Ring-disk Electrode Research Articles

Using Complex Collection Efficiency for Detection of Oxidation Products – Examples from Hydrogen Evolution, Oxygen Evolution and CO2 Reduction

Electrochemical reactions can often yield a distribution of products, for example for CO2 reduction on Cu and methanol oxidation on Pt. This distribution varies with (over)potential and catalyst type, and usually requires the experimenter to incorporate detectors such as mass spectrometry or gas chromatography into their experimental setup. Electrochemical methods also exist, such as using the rotating ring-disk electrode or the analogous double channel electrodes. However, these methods suffer from unsteady measurements, and are hard to do with the desired accuracy. Techniques using alternating current perturbations may be an improvement here and preliminary work on transfer functions for product detection show promise, here called complex collection efficiency (CCE)[1]. This technique has the advantage that it reduces or removes the influence of parasitic currents on the electrodes and can yield the product distribution as a function of current density and/or electrode potential. In this work, CCE was tested on a lab made microfluidic system with double channel electrodes and measurements were done on ideal reactions with a single product, the hydrogen evolution reaction and oxygen evolution reaction to optimize and calibrate the setup, see Figure 1. Finally, CO2 reduction on Ag was attempted and compared with literature values as this primarily yields CO and H2 as the reaction products. Holm, T., J.A. Diaz Real, and W. Mérida, Numerical modeling of complex collection efficiency for double channel electrodes. Journal of Electroanalytical Chemistry, 2019. 842: p. 115-126. Figure 1

(Keynote) Mechanistic Understanding of the Activity of Atomically Dispersed Transition Metal-Nitrogen-Carbon Catalysts in Oxygen, Carbon Dioxide or Nitrogen Electro-Reduction

Over the last two decades, platinum group metal-free (PGM-free) catalysts are attracting increasing attention and finding applications in several important process across many electrochemical energy technologies. Among those PGM-free materials, atomically dispersed (AD) transition metal-nitrogen-carbon (M-N-C) catalysts are gaining exceptional popularity as they demonstrate very high (for this class of materials) activity in oxygen reduction reaction (ORR)1 and are the only cathode catalysts suitable for both proton exchange membrane fuel cells (PEMFC) and alkaline, including anion/hydroxyl exchange membrane fuel cells (AFC, AEMFC/HEMFC). Over the last few years, M-N-C catalysts have shown promising activity in carbon dioxide reduction reaction (CO2RR).2 In this case, varying the transition metal in M-N-Cs opens routes for controlling the selectivity towards a list of C1 and C2 products. There are recent reports on catalytic activity of AD M-N-C materials in direct electro-reduction of molecular nitrogen (N2RR) or reactions of reduction of nitrates, nitrites or various nitrogen oxides (NOx). We have systematically investigated all these processes having as a base the M-N-C catalysts synthesized by sacrificial support method (SSM) – a hard template approach with transition metal salt and charge-transfer organic salt (nicarbazin) mixed by ball-milling, pyrolyzed at high temperature in inert atmosphere and then etched in HF after cooling. In most cases a secondary (similar) pyrolysis was performed to refine the material and ensure its AD character.The makeup and structure of the active site/sites of the AD M-N0C electrocatalysts, including geometry (coordination) and chemistry (composition and oxidation state) remain contentious to this day. There is an emerging agreement however, that the transition metal (at least for the 2nd row transitions meals) is immediately associated with (liganded by) the nitrogen functionalities, displayed on the surface if the carbonaceous substrate. It is almost universally accepted that N-coordinated AD transition metal ions, either as in-plane or edge-type defect in “graphene” sheet, are the main/principal active sites. This is often combined with a broadly accepted hypothesis that micro-porous surface area plays a critical role forming edge-type, intercalational active sites while meso-porous interface is most-likely associated with the in-plane, substitutional AD metal sites. Candidate structures participating in reativity towards O2, CO2 or nitrogen species include a list of nitrogen-containg and oxygen-containng moeties in the carbonaceous matrix. The carbon itself displays various degrees of graphitization, depending on the transition metal used in M-N-C synthesis. Additional complexity in this calss of caralysts study comes from the fact that many samples are not strictly AD materials. They often contain incorporated metal nano-particles, corresponding (native) oxides and/or carbides and nitrides (oxocabides and oxonitrides have been observed as well).These “unrefined” M-N-C materials are often used in practice and the corresponding nano-particle components of the de-facto nanocomposites do alter substantially the reactivity and selectivity of the catalysts in all these electro-reduction reactions.This talk discusses the mechanistic aspects of M-N-C catalysts in ORR, CO2RR, N2RR and electroreduction of nitrogen-containing oxo-species, obtained when cross-referencing electrochemical activity results obtained in rotating disk and rotating ring-disk electrodes setting (RDE/RRDE) with those observed in near-ambient pressure X-ray photo-electron spectroscopy (NAP-XPS) and supported by density functional theory calculations of the reagents adsorption on AD transition metal or nitrogen- or oxygen-containing moieties from the carbonaceous matrix of the M-N-Cs. The later are of particular importance as significant reactivity has been observed for most of those processes when metal-free, nitrogen-doped carbon (N-C) catalysts are used.3 We will present a case that outlines the reactivity of M-N-C in those important electro-reduction reactions in terms of (i) role of the AD transition metal, (ii) role of the surface N-groups as co-catalysts/alternative sites (iii) role of surface oxides as co-catalysts or hydrophilic/hydrophobic properties descriptor, the last being also critically dependent on morphology.4

