Metal Nitrogen Carbon Catalysts Research Articles

General and Ultrafast Photothermal Synthesis of Atomic Metal‐Nitrogen‐Carbon Catalysts for H2O2 Electrosynthesis

AbstractAtomic metal‐nitrogen‐carbon catalysts (M‐N‐Cs) are promising electrocatalysts for the two‐electron oxygen reduction reaction (2e− ORR) for the green production of H2O2. However, the conventional synthetic methods of M‐N‐Cs typically suffer from prolonged possessing time and high energy consumption. Herein, a flash light irradiation‐assisted transient pyrolysis strategy is reported for the rapid and facile synthesis of a Co‐N‐C catalyst consisting of atomic Co‐N4 sites supported on graphene. The resultant Co‐N‐C is highly active and selective as an electrocatalyst for the 2e− ORR, with a H2O2 selectivity up to 94.2%, high mass activity of 113.9 A g−1, high turnover frequency of 34.8 s−1, and an average H2O2 production rate of 12.1 mol g−1 h−1 with an accumulated H2O2 concentration up to 15.2 mmol L−1 when evaluated with a three‐phase flow cell setup. Additionally, this synthetic strategy can be readily expanded to prepare other types of M‐N‐Cs (M = Fe, Ni, Cu, and noble metal Ir) with similar M‐N4 configurations by simply changing the metal precursors, demonstrating the generality of this method. The rapid and general flash‐assisted synthetic strategy for M‐N‐Cs can provide a readily accessible material library and platform for investigating their catalytic properties in different energy conversion processes.

Atomically dispersed Mn–Nx catalysts derived from Mn‐hexamine coordination frameworks for oxygen reduction reaction

AbstractMetal‐organic frameworks recently have been burgeoning and used as precursors to obtain various metal–nitrogen–carbon catalysts for oxygen reduction reaction (ORR). Although rarely studied, Mn–N–C is a promising catalyst for ORR due to its weak Fenton reaction activity and strong graphitization catalysis. Here, we developed a facile strategy for anchoring the atomically dispersed nitrogen‐coordinated single Mn sites on carbon nanosheets (MnNCS) from an Mn‐hexamine coordination framework. The atomically dispersed Mn–N4 sites were dispersed on ultrathin carbon nanosheets with a hierarchically porous structure. The optimized MnNCS displayed an excellent ORR performance in half‐cells (0.89 V vs. reversible hydrogen electrode (RHE) in base and 0.76 V vs. RHE in acid in half‐wave potential) and Zn–air batteries (233 mW cm−2 in peak power density), along with significantly enhanced stability. Density functional theory calculations further corroborated that the Mn–N4–C12 site has favorable adsorption of *OH as the rate‐determining step. These findings demonstrate that the metal‐hexamine coordination framework can be used as a model system for the rational design of highly active atomic metal catalysts for energy applications.

Fe-N-C Oxygen Reduction Catalysts Via Chemical Vapor Deposition in Fluidized Bed Reactor

Metal nitrogen carbon catalysts (MNC) are an attractive substitute for platinum-based catalysts in the oxygen reduction reaction (ORR) in PEM fuel cells. However, state-of-the-art MNC catalysts still suffer from low activity and instability. Li et al. [1] demonstrated the use of zinc imidazolate frameworks (ZIF-8) in chemical vapor deposition (CVD) to form active sites without unwanted particle formation or additional treatment steps. In this method, gaseous iron chloride exchanges the zinc in the precatalyst to form active sites.In this work, we introduce a vertical fluidized bed reactor (FBR) for high-scale synthesis of FeNC via CVD, allowing for uniform dispersion of the precatalyst and better zinc-iron exchange. Furthermore, we investigated the influence of differently sized ZIF-8 on the CVD and the formation of the FeNC catalyst.The final catalysts were characterized for their electrochemical activity in a rotating ring disc setup, their site density via CO-chemisorption, and their respective turnover frequency. The physicochemical characterization includes the analysis of surface area and pore structure via nitrogen physisorption, SEM, and TEM, as well as surface structure and compositional analysis via elemental analysis, ICP-OES, and XPS.The exchange of Zn was in the range of 90%, resulting in an iron content of 3.8 wt% with only the formation of desired Fe-Nx active sites. STEM-EDX imaging shows an even distribution of iron and nitrogen in the catalyst, with mean particle sizes of 110 nm. In rotating disc electrode experiments with 0.5 M sulfuric acid, mass activities of 0.23 A/g at 0.9 V vs RHE were achieved.Commercially nano-scaled ZIF-8 with mean particle sizes of 500 nm, synthesized particles with mean sizes of 110 nm were tested, compared to the commercial ZIF-8 Basolite Z1200 with mean particle sizes of 4.9 µm. We could show improved activity in RRDE of the nano-scaled ZIF-8 materials compared to the macro-scaled Basolite.Applying this highly scalable synthesis method for MNC catalysts could be a viable way to achieve high-performance catalysts for ORR.[1] Li Jiao, Jingkun Li, Lynne Larochelle Richard, Qiang Sun, Thomas Stracensky, et al.. Chemical vapour deposition of Fe–N–C oxygen reduction catalysts with full utilization of dense Fe–N4 sites. Nature Materials, 2021, 20, pp.1385-1391.

