Fe/N/C non-precious catalysts for PEM fuel cells: Influence of the structural parameters of pristine commercial carbon blacks on their activity for oxygen reduction

Atomic metal coordinated to nitrogen-doped carbon electrocatalysts for proton exchange membrane fuel cells: a perspective on progress, pitfalls and prospectives

Proton exchange membrane fuel cells require reduced construction costs to improve commercial viability, which can be fueled by elimination of platinum as the O2 reduction electrocatalyst. The past 10 years has seen significant developments in synthesis, characterisation, and electrocatalytic performance of the most promising alternative electrocatalyst; single metal atoms coordinated to nitrogen-doped carbon (M-N-C). In this Perspective we recap some of the important achievements of M-N-Cs in the last decade, as well as discussing current knowledge gaps and future research directions for the community. We provide a new outlook on M-N-C stability and atomistic understanding with a set of original density functional theory simulations.

Review on the Degradation Mechanisms of Metal-N-C Catalysts for the Oxygen Reduction Reaction in Acid Electrolyte: Current Understanding and Mitigation Approaches

One bottleneck hampering the widespread use of fuel cell vehicles, in particular of proton exchange membrane fuel cells (PEMFCs), is the high cost of the cathode where the oxygen reduction reaction (ORR) occurs, due to the current need of precious metals to catalyze this reaction. Electrochemists tackle this issue in the short/medium term by developing catalysts with improved utilization or efficiency of platinum, and in the longer term, by developing catalysts based on Earth-abundant elements. Considerable progress has been achieved in the initial performance of Metal-nitrogen-carbon (Metal-N-C) catalysts for the ORR, especially with Fe-N-C materials. However, until now, this high performance cannot be maintained for a sufficiently long time in an operating PEMFC. The identification and mitigation of the degradation mechanisms of Metal-N-C electrocatalysts in the acidic environment of PEMFCs has therefore become an important research topic. Here, we review recent advances in the understanding of the degradation mechanisms of Metal-N-C electrocatalysts, including the recently identified importance of combined oxygen and electrochemical potential. Results obtained in a liquid electrolyte and a PEMFC device are discussed, as well as insights gained from in situ and operando techniques. We also review the mitigation approaches that the scientific community has hitherto investigated to overcome the durability issues of Metal-N-C electrocatalysts.

Effective Approaches for Designing Stable M-Nx /C Oxygen-Reduction Catalysts for Proton-Exchange-Membrane Fuel Cells

The large-scale commercialization of proton-exchange-membrane fuel cells (PEMFCs) is extremely limited by their costly platinum-group metals (PGMs) catalysts, which are used for catalyzing the sluggish oxygen reduction reaction (ORR) kinetics at the cathode. Among the reported PGM-free catalysts so far, metal-nitrogen-carbon (M-N_x /C) catalysts hold a great potential to replace PGMs catalysts for the ORR due to their excellent initial activity and low cost. However, despite tremendous progress in this field in the past decade, their further applications are restricted by fast degradation under practical conditions. Herein, the theoretical fundamentals of the stability of the M-N_x /C catalysts are first introduced in terms of thermodynamics and kinetics. The primary degradation mechanisms of M-N_x /C catalysts and the corresponding mitigating strategies are discussed in detail. Finally, the current challenges and the prospects for designing highly stable M-N_x /C catalysts are outlined.

A Top-Down Templating Strategy toward Functional Porous Carbons

Nanostructured carbon materials with high porosity and desired chemical functionalities are of immense interest because of their wide application potentials in catalysis, environment, and energy storage. Herein, a top-down templating strategy is presented for the facile synthesis of functional porous carbons, based on the direct carbonization of diverse organic precursors with commercially available metal oxide powders. During the carbonization, the metal oxide powders can evolve into nanoparticles that serve as in situ templates to introduce nanopores in carbons. The porosity and heteroatom doping of the prepared carbon materials can be engineered by varying the organic precursors and/or the metal oxides. It is further demonstrated that the top-down templating strategy is applicable to prepare carbon-based single-atom catalysts with iron-nitrogen sites, which exhibit a high power density of 545 mW cm^-2 in a H₂ -air proton exchange membrane fuel cell.

