Transfer Oxygen Reduction Reaction Research Articles

Iron-based graphene composite for oxygen reduction reaction in alkaline media: Electrocatalytic activity and lifetime evaluation

The development of electrocatalysts with high catalytic performance and low costs for oxygen reduction reaction (ORR) is still challenging. Herein, an overall solution for ORR in alkaline media, from the catalyst synthesis to catalyst regeneration and to the development of a rapid, reliable and easy approach for electrode surface evaluation, is presented. We focused on the development and characterization of an efficient material for ORR in alkaline media, α-Fe2O3 N-doped graphene (Fe-N-Gr). The associative pathway for the four electron transfer ORR mechanism is sustained by the Raman spectra recorded from the electrode surface. We highlighted the possibility of catalyst regeneration by a simple electrochemical method. After two regeneration rounds and 1500 cycles in O2-saturated 1 M NaOH, the catalyst still retains 40.8 % catalytic activity. Finally, as a part of the overall solution, we demonstrated that a methodology based on Raman spectroscopic measurements and machine learning algorithms can be applied for graphene-based catalysts lifetime investigation.

Coupling Single-Ni-Atom with Ni–Co Alloy Nanoparticle for Synergistically Enhanced Oxygen Reduction Reaction

Developing nonprecious metal-based oxygen reduction reaction (ORR) electrocatalysts with superior activity and durability is crucial for commercializing proton-exchange membrane (PEM) fuel cells. Herein, we report a metal-organic framework (MOF)-derived unique N-doped hollow carbon structure (NiCo/hNC), comprising of atomically dispersed single-Ni-atom (NiN4) and small NiCo alloy nanoparticles (NPs), for highly efficient and durable ORR catalysis in both alkaline and acidic electrolytes. Density functional theory (DFT) calculations reveal the strong coupling between NiN4 and NiCo NPs, favoring the direct 4e- transfer ORR process by lengthening the adsorbed O-O bond. Moreover, NiCo/hNC as a cathode electrode in PEM fuel cells delivered a stable performance. Our findings not only furnish the fundamental understanding of the structure-activity relationship but also shed light on designing advanced ORR catalysts.

Electrodeposited Cobalt Nanosheets on Smooth Silver as a Bifunctional Catalyst for OER and ORR: In Situ Structural and Catalytic Characterization.

Developing earth-abundant, cost-effective, and active bifunctional electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is key to boosting sustainable energy systems such as electrolyzers and lithium-air batteries. However, the performance of promising cobalt-based materials is impaired by the external effects of binders and carbon additives as well as inhomogeneous electrode fabrication. In this work, binder- and carbon-free flower-like Co-decorated Ag catalytic nanosheets were in situ-synthesized via a simple electrodeposition approach. The morphology, composition, and structure of Co/Ag before and after OER were characterized using scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD). Co/Ag thin film electrodes with various Co contents exhibited a bifunctional activity toward ORR and OER due to a synergistic effect. XPS analysis suggested the formation of Co3O4 as the main active species for OER. In particular, Co (83%)/Ag surface revealed a 60 mV lower ORR overpotential than a pure Ag surface and even lower than drop-casted Co3O4 nanoparticles on Ag surface. Only 1.5% peroxide was generated, suggesting a four-electron transfer ORR. In addition, the OER onset potential on Co/Ag is 60 mV less than Co3O4. Tafel slopes of 71 and 75 mV dec-1 were obtained for ORR and OER, respectively. Importantly, the three-dimensional (3D) growth mechanism of a cobalt layer (∼1 nm) on a well-defined atomic smooth Ag surface is unraveled by in situ electrochemical scanning tunneling microscopy (EC-STM). EC-STM suggests that Co prefers to nucleate at the step edges of Ag and grows in a 3D, forming nanoparticles, where the deposition/dissolution process of the Co adlayer on Ag is reversible. This investigation may provide insights into design strategies of efficient oxygen electrocatalysts.

