Two-dimensional iron-phthalocyanine (Fe-Pc) monolayer as a promising single-atom-catalyst for oxygen reduction reaction: a computational study

First-principle calculations study of the ORR/OER electrocatalytic activity of ruthenium polyphthalocyanine axially modified with aliphatic thiol groups

In this study, the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalytic activity of ruthenium polyphthalocyanine axially modified with different aliphatic thiol groups, RuPPc-SR (SR = -SCH3, -SC2H5, -SC3H7, -SC4H9, -SC5H11, and -SC6H13), in an acidic medium were simulated using DFT. All -SR groups can effectively enhance the ORR and OER catalytic activities of RuPPc. The ORR and OER overpotentials of RuPPc-SC4H9 are 0.237 V and 0.436 V, respectively, which are far lower than those of RuPPc (0.960 V and 0.903 V). For RuPPc-SC4H9, the four C and S atoms of the -SC4H9 chain and Ru atom are coplanar, and thus, conjugate effects and inductive effects exist between the -SC4H9 chain and Ru atom. This makes the Ru atom exhibit the least positive Bader charge and smallest spin density, and the anti-bonding orbitals of dxz, dyz, and dz2 of the Ru atom shift below the Fermi level (Ef). This makes the adsorption strength of RuPPc-SC4H9 toward ORR and OER intermediates the weakest, which accelerates the reaction process, thus resulting in better ORR and OER catalytic activity. Therefore, the introduction of the aliphatic thiol groups might effectively improve the OER/ORR catalytic activity of RuPPc.

Regulating the frontier orbital of iron phthalocyanine with nitrogen doped carbon nanosheets for improving oxygen reduction activity

Iron phthalocyanine (FePc) has attracted widespread attention for its tunable electronic structure. However, the Fe-N sites suffer from undesirable oxygen reduction activity due to the symmetric geometries. A suitable substrate was thus needed to induce electron redistribution around Fe-N to improve the activity. Herein, ultrathin nitrogen-doped carbon nanosheets (N-CNSs) were prepared by a simple high temperature pyrolysis. Then iron phthalocyanine was loaded on the ultrathin nitrogen-doped carbon nanosheets (FePc@N-CNSs) by a low-cost and simple solution method. This composite catalyst shows an excellent ORR activity with a half potential of 0.88 V, an onset potential of 0.99 V and durability superior to commercial Pt/C. When used as an air cathode catalyst for rechargeable zinc-air batteries, FePc@N-CNS modified batteries outperform Pt/C + RuO2 modified batteries with higher power density and superior constant current charge-discharge cycling stability of 37 hours. The regulated electronic structure of FePc by the N-CNS substrate was revealed further by DFT calculations, which explained the enhanced adsorption of the active center to the intermediates and the increased ORR performance.

Strontium doped Fe-based porous carbon for highly efficient electrocatalytic ORR and MOR reactions

The catalytic activity improvement of Fe-based active sites derived from metal organic frameworks toward oxygen reduction reaction (ORR) and methanol oxidation reaction (MOR) remains a major challenge. In this study, the growth of strontium decorated 2-methylimidazole zinc salt (Sr/ZIF-8) is prepared as a carrier to vapor deposited iron formation Sr doped Fe-based nitrogen-doped carbon framework (named as Sr/FeNC). After high-temperature pyrolysis and vapor deposition, strontium carbonate nanocrystals are evenly dispersed on the shrunk dodecahedron carbon frame and multitudinous Fe-based active catalytic sites are embedded in carbon skeleton. The optimal Sr/FeNC-2 catalyst demonstrates the outstanding ORR performance in terms of a half-wave potential of 0.851 V and an onset potential of 0.90 V, while Sr/FeNC-2 exhibits a high current density of 18.2 mA cm-2 and a lower Tafel slope of 21 mV dec-1 in MOR. The exceptional catalytic activity could be ascribed to the synergistic coupling effect of strontium compounds with Fe-based catalytic sites (Fe-Nx, Fe, and iron oxide). In particular, the formation of SrCO3 affects the bonding configuration of the iron species sites, leading to an optimization of the electronic structure within the multihole carbon matrix. The synthetic approach presents a prospective strategy for future endeavors in developing innovative and advanced bifunctional catalysts for ORR and MOR.

Modulating the Structure and Composition of Single-Atom Electrocatalysts for CO2 reduction

Electrochemical CO2 reduction reaction (eCO2 RR) is a promising strategy to achieve carbon cycling by converting CO2 into value-added products under mild reaction conditions. Recently, single-atom catalysts (SACs) have shown enormous potential in eCO2 RR due to their high utilization of metal atoms and flexible coordination structures. In this work, the recent progress in SACs for eCO2 RR is outlined, with detailed discussions on the interaction between active sites and CO2 , especially the adsorption/activation behavior of CO2 and the effects of the electronic structure of SACs on eCO2 RR. Three perspectives form the starting point: 1) Important factors of SACs for eCO2 RR; 2) Typical SACs for eCO2 RR; 3) eCO2 RR toward valuable products. First, how different modification strategies can change the electronic structure of SACs to improve catalytic performance is discussed; Second, SACs with diverse supports and how supports assist active sites to undergo catalytic reaction are introduced; Finally, according to various valuable products from eCO2 RR, the reaction mechanism and measures which can be taken to improve the selectivity of eCO2 RR are discussed. Hopefully, this work can provide a comprehensive understanding of SACs for eCO2 RR and spark innovative design and modification ideas to develop highly efficient SACs for CO2 conversion to various valuable fuels/chemicals.

