Steady-state Polarization Curves Research Articles

High-Performance Ru 2 P Anodic Catalyst for Alkaline Polymer Electrolyte Fuel Cells

High-Performance Ru ₂ P Anodic Catalyst for Alkaline Polymer Electrolyte Fuel Cells

Preparation and performance of an Al/TiB2 + 10%Ti4O7/β-PbO2 as a composite anodic material for electrowinning of non-ferrous metals

Electrodes are the key material in the electrolysis of non-ferrous metals, and their selection and preparation can be a difficult problem in the hydrometallurgical industry. In this paper, starting from the selection of electrode materials and structural design, an Al/TiB2 + 10%Ti4O7-coating composite material was prepared by plasma spraying technology, and a β-PbO2 coating was prepared by electrochemical deposition. The phase composition of the coating was analyzed by X-ray diffractometer, and the structure of the coating was observed by scanning electron microscopy. Results show that the electrode prepared by electrodeposition at a current density of 0.03 A cm−2 has a more compact structure and more uniform grain size. Through steady-state polarization curve, cyclic voltammetry, linear sweep voltammetry, and electrochemical impedance spectroscopy, the electrochemical performance of the electrode was studied. Through porosity measurements, it was found that the composite electrode material prepared by the plasma spraying method under the parameters of a spraying power of 36 kW, powder feeding rate of 30 g/min, spraying distance of 105 mm, and argon gas flow rate of 2.6 m3/h greatly reduces the charge resistance in the double-layer structure on the electrode surface, thereby accelerating the charge transfer rate. Plasma spraying and electrochemical deposition have been used to successfully prepare Al/TiB2 + 10%Ti4O7/β-PbO2 composite electrode materials with good corrosion resistance and electrochemical catalytic activity. Compared to Ti/β-PbO2 and Pb-(0.5 wt%)Ag/β-PbO2, the corrosion resistance and polarization potential increased by 83.6% and 93.0% and negatively shifted by 517.37 mV and 587.12 mV, respectively, and the catalytic activity was also significantly improved.

The determination of the HOR/HER reaction mechanism from experimental kinetic data.

Hydrogen oxidation and evolution are important processes from both a fundamental and applied perspective. In interpreting experimental kinetic data, few studies have explicitly accounted for the impact of H* coverage and mass transport, which lead to discrepancies in the kinetic parameters and the resultant reaction mechanism. Here, we present how to determine the kinetic parameters accounting for both effects. We discuss the use of the kinetic parameters towards mechanistic interpretations for HOR/HER and show that, in general, knowledge of the coverage of H* or activation energies may be required to assign a reaction mechanism. We apply these ideas to activity data of several HOR and HER electrocatalysts, such as Au, Pt, MoS2, and CoP.

Secondary Reduction of Refractory Metal near Smooth Cathode during the Salt Melt Electrolysis. 3. Numerical Reproducing of Natural Experiment with the Nb Cathodic Reduction in KNaCl2–NaF–K3NbF7 Melt

In terms of the model of steady-state process of a refractory-metal ion secondary reduction with an alkali (or alkali-earth) metal formed on a smooth cathode during salt melt electrolysis, numerical reproducing of a natural experiment is carried out. The model calculations are based on the polarization measured during the cathodic reduction of Nb in chloride―fluoride melt: [Nb] = 2.4 mol %, 800°C, current density i up to 5.4 A/cm2. On a smooth cathode in the overvoltage range from 0.21 to 0.41 V (i = 0.75―1.17 A/cm2), the reduction mechanism is shown to completely change from primary to the secondary reduction. The secondary reduction zone is located inside the diffusion layer and moves away from the cathode with increasing i. Even at maximal i, this zone remains inside δ (δ is the thickness of the diffusion layer): with i = 0.96 A/cm2, the coordinate of the maximum of the secondary reduction rate xm = 0; with i = 5.4 A/cm2, xm = 0.93δ (in the experiment, the value of i at which xm ≥ δ was not achieved). At i = 5.4 A/cm2, the fraction of secondary reduction in the bulk is insignificant, ≈10−5 A/cm2. The δ vs. E curve is calculated: when E is shifted from the equilibrium value toward the negative values, |i| increased from zero and higher, whereas δ first decreased by an order of magnitude and then increased. Correspondingly, the limiting diffusion current is not constant at a constant Nb concentration in the bulk. This is the reason of uncertain limiting current on steady-state cathodic polarization curves during the electrolysis of metals in melts. A hypothesis is proposed on the dependence of the Nernst diffusion layer thickness near the cathode on the current density exceeding the limiting diffusion current density under the conditions of secondary reduction.