A Critical Reassessment of the Mechanism of the Oxygen Reduction Reaction on Au(poly) in Aqueous Alkaline Electrolytes

The oxygen reduction reaction, ORR, in aqueous electrolytes, may rank among the most studied heterogeneous electron transfer processes. Despite the extraordinary efforts of numerous research groups worldwide, critical questions still remain unanswered regarding critical aspects of this technologically important reaction and the dependence of its mechanism and rates on the nature of the electrode material. In their pioneering studies, Zurilla et al.1 examined the ORR on polycrystalline gold, Au(poly), in alkaline solutions, using rotating ring-disk electrode, RRDE, techniques. According to these authors, the experimental evidence collected was consistent with a mechanism involving an initial one-electron transfer to yield adsorbed superoxide ion, O2 -(ads), which, subsequently, underwent a second order heterogeneous dismutation generating solution phase peroxide and oxygen, O2(aq) and HO2 -(aq). The same mechanism was later invoked by Adzic et al.2 for the ORR on Au(100), a surface that displayed extraordinary activity compared to other low index faces of Au. More recently, Ignaczak et al.3 put forward theoretical arguments that support the view that the initial RDS is actually of the outer sphere type, yielding a solvated species, i.e. O2 -(aq), which is then followed by a subsequent, fast, outer sphere one electron-transfer, generating solution phase peroxide, HO2 -(aq), and not by a second order dismutation of as was postulated earlier in the literature. This is shown as the ‘outer sphere pathway’ in Scheme 1. Scheme 1: Proposed inner and outer sphere pathways for ORR on Au in basic solution.This presentation will describe the use of RRDE methods to examine the kinetics of the ORR on Au(poly) in 0.1 M NaOH + 0.9 M NaClO4 aqueous electrolytes containing oxygen in one case and peroxide in the other. The results obtained were found to be consistent with Ignaczak et al.’s mechanism, or equivalently an inner sphere pathway (Scheme 1) which includes a fast equilibrium between O2 -(aq), HO2 -(aq) and their corresponding adsorbed counterparts, O2 -(ads) i.e. and HO2 -(ads) however, a critical assessment of in situ vibrational spectroscopy data published in the literature has raised questions regarding the assignment of the spectral features reported for these adsorbed species. Also considered in this model is the direct reduction of HO2 -(aq) and O2(aq) to generate OH-(aq). Quantitative analyses of data collected in O2-saturated and Ar-purged containing solutions made it possible to determine a unique set of kinetic rate constants for the various steps in the proposed mechanism over a wide potential range.

