Encapsulated iron-based oxygen reduction electrocatalysts by high pressure pyrolysis

Non-precious metal catalysts (NPMCs) are candidate materials to replace platinum for proton exchange membrane fuel cells (PEMFCs). Herein we reported a type of iron-based NPMCs prepared by high pressure pyrolysis for the oxygen reduction reaction (ORR) in acidic media. The catalysts are in form of carbon microspheres in a sub-microscale consisting of iron-containing nanoparticles encapsulated by graphitic layers. By tailoring temperatures and duration of pyrolysis, the best ORR catalyst was achieved at 700 °C and 75 min, which exhibits an onset potential of 0.85 V at 0.1 mA cm^?2 and a half-wave potential of 0.72 V in acid media. After 10,000 potential cycles, only 25 mV shift of half-wave potential is observed showing excellent stability. An analogue material prepared from nitrogen-free precursors shows significant electrochemical activity though it is much lower than that from the nitrogen containing precursors and can be improved by post treatment in ammonia. These results indicate the contribution to the catalysis from surface nitrogen functionalities and encapsulated metal-containing nanoparticles.     

Comparative investigation on the properties of carbon-supported cobalt-polypyrrole pyrolyzed at various conditions as electrocatalyst towards oxygen reduction reaction

A series of non-precious metal catalysts named as Co-PPy-TsOH/C towards oxygen reduction reaction (ORR) were synthesized by pyrolyzing carbon supported cobalt-polypyrrole at various temperatures for diverse durations. The catalytic activity of these catalysts was evaluated with electrochemical techniques of cyclic voltammetry, rotating disk electrode and rotating ring-disk electrode. Physicochemical techniques, such as XRD, TEM and XPS, were employed to characterize the structure/morphology of the catalysts in order to understand the effects of pyrolysis conditions on the ORR activity. The results showed that both pyrolysis temperature and the duration have essential effects on the structure/morphology as well as ORR activity of the Co-PPy-TsOH/C catalysts, pyrolyzing the precursor at 800 °C for 2 h is the optimal condition to synthesize the catalyst with the best ORR performance.     

Mesoporous carbon based non-precious metal catalysts for oxygen reduction reaction: Effect of metal species and valence state

In this work, mesoporous carbon decorated by non-precious metals (FeSO₄, Fe₂(SO₄)₃ and CoSO₄) have been successfully prepared using a simple one-pot process. A low-cost and easily available nitrogen-containing polymer [poly(ethyleneimine), (PEI)] was chosen as carbon precursor, and nano-silica was employed as a template to provide a large specific area and abundant active sites for the catalyst. The FeSO₄-PEI was found to exhibit excellent ORR activity in 0.1 M KOH in terms of onset potential and half wave potential of 0.178V and 0.05V (vs. SHE), respectively. FeSO₄-PEI also shows remarkable ORR performance even in 0.5 M H₂SO₄ with an onset potential and a half wave potential of 0.807V and 0.654V (vs. SHE), respectively. Durability test demonstrates that the FeSO₄-PEI only has a small negative shift about 19 mV in 0.1 M KOH and 30 mV in 0.5 M H₂SO₄ aqueous solution, suggesting an outstanding stability for oxygen reduction. Combined with the physical characterizations of SEM-EDS, XRD, XPS and N₂ adsorption-desorption isotherm, FeSO₄ verifies strong ability to complex with PEI, which formed more efficient active sites when compared to Fe₂(SO₄)₃-PEI and CoSO₄-PEI catalysts. The good catalytic properties could be attributed to the unique nature of metal species, the high specific surface area and the favorable degree of graphitization.     

Astragali Radix-derived nitrogen-doped porous carbon: An efficient electrocatalyst for the oxygen reduction reaction

The development of biomass-derived nitrogen-doped porous carbons (NPCs) for the oxygen reduction reaction (ORR) is important for sustainable energy systems. Herein, NPCs derived from Astragali Radix (AR) via a cost-effective strategy are reported for the first time. The as-prepared AR-950-5 catalyst shows a stacked layer-like structure and porosity. Notably, the optimized AR-950-5 delivers catalytic activity comparable to that of commercial Pt/C (C-Pt/C), with high onset potential, positive half-wave potential and large limiting current density. It also displays superior long-term stability and methanol tolerance for ORR. This work will pave the way for a new approach in the development of highly active and low-cost NPCs for fuel cells.     

