Rotating Ring Disk Electrode Research Articles

Cover Feature: Interfacial pH Measurements Using a Rotating Ring‐Disc Electrode with a Voltammetric pH Sensor (ChemElectroChem 1/2022)

The Cover Feature illustrates a Rotating Ring-Disc Electrode in contact with an electrolyte. In this work, this electrode is functionalized with a pH sensing monolayer and is used to measure the interfacial pH during hydrogen evolution under controlled mass transport conditions. More information can be found in the Article by M C. O. Monteiro, X. Liu et al.

A new spin on electrochemistry in the undergraduate lab

Abstract The growing interest in electrochemistry over recent years has sparked an increase in the popularity of various electrochemical techniques, including more advanced methods, that have previously been overlooked in academia and industry. This makes comprehensive hands-on experience in electrochemistry a highly demanded addition to chemistry graduates. However, many students do not receive sufficient training in the theory and experimental design to confidently use and apply various electrochemical techniques throughout their undergraduate, and sometimes even in graduate studies. Here we summarize the theory and practical applications for both rotating disk electrode (RDE) and rotating ring disk electrode (RRDE) techniques. The different modes of operation of rotating ring disk voltammetry, methodologies of data analysis and interpretation as well as the scope of the information that can be extracted from the RDE/RRDE are discussed. Proposed modifications of the laboratory curriculum will allow students to examine and learn valuable information about the reactions on the surface of the electrode/liquid interface. This information will allow chemists to confidently use RDE and RRDE techniques for a wide range of research and development targets. Furthermore, incorporating these techniques into existing chemistry laboratories will help chemistry educators to enrich the undergraduate chemistry curriculum and improve students’ learning outcomes.

Dual Electrode Detection in LC‐EC Analysis of Sanger‐tagged Amino Acids: Electrochemical Reduction of Aromatic Nitro Groups

AbstractAnalysis of amino acids is crucial to protein structure elucidation. Amino acids, amenable to separation by liquid chromatography (LC), can be detected by sensitive detectors used in liquid chromatograph including absorbance, fluorescence or electrochemical detector when derivatized. Derivatization of amino acids using suitable reagents enhances the separation and detection of amino acids using liquid chromatography. Sanger's reagent (1‐fluoro‐2,4‐dinitrobenzene, DNFB) makes amino acids suitable for absorbance detection. However, little has been done in terms of liquid chromatography with electrochemical detection (LC‐EC) of the DNFB derivatized amino acids. Furthermore, sensor development based on electrochemistry of nitroaromatics has increased dramatically for explosive detection. Since nitroaromatics are electrochemically active, it is essential to determine the reduction pathway. Cyclic voltammetry (CV), rotating disk electrode (RDE) and rotating ring disk electrode (RRDE) experiments have been performed on nitrobenzene (NB) and N‐methyl‐2,4‐dinitroaniline (NMDNA). The results showed that the nitro group was reduced to hydroxylamine in aqueous solutions by addition of four electrons and four protons per nitro group and the hydroxylamine can be reversibly oxidized into a nitroso by removal of two electrons and two protons. Electrochemical detection of the nitro groups is appropriate for dual electrode detection of DNFB labeled amino acids in which reduction can occur on an upstream electrode and corresponding oxidation of the reduction product occurs on the downstream electrode.

Dye-synthesized N, S co-doped carbon via plasma engineering as metal-free oxygen reduction reaction electrocatalysts