Cobalt (iron), nitrogen and carbon doped mushroom biochar for high-efficiency oxygen reduction in microbial fuel cell and Zn-air battery

To improve the oxygen reduction reaction (ORR) kinetics in microbial fuel cell (MFC) and Zn-air battery, cobalt (iron), nitrogen and carbon doped mushroom biochar, i.e. CoNC@MC, FeNC@MC and CoFeNC@MC were prepared using metal-organic frameworks and mushroom as the precursors. The catalysts showed a mesopore-dominant structure with abundant Metal-N, Pyridinic-N and Graphitic-N catalytic active sites. Rotating disk electrode tests revealed that FeNC@MC performed the best ORR activity in neutral and alkaline solutions, which followed the four-electron transfer ORR pathway. Acid leaching could further improve the ORR activity of FeNC@MC in neutral conditions. In MFCs, the FeNC@MC and acid-leached FeNC@MC yielded maximum power densities of 1052.5 mW m−2 and 1126.3 mW m−2 respectively. The above catalysts also produced high power densities of 94.0 mW cm−2 and 93.7 mW cm−2 in the Zn-air battery, with high stability and good methanol tolerance. This article provides a new strategy to prepare low-cost and efficient non-noble ORR catalysts for microbial fuel cell and Zn-air battery.

N-Doped Colloid Imprinted Carbons As Promising ORR Catalysts for Alkaline Applications

The oxygen reduction reaction (ORR) has long been of interest in relation to its many energy applications and interesting multi-pathway mechanisms. The ORR is a key reaction in a range of electrochemical energy conversion and storage devices, such as hydrogen fuel cells and metal-air batteries, respectively. These devices are expected to play an ever-increasing role in the global transition to net zero emissions. Metal-air batteries, such as Zn/air batteries, can operate at room temperature, use recyclable materials, are environmentally friendly, and are preferred in relation to consumer safety than batteries relying on organic solvents and reactive electrode species.1 High rates of the ORR are crucial for the development of high performance Zn/air batteries and catalysts play a significant role, with lowering of the catalyst cost also of increasing importance.The costs of conventional Pt-based ORR catalysts are high. Therefore, metal-free carbon-based ORR electrocatalysts are viewed as increasingly promising alternatives, especially as they are lower in cost due to the availability of the precursor materials. Carbon is also a good electrical conductor and support material, is chemically stable, and can have large surface areas. However, the ORR kinetics are sluggish on carbon and chemical and physical modifications are required to enhance its activity. In typical Zn/air batteries, the ORR occurs at the three-phase boundary (TPB) formed between the solid electrode, liquid electrolyte, and gaseous oxygen. The porosity of the catalyst layer, the wettability of the catalyst/electrolyte interface, and the gas permeability and hydrophobicity of the gas diffusion layer (GDL) are thus also important, significantly influencing the cathode performance and durability. The catalyst layer (CL) must therefore be constructed with both a high-performance catalyst and an optimized TPB length to provide high performance without compromising durability.In the current work, we have doped nitrogen into the lattice of a family of nanoporous colloid imprinted carbon (CIC) powders to increase its ORR activity. The CICs are unique for their versatility in terms of pore size control and ease of surface functionalization.2 Pore sizes in the range of 12 to 100 nm were examined and their effect on the ORR activity and mass transport limitations were investigated. To carry out N-doping, the CICs were exposed to ammonia at 800 ˚C for 7 hr. Catalyst inks were then prepared by mixing the CICs with a binder in an isopropyl alcohol/water solution. Aliquots of the ink were drop-casted on the disc of an RRDE system, or were spray coated or drop-casted on a GDL to determine the ORR performance in an in-house Zn/air battery testing cell, with the N-doped CIC catalyst layer sandwiched between an electrolyte chamber and a graphite current collector. In this setup, O2 gas was flowed through the pores in the GDL to the catalyst/electrolyte interface, a Zn wire installed in the electrolyte chamber was used as the reference electrode, and a Ni sponge was used as the counter electrode.The RRDE experiments showed that, after N doping of the CIC powders, the production of peroxide decreased significantly and the ORR onset potential increased to a very respectable value of ca. 0.9 V vs RHE, indicating the successful activation of the ORR. Electron transfer numbers were found to be greater than 3.5, indicating that either a direct or pseudo- 4 electron transfer ORR pathway is dominant. In agreement with the literature, the ORR currents in the kinetic regions increased linearly with mass loading, expected to be proportional to the total active N-doped CIC surface area.3 The N-doped CIC samples retained excellent performance up to a loading of 0.350 μg/cm2 without losing mechanical stability.Similar N-doped CICs and binders of different hydrophobicity were tested in the Zn/air battery testing system. Electrodes made with a hydrophobic NCS microporous layer (MPL) showed much better ORR performance and durability than hydrophilic NCS MPLs. Although electrodes made using hydrophobic polytetrafluoroethylene (PTFE) as the binder in the catalyst layer showed a similar initial performance to those made using hydrophilic Nafion binders, the PTFE based electrodes exhibited better durability. Furthermore, the temperature and pressure used during electrode fabrication were also found to have a significant impact on the binder distribution and ORR performance. Once optimized, a very good correlation was obtained between the N-doped CIC catalyst performance in the RRDE setup and in the Zn/air battery testing system. References J. Pan et al., Adv. Sci., 5, 1700691 (2018).X. Li et al., ACS Appl. Mater. Interfaces, 10, 2130–2142 (2018).N. Gavrilov et al., J. Power Sources, 220, 306–316 (2012).