(Digital Presentation) Structure-Activity Relationship of M-N-C Electrocatalysts Synthesized Using a General Solution-Phase Coordination Approach

Metal- and nitrogen-doped carbon (M-N-C) catalysts have great potential in heterogeneous catalysis. They are used to catalyze various crucial electrochemical reactions, such as the oxygen reduction reaction (ORR), the hydrogen evolution reaction (HER), the oxygen evolution reaction (OER), and the CO2 reduction reaction (CO2RR).1-4 The nitrogen-coordinated metal sites (MNx) have been considered the main active sites in these M-N-C catalysts. However, the synthesis of MNx moieties often undergoes a high-temperature heat-treatment step, resulting in low site density. On the other side, the coordination environment in the MNx sites is also affected by the metal species which induced the site formation. Therefore, it is challenging to single out the role of central metal in the structure-activity relationships for these MNx sites.In this presentation, we will discuss our recent progress in the development of a solution-phase coordination synthesis approach targeting M-N-C catalysts with high active-site density and well-defined coordination environments.5, 6 A series of M-N-C catalysts are synthesized via this approach by coordinating electroactive target metal ions with the nitrogen-coordinated metal-vacancy (MVNx) sites in N-C templated by sacrificial metals. With a combined experimental and computational approach, we explore the role of sacrificial metals, including s-, p-, 3d-, 4d-, and f-block metals, in forming various MVNx sites with unique coordination configurations. The structure-activity relationship between the coordination environment and the catalytic activity for ORR, HER, OER, and CO2RR is established by comparing the MNx site structures induced by various sacrificial metals. Furthermore, we will present the activity series of metal centers in M-N-C catalysts with the same and well-defined coordination environment for ORR, HER, OER, and CO2RR. These results will guide the future development of M-N-C catalysts. Acknowledgments Financial support from the State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences is greatly appreciated. This study is financially supported by the Shanxi Province grant (Grant No. 20210302123011, 202203021212007, and 202103021224442), and Key Research and Development (R&D) Projects of Shanxi Province (202102070301018). References J. Cui, Q. Chen, X. Li and S. Zhang. Recent advances in non-precious metal electrocatalysts for oxygen reduction in acidic media and PEMFCs: an activity, stability and mechanism study. Green Chemistry, 23, 6898 (2021).H. He, H. H. Wang, J. Liu, X. Liu, W. Li and Y. Wang. Research Progress and Application of Single-Atom Catalysts: A Review. Molecules, 26, 6501 (2021).U. Martinez, S. Komini Babu, E. F. Holby, H. T. Chung, X. Yin and P. Zelenay. Progress in the Development of Fe-Based PGM-Free Electrocatalysts for the Oxygen Reduction Reaction. Advanced Materials, 31, 1806545 (2019).Y. Wang, X. Cui, L. Peng, L. Li, J. Qiao, H. Huang and J. Shi. Metal–Nitrogen–Carbon Catalysts of Specifically Coordinated Configurations toward Typical Electrochemical Redox Reactions. Advanced Materials, 33, e2100997 (2021).M. Huang, R. Ding, J. Yang, W. Shi, S. Shi, L. Chen, S. Liu and X. Yin. Formation of Nitrogen-Coordinated Metal Sites (M = Fe, Co) via Solution-Phase Coordination on Nickel- And Nitrogen-Co-Doped Carbon Templates with Metal Vacancy-Nx Sites. Journal of The Electrochemical Society, 169, 106507 (2022).M. Huang, L. Chen, R. Ding, W. Shi, Q. Qin, J. Yang, S. Liu and X. Yin. Solution-Phase Synthesis of Co-N-C Catalysts Using Alkali Metals-Induced N-C Templates with Metal Vacancy-Nx sites. ChemRxiv, doi: 10.26434/chemrxiv-2022-xnh88 (2022).