Emerging metal-free heterogeneous element-doped carbon-based catalysts have shown advantages of high catalytic efficiency and low cost, and are regarded as a promising alternative to metal catalysts in alkaline-based fuel cells and metal–air batteries. Methylene blue, commonly used to stain specimens, has been causing serious marine pollution and should be considered for eco-friendly recycling. In this study, methylene blue was chosen as an additive and precursor for N- and S-doped carbon nanoparticles and was dissolved in quinoline (C9H7N) to synthesize N, S co-doped carbon electrocatalysts via plasma engineering. Based on the electrochemical analysis conducted using a rotating ring disk electrode system, compared to the carbon catalyst synthesized from pure quinoline, the oxygen reduction reaction (ORR) performance was enhanced by increasing the amount of methylene blue (E onset = 0.78 V (vs RHE) at 100% quinoline, 0.79 V (vs RHE) at 1 mM MBQ-G, 0.84 V (vs RHE) at 2 mM MBQ-G, and 0.86 V (vs RHE) at 3 mM MBQ-G). From the electrochemical results, the onset potential (E onset), half-wave potential (E 1/2), and Tafel slope of 3 mM MBQ-G showed the best performance among all the carbon-based catalysts. In addition, the durability properties of 3 mM methylene blue (declined 30 mV after 6000 cycles) is superior to that of the benchmark ORR catalysts of 20 wt.% Pt/C (declined 60 mV after 6000 cycles). Through this study, we have successfully shown the possibility to effectively recycle methylene blue, which often causes marine and water pollution in the dyeing industry, as a useful precursor in carbon-based catalytic materials.

Simulation studies on a rotating ring disk electrode: Effects of ionic migration with kinetic complication‐disk results

AbstractIn continuation to my previous work (Guha S. AIChE J. 2013;59(4):1390‐1399), in this work, effects of ionic migration are evaluated for disk region of a rotating ring disk electrode system by numerically solving complex differential equations, developed for mass transfer along with kinetic complication in presence of ionic migration under limiting current condition. The system for simulation is 0.01 M Fe2(SO4)3 solution with H2SO4 as supporting electrolyte. Simulation cases are presence and absence of ionic migration with kinetic complication (oxidation of Fe2+ to Fe3+ under O2 pressure). Results show that concentration boundary layer thickness of reactant Fe3+ reduces appreciably and steady‐state disk current reduces substantially in presence of migration. Simulated steady‐state disk current in absence of migration case agrees well with published data. Results indicate higher Fe2+ concentration in presence of migration and thereby higher rate of oxidation of Fe2+ to Fe3+ at all rate constant values.

Interfacial pH Measurements Using a Rotating Ring‐Disc Electrode with a Voltammetric pH Sensor

AbstractElectrochemical reactions in which H+ or OH− ions are produced or consumed, affect the pH near the electrode surface. Probing the pH locally is therefore highly desired to understand and model the reaction environment under operando conditions. We carried out interfacial pH measurements under mass transport control using a rotating ring‐disc electrode (RRDE) coupled with our recently developed voltammetric pH sensor. The interfacial disc pH is detected by functionalizing the gold ring with a hydroxylaminothiophenol (4‐HATP)/4‐nitrosothiophenol (4‐NSTP) redox couple. As protons only have to interact with a monolayer containing the 4‐HATP/4‐NSTP, the sensitivity and time resolution that can be achieved are superior to potentiometric sensors. We used hydrogen evolution as a model reaction and performed measurements in buffered and unbuffered electrolytes. The effects of the current density, potential, the buffer capacity of the electrolyte and rotation rate on the local pH were investigated. This work shows a reliable and sensitive method for accurately probing the reaction environment under well‐defined mass transport conditions, over a wide pH range.

X-ray Structure Determination, Antioxidant Voltammetry Studies of Butein and 2',4'-Dihydroxy-3,4-dimethoxychalcone. Computational Studies of 4 Structurally Related 2',4'-diOH Chalcones to Examine Their Antimalarial Activity by Binding to Falcipain-2.

Malaria is a huge global health burden with resistance to currently available medicines resulting in the search for newer antimalarial compounds from traditional medicinal plants in malaria-endemic regions. Previous studies on two chalcones, homobutein and 5-prenylbutein, present in E. abyssinica, have shown moderate antiplasmodial activity. Here, we describe results from experimental and computational investigations of four structurally related chalcones, butein, 2′,4′-dihydroxy-3,4-dimethoxychalcone (DHDM), homobutein and 5-prenylbutein to elucidate possible molecular mechanisms by which these compounds clear malaria parasites. The crystal structures of butein and DHDM show that butein engages in more hydrogen bonding and consequently, more intermolecular interactions than DHDM. Rotating ring-disk electrode (RRDE) voltammetry results show that butein has a higher antioxidant activity towards the superoxide radical anion compared to DHDM. Computational docking experiments were conducted to examine the inhibitory potential of all four compounds on falcipain-2, a cysteine protease that is involved in the degradation of hemoglobin in plasmodium-infected red blood cells of the host. Overall, this work suggests butein as a better antimalarial compound due to its structural features which allow it to have greater intermolecular interactions, higher antioxidant activity and to create a covalent complex at the active site of falcipain-2.