Investigation of the stability of the boron-doped diamond support for Co3O4-based oxygen evolution reaction catalysts synthesized through in situ autocombustion method

• The In Situ Autocombustion allows easy preparation of oxide–carbon composites for OER. • Co 3 O 4 nanoparticles in composites are smaller and less agglomerate than bulk one. • The Co 3 O 4 -BDD composite is a competitive electrocatalyst for the alkaline OER. • Higher stability of the BDD compared either to Vulcan or Sibunit conventional carbons. • BDD is a promising support for alkaline or anion exchange membrane electrolysis cells. In this work we studied the anodic stability of nanostructured Boron-Doped Diamond (BDD) in view of its eventual utilisation as support for metal oxide–carbon composites prepared by the In Situ Autocombustion (ISAC) method. The anodic behaviour of the BDD was investigated by the rotating ring-disc electrode (RRDE) method in 1 M NaOH electrolyte to determine its corrosion current and compare the latter with that of the commercial Vulcan XC-72 carbon from Cabot and Sibunit pyrolytic carbon, the latter known for its high stability against electrochemical corrosion. The BDD has proven to be significantly more stable than either the Sibunit or the Vulcan carbon. The Co 3 O 4 -BDD and Co 3 O 4 -Sibunit composites prepared by the ISAC method were then tested as catalysts of the oxygen evolution reaction (OER). The Co 3 O 4 -BDD composites appear to be competitive electrocatalysts for the OER in alkaline medium, showing activity comparable to the literature and much higher support stability towards oxidation. Further research is required to decrease the size of Co 3 O 4 spinel nanoparticles and improve their distribution over the BDD support and hence increase the mass-specific OER activity of the BDD-based ISAC composites.

Versatile DIY Route for Incorporation of a Wide Range of Electrode Materials into Rotating Ring Disk Electrodes.

Rotating ring disk electrodes (RRDEs) are a powerful and versatile tool for mechanistically investigating electrochemical reactions at electrode surfaces, particularly in the area of electroanalysis and catalysis. Despite their importance, only limited electrode materials (typically glassy carbon, platinum, and gold) and combinations thereof are available commercially. In this work, we present a method employing three-dimensional (3D) printing in conjunction with machined brass components to produce housing, which can accommodate any electrode material in, e.g., pressed powdered pellet, wafer, rod, foil, or vapor deposited onto a conductive substrate form. In this way, the range and usability of RRDEs is extended. This custom do-it-yourself (DIY) approach to fabricating RRDEs also enables RRDEs to be produced at a significant fraction of the cost of commercial RRDEs. To illustrate the versatility of our approach, coplanar boron-doped diamond (BDD) RRDEs are fabricated for the first time using the approach described. Experimental collection efficiencies for the redox couple FcTMA+/FcTMA2+ are found to be very close to those predicted theoretically. BDD electrodes serve as an ideal electrocatalyst support due to their low background currents, wide solvent potential window in aqueous solution, and chemical and electrochemical stability in acid and alkali solutions. The BDD RRDE configuration is employed to investigate the importance of surface-incorporated nondiamond carbon in BDD on hydrogen peroxide generation via the oxygen reduction reaction in acid solutions.

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.