Theoretical Study on ORR/OER Bifunctional Catalytic Activity of Axial Functionalized Iron Polyphthalocyanine

Designing efficient ORR/OER bifunctional electrocatalysts is very significant for reducing energy consumption and environmental protection. Hence, we studied the ORR/OER bifunctional catalytic activity of iron polyphthalocyanine (FePPc) coordinated by a series of axial ligands which has different electronegative coordination atom (FePPc-L) (L = -CN, -SH, -SCH3, -SC2H5, -I, -Br, -NH2, -Cl, -OCH3, -OH, and -F) in alkaline medium by DFT calculations. Among all FePPc-L, FePPc-CN, FePPc-SH, FePPc-SCH3, and FePPc-SC2H5 exhibit excellent ORR/OER bifunctional catalytic activities. Their ORR/OER overpotential is 0.256 V/0.234 V, 0.278 V/0.256 V, 0.280 V/0.329 V, and 0.290 V/0.316 V, respectively, which are much lower than that of the FePPc (0.483 V/0.834 V). The analysis of the electronic structure of the above catalysts shows that the electronegativity of the coordination atoms in the axial ligand is small, resulting in less distribution of dz2, dyz, and dxz orbitals near Ef, weak orbital polarization, small charge and magnetic moment of the central Fe atom, and weak adsorption strength for *OH. All these prove that the introduction of axial ligands with appropriate electronegativity coordinating atoms can adjust the adsorption of catalyst to intermediates and modify the ORR/OER bifunctional catalytic activities. This is an effective strategy for designing efficient ORR/OER bifunctional electrocatalysts.

Electrocatalytic Urea Synthesis via N2 Dimerization and Universal Descriptor

Electrocatalytic urea synthesis through N2 + CO2 coreduction and C-N coupling is a promising and sustainable alternative to harsh industrial processes. Despite considerable efforts, limited progress has been made due to the challenges of breaking inert N≡N bonds for C-N coupling, competing side reactions, and the absence of theoretical principles guiding catalyst design. In this study, we propose a mechanism for highly electrocatalytic urea synthesis using two adsorbed N2 molecules and CO as nitrogen and carbon sources, respectively. This mechanism circumvents the challenging step of N≡N bond breaking and selective CO2 to CO reduction, as the free CO molecule inserts into dimerized *N2 and binds concurrently with two N atoms, forming a specific urea precursor *NNCONN* with both thermodynamic and kinetic feasibility. Through the proposed mechanism, Ti2@C4N3 and V2@C4N3 are identified as highly active catalysts for electrocatalytic urea formation, exhibiting low onset potentials of -0.741 and -0.738 V, respectively. Importantly, taking transition metal atoms anchored on porous graphite-like carbonitride (TM2@C4N3) as prototypes, we introduce a simple descriptor, namely, effective d electron number (Φ), to quantitatively describe the structure-activity relationships for urea formation. This descriptor incorporates inherent atomic properties of the catalyst, such as the number of d electrons, the electronegativity of the metal atoms, and the generalized electronegativity of the substrate atoms, making it potentially applicable to other urea catalysts. Our work advances the comprehension of mechanisms and provides a universal guiding principle for catalyst design in urea electrochemical synthesis.

Interface engineering of transition metal-nitrogen-carbon by graphdiyne for boosting the oxygen reduction/evolution reactions: A computational study

Exploring high-efficiency electrocatalysts to boost the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is pivotal to the large-scale applications for clean and renewable energy technologies, such as fuel cells, water splitting, and metal-air batteries. Herein, by means of density functional theory (DFT) computations, we proposed a strategy to modulate the catalytic activity of transition metal-nitrogen-carbon catalysts through their interface engineering with graphdiyne (TMNC/GDY). Our results revealed that these hybrid structures exhibit good stability and excellent electrical conductivity. Especially, CoNC/GDY was identified as a promising bifunctional catalyst for ORR/OER with rather low overpotentials in acidic conditions according to the constant-potential energy analysis. Moreover, the volcano plots were established to describe the activity trend of the ORR/OER on TMNC/GDY using the adsorption strength of the oxygenated intermediates. Remarkably, the d-band center and charge transfer of the TM active sites can be utilized to correlate the ORR/OER catalytic activity and their electronic properties. Our findings not only suggested an ideal bifunctional oxygen electrocatalyst, but also provided a useful strategy to obtain highly efficient catalysts by interface engineering of two-dimensional heterostructures.

A facile synthesis of FeS/Fe3C nanoparticles highly dispersed on in situ grown N-doped CNTs as cathode electrocatalysts for microbial fuel cells

A novel composite of iron sulfide, iron carbide and nitrogen carbides (Nano-FeS/Fe3C@NCNTs) as a cathode electrocatalyst for microbial fuel cells (MFCs) is synthesized by a one-pot solid state reaction, which yields a unique configuration of FeS/Fe3C nanoparticles highly dispersed on in situ grown nitrogen-doped carbon nanotubes (NCNTs). The highly dispersed FeS/Fe3C nanoparticles possess large active sites, while the NCNTs provide an electronically conductive network. Consequently, the resultant Nano-FeS/Fe3C@NCNTs exhibit excellent electrocatalytic activity towards the oxygen reduction reaction (ORR), with a half-wave potential close to that of Pt/C (about 0.88 V vs. RHE), and enable MFCs to deliver a power density of 1.28 W m-2 after two weeks' operation, which is higher than that of MFCs with Pt/C as the cathode electrocatalyst (1.02 W m-2). Theoretical calculations and experimental data demonstrate that there is a synergistic effect between Fe3C and FeS in Nano-FeS/Fe3C@NCNTs. Fe3C presents a strong attraction and electron-donating tendency to oxygen molecules, serving as the main active component, while FeS reduces charge transfer resistance by transferring electrons to Fe3C, synergistically improving the kinetics of the ORR and power density of MFCs.