Mn-Ni-Co-O Spinel Oxides towards Oxygen Reduction Reaction in Alkaline Medium: Mn0.5Ni0.5Co2O4/C Synergism and Cooperation

Mn-doped spinel oxides MnxNi1−xCo2O4 (x = 0, 0.3, 0.5, 0.7, and 1) were synthesized using the citric acid-assisted sol–gel method. The Mn0.5Ni0.5Co2O4 (x = 0.5) supported on carbon nanosheets, Mn0.5Ni0.5Co2O4/C, was also prepared using the same method employing NaCl and glucose as a template and carbon source, respectively, followed by pyrolysis under an inert atmosphere. The electrocatalytic oxygen reduction reaction (ORR) activity was performed in alkaline media. Cyclic voltammetry (CV) was used to investigate the oxygen reduction performance of MnxNi1−xCo2O4 (x = 0, 0.3, 0.5, 0.7, and 1), and Mn0.5Ni0.5Co2O4 was found to be the best-performing electrocatalyst. Upon supporting the Mn0.5Ni0.5Co2O4 on a carbon sheet, the electrocatalytic activity was significantly enhanced owing to its large surface area and the improved charge transfer brought about by the carbon support. Rotating disk electrode studies show that the ORR electrocatalytic activity of Mn0.5Ni0.5Co2O4/C proceeds via a four-electron pathway. Mn0.5Ni0.5Co2O4/C was found to possess E1/2(V) = 0.856, a current density of 5.54 mA cm−2, and a current loss of approximately 0.11% after 405 voltammetric scan cycles. This study suggests that the interesting electrocatalytic performance of multimetallic transition metal oxides can be further enhanced by supporting them on conductive carbon materials, which improve charge transfer and provide a more active surface area.

Metal-Nitrogen-Carbon Catalysts of Specifically Coordinated Configurations toward Typical Electrochemical Redox Reactions

Metal-nitrogen-carbon (M-N-C) material with specifically coordinated configurations is a promising alternative to costly Pt-based catalysts. In the past few years, great progress is made in the studies of M-N-C materials, including the structure modulation and local coordination environment identification via advanced synthetic strategies and characterization techniques, which boost the electrocatalytic performances and deepen the understanding of the underlying fundamentals. In this review, the most recent advances of M-N-C catalysts with specifically coordinated configurations of M-N_x (x = 1-6) are summarized as comprehensively as possible, with an emphasis on the synthetic strategy, characterization techniques, and applications in typical electrocatalytic reactions of the oxygen reduction reaction, oxygen evolution reaction, hydrogen evolution reaction, CO₂ reduction reaction, etc., along with mechanistic exploration by experiments and theoretical calculations. Furthermore, the challenges and potential perspectives for the future development of M-N-C catalysts are discussed.

Efficient oxygen reduction electrocatalyst derived from facile Fe,N-surface treatment of carbon black

The development of nonprecious metal-based electrocatalysts for oxygen reduction reaction (ORR) is a central task in renewable electrochemical energy conversion and storage technologies. Iron-nitrogen doped carbon-based (Fe-N/C) materials are promising alternatives to Pt-based ORR electrocatalysts. Owing to large specific surface area and outstanding electrical conductivity, carbon black is an inborn support for electrocatalysts. Unfortunately, the direct incorporation of Fe-N_x moieties onto the surface of carbon black has not been realized to date. Herein, Fe-N_x moieties are directly incorporated onto the surface of carbon black through surface modification and the following Fe and N co-doping. The obtained Fe and N co-doped carbon back (Fe-N/CB) catalyst has very large specific surface area and abundant accessible Fe-N_x moieties. As a result, Fe-N/CB electrocatalyst exhibits a more positive half-wave potential (0.86 V) than Pt/C. The Fe-N/CB catalyst also displays better stability and methanol resistance than Pt/C. The Zn-air battery with Fe-N/CB as cathodic catalyst shows a maximum power density of 68 mW cm^-2 and a specific capacity of 676 mAh g_Zn^-1. Our finding provides a convenient and low-cost approach to fabricating efficient M-N/C-based catalysts and will be helpful to the development of renewable electrochemical energy conversion and storage technologies.