Durable Hybrid Electrocatalysts for Proton Exchange Membrane Fuel Cells

Developing transition metal and nitrogen co-doped carbon (Me-N-C) catalysts with comparable ORR activity is a feasible approach to addressing the material cost issue of proton exchange membrane fuel cells (PEMFCs).1-3 However, the low durability of Me-N-C hinders their practical application. It was found that the half-wave potential of Fe-N-C catalysts decreased significantly (16-30 mV) only after 10000 cycles between 0.6 and 1.0 V in acidic electrolytes,4-5 and above 50% current density loss only after 20 h at 0.4 V in sing cell tests.6-7Herein, we rationally designed a hybrid Pt-Fe-N-C electrocatalyst consisting of abundant Pt and Fe single atoms homogeneously dispersed on the nitrogen-doped carbon support and a small amount of Pt-Fe alloy nanoparticles. The distributions of single atoms and morphologies of carbon framework were obtained with high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The oxidation state and chemical bond information was characterized by X-ray photoelectron spectroscopy and X-ray absorption spectroscopy. The optimized Pt-Fe-N-C consisting of 0.21wt.% Pt and 2.4 wt.% Fe demonstrated unprecedented durability upon potential cycling with no change in the half-wave potential after 70000 cycles between 0.6 and 1.0 V (Figure 1a). The higher durability was also confirmed in the fuel cell testing. As shown in Figure 1b, after keeping the voltage at 0.4 V for 85 h, the current density of the fuel cell using Pt-Fe-N-C as the cathode material only dropped by 20%. For comparison, the current density dropped by more than 50% after only 23 h for the fuel cell using Fe-N-C as the cathode material.This work demonstrates the feasibility of improving the durability of Fe-N-C material via ultra-low Pt doping. In addition, it highlights the importance of the synergy effect between Pt and Fe single atoms and the strong interaction between active sites and carbon support, which may shed light on improving the durability of non-precious metal catalysts via a hybrid structure.Figure 1. (a) Steady-state polarization curves of oxygen reduction reaction activity Pt-Fe-N-C after 70000 cycles in 0.1 M HClO4 electrolyte. The catalyst loading is 0.51 mg cm-2. (b) Long-term durability test of Pt-Fe-N-C and Fe-N-C catalyst at 0.4 V (1 bar H2/1 bar air; anode Pt loading: 0.1 mg cm-2; cathode catalyst loading: 3.2 mg cm-2; cell temperature 80 °C; 100% RH).References(1)Wu, G.; More, K. L.; Johnston, C. M.; Zelenay, P., High-Performance Electrocatalysts for Oxygen Reduction Derived from Polyaniline, Iron, and Cobalt. Science 2011, 332, 443-447.(2)Chung, H. T.; Cullen, D. A.; Higgins, D.; Sneed, B. T.; Holby, E. F.; More, K. L.; Zelenay, P., Direct Atomic-Level Insight into the Active Sites of a High-Performance PGM-Free ORR Catalyst. Science 2017, 357, 479-484.(3)Proietti, E.; Jaouen, F.; Lefèvre, M.; Larouche, N.; Tian, J.; Herranz, J.; Dodelet, J.-P., Iron-Based Cathode Catalyst with Enhanced Power Density in Polymer Electrolyte Membrane Fuel Cells. Nat. Commun. 2011, 2, 416.(4)Xiao, F.; Xu, G.-L.; Sun, C.-J.; Xu, M.; Wen, W.; Wang, Q.; Gu, M.; Zhu, S.; Li, Y.; Wei, Z., Nitrogen-Coordinated Single Iron Atom Catalysts Derived from Metal Organic Frameworks for Oxygen Reduction Reaction. Nano Energy 2019, 61, 60-68.(5)Merzougui, B.; Hachimi, A.; Akinpelu, A.; Bukola, S.; Shao, M., A Pt-Free Catalyst for Oxygen Reduction Reaction Based on Fe–N Multiwalled Carbon Nanotube Composites. Electrochimica Acta 2013, 107, 126-132.(6)Chen, J.; Yan, X.; Fu, C.; Feng, Y.; Lin, C.; Li, X.; Shen, S.; Ke, C.; Zhang, J., Insight into the Rapid Degradation Behavior of Nonprecious Metal Fe–N–C Electrocatalyst-Based Proton Exchange Membrane Fuel Cells. ACS Appl. Mater. Interfaces 2019, 11, 37779-37786.(7)Choi, J.-Y.; Yang, L.; Kishimoto, T.; Fu, X.; Ye, S.; Chen, Z.; Banham, D., Is the Rapid Initial Performance Loss of Fe/N/C Non Precious Metal Catalysts Due to Micropore Flooding? Energy Environ. Sci. 2017, 10, 296-305. Figure 1

