Electrochemical Oxygen Reduction Research Articles

Efficient Electrochemical Oxygen Reduction to Hydrogen Peroxide Production from Nickel-Nitrogen-Doped Mesoporous Graphene Catalyst

Hydrogen peroxide(H2O2) is an excellent oxidizing agent with various uses in industrial bleaching, chemical synthesis, and wastewater treatment. Because of its wide range of applications, the global market demand is rapidly increasing. At present, over 95 % of H2O2 is produced through the anthraquinone process which is an environmentally unfriendly process. Therefore, numerous studies have focused on replacing this process for H2O2 with more eco-friendly processes. One of the alternative methods is the electrochemical production of H2O2 through oxygen reduction reaction(ORR), which offers an economical and environmentally friendly route to H2O2.The oxygen reduction reaction involves a multielectron transfer process in which O2 can be reduced to produce H2O in a four-electron process or undergo a two-electron pathway to form H2O2. However, the design of selective and active electrocatalysts remains a challenge due to competitive reaction pathways between two-electron and four-electron reduction.We develop 3D mesoporous graphene with high H2O2 production in neutral electrolytes by adjusting the pore size and regulating the atmosphere of Nickel single atom. This catalyst has 3D interconnected hollow N-doped graphene shells and Nickel single atoms will be atomically dispersed on N-doped graphene shells. The mesoporous structure with a large surface area accelerates mass transport. Moreover, electron-poor Nickel atoms yield high selectivity above 90 % and high activity for electrochemical oxygen reduction to produce H2O2 in neutral conditions. This result represents a benchmark for the two-electron ORR electrocatalyst in the neutral conditions.

Insights on Oxygen Reduction Reaction to H2O2: The role of functional groups and textural properties on the activity and selectivity of doped carbon electrocatalysts

Hydrogen peroxide (H2O2) is a key molecule in chemical industrial processes and alterative processes and catalysts are needed to achieve a green in-situ generation. The electrochemical two-electron oxygen reduction (2e-ORR) reactions are one of the most attractive routes along with the water oxidation or the direct synthesis. Here, we focused on the 2e-ORR, summarizing the recent advances in the finalization of catalysts for specifically catalyse the generation of H2O2 and technical measures to improve material screening, especially to obtain reliable activity and selectivity data. Firstly, a brief overview of metal-free catalysts is given and secondly a focus on how different strategies are adopted to improve activity and selectivity is reported. Such strategies include to tune the thermodynamic energy barriers by changing the material composition/functionalization and/or to facilitate mass transfer for reactants and products by acting on morphology. Finally, a brief overview of challenges in ink and electrode preparation for the correct screening of H2O2 electrocatalysts is reported.

Mixing Ionic Liquids Affects the Kinetics and Thermodynamics of the Oxygen/Superoxide Redox Couple in the Context of Oxygen Sensing.

The electrochemical oxygen reduction reaction is vital for applications such as fuel cells, metal air batteries and for oxygen gas sensing. Oxygen undergoes a 1-electron reduction process in dry ionic liquids (ILs) to form the electrogenerated superoxide ion that is solvated and stabilized by IL cations. In this work, the oxygen/superoxide (O2/O2 •-) redox couple has been used to understand the effect of mixing ILs with different cations in the context of developing designer electrolytes for oxygen sensing, by employing cyclic voltammetry at both gold and platinum electrodes. Different cations with a range of sizes, geometries and aromatic/aliphatic character were studied with a common bis(trifluoromethylsulfonyl)imide ([NTf2]-) anion. Diethylmethylsulfonium ([S2,2,1]+), N-butyl-N-methylpyrrolidinum ([C4mpyrr]+) and tetradecyltrihexylphosphonium ([P14,6,6,6]+) cations were mixed with a common 1-butyl-3-methylimidazolium ([C4mim]+) cation at mole fractions (x) of [C4mim]+ of 0, 0.2, 0.4, 0.6, 0.8, and 1. Both the redox kinetics and thermodynamics were found to be highly dependent on the cation structure and the electrode material used. Large deviations from "ideal" mixtures were observed for mixtures of [C4mim][NTf2] with [C4mpyrr][NTf2] on gold electrodes, suggesting a much higher amount of [C4mim]+ ions near the electrode surface despite the large excess of [C4mpyrr]+ in the bulk. The electrical double layer structure was probed for a mixture of [C4mim]0.2[C4mpyrr]0.8[NTf2] using atomic force microscopy measurements on Au, revealing that the first layer was more like [C4mim][NTf2] than [C4mpyrr][NTf2]. Unusually fast kinetics for O2/O2 •- in mixtures of [C4mim]+ with [P14,6,6,6]+ were also observed in the electrochemistry results, which warrants further follow-up studies to elucidate this promising behavior. Overall, it is important to understand the effect on the kinetic and thermodynamic properties of electrochemical reactions when mixing solvents, to aid in the creation of designer electrolytes with favorable properties for their intended application.