Effect of porosity and active area on the assessment of catalytic activity of non-precious metal electrocatalyst for oxygen reduction

We describe a method to study porous thin-films deposited onto rotating disc electrodes (RDE) applied to non-platinum group electrocatalyst obtained by pyrolysis of iron phthalocyanine and carbon, FePc/C. The electroactive area and porous properties of the thin film electrodes were obtained using electrochemical impedance spectroscopy under the framework of de Levie impedance model. The electrocatalytic activity of different electrodes was correlated to the total electroactive area (A_p) and the penetration ratio parameter through the film under ac current. The cylindrical pore model was extended to the RDE boundary conditions and derived in a Koutecky-Levich type expression that allowed to separate the effect of the electroactive area and structural properties. The resulting specific electrocatalytic activity of FePc/C heat treated at different temperatures was correlated to FePc surface concentration.

Development of a highly active FeNC catalyst with the preferential formation of atomic iron sites for oxygen reduction in alkaline and acidic electrolytes

Nitrogen-doped porous carbons containing atomically dispersed iron are prime candidates for substituting platinum-based catalysts for oxygen reduction reaction (ORR) in fuel cells. These carbon catalysts are classically synthesizedviacomplicated routes involving multiple heat-treatment steps to form the desired Fe-N_x sites. We herein developed a highly active FeNC catalyst comprising of exclusive Fe-N_x sites by a simplified solid-state synthesis protocol involving only a single heat-treatment. Imidazole is pyrolyzed in the presence of an inorganic salt-melt resulting in highly porous carbon sheets decorated with abundant Fe-N_x centers, which yielded a high density of electrochemically accessible active sites (1.36 × 10¹⁹ sites g^-1) as determined by the in situ nitrite stripping technique. The optimized catalyst delivered a remarkable ORR activity with a half-wave potential (E_1/2) of 0.905 V_RHE in alkaline electrolyte surpassing the benchmark Pt catalyst by 55 mV. In acidic electrolyte, an E_1/2 of 0.760 V_RHE is achieved at a low loading level (0.29 mg cm^-2). In PEMFC tests, a current density of 2.3 mA cm^-2 is achieved at 0.90 V_iR-free under H₂-O₂ conditions, reflecting high kinetic activity of the optimized catalyst.

Improved Electrocatalytic Activity and Durability of Pt Nanoparticles Supported on Boron-Doped Carbon Black

A facile strategy is proposed to synthesize boron-doped ECP600 carbon black (B-ECP600), and the catalyst of Pt supported on boron-doped ECP600 (Pt/B-ECP600) shows smaller particle sizes and a higher electrochemical surface area (95.62 m2·gPt−1) and oxygen reduction reaction activity (0.286 A·mgPt−1 for mass activity; 0.299 mA·cm−2 for area specific activity) compared to the catalyst of Pt supported on ECP600 (Pt/ECP600). The results show that the boron doping of the carbon supports plays an important role in controlling the size and dispersion of Pt nanoparticles and the O2 adsorption/dissociation of the oxygen reduction reaction. A further accelerated durability test proves that boron doping can greatly enhance the stability of carbon support and thus improves the electrochemical performance of the catalyst during the long-time running. All these results suggest boron-doped carbon has great potential for application in fuel cells.

Thermal and Physical Characterization of PEG Phase Change Materials Enhanced by Carbon-Based Nanoparticles

This paper presents the preparation and thermal/physical characterization of phase change materials (PCMs) based on poly(ethylene glycol) 400 g·mol^-1 and nano-enhanced by either carbon black (CB), a raw graphite/diamond nanomixture (G/D-r), a purified graphite/diamond nanomixture (G/D-p) or nano-Diamond nanopowders with purity grades of 87% or 97% (nD87 and nD97, respectively). Differential scanning calorimetry and oscillatory rheology experiments were used to provide an insight into the thermal and mechanical changes taking place during solid-liquid phase transitions of the carbon-based suspensions. PEG400-based samples loaded with 1.0 wt.% of raw graphite/diamond nanomixture (G/D-r) exhibited the lowest sub-cooling effect (with a reduction of ~2 K regarding neat PEG400). The influences that the type of carbon-based nanoadditive and nanoparticle loading (0.50 and 1.0 wt.%) have on dynamic viscosity, thermal conductivity, density and surface tension were also investigated in the temperature range from 288 to 318 K. Non-linear rheological experiments showed that all dispersions exhibited a non-Newtonian pseudo-plastic behavior, which was more noticeable in the case of carbon black nanofluids at low shear rates. The highest enhancements in thermal conductivity were observed for graphite/diamond nanomixtures (3.3-3.6%), while nano-diamond suspensions showed the largest modifications in density (0.64-0.66%). Reductions in surface tension were measured for the two nano-diamond nanopowders (nD87 and nD97), while slight increases (within experimental uncertainties) were observed for dispersions prepared using the other three carbon-based nanopowders. Finally, a good agreement was observed between the experimental surface tension measurements performed using a Du Noüy ring tensiometer and a drop-shape analyzer.

