Non-precious Oxygen Reduction Reaction Research Articles

Tuning the atomic configuration environment of MnN4 sites by Co cooperation for efficient oxygen reduction

Carbon-based N-coordinated Mn (Mn-Nx/C) single-atom electrocatalysts are considered as one of the most desirable non-precious oxygen reduction reaction (ORR) candidates due to their insignificant Fenton reactivity, high abundance, and intriguing electrocatalytic performance. However, current Mn-Nx/C single-atom electrocatalysts suffer from high overpotentials because of their low intrinsic activity and unsatisfactory chemical stability. Herein, through an in-situ polymerization-assisted pyrolysis, the Co as a second metal is introduced into the Mn-Nx/C system to construct Co, Mn-Nx dual-metallic sites, which atomically disperse in N-doped 1D carbon nanorods, denoted as Co, Mn-N/CNR and hereafter. Using electron microscopy and X-ray absorption spectroscopy (XAS) techniques, we verify the uniform dispersion of CoN4 and MnN4 atomic sites and confirm the effect of Co doping on the MnN4 electronic structure. Density functional theory (DFT) calculations further elucidate that the energy barrier of rate-determining step (*OH desorption) decreases over the 2 N-bridged MnCoN6 moieties related to the pure MnN4. This work provides an effective strategy to modulate the local coordination environment and electronic structure of MnN4 active sites for improving their ORR activity and stability.

N-doping carbon-coated Cu@C(N) nanocapsules synthesized by arc plasma toward high-performance ORR electrocatalyst

The development of efficient and cost-effective methods for the synthesis of non-precious metal as the catalytic material for oxygen reduction reaction (ORR) is addressing a key barrier to fuel cell deployment in a wide range of applications. In this study, core-shell carbon-coated copper nanoparticles (Cu@C) were prepared using arc discharge plasma under argon and hydrogen atmospheres. And nitrogen doping was achieved during the subsequent heat treatment using dicyandiamide as the nitrogen source. The results showed that the catalysts had better ORR catalytic activity, better stability and better methanol tolerance than Pt/C, and the ORR reaction process was nearly four-electron pathway. The successful doping of nitrogen increased the reaction sites. And the addition of copper core improved the conductivity of the catalytic material. Meanwhile, the N-C shell layer protected the Cu nanoparticles from dissolving in aqueous solutions or being chemically oxidized, facilitating the electronic interaction of the core Cu nanoparticles with the N-C shell layer. This study provides an insight into the preparation of high-efficiency non-precious metal-based ORR electrocatalysts by DC arc plasma, a beneficial method.

Durability Assessment of Transition Metal Oxygen Electroreduction Catalysts with on-Line Flow Cell Trace Elemental Analysis