The Effect of Temperature on the Cation‐Promoted Electrochemical CO2 Reduction on Gold

AbstractElectrochemical reactions in general, and the CO2 reduction reaction (CO2RR) in particular, are commonly studied at room temperature. However, practical electrolysers may use elevated temperatures. There is currently a lack of fundamental understanding of the effect of temperature on the CO2RR. Here, we performed temperature‐dependent studies on the CO2RR on a relatively simple electrode material, namely gold, to obtain insights into how temperature influences this reaction and the competing hydrogen evolution reaction. A rotating ring disk electrode setup was used to show that the CO2RR activity and selectivity increased with temperature, and to obtain kinetic parameters such as the apparent activation energy and transfer coefficient. The magnitude of the temperature effect and the activation energy was affected by both the cation identity and concentration in the electrolyte. Moreover, the positive effect of temperature on the kinetics of the CO2RR was counteracted by the lower CO2 solubility, making efficient mass transport even more important at higher temperatures.

Nickel single atoms anchored on ultrathin carbon nitride for selective hydrogen peroxide generation with enhanced photocatalytic activity

Photocatalysis technology provides a promising and effective method for hydrogen peroxide (H2O2) production. Nevertheless, the photocatalytic H2O2 production activity is greatly regulated by the two-electron oxygen reduction reaction (2e− O2RR). Herein, we demonstrate a robust single atom photocatalyst (NiCN-x, Ni single atoms anchored on ultrathin g-C3N4) for artificial H2O2 production under visible light irradiation. A high H2O2 generation rate of 27.11 mmol g−1h−1 and an apparent quantum yield (AQY) of 8.56 % at 400 nm are both obtained in the optimal NiCN-4 photocatalytic system. Furthermore, NiCN-4 catalyst shows relatively high selectivity of H2O2 with 87.3 % through using a rotating ring-disk electrode (RRDE) measurement. Furthermore, NiCN-4 shows enhanced photocatalytic TC decomposition with degradation efficiency of 89.4 %. Based on DFT calculation, experimental results and advanced characterizations, we find that the high photocatalytic performance is attributed to the boosting 2e−O2RR process with the formation of Ni-μ-peroxide (Ni-OOH) and the unique electronic feature of Ni-Nx coordination sites. This study offers a new blueprint for the fabrication of single atom catalysts (SACs) to achieve artificial photosynthesis of H2O2 with high selectivity and degradation of organic pollutants.

Homogeneous Catalytic Reduction of O2 to H2O by a Terpyridine-Based FeN3O Complex.

We report a new terpyridine-based FeN3O catalyst, Fe(tpytbupho)Cl2, which reduces O2 to H2O. Variable concentration and variable temperature spectrochemical studies with decamethylferrocene as a chemical reductant in acetonitrile solution enabled the elucidation of key reaction parameters for the catalytic reduction of O2 to H2O by Fe(tpytbupho)Cl2. These mechanistic studies suggest that a 2 + 2 mechanism is operative, where hydrogen peroxide is produced as a discrete intermediate, prior to further reduction to H2O. Consistent with this proposal, the spectrochemically measured first-order rate constant k (s-1) value for H2O2 reduction is larger than that for O2 reduction. Further, significant H2O2 production is observed under hydrodynamic conditions in rotating ring-disk electrode measurements, where the product can be swept away from the cathode surface before further reduction occurs.

Towards a Generalized ORR Mechanism in M2+‐Containing DMSO: Oxygen Reduction and Evolution in Ca2+‐Containing DMSO on Atomically Smooth and Rough Pt

AbstractThe oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in Ca2+ containing dimethyl sulfoxide (DMSO) on atomically smooth Pt(111) and Pt(100) and rough Pt surfaces is reported. As demonstrated by cyclic voltammetry and XPS, the bromine adlayer used to protect Pt single crystals against ambient air and solvent vapor is desorbed in DMSO and does not affect the following measurements. Cyclic voltammetry, differential electrochemical mass spectrometry (DEMS), and rotating ring disk electrode (RRDE) investigations with variation of the electrode surface roughness and atomically surface structure show, that on Pt electrodes the CaO2 adsorbate layer formation determines the ORR product distribution. On Pt electrodes, calcium peroxide is formed on the clean electrode, whereas calcium superoxide is formed at the adsorbate covered electrode. We furthermore identified four key parameters, which strongly affect the ORR product distribution: 1) The electrode oxide interaction: A strong interaction increases superoxide contribution; 2) The alkaline earth metal oxide interaction: A strong interaction increases peroxide contribution; 3) The electrode surface area: A large electrode surface area increases peroxide contribution; 4) Electrode surface defects: Defects increase superoxide contribution. Finally, reviewing earlier results of our group, we provide a more general mechanism for the oxygen reduction alkaline earth metal cation containing DMSO, for a variety of electrode materials.

