activity of PCN5 is further evaluated in a closer-to-application scenario, showing an increase of 18% in the burn rate of AP-Al-HTPB (with 2 wt% of PCN5) solid composite propellant. The newly developed template- and additive-free self-propagating combustion synthetic methodology using specially designed nitrogen-rich precursors should provide a novel platform for the preparation of non-graphitic PCNs with a variety of building block chemistries, morphologies, and properties suitable for a broad range of technologies.

Reversible Hydrogen Storage Media by g-CN Monolayer Decorated with NLi4: A First-Principles Study

A two-dimensional graphene-like carbon nitride (g-CN) monolayer decorated with the superatomic cluster NLi₄ was studied for reversible hydrogen storage by first-principles calculations. Molecular dynamics simulations show that the g-CN monolayer has good thermal stability at room temperature. The NLi₄ is firmly anchored on the g-CN monolayer with a binding energy of -6.35 eV. Electronic charges are transferred from the Li atoms of NLi₄ to the g-CN monolayer, mainly due to the hybridization of Li(2s), C(2p), and N(2p) orbitals. Consequently, a spatial local electrostatic field is formed around NLi₄, leading to polarization of the adsorbed hydrogen molecules and further enhancing the electrostatic interactions between the Li atoms and hydrogen. Each NLi₄ can adsorb nine hydrogen molecules with average adsorption energies between -0.152 eV/H₂ and -0.237 eV/H₂. This range is within the reversible hydrogen storage energy window. Moreover, the highest achieved gravimetric capacity is up to 9.2 wt%, which is superior to the 5.5 wt% target set by the U.S. Department of Energy. This study shows that g-CN monolayers decorated with NLi₄ are a good candidate for reversible hydrogen storage.

Computational Study on the Catalytic Performance of Single-Atom Catalysts Anchored on g-CN for Electrochemical Oxidation of Formic Acid

The electrochemical formic acid oxidation reaction (FAOR) has attracted great attention due to its high volumetric energy density and high theoretical efficiency for future portable electronic applications, for which the development of highly efficient and low-cost electrocatalysts is of great significance. In this work, taking single-atom catalysts (SACs) supported on graphitic carbon nitrides (g-CN) as potential catalysts, their catalytic performance for the FAOR was systemically explored by means of density functional theory computations. Our results revealed that the strong hybridization with the unpaired lone electrons of N atoms in the g-CN substrate ensured the high stability of these anchored SACs and endowed them with excellent electrical conductivity. Based on the computed free energy changes of all possible elementary steps, we predicted that a highly efficient FAOR could be achieved on Ru/g-CN with a low limiting potential of −0.15 V along a direct pathway of HCOOH(aq) → HCOOH* → HCOO* → CO2* → CO2(g), in which the formation of HCOO* was identified as the potential-determining step, while the rate-determining step was located at the CO2* formation, with a moderate kinetic barrier of 0.89 eV. Remarkably, the moderate d-band center and polarized charge of the Ru active site caused the Ru/g-CN catalyst to exhibit an optimal binding strength with various reaction intermediates, explaining well its superior FAOR catalytic performance. Hence, the single Ru atom anchored on g-CN could be utilized as a promising SAC for the FAOR, which opens a new avenue to further develop novel catalysts for a sustainable FAOR in formic-acid-based fuel cells.

Rational Design of Carbon Nitride Photoelectrodes with High Activity Toward Organic Oxidations

Carbon nitride (CN_x ) is a light-absorber with excellent performance in photocatalytic suspension systems, but the activity of CN_x photoelectrodes has remained low. Here, cyanamide-functionalized CN_x (^NCN CN_x ) was co-deposited with ITO nanoparticles on a 1.8 Å thick alumina-coated FTO electrode. Transient absorption spectroscopy and impedance measurements support that ITO acts as a conductive binder and improves electron extraction from the ^NCN CN_x , whilst the alumina underlayer reduces recombination losses between the ITO and the FTO glass. The Al₂ O₃ |ITO : ^NCN CN_x film displays a benchmark performance for CN_x -based photoanodes with an onset of -0.4 V vs a reversible hydrogen electrode (RHE), and 1.4±0.2 mA cm^-2 at 1.23 V vs RHE during AM1.5G irradiation for the selective oxidation of 4-methylbenzyl alcohol. This assembly strategy will improve the exploration of CN_x in fundamental and applied photoelectrochemical (PEC) studies.