Proton exchange membrane hydrogen fuel cell (PEMFC) cathodes for commercial applications are typically based on platinum group materials that exhibit remarkable oxygen reduction reaction (ORR) activity and stability. Despite this performance, the high cost of such platinum electrodes prevents a more rapid transition to a more sustainable hydrogen economy involving PEMFCs. Platinum-free transition metal ORR catalysts have been identified and tested as high activity, low-cost alternatives with well-established protocols, though understanding the durability of these materials would require more experimental efforts. Techniques for measuring catalyst degradation typically involve activity loss measurements, surface-sensitive microscopy and spectroscopy, and liquid aliquot collection for metal loss quantification that are all performed post-catalysis for extended periods of time. Experiments using this approach provide valuable information about overall catalyst material degradation but usually cannot elucidate mechanistic or potential-dependent phenomena that may occur on shorter time scales. More rigorous experiments with synchrotron radiation have been utilized to investigate the degradation mechanisms of platinum-based fuel cell catalysts but studies on other transition metal materials are less widespread due to the high time requirements and limited accessibility of synchrotron beamlines for such experiments. In our work, we utilize an on-line electrochemical flow cell coupled with an in-house inductively coupled plasma-mass spectrometer (ICP-MS) for evaluation of fuel cell catalyst degradation in acidic electrolyte environments. Oxygen and nitrogen gas saturated electrolytes with a constant pH of 1 but differing chemical identity are flowed through the cell and across a working electrode composed of non-platinum transition metals. Dissolved metal ions are convectively transported to the ICP-MS where they are measured with up to part per trillion resolution with a time delay of ~4 seconds, enabling millisecond resolution of transient catalyst surface degradation. We evaluate the oxygen mass transport capabilities of our custom-machined flow cell against a commercial flow cell and compare to the much more established rotating disk electrode (RDE) technique for distinguishing ORR activity. Cyclic voltammetry measurements in oxygen-saturated electrolyte on various metal foils indicate potential dependent corrosion in agreement with what is expected from a Pourbaix (potential-pH) diagram of stability. In contrast, potential cycling these materials in the ORR-relevant range reveals degradation during ORR electrocatalysis that increases with ORR current to differing amounts as a function of the metal in a way that has not been previously characterized. Notably, degradation appears to be suppressed in the ORR potential range when saturating the electrolyte with nitrogen compared to oxygen, suggesting that surface-bound oxygen and ORR intermediates have a potential-dependent role in enabling degradation. Probing degradation during ORR conditions further with varied cyclic voltammetry scan rates and directions reveals more fundamental information about the timescales of catalytic corrosion processes and may inform the development of activation protocols that enhance the lifetime of ORR catalyst materials during operation. Through electrochemical flow cell testing in various pH 1 electrolytes, we make suggestions for electrolyte compositions and catalyst operation potential ranges that maintain activity and stability to advise fuel cell catalyst researchers in developing platinum-free PEMFC cathode materials. We are also developing additional flow cell configurations to allow testing of other non-precious ORR catalyst morphologies such as nanoparticle catalysts on carbon supports. The on-line ICP-MS flow cell architecture accelerates materials stability and degradation experiments for rapid assessment of platinum-free ORR electrocatalysts through in-house experiments and direct comparability to established electrochemical techniques.

Developing First Row Transition Metal Antimonate Oxynitride and Oxysulfide Nanoparticles As Oxygen Reduction Electrocatalysts

Hydrogen fuel cells (FCs) are a promising avenue for replacing global dependence on carbon-intensive fuels especially in the transportation sector. The cathode of a FC carries out the oxygen reduction reaction (ORR), and the kinetics of the ORR is one of the primary sources of inefficiency in commercial FC devices. Commercial FC cathodes are based on expensive platinum-based catalyst architectures owing to their high activity, and therefore identifying highly active, non-precious ORR catalysts could accelerate the deployment of hydrogen fuel cells into the energy landscape. Theory calculations and experiments indicate first-row transition metal antimonates (MSbOx, M = Mn, Fe, Cr, Ni) have highly desirable ORR activity and stability characteristics. Furthermore, the material systems in this family have been extended to ternary and quaternary compositions for the ORR through the inclusion of additional transition metals to identify further opportunities for activity enhancement. In this work, we develop (oxy)sulfides and (oxy)nitrides of first-row transition metals such as manganese (MnSbOxSy and MnSbOxNz, respectively) to extend this class of materials towards enhanced activity when compared to previously reported antimonate materials. These nanoscale electrocatalysts are synthesized via colloidal means followed by high temperature surface modification under various reactive atmospheres (NH3, H2S, O2). Precise control of temperature ramps and holds enables modulation of the degree of oxidation, sulfidation, or nitridation, resulting in fully oxidized manganese antimonate nanocrystal cores with thin polycrystalline shells of (oxy)sulfide and (oxy)nitride. We tune the synthetic parameters of these materials and evaluate their enhanced electrochemical ORR activity in both alkaline and acidic conditions that are relevant to the operating conditions of anion-exchange membrane and proton-exchange membrane FCs, respectively. We perform extensive bulk and surface characterization with techniques such as X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and gas adsorption BET surface area analysis. We find that converting the pure oxide into oxysulfide or oxynitride causes significant alterations to crystallinity and is accompanied by the appearance of additional crystalline phases that were not present previously as seen in XRD diffractograms. Transmission electron micrographs reveal polycrystalline nanocrystals of roughly 50 nm diameter for all compositions and show varying internal contrast patterns. The pure oxide nanocrystals form a thin shell of ~4 nm thickness after sulfidation or nitridation, and this shell may be converting into an active phase exposed to electrolyte during ORR testing and is expected to be significant for understanding the observed activity enhancement. XPS spectra indicate shifts in Sb and Mn peaks that are suggestive of changing surface-level oxidation state. We also utilize nanometer-resolution elemental mapping with TEM coupled to energy dispersive X-ray spectroscopy (EDS) to visualize the distribution of elements both before and after catalysis to understand how degradation affected observed activity trends. Lastly, on-line degradation measurements during potential cycling on an electrochemical flow cell coupled to inductively coupled plasma-mass spectrometry (ICP-MS) reveal potential and current density dependence of corrosion to provide insight into degradation mechanisms. Utilization of a diverse set of physical and electrochemical characterization techniques provides a thorough description of how manganese antimonate (oxy)sulfides and (oxy)nitrides carry out the ORR and could inform the future development of advanced non-precious FC cathode catalysts to drive down device cost.