NiFe layered double hydroxide as an efficient bifunctional catalyst for electrosynthesis of hydrogen peroxide and oxygen

Two electron oxygen reduction reaction to produce hydrogen peroxide (H2O2) is a promising alternative technique to the multistep and high energy consumption anthraquinone process. Herein, Ni–Fe layered double hydroxide (NiFe-LDH) has been firstly demonstrated as an efficient bifunctional catalyst to prepare H2O2 by electrochemical oxygen reduction (2e− ORR) and oxygen evolution reaction (OER). Significantly, the NiFe-LDH catalyst possesses a high faraday efficiency of 88.75% for H2O2 preparation in alkaline media. Moreover, the NiFe-LDH catalyst exhibits excellent OER electrocatalytic property with small overpotential of 210 mV at 10 mA cm−2 and high stability in 1 M KOH solution. On this basis, a new reactor has been designed to electrolyze oxygen and generate hydrogen peroxide. Under the ultra-low cell voltage of 1 V, the H2O2 yield reaches to 47.62 mmol gcat−1 h−1. In order to evaluate the application potential of the bifunctional NiFe-LDH catalyst for H2O2 preparation, a 1.5 V dry battery has been used as the power supply, and the output of H2O2 reaches to 83.90 mmol gcat−1 h−1. The excellent electrocatalytic properties of 2e− ORR and OER make NiFe-LDH a promising bifunctional electrocatalyst for future commercialization. Moreover, the well-designed 2e− ORR-OER reactor provides a new strategy for portable production of H2O2.

BaCO3 Nanoparticles-Modified Composite Cathode with Improved Electrochemical Oxygen Reduction Kinetics for High-Performing Ceramic Fuel Cells

The effects of the electrochemical oxygen reduction reaction (ORR) on the surface of single-phase perovskite cathodes are well understood, but its potential for use in a complex system consisting of different material types is unexplored. Herein, we report how BaCO3 nanoparticles-modified La0.6Sr0.4Co0.2Fe0.8O3-δ-Gd0.2Ce0.8O2-δ (LSCF–GDC)-composite cathodes improved the electrochemical oxygen reduction kinetics for high-performing ceramic fuel cells. Both X-ray diffraction (XRD) and thermogravimetric analysis (TGA) studies reveal that BaCO3 is stable, and that it does not show any solid-state reaction with LSCF–GDC at SOFCs’ required operating temperature. The electrochemical conductivity relaxation (ECR) study reveals that during the infiltration of BaCO3 nanoparticles into LSCF–GDC, the surface exchange kinetics (Kchem) are enhanced up to a factor of 26.73. The maximum power density of the NiO-YSZ anode-support cell is increased from 1.08 to 1.48 W/cm2 via surface modification at 750 °C. The modified cathode also shows an ultralow polarization resistance (Rp) of 0.027 Ω.cm2, which is ~4.4 times lower than that of the bare cathode (~0.12 Ω.cm2) at 750 °C. Such enhancement can be attributed to the accelerated oxygen surface exchange process, possibly through promoting the dissociation of oxygen molecules via the infiltration of BaCO3 nanoparticles. The density functional theory (DFT) illustrates the interaction mechanism between oxygen molecules and the BaCO3 surface.

Optimizing Catalysis by Combining Molecular Scaling Relationships: Iron Porphyrin-Catalyzed Electrochemical Oxygen Reduction as a Case Study

Optimizing Catalysis by Combining Molecular Scaling Relationships: Iron Porphyrin-Catalyzed Electrochemical Oxygen Reduction as a Case Study

Electrokinetic Insights into the Triple Ionic and Electronic Conductivity of a Novel Nanocomposite Functional Material for Protonic Ceramic Fuel Cells.