In Situ Mechanistic Insights for the Oxygen Reduction Reaction in Chemically Modulated Ordered Intermetallic Catalyst Promoting Complete Electron Transfer.

The well-known limitation of alkaline fuel cells is the slack kinetics of the cathodic half-cell reaction, the oxygen reduction reaction (ORR). Platinum, being the most active ORR catalyst, is still facing challenges due to its corrosive nature and sluggish kinetics. Many novel approaches for substituting Pt have been reported, which suffer from stability issues even after mighty modifications. Designing an extremely stable, but unexplored ordered intermetallic structure, Pd2Ge, and tuning the electronic environment of the active sites by site-selective Pt substitution to overcome the hurdle of alkaline ORR is the main motive of this paper. The substitution of platinum atoms at a specific Pd position leads to Pt0.2Pd1.8Ge demonstrating a half-wave potential (E1/2) of 0.95 V vs RHE, which outperforms the state-of-the-art catalyst 20% Pt/C. The mass activity (MA) of Pt0.2Pd1.8Ge is 320 mA/mgPt, which is almost 3.2 times better than that of Pt/C. E1/2 and MA remained unaltered even after 50,000 accelerated degradation test (ADT) cycles, which makes it a promising stable catalyst with its activity better than that of the state-of-the-art Pt/C. The undesired 2e- transfer ORR forming hydrogen peroxide (H2O2) is diminished in Pt0.2Pd1.8Ge as visible from the rotating ring-disk electrode (RRDE) experiment, spectroscopically visualized by in situ Fourier transform infrared (FTIR) spectroscopy and supported by computational studies. The effect of Pt substitution on Pd has been properly manifested by X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS). The swinging of the oxidation state of atomic sites of Pt0.2Pd1.8Ge during the reaction is probed by in situ XAS, which efficiently enhances 4e- transfer, producing an extremely low percentage of H2O2.