Boron doping induced electronic reconfiguration of Fe-Nx sites in N-doped carbon matrix for efficient oxygen reduction reaction in both alkaline and acidic media

Developing non-precious metal-based catalysts as the substitution of precious catalysts (Pt/C) in oxygen reduction reaction (ORR) is crucial for energy devices. Herein, a template and organic solvent-free method was adopted to synthesize Fe, B, and N doped nanoflake-like carbon materials (Fe/B/N–C) by pyrolysis of monoclinic ZIF-8 coated with iron precursors and boric acid. Benefiting from introducing B into Fe–N–C, the regulated electron cloud density of Fe-Nx sites enhance the charge transfer and promotes the ORR process. The as-synthesized Fe/B/N–C electrocatalyst shows excellent ORR activity of a half-wave potential (0.90 V vs 0.87 V of Pt/C), together with superior long-term stability (95.5% current density retention after 27 h) in alkaline media and is even comparable to the commercial Pt/C catalyst (with a half-wave potential of 0.74 V vs 0.82 V of Pt/C) in an acidic electrolyte. A Zn-air battery assembled with Fe/B/N–C as ORR catalyst delivers a higher open-circuit potential (1.47 V), specific capacity (759.9 mA h g−1Zn at 10 mA cm−2), peak power density (62 mW cm−2), as well as excellent durability (5 mA cm−2 for more than 160 h) compared to those with commercial Pt/C. This work provides an effective strategy to construct B doped Fe–N–C materials as nonprecious ORR catalyst. Theoretical calculations indicate that introduction of B could induce Fe-Nx species electronic configuration and is favorable for activation of OH∗ intermediates to promote ORR process.

Synthesis and electrocatalytic properties of M (Fe, Co),N co-doped porous carbon frameworks for efficient oxygen reduction reaction

