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Bioinspired transition-metal catalysts seek to mimic the specific active site of metalloenzymes, as the multicopper oxidase, that can efficiently reduce dioxygen to water via a complete 4-electrons mechanism at low overpotential. However, the multicopper oxidase enzymes lack stability under operando conditions, hampering their application in fuel cell electrodes. Bioinspired multicopper catalysts present a remarkable electrocatalytic activity for the oxygen reduction reaction (ORR), where the structure and electronic properties of the ligands play a fundamental role. In this work, we explore the instantaneous catalytic activity and its evolution under operando conditions of two multicopper catalysts with different ligand flexibility, pyridyl ligand (CuL1), and pyridylmethyl ligand (CuL2), by conventional electrochemical techniques and scanning electrochemical microscopy (SECM). Both catalysts present similar instantaneous electrocatalytic activity with no significant role of the ligand, but there is a change in the mechanism. While the rigidity of CuL1 reduces the dioxygen via direct 4e-, the catalyst with higher flexibility (CuL2) follows a 2e−x 2e− mechanism. The production of H2O2 as ORR byproduct evaluated by rotating ring-disk electrode (RRDE) and stability test evaluated under ORR operating conditions by SECM imaging of both catalysts demonstrated a higher decrease in catalytic activity and higher H2O2 production in CuL2 than in CuL1, which evidences a ligand structure-reactivity correlation. These results contribute to the rational design of next generation of copper catalysts and open the door to a new methodology to evaluate the activity evolution under operando conditions by SECM, This work has been supported by ECOS Sud-Chile CONICYT program (Project C17E10/170037), FONDECYT Project 1161117, FONDECYT Project 11221073, FONDECYT Postdoctoral Project 3170330, and ACT192175.

The effect of nitrogen (N) and iron (Fe) on oxygen reduction reaction (ORR) was investigated by using carbon nanowalls (CNWs) as carbon matrix. The dopings of N and Fe into CNWs are carried out by N plasma and Fe sputtering, respectively. The onset potentials corresponding to ORR activities are evaluated from steady-state curve of cyclic voltammogram. The composition and structure of CNW catalysts before and after ORR measurements are characterized by X-ray photoelectron spectroscopy (XPS). The onset potential increases in the order of non-, N- and Fe-N-doped CNW catalysts. The ORR activity for N-doped CNW catalysts is ascribed to pyridinic N stabilized near the surface of carbon matrix. The higher ORR activity for Fe-N-doped CNW catalysts is attributed to trace Fe-N rather than pyridinic N near the surface. Additionally, it is shown that such ORR activities due to pyridinic N and Fe-N near the surface are further promoted by Fe and Fe-N incorporated beneath the surface. These knowledges provide effective strategy for the synthesis of Fe-N-doped carbon catalysts with higher ORR activities.