Electrochemical redox kinetic behavior of S8 and Na2Sn (n = 2, 4, 6, 8) on vulcan XC-72R carbon in a flowing-electrolyte system

Many studies have shown that sulfur electrodes in room-temperature sodium–sulfur (RT Na–S) batteries undergo a liquid-phase electrochemical reaction. Moreover, the electrochemical redox kinetic behavior of S8 and sodium polysulfides (Na2S8, Na2S6, Na2S4, and Na2S2) is important in understanding the macroscopic kinetics of sulfur electrodes. In this study, a three-electrode electrochemical test system is built using a two-chamber micro-electrolysis cell with a flowing electrolyte and Nafion membrane. Electrochemical kinetic parameters, such as the onset redox potentials, exchange current densities, reaction orders, and apparent activation energies of the sodium polysulfides during the redox process are obtained from a steady-state polarization curve. The onset redox potentials of S8, Na2S8, Na2S6, Na2S4, and Na2S2 on Vulcan XC-72R carbon are 2.37, 2.23, 2.11, 1.99, and 1.83 V (vs. Na/Na+), respectively. Furthermore, the relationship between the discharge capacity of sodium polysulfides and their onset redox potentials is established. The aforementioned kinetic parameters are vital in elucidating the formation of discharge/charge platforms and sloping regions in an RT Na–S battery. The results of this work provide a basis to further understand the performance of RT Na–S batteries.

Nanocomposites formed by combination of urchin like NiS with Ni-nanoparticles/N-doped nanoporous carbon, derived from nickel organic framework, and decorated with RuO2 nanoparticles: Construction and kinetics for hydrogen evolution reaction

Fabrication of active, stable, and cost effective electrocatalysts, as alternative cathodes to Pt or Pt-based materials for catalytic hydrogen production, is important for different industrial purposes, such as hydrogen therapy in medicine, syntheses in pharmaceutical and nutrient, and clean carbon-free energy carrier in industry.Herein, construction, characterization and examination of an efficient electrocatalyst for hydrogen evolution reaction (HER) in alkaline media is reported. The catalyst system is based on nanocomposites formed of urchin-like nickel sulfide incorporated with nickel nanoparticles and nitrogen doped-nanoporous carbon, and decorated electrochemically with ruthenium (IV) oxide on the glassy carbon electrodes, leading to GCE-NiS/Ni/NNPC/RuO2 system.Physicochemical characteristics of the prepared composite systems are studied by several surface, solution and electrochemical methods, from which the system is characterized.Then, electrocatalytic activity of the GCE modified with the composite, is studied for the HER by means of the steady-state linear sweep voltammetry (LSV), polarization curves (Tafel plots), and electrochemical impedance spectroscopy (EIS). The obtained data are analyzed by using appropriate equivalent circuit models, from which the HER kinetics are quantitatively determined.The results revealed that the electrode modified with the prepared composite, GCE-NiS/Ni/NNPC/RuO2, is an efficient electrocatalyst for the HER in alkaline solution, where an overpotential of −82.54 mV at the current density of 20 mA cm‒2 (η20 =− 82.54 mV) with Tafel slope of 49.88 mV dec‒1 is observed. These activities are very close to those obtained on Pt-based electrode materials. The origin of the enhancement in the HER activity can be attributed to (i) the high surface area of the urchin-like structure along with nanoporous carbon, and (ii) the synergistic chemical coupling effects between the NiS, Ni/NNPC and RuO2nanoparticles.