Porous carbon polyhedrons with exclusive Metal-NX moieties for efficient oxygen reduction reaction

This work has manufactured a series of M-NX/C (M = Fe, Cu, Ni) ORR catalysts by the pyrolysis of metal tetraphenylporphyrin (MTPP) adsorbed onto N-doped porous carbon polyhedrons (NPCP) derived from ZIF-8. The comprehensive characterization data shows that the prepared M-NX/C catalysts have hierarchical porous structure with high specific surface area and abundant M-NX moieties. All prepared M-NX/C catalysts exhibit good ORR performance. The ORR half-wave potentials E1/2 of Fe-NX/C, Cu-NX/C and Ni-NX/C catalyst are 0.885, 0.801 and 0.755 V respectively. The rotating ring-disk electrode (RRDE) test reveals that the ORR of Fe-NX/C is a four electron process. While a two and four electron mixing ORR process are observed for Cu-NX/C and Ni-NX/C. In an alkaline medium, E1/2 of Fe-NX/C is observed 44 mV higher than that of commercial Pt/C. Also, Fe-NX/C catalyst has outstanding long-term durability and good methanol resistance.

Agglomerate Size Effect on the PEMFC Performance of a Non-Noble Metal Oxygen Reduction Catalyst

Proton exchange membrane fuel cells (PEMFCs) are expected to be a key contributor to environmentally friendly transportation, but the high cathode Pt-loadings required to catalyze the oxygen reduction reaction (ORR) inhibit their widespread commercialization. In order to address this issue, tremendous efforts are being devoted to develop inexpensive, non-noble metal catalysts (NNMCs), and novel syntheses established in recent years have led to NNMCs with initial ORR-activities comparable to those of Pt-based materials [1]. However, NNMC cathode loadings need to be much higher (≈ 4 mgNNMC·cm−2) than those of Pt-based catalyst layers (CLs, with loadings ≤ 0.3 mgPt·cm−2) to compensate for their intrinsically lower ORR-activity compared to Pt. Therefore, the resulting, thick NNMC CLs (up to ≈ 100 μm) suffer from mass transport issues [2] which prevent them from reaching the high current densities at high potentials required for automotive applications. Moreover, NNMC CLs display a remarkably poor durability for which the reasons remain under debate [3].In this context, our group recently proposed a novel synthesis [4] that yields NNMCs with a high initial ORR-activity in PEMFC tests (≈ 10 A·g−1 at 0.8 V in H2:O2 at 80 °C, 1.5 barabs, and 100% relative humidity) but poor mass transport properties. The latter was caused by the NNMC’s large particle size, which we have efficiently reduced by substituting the dry ball-milling (BM) originally used to mix the catalyst precursors [4] with wet BM (i.e., in the presence of a solvent). This new approach resulted in a reduction of the NNMC aggregate size from > 2 μm to ≈ 100 nm (see Fig. 1). The electrochemical behavior of both kinds of NNMCs was subsequently studied through rotating ring disk electrode (RRDE) voltammetry measurements at various catalysts loadings and temperatures (50 – 500 μgNNMC·cm−2 and 20 – 50 °C, respectively), which revealed the negligible effect of the former parameter on the catalysts’ ORR-activity and H2O2-yields, along with a significant enhancement in the limiting current of the NNMC with a smaller particle size. Subsequently, PEMFC tests at temperatures between 40 and 80 °C were conducted for both kinds of NNMCs with different ionomer-to-catalyst ratios, including electrochemical impedance spectroscopy measurements from which we estimated the CLs’ proton-transfer resistance [5]. These in terms allowed us to decouple the different overpotential contributions to the overall PEMFC performance, which confirmed the enhanced mass transport properties of the wet-milled NNMC, and allowed us to compare the kinetic parameters of both materials in RRDE vs. fuel cell configurations. Acknowledgments: The authors acknowledge the financial assistance of the Swiss National Science Foundation through Sinergia grant CRSII5_18033.