Carbon skeleton materials derived from rare earth phthalocyanines (MPcs) (M = Yb, La) used as high performance anode materials for lithium-ion batteries

In this work, novel carbon skeleton materials were prepared by high-temperature carbonization of rare earth phthalocyanines (MPcs) (M = Yb, La) under a nitrogen atmosphere. The resulting carbon materials of YbPc-900 (carbonisation temperature of 900 °C for 2 h) and LaPc-1000 (carbonization temperature of 1000 °C for 2 h) have a graphite-layered structure in predominantly ordered states, with a smaller particle size, a larger specific surface area and a higher degree of hard carbonization compared to those of the uncarbonized sample. As a result, the batteries using the YbPc-900 and LaPc-1000 carbon skeleton materials as electrodes display excellent energy storage behaviors. The initial capacities of the YbPc-900 and LaPc-1000 electrodes at 0.05 A g^-1 were 1100 and 850 mA h g^-1, respectively. After 245 cycles and 223 cycles, the capacities remain at 780 and 716 mA h g^-1 with retention ratios of 71% and 84%. At a high rate of 1.0 A g^-1, the initial capacities of the YbPc-900 and LaPc-1000 electrodes were 400 and 520 mA h g^-1, respectively, and after 300 cycles, the capacities can still remain at 526 and 587 mA h g^-1 with retention ratios of 131.5% and 112.8%, respectively, which were much higher than those of the pristine rare earth phthalocyanine (MPc) (M = Yb, La) electrodes. Moreover, better rate capabilities were also observed during the YbPc-900 and LaPc-1000 electrode tests. The capacities of the YbPc-900 electrode at 0.05, 0.1, 0.2, 0.5, 1 and 2C were 520, 450, 407, 350, 300 and 260 mA h g^-1 respectively, which were higher than those of the YbPc electrode (550, 450, 330, 150, 90 and 40 mA h g^-1). Similarly, the rate performance of the LaPc-1000 electrode at different rates was also significantly improved compared to that of the pristine LaPc electrode. In addition, the initial Coulomb efficiencies of the YbPc-900 and LaPc-1000 electrodes were also greatly improved compared to those of pristine YbPc and LaPc electrodes. After carbonization, the YbPc-900 and LaPc-1000 carbon skeleton materials derived from rare earth phthalocyanines (MPcs) (M = Yb, La) exhibit improved energy storage behaviors, which would provide new ideas for developing novel organic carbon skeleton negative materials for lithium ion batteries.

Iron(II) Phthalocyanine Adsorbed on Defective Graphenes: A Density Functional Study

The adsorptions of iron(II) phthalocyanine (FePc) on graphene and defective graphene were investigated systematically using density functional theory. Three types of graphene defects covering stone-wales (SW), single vacancy (SV), and double vacancy (DV) were taken into account, in which DV defects included DV(5-8-5), DV(555-777), and DV(5555-6-7777). The calculations of formation energies of defects showed that the SW defect has the lowest formation energy, and it was easier for DV defects to form compared with the SV defect. It is more difficult to rotate or move FePc on the surface of defective graphenes than on the surface of graphene due to bigger energy differences at different sites. Although the charge analysis indicated the charge transfers from graphene or defective graphene to FePc for all studied systems, the electron distributions of FePc on various defective graphenes were different. Especially for FePc@SV, the d xy orbital of Fe in the conduction band moved toward the Fermi level about 1 eV, and the d xz of Fe in the valence band for FePc@SV also moved toward the Fermi level compared with FePc@graphene and other FePc@defective graphenes. Between the planes of FePc and defective graphene, the electron accumulation occurs majorly in the position of the FePc molecular plane for FePc@SW, FePc@DV(5-8-5), and FePc@DV(5555-6-7777) as well as FePc@graphene. However, electrons were accumulated on the upper and lower surfaces of the FePc molecular plane for FePc@SV and FePc@DV(555-777). Thus, the electron distribution of FePc can be modulated by introducing the interfaces of different defective graphenes.

Electrochemical Reactors for Continuous Decentralized H2 O2 Production

The global utilization of H₂ O₂ is currently around 4 million tons per year and is expected to continue to increase in the future. H₂ O₂ is mainly produced by the anthraquinone process, which involves multiple steps in terms of alkylanthraquinone hydrogenation/oxidation in organic solvents and liquid-liquid extraction of H₂ O₂ . The energy-intensive and environmentally unfriendly anthraquinone process does not meet the requirements of sustainable and low-carbon development. The electrocatalytic two-electron (2 e^- ) oxygen reduction reaction (ORR) driven by renewable energy (e.g. solar and wind power) offers a more economical, low-carbon, and greener route to produce H₂ O₂ . However, continuous and decentralized H₂ O₂ electrosynthesis still poses many challenges. This Minireview first summarizes the development of devices for H₂ O₂ electrosynthesis, and then introduces each component, the assembly process, and some optimization strategies.