Effect of transition metal (M: Fe,Co or Mn) for the oxygen reduction reaction with non-precious metal catalysts in acid medium

The effect of the metal for the oxygen reduction reaction (ORR) in acid medium with non-precious metal catalysts has been investigated. A series of non-precious metal catalysts with typical formulation M/N/C with M being Mn, Co or Fe have been prepared by incorporating N onto an active carbon matrix by means of thermal treatments under inert atmospheres. The N-containing active carbons were further treated with the M-containing precursors based upon Mn, Co or Fe phthalocyanines and thermally treated under inert atmosphere. The performance for the ORR in acid medium of all of the catalysts has been evaluated by means of electrochemical techniques. The activity, both in terms of onset potential for the ORR and maximum current density at representative potentials between 900 and 700 mV follows the trend Fe > Co > Mn. In addition, the performance of the Fe-based catalysts obtained during the different stages of the catalyst preparation has been also evaluated. The catalysts obtained after the pyrolysis step are the only ones showing measurable rates for the ORR. Although the amount of N and Fe incorporated onto the carbon matrix decreases the pyrolysis treatment, this treatment leads to the formation of the real active sites for the ORR irrespectively of the nature of the transition metal.     

A derivative of mesoporous oxygen reduction reaction electrocatalysts from citric acid and dicyandiamide

It remains urgent to make continuous efforts on well-designed and highly active non-precious metal (NPM) electrocatalysts for the cathodic oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs), thus helping greatly reduce the fuel cell cost. Due to an unsatisfied stability caused by Fenton reaction for Fe-based materials, Co-based materials bear much more expectations as one type of NPM electrocatalysts to be applied in the ORR. Here we report a novel strategy to synthesize a series of mesoporous nitrogen-doped carbon-supported cobalt electrocatalysts (Co-DCD-CA), which takes full advantage of electrostatic interaction between carboxyl in citric acid (CA) and amidogen in dicyandiamide (DCD) as well as chelating interaction between citric acid and cobalt cation. When CA is employed as carbon source, the optimal derivative of the Co-DCD-2-CA-900 electrocatalyst exhibits a higher ORR activity with a half-wave potential at 0.75 V, which is 60 mV higher than that prepared using Ketjenblack EC 300 J (Co-DCD-2-EC-900) as the carbon support. Besides, the effects of pyrolysis temperature as well as DCD to CA ratio on the ORR activity are detailedly investigated.     

Recent progress in heteroatom doped carbon based electrocatalysts for oxygen reduction reaction in anion exchange membrane fuel cells

Oxygen reduction reaction (ORR) is a key step in many electrochemical devices such as fuel cells and metal-air batteries. However, the reaction proceeds at a significant overpotential requiring Pt-based catalysts. The scarcity and economical challenges associated with Pt is one of the major limitations for the commercialization of the devices. In this context, the electrochemical research community is constantly exploring other low-cost and earth-abundant materials as ORR catalysts. Carbon nanomaterials are identified as promising electrocatalysts due to their superb electronic conductivity together with high specific surface area. However, the low reactivity of carbon is the major limiting factor in the fabrication of ORR catalysts. Recent studies have proved that chemical modification of the carbon network (substitution of foreign atoms, Ex: N, S, B, F, P) could alter the reactivity of carbon nanomaterials for ORR. Many doping strategies have been proposed including single atom doping, co-doping and multi-atom doping. The heteroatom doped carbons have delivered promising results towards ORR in alkaline media. This review presents a rational approach of doping methods and the electrochemical properties of heteroatom doped carbons, and we believe that this review could be a guiding material to design advanced non-noble catalysts for ORR in the coming future.     

Mesoporous spinel NiFe oxide cubes as advanced electrocatalysts for oxygen evolution

Oxygen evolution reaction (OER) is the rate-controlling step of the electrochemical water splitting. The slow kinetics hinders large-scale H₂ production. Herein, the spinel NiFe oxides were prepared by directly pyrolyzing nickel hexacynoferrate precursors in air. The NiFe oxides were presented as mesoporous nanocubes with a specific surface area of 125 m² g⁻¹. The mesoporous spinel NiFe oxide nanocubes can afford a geometric current of 10 mA cm⁻² at a low overpotential of a 0.24 V and a small Tafel slope of 41 mV dec⁻¹ in alkaline solution. The specific activity can reach up to 0.37 mA cm⁻² with a turnover frequency of 0.93 s⁻¹. The superior OER activity of the NiFe oxide nanocubes (NiFeO NCs) can outperform those of the state-of-the-art IrO₂ catalysts, and compare favorably with other spinel transition metal oxides reported recently under identical condition. NiFeO NCs also show a long-term durability without significant loss of the OER activity. Our works provide a new strategy to develop efficient, robust and earth-abundant spinel NiFe oxides as advanced OER electrocatalysts to replace the expensive commercial IrO₂ catalysts for water splitting in the industrial scale.     