Recycling synthesis of single-atom Zn-nitrogen-carbon catalyst for electrocatalytic reduction of O2 to H2O2

Portable electrochemical synthesis of H2O2, which is widely used for disinfection, sterilization, and waste treatment, has attracted increasing attention. However, low atomic utilization efficiency and by-product disposal during the synthesis of electrocatalysts are inevitable. Based on the advantages of single-atom catalysts (SACs) towards H2O2 synthesis via a 2e− transfer oxygen reduction reaction, this work presents the synthesis of ZnNC SACs as excellent oxygen reduction catalysts with high atomic use efficiency and almost no generated waste by using formamide and metal powder as precursors. The ZnNC SACs exhibit H2O2 selectivity exceeding 80% when working continuously for 10,000 s. The synthetic strategy described in this paper is intended to be used as a supplement to the synthesis of metal-nitrogen-carbon SACs with high atomic utilization efficiency and low waste generation for environmental and chemical applications.

Microwave-assisted synthesis of well-defined nitrogen doping configuration with high centrality in carbon to identify the active sites for electrochemical hydrogen peroxide production

While nitrogen (N)-doped carbon can facilitate the four-electron transfer in oxygen reduction reaction (ORR), its effect on the two-electron path is yet to be investigated. To date, the random N-C bond configurations in N-doped carbons make it difficult to identify the active sites for electrochemical reactions. Here, we report a preparation of well-controlled N-C configurations with high centrality as model catalysts for identification of active sites for two-electron transfer ORR. Specifically, the synthesized Pyrrolic-N doped single-wall carbon nanotube and graphene utilizing a microwave-assisted pulse heating method, exhibit excellent activity for two-electron path (H2O2 selectivity of 93.5% and 98.35%). X-ray adsorption near-edge structure spectroscopy measurements indicate that carbon atoms adjacent to Pyrrolic-N are the active sites for two-electron transfer. This work provides a viable way for the fabrication of tailored N configurations in carbon materials and significantly advances our understanding of the N moieties for the electrochemical H2O2 generation.

Coupled intramolecular/heterointerfacial electron transfer in polyelectrolyte-shielded Iso-type black phosphorus hetero-structure boosts oxygen reduction kinetics

Searching new structured black phosphorus (BP) and exploring intriguing functions and applications have become a hot topic so far. Here, we introduce a novel Iso-type black phosphorus heterostructure guided by first principle calculation, which features unique heterointerface and electronic coupling interaction via stacking assembly of exfoliated black phosphorus (EBP) and amine-functionalized EBP (N-EBP). Inspired by the theoretical results, we constructed the Iso-type heterostructure comprising of ultrathin exfoliated few-layered EBP and N-EBP, both of which were derived from identical bulk BP. The purposive amine-functionalization not only creates positively-charged P atoms on N-EBP as effective active sites via N-induced intramolecular electron transfer (IET) but also endows N-EBP with lower work function relative to EBP, while the unique EBP/N-EBP Iso-type heterostructure engenders directional heterointerfacial electron transfer (HET). The coupled IET/HET effects optimize the charge redistribution to afford favorable O2 adsorption. In this case, our unique strategy for the first time exploits the inherent catalytic capability of BP toward the oxygen reduction reaction (ORR) and enables the first use of BP as metal-free ORR catalysts for Zn-air cells. The newly-designed heterostructure facilitates a 4-e− transfer ORR relative to inactive EBP or N-EBP. Importantly, the polymer-shielded heterostructure acts as efficient air electrodes to endow a primary Zn-air cell with high stability, large capacity and high energy density—superior to the commercial Pt/C-enabled cell. This study as the first report on metal-free BP-based ORR catalysts and air electrodes not only extends BP’s application scopes but also renders new insight toward design of electronically-coupled superstructures for energy-related applications.