Heteroatom-Doped Graphites Oxygen Reduction Catalysts for Anion Exchange Membrane Fuel Cells

Catalysts for the oxygen reduction reaction (ORR) are crucial for electrochemical energy conversion and storage devices. The development of active, ultra-low-cost, critical raw materials-free catalysts is vital for the success of the anion exchange membrane fuel cell (AEMFC) technology [1]. In this work, we present metal-free oxygen reduction catalysts based on heteroatoms (I, N, S or B)-doped graphites by a scalable and reproducible one-step planetary ball milling protocol [2]. The distribution and the morphology of these ultra-low cost catalysts are characterized by advanced electron microscopy showing uniform distribution of heteroatoms in the graphite matrix. We have tested the catalysts for ORR in 0.1 M KOH solution in a rotating ring disk electrode (RRDE) and doped-graphites showed enhanced ORR performance relative to the pristine graphite. Among these, the N-graphite exhibiting the highest ORR onset potential of 0.87 V with a low peroxide yield of 15%@0.1 V. Good ORR kinetic (jk,s ) and mass (ik,m ) activities were also obtained. The catalysts also show remarkable stability in both RDE stress and corrosion tests. Finally, we showed good performance in operando H2-O2 AEMFC, reaching ~350 mW cm‒ 2 at 60 oC with a high voltage efficiency. These results encourage further development of metal-free ultra-low-cost and stable catalysts, easily scalable for AEMFC cathode production.

Is the Degradation of Fe-N-C Catalysts in Proton Exchange Fuel Cells Caused By Hydrogen Peroxide Formation?

In fuel cells chemical energy is directly converted into electricity. Based on this, fuel cells are attractive for automotive propulsion. However, a major drawback that hinders the widespread use of fuel cells are the costs. A major cost contributor in today’s state of the art fuel cells is the catalyst; namely platinum and its alloys that are used to catalyse the hydrogen oxidation reaction and the oxygen reduction reaction (ORR) [1]. Most of the expensive metal is required for the cathodic ORR. However, Fe-N-C catalysts are a potential alternative to Pt/C as they reach promising activity while the stability still needs to be improved. There seems to be a trade-off between activity and stability for this kind of catalysts. Beside other causes of degradation, it is discussed that hydrogen peroxide formation is one of the driving forces [2]. While molecular FeN4 centres are assumed to reduce oxygen to water, other phases are suggested to contribute to hydrogen peroxide formation. Such side phases are e.g. iron or iron carbide which are found in the catalysts after high temperature pyrolysis or if insufficient acid leaching was applied [3].However, there is no systematic study if under fuel cell conditions indeed hydrogen peroxide formation from these side phases is at the origin of instability or maybe the leaching of iron species (from these side phases) with subsequently induced Fenton’s reaction might cause the degradation. The knowledge on this is required for future catalyst design and therefore given attention in this work.In order to address this point, small quantities of an FeNC catalyst were post-modified with different metals as e.g. Pt, Pd, Ag in order to tune the selectivity for hydrogen peroxide formation of the resulting catalyst while still keeping the iron-related composition the same.The catalysts were characterized by cyclic voltammetry and rotating ring disc electrode experiments in order to see to what extent specific adsorption occurs and to determine the ORR activity and selectivity in aqueous electrolyte (0.1M H2SO4).Besides, polarization curves were measured in H2/O2 fuel cells and potentiostatic stability tests (12h) were performed at U = 0.6 V to identify activity and stability under operating conditions.We will discuss to what extent hydrogen peroxide formation correlates with the degradation of FeNC catalysts in proton exchange fuel cells.