Hollow structured Zn0.76Co0.24S–Co9S8 composite with two-phase synergistic effect as bifunctional catalysts

It is of great significance to develop efficient and inexpensive oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) bifunctional catalysts for the current energy crisis. But this is still a long-term and formidable challenge. In this work, a two-phase synergistic effect between transition metal sulfides is applied in the design of ORR/OER bifunctional catalysts. Co-based zeolite imidazolate framework (MOF) is used as a precursor to prepare the hollow Co₉S₈ through vulcanization and heat treatment. At the same time, the phase transformation occurs to form some Zn_0.76Co_0.24S on surface during the heat treatment due to the Zn-polydopamine coating. The hollow structure Zn_0.76Co_0.24S–Co₉S₈ composite has been verified and promotes the diffusion of reactants and products, and the interaction between two phases greatly promotes the catalytic reactions. The coating of Zn-polydopamine forms uniform carbon layer on surface, enhancing the conductivity of Zn_0.76Co_0.24S–Co₉S₈ composite. The Zn_0.76Co_0.24S–Co₉S₈ composite shows much better performance of ORR and OER compared to any of them. The overpotential of OER at a current density of 10 mA/cm² is 330 mV, and the half-wave potential of ORR is 0.83 V. Additionally, the Zn_0.76Co_0.24S–Co₉S₈ composite displays better cyclic stability. The synergistic effect of Zn_0.76Co_0.24S and Co₉S₈ can be considered as the foremost factor for the improvement of catalyst performance, which provides new possibilities for the development of non-precious metal bifunctional catalysts.     

Microwave assisted synthesis of ruthenium electrocatalysts for oxygen reduction reaction in the presence and absence of aqueous methanol

In this work we present the synthesis of ruthenium based electrocatalysts for oxygen reduction reaction in 0.5 mol L⁻¹ H₂SO₄, using microwave irradiation at different power, time and temperature conditions. Ru₃(CO)₁₂ and 1,2-dichlorobenzene were used as precursor and solvent respectively. The materials obtained were structurally characterized by FT-IR spectroscopy and X-ray diffraction; their chemical composition was determined by energy-dispersive spectroscopy analysis. The rotating disk electrode technique was used for the electrochemical characterization of the catalysts; the oxygen reduction reaction was performed in the presence and absence of aqueous methanol solutions. The electrocatalytic activity towards the oxygen reduction reaction is similar to that of ruthenium catalysts synthesized using a conventional process reported in the literature.     

Iron and boron-doped carbonized zeolitic imidazolate frameworks as efficient oxygen reduction electrocatalysts for Al-Air batteries

Porous boron-bearing Fe-nitrogen doped carbon electrocatalysts (Fe-BNC) are prepared by pyrolysis treatment of Fe/B co-doped zeolitic imidazolate frameworks (ZIFs). The as-obtained Fe-BNC catalysts with a high surface area (1300 m² g⁻¹) favor a 4-electron reduction pathway for efficient oxygen reduction reaction (ORR). The Fe-BNC catalysts demonstrate a half-wave potential of ∼0.85 V vs RHE comparable to that of Pt/C catalyst and high stability in 0.1 M KOH. The dopant of little boron and iron into nitrogen-doped carbon results in the high surface area, enhanced surface polarities, electronic properties and exposing more active sites to introduce a synergistic effect for enhanced ORR performance. Moreover, Fe-BNC electrocatalysts used as air cathode for Al-air batteries exhibit a high peak power density of 195.2 mw cm⁻² and excellent stability even after discharging for 24 h at room temperature, revealing an excellent performance in application of metal-air batteries and other energy converting devices.     

Recent advances in metal-organic frameworks-derived carbon-based electrocatalysts for the oxygen reduction reaction

The oxygen reduction reaction (ORR) plays a crucial role in green-energy-related technologies including fuel cells and metal-air batteries. The rational design of cost-effective and high-efficiency electrocatalysts is of paramount importance to accelerate the sluggish kinetics of ORR. Recently, owing to the structural flexibility, high porosity, and large surface area, metal-organic frameworks (MOF)-derived carbon-based electrocatalysts have drawn significant attention as competent ORR catalysts. This review systematically summarizes recent progress toward advanced MOF-derived carbon-based electrocatalysts for ORR from the aspects of composition engineering, structural engineering, and the practical application in fuel cells and metal-air batteries, with the emphasis on the synthetic methods and the engineering mechanisms to achieve the enhanced ORR performance. Finally, some existing challenges and future perspectives for MOF-derived carbon-based electrocatalysts are proposed.     