Simultaneous sulfamethoxazole degradation with electricity generation by microbial fuel cells using Ni-MOF-74 as cathode catalysts and quantification of antibiotic resistance genes

Antibiotic wastewater presents serious challenges in water treatment. Metal-organic frameworks (MOFs) have received significant attention as promising precursors and sacrificial templates in the preparation of porous carbon-supported catalysts. Herein, we investigated the sulfamethoxazole (SMX) degradation and electrochemical performance of microbial fuel cells (MFCs) that applied as-prepared Ni-MOF-74 and Ni–N–C (Ni-MOF-74 underwent pyrolysis treatment at different temperatures) as air-cathode catalyst. Firstly, the electrocatalytic activity towards oxygen reduction reaction (ORR) of the catalyst was investigated by rotating disk electrode. The results showed that electron transfer number for Ni-MOF-74 was 2.12, while that of 800Ni–N–C was 3.44, which was close to four-electron reduction. Applying Ni-MOF-74 in MFCs, a maximum power density of 446 mW/m2 was obtained, which was close to that of 800Ni–N–C. Besides, using Ni-MOF-74 as cathode catalyst, a chemical oxygen demand removal rate of about 84% was obtained, and the degradation rate of 10 mg/L SMX was 61%. The degradation rate decreased with increasing antibiotic concentration, but the average degradation efficiency increased stepwise. Additionally, the relative abundance of resistant gene sul1 in the reactors of the new catalytic material was about 62% lower than that of sul1 in the control (Pt/C) reactors, and the relative abundance of sul2 was about 73% lower. Moreover, cost assessments related to the catalyst performance are presented. The findings of this study demonstrated that Ni-MOF-74 could be considered as a two-electron transfer ORR catalyst, and offers a promising technique for preparation of Ni–N–C for use as four-electron transfer ORR catalysts. In comparison, Ni-MOF-74 could be a promising ORR catalyst of MFCs for antibiotic degradation.

First demonstration of phosphate enhanced atomically dispersed bimetallic FeCu catalysts as Pt-free cathodes for high temperature phosphoric acid doped polybenzimidazole fuel cells

Phosphate poisoning of Pt electrocatalysts is one of the major barriers that constrains the performance of phosphoric acid-doped polybenzimidazole (PA/PBI) membrane fuel cells. Herein, we developed new atomically dispersed bimetallic FeCu coordinated with nitrogen-doped carbon nanotubes (FeCu/N-CNTs) as Pt-free oxygen reduction reaction (ORR) electrocatalysts. The cell with FeCu/N-CNTs cathodes delivers a peak power density of 302 mWcm−2 at 230℃, similar to that using Pt/C electrocatalysts (1 mgPt cm−2) but with a much better stability. In contrast to phosphate poisoning of Pt/C, FeCu/N-CNTs show PA enhanced activities. DFT calcualtions indicate that phosphate promotion effect results from the stronger binding of phosphate on Cu sites, which decreases the activation energy barrier for the cleavage of the O2 double bond and provides local protons to facilitate the proton-coupled electron transfer ORR. The results also show that FeCu/N-CNTs have a much better activity for ORR as comapre to Fe single atom catalysts coordinated with nitrogen-doped carbon nanotubes, Fe/N-CNTs. This study demonstrates the promising potential of bimetallic FeCu/N-CNTs as true Pt-free, highly active and durable cathodes for PA/PBI based high temperature polymer electrolyte fuel cells.

Electrochemical Oxygen Reduction to Hydrogen Peroxide via a Two‐Electron Transfer Pathway on Carbon‐Based Single‐Atom Catalysts

AbstractElectrochemical reduction of oxygen is considered as a new strategy to achieve decentralized preparation of hydrogen peroxide (H2O2) in a green manner. As a promising new type of catalytic material, carbon‐based single‐atom catalysts can achieve wide‐range adjustments of the electronic structure of the active metal centers while also maximize the utilization of metal atoms, toward electrochemical production of H2O2 from the selective two‐electron transfer oxygen reduction reaction (ORR). Herein, starting from the reviewing of characterizing methods and reaction mechanisms of ORR via two‐electron and four‐electron transfer pathways, the vital role of binding strength between OOH intermediate and active sites in determining the activity and selectivity towards H2O2 production is revealed and illustrated. Currently reported carbon‐based single‐atom catalysts for H2O2 production are systematically summarized and critically reviewed. Moreover, with the underpinning chemistry to improve the overall efficiency, three aspects concerning the central metal atoms, coordinated atoms, and environmental atoms are comprehensively analyzed. Based on the understanding of the most current progresses, some predictions for future H2O2 production via electrochemical routes are offered, which include catalyst designs at atomic levels, new synthesis strategies and characterization techniques, as well as interfacial superwetting interaction engineering at electrode and device levels.