Effect of the Axial Halogen Ligand on the Oxygen Reduction Reaction Performance of Transition Metal–Nitrogen–Carbon Catalysts

Effect of the Axial Halogen Ligand on the Oxygen Reduction Reaction Performance of Transition Metal–Nitrogen–Carbon Catalysts

A General Strategy to Remove Metal Aggregates toward Metal-Nitrogen-Carbon Catalysts with Exclusive Atomic Dispersion.

Metal- and nitrogen-doped nanocarbons (M-N-Cs) are promising alternatives to precious metals for catalyzing electrochemical energy conversion processes. However, M-N-Cs synthesized by high-temperature pyrolysis frequently suffer from compositional heterogeneity with the simultaneous presence of atomically dispersed M-Nx sites and crystalline metal nanoparticles (NPs), which hinders the identification of active sites and rational optimization in performance. Herein, a universal and efficient strategy is reported to obtain both precious- and nonprecious-metal-based M-N-Cs (M = Pt, Fe, Co, Ni, Mn, Cu, Zn) with exclusive atomic dispersion by making use of ammonium iodide as the etchant to remove excessive metal aggregates at high temperature. Taking Pt-N-C as a proof-of-concept demonstration, the complete removal of Pt NPs in Pt-N-C enables clarification on the contributions of the atomic Pt-Nx moieties and Pt NPs to the catalytic activity toward the hydrogen evolution reaction. Combined electrochemical measurements and theoretical calculations identify that the atomic Pt-Nx moieties by themselves possess negligible activity, but they can significantly boost the activity of the Pt NPs via the synergistic effect.

The synthesis of iron-nitrogen sites embedded in electrospun carbon nanofibers with an excellent oxygen reduction reaction activity in alkaline/acidic media

Metal-nitrogen carbon catalysts have received great attention in the field of gas-evolving electrocatalysis due to their high activity, large specific surface area and efficient gas diffusion paths. A solution of porphyrin iron, g-C3N4 and polyacrylonitrile in N,N-dimethylformamide was sonicated and electrospun into doped polyacrylonitrile nanofibers (NFs), and the NFs were then stabilized and carbonized at 900 °C to prepare Fe-N/CNF catalyst for oxygen reduction reaction (ORR). It was found that the addition of g-C3N4 to the electrospinning precursor led to the formation of abundant Fe-N species in Fe3+ and Fe2+ valence states, while Fe3C nanoparticles were formed without adding g-C3N4. Compared to Fe3C/CNF prepared without g-C3N4, the Fe-N/CNF catalyst presents an 4e− improved oxygen reduction reaction activity in both alkaline and acidic media. Furthermore, as a cathode in Zn-air batteries, the Fe-N/CNF catalyst exhibits high performance with an open-circuit voltage of 1.49 V, a power density of 146 mW cm−2 and a specific capacity of 703 mAh g-1. This work suggests a way to prepare metal-nitrogen-carbon catalysts for energy-related electrocatalytic applications.

Atomically Dispersed Metal–Nitrogen–Carbon Catalysts with d-Orbital Electronic Configuration-Dependent Selectivity for Electrochemical CO2-to-CO Reduction