Triple ionic and electronic conductivity (TIEC) in cathode materials for protonic ceramic fuel cells (PCFCs) is a desirable feature that enhances the spatial expansion of active reaction sites for electrochemical oxygen reduction reaction. The realization of optimal TIEC in single-phase materials, however, is challenging. A facile route that facilitates the optimization of TIEC in PCFC cathodes is the strategic development of multiphase cathode materials. In this study, a cubic-rhombohedral TIEC nanocomposite material with the composition Ba(CeCo)0.4 (FeZr)0.1 O3- δ (BCCFZ) is designed via self-assembly engineering. The material consists of a mixed ionic and electronic conducting phase, BaCo1-( x + y + z ) Cex Fey Zrz O3- δ (M-BCCFZ), and a dominant proton-conducting phase, BaCe1-( x + y + z ) Cox Zry Fez O3- δ (H-BCCZF). The dominant cerium-rich H-BCCFZ phase enhances the material's oxygen vacancy concentration and the proton defects formation and transport with a low enthalpy of protonation of -30 ± 9kJ mol-1 . The area-specific resistance of the BCCFZ symmetrical cell is 0.089 Ω cm2 at 650 °C in 2.5% H2 O-air. The peak power density of the anode-supported single cell based on BCCFZ cathode reaches 1054mW cm-2 at 650 °C with good operation stability spanning over 500h at 550 °C. These promote BCCFZ as a befitting cathode material geared toward PCFC commercialization.

Electroreduction of oxygen on iron- and cobalt-containing nitrogen-doped carbon catalysts prepared from the rapeseed press cake

• Nitrogen, Co- and/or Fe-doped nanocarbons prepared from rapeseed press cake. • ORR activity of Fe-containing catalysts is close to that of Pt/C in 0.1 M KOH. • Inhibition of ORR by CN − ions shows the existence of MN x centres. • Catalysts show high methanol tolerance and good stability. • In AEMFC , P max = 131 mW cm −2 achieved with bimetallic catalyst. Transition metal-containing nitrogen-doped carbon catalysts for the electrochemical oxygen reduction reaction (ORR) were prepared from inexpensive biomass - rapeseed press cake; cobalt and/or iron salts and dicyandiamide were employed as metal and nitrogen sources. After acid treatment, the catalysts showed excellent electrocatalytic ORR activity in rotating disk electrode experiments in 0.1 M KOH solution, comparable to that of Pt/C (20 wt%) catalyst; good tolerance to methanol and high stability in short-time tests. The bimetallic catalyst displayed moderate performance as cathode material in anion exchange membrane fuel cell (AEMFC) test, by reaching the peak power density of 131 mW cm −2 .

Elucidation and modulation of active sites in holey graphene electrocatalysts for H2O2 production

AbstractSelective electrochemical oxygen reduction (ORR) toward a two‐electron (2e−) pathway is an eco‐friendly alternative method for H2O2 synthesis to replace the energy‐intensive anthraquinone oxidation process. Carbon‐based electrocatalysts (CBEs) show great potential for practical H2O2 synthesis. However, their complex structures make it challenging to determine the nature of active sites and to precisely control them. Herein, we show that precise modulation of the chemistry and structures of holey graphene with edge sites enriched by oxygen‐containing functional groups can facilitate 2e− ORR. These combined functionalities could improve ORR performance under various pH conditions, for example, resulting in an average of 95% H2O2 selectivity, ~97% Faraday efficiency, high productivity of 2360 mol kgcat−1 h−1 in alkaline media. Density functional theory calculations on the oxygen functional groups at the edge sites revealed the most active site for 2e− ORR is a synergy between ether (COC) and carbonyl (CO) functional groups with nearly zero overpotential.image

A novel trickle bed electrochemical reactor design for efficient hydrogen peroxide production

