Effective Electrocatalyst For Oxygen Reduction Reaction Research Articles

Surfactant-driven shape evolution to sub-3 nm Pt-rich Pt3Ni dodecahedrons as efficient electrocatalyst for oxygen reduction reaction

BackgroundDeveloping an economical and durable high-activity Pt-based electrocatalyst to alleviate the slow kinetics of the oxygen reduction reaction (ORR) is crucial for fuel cells. The shape-controllable Pt3Ni alloy catalyst with high electrocatalytic properties is regarded as one of the most efficacious electrocatalysts capable of accelerating ORR kinetics. MethodsHere, sub-3 nm Pt-rich Pt3Ni dodecahedrons (DCs) has been developed via surfactant-driven shape evolution route as effective electrocatalysts for ORR. Significant findingsThe Pt3Ni DCs exhibit 4.3-fold improvement for mass activity and 4.7-fold improvement for specific activity respect to pristine Pt/C catalyst. Moreover, the remaining specific activity on Pt3Ni DCs is 6.7 times larger relative to pristine Pt/C catalyst after 20,000 scanning cycles. Such electrocatalytic performance enhancement is attributed to the ensemble effect including the dodecahedrons shape and synergistic effect between Pt and Ni atoms. We believe that our synthetic approach would provide new avenues toward the formation of polyhedron structures.

Boost oxygen reduction reaction performance by tuning the active sites in Fe-N-P-C catalysts

Cost-effective atomically dispersed Fe-N-P-C complex catalysts are promising to catalyze the oxygen reduction reaction (ORR) and replace Pt catalysts in fuel cells and metal-air batteries. However, it remains a challenge to increase the number of atomically dispersed active sites on these catalysts. Here we report a highly efficient impregnation-pyrolysis method to prepare effective ORR electrocatalysts with large amount of atomically dispersed Fe active sites from biomass. Two types of active catalyst centers were identified, namely atomically dispersed Fe sites and FexP particles. The ORR rate of the atomically dispersed Fe sites is three orders of magnitude higher than it of FexP particles. A linear correlation between the amount of the atomically dispersed Fe and the ORR activity was obtained, revealing the major contribution of the atomically dispersed Fe to the ORR activity. The number of atomically dispersed Fe increases as the Fe loading increased and reaching the maximum at 1.86 wt% Fe, resulting in the maximum ORR rate. Optimized Fe-N-P-C complex catalyst was used as the cathode catalyst in a homemade Zn-air battery and good performance of an energy density of 771 Wh kgZn−1, a power density of 92.9 mW cm−2 at 137 mA cm−2 and an excellent durability were exhibited.

MnO2 coated with graphene by galvanostatic electrodeposition and its enhanced electrocatalysis for oxygen reduction

Manganese oxides (MnOx) are expected to be highly active catalysts for O2 electroreduction. However, MnOx are prone to aggregation, thus reducing their catalytic active sites. Herein, a composite of spherical MnO2 coated with reduced graphene oxide (denoted as MnO2@RGO) was fabricated by a facile and green step galvanostatic electrodeposition. The obtained MnO2@RGO is composed of a RGO lamina shell (10 nm) and spherical MnO2 core (100 nm), which were confirmed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Brunner–Emmet–Teller (BET) measurement, Raman, and X-ray photoelectron spectroscopy (XPS). The obtained composite exhibited a high specific surface area of 251.3 m2 g−1. The as-prepared MnO2@RGO composite displayed a more positive onset potential of 0.86 V vs. reversible hydrogen electrode (RHE), higher kinetic current density (11.23 mA cm−2 at 0.2 V), and higher electron transfer number in OH− solution (n ≈ 3.94) for oxygen reduction reaction (ORR) compared to MnO2 NPs scattered on the surface of RGO (MnO2/RGO), bulk MnO2 NPs, and RGO. The enhanced activity was further demonstrated by the lower Tafel slope (72 mV dec−1). Moreover, MnO2@RGO exhibited greater “tolerance to methanol,” “anti-CO poisoning” ability, and catalytic stability compared to commercial Pt/C catalyst. The results indicated that the prepared MnO2@RGO composite can be a potential low-cost and effective electrocatalyst for ORR. MnO2 coated with reduced graphene oxide has been firstly prepared by step galvanostatic process and shows enhanced catalytic activity and stability for oxygen reduction reaction.