DFT Study on the CO2 Reduction to C2 Chemicals Catalyzed by Fe and Co Clusters Supported on N-Doped Carbon

The catalytic conversion of CO₂ to C₂ products through the CO₂ reduction reaction (CO₂RR) offers the possibility of preparing carbon-based fuels and valuable chemicals in a sustainable way. Herein, various Fe_n and Co₅ clusters are designed to screen out the good catalysts with reasonable stability, as well as high activity and selectivity for either C₂H₄ or CH₃CH₂OH generation through density functional theory (DFT) calculations. The binding energy and cohesive energy calculations show that both Fe₅ and Co₅ clusters can adsorb stably on the N-doped carbon (NC) with one metal atom anchored at the center of the defected hole via a classical MN₄ structure. The proposed reaction pathway demonstrates that the Fe₅-NC cluster has better activity than Co₅-NC, since the carbon–carbon coupling reaction is the potential determining step (PDS), and the free energy change is 0.22 eV lower in the Fe₅-NC cluster than that in Co₅-NC. However, Co₅-NC shows a better selectivity towards C₂H₄ since the hydrogenation of CH₂CHO to CH₃CHO becomes the PDS, and the free energy change is 1.08 eV, which is 0.07 eV higher than that in the C-C coupling step. The larger discrepancy of d band center and density of states (DOS) between the topmost Fe and sub-layer Fe may account for the lower free energy change in the C-C coupling reaction. Our theoretical insights propose an explicit indication for designing new catalysts based on the transition metal (TM) clusters supported on N-doped carbon for multi-hydrocarbon synthesis through systematically analyzing the stability of the metal clusters, the electronic structure of the critical intermediates and the energy profiles during the CO₂RR.

Metal-Decorated Phthalocyanine Monolayer as a Potential Gas Sensing Material for Phosgene: A First-Principles Study

Research into a gas sensing material with excellent performance to detect or remove toxic phosgene (COCl2) is of great significance to environmental and biological protection. In the present work, the adsorption performance of COCl2 on pristine phthalocyanine (Pc) and metal-decorated Pc (MePc, Me = Cu, Ga, and Ru) monolayers was studied by first-principles calculations. The results show that the absorption process of COCl2 on pristine Pc and CuPc both belong to physisorption, indicating that they are not suitable gas sensing materials for COCl2. When Pc sheets are decorated by Ga and Ru atoms, the adsorption of COCl2 is changed into chemisorption, and the corresponding adsorption energies are -0.57 and -0.50 eV for GaPc and RuPc, respectively. The microcosmic mechanism between COCl2 and adsorbents (GaPc, RuPc) was clarified by the analysis of the density of states, the charge density difference, and the Hirshfeld charge. In addition, the COCl2 adsorption results in a significant conductivity variation of the RuPc monolayer, demonstrating it exhibits a high sensitivity to the COCl2 molecule. Meanwhile, quick desorption processes were noticed at various temperatures for the COCl2/RuPc system. Consequently, the RuPc monolayer can be considered as a potential candidate for phosgene sensors because of the moderate adsorption strength, high sensitivity, and fast desorption speed.

Identification of Catalytic Active Sites for Durable Proton Exchange Membrane Fuel Cell: Catalytic Degradation and Poisoning Perspectives

Recent progress in synthetic strategies, analysis techniques, and computational modeling assist researchers to develop more active catalysts including metallic clusters to single-atom active sites (SACs). Metal coordinated N-doped carbons (M-N-C) are the most auspicious, with a large number of atomic sites, markedly performing for a series of electrochemical reactions. This perspective sums up the latest innovative and computational comprehension, while giving credit to earlier/pioneering work in carbonaceous assembly materials towards robust electrocatalytic activity for proton exchange membrane fuel cells via inclusive performance assessment of the oxygen reduction reaction (ORR). M-N_x -C_y are exclusively defined active sites for ORR, so there is a unique possibility to intellectually design the relatively new catalysts with much improved activity, selectivity, and durability. Moreover, some SACs structures provide better performance in fuel cells testing with long-term durability. The efforts to understand the connection in SACs based M-N_x -C_y moieties and how these relate to catalytic ORR performance are also conveyed. Owing to comprehensive practical application in the field, this study has covered very encouraging aspects to the current durability status of M-N-C based catalysts for fuel cells followed by degradation mechanisms such as macro-, microdegradation, catalytic poisoning, and future challenges.

Role of the Supporting Surface in the Thermodynamics and Cooperativity of Axial Ligand Binding to Metalloporphyrins at Interfaces

Abstract:

: Metalloporphyrins have been shown to bind axial ligands in a variety of environments including the vacuum/solid and solution/solid interfaces. Understanding the dynamics of such interactions is a desideratum for the design and implementation of next generation molecular devices which draw inspiration from biological systems to accomplish diverse tasks such as molecular sensing, electron transport, and catalysis to name a few. In this article, we review the current literature of axial ligand coordination to surface-supported porphyrin receptors. We will focus on the coordination process as monitored by scanning tunneling microscopy (STM) that can yield qualitative and quantitative information on the dynamics and binding affinity at the single molecule level. In particular, we will address the role of the substrate and intermolecular interactions in influencing cooperative effects (positive or negative) in the binding affinity of adjacent molecules based on experimental evidence and theoretical calculations.

Porous organic polymers for electrocatalysis

Porous organic polymers (POPs) composed of organic building units linked via covalent bonds are a class of lightweight porous network materials with high surface areas, tuneable pores, and designable components and structures. Owing to their well-preserved characteristics in terms of structure and composition, POPs applied as electrocatalysts have shown promising activity and achieved considerable advances in numerous electrocatalytic reactions, including the hydrogen evolution reaction, oxygen evolution reaction, oxygen reduction reaction, CO₂ reduction reaction, N₂ reduction reaction, nitrate/nitrite reduction reaction, nitrobenzene reduction reaction, hydrogen oxidation reaction, and benzyl alcohol oxidation reaction. Herein, we present a systematic overview of recent advances in the applications of POPs in these electrocatalytic reactions. The synthesis strategies, specific active sites, and catalytic mechanisms of POPs are summarized in this review. The fundamental principles of some electrocatalytic reactions are also concluded. We further discuss the current challenges of and perspectives on POPs for electrocatalytic applications. Meanwhile, the possible future directions are highlighted to afford guidelines for the development of efficient POP electrocatalysts.