The exploitation of high efficiency non-precious metal electrocatalysts towards oxygen reduction reaction (ORR) is great significant for large-scale commercialization of next-generation fuel cells. In this work, we designed and fabricated a series of porous carbazole-based N and M (Co, Fe) doped carbon framework catalysts which were obtained by the pyrolysis of a N-rich hypercrosslinked polymers derived from Friedel–Crafts reaction (abbreviated as TSP-HCP-900) followed by the incorporation of metal into the as-resulted N rich carbon (abbreviated as M-TSP-HCP-900, M = Co, Fe). Based on the high specific surface, excellent porosity, large pyridine nitrogen content and synergistic catalytic effects between the N dopants and metal nanoparticles, the M-TSP-HCP-900 exhibited superior ORR catalytic activity. Among them, the Co-TSP-HCP-900 possesses better electrocatalysis, i.e., a high diffusion limiting current density of 4.74 mA cm −2 , half-wave potential of 0.8 V (vs. RHE, the same below) and onset potential of 0.9 V were found, respectively. In addition, this catalyst also discloses an excellent methanol tolerance, better durability and a biased 4e − reaction pathway, which are comparable to state-of-the-art Pt/C catalysts. Taking advantage of mentioning above, the M-TSP-HCP-900 may hold great potentials as promising alternative of precious metal catalysts for electrochemical energy conversion and storage. • Transition metal and nitrogen co-doped carbon (M-N/C) were prepared as non-precious ORR electrocatalysts. • It yields a half-wave potential of 0.80 V and onset potential of 0.90 V (vs. RHE) in alkaline medium. • It exhibits a high diffusion limiting current density of 4.74 mA cm −2 . • It shows better stability and methanol resistance than that of commercial 20 wt% Pt/C.

Rich and uncovered FeNx atom clusters anchored on nitrogen-doped graphene nanosheets for highly efficient and stable oxygen reduction reaction

It is still challenging but essential to synthesize non-noble catalysts with rich and uncovered catalytic active sites towards oxygen reduction reaction (ORR). And the limited research of the ORR properties of FeNx atom clusters also hinders their practical implementations. Here, we report a two-dimensional (2D) non-precious ORR catalyst constructed by abundant and fully exposed FeNx atom clusters anchored on nitrogen-doped graphene nanosheets (Fe-N-C/NG), which derives from three-dimensional (3D) ZIF-8, 2D graphene oxide nanosheet, and ferrocene. The optimized Fe-N-C/NG delivers outstanding ORR activities (Eonset = 0.97 V, E1/2 = 0.84 V, Jd = 5.48 mA cm−2), excellent Tafel slope (57.7 mV dec−1), remarkable long-term durability, and strong methanol tolerance in an alkaline electrolyte. Its superior catalytic activity is possibly attributed to its unique conversion of nanoarchitecture and the synergistic effects: the change of morphology effectively utilizes the advantages of 2D graphene oxide and 3D ZIF-8, which guarantees sufficient and reachable active sites; the 2D porous nanoarchitecture of Fe-N-C/NG promotes the ion diffusion and mechanical stability; both the FeNx atom clusters and N-doped graphene nanosheets facilitate the catalytic activity. This work introduces an innovative strategy to rationally design the nanoarchitecture and facilely synthesize nonprecious ORR catalysts with sufficient and uncovered active sites for high efficiency and durability.

A novel 3D hybrid carbon-based conductive network constructed by bimetallic MOF-derived CNTs embedded nitrogen-doped carbon framework for oxygen reduction reaction

The rational design of novel non-precious oxygen reduction reaction (ORR) electrocatalysts with good catalytic activity for energy conversion devices is under the spotlight of current research. Herein, we designed and fabricated a novel 3D Co-derived CNTs embedded nitrogen-doped carbon framework (denoted as Co-CNTs@NCFT, where T represents the pyrolysis temperature) as an efficient ORR catalyst by pyrolysis of the bimetallic metal-organic framework (MOF) in the presence of graphene oxide (GO). The optimized catalyst furnishes large specific surface area, outstanding electrical conductivity and rapid mass transport rates during reactions. Because of these synergistic advantages, the Co-CNTs@NCF900 exhibits excellent ORR catalytic activity in terms of a high onset potential of 0.96 V and a half-wave potential of 0.84 V in alkaline electrolyte, outperforming the commercial Pt/C. Furthermore, the Co-CNTs@NCF900 manifests good stability and methanol tolerance comparable to the commercial Pt/C catalyst. The presented strategy open up a new avenue for the synthesis of novel electrocatalysts derived from MOF materials for energy-related applications.