A variety of atomically dispersed transition-metal-anchored nitrogen-doped carbon (M–N–C) electrocatalysts have shown encouraging electrochemical CO2 reduction reaction (CO2RR) performance, with the underlying fundamentals of central transition-metal atom determined CO2RR activity and selectivity yet remaining unclear. Herein, a universal impregnation-acid leaching method was exploited to synthesize various M–N–C (M: Fe, Co, Ni, and Cu) single-atom catalysts (SACs), which revealed d-orbital electronic configuration-dependent activity and selectivity toward CO2RR for CO production. Notably, Ni–N–C exhibits a very high CO Faradaic efficiency (FE) of 97% at −0.65 V versus RHE and above 90% CO selectivity in the potential range from −0.5 to −0.9 V versus RHE, much superior to other M–N–C (M: Fe, Co, and Cu). With the d-orbital electronic configurations of central metals in M–N–C SACs well elucidated by crystal-field theory, Dewar–Chatt–Duncanson (DCD) and differential charge density analysis reveal that the vacant outermost d-orbital of Ni2+ in a Ni–N–C SAC would benefit the electron transfer from the C atoms in CO2 molecules to the Ni atoms and thus effectively activate the surface-adsorbed CO2 molecules. However, the outermost d-orbital of Fe3+, Co2+, and Cu2+ occupied by unpaired electrons would weaken the electron-transfer process and then impede CO2 activation. In situ spectral investigations demonstrate that the generation of *COOH intermediates is favored over Ni–N–C SAC at relatively low applied potentials, supporting its high CO2-to-CO conversion performance. Gibbs free energy difference analysis in the rate-limiting step in CO2RR and hydrogen evolution reaction (HER) reveals that CO2RR is thermodynamically favored for Ni–N–C SAC, explaining its superior CO2RR performance as compared to other SACs. This work presents a facile and general strategy to effectively modulate the CO2-to-CO selectivity from the perspective of electronic configuration of central metals in M–N–C SACs.

Carboxyl induced ultrahigh defects and boron/nitrogen active centers in cobalt-embedded hierarchically porous carbon nanofibers: The stable oxygen reduction reaction catalysis in acid

Transition metal-nitrogen-carbon (MNC) type catalysts have been considered a promising alternative to noble metals for oxygen reduction reaction (ORR) electrocatalysis. Nevertheless, poor stabilities of MNC catalysts in acidic solutions limit their commercialization. In this study, we design and synthesize novel three-dimensional (3D) cobalt (Co) nanoparticles encapsulated in ultrahigh content of boron (B) and nitrogen (N) -doped hierarchically porous carbon nanofibers (denoted as Co@BN-PCNFs) by carbonizing the 3D acetic acid/cobalt nitrate/4-hydroxybenzeneboronic acid/polyvinylpyrrolidone precursor networks woven using electrospinning method under a nitrogen atmosphere. The optimal Co@BN-PCNFs-900 catalyst has abundant micro/mesopores and numerous topological defects and exhibits the largest surface area. Under the synergistic effect of oxygen-containing acetic acid molecules and the electrospinning technology, 5.87 at.% of B and 5.91 at.% of N atoms were doped into carbon nanofibers. Specifically, B/N electrocatalytic active centers (including BC3, pyridinic-N/CoNC, pyrrolic-N, and graphitic-N) of approximately 8.70 at.% were successfully introduced into the skeletons of Co@BN-PCNFs-900. In 0.1 M KOH, the ORR onset potential (Eonset) and half-wave potential (E1/2) of Co@BN-PCNFs-900 were ∼ 64 and ∼ 63 mV, respectively, more positive than those of 20 wt% Pt/C. Additionally, in 0.5 M H2SO4, the ORR Eonset and E1/2 values of Co@BN-PCNFs-900 were only ∼ 11 and ∼ 7 mV, respectively, more negative than those of 20 wt% Pt/C. As the 3D hierarchically porous architectures, topological carbon edges, BC3, and partial NC/CoNC are relatively stable, the Co@BN-PCNFs-900 exhibits excellent stability toward ORR catalysis in both acidic and basic media. These favorable properties of Co@BN-PCNFs-900 nanofibers make them the best non-noble metal-based carbonaceous electrocatalysts for ORR in acidic electrolytes.

Transition metal (Fe, Co, Mn, Cu) containing nitrogen-doped porous carbon as efficient oxygen reduction electrocatalysts for anion exchange membrane fuel cells

Delving into highly active and cost-efficient electrocatalysts for oxygen reduction reaction (ORR) is crucial for the large-scale application of polymer electrolyte fuel cells. Anion exchange membrane fuel cells (AEMFCs) are promising clean energy devices owing to their mild reaction conditions and the high probability of employing Pt-free catalysts for ORR. Developing the promising non-Pt ORR catalysts for AEMFC is still of great importance. Herein, we report the transition metal (Fe, Co, Mn, and Cu) impregnated melamine-phloroglucinol-formaldehyde (MPF) polymeric networks to derive metal-nitrogen-carbon (MNC) electrocatalysts via a robust synthesis route. The catalysts are screened through variable metal contents and different pyrolysis temperature optimizations by virtue of their ORR activity. The controlled synthesis method resulted to the prominent textural properties of the catalysts with efficient active centers to enhance the ORR performance. Amongst, iron-doped (MPF/Fe), and cobalt-doped (MPF/Co) catalysts are performing better in terms of half-wave potential (E1/2) values of 0.81 and 0.80 V vs RHE which is attributed to the highly active M−Nx sites and hierarchical porous structure of catalysts. Outstanding electrochemical stability in half-cell and high-power density in an AEMFC (up to 347 mW cm−2) made the present work drive to the development of highly efficient MNC catalysts for fuel cell applications.