A specific and environmentally benign electrochemical reactor that combines the property of gas diffusion electrode (GDE) with trickling flow property in trickle bed electrochemical reactor (TBER) is designed for intensive hydrogen peroxide (H2O2) production. Compared with other reactors a significant feature of this reactor is the capability to uniformly distribute the oxygen gas all over the cathode bed avoiding oxygen gas transport limitations, electrode limited area, and continuous electrolyte leakage. This reactor consists of two plates as anodes and two porous layers composed of carbon black and polytetrafluoroethylene (C-PTFE) mixture that are pasted on two stainless steel meshes(SSM) to form cathode beds (C-PTFE-SSM). The cathodes are placed on both cathode frame sides which were designed to comprise two separated cathodes. The oxygen gas can flow easily through the space between cathodes and diffuses to the solid-liquid interface to be electrochemically reduced to form H2O2. H2O2 was in situ electrosynthesized in an alkaline solution by the oxygen electrochemical reduction on the cathode surface. All operating conditions that affect the process were systematically investigated. The highest concentration of H2O2 of 46.56 mM was achieved in 40 min and 1.0 V at room temperature, whereas at 0 °C the generated peroxide was 57.86 mM. This significant performance is attributed to the uniform oxygen distribution on the electrode bed making an easy access for gas transport to the electrolyte-cathode interface. The low temperature increases oxygen solubility which increases the production rate as well. Power consumption was also estimated and found to be 4 kWh/kg which is economically beneficial. At different applied cell voltages and best-operating conditions, a kinetic study was conducted for H2O2 electrosynthesized.

A novel chemical approach for synthesizing highly porous graphene analogue and its composite with Ag nanoparticles for efficient electrochemical oxygen reduction

Single-step phase transformation from amorphous to crystalline material avoiding drastic reaction condition is a novel approach in the synthetic and applied materials chemistry research. Both conjugated microporous polymers (CMPs) and graphene/graphene analogues are very demanding materials in the energy research. Depending upon the nature of the graphene material like amorphous, crystalline, functionalized, single-layered and multi-layered, its property and potential application can be largely tuned. In this perspective, 2D carbon allotropes like graphene and its analogues have attracted a huge attention due to their unique electrical, thermal, mechanical, and physico-chemical properties. On the other hand, CMPs are another class of porous nanomaterial synthesized through the polycondensation/coupling of reactive monomeric building units via bottom-up chemistry approach, where extended π-conjugation prevails in the entire organic framework. To the best of our knowledge, herein we first demonstrated a novel methodology for amorphous CMP to crystalline graphene analogue through single step chemical transformation avoiding drastic reaction conditions. For this purpose, we have synthesized a new pyrene-naphthalene based spherical amorphous conjugated microporous polymer, designated as pyrene-naphthalene CMP (PNC), which upon chemical treatment (oxidative C-C coupling reaction) transforms into a highly porous crystalline multilayer porous nanographene sheet-like material pyrene-naphthalene-graphene (PNG). The BET surface area has been drastically enhanced from 600 m2/g for the CMP to 1414 m2/g for this graphene analogue PNG. As analysed by scanning tunnelling spectroscopy (STS), the PNG material has showed atomic level graphitic images with almost zero bandgap, which suggested highly conductive nature for this crystalline graphene analogue. This material has been further explored in the electrochemical oxygen reduction reaction (ORR) by grafting silver nanoparticles (AgNPs) onto the interlayer nanosheets of PNG. The ORR results displayed an admirable half-wave potential (E1/2) and onset potential of 0.78 V and 0.984 V, respectively. The electrocatalyst achieved a very high current density of −0.53 mA cm−2 under the basic electrolyte medium (0.1 M KOH). An efficient ORR catalyst is very demanding towards the fabrication of metal-air batteries as well as fuel cells, which are promising for clean and green energy production.

Electrochemical oxygen reduction reaction using carbon nanotubes–graphene–manganese salen composite material

Electrochemical oxygen reduction reaction using carbon nanotubes–graphene–manganese salen composite material

Constructing Interfacial Boron‐Nitrogen Moieties in Turbostratic Carbon for Electrochemical Hydrogen Peroxide Production

AbstractThe electrochemical oxygen reduction reaction (ORR) provides a green route for decentralized H2O2 synthesis, where a structure–selectivity relationship is pivotal for the control of a highly selective and active two‐electron pathway. Here, we report the fabrication of a boron and nitrogen co‐doped turbostratic carbon catalyst with tunable B−N−C configurations (CNB‐ZIL) by the assistance of a zwitterionic liquid (ZIL) for electrochemical hydrogen peroxide production. Combined spectroscopic analysis reveals a fine tailored B−N moiety in CNB‐ZIL, where interfacial B−N species in a homogeneous distribution tend to segregate into hexagonal boron nitride domains at higher pyrolysis temperatures. Based on the experimental observations, a correlation between the interfacial B−N moieties and HO2− selectivity is established. The CNB‐ZIL electrocatalysts with optimal interfacial B−N moieties exhibit a high HO2− selectivity with small overpotentials in alkaline media, giving a HO2− yield of ≈1787 mmol gcatalyst−1 h−1 at −1.4 V in a flow‐cell reactor.