A Lactate/Oxygen Biofuel Cell: The Coupled Lactate Oxidase Anode and PGM-Free Fe-N-C Cathode.

The rapid development of both wearable and implantable biofuel cells has triggered more and more attention on the lactate biofuel cell. The novel lactate/oxygen biofuel cell (L/O-BFC) with the direct electron transfer (DET)-type lactate oxidase (LOx) anode and the platinum group metal (PGM)-free Fe-N-C cathode is designed and constructed in this paper. In such a reasonable design, the surface-controlled direct two-electron electrochemical reaction of the lactate oxidase was determined by cyclic voltammetry (CV) on the carbon nanotube (CNT) modified electrode with favorable high electrochemical active surface area and electronic conductivity. Additionally, the biosensor based on DET-type LOx modified electrode impressively presented linear response to lactate with different concentrations from 0.000 mM to 12.300 mM. In particular, the apparent Michealis-constant (KMapp) calculated as 0.140 mM clearly indicates that LOx on CNT has strong affinity to the substrate lactate. Meanwhile, 4e- transfer oxygen reduction reaction (ORR) was proven to take place on the Fe-N-C catalysts inthe 0.1 M PBS system, indicating the advantage by using the Fe-N-C catalysts at the cathode of L/O-BFC. Last but not least, the L/O-BFC with the direct electron transfer (DET)-type lactate oxidase(LOx) anode and the Fe-N-C cathode produced an superior open circuit potential (OCP) of 0.264 V and a maximum output power density (OPD) of 24.430 μW cm-2 in O2 saturated 95.020 mM lactate solution. The above results will not only bring about significant interest in developing a DET-type biofuel cell, but also offer guiding direction to explore novel catalyst materials for the biofuel cell. This work enriches the research content and may push developments of the implantable and wearable biofuel cell forward.

Cobalt sulfides as efficient catalyst towards oxygen reduction reactions

Developing low-cost and high performance catalysts to replace precious metal based catalysts for oxygen reduction reaction (ORR) is one of the most feasible ways to promote the commercial application of fuel cells. In this work, flower-like CoS and octahedral CoS2 are synthesized by a facile one-pot hydrothermal method without any adjunction of surfactants or follow-up thermolysis, their catalytic performance towards ORR in alkaline electrolyte are comparatively investigated. The results reveal that CoS2 outperforms CoS owing to the higher electron density around SS bond of S22- in the crystal structure, which promotes the adsorption of oxygen on catalyst surface and facilitates the breakage of OO bond in oxygen, leading to direct 4-electron transfer ORR. When CoS2 particles are dispersed on the surface of rGO with large surface area, their ORR performance could be further improved.

Morphology dependent facile synthesis of manganese oxide nanostructures for oxygen reduction reaction

The present work demonstrates the morphology dependent synthesis of manganese oxide using simple hydrothermal method just by adjusting the reaction temperature. The structural and morphological studies confirm the formation of manganese oxide with different phases and anisotropic structures of manganese oxide. Interestingly, electrochemical studies exhibit significant oxygen reduction reaction (ORR) kinetics by manganese oxide nanowires with improved onset potential of +0.83 V versus reversible hydrogen electrode (RHE) and current density of 1.75 mA/cm2 in alkaline condition. Among three different morphologies, nanowires show better catalytic activity due to improved diffusion path length and higher aspect ratio. However, all the three samples follow two (2) - electron transfer ORR kinetics, though four electron transfer kinetics has been of great interest for energy conversion applications. Experimental results represent that the reaction temperature plays a key role in forming different shapes and affect the crystal phase and growth process thereby affecting electrochemical properties.