Electrocatalysis Mechanism and Structure-Activity Relationship of Atomically Dispersed Metal-Nitrogen-Carbon Catalysts for Electrocatalytic Reactions.

Atomically dispersed metal-nitrogen-carbon catalysts (M-N-C) have been widely used in the field of energy conversion, which has already attracted a huge amount of attention. Due to their unsaturated d-band electronic structure of the center atoms, M-N-C catalysts can be applied in different electrocatalytic reactions by adjusting their own microscopic electronic structures to achieve the optimization of the structure-activity relationship. Consequently, it is of great significance for the revelation of electrocatalytic mechanism and structure-activity relationship of M-N-C catalysts. Thus, this review first introduces the relative research methods, including in situ/operando characterization techniques and theoretical calculation methods. Furthermore, clarifying the electrocatalytic mechanism and structure-activity relationship of M-N-C catalysts in different electrochemical energy conversion reactions is focused. Moreover, the future research directions are pointed out based on the discussion. This review will provide good guidance to systematically study the catalytic mechanism of single-atom catalysts and reasonably design the single-atom catalysts.

Metal–Nitrogen–Carbon Catalysts by Dynamic Template Removal for Highly Efficient and Selective Electroreduction of CO2

The renewable electroreduction of CO2 to CO is a key component of future clean energy scenarios. These scenarios allow for the recycling of carbon emissions into value-added chemicals which achieves the joint goal of reducing greenhouse gase(s) while producing valuable chemical product(s). A catalyst which has a high activity and selectivity for the electroreduction of CO2 to CO is highly desired for these applications. Nonprecious metal catalysts (non-PGM) and specifically metal–nitrogen–carbon (M–N–C) catalysts are prime cathode candidates as they are selective for CO and H2 formation with only trace amounts of other products such as CH4. The traditional method of production of atomically dispersed M–N–C proceeds either through a sacrificial poymer approach or through hard-templating the catalyst with silica. The other is through the direct pyrolysis of nonabundant metal macrocycles such as MOF-based precursors via a soft-templating approach. These syntheses have substantial industrial limitations as they require harsh acid or basic solvents for postpyrolytic removal of the support or they require rare chemical precursors. The method herein uses mechanochemical mixing of a fluorine-containing polymer with common pyrolytic precursors for the in situ removal of the template during the first pyrolysis. Further ball-milling and post-treatment in ammonia atmosphere yield a highly selective catalyst for CO2 reduction. The role of the metal center in these M–N–C catalysts in promoting CO2 reduction is explored (M = Fe, Ni, Co, Mn) vs the performance of metal-free N–C. A mechanistic pathway for CO2 reduction on the various M–N–C catalysts is suggested. The champion catalyst in terms of overall selectivity/activity (Ni–N–C) boasts a 98.9 ± 0.2% faradaic efficiency for CO formation (FEco) at −1.1 V vs RHE and an unmatched selectivity for CO2 reduction (FEco > 85%) even at low overpotential (E = −0.3 V vs RHE) compared to traditional Ni–N–C. The catalysts synthesized here present a promising class of electrocatalysts which may be explored for a range of electrocatalytic applications.

Metal–nitrogen–carbon catalysts towards acidic ORR in PEMFC: fundamentals, durability challenges, and improvement strategies

Metal-Nitrogen-Carbon (M-N-C) materials are the most promising Platinum-group-metal (PGM)-free catalysts in replacing the high-cost and scarce Pt catalysts in proton exchange membrane fuel cells (PEMFCs). However, while striking improvement of M-N-C catalysts has been reached in activity, the headache degradation problems hinder their real-world application. Herein, we present a comprehensive overview of the durability of the M-N-C catalyst for oxygen reaction reduction (ORR). The fundamental understanding and identification of the ORR performance of M-N-C catalysts are discussed. Meanwhile, the standard methods to evaluate and predict the ORR performance of the PGM-free catalysts are suggested. We mainly introduce the durability challenges of the M-N-C catalyst and explain the inactivation mechanism in detail. The proposed solution and useful strategies to alleviate catalyst degradation are systematically summarized to overcome the durability bottlenecks.