Study of the initial stage of manganese dioxide electrodeposition on a rotating ring-disk electrode

The initial stage of electrodeposition of manganese dioxide has been studied on the platinum disk electrode and on the ring-disk electrode. Two current maxima were observed on the anodic voltammograms. It has been shown that the first rise in the current is associated with the formation of manganese dioxide (MD) nuclei on the electrode surface, and the second with their growth at the border between MnO2 and bare platinum due to an increase in the perimeter of the nuclei. Electrodeposition on bare platinum proceeds at a higher rate than on the surface of growing MD. This is explained by the difference in the reaction mechanisms on these substrates. The first stage of the process on the growing MD is the chemisorption of Mn(II) ions with the formation of Mn(III) ions, which are then electrochemically oxidized to MD. Chemisorption is the slowest step in the whole process.

Potentiometric Rotating Ring Disk Electrode Study of Interfacial pH during CO2 Reduction and H2 Generation in Neutral and Weakly Acidic Media

Potentiometric Rotating Ring Disk Electrode Study of Interfacial pH during CO₂ Reduction and H₂ Generation in Neutral and Weakly Acidic Media

Synthesis, characterization, and degradation study of Mn-based phosphate frameworks (Na3MnTi(PO4)3, Na3MnPO4CO3, Na4Mn3(PO4)2P2O7) as aqueous Na-ion battery positive electrodes

Aqueous electrolyte Na-ion batteries based on abundant materials offer one of the attractive alternatives to replace conventional Li-ion batteries in the field of stationary electricity storage. However, the search for and development of stable, low-cost and compatible positive electrode materials remains a significant challenge. Mn-based materials are deemed to be among the most suitable due to their abundance, environmental compatibility and suitable electrochemical properties. In this work, three different phosphate and mixed phosphate framework compounds Na3MnTi(PO4)3, Na3MnPO4CO3, and Na4Mn3(PO4)2P2O7 are successfully prepared and comprehensively characterized as potential aqueous Na-ion positive electrode materials using a wide range of methods such as impedance spectroscopy, cyclic voltammetry, galvanostatic charge/discharge cycling. The degradation of these materials is studied by rotating ring-disc electrode technique. The results clearly show that charge capacity loss observed in these materials is related to the electrochemical and electrochemically-induced chemical dissolution of manganese from the electrode. The results obtained using a combination of different methods also indicate that a significant part of Mn is electrochemically inactive or inaccessible in such Mn-based phosphate frameworks. The fraction of electrochemically inactive Mn also tends to increase during the electrochemical operation, limiting the performance of these materials as potential battery electrodes in aqueous electrolytes.

Air-breathing cathodes for microbial fuel cells based on iron-nitrogen-carbon electrocatalysts

This work reports the development of an iron-nitrogen-carbon electrocatalyst (Fe-N-C) synthesized by functionalization of carbon support using low-cost Fe- and N-based precursors in a wet impregnation procedure followed by a pyrolysis treatment under an inert atmosphere. Structure and surface chemistry were investigated by Raman and X-ray photoelectron spectroscopy (XPS), which indicated an efficient interaction of precursors with the carbon support during the wet-impregnations step, which allows obtaining a carbonized material with a high content of active sites based on Fe-Nx moieties. This led to Fe-N-C materials with high catalytic activity towards oxygen reduction at neutral pH, as demonstrated by cyclic voltammetry (CV) and hydrodynamic linear sweep voltammetry with rotating ring disk electrode (LSV-RRDE). The Fe-N-C electrocatalyst was incorporated in air-breathing cathodes and performance was optimized in terms of oxygen reduction activity and stability. Such cathodes were assembled in single-chamber microbial fuel cell prototypes, and electrical power and voltage generation were evaluated over time.