Expanding Opportunities for Urea Electro-oxidation for Both Hydrogen Energy Storage and Cleaner Water

Oxidation of urea over nickel-catalysts is capable of such activity as to potentially disrupt several industrial and scientific fields, such as fuel cell power, and environmental pollution remediation. Though well suited for photoelectrochemical applications the reaction has commonly observed a loss in activity coinciding with the onset of oxygen evolution, leaving a narrow ~0.3 V electrochemical window. This narrow window limits many options for semiconductor substrates in a photoactive system. By adsorbing Ni-ions onto high surface area carbon black (CB) electrodes, instead of using metallic Ni catalysts, the loss in activity is avoided. The resulting oxidation window is expanded to over 1 V positive of urea’s oxidation onset potential. In addition to the expanded oxidation window, the use of a specially designed rotating-ring disc electrode is explored to electrochemically modify the local pH environment to limit the alkalinity of the bulk aqueous system. It is shown that with proper system parameters urea electro-oxidation is able to operate at 1 mA/cm2 despite the bulk solution’s neutral pH. This allows for a potentially additive-free use of this technology in environmental applications, or austere locations.

Compositionally Tuned Trimetallic Thiospinel Catalysts for Enhanced Electrosynthesis of Hydrogen Peroxide and Built-In Hydroxyl Radical Generation

On-site electrochemical production of hydrogen peroxide (H2O2), an oxidant and disinfectant with growing demand, could be realized through the selective two-electron oxygen reduction reaction (2e– ORR), but the widespread adoption of this method depends on robust and efficient electrocatalysts. Current catalysts have been limited by cost or toxicity and lack well-defined structures that can facilitate systematic tuning of activity and selectivity. Here, we demonstrate a series of CuCo2–xNixS4 (0 ≤ x ≤ 1.2) thiospinel catalysts for 2e– ORR with variable compositions that can be synthesized via hydrothermal conversion. Rotating ring disk electrode measurements show that these catalysts have high selectivity for 2e– ORR (>60%) and that their activity can be improved by increasing the nickel content without compromising selectivity. An acid treatment step is critical prior to employing the optimized CuCo0.8Ni1.2S4 catalyst for bulk electrosynthesis of H2O2 in 0.05 M H2SO4 solution. Various structural analyses, including synchrotron X-ray spectroscopy, confirm that the catalysts retain the spinel structure after acid treatment and H2O2 electrosynthesis. The acid treatment likely leaches the soluble copper species from the as-synthesized catalysts that would catalyze an electro-Fenton process to consume H2O2, generate hydroxyl radicals, and therefore prevent the accumulation of H2O2. This work demonstrates a general strategy for systematic tuning of metal compound catalysts for practical H2O2 electrosynthesis and facile generation of hydroxyl radicals.

Enhanced oxygen reduction and methanol oxidation reaction over self-assembled Pt-M (M = Co, Ni) nanoflowers

Herein, we introduce a facile approach to synthesize a unique class of Pt-M (M = Ni, Co) catalysts with a nanoflower structure for boosting both oxygen reduction reaction (ORR) and methanol oxidation reaction (MOR). By controlling the surface-active agents, we modified the functional groups surrounding the Pt atoms, tuned the alloying of Pt and the transition metals Ni and Co, and prepared two different kinds of nanodendrites. Their successful synthesis depends on the selection and amount of surfactants (hexadecyltrimethylammonium bromide (CTAB), Polyvinylpyrrolidone (PVP)). Besides, by controlling reaction time, we also explored the forming procedures for Pt-Co globularia nanodendrite (Pt-Co GND) and Pt-Ni petalody nanodendrite (Pt-Ni PND). Our investigation highlights the importance of complex nanoarchitecture, which enables surface and interface modification to achieve excellent catalytic performance in fuel cell electrocatalysis. The characterization of the as-prepared catalysts reveals a high electrochemical surface area and mass activity (2041 mAmgPt-1and 950 mAmgPt-1 for Pt-Co GND and Pt-Ni PND, respectively, for ORR). Furthermore, Pt-Co GND showed a high MOR activity, with a mass activity value recorded at 1615 mAmgPt-1 which is far superior to that for Pt/C. Moreover, both catalysts retain high activity after accelerated durability tests (ADTs). The electron transfer number was calculated by performing the rotating ring-disk electrode (RRDE) measurements. Due to abundant active sites of Pt, both Pt-Co GND and Pt-Ni PND exhibit a 4e− pathway for ORR with electron transfer number of >3.95.