The role of arginine as nitrogen doping and carbon source for enhanced oxygen reduction reaction

Doped carbon nanostructures as non-precious metal (NPM) catalysts for oxygen reduction reaction (ORR) in acid medium are mainly synthesized using 5, 10, 15, 20-tetrakis (4-methoxyphenyl)-porphyrin-Fe (III) chloride (Fe-TMPP) as doping and carbon sources. In this study, the doped carbon nanostructures used as cathode NPM catalysts for ORR are prepared using a mixture of iron phthalocyanine (FePc) and arginine as doping and carbon sources. The morphology and composition of the as-prepared samples are characterized using field-emission scanning electron microscopy, field-emission transmission electron microscopy, and energy dispersive X-ray (EDX) spectroscopy. The crystal and pore structures are analyzed using X-ray diffraction method, Raman spectroscopy, and nitrogen adsorption/desorption method. The sample prepared using a precursor mixture with a proper ratio of FePc and arginine exhibits significantly superior ORR performance, i.e. high specific activity, enhanced half-wave potential, and improved stability in an acid medium, as even compared to a commercial Pt/C. The improved ORR properties is mainly attributed to high portion of pyridinic N state with a relatively high specific surface area, which can result from the FePc precursor surrounded by the fused arginine.     

Enhancement of oxygen reduction activity with addition of carbon support for non-precious metal nitrogen doped carbon catalyst

An improved synthesis scheme of non-precious metal N-doped carbon catalysts for oxygen reduction reaction is reported. The non-precious metal N-doped carbon catalysts were prepared by pyrolysis of the mixture (phenol resin, Ketjen black carbon support and cobalt phenanthroline complex). The drastic improvement of distribution state of Ketjen black supported non-precious metal N-doped carbon catalysts was observed by means of transmission electron microscopy (TEM). In addition, the non-precious metal N-doped carbon catalyst synthesized with Ketjen black carbon support showed much higher oxygen reduction reaction (ORR) activity relative to unsupported non-precious metal N-doped carbon catalyst in O₂-saturated 0.5 mol l⁻¹ H₂SO₄ at 35 °C. Moreover, the highest ORR activity was obtained with addition of optimum amount of Ketjen black carbon support was thirtyfold compared to unsupported non-precious metal N-doped carbon catalyst at 0.7 V. Similarly, the performance of a polymer electrolyte fuel cell (PEFC) using the non-precious metal N-doped carbon catalyst as the cathode electrode catalyst was obviously better than that of the non-precious metal N-doped carbon catalyst before optimization. Microstructure, specific surface area and surface composition of the non-precious metal N-doped carbon catalysts were analyzed by XRD, XPS and BET measurement with nitrogen physisorption, respectively.     

Peat as a carbon source for non-platinum group metal oxygen electrocatalysts and AEMFC cathodes

Naturally abundant well-decomposed peat was used as a precursor for synthesizing several non-platinum group metal-type oxygen electrocatalysts. The materials were studied in an alkaline environment, where it was discovered that the oxygen evolution (OER) and the oxygen reduction (ORR) activity of the catalysts can be severely influenced by changing the parameters of the peat carbonization procedure. High OER activity was achieved with a minimally treated catalyst which seemed to be because of a Co-rich FeCo alloy species. In both rotating disc electrode and anion exchange membrane fuel cell experiments, the catalyst based on ZnCl₂-activated peat-derived carbon showed superior ORR performance with a peak power density of 51 mW cm^?2. It was found that the peak power densities of the catalysts correlated with several physical parameters. Above all, we demonstrate the possibility of fabricating advanced functional carbon materials for oxygen electrocatalysis from peat.     