The kinetics of hydrogen evolution reaction accompanied by hydrogen absorption reaction with consideration of subsurface hydrogen as an adsorbed species: Polarization curve

The scheme for hydrogen evolution reaction with hydrogen absorption in parallel is considered using the concept of hydrogen adsorbed in the subsurface. The steady-state polarization curves are calculated for cases when hydrogen evolution occurs according to the Volmer–Heyrovsky and Volmer–Tafel mechanisms with the Langmuir hydrogen adsorption isotherm. It is shown that at low current densities, j, the presence of hydrogen absorption results in an increase in the slopes dη/dlnj (η is the overpotential).

Validation of 1D porous electrode theory using steady-State measurements of flooded electrodes at variable electrolyte compositions

Steady-state measurements were performed using flow-through cells and electrolyte solutions spanning a wide range of concentrations of ferrous/ferric ions as active species and supporting salts (hydrochloric acid, sodium chloride, cesium chloride). Voltage losses obtained from steady-state polarization curves and impedance measurements were analyzed using Newman’s porous electrode model. The results showed that the model accurately predicts the electrode performance for commonly used concentrated electrolyte solutions but deviates substantially for low levels of supporting electrolyte (particularly for sodium chloride). Effects of mass transport and diffuse double layer effects are discussed to account for the nonlinear voltage losses.

Investigation of oxygen evolution reaction kinetic process and kinetic parameters on iridium electrode by electrochemistry impedance spectroscopy analysis

Though metallic iridium catalyst is broadly used as an efficient OER catalyst due to its high OER activity, however, there is a current dearth of researches on its OER mechanism and kinetic parameters. The present study aims to investigate the OER kinetic process and obtain kinetic parameters of the iridium catalyst. The OER rate determining step was investigated by EIS measurements. Meanwhile, OER kinetic parameters were obtained by both steady-state polarization curve measurement and microkinetic modelling. Results show that the rate determining step of OER is the adsorption of Ir-OHads species on active sites. Values of both Tafel slope and exchange current density are important kinetic parameters which are about 48.6 mV/dec and 1.88 × 10−11 A/cm2 according to the microkinetic modelling. The obtained parameters could be referred in the novel OER catalysts investigation. The approaches for OER kinetic process study and kinetic parameters evaluation are useful in the investigation of other catalysts.

Realising the activity benefits of Pt preferential (111) surfaces for ethanol oxidation in a nanowire electrocatalyst