Graphene in situ composite metal phthalocyanines (TN-MPc@GN, M = Fe, Co, Ni) with improved performance as anode materials for lithium ion batteries

In view of the disadvantage of the limited active site utilization due to the easy aggregation of phthalocyanine compounds, three kinds of graphene composite metal phthalocyanines (TN-MPc@GN, M = Fe, Co, Ni) were prepared using an in situ composite method, and their electrochemical properties were investigated as anode materials for lithium-ion batteries.

Catalytic activity, thermal stability and structural evolution of PdCu single-atom alloy catalysts: the effects of size and morphology

Single-atom alloys (SAAs) have been emerging as an important field of research in electrocatalysis owing to extremely high atom utilization, unique structure and high catalytic activity. In this work, the catalytic properties and thermal stability of PdCu SAAs with a crown-jewel (CJ) structure are studied by density functional theory (DFT) calculations and the molecular dynamics (MD) simulation method. The DFT results reveal that CJ-structured PdCu SAAs show excellent HER and ORR catalytic performance, and can be regarded as a promising alternative to Pt catalysts towards the ORR or HER. Additionally, we attempt to explain the high catalytic activity of PdCu SAAs by electronic structure analysis. In addition, MD simulation results confirm the thermal stability of CJ-structured PdCu. More importantly, we found that CJ-structured PdCu clusters undergo a structural transformation from cuboctahedral (Cubo) to icosahedral (Ico) structure by heating or after the adsorption of reaction intermediate, which indicates that Cubo is less stable than the Ico structure. Besides, Cubo-Ico transformation is size-dependent and only found in small clusters. Furthermore, the effects of size and morphology on melting properties are discussed. The melting point increases as cluster size increases, which agrees well with Pawlow's law.

Single-Atom (Iron-Based) Catalysts: Synthesis and Applications

Supported single-metal atom catalysts (SACs) are constituted of isolated active metal centers, which are heterogenized on inert supports such as graphene, porous carbon, and metal oxides. Their thermal stability, electronic properties, and catalytic activities can be controlled via interactions between the single-metal atom center and neighboring heteroatoms such as nitrogen, oxygen, and sulfur. Due to the atomic dispersion of the active catalytic centers, the amount of metal required for catalysis can be decreased, thus offering new possibilities to control the selectivity of a given transformation as well as to improve catalyst turnover frequencies and turnover numbers. This review aims to comprehensively summarize the synthesis of Fe-SACs with a focus on anchoring single atoms (SA) on carbon/graphene supports. The characterization of these advanced materials using various spectroscopic techniques and their applications in diverse research areas are described. When applicable, mechanistic investigations conducted to understand the specific behavior of Fe-SACs-based catalysts are highlighted, including the use of theoretical models.

Recent advances in electrocatalysis with phthalocyanines

Applications of phthalocyanines (Pcs) in electrocatalysis-including the oxygen reduction reaction (ORR), the carbon dioxide reduction reaction (CO₂RR), the oxygen evolution reaction (OER), and the hydrogen evolution reaction (HER)-have attracted considerable attention recently. Pcs and their derivatives are more attractive than many other macrocycles as electrocatalysts since, although they are structurally related to natural porphyrin complexes, they offer the advantages of low cost, facile synthesis and good chemical stability. Moreover, their high tailorability and structural diversity mean Pcs have great potential for application in electrochemical devices. Here we review the structure and composition of Pcs, methods of synthesis of Pcs and their analogues, as well as applications of Pc-based heterogeneous electrocatalysts. Optimization strategies for Pc-based materials for electrocatalysis of ORR, CO₂RR, OER and HER are proposed, based on the mechanisms of the different electrochemical reactions. We also discuss the structure/composition-catalytic activity relationships for different Pc materials and Pc-based electrocatalysts in order to identify future practical applications. Finally, future opportunities and challenges in the use of molecular Pcs and Pc derivatives as electrocatalysts are discussed.

Substrate strain tunes operando geometric distortion and oxygen reduction activity of CuN2C2 single-atom sites

Single-atom catalysts are becoming increasingly significant to numerous energy conversion reactions. However, their rational design and construction remain quite challenging due to the poorly understood structure–function relationship. Here we demonstrate the dynamic behavior of CuN₂C₂ site during operando oxygen reduction reaction, revealing a substrate-strain tuned geometry distortion of active sites and its correlation with the activity. Our best CuN₂C₂ site, on carbon nanotube with 8 nm diameter, delivers a sixfold activity promotion relative to graphene. Density functional theory and X-ray absorption spectroscopy reveal that reasonable substrate strain allows the optimized distortion, where Cu bonds strongly with the oxygen species while maintaining intimate coordination with C/N atoms. The optimized distortion facilitates the electron transfer from Cu to the adsorbed O, greatly boosting the oxygen reduction activity. This work uncovers the structure–function relationship of single-atom catalysts in terms of carbon substrate, and provides guidance to their future design and activity promotion.

The rational design of single-atom catalysts is challenging. This work reveals a substrate-strain tuned geometry distortion of CuN₂C₂ single-atom site, which greatly boosts oxygen reduction activity by facilitating electron transfer to adsorbed O.

