Heteroatoms doped C3N4 as high performance catalysts for the oxygen reduction reaction

Different kinds of carbon nitride were successfully synthesized through pyrolyzing the precursors. Their physical properties were characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. While the electrochemical properties were measured by cyclic voltammetry, linear sweep voltammetry, electrochemical impedance spectroscopy and chronoamperometry. The results showed that the sulfur-doped carbon nitride exhibited better electrochemical catalytic properties towards oxygen reduction reaction. Most importantly, the onset potential of sulfur-doped carbon nitride was 0.77 V (vs. RHE), which positively shifted 40 mV than that of the carbon nitride. The calculation of kinetics parameters showed that it occurred through an approximately four electron pathway with a lower Tafel slope (115 mV/decade). Furthermore, the sulfur-doped carbon nitride also presented excellent stability and methanol tolerance.     

Preparation of non-precious metal catalysts for PEMFC cathode from pyrolyzed vitamin B12

In this study, the effect of non-precious metal catalysts in the form of pyrolyzed Vitamin B12 that is supported by carbon black on oxygen reduction reaction (ORR) is examined. Pyrolysis was carried out at temperatures of 300 °C (py-B12/C-300), 500 °C (py-B12/C-500), 700 °C (py-B12/C-700) and 900 °C (py-B12/C-900) in an N₂-atmosphere. The ring-rotating disk electrode technique revealed that the electron-transfer numbers of py-B12/C-300, py-B12/C-500, py-B12/C-700 and py-B12/C-900 are 3.02, 3.42, 3.90 and 3.57, respectively: py-B12/C-700 exhibits near four-electron transfer. The X-ray absorption spectra demonstrate that during the pyrolysis, as the Co oxidation state of py-B12-700 is changed from Co(III) to Co(II), the Co coordination number changes from 6 to 4, suggesting that the structure is a square-planar Co–N₄ chelate. However, the Co–N₄ chelate is decomposed as the pyrolysis temperature increases to 900 °C, resulting in a loss of ORR activity. The H₂–O₂ PEMFC that uses py-B12/C-700 provides excellent performance, substantially outperforming py-CoTMPP/C.     

Nitrogen doped amorphous carbon as metal free electrocatalyst for oxygen reduction reaction

A non metal catalyst for the oxygen reduction reaction is prepared by simply pyrolyzing ion exchange resin D113 in NH₃. The product is nitrogen doped amorphous carbon. The pyrolysis of D113 exchanged with iron ion results in nitrogen doped graphitic carbon. The amorphous carbon is easier to be doped by NH₃ with higher nitrogen content. The nitrogen doped amorphous carbon is more active than graphitized carbon, together with much improved stability. The higher activity is explained by the higher total nitrogen content and higher pyridinic/graphitic nitrogen percentage. The higher stability is because there is no loss or dissolution of the active sites. The results of this work prove metal element and graphitization of carbon are not necessary factors for nitrogen doped carbon as non noble metal catalyst for the oxygen reduction reaction.     

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.     

Nitrogen doped lotus stem carbon as electrocatalyst comparable to Pt/C for oxygen reduction reaction in alkaline media

The nitrogen doped carbon with high content of pyridine N and porous structure indicates high activity for oxygen reduction reaction (ORR). In this paper, nitrogen doped lotus stem carbon (N-LSC) with 6.3 at% of N (containing 52 at% of pyridine N) and porous structure is developed by using lotus stem as carbon source and dopamine hydrochloride as nitrogen source. The ORR activity, stability and methanol tolerance are characterized. The results show that the N-LSC has comparable activity to Pt/C, and much better methanol tolerance and stability than Pt/C. The porous structure and high content of pyridine N are believed to lead to the high ORR performances of the N-LSC.     

Highly active Pt decorated Pd/C nanocatalysts for oxygen reduction reaction

This article describes findings of the correlation between the atomic scale surface structure and the electrocatalytic performance of nanoengineered Pt-Pd/C catalysts for oxygen reduction reaction (ORR), aiming at providing a new fundamental insight into the role of the detailed atomic decorated structure of the catalysts in fuel cell reactions. Carbon-supported Pt decorated Pd nanoparticles (donated as Pt-Pd/C), with Pt coverage close to a monolayer, were prepared from a simple galvanic replacement reaction between Pd/C particles and PtCl₄^2? at room temperature. The decorated architecture was confirmed by extensive microstructural characterization techniques, including TEM, XRD, XPS, HAADF-STEM, ICP and HS-LEIS. The catalysts were also examined for their intrinsic kinetic activities towards oxygen reduction reaction. The results have shown that the Pt-Pd/C catalysts are highly active towards molecular oxygen electrocatalytic reduction. These findings have profound implications to the design and nanoengineering of decorated surfaces of catalysts for oxygen reduction reaction.     