Constructing Interfacial Boron-Nitrogen Moieties in Turbostratic Carbon for Electrochemical Hydrogen Peroxide Production.

The electrochemical oxygen reduction reaction (ORR) provides a green route for decentralized H2O2 synthesis, where a structure–selectivity relationship is pivotal for the control of a highly selective and active two‐electron pathway. Here, we report the fabrication of a boron and nitrogen co‐doped turbostratic carbon catalyst with tunable B−N−C configurations (CNB‐ZIL) by the assistance of a zwitterionic liquid (ZIL) for electrochemical hydrogen peroxide production. Combined spectroscopic analysis reveals a fine tailored B−N moiety in CNB‐ZIL, where interfacial B−N species in a homogeneous distribution tend to segregate into hexagonal boron nitride domains at higher pyrolysis temperatures. Based on the experimental observations, a correlation between the interfacial B−N moieties and HO2 − selectivity is established. The CNB‐ZIL electrocatalysts with optimal interfacial B−N moieties exhibit a high HO2 − selectivity with small overpotentials in alkaline media, giving a HO2 − yield of ≈1787 mmol gcatalyst −1 h−1 at −1.4 V in a flow‐cell reactor.

Second-Sphere Hydrogen-Bond Donors and Acceptors Affect the Rate and Selectivity of Electrochemical Oxygen Reduction by Iron Porphyrins Differently.

The factors that control the rate and selectivity of 4e-/4H+ O2 reduction are important for efficient energy transformation as well as for understanding the terminal step of respiration in aerobic organisms. Inspired by the design of naturally occurring enzymes which are efficient catalysts for O2 and H2O2 reduction, several artificial systems have been generated where different second-sphere residues have been installed to enhance the rate and efficiency of the 4e-/4H+ O2 reduction. These include hydrogen-bonding residues like amines, carboxylates, ethers, amides, phenols, etc. In some cases, improvements in the catalysis were recorded, whereas in some cases improvements were marginal or nonexistent. In this work, we use an iron porphyrin complex with pendant 1,10-phenanthroline residues which show a pH-dependent variation of the rate of the electrochemical O2 reduction reaction (ORR) over 2 orders of magnitude. In-situ surface-enhanced resonance Raman spectroscopy reveals the presence of different intermediates at different pH's reflecting different rate-determining steps at different pH's. These data in conjunction with density functional theory calculations reveal that when the distal 1,10-phenanthroline is neutral it acts as a hydrogen-bond acceptor which stabilizes H2O (product) binding to the active FeII state and retards the reaction. However, when the 1,10-phenanthroline is protonated, it acts as a hydrogen-bond donor which enhances O2 reduction by stabilizing FeIII-O2.- and FeIII-OOH intermediates and activating the O-O bond for cleavage. On the basis of these data, general guidelines for controlling the different possible rate-determining steps in the complex multistep 4e-/4H+ ORR are developed and a bioinspired principle-based design of an efficient electrochemical ORR is presented.

Synergistic Effects in N,O-Comodified Carbon Nanotubes Boost Highly Selective Electrochemical Oxygen Reduction to H2 O2.

Electrochemical 2‐electron oxygen reduction reaction (ORR) is a promising route for renewable and on‐site H2O2 production. Oxygen‐rich carbon nanotubes have been demonstrated their high selectivity (≈80%), yet tailoring the composition and structure of carbon nanotubes to further enhance the selectivity and widen working voltage range remains a challenge. Herein, combining formamide condensation coating and mild temperature calcination, a nitrogen and oxygen comodified carbon nanotubes (N,O‐CNTs) electrocatalyst is synthesized, which shows excellent selective (>95%) H2O2 selectivity in a wide voltage range (from 0 to 0.65 V versus reversible hydrogen electrode). It is significantly superior to the corresponding selectivity values of CNTs (≈50% in 0–0.65 V vs RHE) and O‐CNTs (≈80% in 0.3–0.65 V vs RHE). Density functional theory calculations revealed that the C neighbouring to N is the active site. Introducing O‐related species can strengthen the adsorption of intermediates *OOH, while N‐doping can weaken the adsorption of in situ generated *O and optimize the *OOH adsorption energy, thus improving the 2‐electron pathway. With optimized N,O‐CNTs catalysts, a Janus electrode is designed by adjusting the asymmetric wettability to achieve H2O2 productivity of 264.8 mol kgcat –1 h–1.