S-doped M-N-C catalysts for the oxygen reduction reaction: Synthetic strategies, characterization, and mechanism

The systematic summarization of the synthesis-structure-performance-mechanism correlations provides guidance for the rational design of S-doped M-N-C catalysts for ORR. • Recent review on preparation of S-doped M-N-C catalysts for ORR. • Effects of doping S atoms on the ORR performance of M-N-C catalysts. • The advanced characterization methods and computational methods for S-doped M-N-C catalysts are introduced. • The future development direction of M-N-C catalysts is described. Due to the strong electronegativity of the N atom, the adsorption energy of the transition metal –nitrogen active site for the ORR intermediate is too high to facilitate the subsequent elementary reactions. It is worth noting that introducing sulfur atoms in transition metal–nitrogen–carbon catalysts can optimize the charge distribution of nitrogen atoms around metal sites to improve the electronic structure and structural defects of the catalysis, so as to boost the adsorption and transfer of oxygen-containing intermediates. Meanwhile, sulfur-doping can also inhibit metal nanoparticle formation and promote hierarchical porous carbon formation. This review will afford the S-doped M-N-C catalysts rational design principle and strategies and introduce the corresponding ORR electrochemical properties. We will combine advanced characterization techniques and DFT calculation to discuss the coordination environment of active sites and the ORR mechanism of S-doped M-N-C catalysts. Finally, a general perspective on the challenges and opportunities for S-doped M-N-C catalysts will be proposed to facilitate further development.

Enriched sp2-Hybridized C Atoms toward the Tradeoff between Activity, Conductivity and Stability of Spherical Porous Metal–Nitrogen–Carbon Catalysts for Rechargeable Zinc–Air Batteries

Enriched sp²-Hybridized C Atoms toward the Tradeoff between Activity, Conductivity and Stability of Spherical Porous Metal–Nitrogen–Carbon Catalysts for Rechargeable Zinc–Air Batteries

Electrochemical Trends of Atomically Dispersed Metal-Nitrogen-Carbon Materials As Oxygen Reduction Reaction Catalysts and Active Supports

Platinum-based catalysts currently remain the best option for polymer electrolyte membrane fuel cell (PEMFC) applications. Meanwhile, atomically dispersed metal-nitrogen-carbon (MNC) materials have been the frontrunner for platinum-group metal free catalysts for the oxygen reduction reaction (ORR)1. Considering that the platinum-based nanoparticles are preferably supported on carbonaceous materials, it would be intuitive to use MNC catalysts as a support for platinum nanoparticles to further increase the activity and performance of the catalyst layer in a PEMFC. In this work, we synthesized and investigated a series of MNC materials (Mn-N-C, Fe-N-C, Co-N-C, Ni-N-C, and Cu-N-C) using a previously described method2 to elucidate the electrochemical trends of possible interactions between the MNC materials as a support and standard Pt nanoparticles. Recent works3,4 have shown that combining MNC catalysts with Pt catalysts not only improves the activity of the overall composite catalyst, but also improves the stability of both counterparts. As shown in Figure 1, the supported Pt nanoparticles on MNC materials outperforms the same type of Pt nanoparticles when supported on an industry standard carbon.Reference Asset, T. & Atanassov, P. Iron-Nitrogen-Carbon Catalysts for Proton Exchange Membrane Fuel Cells. Joule 4, 33–44 (2020).Murphy, E. et al. Using Atomically Dispersed Bimetallic Active Sites to Probe the Nitrate Reduction Mechanism to Ammonia. ECS Meet. Abstr. MA2021-02, 1559 (2021).Ao, X. et al. Atomically dispersed Fe–N–C decorated with Pt-alloy core–shell nanoparticles for improved activity and durability towards oxygen reduction. Energy Environ. Sci. 13, 3032–3040 (2020).Qiao, Z. et al. Atomically dispersed single iron sites for promoting Pt and Pt3Co fuel cell catalysts: performance and durability improvements. Energy Environ. Sci. (2021). Figure 1

Fe- and Co-Containing Nitrogen-Doped Nanocarbon Catalysts from 5-Methylresorcinol for Anion Exchange Membrane Fuel Cells