Gram-scale synthesis of alkoxide-derived nitrogen-doped carbon foam as a support for Fe–N–C electrocatalysts

Non-platinum group metal (non-PGM) catalysts for the oxygen reduction reaction (ORR) are set to reduce the cost of polymer electrolyte membrane fuel cells (PEFCs) by replacing platinum at the cathode. We previously developed unique nitrogen-doped carbon foams by template-free pyrolysis of alkoxide powders synthesized using a high temperature and high pressure solvothermal reaction. These were shown to be effective ORR electrocatalysts in alkaline media. Here, we present a new optimised synthesis protocol which is carried out at ambient temperature and pressure, enabling us to safely increase the batch size to 2 g, increase the yield by 60%, increase the specific surface area to 1866 m2 g−1, and control the nitrogen content (between 1.0 and 5.2 at%). These optimized nitrogen-doped carbon foams are then utilized as effective supports for Fe–N–C catalysts for the ORR in acid media, whilst multiphysics modelling is used to gain insight into the electrochemical performance. This work highlights the importance of the properties of the carbon support in the design of Pt-free electrocatalysts.

Facile loading carbon dots on Co3O4 as an enhanced oxygen reduction reaction catalyst

One of the crucial issues in fuel cells is to explore the highly-efficient, long-term stable and low-cost catalysts for the oxygen reduction reaction (ORR). Here, the carbon dots/Co3O4 (CDs/Co3O4) is reported as an effective electrocatalyst for ORR. The CDs/Co3O4 was synthesized through an in-situ two-step heating strategy with the graphitic carbon nitride (g-C3N4), cobalt nitrate hexahydrate and organic base as raw materials. The carbon dots (CDs) are derived from the C source of g-C3N4, which are in situ introduced into the Co3O4 to form the CDs/Co3O4 composite. Consequently, the introduction of CDs endows enhanced electrocatalytic activity of the CDs/Co3O4 composite.

Aluminum and Nitrogen Codoped Graphene: Highly Active and Durable Electrocatalyst for Oxygen Reduction Reaction

The development of a highly active and exceedingly durable electrocatalyst at low cost for the oxygen reduction reaction (ORR) is extremely desirable but remains to be a grand challenge. Over the past decade, the transitional-metal (e.g., Fe, Co, Ni) and N codoped graphene materials have attracted most attention as the state-of-the-art nonprecious-metal-based effective electrocatalyst for ORR but still entail unsatisfactory issues such as moderate activity and life. Herein, the main-group-metal Al and N codoped graphene (ANG) is successfully fabricated via thermal annealing treatment of N-doped graphene with aluminum tri-(8-hydroxyquinoline). As a highly effective electrocatalyst for ORR, the as-prepared ANG exhibits not only high electrocatalytic activity that even outperforms the commercial Pt/C but also good durability in both three-electrode cell and Zn-air battery. Theoretical calculations show that the inhomogeneous charge density distribution and the interaction between Al and N are mainly responsi...

Anisotropic N-Graphene-diffused Co3O4 nanocrystals with dense upper-zone top-on-plane exposure facets as effective ORR electrocatalysts

We provide strong evidence of the effectiveness of homogenously self-propelled particle-in-particle diffusion, interaction and growth protocol. This technique was used for one-pot synthesis of novel nitrogen-graphene oxide (N-GO)/Co3O4 nanocrystals with cuboid rectangular prism-shaped nanorods (NRs) along {110}-plane and truncated polyhedrons with densely-exposed, multi-facet sites along {311} and {111} planes. These hierarchal nanocrystals create electrode catalyst patterns with vast-range accessibility to active Co2+ sites, a vascular system for the transport and retention of captured O2 molecule interiorly, and low adsorption energy and dense electron configuration surfaces during the oxygen reduction reaction (ORR). The superior electrocatalytic ORR activity of the N-GO/Co3O4 polyhedron nanocrystals in terms of electrochemical selectivity, durability and stability compared with NRs or commercial Pt/C catalysts confirms the synergetic contribution of multi-functional, dense-exposed, and actively topographic facets of polyhedrons to significantly activate the catalytic nature of the catalyst. Our findings show real evidence, for the first time that not only the large number of catalytically active Co2+ cations at the top surface layer but also the dense location of active Co2+ sites on the upper-zone top-on-plane exposure, and the electron density configuration and distribution around the Co2+ sites were important for effective ORR.