The commercialisation of direct ethanol fuel cells (DEFCs) is hindered by the low activity of the currently available electrocatalysts that are mainly carbon-supported nanoparticles (NPs). Here, we synthesised carbon-supported Pt nanowires (Pt/C NWs) by chemical reduction of metallic precursors at room temperature without using surfactants, templates, or stabilising agents. The synthesised Pt NWs were supported on high surface area carbon (Vulcan XC-72 R) with 40 wt%. Of metal loading concerning the support. The electroactivity of the synthesised NWs catalyst towards ethanol and CO (the dominant intermediate species) oxidation is investigated and tested in comparison with a commercial nanoparticle Pt/C electrocatalyst (named: Pt/C NPs). The electrooxidation of ethanol was investigated in acid media by CO-stripping, cyclic voltammetry, derivative voltammetry, chronoamperometry, steady-state polarisation curves, and in situ Fourier transform infrared spectroscopy (FTIR) experiments. The results showed that the synthesised Pt NWs are much more active than the Pt/C NPs catalyst, for both ethanol and CO-monolayer oxidation in acidic media. In situ FTIR data revealed that Pt NWs catalyst favours the formation of CO2 and acetic acid. This activity is mainly due to the existence of extended terraces, resulting in enhanced mobility of OHads and COads, facilitating the removal of COads from the catalyst surface and allowing better ethanol adsorption for further oxidation. Hence, 5-fold higher current density for ethanol oxidation is obtained. The NW morphology of Pt nanocatalysts results in very active materials towards ethanol oxidation and may provide a promising means of increasing the performance of anodes for DEFCs.

Oxidation of Sulphites by Electrodes Made of Novel Materials for Use in Microbial Fuel Cells

Catalysts based on novel carbon forms (Fullerenes С60/С70, Higher Fullerenes and Double Wall Carbon Nanotubes) were applied to facilitate the process of oxidation of sulfites to sulfates. The electrodes with incorporated new catalysts are able to desulphurize and remove toxic pollutants. The electrochemical characterization included steady state polarization curve analysis. The electrodes that incorporate fullerene structures were found to yield the highest current densities. A short overview considers the existing sulfur oxidizing microbes that should facilitate the creation of a workable microbial fuel cell.

Импеданс реакции выделения водорода по механизму Фольмера-Гейровского при различной форме зависимости теплоты адсорбции водорода от степени заполнения

The steady-state polarization curves and overpotential dependences of the equivalent circuit elements (R1, R2, C2) were calculated for the hydrogen evolution at nonlinear variation of the free energy of hydrogen adsorption DGadswith increasing coverage q. It was shown that the polarization curves depend weakly on the function g(q) = DGads/RT while the values of the Faradaic impedance parameters depend markedly on g(q). When DGads is coverage-dependent, the following differences from Langmuir adsorption are observed: (i) inductive behavior of impedance at small rate constant of the Heyrovsky reaction compared with rate constant of the Volmer reaction and at equal transfer coefficients of the steps; (ii) logR1–h curve is off parallel with logR2–h curve at high cathodic polarizations.

Applications of cathodic Co100-XNiX (x = 0, 30, 70, and 100) electrocatalysts chemically coated with Pt for PEM fuel cells

The present work comprises a study about the synthesis, characterization, and performance evaluation of four Co100-X-NiX (x = 0, 30, 70, and 100) electrocatalysts coated with Pt with potential catalytic activity towards ORR. Electrocatalysts were prepared through a two-stage process combining the versatile high-energy ball milling synthesis and the galvanic displacement method. On a first stage, high-energy ball milling was used to produce nanoparticles of non-noble transition metals, (M = Co100 and Ni100) and (BM = Co30Ni70and Co70Ni30). On a second stage, galvanic displacement reactions via chemical reflux treatment were employed to promote a Pt coating over the milled nanoparticles. The four electrocatalysts were dispersed on Vulcan carbon maintaining a metal:carbon (wt.%) ratio of 50:50 each one. The result was two type of systems: (M-Pt/C = Co100–Pt/C and Ni100–Pt/C) and (BM-Pt/C = Co30Ni70–Pt/C and Co70Ni30–Pt/C). All the electrocatalysts presented a final composition: ~10 wt%. of Pt, ~40 wt% of Co100-xNix, and ~50 wt% of Vulcan carbon. Physical characterization of the synthesized electrocatalysts involved XRD, STEM, AAS-ICP-MS and EDS-SEM studies confirming the formation of homogeneously-distributed metallic nanoparticles on the carbon. The electrochemical evaluation, through cyclic voltammetry and steady-state polarization curves using rotating disk electrode technique, was performed for the four synthesized electrocatalysts and for a commercial platinum-loaded carbon black (Pt Etek 20 wt%) catalysts. All four synthesized electrocatalysts were electroactive for ORR. Moreover, the Ni100–Pt/C electrocatalyst presented the highest mass activity whereas Co30Ni70–Pt/C electrocatalyst showed the major stability after 3000 cycles of accelerated degradation test than the others. Two membrane electrode assemblies (MEAs) for PEM fuel cell were fabricated with the synthesized Ni100–Pt/C and Co30Ni70–Pt/C electrocatalysts used as cathodes. Results under single-fuel cell operation showed the same performance trend for those materials that observed under electrochemical glass cell conditions.