peer-reviewed The full text of this article will not be available in ULIR until the embargo expires on the 18/11/2021 The influence of the bipolar electrode on the voltammetry observed with a closed bipolar electrochemical cell (CBPEC) goes far beyond simply conducting electrons between the two electrolyte solutions. The surface of each pole of the bipolar electrode may contain redox active functional groups that generate misleading or interfering electrochemical responses. Herein, a 4-electrode CBPEC configuration was studied with the opposite poles of the bipolar electrode resting in separate aqueous and organic electrolyte solutions. Using gold mesh wire electrodes as the poles, we systematically investigated the many experimental variables that influence the observed voltammetry upon addition of a reductant (decamethylferrocene) to the organic phase. External bias of the driving electrodes forced electrons released by decamethylferrocene at the organic pole to flow along the bipolar electrode and reduce redox active surface functional groups at the aqueous pole, such as oxide or hydroxide groups, or carry out the oxygen reduction reaction (ORR) or hydrogen evolution reaction (HER). The 4-electrode CBPEC configuration diminishes capacitive currents, permitting observation of voltammetric signals from electron transfer processes related to surface functional groups at the aqueous pole at much lower scan rates than possible with working electrodes in conventional 3-electrode electrochemical cells. Surface modification, by oxidative or reductive electrochemical pre-treatment, changes the potential window experienced by the aqueous pole in the 4-electrode CBPEC in terms of its position versus the standard hydrogen electrode (SHE) and dynamic range. In a related observation, the electrochemical responses from the surface functional groups on the aqueous pole completely disappear after oxidative pre-treatment, but remain after reductive pre-treatment. The flow of electrons from decamethylferrocene to the surface of the aqueous pole is limited in magnitude, by the decamethylferrocene concentration, and kinetically limited, due to decamethylferrocene diffusion to the organic pole, in comparison to the infinite supply of electrons delivered to the surface of a working electrode in a 3-electrode cell. This unique feature of the 4-electrode CBPEC facilitates a very gradual evolution of the surface chemistry at the aqueous pole, for example from fully oxidised after oxidative pre-treatment to a more reduced state after repetitive cyclic voltammetry cycling. Perspective applications of this slow, controlled release of electrons to the electrode surface include spectroelectrochemical analysis of intermediate states for the reduction of metal salts to nanoparticles, or conversion of CO2 to reduced products at catalytic sites. The use of indium tin oxide (ITO) electrodes in CBPEC experiments for specific reactions is recommended to avoid misleading or interfering electrochemical responses from redox active functional groups prevalent on metallic surfaces. However, the electronic bridge to implement entirely depends on the reaction under study, as ITO also has drawbacks such as a lack of electrocatalytic activity and the requirement of an overpotential due to its semiconducting nature. ACCEPTED peer-reviewed

Intermetallic alloy has become a hot topic in heterogeneous catalysis due to improved activity and stability. We report here the results of Pd monolayer supported on face-centered tetragonal (fct) Pd-M (M = Ni or Fe) intermetallic alloy as cathode materials by first principle calculations. Adsorbed O2 is found to undergo hydrogenation process into OOH instead of direct dissociative mechanism. The oxygen reduction reaction (ORR) proceeds via four-electron transfer pathway on both surfaces. The enhanced activities are ascribed to moderate adsorption strength for ORR intermediates due to compression strain and substrate electron transfer. The dissolution potential analysis indicates that both Pd/PdNi(111) and Pd/PdFe(111) are stable under operating condition. Gibbs free energy analysis and barrier calculations suggest that the proposed Pd monolayer structures are promising for cathode material. These results can be useful in designing intermetallic Pd-M alloy as ORR electrocatalysts.

The Electro-Fenton process for in-situ H(2)O(2) electrogeneration is impacted by low O(2) utilization efficiency (

In this work, a platinum group metal-free (PGM-free) catalyst based on iron as transitional metal and Nicarbazin (NCB) as low cost organic precursor was synthesized using Sacrificial Support Method (SSM). The catalyst was then incorporated into a large area air-breathing cathode fabricated by pressing with a large diameter pellet die. The electrochemical tests in abiotic conditions revealed that after a couple of weeks of successful operation, the electrode experienced drop in performances in reason of electrolyte leakage, which was not an issue with the smaller electrodes. A decrease in the hydrophobic properties over time and a consequent cathode flooding was suspected to be the cause. On the other side, in the present work, for the first time, it was demonstrated the proof of principle and provided initial guidance for manufacturing MFC electrodes with large geometric areas. The tests in MFCs showed a maximum power density of 1.85 W m−2. The MFCs performances due to the addition of Fe-NCB were much higher compared to the iron-free material. A numerical model using Nernst-Monod and Butler-Volmer equations were used to predict the effect of electrolyte solution conductivity and distance anode-cathode on the overall MFC power output. Considering the existing conditions, the higher overall power predicted was 3.6 mW at 22.2 S m−1 and at inter-electrode distance of 1 cm., Graphical abstract Iron-Nicarbazin derived platinum group metal-free electrocatalyst was incorporated into a scalable-size air-breathing cathodes and tested in microbial fuel cells. Maximum power density of 1.85 W m−2 was achieved initially and then decreased to 1.25 W m−2 after 22 days operations.Image 1, Highlights • Fe-based material was used as catalyst for ORR in MFCs. • The catalyst was incorporated into a scalable-size air breathing cathode. • MFC had an initial power density of 1.85 W m−2. • The effect of solution conductivity and electrodes distance is evaluated on MFC performance. • Higher power predicted was 3.6 mW at 22.2 S m−1 and at electrode distance of 1 cm.