Insulating Boron Nitride Nanosheet on Inert Gold Substrate As a Highly Efficient Electrocatalyst for Hydrogen Evolution Reaction

Although hydrogen evolution reaction (HER) takes place readily at electrodes of Pt group metals and their alloys, for water electrolysis to become a practical hydrogen production process, cost reduction is essential and the use of expensive and less abundant Pt group metals must be avoided. Recently, inorganic catalysts such as nano-sized metal dichalcogenides such as MoS2 and WS2, which are semiconductors with layered structure, have been found to have high activity for HER and have drawn great attention due to their low cost and high chemical stability. Hexagonal boron nitride (h-BN) monolayer has similar geometric structure to that of graphene but it is an insulator with a wide band gap (˜5.8eV) and, therefore, is not considered as an electrocatalytic material. We, however, theoretically demonstrated that the band gap of h-BN monolayer is considerably reduced and h-BN becomes semiconducting if some defects are introduced and/or it is placed on metal substrates [1]. Theoretical investigation also suggested that h-BN would act as an effective electrocatalyst for oxygen reduction reaction (ORR) [2]. It was experimentally proved that Au electrodes modified with various types of h-BN indeed act as effective ORR electrocatalysts, although the overpotential for ORR is still high compared with Pt electrode and oxygen is mainly reduced to H2O2 via 2-electron process [2-4]. Considering the electrocatalytic activity of BN for ORR and relatively large hydrogen adsorption/storage ability of BN, electrocatalytic activity of BN nanosheet (BNNS) on Au electrode is examined [5]. Figure 1 shows linear sweep voltammograms (LSVs) of bare and BNNS modified Au, glassy carbon (GC), and Pt (v, vi) electrodes obtained in Ar saturated 0.5 M H2SO4 solution by scanning the potential negatively with 1 mV/s. Overpotential at Au electrode is reduced by ca. 250 mV by the BNNS modification, showing that BNNS indeed acts as an electrocatalyst for HER at Au electrode as does for ORR, although BNNS shows almost negative and no effect at Pt and GC electrodes, respectively, as observed for ORR.DFT calculations were performed to obtain free energy diagram for HER at various 2D BN nano-systems on Au(111) at equilibrium potential. Properties of the h-BN monolayer towards H adsorption are considerably modified by Au(111) substrate. Atoms at the edges of BNNS islands on Au(111) are found to provide energetically favored sites for adsorbed hydrogen, i.e., intermediate state, of HER and can act as good electrocatalytic sites for HER. Based on this theoretical consideration, further improvement of electrocatalytic activity for HER can be expected by increasing the fraction of atoms at the edges of BNNS on Au substrate. Fraction of edge atoms should be increased by decreasing the size of each BNNS at a given amount of BNNS on the surface. The overpotential decreased by decreasing the size of the BNNS as expected. The best result was obtained for the Au(111) electrode modified with the smallest BNNS used in the present study (BNNS(0.1-0.22 µm)/Au). The overpotential at BNNS(0.1-0.22 µm)/Au electrode is only 30 and 40 mV larger than that at Pt electrode at 5 mAcm-2 and 15 mAcm-2.

Biomass-Derived Porous Fe3C/Tungsten Carbide/Graphitic Carbon Nanocomposite for Efficient Electrocatalysis of Oxygen Reduction.

The oxygen-reduction reaction (ORR) draws an extensive attention in many applications, and there is a growing interest to develop effective ORR electrocatalysts. Iron carbide (Fe3C) is a promising alternative to noble metals (e.g., platinum), but its performances need further improvement, and the real role of the Fe3C phase remains unclear. In this study, we synthesize Fe3C/tungsten carbide/graphitic carbon (Fe3C/WC/GC) nanocomposites, with waste biomass (i.e., pomelo peel) serving as carbon source, using a facile, one-step carbon thermal-reduction method. The nanocomposite is characterized by a porous structure consisting of uniform Fe3C nanoparticles encased by graphitic carbon (GC) layers with highly dispersed nanosized WC. The Fe3C provides the active sites for ORR, while the graphitic layers and WC nanoparticles can stibilize the Fe3C surface, preventing it from dissociation in the electrolyte. The Fe3C/WC/GC nanocomposite is highly active, selective, and stable toward four-electron ORR in pH-neutral electrolyte, which results in a 67.82% higher power density than that of commercial Pt/C and negligible voltage decay during a long-term phase of a 33 cycle (2200 h) operation of a microbial fuel cell (MFC). The density functional theory (DFT) calculations suggest high activity for splitting the O-O bond of molecular oxygen on the surface of Fe3C.