Impact of Fe(III) on the Autocatalytic Processes of Corrosion of Stainless Steels in Concentrated Nitric Acid

This study takes place in the industrial context of the spent nuclear fuel reprocessing plants, in which hot and concentrated nitric acid is used in the process to dissolve nuclear fuel. This imply a high oxidative solution, due to nitric acid but also to species resulting from the dissolution of the fuel and/or the equipment. Especially where there is stagnant solution in which different nitrogenous species can accumulate, causing the autocatalytic reduction reactions of nitric acid. These mechanisms can be responsible of the corrosion processes of the equipment. Then, in order to have resistant equipment to this acidic and oxidizing medium, stainless steels were chosen. The purpose of this work is to investigate the corrosion mechanisms of stainless steels in such aggressive medium. Particularly, it was established that the presence of Fe(III) (corrosion product) accelerates the corrosion of stainless steels, due to the rise of the cathodic reactions [1, 2]. Thus, this work was focused on the specific impact of Fe(III) on the corrosion processes, including autocatalytic reduction reactions of nitric acid. Indeed, it was not clear if this acceleration of the cathodic processes was induced by a direct reduction of Fe(III) to Fe(II) at the surface of the material or if it was an indirect impact of Fe(III) by a catalyse of the nitric acid reduction process (involving reduced species of nitric acid such as HNO2, NO2 and NO) (Figure 1). In order to investigate this point, we coupled stationary electrochemical techniques (chronoamperometry, steady-state polarisation curves) or dynamic electrochemical techniques (electrochemical impedance spectroscopy) with local analytical techniques (RAMAN microspectroscopy, Scanning ElectroChemical Microscope). These studies were completed with the analysis of the gaseous products of the reactions (NO2 and NO production) by mass and IR spectroscopies. It was shown that the direct reduction of Fe(III) to Fe(II) at the surface of stainless steel has a minor contribution to the corrosion. Thus, the role of Fe(III) is essentially related to its catalytic effect on the nitric acid reduction reaction (Figure 1). From the different measurements performed, the following mechanism of interaction of Fe(III) with the nitric acid reduction was proposed (Figure 1) and the kinetic constants of some elementary steps were determined. [1] Fauvet, P., et al. (2008). "Corrosion mechanisms of austenitic stainless steels in nitric media used in reprocessing plants." Journal of Nuclear Materials 375(1): 52. [2] Fauvet, P. (2012). Corrosion issues in nuclear fuel reprocessing plants. Nuclear corrosion science and engineering. D. Feron, Woodhead Publishing. 22: 679-728. [3] Lange, R., Phénomènes de couplage acier 304L – platinoïdes dans les milieux de dissolution des combustibles usés. 2012, UNIVERSITÉ PIERRE ET MARIE CURIE. p. 174. Figure 1

Porous Ni Photocathodes Obtained by Selective Corrosion of Ni-Cu Films: Synthesis and Photoelectrochemical Characterization

In this work, a dealloying technique is proposed as a synthesis method to obtain highly porous Nickel electrodes starting from Ni-Cu co-deposit: pulsed corrosion is applied adopting different corrosion and relaxation times. Different morphologies, pore size distribution and residual copper amount were obtained depending on the corrosion conditions. For the developed electrodes, the surface roughness factor, Rf, was evaluated by electrochemical impedance spectroscopy (EIS). The hydrogen evolution reaction (HER) on these electrodes was evaluated by means of steady-state polarization curves, and the related parameters were derived by Tafel analysis. Finally, a thin layer of NiO on the porous structures was obtained to exploit the semiconductor characteristic of the oxide, so that an extra-photopotential was obtained by the simulated solar light action. Results demonstrate greater apparent activity of the developed electrodes towards HER in comparison with commercial smooth Ni electrode, which can be mainly attributed to the large Rf obtained with the proposed technique.