CO2 reduction on a nanostructured La0.5Ba0.5CoO3 perovskite: Electrochemical characterization and DFT calculations

A La0.5Ba0.5CoO3 perovskite was evaluated as a catalyst for the electrochemical reduction of CO2. Nanocrystalline structures of La0.5Ba0.5CoO3 were synthesized using polycarbonate porous templates by microwave irradiation. Electrochemical characterization was carried out by linear sweep voltammetry, cyclic voltammetry and chronoamperometry. The electroreduction of CO2 produced HCOOH with a faradaic efficiency of 30% at − 0.9 V vs SHE as determined by ionic chromatography. The rotating ring-disc electrode configuration was used to further study the electroreduction processes. A faradaic efficiency of 85% at − 1.2 V vs SHE attributed to CO, was obtained from the disk and ring currents. In parallel, the adsorption process of CO2 and plausible reduction pathways to HCOOH and CO were studied by DFT calculation on the (001) and (110) orientations of the perovskite. The CO2 molecule strongly adsorbs at the oxygen surface sites through the C atom in a bent configuration, determining the *COOH species as the only possible intermediate. The formation and release of HCOOH becomes feasible under an applied potential of − 0.8 V and − 0.7 V vs SHE for the (001) and (110) orientations, respectively. On the other hand, the calculation showed that the release of CO is only feasible at the (001) surface plane for potentials below − 0.9 V vs SHE. The calculated applied potentials are in good agreement, both in trend and magnitude, with the experimental observations despite the model approximation of neglecting solvent effects.

Engineered biochar derived from pyrolyzed waste tea as a carbon support for Fe-N-C electrocatalysts for the oxygen reduction reaction

Platinum metal group (PGM-free) electrocatalysts for oxygen reduction reaction (ORR) were synthesized by ball milling of activated biochar and Fe(II) phthalocyanine. The biochar used as a carbon support was produced from pyrolysis of waste tea leaves at 1500 °C in argon atmosphere. The pyrolyzed waste tea was then activated with CO2 or urea. FE-SEM, HR-TEM, XPS, and Raman analyses were performed to investigate the morphology and the physicochemical properties of the electrocatalysts. The ORR activity and methanol tolerance of the Fe-N-C electrocatalysts were tested in rotating ring disk electrode (RRDE), showing promising results in terms of mass activity, onset and half-wave potential in an alkaline environment. Two different short potential cycling protocols demonstrated the high stability of these Fe-N-C electrocatalysts, especially when compared with a 20 wt. % commercial Pt/C electrocatalyst.

CuO–NiO binary transition metal oxide nanoparticle anchored on rGO nanosheets as high-performance electrocatalyst for the oxygen reduction reaction

To replace the existing noble-metal-based catalysts, developing highly efficient, stable electrocatalysts for oxygen reduction reactions for the increased current generation with lower overpotential is a demanding undertaking. In the present work, CuO–NiO/rGO nanocomposites were prepared using simple, cost-effective Co-precipitation methods. They act as highly effective electrocatalysts for oxygen reduction reactions in an alkaline medium. The structural characterizations demonstrate that prepared nanoparticles (≈13 nm) are tightly and effectively organized on reduced graphene oxide sheets. The electrochemical properties of the CuO, NiO nanoparticles and CuO–NiO, CuO–NiO/rGO nanocomposites were investigated. The results of the CuO–NiO/rGO nanocomposites revealed the high current density (2.9 × 10−4 mA cm−2), lower Tafel slope (72 mV dec−1) and low hydrogen peroxide yield (15%) when compared to other prepared materials (CuO, NiO, and CuO–NiO). The reduced graphene oxide increases an electron transfer during the ORR process, while the CuO–NiO has variable oxidation states that promote electro-rich features. With the combination of CuO–NiO and rGO, the hybrid electrocatalysts specific surface area and charge transfer rate drastically increase. The investigations of the rotating ring-disk electrodes experiments indicate that the oxygen reduction process takes place on CuO–NiO/rGO through an efficient four-electron pathway. Our results propose a new approach to creating highly efficient and long-lasting electrocatalysts.