Self-assembly strategy for Co/N-doped meso/microporous carbon toward superior oxygen reduction catalysts

Proton-exchange-membrane fuel cells (PEMFCs) are of considerable interest as a sustainable, renewable, efficient, and eco-friendly electrochemical conversion systems of energy. Sluggish oxygen reduction reaction (ORR) as the cathodic reaction have mainly dragged down the PEMFCs in practical use. The most effective catalyst for ORR today is a Pt-based catalyst, which have a high scarcity value and low durability for methanol. Therefore, major efforts have recently been devoted to exploring transition metal and hetero-atom doped carbon materials in fuel cells for large-scale and commercial applications. Herein, we report a facile synthesis route of Co-N doped meso/microporous carbon catalysts (Co/N/MPC-act) via self-assembly of resorcinol-based resin, triblock co-polymer and Co containing ionic liquid followed by CO 2 activation. The obtained catalyst exhibited outstanding oxygen reduction activity in an alkaline medium. It yields an onset potential of 0.960 V (vs. RHE) and a limiting current density of 5.0 mA/cm 2 , which are comparable to that of the commercial Pt/C catalyst. This high activity is contributed by the combination of inter-connected meso/microporous structures and highly dispersed CoN x species with metallic Co. The Co/N/MPC-act catalyst exhibited excellent long-term operation stability and methanol tolerance, making it a promising non-precious metal-based ORR catalyst for PEMFCs. • Co/N doped meso-micro porous carbon (Co/N/MPC-act) was prepared by self-assembly process and CO 2 activation. • Co/N/MPC-act contains high content of pyridinic-N and Co-N. • Co/N/MPC-act exhibits an impressive high catalytic performance for ORR. • Co/N/MPC-act shows better durability and tolerance to methanol for ORR than 20 wt% Pt/C.

An ultra-dispersive, nonprecious metal MOF\u2013FeZn catalyst with good oxygen reduction activity and favorable stability in acid

Development of the more stable nonprecious oxygen reduction reaction (ORR) catalyst is of great significance nowadays. Herein, a high-performance iron-doped integral uniform macrocyclic organic framework (MOF–FeZn) catalyst is synthesized through a combined hydrothermal and pyrolysis process, showing favorable ORR activity and stability in acid. This as-synthesized MOF–FeZn catalyst displays high porous and graphitic structures with sufficient catalytic active dopants of pyridinic N, Fe–N, pyrrolic N, graphitic N, making it a promising ORR candidate catalyst with high electrochemical stability. The onset potential, half-wave potential and limited diffusion current density of MOF–FeZn are 0.93 V @ 0.1 mA cm−2, 0.768 V@ 2.757 mA cm−2 and 5.5 mA cm−2, respectively, which are comparable to the state-of-the-art nonprecious catalyst and commercial Pt/C. ORR catalysis on MOF–FeZn follows the nearly four-electron path. What is more, MOF–FeZn can sustain the 10,000 cycles electrochemical potential cycling process in acid with the half-wave potential changed only 21 mV, superior to the reduction of 149 mV for Pt/C. The well-developed integral uniform structures, homogeneously dispersed carbides and nitrides protected by the highly graphitic carbon layers and the better agglomeration suppression of nanoparticles by the confined graphitic carbon layers on catalyst can significantly enhance the catalytic activity and stability of MOF–FeZn.

Co-Zn-MOFs Derived N-Doped Carbon Nanotubes with Crystalline Co Nanoparticles Embedded as Effective Oxygen Electrocatalysts.