Catalytic Role of Adsorption of Electrolyte/Molecules as Functional Ligands on Two-Dimensional TM-N4 Monolayer Catalysts for the Electrocatalytic Nitrogen Reduction Reaction

Two-dimensional single-atom catalysts (2D SACs) have been widely studied on the nitrogen reduction reaction (NRR). The characteristics of 2D catalysts imply that both sides of the monolayer can be catalytic sites and adsorb electrolyte ions or molecules from solutions. Overstrong adsorption of electrolyte ions or molecules on both sides of the catalyst site will poison the catalyst, while the adsorbate on one side of the catalytic site will modify the activity and selectivity of the other side for NRR. Discovering the influence of adsorption of electrolyte ions or molecules as a functional ligand on catalyst performance on the NRR is crucial to improve NRR efficiency. Here, we report this work using the density functional theory (DFT) method to investigate adsorption of electrolyte ions or molecules as a functional ligand. Among all of the studied 18 functional ligands and 3 transition metals (TMs), the results showed that Ru&F, Ru&COOH, and Mo&H₂O combinations were screened as electrocatalysis systems with high activity and selectivity. Particularly, the Mo&H₂O combination possesses the highest activity with a low ΔG_MAX of 0.44 eV through the distal pathway. The superior catalytic performance of the Mo&H₂O combination is mainly attributed to the electron donation from the metal d orbital. Furthermore, the functional ligands can occupy the active sites and block the competing vigorous hydrogen evolution reaction. Our findings offer an effective and practical strategy to design the combination of the catalyst and electrolyte to improve electrocatalytic NRR efficiency.

Cooperative Single-Atom Active Centers for Attenuating the Linear Scaling Effect in the Nitrogen Reduction Reaction

Cooperative effects of adjacent active centers are critical for single-atom catalysts (SACs) as active site density matters. Yet, how it affects scaling relationships in many important reactions such as the nitrogen reduction reaction (NRR) is underexplored. Herein we elucidate how the cooperation of two active centers can attenuate the linear scaling effect in the NRR through a first-principle study on 39 SACs comprised of two adjacent (∼4 Å apart) four N-coordinated metal centers (MN₄ duo) embedded in graphene. Bridge-on adsorption of dinitrogen-containing species appreciably tilts the balance of adsorption of N₂H and NH₂ toward N₂H and thus substantially loosens the restraint of scaling relationships in the NRR, achieving low onset potential (V) and direct N≡N cleavage (Mo, Re) at room temperature, respectively. The potential of the MN₄ duo in the NRR provides new insight into circumventing the limitations of scaling relationships in heterogeneous catalysis.

Reconstructing 1D Fe Single-atom Catalytic Structure on 2D Graphene Film for High-Efficiency Oxygen Reduction Reaction

The ordinary intrinsic activity and disordered distribution of metal sites in zero/one-dimensional (0D/1D) single-atom catalysts (SACs) lead to inferior catalytic efficiency and short-term endurance in the oxygen reduction reaction (ORR), which restricts the large-scale application of hydrogen-oxygen fuel cells and metal-air batteries. To improve the activity of SACs, a mild synthesis method was chosen to conjugate 1D Fe SACs with 2D graphene film (Fe SAC@G) that realized a composite structure with well-ordered atomic-Fe coordination configuration. The product exhibits outstanding ORR electrocatalytic efficiency and stability in 0.1 M KOH aqueous solution. DFT-D computational results manifest the intrinsic ORR activity of Fe SAC@G originated from the newly-formed FeN₄ -O-FeN₄ bridge structure with moderate adsorption ability towards ORR intermediates. These findings provide new ways for designing SACs with high activity and long-term stability.

Two-dimensional transition metal phthalocyanine sheet as a promising electrocatalyst for nitric oxide reduction: a first principle study

Electrochemical reduction is a promising technology to treat polluted water contaminated by nitrate and nitrite ions under mild conditions. NO is an important intermediate species and determines selectivity toward different product and rate of whole reaction. However, the most studied NOER electrocatalysts are noble pure metal, which face problems of low utilization and high cost. Herein, by means of density functional theory computations, catalytic performance of 2D TM-Pc sheets (TM = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Nb, Mo, Ru) as NOER catalysts were systematically evaluated. Among all the studied 2D TM-Pc sheets, our results revealed 2D Co-Pc sheet was identified as the best NOER catalyst, for a proper NO absorption energy and its relatively low limiting potential. The final reduction product of NOER is either NH₃ at low coverages with energy input of 0.58 eV or N₂O at high coverages with no energy barrier. Moreover, 2D Co-Pc sheet can efficiently suppress the competing HER. This study could not only provide a new approach for electrochemical denitrification to resolve environmental pollution but also be useful for valuable ammonia production.

Ultrasonic Plasma Engineering Toward Facile Synthesis of Single-Atom M-N4/N-Doped Carbon (M = Fe, Co) as Superior Oxygen Electrocatalyst in Rechargeable Zinc-Air Batteries

As bifunctional oxygen evolution/reduction electrocatalysts, transition-metal-based single-atom-doped nitrogen-carbon (NC) matrices are promising successors of the corresponding noble-metal-based catalysts, offering the advantages of ultrahigh atom utilization efficiency and surface active energy. However, the fabrication of such matrices (e.g., well-dispersed single-atom-doped M-N₄/NCs) often requires numerous steps and tedious processes. Herein, ultrasonic plasma engineering allows direct carbonization in a precursor solution containing metal phthalocyanine and aniline. When combining with the dispersion effect of ultrasonic waves, we successfully fabricated uniform single-atom M-N₄ (M = Fe, Co) carbon catalysts with a production rate as high as 10 mg min^-1. The Co-N₄/NC presented a bifunctional potential drop of ΔE = 0.79 V, outperforming the benchmark Pt/C-Ru/C catalyst (ΔE = 0.88 V) at the same catalyst loading. Theoretical calculations revealed that Co-N₄ was the major active site with superior O₂ adsorption-desorption mechanisms. In a practical Zn-air battery test, the air electrode coated with Co-N₄/NC exhibited a specific capacity (762.8 mAh g^-1) and power density (101.62 mW cm^-2), exceeding those of Pt/C-Ru/C (700.8 mAh g^-1 and 89.16 mW cm^-2, respectively) at the same catalyst loading. Moreover, for Co-N₄/NC, the potential difference increased from 1.16 to 1.47 V after 100 charge-discharge cycles. The proposed innovative and scalable strategy was concluded to be well suited for the fabrication of single-atom-doped carbons as promising bifunctional oxygen evolution/reduction electrocatalysts for metal-air batteries.