Redox mediation at poly(o-aminophenol) coated electrodes: mechanistic diagnosis from steady state polarization curves

In this work, the mediated reduction of Fe(CN)63- and Fe3+ in poly(o-aminophenol) coated electrodes is analyzed by means of a diagnosis diagram based on the features of the steady state current-potential curves. This analysis allows to identify the current determining process and to reproduce the experimental characteristics of the polarization curve from the relevant kinetic and thermodynamic parameters with a minimum amount of experimental measurements.

Investigation of Earth-Abundant Oxygen Reduction Electrocatalysts for the Cathode of Passive Air-Breathing Direct Formate Fuel Cells

The development of direct formate fuel cells encounters important obstacles related to the sluggish oxygen reduction reaction (ORR) and low tolerance to formate ions in Pt-based cathodes. In this study, electrocatalysts formed by earth-abundant elements were synthesized, and their activity and selectivity for the ORR were tested in alkaline electrolyte. The results showed that carbon-encapsulated iron-cobalt alloy nanoparticles and carbon-supported metal nitrides, characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD), do not present significant activity for the ORR, showing the same half-wave potential of Vulcan carbon. Contrarily, nitrogen-doped carbon, synthesized using imidazole as the nitrogen source, showed an increase in the half-wave potential, evidencing an influential role of nitrogen in the ORR electrocatalysis. The synthesis with the combination of Vulcan, imidazole, and iron or cobalt precursors resulted in the formation of nitrogen-coordinated iron (or cobalt) moieties, inserted in a carbon matrix, as revealed by X-ray absorption spectroscopy (XAS). Steady-state polarization curves for the ORR evidenced a synergistic effect between Fe and Co when these two metals were included in the synthesis (FeCo-N-C material), showing higher activity and higher limiting current density than the materials prepared only with Fe or Co. The FeCo-N-C material presented not only the highest activity for the ORR (approaching that of the state-of-the-art Pt/C) but also high tolerance to the presence of formate ions in the electrolyte. In addition, measurements with FeCo-N-C in the cathode of an passive air-breathing direct formate fuel cells, (natural diffusion of formate), showed peak power densities of 15.5 and 10.5 mW cm−2 using hydroxide and carbonate-based electrolytes, respectively, and high stability over 120 h of operation.

Understanding how the oxygen evolution reaction kinetics influences electrochemical wastewater oxidation

The electrochemical oxidation of organic species in wastewater often occurs in parallel with the oxygen evolution reaction. At electrocatalytic anodes (e.g.IrO2), the organic species is oxidized by the surface bound oxygen species, S–O, an intermediate of the oxygen evolution reaction. Previously, the rate of organic oxidation by the S–O species has been assumed to be much faster than the combination of S–O to form O2, but here a model is developed without this assumption and solved numerically for a wide range of rate constants to determine steady-state polarisation curves for oxygen evolution and the rate of the oxidation of organics. Experimental oxygen evolution polarisation curves measured on IrO2 anodes are fit to a model for the electrochemical oxide pathway to understand how the kinetics of the individual steps in the oxygen evolution reaction influence the rate of oxidation of organics. Multiple sets of rate constants describe the same steady-state polarisation curve with unique surface oxide coverage vs. potential behaviour. While this complicates the extraction of rate constants from oxygen evolution polarisation curves, it enables the study of influence of S–OH and S–O surface coverages on the organic oxidation rate while keeping the overall oxygen evolution kinetics constant. By comparing the organic oxidation over a range of kinetic parameters, it can be concluded that the organic oxidation reaction only alters the current density when the organic oxidation rate constant is many times larger than the rate constant for the competing step in the oxygen evolution reaction. Our model also shows that organics can be oxidized with by molecular O2 via an S–O mediated process, although as this only occurs when the organic oxidation rate constant is very large and at low overpotentials where the specific oxidation rate is very low. The model also shows that organic oxidation reaction is almost entirely kinetically limited at low overpotentials and only becomes mass transport limited when the rate constant for the organic oxidation reaction is extremely large. The model is validated by comparing simulations against data for 4-nitrophenol oxidation at a 90 wt% SnO2-Sb2O5- 10 wt% IrO2 anode and is shown to correctly describe the potential dependence of the apparent first-order rate constant. From this, we conclude that only one type of surface coverage behaviour can describe the experimental results and that the specific rate constant for 4-nitrophenol oxidation on this anode is approximately 2.3 × 104 cm3 mol−1 s−1.