Electrochemical oxygen reduction reaction (ORR) represents a crucial cathodic process of different fuel cells. The development of highly active and stable noble metal-free ORR electrocatalysts remains as one of major challenges. Herein, we report cobalt/nitrogen co-doped porous carbon materials (Co-N-C) from well-designed bimetal-organic frameworks (Zn100-xCox-ZIF) as efficient ORR electrocatalysts in both pH-neutral and alkaline solutions. The compositional and structural features, and the corresponding ORR activity can be tailored by tuning the Zn/Co ratio in the precursor. Remarkable features of large surface-area and suitable graphitization degree and well-dispersed Co-N-C moieties, enable a high catalytic efficiency. The optimized electrocatalyst registers considerable ORR performance with a high half-wave potential of 0.65 V vs RHE and a saturated current density of 5.30 mA cm−2 close to the value of the commercial Pt/C in 0.1 M pH 7.0 PBS, and considerable operational stability and tolerance to glucose. A preliminary one-compartment hybrid glucose/O2 enzymatic biofuel cell, consisting of Co-N-C-10 abiotic cathode and glucose oxidising bioanode, is constructed and tested. Furthermore, Co-N-C also presents reasonable ORR electrocatalytic activity and operational stability in alkaline condition.

The oxygen reduction reaction (ORR) at different pH values has been studied at platinum single crystal electrodes using the hanging meniscus rotating disk electrode (HMRDE) configuration. The use of NaF/HClO4 mixtures allows investigating the reaction up to pH = 6 in solutions with enough buffering capacity and in the absence of anion specific adsorption. The analysis of the currents shows that the kinetic current density measured at 0.85 V for the Pt(111) electrode follows a volcano curve with the maximum located around pH = 9. This maximum activity for pH = 9 can be related to the effects of the electrode charge and/or water structure in the ORR. On the other hand, the catalytic activity for the other basal planes shows a monotonic behavior with a small dependence of the activity with pH. For stepped surfaces with (111) terraces, the behavior with pH changes gets closer to that of the Pt(111) surface as the terrace length increases. Additionally, the ORR curves show a dependence of the limiting diffusion current with pH. It is observed that the limiting current density diminishes as the pH increases in a potential region where hydrogen peroxide is readily reduced. These results suggest the existence of a bifurcation point in the mechanism previous to peroxide formation, in which OOH• is proposed as the bifurcation intermediate. The reduction of OOH• requires proton addition and would be more difficult at neutral pH values, justifying the diminution of the limiting currents. This work has been financially supported by the MCINN-FEDER (Spain) and Generalitat Valenciana (Feder) through projects CTQ2016-76221-P and PROMETEOII/2014/013, respectively. VBM thankfully acknowledges to MINECO the award of a pre-doctoral grant (BES-2014-068176, project CTQ2013-44803-P).

Graphene-supported NiMo and CoMo carbides with monodisperse feature were prepared and their electrocatalytic activity toward for the ORR was investigated. The NiMo carbide/G and CoMo carbide/G are found to be different in main carbide species (i.e., MoC and Mo2C, respectively) and in carbide particle size distribution. The ORR tests revealed that CoMo carbide/G exhibit better electrocatalytic performance than NiMo carbide/G in terms of on-set potential, limiting current density and durability. Our present results may open new way for the synthesis of molybdenum carbides, and also suggested that strategies for tuning the molybdenum carbide species within bi-metallic carbides may lead to further enhancements in ORR activity. MOE (Min. of Education, S’pore)