Urea-assisted synthesis of a Fe nanoparticle modified N-doped three-dimensional porous carbon framework for a highly efficient oxygen reduction reaction

A rapid gas foaming process is developed to prepare a novel Fe nanoparticle modified N-doped 3D porous carbon framework (Fe/3DNC) catalyst with a controllable morphology and catalytic efficiency through regulation of the urea content.

Hierarchically porous carbons as supports for fuel cell electrocatalysts with atomically dispersed Fe-N x moieties

The development of high-performance non-platinum group metal (non-PGM) catalysts for the oxygen reduction reaction (ORR) is still of significance in promoting the commercialization of proton exchange membrane fuel cells (PEMFCs). In this work, a "hierarchically porous carbon (HPC)-supporting" approach was developed to synthesize highly ORR active Fe-phenanthroline (Fe-phen) derived Fe-N x -C catalysts. Compared to commercial carbon black supports, utilizing HPCs as carbon supports can not only prevent the formation of inactive iron nanoparticles during pyrolysis but also optimize the porous morphology of the catalysts, which eventually increases the amount of reactant-accessible and atomically dispersed Fe-N x active sites. The prepared catalyst therefore exhibits a remarkable ORR activity in both half-cells (half-wave potential of 0.80 V in 0.5 M H2SO4) and H2-air PEMFCs (442 mA cm-2 at a working voltage of 0.6 V), making it among the best non-PGM catalysts for PEMFCs.

MOF-Derived Isolated Fe Atoms Implanted in N-Doped 3D Hierarchical Carbon as an Efficient ORR Electrocatalyst in Both Alkaline and Acidic Media

In order to improve the catalytic performance of oxygen reduction reaction (ORR), it is pivotal to increase the density and accessibility of the active sites. Herein, we have developed a template-free melamine-assisted cocalcined strategy to afford Fe-embedded and N-doped carbons (Fe-N-C) with not only high density of atomically dispersed Fe-N_x active sites but also abundant three-dimensional interconnected mesopores by directly pyrolyzing Fe-ZIF-8 covered with a controllable melamine layer. It is demonstrated that the introduction of melamine in the precursor plays a key role in constructing various carbonized products with controllable morphology, porosity, and components. With an optimal mass ratio 1:1 of melamine to Fe-ZIF-8, the resultant Fe@MNC-1 exhibits excellent ORR activity and stability, which exceeds 20 wt % commercial Pt/C catalyst (with a half-wave potential of 0.88 V vs 0.85 V) in an alkaline electrolyte and is even comparable to the commercial Pt/C catalyst (with a half-wave potential of 0.78 V vs 0.80 V) in an acidic electrolyte. To the best of our knowledge, Fe@MNC-1 can be ranked among the best nonprecious metal electrocatalysts for ORR in both alkaline and acidic media. The present synthetic strategy may provide a new opportunity for the design and construction of metal-organic framework-derived nanomaterials with rational composition and a desired porous structure to boost their electrocatalytic performance.

Bottom-Up Construction of Active Sites in a Cu-N4-C Catalyst for Highly Efficient Oxygen Reduction Reaction

Bottom-up construction of efficient active sites in transition metal-nitrogen-carbon (M-N-C) catalysts for oxygen reduction reaction (ORR) from single molecular building blocks remains one of the most difficult challenges. Herein, we report a bottom-up approach to produce a highly active Cu-N₄-C catalyst with well-defined Cu-N₄ coordination sites derived from a small molecular copper complex containing Cu-N₄ moieties. The Cu-N₄ moieties were found to be covalently integrated into graphene sheets to create the Cu-N₄ active sites for ORR. Furthermore, the activity was boosted by tuning the structure of active sites. We find that the high ORR activity of the Cu-N₄-C catalyst is related to the Cu-N₄ center linked to edges of the graphene sheets, where the electronic structure of the Cu center has the right symmetry for the degenerate π* orbital of the O₂ molecule. These findings point out the direction for the synthesis of the M-N-C catalysts at the molecular level.