The oxygen reduction reaction (ORR) is a crucial step in fuel cells and metal-air batteries. It is necessary to expand the range of efficient non-precious ORR electrocatalysts on account of the low abundance and high cost of Pt/C catalysts. Herein, we synthesized crystalline cobalt-embedded N-doped carbon nanotubes (Co@CNTs-T) via facile carbonization of Co/Zn metal-organic frameworks (MOFs) with dicyandiamide at different temperatures (t = 600, 700, 800, 900 °C). Co@CNTs- 800 possessed excellent ORR activities in alkaline electrolytes with a half wave potential of 0.846 V vs. RHE (Reversible Hydrogen Electrode), which was comparable to Pt/C. This three-dimensional network, formed by Co@CNTs-T, facilitated electron migration and ion diffusion during the ORR process. The carbon shell surrounding the Co nanoparticles resulted in Co@CNTs-800 being stable as an electrocatalyst. This work provides a new strategy to design efficient and low-cost oxygen catalysts.

Cobalt Nanoparticles with Narrow Size Distribution Anchored to Flower-Like Carbon Microspheres for Enhanced Oxygen Reduction Catalysis

The development of efficient non-precious ORR electrocatalysts is crucial for the practical applications of fuel cells and metal-air batteries. In this work, a highly active and durable electrocatalyst for oxygen reduction reaction (ORR) was developed. Flower-like carbon microspheres coated with crystalline cobalt nanoparticles (Co@CMS) were successfully synthesized by the carbonization of Co-organic framework (Co-MOF). The Co nanoparticles, with narrow size distribution, are strongly anchored to the carbon support. Co@CMS possesses good catalytic ability for ORR in alkaline solution with a reduction peak potential ( E p, c ) of 0.75 V vs. RHE, which was comparable to Pt/C. The unique 3D porous structure of Co@CMS enhanced the electron transfer and electrolyte diffusion during the ORR. The ORR on Co@CMS is a diffusion-controlled process and follows a 4-electron-transfer mechanism.

Atomic-Level Modulation of Electronic Density at Cobalt Single-Atom Sites Derived from Metal-Organic Frameworks: Enhanced Oxygen Reduction Performance.

Demonstrated here is the correlation between atomic configuration induced electronic density of single-atom Co active sites and oxygen reduction reaction (ORR) performance by combining density-functional theory (DFT) calculations and electrochemical analysis. Guided by DFT calculations, a MOF-derived Co single-atom catalyst with the optimal Co1 -N3 PS active moiety incorporated in a hollow carbon polyhedron (Co1 -N3 PS/HC) was designed and synthesized. Co1 -N3 PS/HC exhibits outstanding alkaline ORR activity with a half-wave potential of 0.920 V and superior ORR kinetics with record-level kinetic current density and an ultralow Tafel slope of 31 mV dec-1 , exceeding that of Pt/C and almost all non-precious ORR electrocatalysts. In acidic media the ORR kinetics of Co1 -N3 PS/HC still surpasses that of Pt/C. This work offers atomic-level insight into the relationship between electronic density of the active site and catalytic properties, promoting rational design of efficient catalysts.

Tailored synthesis of hybrid iron-nitrogen-graphene with reduced carbon xerogel as an efficient electrocatalyst towards oxygen reduction

In this study, a non-precious metal-based electrocatalyst consisting of nitrogen-doped iron-coated reduced graphene oxide (FeNG) on carbon xerogel towards oxygen reduction reaction (ORR) in alkaline media is reported. Herein, we describe a facile three-step synthesis route towards enhanced ORR activity. The effect of pyrolysis temperature and the resulting structural variations of the designated catalyst towards ORR were investigated. The as-synthesized carbon xerogel samples were reduced (rCX) and then pyrolyzed at different temperatures, viz., 700, 900, and 1100 °C, followed by the incorporation of FeNG, and their performance towards ORR was studied. The resultant rCXFeNG (reduced carbon xerogel-iron-nitrogen-doped graphene) catalyst pyrolyzed at an optimum temperature of 1100 °C (rCXFeNG-1100) showed enhanced electrocatalytic performance towards ORR and exhibited an onset potential of 0.84 V vs. RHE (reversible hydrogen electrode). Besides, it is remarkable that rCXFeNG-1100 delivers a limiting current density of 5.55 mA cm−2, which is fairly equivalent to that of the commercial Pt/C electrocatalyst. It is noteworthy that the rCXFeNG-1100 electrocatalyst showed a four-electron transfer pathway for ORR and showed better stability and improved durability outperforming the commercial Pt/C electrocatalyst. The present study opens up a promising approach for the design and fabrication of cost-effective non-precious ORR electrocatalysts for alkaline polymer electrolyte fuel cells.