Kinetic Insights into the Mechanism of Oxygen Reduction Reaction on Fe2O3/C Composites.

Since the inception of cobalt phthalocyanine for oxygen reduction reaction (ORR), non-platinum group metals have been the central focus in the area of fuel-cell electrocatalysts. Besides Fe-Nx active sites, a large variety of species are formed during the pyrolysis, but studies related to their ORR activity have been given less importance in the literature. Fe2O3 is one among them, and this study describes the role of Fe2O3 in the ORR. The Fe2O3 is carefully synthesized on various carbon supports and characterized using X-ray photoelectron spectroscopy (XPS) spectra, high-resolution transmission electron microscopy (HRTEM) images, and surface area analysis. The characterization techniques reveal that the Fe2O3 nanoparticles are present in the pores of the carbon supports, having a particle size ranging from 4 to 15 nm. The current density of the ORR on Fe2O3/C catalysts is increased compared with bare carbon supports, as discerned from the rotating ring-disk electrode (RRDE) voltammetry experiments, demonstrating the role of size-confined Fe2O3 nanoparticles. The overall number of electrons in the ORR is increased by the introduction of Fe2O3 on the carbon support. Based on the kinetic analysis, the ORR on Fe2O3/C follows a pseudo-4-electron or 2+2-electron ORR, where the first 2-electron ORR to H2O2 and second 2-electron H2O2 reduction reaction (HPRR) to H2O are assigned to the graphitic carbon (carbon defects) and Fe2O3 active sites, respectively. Theoretical studies indicate that the role of Fe2O3 is to decrease the free energy of O2 adsorption and reduce the energy barrier for the reduction of *OOH to OH-. The onset potential estimated from the free energy diagram is 0.42 V, matching with the HPRR activity demonstrated using the potential-dependent rate constants plot. Fe2O3/C shows higher stability by retaining 95% of the initial activity even after 20 000 cycles.

Systematic study of precursor effects on structure and oxygen reduction reaction activity of FeNC catalysts.

In this work, the effect of porphyrin loading and template size is varied systematically to study its impact on the oxygen reduction reaction (ORR) activity and selectivity as followed by rotating ring disc electrode experiments in both acidic and alkaline electrolytes. The structural composition and morphology are investigated by 57Fe Mössbauer spectroscopy, transmission electron microscopy, Raman spectroscopy and Brunauer-Emmett-Teller analysis. It is shown that with decreasing template size, specifically the ORR performance towards fuel cell application gets improved, while at constant area loading of the iron precursor (here expressed in number of porphyrin layers), the iron signature does not change much. Moreover, it is well illustrated that too large area loadings result in the formation of undesired side phases that also cause a decrease in the performance, specifically in acidic electrolyte. Thus, if the impact of morphology is the focus of research it is important to consider the area loading rather than its weight loading. At constant weight loading, beside morphology the structural composition can also change and impact the catalytic performance. This article is part of the theme issue 'Bio-derived and bioinspired sustainable advanced materials for emerging technologies (part 2)'.