Electrocatalytic Activity of Amorphous Ni-Nb-Y Alloys for the HER in Alkaline Water Electrolysis

Introduction: The recent development of anion exchange membranes (AEM) has renewed interest in alkaline water electrolysis for large-scale production of hydrogen gas. These systems can achieve current densities comparable to PEM-based systems while enabling the use of non-PGMs as catalyst materials. New electrocatalysts are required to interface with such AEMs in advanced alkaline water electrolyser designs. Amorphous alloys have previously been proposed as electrocatalysts due to their unique atomic structure, which results in electrochemical properties particularly suitable for catalysis. These properties include a high density of co-ordinatively unsaturated sites, a much greater solute solubility allowing for highly tunable homogeneous compositions, and a high stability against surface segregation of solute atoms [1-3]. To date, amorphous alloys have been underutilized as catalysts due to the lack of suitable methods to increase their electrochemically active surface area. Mechanical alloying and subsequent dealloying can be used to overcome this issue to yield 3D mesoporous and nanoporous amorphous structures. The present work assesses the role of structure and chemistry on the catalytic activity of Ni-Nb-Y amorphous alloys towards the hydrogen evolution reaction. Ni-based alloys exhibit some of the highest activity among the non-PGM catalysts while yttrium and niobium are excellent glass formers and offer interesting catalytic performance, particularly yttrium which has shown high catalytic activity when alloyed with nickel [4, 5]. Experimental Methods: Cryogenic ball milling of elemental powders was used to synthesize a novel high surface area amorphous Ni79.2Nb12.5Y8.3 alloy. Composite amorphous alloys with minor secondary crystalline phases (Ni5Y and Y2O3) were also synthesized and evaluated. X-ray diffraction and electron microscopy were used to analyze the alloy structures and particle morphology. Working electrodes were prepared by synthesizing catalyst suspensions consisting of the alloyed powder along with Nafion® 117 solution and iso-propanol and drop casting said suspensions on glassy carbon electrodes. Electrochemical characterization was assessed through steady state polarization (Tafel) curves, cyclic voltammetry, and AC impedance spectroscopy. Results and Discussion: The Ni79.2Nb12.5Y8.3 amorphous alloy exhibited improved intrinsic catalytic performance compared to nickel, and the addition of minor secondary crystalline phases, namely Ni5Y, resulted in even greater intrinsic performance. Electrochemical testing revealed this trend to be consistent even when testing amorphous composite materials containing higher (Ni81.3Nb6.3Y12.5) and lower (Ni77.1Nb18.8Y4.2) Y content. Tafel measurements for Ni-Nb-Y alloys can be seen in figure 1 with corresponding data summarized in table 1. These alloys highlight the improved catalytic performance to be a result of the secondary Ni5Y phase as opposed to the total alloying content. These results warrant further electrochemical characterization of multiphase amorphous nickel alloys with finely dispersed crystallinity for catalyst development. Conclusions and Future Work: The mechanical alloying process can be used to produce highly tunable chemical compositions where then the addition of finely dispersed secondary crystalline phases result in improved catalytic activity. The preliminary electrochemical results illustrate that mechanically alloyed Ni-based amorphous materials are promising catalyst precursors for clean electrochemical hydrogen production.