The Effect of an External Magnetic Field on the Electrocatalytic Activity of Heat-Treated Cyanometallate Complexes towards the Oxygen Reduction Reaction in an Alkaline Medium.

This work focuses on the development of an electrocatalytic material by annealing a composite of a transition metal coordination material, iron hexacyanoferrate (Prussian blue) immobilized on carboxylic-acid-functionalized reduced graphene oxide. Pyrolysis at 500 °C under a nitrogen atmosphere formed nanoporous core–shell structures with efficient activity, which mostly included iron carbide species capable of participating in the oxygen reduction reaction in alkaline media. The physicochemical properties of the iron-based catalyst were elucidated using transmission electron microscopy, X-ray diffraction, Mössbauer spectroscopy, and various electrochemical techniques, such as cyclic voltammetry and rotating ring–disk electrode (RRDE) voltammetry. To improve the electroreduction of oxygen over the studied catalytic material, an external magnetic field was utilized, which positively shifted the potential by ca. 20 mV. The formation of undesirable intermediate peroxide species was decreased compared with the ORR measurements without an external magnetic field.

Two-Dimensional Metal–Organic Frameworks with Unique Oriented Layers for Oxygen Reduction Reaction: Tailoring the Activity through Exposed Crystal Facets

Two-Dimensional Metal–Organic Frameworks with Unique Oriented Layers for Oxygen Reduction Reaction: Tailoring the Activity through Exposed Crystal Facets

Probing Oxygen-to-Hydrogen Peroxide Electro-Conversion at Electrocatalysts Derived from Polyaniline.

Hydrogen peroxide (H2O2) is a key chemical for many industrial applications, yet it is primarily produced by the energy-intensive anthraquinone process. As part of the Power-to-X scenario of electrosynthesis, the controlled oxygen reduction reaction (ORR) can enable the decentralized and renewable production of H2O2. We have previously demonstrated that self-supported electrocatalytic materials derived from polyaniline by chemical oxidative polymerization have shown promising activity for the reduction of H2O to H2 in alkaline media. Herein, we interrogate whether such materials could also catalyze the electro-conversion of O2-to-H2O2 in an alkaline medium by means of a selective two-electron pathway of ORR. To probe such a hypothesis, nine sets of polyaniline-based materials were synthesized by controlling the polymerization of aniline in the presence or not of nickel (+II) and cobalt (+II), which was followed by thermal treatment under air and inert gas. The selectivity and faradaic efficiency were evaluated by complementary electroanalytical methods of rotating ring-disk electrode (RRDE) and electrolysis combined with spectrophotometry. It was found that the presence of cobalt species inhibits the performance. The selectivity towards H2O2 was 65–80% for polyaniline and nickel-modified polyaniline. The production rate was 974 ± 83, 1057 ± 64 and 1042 ± 74 µmolH2O2 h−1 for calcined polyaniline, calcined nickel-modified polyaniline and Vulcan XC 72R (state-of-the-art electrocatalyst), respectively, which corresponds to 487 ± 42, 529 ± 32 and 521 ± 37 mol kg−1cat h−1 (122 ± 10, 132 ± 8 and 130 ± 9 mol kg−1cat cm−2) for faradaic efficiencies of 58–78%.