Regulating the electronic properties of MoSe2 to improve its CO2 electrocatalytic reduction performance via atomic doping

The atomic environment should heavily influence the performance of CO₂ reduction, and the regulated electronic property of reaction intermediates and metals (Cu) is responsible for the high catalytic performance of CH₄ production.

Mechanistic insight into photocatalytic CO2 reduction by a Z-scheme g-C3N4/TiO2 heterostructure

The Z-scheme g-C₃N₄/TiO₂ heterostructure has remarkable catalytic activity for reducing CO₂ to CH₄ and CH₃OH.

CO2 electrochemical reduction boosted by the regulated electronic properties of metalloporphyrins through tuning an atomic environment

The selectivity of metalloporphyrins can be regulated via doping ions, and the reduction products are changed from HCOOH to HCHO or CH₄ after ion doping, which is attributed to the regulated electronic structure of the metal center.

A No-Sweat Strategy for Graphene-Macrocycle Co-assembled Electrocatalyst toward Oxygen Reduction and Ambient Ammonia Synthesis

Toward the realm of sustainable energy, the development of efficient methods to enhance the performance of electrocatalysts with molecular level perception has gained immense attention. Inspite of untiring attempts, the production cost and scaling-up issues have been a step back toward the commercialization of the electrocatalysts. Herein, we report a one-pot electrophoretic exfoliation technique with minimum time and power input to synthesize iron phthalocyanine functionalized high-quality graphene sheets (G-FePc). The π-stacked co-assembly excels in oxygen reduction performance (major criterion for fuel cells) with a high positive E_1/2 of 0.91 V (vs RHE) and a reproducible reduction peak potential of 0.90 V (vs RHE). An overpotential as low as 29 mV dec^-1 and complete tolerance toward the methanol crossover effect confirm the authentication of the catalytic performance of our designed catalyst G-FePc. The catalyst simultaneously exhibits hydrogen storage efficacy by means of nitrogen fixation, yielding 27.74 μg h^-1 mg_cat^-1 NH₃ at a potential of -0.3 V (vs RHE) in an acidic electrolyte. The structure-function relationship of the catalyst is revealed via molecular orbital chemistry for the bonding of the Fe(II) active center with O₂ and N₂ during catalysis.

Density functional theory study on a nitrogen-rich carbon nitride material C3N5 as photocatalyst for CO2 reduction to C1 and C2 products

A new-type nitrogen-rich carbon nitride material C₃N₅ has been synthesized recently, in which the C:N ratio increases from 3:4 in g-C₃N₄ to 3:5 due to the introduction of azo linkage (NN) connecting segments in two C₆N₇ units. Herein, C₃N₅ as a photocatalyst for CO₂ reduction was investigated by density functional theory methods. The electronic and optical properties indicate that C₃N₅ has a longer visible-light region with 2.0 eV of band gap in comparison with g-C₃N₄. The spatial distributions of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) show that the π network of C₃N₅ is extended by introducing -NN- linkage, which results in much higher photocatalytic efficiency than g-C₃N₄. The Gibbs free energies for possible CO₂ reaction paths on C₃N₅ were computed. The results show that CO₂ can be reduced to CH₄ with a low limiting potential of -0.54 V and to CH₃CH₂OH with a low limiting potential of -0.61 V, which all driven by solar energy. The present work is expected to provide useful guide for new-type nitrogen-rich C₃N₅ as promising photocatalyst for CO₂ reduction reaction (CO₂RR).

Theoretical Understandings of Graphene-based Metal Single-Atom Catalysts: Stability and Catalytic Performance

Research on heterogeneous single-atom catalysts (SACs) has become an emerging frontier in catalysis science because of their advantages in high utilization of noble metals, precisely identified active sites, high selectivity, and tunable activity. Graphene, as a one-atom-thick two-dimensional carbon material with unique structural and electronic properties, has been reported to be a superb support for SACs. Herein, we provide an overview of recent progress in investigations of graphene-based SACs. Among the large number of publications, we will selectively focus on the stability of metal single-atoms (SAs) anchored on different sites of graphene support and the catalytic performances of graphene-based SACs for different chemical reactions, including thermocatalysis and electrocatalysis. We will summarize the fundamental understandings on the electronic structures and their intrinsic connection with catalytic properties of graphene-based SACs, and also provide a brief perspective on the future design of efficient SACs with graphene and graphene-like materials.

Transition metal-N4 embedded black phosphorus carbide as a high-performance bifunctional electrocatalyst for ORR/OER

Designing highly active electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is an important challenge in energy conversion and storage technology. In this work, based on computational screening over doping of 23 kinds of transition metals (TMs), we use first-principles study to explore the ORR and OER activity of TM-N₄ embedded black phosphorus carbide monolayer (b-PC). The results show that its catalytic performance highly depends on the number of electrons in the d orbital and the number of valence electrons of introduced TM atom. Moreover, we found that Co-N₄-bPC (η_ORR = 0.31 V; η_OER = 0.22 V), Rh-N₄-bPC (η_ORR = 0.33 V; η_OER = 0.62 V), and Ir-N₄-bPC (η_ORR = 0.21 V; η_OER = 0.21 V) can be promising candidates as bifunctional catalysts for both the ORR and OER and can be comparable or superior to TM-N₄-graphene in terms of overpotential. They experience no structural distortion at 500 K. Moreover, the exfoliation energy of b-PC is lower than that of graphene, and these three promising candidates show much lower formation energy than TM-N₄-graphene. Our study provides a systematical method for designing and developing high performance 2D material-based single atom catalysts (SACs) beyond graphene.