Nitrogen-modified metal-organic framework-based carbon: An effective non-precious electrocatalyst for oxygen reduction reaction

Metal-organic framework (MOF) based composite materials have attracted significant research interest in the electrocatalytic field. In this study, a carbonized polypyrrole coated MOF composite electrocatalyst (C-MIL-101@PPy) is developed, which is a nitrogen-modified MOF-based carbon used as non-precious oxygen reduction reaction (ORR) electrocatalyst. Physical characterizations prove that the C-MIL-101@PPy electrocatalyst has high nitrogen doping (2.53 at.%) and effective graphite and pyridinium nitrogen doping (86.6%). When compared with commercial Pt/C, as-obtained C-MIL-101@PPy electrocatalyst shows good ORR performance (∆E1/2 = −22 mV) with good stability (87.8%) and methanol tolerance. All results indicate that MOF-based C-MIL-101@PPy electrocatalyst is an effective and promising electrocatalyst for ORR.

Unveiling the Axial Hydroxyl Ligand on Fe-N4-C Electrocatalysts and Its Impact on the pH-Dependent Oxygen Reduction Activities and Poisoning Kinetics.

Fe—N—C materials have shown a promising nonprecious oxygen reduction reaction (ORR) electrocatalyst yet their active site structure remains elusive. Several previous works suggest the existence of a mysterious axial ligand on the Fe center, which, however, is still unclarified. In this study, the mysterious axial ligand is identified as a hydroxyl ligand on the Fe centers and selectively promotes the ORR activities depending on different Fe—N4—C configurations, on which the adsorption free energy of the hydroxyl ligand also differs greatly. The selective formation of hydroxyl ligand on specific Fe—N—C configurations can resolve contradictories between previous theoretical and experimental results regarding the ORR activities and associated active configurations of Fe—N—C catalysts. It also explains the pH‐dependent ORR activities and, moreover, a previously unreported pH‐dependent poisoning kinetics of the Fe—N—C catalysts.

Functionalized halloysite template-assisted polyaniline synthesis high-efficiency iron/nitrogen-doped carbon nanotubes towards nonprecious ORR catalysts

The iron-functionalized halloysite nanotudes is synthesized as a tubular template from an iron-nitrogen co-doped carbon nanocatalyst derived from the polyaniline pyrolysis of Oxygen reduction reaction (ORR). The use of iron-doped sites exposes the surface of the material to more graphite defects, effectively increasing the specific surface area of the carbon nanotubes and optimizing the density of the nitrogen-containing active sites. We have found that this iron-nitrogen co-doped carbon nanotube exhibits a superior ORR performance with a half-wave potential of 0.77 V in the 0.1 mol L−1 KOH aqueous solution. The electrochemical stability test was carried out at 1600 rpm. The half-wave potential showed only a degradation of 8 mV, which was better than the commercial 20% Pt/C (18 mV). This study developed an economical method for the direct synthesis of iron-nitrogen co-doped carbon nanotubes using a hard template.