Stability of hemin/C electrocatalyst for oxygen reduction reaction

Hemin has been reported to be an effective electrocatalyst for mediating the oxygen reduction reaction. In this work, the stability of hemin/C is extensively investigated in both acid and alkaline media by the electrochemical methods. It is found that the pristine hemin/C yields significant change in the composition and the electrochemical features when it undergoes the potential cycling in acid media. In comparison, the catalyst shows superior stability in alkaline media. The pyrolysis can improve the stability of the hemin/C catalyst by removing the organic groups in hemin; however, the heat treatment cannot prevent the metal ion loss in acid media. Finally, the acid-leaching experiment reveals that the active center for the 4-electron reaction tends to get lost in acid, indicating that the iron metal ion should be involved in catalyzing the 4-electron reduction reaction. Furthermore, the XPS result indicates that the element N is also involved in the active center. Therefore, it can be concluded that the Fe–N contributes to the active center for the complete reduction of oxygen in alkaline media.     

Nitrogen doped porous carbon with iron promotion for oxygen reduction reaction in alkaline and acidic media

Nitrogen doped water-hyacinth graphite with little iron (NFe-WHG) is synthesized by using water hyacinth as carbon source, dopamine hydrochloride as N source and Fe(NO₃)₃ as Fe source. The water hyacinth is carbonized to porous carbon; the addition of Fe increases pore diameter, graphitization degree, total N and pyridinic N content. The characterizations indicate that the doping N contributes great on ORR activity, yet the residual Fe species themselves show inconspicuous catalytic effect on ORR. The NFe-WHG with the above features displays superior ORR activity in alkaline media and comparable ORR activity to commercial Pt/C in acidic media. Due to the graphite matrix and that most of the Fe species have been removed, the NFe-WHG shows excellent stability in both alkaline and acidic media with excellent anti-methanol and anti-CO performances.     

Waste wine mash-derived doped carbon materials as an efficient electrocatalyst for oxygen reduction reaction

Oxygen reduction reaction (ORR) is a core reaction of fuel cell and metal-air cell. In recent years, it has been a hot topic to study non-precious metal catalysts for ORR. Herein, we have used waste wine mash-derived carbon, melamine and ferric chloride to prepare a Fe- and N- co-doped carbon catalyst. The specific surface area of the catalyst is up to 1066.6 m² g⁻¹. And its wave potential is 15 mV higher than that of commercial Pt/C catalyst. The ORR on our catalyst followed a four-electron pathway; and it has high stability and high impressive immunity to methanol. After continuous oxygen reduction of 30,000s, the retention rate is 90%.     

Thermally-induced arrangement of cobalt and iron in the ZIF-derived Fe,Co,Zn-N/C catalysts for the oxygen reduction reaction

Heteroatom-doped carbon materials (HDCM) are perspective Pt-free alternatives for applications in fuel cells. The Fe,Co,Zn-N/C catalysts were obtained by pyrolysis (at 700 °C in Ar) of sacrificial bimetallic zeolitic imidazolate frameworks (Co,Zn-ZIF), prepared with different Co/Zn ratio by a microwave-assisted solvothermal synthesis (at 140 °C in DMF for 2 h). Co,Zn-ZIF hybrids were impregnated with a Fe^II-phenanthroline complex before the pyrolysis. The structural properties of prepared materials were assessed primarily by X-ray diffraction (XRD) and transmission electron microscopy (TEM), while X-ray fluorescence (XRF) and the scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy (STEM-EDS) mapping and were used for the elemental content analysis. Because in the obtained HDCM both Fe and Co participate in formation of the bamboo-like structures, synchrotron-based X-ray absorption spectroscopy (XAS) studies were performed at their K-edges. The results of in situ XAS measurements during carbonization of Fe,Co,Zn-ZIF upon heating (up to 500 °C in Ar) as well as operando XAS measurements during the electrochemical cycling of HDCM are reported. The registered changes in the oxidative state of Fe and Co (XANES) and in their coordinative environment (EXAFS) were analyzed. The study is complemented by the electrochemical tests of the synthesized HDCM (in 0.1 M HClO₄ solution) towards the oxygen reduction reaction, demonstrating their high efficiency and stability in acidic medium.     