Replacing fossil fuels with alternative energy sources has a growing scientific and societal interest because of the the environmental concerns related with their utilization. Low-temperature polymer electrolyte fuel cells have become a subject of intense research as the future cornerstone of renewable energy-based economy.Platinum nanoparticles supported on high-area carbon materials (Pt/C) are commonly used as the cathode catalyst in low-temperature fuel cells. However, the high cost of precious metal-based catalysts limits the commercial viability of such fuel cells. Considerable effort has been directed to design efficient and cheap electrocatalysts through the last decades and transition metal-containing nitrogen-doped carbon nanomaterials have shown the most promising results. A simple strategy to prepare these catalyst materials is high-temperature pyrolysis of organic carbon precursors in the presence of nitrogen and metal sources, where carbonization and N-doping occur simultaneously.1 In this work, highly active oxygen reduction reaction (ORR) electrocatalysts from 5-methylresorcinol, dicyandiamide (DCDA) and Fe and/or Co salts were prepared by a simple one-pot synthesis procedure.2 It has been proposed that DCDA polymerizes into graphitic carbon nitride at around 550 °C, which acts as a reactive template during carbonization of organic precursors, decomposing above 750 °C to produce the micro- and macropores necessary for efficient mass transfer and yielding N-doped graphene-like carbon structures.3 The SEM and TEM analysis revealed that the materials are composed of wrinkled carbon structures doped with nitrogen and metals, and also contain metal nanoparticles encapsulated in N-doped carbon layers. According to the XRD analysis, these nanoparticles are mainly composed of Co, Fe/Fe3O4 and FeCo alloy for the CoNC, FeNC and FeCoNC catalysts, respectively. The specific surface area of the materials was between 290 and 410 m2 g−1. The XPS analysis revealed that the catalysts contain metal-coordinated N centers, while pyridinic nitrogen is the most abundant N species in all materials. Electrocatalytic activity for ORR in alkaline media was evaluated via the rotating disk electrode (RDE) method. The ratio of the precursors was optimized and bimetallic catalysts were found to be more active toward ORR than their monometallic counterparts. Acid treatment slightly increased the catalysts’ ORR activity. The acid-treated bimetallic catalyst (FeCoNC-at) showed exceptional ORR performance, comparable to that of commercial Pt/C (20 wt%) (Figure 1a). This can be attributed to the high surface metal and nitrogen contents, which at least partly are present as nitrogen-coordinated metal (M-Nx) centers, as confirmed by inhibition of ORR in the presence of cyanide ions. The catalyst also demonstrated a rather high stability in short-time tests (15000 potential cycles) and good tolerance to methanol. The FeCoNC-at catalyst was further tested in an anion exchange membrane fuel cell (AEMFC) with hexamethyl-p-terphenyl poly(benzimidazolium) (HMP-PMBI) membrane,4 where a high value of peak power density (P max= 415 mW cm–2) was achieved (Figure 1b).2 References Sarapuu, E. Kibena-Põldsepp, M. Borghei, K. Tammeveski, Electrocatalysis of oxygen reduction on heteroatom-doped nanocarbons and transition metal–nitrogen–carbon catalysts for alkaline membrane fuel cells, J. Mater. Chem. A 6, 776–804 (2018).Kisand, A. Sarapuu, D. Danilian, A. Kikas, V. Kisand, M. Rähn, A. Treshchalov, M. Käärik, M. Merisalu, P. Paiste, J. Aruväli, J. Leis, V. Sammelselg, S. Holdcroft, K. Tammeveski, Transition metal-containing nitrogen-doped nanocarbon catalysts derived from 5-methylresorcinol for anion exchange membrane fuel cell application, J. Colloid Interface Sci. 584, 263–274 (2021).-T. Ren, Z.-Y. Yuan, A universal route to N-coordinated metals anchored on porous carbon nanosheets for highly efficient oxygen electrochemistry, J. Mater. Chem. A 7, 13591–13601 (2019).G. Wright, J. Fan, B. Britton, T. Weissbach, H.-F. Lee, E. A. Kitching, T. J. Peckham, S. Holdcroft, Hexamethyl-p-terphenyl poly(benzimidazolium): a universal hydroxide-conducting polymer for energy conversion devices, Energy Environ. Sci. 9, 2130–2142 (2016). Figure 1

Transition Metal and Nitrogen-Doped Mesoporous Carbons As Cathode Catalysts for Anion-Exchange Membrane Fuel Cells