Facile synthesis of sewage sludge-derived in-situ multi-doped nanoporous carbon material for electrocatalytic oxygen reduction.

Developing efficient, low-cost, and stable carbon-based catalysts for oxygen reduction reaction (ORR) to replace the expensive platinum-based electrocatalysts remains a major challenge that hamper the practical application of fuel cells. Here, we report that N, Fe, and S co-doped nanoporous carbon material, derived via a facile one-step pyrolysis of sewage sludge, the major byproduct of wastewater treatment, can serve as an effective electrocatalyst for ORR. Except for the comparable catalytic activity with commercial 20% Pt/C via a nearly four-electron transfer pathway in both alkaline and acid medium, the as-synthesized co-doped electrocatalyst also exhibits excellent methanol crossover resistance and outstanding long-term operation stability. The organic compounds in sewage sludge act as the carbon source and the in-situ N and S dopant in the fabrication, while the inorganic compounds serve as the in-built template and the in-situ Fe dopant. Our protocol demonstrates a new approach in the economic and eco-friendly benign reuse of sewage sludge, and also provides a straightforward route for synthesizing excellent carbon-based electrocatalysts as promising candidates for ORR directly from a type of waste/pollution.

Shape-Controlled Nanocrystals and Core-Shell for Oxygen Reduction Reaction

The slow kinetics of oxygen reduction reaction (ORR) is one of the main obstacles hindering the commercialization of low-temperature fuel cells.1 Platinum is by far the most effective electrocatalyst for ORR, but it is an extremely rare and expensive element. One way to address this problem is to increase the ORR activity of a metallic catalyst by optimizing the shape of constituent nanoparticles and thus maximizing the expression of facets most reactive towards ORR reaction.2-3 Another approach is to increase the utilization of expensive noble metal atoms by developing core-shell catalysts.4-5 In this study, the structural dependence of palladium nanocrystals for ORR, as well as the impact of the structure of the Pd core on the ORR activity of Pt shells in core-shell catalysts was studied in details. The carbon supported Pd nanocubes and octahedra with an edge length of 5-6 nm were synthesized and their ORR activities were measured using a rotating disk electrode technique.6 The activity enhancement of cubic Pd in a HClO4 solution was about 10 and 6 times over octahedral and conventional Pd, respectively. Interestingly, no difference in ORR activity was observed in an alkaline solution among the three samples (Figure 1). Two (octahedral and conventional) of the three Pd samples showed an increase in activity when an atomic thick Pt shell was deposited on their surfaces. Surprisingly, the activity decreased slightly for the Pd cubes after the deposition of a Pt shell.The density functional theory (DFT) calculations were conducted to explain this observation and will be discussed in the meeting.References Electrocatalysis in Fuel Cells: A Non- and Low- Platinum Approach; Shao, M., Ed.; Springer: London, 2013.N. M. Markovic, R. R. Adzic, B. D. Cahan and E. B. Yeager, J. Electroanal. Chem. 1994, 377, 249.V. R. Stamenkovic, B. Fowler, B. S. Mun, G. Wang, P. N. Ross, C. A. Lucas and N. M. Markovic, Science 2007, 315, 493.Adzic, R. R.; Zhang, J.; Sasaki, K.; Vukmirovic, M. B.; Shao, M.; Wang, J. X.; Nilekar, A. U.; Mavrikakis, M.; Valerio, J. A.; Uribe, F. Topics in Catalysis 2007, 46, (3-4), 249.Shao, M. H.; Shoemaker, K.; Peles, A.; Kaneko, K.; Protsailo, L. J. Am. Chem. Soc. 2010, 132, (27), 9253.Shao, M.; Yu, T.; Odell, J.; Jin, M.; Xia, Y. Chem . Commun. 2011, 47, 6566.