DFT-based study of single transition metal atom doped g-C3N4 as alternative oxygen reduction reaction catalysts

In this paper, the stability and the oxygen reduction reaction (ORR) catalytic activity of single transition metal atom doped g-C₃N₄ catalysts, M-C₃N₄ (M = Mn, Fe, Co, Ni, Cu, Rh, Pd, Ag, Pt, Au), were investigated in detail by performing density functional theory (DFT) calculations. The results of binding energy reveal all M-C₃N₄ are thermodynamically stable. Further dynamic calculations demonstrate they are also dynamically stable except Au-C₃N₄. Then, through comparing the value of overpotentials, we found that most of M-C₃N₄ exhibit no ORR catalytic activity except for Ag-C₃N₄ and Pd-C₃N₄, both of which have somewhat catalytic properties but still inferior to Pt(111). It may be caused by the strong adsorption between ORR intermediates (OOH, O, OH) and M-C₃N₄. We further preformed DFT calculation for the high-valent metal complexes of g-C₃N₄ (M-OH-C₃N₄) and the significant enhancement of activity is obtained. Due to the additional OH group, the overall adsorption energies of ORR intermediates on M-OH-C₃N₄ have been decreased and become more close to those on Pt(111), and ORR mechanisms have also been changed. In addition, the overpotentials of ORR on Ni-OH-C₃N₄ and Cu-OH-C₃N₄ are much close to that on the Pt(111), indicating that they possess the catalytic activity comparable to precious Pt catalyst.     

Transition metal carbides coupled with nitrogen-doped carbon as efficient and stable Bi-functional catalysts for oxygen reduction reaction and hydrogen evolution reaction

Developing non-precious metal catalysts for oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER) is crucial for proton exchange membrane fuel cell (PEMFC), metal-air batteries and water splitting. Here, we report a in-situ simple approach to synthesize ultra-small sized transition metal carbides (TMCs) nanoparticles coupled with nitrogen-doped carbon hybrids (TMCs/NC, including WC/NC, V₈C₇/NC and Mo₂C/NC). The TMCs/NC exhibit excellent ORR and HER performances in acidic electrolyte as bi-functional catalysts. The potential of WC/NC at the current density of 3.0 mA cm^?2 for ORR is 0.814 V (vs. reversible hydrogen electrode (RHE)), which is very close to Pt/C (0.827 V), making it one of the best TMCs based ORR catalysts in acidic electrolyte. Besides, the TMCs/NC exhibit excellent performances toward HER, the Mo₂C/NC only need an overpotential of 80 mV to drive the current density of 10 mA cm^?2, which is very close to Pt/C (37 mV), making it the competitive alternative candidate among the reported non-precious metal HER catalysts.     

Activity and stability of non-precious metal catalysts for oxygen reduction in acid and alkaline electrolytes

The activity and stability of non-precious metal catalysts (NPMCs) for the oxygen reduction reaction (ORR) in both acid and alkaline electrolytes were studied by the rotating disk electrode technique. The NPMCs were prepared through the pyrolysis of cobalt-iron-nitrogen chelate followed by combination of pyrolysis, acid leaching, and re-pyrolysis. In both environments, the catalysts heat-treated at 800-900 °C exhibited relatively high activity. Particularly, an onset potential of 0.92 V and a well-defined limiting current plateau for the ORR was observed in alkaline medium. The potential cycling stability test revealed the poor stability of NPMCs in acid solution with an exponential increase in the performance degradation as a function of the number of potential cycling. In contrast, the NPMCs demonstrated exceptional stability in alkaline solution. The numbers of electron transferred during the ORR on the NPMCs in acid and alkaline electrolytes were 3.65 and 3.92, respectively, and these numbers did not change before and after the stability test. XPS analysis indicated that the N-containing sites of catalysts are stable before and after the stability test when in alkaline solution but not in acid solution.     

Iron-based sulfur and nitrogen dual doped porous carbon as durable electrocatalysts for oxygen reduction reaction