Energy is an essential part of our lives and causes significant impact on the environment. As hydrogen is clean, cost-efficient and easily producible, then it is believed to become an essential player in the energy field.1 The proton exchange membrane fuel cells fueled by hydrogen are already used in the transport sector, but their further commercialization is limited by the excess usage of precious metals. To ease the search for replacements, the harsh acidic environment can be changed to alkaline, thus giving opportunity to use non-precious metal catalysts.2 Especially for the cathodic oxygen reduction reaction (ORR), different transition metal-containing nitrogen-doped carbon-based materials have shown great promise.3 This gives an opportunity for the rise of anion exchange membrane fuel cells (AEMFCs) as future hydrogen-based energy devices.2,3 In this work, a novel catalyst support based on mesoporous carbon (MPC, from Pajarito Powder, LLC) is used to produce M−N−C type catalyst materials. Such mesoporous structure is useful for better mass transport in the catalyst layer during AEMFC operation. This MPC support is mixed with 1,10-phenanthroline as nitrogen source and transition metal acetates, followed by high-temperature pyrolysis at 800 °C in an inert atmosphere. Five M-N-C materials are prepared with the following transition metal combinations: Co, Fe, CoFe, CoMn, and FeMn.Several physico-chemical characterization methods (SEM-EDX, STEM, XPS, MP-AES, Raman spectroscopy, and N2 physisorption) are employed to study the materials. These indeed proved that all five catalyst materials have feasible mesoporous structure (pore diameters predominantly 7-8 and 25-35 nm) and the doping with transition metals (content ca. 1 wt%) and nitrogen (ca. 2.3 at%) has been a success. The electrochemical testing to study the ORR pathway and activity of these materials in alkaline media was done using the RRDE method. CoFe-N-MPC, Fe-N-MPC and FeMn-N-MPC catalysts showed similar and excellent electrocatalytic performance by obtaining half-wave potential of 0.9 V vs RHE. The lowest peroxide yield was obtained with Fe-N-MPC and FeMn-N-MPC. The latter two catalyst materials also showed very good performance as cathode catalysts in an H2/O2 AEMFC together with an HMT-PMBI4 membrane, obtaining peak power density of >470 mW cm–2. This indicates that the M-N-MPC materials are promising cathode catalysts for the AEMFC application.References Nazir, H.; Louis, C.; Jose, S.; Prakash, J.; Muthuswamy, N.; Buan, M.E.M.; Flox, C.; Chavan, S.; Shi, X.; Kauranen, P.; Kallio, T.; Maia, G.: Tammeveski, K.; Lymperopoulos, N.; Carcadea, E.; Veziroglu, E.; Iranzo, A.; Kannan, A.M. Is the H2 economy realizable in the foreseeable future? Part I: H2 production methods. J. Hydrogen Energy 2020, 45, 13777-13788, DOI: 10.1016/j.ijhydene.2020.03.092Gottesfeld, S.; Dekel, D. R.; Page, M.; Bae, C.; Yan, Y. S.; Zelenay, P.; Kim, Y. S. Anion exchange membrane fuel cells: current status and remaining challenges. Power Sources 2018, 375, 170-184, DOI: 10.1016/j.jpowsour.2017.08.010Sarapuu, A.; Kibena-Põldsepp, E.; Borghei, M.; Tammeveski, K. Electrocatalysis of oxygen reduction on heteroatom-doped nanocarbons and transition metal–nitrogen–carbon catalysts for alkaline membrane fuel cells. J. Mater. Chem. A 2018, 6, 776-804, DOI: 10.1039/C7TA08690C.Wright, A.G.; Fan, J.T.; Britton, B.; Weissbach, T.; Lee, H.F.; Kitching, E.A.; Peckham, T.J.; Holdcroft, S. Hexamethyl-p-terphenyl poly(benzimidazolium): a universal hydroxide-conducting polymer for energy conversion devices. Energy Environ. Sci., 2016, 9, 2130-2142, DOI: 10.1039/C6EE00656F.

Study of Non-Noble-Metal-Based Metal–Nitrogen–Carbon Catalysts for Formic Acid Dehydrogenation

Study of Non-Noble-Metal-Based Metal–Nitrogen–Carbon Catalysts for Formic Acid Dehydrogenation

Theoretical study of p-block metal–nitrogen–carbon single-atom catalysts for the oxygen reduction reaction

A ‘double-peak’ ORR volcano plot is found for the p-block metal–nitrogen–carbon catalysts by considering both pristine and OH* self-modifying sites.