Iron phthalocyanine with coordination induced electronic localization to boost oxygen reduction reaction

Iron phthalocyanine (FePc) is a promising non-precious catalyst for the oxygen reduction reaction (ORR). Unfortunately, FePc with plane-symmetric FeN₄ site usually exhibits an unsatisfactory ORR activity due to its poor O₂ adsorption and activation. Here, we report an axial Fe-O coordination induced electronic localization strategy to improve its O₂ adsorption, activation and thus the ORR performance. Theoretical calculations indicate that the Fe-O coordination evokes the electronic localization among the axial direction of O-FeN₄ sites to enhance O₂ adsorption and activation. To realize this speculation, FePc is coordinated with an oxidized carbon. Synchrotron X-ray absorption and Mössbauer spectra validate Fe-O coordination between FePc and carbon. The obtained catalyst exhibits fast kinetics for O₂ adsorption and activation with an ultralow Tafel slope of 27.5 mV dec^-1 and a remarkable half-wave potential of 0.90 V. This work offers a new strategy to regulate catalytic sites for better performance.

Surface Modification Strategy for Promoting the Performance of Non-noble Metal Single-Atom Catalysts in Low-Temperature CO Oxidation

As a bridge between homogeneous and heterogeneous catalyses, single-atom catalysts (SACs), especially the noble metal atoms, have received extensive attention from both the fundamental and applied perspectives recently. High cost and difficulty in synthesis are considerable factors, however, limiting the development and practical applications of SACs. Thus, seeking for non-noble SACs for substituting the noble ones is not only of vital importance but also a long-standing challenge. Herein, a surface modification strategy by introducing an oppositely charged dopant and inducing the charge transfer between the SAC and the substrate was proposed to improve the stability and catalytic performance of the non-noble Cu SAC. Using first-principles density functional theory (DFT) calculations, it was demonstrated that the introduction of C in the MoS₂ monolayer (C:MoS₂, experimentally available) can assist in stabilizing Cu and make it more positively charged, which will facilitate the adsorption of the reactants and further enhance the activity for CO oxidation. Strikingly, our results show that CO oxidation over Cu-C:MoS₂ is more favorable than over the Pt atom deposited on the pristine MoS₂ (Pt-MoS₂), exhibiting its potential in noble metal substitution and low-temperature CO oxidation. Additionally, Cu-C:MoS₂ was observed to have a response to visible light, which manifests that it may be a promising photocatalyst. The strategy proposed here provides an efficient route to regulate the electronic structures of SACs through charge transfer, which further promotes the reactivity of the non-noble metal SACs. We hope that this strategy can contribute to design more SACs with low cost and high efficiency, which will be beneficial for their practical applications.

Computational screening of transition metal-doped phthalocyanine monolayers for oxygen evolution and reduction

Rationally designing efficient, low-cost and stable catalysts toward the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR) is of significant importance to the development of renewable energy technologies. In this work, we have systematically investigated a series of potentially efficient and stable single late transition metal atom doped phthalocyanines (TM@Pcs, TM = Mn, Fe, Co, Ni, Cu, Ru, Rh, Pd, Ir and Pt) as single-atom catalysts (SACs) for applications toward the OER and ORR through a computational screening approach. Our calculations indicate that TM atoms can tightly bind with Pc monolayers with high diffusion energy barriers to prevent the isolated atoms from clustering. The interaction strength between intermediates and TM@Pc governs the catalytic activities for the OER and ORR. Among all the considered TM@Pc catalysts, Ir@Pc and Rh@Pc were found to be efficient OER electrocatalysts with overpotentials η OER of 0.41 and 0.44 V, respectively, and for the ORR, Rh@Pc exhibits the lowest overpotential η ORR of 0.44 V followed by Ir@Pc (0.55 V), suggesting that Rh@Pc is an efficient bifunctional catalyst for both the OER and ORR. Moreover, it is worth noting that the Rh@Pc catalyst can remain stable against dissolution under the pH = 0 condition. Ab initio molecular dynamic calculations suggest that Rh@Pc could remain stable at 300 K. Our findings highlight a novel family of two-dimensional (2D) materials as efficient and stable SACs and offer a new strategy for catalyst design.

Metallic single-atoms confined in carbon nanomaterials for the electrocatalysis of oxygen reduction, oxygen evolution, and hydrogen evolution reactions

Recent advances of single-atom-based carbon nanomaterials for the ORR, OER, HER, and bifunctional electrocatalysis are covered in this review article.

Improving the Activity of M-N4 Catalysts for the Oxygen Reduction Reaction by Electrolyte Adsorption

Metal and nitrogen codoped carbons (M-N/Cs) have emerged as promising alternatives to platinum-based catalysts for the oxygen reduction reaction (ORR). DFT calculations are used to investigate the adsorption of anions and impurities from the electrolyte on the active site, modeled as an M-N₄ motif embedded in a planar carbon sheet (M=Cr, Mn, Fe, Co). The two-dimensional catalyst structure implies that each metal atom has two potential active sites, one on each side of the sheet. Adsorption of anions or impurities on both sites results in poisoning, but adsorption on one of the sites leads to a modified ORR activity on the remaining site. The calculated adsorption energies show that a number of species adsorb only on one of the two sites under realistic experimental conditions. Furthermore, a few of these adsorbates modify the adsorption energies of the ORR intermediates on the remaining site, in such a way that the limiting potential is improved.