In Situ Self-Template Synthesis of Fe-N-Doped Double-Shelled Hollow Carbon Microspheres for Oxygen Reduction Reaction

Herein, we reported a special Fe-N-doped double-shelled hollow carbon microsphere (Fe-N-DSC) which was prepared by a facile, in situ polymerization followed by pyrolysis. With porous ferroferric oxide (Fe₃O₄) hollow microspheres as the templates, where pyrrole monomers were dispersed around the outer surface and prefilled the interior space. By adding hydrochloric acid, Fe³⁺ ions were released to initiate polymerization of pyrrole on both the outer and inner surfaces of Fe₃O₄ microspheres until they were completely dissolved, resulting in the Fe-containing polypyrrole double-shelled hollow carbon microspheres (Fe-PPY-DSC). The Fe-PPY-DSC was then pyrolyzed to generate the Fe-N-DSC. The Fe₃O₄ hollow microspheres played trifunctional roles, i.e., the template to prepare a double-shelled hollow spherical structure, the initiator (i.e., Fe³⁺ ions) for the polymerization of pyrrole, and the Fe source for doping. The Fe-N-DSC exhibited a superior catalytic activity for oxygen reduction as comparable to commercial Pt/C catalysts in both alkaline and acidic media. The high catalytic performance was ascribed to the special porous double-shelled hollow spherical structure, which provided more active sites and was beneficial to a high-flux mass transportation.

Identification of catalytic sites in cobalt-nitrogen-carbon materials for the oxygen reduction reaction

Single-atom catalysts with full utilization of metal centers can bridge the gap between molecular and solid-state catalysis. Metal-nitrogen-carbon materials prepared via pyrolysis are promising single-atom catalysts but often also comprise metallic particles. Here, we pyrolytically synthesize a Co-N-C material only comprising atomically dispersed cobalt ions and identify with X-ray absorption spectroscopy, magnetic susceptibility measurements and density functional theory the structure and electronic state of three porphyrinic moieties, CoN₄C₁₂, CoN₃C_10,porp and CoN₂C₅. The O₂ electro-reduction and operando X-ray absorption response are measured in acidic medium on Co-N-C and compared to those of a Fe-N-C catalyst prepared similarly. We show that cobalt moieties are unmodified from 0.0 to 1.0 V versus a reversible hydrogen electrode, while Fe-based moieties experience structural and electronic-state changes. On the basis of density functional theory analysis and established relationships between redox potential and O₂-adsorption strength, we conclude that cobalt-based moieties bind O₂ too weakly for efficient O₂ reduction.Nitrogen-doped carbon materials with atomically dispersed iron or cobalt are promising for catalytic use. Here, the authors show that cobalt moieties have a higher redox potential, bind oxygen more weakly and are less active toward oxygen reduction than their iron counterpart, despite similar coordination.

Single-Atomic Ruthenium Catalytic Sites on Nitrogen-Doped Graphene for Oxygen Reduction Reaction in Acidic Medium

The cathodic oxygen reduction reaction (ORR) is essential in the electrochemical energy conversion of fuel cells. Here, through the NH₃ atmosphere annealing of a graphene oxide (GO) precursor containing trace amounts of Ru, we have synthesized atomically dispersed Ru on nitrogen-doped graphene that performs as an electrocatalyst for the ORR in acidic medium. The Ru/nitrogen-doped GO catalyst exhibits excellent four-electron ORR activity, offering onset and half-wave potentials of 0.89 and 0.75 V, respectively, vs a reversible hydrogen electrode (RHE) in 0.1 M HClO₄, together with better durability and tolerance toward methanol and carbon monoxide poisoning than seen in commercial Pt/C catalysts. X-ray adsorption fine structure analysis and aberration-corrected high-angle annular dark-field scanning transmission electron microscopy are performed and indicate that the chemical structure of Ru is predominantly composed of isolated Ru atoms coordinated with nitrogen atoms on the graphene substrate. Furthermore, a density function theory study of the ORR mechanism suggests that a Ru-oxo-N₄ structure appears to be responsible for the ORR catalytic activity in the acidic medium. These findings provide a route for the design of efficient ORR single-atom catalysts.