Medulla stachyuri-derived iron and nitrogen co- doped 2D porous carbon-flakes for highly efficient oxygen reduction electrocatalysis and supercapacitors

Iron and nitrogen co-doped two-dimensional (2D) porous carbon-flakes have been fabricated by using foam-like Medulla stachyuri (MS, the stem pith of tetrapanax papyrifer) as both carbon precursor and template and ammonium ferric citrate as iron and nitrogen precursor. The ammonium ferric citrate-impregnated foams are subsequently converted into iron and nitrogen co-doped 2D porous carbon-flakes by pyrolysis at high temperature in an inert atmosphere. The porous carbon-flakes fabricated at 900 °C (MS-Fe-900) possess high surface area (1140.9 m² g⁻¹) and effective Fe/N co-doping (0.22 at.% Fe and 2.02 at.% N). In comparison with Pt/C, MS-Fe-900 exhibits superior ORR activity (E₀ = 968 mV; E_1/2 = 830 mV vs RHE), preferable methanol/CO tolerance and better stability. Furthermore, the MS-Fe-900-based electrode presents high-rate performance (80.1% capacitance retention from 1 to 100 A g⁻¹), and good cycling stability for over 10000 cycles in 6 M KOH electrolyte. This work takes full advantage of the unique structure of biomass and provides a feasible approach to develop cost-efficient and high performance activated carbon materials for ORR electrocatalysis and supercapacitors.     

Iron (II) phthalocyanine/N-doped graphene: A highly efficient non-precious metal catalyst for oxygen reduction

Non-precious metal electrocatalysts (NPMCs) are a promising alternative to platinum-based catalysts towards the large-scale commercial application of hydrogen fuel cells and metal-air batteries. However, hazardous chemicals or high-temperature pyrolysis are generally involved in the synthesis of these highly-active NPMCs, leading to environmental and safety issues, particularly in scaling up. Exploration of low-cost and straightforward strategies to fabricate high-performance catalysts for ORR are urgently needed. Herein, we report a simple approach to fabricate a new class of the NPMCs by immobilizing iron phthalocyanine (FePc) into a surface of nitrogen-doped electrochemical exfoliated graphene (N-GP950). We highlight that at optimum content of 33 wt% FePc in the composite (FePc-33/N-GP950), the ORR performance significantly improves, exhibiting high current density at 0.8 V (5.0 mA cm⁻²), which is comparable to 4.0 mA cm⁻² of commercial Pt/C in an alkaline media. The optimized sample also displays excellent long-term durability. The present study offers a low cost and straightforward strategy to fabricate inexpensive and durable ORR catalysts for practical hydrogen fuel cells applications and metal-air batteries.     

Highly active electrocatalysts for oxygen reduction from carbon-supported copper-phthalocyanine synthesized by high temperature treatment

The active, carbon-supported copper phthalocyanine (CuPc/C) nano-catalyst, as a novel cathode catalyst for oxygen reduction reaction, is synthesized via a combined solvent-impregnation along with the high temperature treatment. The catalytic activities of both pyrolyzed and unpyrolyzed catalysts are screened by linear sweep voltammetry (LSV) employing a rotating disk electrode (RDE) technique to quantitatively obtain the oxygen reduction reaction (ORR) kinetic constants and the reaction mechanisms. The results show that heat-treatment can significantly improve the ORR activity of the catalyst, and the optimal heat-treated temperature is around 800 °C, under which, an onset potential of 0.10 V and a half-wave potential of −0.05 V are achieved in alkaline electrolyte. Besides the ORR kinetic rate is increased, the ORR electron transfer number is also increased from 2.5 to 3.6 with increasing heat-treatment temperature from 600 to 800 °C. Also, the effect of catalyst loading in the catalyst layer on the corresponding ORR activity is also studied, and finds that increasing the catalyst loading, the catalyzed ORR kinetic current density can be significantly increased. For a fully understanding of this heat-treatment temperature effect, XRD, TEM, SEM–EDX, TG and XPS are used to identify the catalyst structure and composition. TG results demonstrated that the presence of Cu prevents phthalocyanine from thermal decomposition, thus contribute to higher nitrogen content which can form more Cu–N_x activity sites for the ORR. XPS analysis indicates that both pyridinic-N and graphitic-N may be responsible for the enhanced ORR activity.     

Highly stable and efficient non-precious metal electrocatalysts of Mo-doped NiOOH nanosheets for oxygen evolution reaction

Active and stable electrocatalysts for oxygen evolution reaction (OER) made from earth-abundant elements are highly attractive. Herein, we report our recent efforts in developing Mo-doped NiOOH (MoNiOOH) nanosheets as highly active and stable OER catalysts. The doping of Mo is found to be conducive for both activity and stability. The MoNiOOH nanosheets need overpotential of only 390 mV to afford the current densities of 100 mA cm^?2 and show no obvious degradation under steady-state current density of 100 mA cm^?2 even after 24 h.