Activation of Oxygen Reduction Reaction on Carbon Supported Ni‐Based Complexes

AbstractThere are unremitting efforts towards developing an effective oxygen reduction reaction (ORR) catalyst as it is seminal for the commercially viable energy conversion and storage devices such as fuel cell and metal‐air batteries. In this study, three carbon‐supported Ni‐based coordination complexes; Ni‐phen/AC, Ni‐bpy/AC and Ni‐bimz/AC (where phen=1,10‐ phenanthroline, bpy=2, 2′‐bipyridine and bimz=benzimidazole) were explored for ORR. Among these, Ni‐bimz forms a 1D‐polymer chain via a succinate bridge, and these chains are stacked by the π‐π interactions and hydrogen bonding. The ORR potentials of these catalysts were studied using cyclic voltammetry technique with a rotating ring disk electrode system, and oxygen adsorption energies (Ead) onto these complexes were calculated using density functional theory (DFT) analysis. The highest onset potential of 0.89 V vs. RHE was observed for Ni‐bimz/AC, whereas 0.88 V and 0.86 V vs. RHE for Ni‐phen/AC and Ni‐bpy/AC catalysts, respectively. The Ead value for O2 via side‐on and end‐on mode over Ni‐phen, Ni‐bpy, and Ni‐bimz complexes, were calculated. The highest negative Ead value of −25.53 eV via side‐on mode was observed for Ni‐bimz complex.

In situ57Fe mössbauer study of a porphyrin based FeNC catalyst for ORR

FeNC catalysts are important substitutes for the oxygen reduction reaction (ORR) in fuel cells. This work reports on an in situ Mössbauer spectroelectrochemical study of a porphyrin-based catalyst. Activity and selectivity towards ORR were determined from rotating ring disc electrode (RRDE) experiments at different loadings in acidic electrolyte and accompanied by H2O2 oxidation reduction measurements in order to identify the contributions to the different ORR pathways as function of potential. The comparison to in situ57Fe Mössbauer spectra enables an assignment of these contributions to the iron signatures. The results indicate that two different “onset potentials” for obtaining the deoxygenated state associated with two different iron environments can be identified and being associated with the selectivity data. Moreover, the in situ data enable the determination of mass-based site density and turn-over frequency data for ORR relevant conditions. As a consequence, this work sheds light on the oxygen reduction reaction mechanism involved in FeNC catalysts.

Investigation on Electron Transfer Process of Oxygen Reduction Reactions Catalyzed by Nitrogen-doped Graphitic Carbon in Acidic and Alkaline Media

The electron transfer number is an important parameter to illustrate oxygen reduction reaction (ORR) pathway and explore reaction processes, which can be obtained by the rotating ring-disk electrode (RRDE) method and the Koutecky−Levich equation. However, the two approaches sometimes lead to different and confusing conclusions. Herein, graphitic carbon, nitrogen-doped graphitic carbon and Pt/C are respectively used as carbon and precious metal catalysts to evaluate the reliability of these two methods to calculate electron transfer numbers in acidic and alkaline media. Furthermore, density functional theory (DFT) calculations are employed to reveal effects of electrical potentials on the 4-electron ORR process in acidic and alkaline media. The results show that the electron transfer number varies with potentials and the RRDE method can provide more objective and actual electron transfer numbers in alkaline and acidic electrolytes from the perspective of experiments and theoretical calculations.

Kinetic Effects of Temperature on Fe–N–C Catalysts for 2e- and 4e-Oxygen Reduction Reactions

Hydrogen peroxide (H2O2) formed via the two-electron oxygen reduction reaction (2e-ORR) on carbon-based platinum group metal-free (PGM-free) catalysts at elevated temperature can cause catalyst degradation in fuel cells. In this work, we studied the effects of temperature on the selectivity of iron- and nitrogen-doped carbon (Fe–N–C) catalyst for 2e- and 4e-ORR in acidic electrolyte using a high-temperature rotating-ring disk electrode. The results of individual-heating experiments showed that H2O2 yield increased slightly with the increase of temperature. In the meantime, the results of sequential heating/cooling experiments suggested that prolonged high-temperature exposure at ORR polarization conditions can lead to catalyst degradation and higher selectivity for 2e-ORR. The temperature effects on the selectivity of Fe–N–C catalyst for 2e- and 4e-ORR was further explained with a kinetic model describing the competitive reactions and the thermodynamics of the system, which suggested that the increase of H2O2 yield with temperature in the individual-heating experiment was due to the promoted 2e-ORR pathway instead of catalyst degradation.