Electrochemical Oxygen Reduction Reaction Performance of Water Hyacinth Derived Porous Non-precious Electrocatalyst in Alkaline Media

This research studied the possibility of converting water hyacinth biomass into the porous non-precious oxygen reduction reaction (ORR) electrocatalyst using a simple, low cost and scalable autogenic pressure method. The electrocatalyst was prepared by thermally annealing water hyacinth root contained ZnCl2 at 700oC under autogenic pressure conditions. The phase of the catalyst was the mixture of carbon and metal oxide. In addition, rough surface morphology and high porosity were clearly observed using scanning electron microscope. The synthesized catalyst was then determined the ORR performance by cyclic voltammetry (CV) and linear sweep voltammetry (LSV) techniques under O2 saturated KOH solution. The ORR performance increased as the catalyst loading was increased and the optimum catalyst loading was found to be 1.5 mg/cm2 which generated the Eonset and E1/2 value of 0.93 V and 0.80 V vs. RHE, respectively. Furthermore, the E1/2 value of the synthesized catalyst was 230 and 130 mV greater than the catalyst synthesized without ZnCl2 and commercial carbon (VXC-72R). ORR durability study suggested that the prepared catalyst was durable to operate ORR for 5000 cycles in alkaline media. These results suggested that the autogenic pressure conditions would be a promising technique to prepare highly active and durable biomass derived ORR electrocatalyst.

Preparation of monodisperse ferrous nanoparticles embedded in carbon aerogels via in situ solid phase polymerization for electrocatalytic oxygen reduction.

Core-shell structured materials constructed by using Fe/Fe3C cores and nitrogen doped carbon shells represent a type of promising non-precious oxygen reduction reaction (ORR) catalyst due to well-established active sites at the interface positions. However, the traditional liquid phase polymerization route for preparing such materials normally leads to a compact macropore-deficient structure with randomly dispersed metallic nanoparticles, which is not beneficial for mass transfer and the formation of a high-density dispersion of active sites. Herein, we report an "in situ solid phase polymerization strategy" in which a frozen block containing uniformly dispersed oligomers is firstly achieved by combining a well-controlled hydrothermal reaction and a subsequent liquid nitrogen-facilitated fast solidification. During the following freeze-dry process, the oligomers in situ polymerize into a 3D highly cross-linked network in the confined space of the ice block which not only effectively avoids the direct stacking of polymerized intermediates, but also prevents the agglomeration of metallic nanoparticles. The finally obtained monodisperse Fe/Fe3C nanoparticles embedded in nitrogen-doped carbon aerogel catalyst, in the ORR, delivers an ultrahigh activity as the half-wave potential and the kinetic current density at 0.9 V reach 0.919 V and 7.83 mA cm-2 respectively in an alkaline solution. Using this route, a range of aerogel materials with improved performances for various applications may be explored.

New Insights on the Surface and Stress Behavior of Manganese-Oxide As Oxygen Reduction Reaction Catalyst in Alkaline Electrolyte

The electro-catalysts for oxygen reduction reaction (ORR) on air-cathode have a decisive impact on metal-air batteries and fuel cells. Much effort has been invested in finding a cost effective electro-catalyst, enabling a decrease in the ORR over-potential and enhancing the power source device discharge performance.[1-2] Manganese-oxides (MnOx) are particularly interesting as non-precious ORR electro-catalysts candidates due to their rich oxidation states, chemical compositions and their variety of crystal structures[1-3]. In order to broaden our understanding on the catalytic mechanism of Manganese-Oxide and its surface chemistry during ORR, we performed unique in-situ electrochemical surface stress (ESS) measurements[4]. The in-situ ESS response was measured on an Au/MnOx electrode in both Ar and O2 saturated environments. A complex stress response was measured during the electrochemical scan caused by various crystal structure transitions[5]. The presence of oxygen resulted in a less compressive stress response during the ORR; suggesting that part of the activated sites on the surface are reduced and re-oxidized (Mn4+↔Mn3+) during the ORR. These observations evidently support the proposed ORR catalytic mechanism[3,5]. In addition, an overall tensile trend was recorded during multiple cycling, pouring some light on the poor mechanical stability of the MnOx film. This insight has major implications on the applicability of MnOx as a catalyst on practical carbon based electrodes.