Low‐Platinum‐Content Quaternary PtCuCoNi Nanotubes with Markedly Enhanced Oxygen Reduction Activity

Polymer electrolyte membrane fuel cells (PEMFCs), among other clean energy sources, represent a highly efficient and potentially inexpensive solution for worldwide ever-growing energy demand and increasing environmental concern, and are expected to be able to find various applications ranging from automotive vehicles to stationary and portable devices. Presently, the widespread commercialization of PEMFCs is, however, substantially hampered by the slow rate of the oxygen reduction reaction (ORR) at the cathode and the relatively high costs resulting from the excessive use of precious metal platinum. Therefore, considerable research efforts have recently been dedicated to the development of low-cost and highly efficient electrocatalysts for the ORR. While great progress has been made towards non-platinum electrocatalysts, such as nitrogen-doped carbon nanotubes/ graphenes, conducting polymers, and other nonprecious metals/alloys, platinum-based catalysts are still more efficient for the ORR. To date, there are several major strategies being developed to reduce the cost of platinum or enhance platinum utilization in electrocatalysts. The most popular one is to alloy Pt with other nonprecious metals, for example, Fe, Co, Ni, Cu, Ti, Bi, Sc, and Y (PtM alloys). A variety of binary PtM alloy nanoparticle electrocatalysts have been investigated, and a 2–10-fold enhancement in ORR activity relative to pure Pt was repeatedly observed. It is generally accepted that this activity improvement can be ascribed to the lattice strain induced by the formation of Pt-skinned surfaces (i.e. PM alloy/Pt core/shell structure) through surface dealloying and the modified electronic structures, which can weaken the interaction between the Pt surface atoms and spectator species so as to increase the number of active Pt sites. The second frequently used approach to enhancing Pt utilization is to design core/shell nanostructures by covering the surface of nonprecious metal nanocatalysts with a Pt shell or monolayer, which can be accomplished either by solution-phase heterogeneous nucleation and growth on nonprecious metal nanoparticles or by Cu underpotential deposition and subsequent galvanic replacement reaction. The third strategy is to use hollow Pt nanostructures as electrocatalysts. In contrast to core–shell structures, the hollow geometry not only allows the reactants to access the external active Pt sites, but also renders the internal catalytically active sites accessible. Moreover, it was experimentally demonstrated very recently that the confinement effect occurring in the cavity of the nanocage catalysts can lead to a much higher frequency factor for the reduction of 4nitrophenol by NaBH4, a model reaction that can be used to evaluate the catalytic activity of metals. So far, a few reports have been dedicated to the development of hollow Pt electrocatalysts for both the ORR and methanol oxidation reaction (MOR). Nevertheless, in contrast with the other two approaches, Pt-based hollow electrocatalysts still remain insufficiently explored. Herein, we report the synthesis of low-platinum-content quaternary PtCuCoNi nanotubes (NTs) by means of template-assisted, one-step electrodeposition and testing of the suitability of these hollow multimetallic NTs as effective ORR electrocatalysts. We found that the as-prepared PtCuCoNi NTs exhibit markedly enhanced ORR activity over commercially available Pt black and Pt/C catalysts because of compositional (i.e. multicomponent alloys) and geometrical (i.e. hollow structure) properties of the material. The quaternary PtCuCoNi NTs were prepared by a onestep direct electrodeposition approach using a porous anodic aluminium oxide (AAO) membrane as the template. Onestep electrodeposition takes advantage of the sputtered annular base electrode and rapid consumption of metal ions at the deposition front as a result of a large deposition current density to grow tubular structures (see Figure S1 in the Supporting Information) without the need to modify the pore walls of the AAO template, and has recently been proved to be a simple and highly efficient way to fabricate metallic and alloy nanotubes. With this method, a growth rate of PtCuCoNi NTs of as high as 5 mmmin 1 was achieved. The electrodeposition was carried out under a condition that favors the reduction of less noble copper ions in the electrolyte (e.g. 0.8 V vs. Ag/AgCl). Afterwards, the AAO membrane was immersed into 10 wt% H3PO4 solution at 45 8C for 5 h. In this process, the porous alumina was completely removed. Meanwhile, mild dealloying also occurred in the as-deposited nanostructures. Figure 1a shows a representative scanning electron microscopy (SEM) image of the as-prepared nanostructures after removal of the AAO template. Unlike the nanoporous PtCo and PtNi nanowires reported previously, compact and robust fibers without discernable nanopores were observed after the acid treatment. Upon closer examination by transmission electron microscopy (TEM), it was found that most fibers actually possess hollow tubular morphology with an average diameter of 50 nm, as evidenced in Figure 1b,c. The [*] Dr. L. Liu, Dr. E. Pippel Max Planck Institute of Microstructure Physics Weinberg 2, 06120 Halle (Saale) (Germany) Fax: (+49)345-551-1223 E-mail: liulif@mpi-halle.de