World plastic production has increased since industrial-scale production began in the 1940s and while large amount of thermoplastic polymers can be effectively recycled and re-used, undifferentiated polymers or thermoset polymers cannot, and as a result, most of these raw materials end up in landfill or energy recovery in incinerators. The synthesis of carbon nanomaterials from conversion of waste polymers is an alternative, promising approach owing to the high added-value of these products. In particular, novel carbon materials, could translate into interesting and cheap material for the catalytic reduction of oxygen (ORR), a fundamental reaction for the production of H2O2 or in fuel cell and metal air batteries. This paper presents the synthesis of iron-nitrogen-carbon (Fe-N-C) electrocatalysts starting from a mix of polyethylene (PE) and polyurethane (PU) by adding FeCl3 as an iron source for promoting Fe-Nx sites formation. The two polymers were mixed according to a solvent assisted process employing p-xylene or with a solvent free process using a Brabender plastograph. The blend of thermoplastic and thermoset polymers was converted into Fe-N-C materials in a two-step pyrolysis, where the temperature of second pyrolysis showed to sensitively affect the chemical and textural properties of the resulting material. Depending on the temperature, and initial PE content, the surface area ranges between 195 and 479 m2 g−1, with a preferential formation of micropores. XPS analysis confirmed that the employment of PU leads to the formation of nitrogen functional groups and Fe-Nx sites, while XRD investigation in heating chamber allowed to follow the formation Fe3C, Fe2O3, γ-Fe and α-Fe with temperature rises between 700–1000°C. The new Fe-N-C catalysts were characterized by linear sweep voltammetry at ring disk electrode showing interesting activity for the ORR in both acid and alkaline electrolyte and a selectivity for H2O2 which deeply depends on the type of electrolyte as well as on nitrogen content and on the surface area of the samples.

Experimental results for the ORR (oxygen reduction reaction) in perchloric acid for ultra low loading Pt/C electrodes have been fitted to a number of different ORR mechanisms. These were accomplished as a function of temperature (280–330 K), oxygen partial pressure( 0.01 p o 2 p o 2 0 1 ) and potential (0.3–1.0 V vs. RHE). A reaction exponent for oxygen of 1 ± 0.1 across the potential range 0.3–0.85 V vs. RHE is confirmed. From the experimental results it is clear that the surface becomes increasingly blocked towards the ORR as overpotential increases (i.e. as the potential decreases from 0.6 to 0.3 V vs. RHE). The double trap model [J.X. Wang, J. Zhang, R.R. Adzic, J. Phys. Chem. A, 111 (2007) 12,702] fails to account for this observation, although we have produced a modified version to include the formation of OOHad intermediates. These intermediates block the electrode at larger overpotentials and lead to a decrease in electrocatalyst performance compared to a Tafel type approximation. Furthermore these intermediates can lead to the formation of hydrogen peroxide at large overpotentials, an experimental observation which is currently poorly described by models. The decreased activity at large overpotentials suggests that blocking of active catalyst sites may be as important to catalyst activity in an operating fuel cell as the absolute performance of the electrode in the low overpotential region as typically measured on an RDE. It may also offer an explanation to the increased losses seen in fuel cell electrodes at lower catalyst loadings – i.e. the loses, which are typically ascribed to increased mass transport loses, may instead result from decreased electrocatalytic performance at high overpotentials.

The hydrogen peroxide reduction reaction (HPRR) is investigated on Pt(111) as well as on Pt(S)[(n-1)(111) × (110)] and Pt(S)[n(111) × (100)] stepped surfaces in 0.1 M HClO4 by means of voltammetric studies using the hanging meniscus rotating disk electrode (HMRDE) configuration. Results point out that there is a direct correlation between the potential value (Einhibition) at which the inhibition of the HPRR at low potential starts and the local potential of maximum entropy (pme) for the terraces. In addition, there is also a correspondence between the potential of the peak (Epeak) that appears at E < 0.3 V vs. RHE for stepped surfaces and the local pme for the steps. Additional experiments for stepped surfaces from acidic to neutral pH confirm this correlation since Einhibition shifts 0.059 V per pH units towards more positive potentials, which is the same observed behavior for the pme from laser-induced temperature-jump experiments. However, adsorbed OH can influence this trend when the pme values are near the region of adsorption of these species. The effect of surface charge on the structure of interfacial water can also influence the current inhibition as inferred from measurements in alkaline media. Finally, ORR measurements and the differences observed with the HPRR results in the same conditions suggest that the formation of H2O2 intermediate is less favored as the pH is increased, and therefore a previous bifurcation point in the mechanism should exist. This work has been financially supported by the MICINN-FEDER (project CTQ2016-76221-P. VBM thankfully acknowledges to MINECO the award of a predoctoral grant (BES-2014-068176).