Trends in Adhesion Energies of Metal Nanoparticles on Oxide Surfaces: Understanding Support Effects in Catalysis and Nanotechnology

Nanoparticles on surfaces are ubiquitous in nanotechnologies, especially in catalysis, where metal nanoparticles anchored to oxide supports are widely used to produce and use fuels and chemicals, and in pollution abatement. We show that for hemispherical metal particles of the same diameter, D, the chemical potentials of the metal atoms in the particles (μ_M) differ between two supports by approximately -2(E_adh,A - E_adh,B)V_m/D, where E_ad,i is the adhesion energy between the metal and support i, and V_m is the molar volume of the bulk metal. This is consistent with calorimetric measurements of metal vapor adsorption energies onto clean oxide surfaces where the metal grows as 3D particles, which proved that μ_M increases with decreasing particle size below 6 nm and, for a given size, decreases with E_adh. Since catalytic activity and sintering rates correlate with metal chemical potential, it is thus crucial to understand what properties of catalyst materials control metal/oxide adhesion energies. Trends in how E_adh varies with the metal and the support oxide are presented. For a given oxide, E_adh increases linearly from metal to metal with increasing heat of formation of the most stable oxide of the metal (per mole metal), or metal oxophilicity, suggesting that metal-oxygen bonds dominate interfacial bonding. For the two different stoichiometric oxide surfaces that have been studied on multiple metals (MgO(100) and CeO₂(111), the slopes of these lines are the same, but their offset is large (∼2 J/m²). Adhesion energies increase as MgO(100) ≈ TiO₂(110) < α-Al₂O₃(0001) < CeO₂(111) ≈ Fe₃O₄(111).

Enhancing Electrocatalytic Performance of Bifunctional Cobalt-Manganese-Oxynitride Nanocatalysts on Graphene

We report the synthesis and characterization of graphenesupported cobalt-manganese-oxynitride nanocatalysts (CoMnON/G) as bifunctional electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). A nitriding treatment of spinel compound CoMnO increased the ORR activity considerably, and the most active material catalyzed the ORR with only a 30 mV half-wave potential difference from the commercial carbon-supported platinum (Pt/C) in alkaline media. In addition to high activity, the catalyst also exhibited an intrinsic stability that outperformed Pt/C. An appropriately designed nitridation thus facilitates new directions for developing active and durable non-precious-metal oxynitride electocatalysts.

Active Site Structures in Nitrogen-Doped Carbon-Supported Cobalt Catalysts for the Oxygen Reduction Reaction

The catalytic mechanism and the nature of active sites are revealed for the oxygen reduction reaction (ORR) with new non-noble-metal nitrogen-doped carbon-supported transition-metal catalysts (metal-N-C catalyst). Specifically, new nitrogen-doped carbon-supported cobalt catalysts (Co-N-C catalysts) are made by pyrolyzing various ratios of the nitrogen-atom rich heterocycle compound, 1-ethyl-3-methyl imidazolium dicyanamide (EMIM-dca) and cobalt salt (Co(NO₃)₂). The ORR activity (J_K at 0.8 V vs RHE, in 0.1 M KOH solution) of a typical catalyst in this family, Co₁₅-N-C800, is 8.25 mA/mg, which is much higher than the ORR activity values of N-C catalysts (0.41 mA/mg). The active site in the catalyst is found to be the Co-N species, which is most likely in the form of Co₂N. Metallic cobalt (Co) particles, Co₃C species, and N-C species are not catalytically active sites, nor do these moieties interact with the Co-N active sites during the catalysis of the ORR. Increasing the Co salt content during the synthesis favors the formation of Co-N active sites in the final catalyst. Higher pyrolysis temperatures (e.g., a temperature higher than 800 °C) do not favor the formation of the Co-N active sites, but cause the formed Co-N active sites to decompose, which, therefore, leads to a lower catalytic activity. This reveals that the control of the parameters that affect the final structure is critical to catalyst performance and, therefore, the effective development of high-performance heteroatom-doped non-noble-metal ORR catalysts.