Electrocatalytic Oxygen Reduction by Dopant‐free, Porous Graphene Aerogel

AbstractPorous graphene aerogel (GA) was prepared by the hydrothermal reduction of graphene oxide (GO). The prepared GA was characterized with the various analytical techniques. Its morphological and electrochemical properties were investigated and compared with those of GO and electrochemically reduced graphene oxide (erGO). Three glassy carbon (GC) electrodes modified with GO, erGO, and GA were employed to study the electrocatalytic oxygen reduction reaction (ORR). The influence of GA porosity on the kinetic parameters of the ORR was also investigated. Whereas two‐electron transfer ORR was observed at the erGO‐ and GO‐modified electrodes, the GA‐modified electrode exhibited drastically enhanced electrocatalytic ORR activity. The remarkable electrocatalytic activity of GA can be attributed to the unique porous effect of GA. The porous GA offers confined spaces for ORR that would drastically increase the residence time of oxygen and thus enhance the contact frequency for the reduction of oxygen. Additional rotating disk electrode voltammetry corroborates that the porous GA facilitates efficient ORR.

Enhanced performance stability of carbon/titania hybrid electrodes during capacitive deionization of oxygen saturated saline water

Capacitive deionization (CDI) is a promising technology for the desalination of brackish water due to its potentially high energy efficiency and its relatively low costs. One of the most challenging issues limiting current CDI cell performance is poor cycling stability. CDI can show highly reproducible salt adsorption capacities (SACs) for hundreds of cycles in oxygen-free electrolyte, but by contrast poor stability when oxygen is present due to a gradual oxidation of the carbon anode. This oxidation leads to increased concentration of oxygen-containing surface functional groups within the micropores of the carbon anode, increasing parasitic co-ion current and decreasing SAC. In this work, activated carbon (AC) was chemically modified with titania to achieve additional catalytic activity for oxygen-reduction reactions on the electrodes, preventing oxygen from participating in carbon oxidation. Using this approach, we show that the SAC can be increased and the cycling stability prolonged in electrochemically highly demanding oxygen-saturated saline media (5mM NaCl). The electrochemical oxygen reduction reaction (ORR) occurring in our CDI cell was evaluated by the number of electron transfers during charging and discharging. It was found that, depending on the amount of titania, different ORR pathways take place. A loading of 15 mass% titania presents the best CDI performance and also demonstrates a favorable three-electron transfer ORR.

Non-Precious ORR Catalyst for PEMFC Application Using Iron, Nitrogen and Sulfur Co-Doped Few Layer Graphene