11 Figuras, 3 Tablas.-- Material suplementario disponible en línea en la página web del editor., Oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) catalysts play an important role in energy conversion and storage devices, but highly active and robust nonprecious metal catalysts are required to address the cost and durability issues. In this work, a modified gel methodology has been used to obtain nanocomposites of perovskite CoTiO3 and N-doped reduced graphene oxide (NrGO). Moreover, the influence of the annealing extent in the synthesis has been studied. The composites show higher activity in oxygen evolution and reduction reactions in alkaline environment compared to sole ilmenite, which is related to a strong interaction between cobalt titanate and NrGO carbon matrix. The annealing duration appears as a key variable to modulate the physicochemical properties and the electrochemical behavior. The composite prepared at the largest duration (3 h) exhibit enhanced bifunctional properties for ORR and OER, with onset potentials of 0.93 V and 1.53 V vs. RHE respectively, which is mainly attributed to higher concentration of nitrogen, larger extent of defects and/or the presence of more oxidized Co species in the nanocomposite., The authors wish to acknowledge the Ministerio de Ciencia, Innovación y Universidades (MICINN) and FEDER for the received funding in the project of reference ENE2017-83976-C2-1-R. Thanks also to the Gobierno de Aragón (DGA) the funding for the Grupo de Investigación Conversión de Combustibles (T06_17R). J. M. Luque also thanks MICINN for its Ph.D. grant. D. Sebastián acknowledges the MICINN for the contract Ramón y Cajal (RyC-2016-20944).

Oxygen reduction reaction (ORR) was studied on a silver polycrystalline electrode in different NaOH concentrations with a help of rotating disc and rotating ring disc electrodes. Soluble reaction intermediate was detected at all alkaline concentrations, but its concentration increased with an increase of the level of impurities. ORR is not NaOH concentration dependent at low concentrations (0.1 and 1 M). In 11 M NaOH ORR onsets at more positive potentials in the region where underpotential silver oxide formation shows less reversible behaviour. The nonlinear frequency response analysis spectra show significant qualitative difference with NaOH concentration indicating a high capability of this method for kinetic mechanism investigations.

Within the search for new catalysts for the oxygen reduction reaction in polymer electrolyte membrane fuel cells, alloys of Pt with other transition metals are of great interest due to their increased specific and especially mass activity. However, the drawback of these catalysts is their reduced stability due to the dissolution of the less-noble metal from the alloy. To resolve the potential dependence of these dissolution processes, we investigate a material library of Pt-Cu thin-film alloys with compositions ranging from 0 at% Cu up to 60 at% Cu. Utilizing our combinatorial scanning flow cell technique coupled to a mass spectrometer several aspects of dissolution are revealed. The onset of relevant Cu dissolution was found to be around 0.9 VRHE, independent of the composition. Although this is well below the onset potential of the Pt dissolution (1.15 VRHE), the two dissolution processes are clearly correlated, especially when the surface is already depleted of Cu. In contrast to Pt, however, Cu preferentially dissolves during anodic polarization rather than during the cathodic sweep. Additionally, at Cu compositions above the parting limit between 50 and 57 at% Cu a breakdown of passivity and massive Cu dissolution leads to porosity formation. The critical potential for the alloy with 57 at% Cu was detected around 1.3 VRHE, which is above the stability potential of Pt. While the absolute activity during the porosity formation increases due to the formation of more active sites in the pores, the specific activity decays to a value close to pure Pt.