The widespread use of fuel cell technology is hampered by the use of expensive and scarce platinum metal in electrodes which is required to facilitate the sluggish oxygen reduction reaction (ORR). In this work, a viable synthetic approach was developed to prepare iron-based sulfur and nitrogen dual doped porous carbon (Fe@SNDC) for use in ORR. Benzimidazole, a commercially available monomer, was used as a precursor for N doped carbon and calcined with potassium thiocyanate at different temperatures to tune the pore size, nitrogen content and different types of nitrogen functionality such as pyridinic, pyrrolic and graphitic. The Fe@SNDC–950 with high surface area, optimum N content of about 5 at% and high amount of pyridinic and graphitic N displayed an onset potential and half-wave potential of 0.98 and 0.83 V vs RHE, respectively, in 0.1 M KOH solution. The catalyst also exhibits similar oxygen reduction reaction performance compared to Pt/C (20 wt%) in acidic media. Furthermore, when compared to commercially available Pt/C (20 wt%), Fe@SNDC–950 showed enhanced durability over 6 h and poison tolerance in case of methanol crossover with the concentration up to 3.0 M in oxygen saturated alkaline electrolyte. Our study demonstrates that the presence of N and S along with Fe-N moieties synergistically served as ORR active sites while the high surface area with accessible pores allowed for efficient mass transfer and interaction of oxygen molecules to the active sites contributing to the ORR activity of the catalyst.     

Nitrogen-modified carbon-based catalysts for oxygen reduction reaction in polymer electrolyte membrane fuel cells

Nitrogen-modified carbon-based catalysts for oxygen reduction were synthesized by modifying carbon black with nitrogen-containing organic precursors. The electrocatalytic properties of catalysts were studied as a function of surface pre-treatments, nitrogen and oxygen concentrations, and heat-treatment temperatures. On the optimum catalyst, the onset potential for oxygen reduction is approximately 0.76 V (NHE) and the amount of hydrogen peroxide produced at 0.5 V (NHE) is approximately 3% under our experimental conditions. The characterization studies indicated that pyridinic and graphitic (quaternary) nitrogens may act as active sites of catalysts for oxygen reduction reaction. In particular, pyridinic nitrogen, which possesses one lone pair of electrons in addition to the one electron donated to the conjugated π bond, facilitates the reductive oxygen adsorption.     

Highly microporous nitrogen doped graphene-like carbon material as an efficient fuel cell catalyst

Nitrogen-doped carbon materials are known to be promising candidates as oxygen reduction reaction electrocatalysts used in fuel cells. However, developing metal-free catalysts with high performance and stability still remains a big challenge. Herein we report a new route by using the Maillard reaction, to caramelize ribose in a dispersing salt matrix, followed by carbonization of this caramel to synthesize metal-free catalysts. This catalytic material has the morphology of microporous nitrogen doped graphene-like carbon, and a highest surface area of 1261 m² g^?1 with a large amount of micropores. Such microporous structure offers numerous defects that generate a large number of reactive sites. As a result, when used as the cathode materials in fuel cells, the fuel cell shows a high power density of 547 mW cm^?2 under 1.0 atm back pressure with good stability with only 12.5% loss after 250 h. Such catalyst has good performance in the class of metal-free oxygen reduction reaction catalysts, and is possible for commercial use.     

Trimetallic PtNiCo nanoflowers as efficient electrocatalysts towards oxygen reduction reaction

Nanostructures of PtNiCo alloy have been prepared using a simple solvothermal process followed by annealing at higher temperature and studied for electrochemical oxygen reduction reaction (ORR) kinetics. PtNiCo/C catalyst has demonstrated an interesting trend of enhancement in the ORR activity along with long-term durability. The specific activity of 2.47 mA cm^?2 for PtNiCo-16h/C (PtNiCo/C prepared at reaction time of 16 h) is ～12 times higher than that of Pt/C (0.2 mA cm^?2). Further, X-ray diffraction, transmission electron microscopy and X-ray photo electron spectroscopy studies have been carried out systematically to understand the phase formation, morphology along with surface defects and elemental analysis respectively. The durability of the catalyst was evaluated over 10,000 potential cycles using standard triangular potential scan in the lifetime regime. Interestingly, after 10k durability cycles, PtNiCo-16h/C electrocatalyst showed enhanced ORR activity (32% higher activity; Im@10k cycles = 0.716 A mg_Pt^?1) and stability compared to commercial Pt/C signifying the retention of Ni and Co due to higher lattice contraction in PtNiCo alloy electrocatalyst.     

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.     

Synergistic improvement of oxygen reduction reaction on gold/cerium-phosphate catalysts

The main challenge in fuel cells lies in improving slow oxygen reduction reaction (ORR) kinetics causing low conversion efficiencies. Here, we introduce the Au/CePO₄-binary nanocomposites as effective oxygen reduction catalysts in alkaline media. The ORR activity comparable with Pt is achieved through the serial 4-electron reduction pathway. The bi-functionality of CePO₄ is suggested to explain the remarkably enhanced activity on the Au/CePO₄ nanocomposites. Significantly, the own catalytic activity of CePO₄ for hydrogen peroxide is demonstrated, validating synergistic effects with Au for complete ORR.