The sluggish kinetics of oxygen reduction reaction (ORR) at the cathode of proton exchange membrane fuel cells (PEMFC) demand a high loading of precious platinum (Pt) or Pt alloy electrocatalysts [1]. The extensive implementation of fuel cell technologies for stationary and vehicular applications, however, demands the replacement of high cost and less abundant Pt with low cost and natural abundant electrocatalysts. Recent studies show the use of various non-precious metals, metal alloys, metal oxides and nitrogen doped carbon as alternative catalysts for ORR [2, 3]. Even though, these non-precious metal catalysts experience severe dissolution and large agglomeration under vigorous PEMFC operational conditions and subsequent catalyst performance degradation. In this work, we developed a facile method for making non-precious ORR catalyst using nitrogen (3.7 at. %), sulfur (2.4 at. %) and iron (1.3 at. %) co-doped porous graphene (Fe-NSG). The ORR activity and durability of catalyst in acidic media was investigated by half (Fig. 1) and full cell electrochemical measurements. RDE measurements of the Fe-NSG catalyst confirm a four electron transfer ORR process with high current density. PEMFC full cell measurements of the catalyst give a maximum power density of 225 mW cm-2 at 80 °C with good stability [4]. The high ORR activity and good stability of Fe-NSG electrocatalyst can be ascribed due to, (a) the comparatively higher electronegativity of doped N and S atoms (N: 3.04, S: 2.58) with respect to carbon (C: 2.55), which induces more charged sites within the graphene lattice favorable for oxygen adsorption and reduction, (b) presence of the large amount of pyridinic nitrogen, which can coordinate with iron cations and increase the density of ORR active Fe-Nx centers, (c) an optimum amount of sulfur within the non-precious catalyst suppress the iron carbide formation, and increases the formation of Fe-N4 ORR active species, (d) the unique confined morphology of Fe nanoparticles within the graphene layers suppress the agglomeration/dissolution of metal particles and increase their interfacial contact and durability. References F. Jaouen, E. Proietti, M. Lefevre, R. Chenitz, J.-P. Dodelet, G. Wu, H. T. Chung, C. M.Johnston and P. Zelenay, Energy & Environmental Science, 2011, 4, 114-130.Z. Chen, D. Higgins, A. Yu, L. Zhang and J. Zhang, Energy & Environmental Science, 2011, 4, 3167-3192.M. Lefèvre, E. Proietti, F. Jaouen and J.-P. Dodelet, Science, 2009, 324, 71-74.B. P. Vinayan, T. Diemant, R. J. Behm and S. Ramaprabhu, RSC Advances, 2015, 5, 66494-66501. Figure 1

Vertically Aligned Carbon Nanosheets As Immobilization Supports for Enzyme Bio-Electrocatalysis

High performance laccase-based biofuel cell cathode has been developed using carbon nanosheet (CNS) with buckypaper (BP) as the substrate electrode. Compared to multi-walled carbon nanotubes (MWNT) based electrode, CNS based electrode exhibited better electrochemical activity for oxygen reduction reaction (ORR) at biologically relevant conditions. The scanning electron microscopy and Raman spectroscopy studies show that CNS was coated on the nano-tubule bundles within the BP. The attenuated total resonance Fourier transform infrared spectroscopy results show that laccase was intimately attached on to the CNS-BP. Electrochemical characterization including cyclic voltammetry and Tafel region sweep were performed to derive the kinetic parameters of ORR at the laccase-CNS-BP cathode. The onset potential obtained from the cyclic voltammogram for laccase ORR at CNS-BP is 20 mV higher than the MWNT based electrodes. The laccase-CNS-BP also exhibited steep ORR cathodic wave in the voltammograms with a Tafel slope of 19 mV decade-1. Steady state cathode polarization curves also reveal that laccase-CNS-BP cathode yielded higher current density than MWNT electrodes. Fig. 1: Schematic representation of four electron transfer ORR at laccase-CNS interface. Acknowledgements: This work was partially supported by the United States Army under Contract No W911NF-13-C-0014.

Iron–Nitrogen Coordination in Modified Graphene Catalyzes a Four‐Electron‐Transfer Oxygen Reduction Reaction

AbstractIt is shown that iron–nitrogen (Fe/N) coordination in an Fe/N‐modified graphene (Fe/N‐graphene) electrocatalyst catalyzes a four‐electron transfer oxygen reduction reaction (ORR). The Fe/N‐graphene is synthesized by heat‐treating graphene oxides in the presence of an Fe/N complex at 900 °C for just 45 s. When the heat‐treatment period is increased, the Fe/N coordination bonds are broken and the number of electrons involved in the ORR decreases. Extended X‐ray absorption fine structure analysis and X‐ray absorption spectroscopy (XAS) show that both a complete sp2 graphitic structure and a high density of Fe/N coordination structures exist in the Fe/N‐graphene material, which is in contrast to the carbon‐based catalyst prepared by using the conventional pyrolysis method. XAS analysis also reveals that back‐donation from Fe atoms coordinated with N to an O antibonding π* orbital occurrs, promoting the four‐electron transfer ORR.