Graphical abstract, Highlights • PGM-free catalysts were synthesized using sacrificial support method. • Catalysts were made with Fe, Co, Mn and Ni as metal center and AAPyr as precursor. • Fe-catalysts showed highest performance for ORR in microbial fuel cell. • Increase in solution conductivity led to a maximum power of 482 ± 5 μWcm−2, The oxygen reduction reaction (ORR) is one of the major factors that is limiting the overall performance output of microbial fuel cells (MFC). In this study, Platinum Group Metal-free (PGM-free) ORR catalysts based on Fe, Co, Ni, Mn and the same precursor (Aminoantipyrine, AAPyr) were synthesized using identical sacrificial support method (SSM). The catalysts were investigated for their electrochemical performance, and then integrated into an air-breathing cathode to be tested in “clean” environment and in a working microbial fuel cell (MFC). Their performances were also compared to activated carbon (AC) based cathode under similar conditions. Results showed that the addition of Mn, Fe, Co and Ni to AAPyr increased the performances compared to AC. Fe-AAPyr showed the highest open circuit potential (OCP) that was 0.307 ± 0.001 V (vs. Ag/AgCl) and the highest electrocatalytic activity at pH 7.5. On the contrary, AC had an OCP of 0.203 ± 0.002 V (vs. Ag/AgCl) and had the lowest electrochemical activity. In MFC, Fe-AAPyr also had the highest output of 251 ± 2.3 μWcm−2, followed by Co-AAPyr with 196 ± 1.5 μWcm−2, Ni-AAPyr with 171 ± 3.6 μWcm−2, Mn-AAPyr with 160 ± 2.8 μWcm−2 and AC 129 ± 4.2 μWcm−2. The best performing catalyst (Fe-AAPyr) was then tested in MFC with increasing solution conductivity from 12.4 mScm−1 to 63.1 mScm−1. A maximum power density of 482 ± 5 μWcm−2 was obtained with increasing solution conductivity, which is one of the highest values reported in the field.

In this report, the preparation of Fe-carbon nitride (CN)-based electrocatalysts (ECs) with a “core-shell” morphology for the oxygen reduction reaction (ORR) is described. The ECs consist of spherical XC-72R carbon nanoparticles, the “cores”, that are covered by a CN matrix, the “shell”, that embeds Fe species in “coordination nests”. The latter consist of hollow cavities in the CN matrix, whose internal surface is covered by N- and C-ligands able to stabilize alloy nanoparticles or active sites. Two families of CN-based ECs are prepared, which are grouped on the basis of the concentration of N atoms in the CN “shell”. Each group comprises of both a “pristine” and an “activated” EC; the latter is obtained from the “pristine” EC by a suitable series of treatments (A) devised to improve the ORR performance. The chemical composition of the CN-based ECs is determined by Inductively-Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) and microanalysis. The thermal stability under both inert and oxidizing atmospheres is gauged by High-Resolution Thermogravimetric Analysis (HR-TGA). The structure is probed by powder X-ray diffraction, and the morphology is inspected by Scanning Electron Microscopy (SEM) and High-Resolution Transmission Electron Microscopy (HR-TEM). The surface area of the CN-based ECs is determined by nitrogen physisorption techniques, and the surface composition is probed by X-ray Photoelectron Spectroscopy (XPS). The electrochemical performance and reaction mechanism of the CN-based ECs in the ORR is investigated in both acid and alkaline environments by cyclic voltammetry with the Thin-Film Rotating Ring-Disk Electrode setup (CV-TF-RRDE). The influence of the preparation parameters and of the treatments on the physicochemical properties, the ORR performance, and reaction mechanism is studied in detail. In the alkaline environment the FeFe2-CNl 900/CA “core-shell” EC shows a remarkable ORR onset potential of 0.908 V vs. RHE which, with respect to the value of 0.946 V vs. RHE of the Pt/C ref., classifies the proposed materials as very promising “Platinum Group Metal-free” ECs for the ORR.