An In-Depth Exploration of the Electrochemical Oxygen Reduction Reaction (ORR) Phenomenon on Carbon-Based Catalysts in Alkaline and Acidic Mediums

Detailed studies of the electrochemical oxygen reduction reaction (ORR) on catalyst materials are crucial to improving the performance of different electrochemical energy conversion and storage systems (e.g., fuel cells and batteries), as well as numerous chemical synthesis processes. In the effort to reduce the loading of expensive platinum group metal (PGM)-based catalysts for ORR in the electrochemical systems, many carbon-based catalysts have already shown promising results and numerous investigations on those catalysts are in progress. Most of these studies show the catalyst materials’ ORR performance as current density data obtained through the rotating disk electrode (RDE), rotating ring-disk electrode (RRDE) experiments taking cyclic voltammograms (CV) or linear sweep voltammograms (LSV) approaches. However, the provided descriptions or interpretations of those data curves are often ambiguous and recondite which can lead to an erroneous understanding of the ORR phenomenon in those specific systems and inaccurate characterization of the catalyst materials. In this paper, we presented a study of ORR on a newly developed carbon-based catalyst, the nitrogen-doped graphene/metal-organic framework (N-G/MOF), through RDE and RRDE experiments in both alkaline and acidic mediums, taking the LSV approach. The functions and crucial considerations for the different parts of the RDE/RRDE experiment such as the working electrode, reference electrode, counter electrode, electrolyte, and overall RDE/RRDE process are delineated which can serve as guidelines for the new researchers in this field. Experimentally obtained LSV curves’ shapes and their correlations with the possible ORR reaction pathways within the applied potential range are discussed in depth. We also demonstrated how the presence of hydrogen peroxide (H2O2), a possible intermediate of ORR, in the alkaline electrolyte and the concentration of acid in the acidic electrolyte can maneuver the ORR current density output in compliance with the possible ORR pathways.

Optimization of H2O2 production in a small-scale off-grid buffer layer flow cell equipped with Cobalt@N-doped graphitic carbon core–shell nanohybrid electrocatalyst

Electrochemical oxygen reduction (ORR) to hydrogen peroxide (H2O2) is emerging as a sustainable approach for the production of ‘green’ H2O2 requiring only oxygen and electricity compared to the energy intensive anthraquinone process. High 2e− selectivity is required in order to boost faradaic and energy efficiency (FE) of the process. Upon correct tuning of their properties, nitrogen-doped carbon materials are excellent candidates as electrocatalyst for H2O2 electrosynthesis due to their chemical and electrochemical resistance and 2e− selectivity. Furthermore, careful cell design and parameter optimization are mandatory for an industrial scale up of the process. In this study, a Cobalt@N-doped graphitic carbon core–shell nanohybrid (CS(Co)-N-GC) electrocatalyst was studied in a buffer layer complete cell equipped with a proton exchange membrane in order to determine the effect of flow rate and potential on process selectivity and energy efficiency. After optimization, the cell was able to produce 0.5 wt% H2O2 with an average FE higher than 40%, an energy consumption lower than 8 kWh/kgH2O2 and a production rate of 1.2 g/h gcat @ 0.3 V vs RHE with the possibility to produce up to 1 wt% H2O2.

Mathematical Model of Hydrogen Peroxide Production in Anode, Cathode, and Membrane of LT-PEMFC

Degradation of proton exchange membrane in low-temperature fuel cells represents one of the main limiting factors for wider adoption of this clean, carbon emission free energy device. In combination with mechanical degradation caused by membrane swelling and shrinkage during water content change, chemical degradation of the membrane is considered to be the main mechanism leading to loss of membrane conductivity, membrane thinning and eventual pinhole formation.Chemical degradation is caused by the attack of reactive radical species on the perfluorinated polymer chains of the membrane, which are formed in Fenton reaction between hydrogen peroxide and ions of transition metals, which are inevitably present in the membrane in small traces. Since the metal ions are recycled in Fenton reactions, the rate of chemical degradation is mainly determined by the production of hydrogen peroxide, formed as a side product to water in reaction between oxygen and protons in the fuel cell. [1]The source of hydrogen peroxide production in low-temperature fuel cells have long been debated in scientific literature. On one hand, the abundance of oxygen in the cathode suggest that the production takes place there, but high local electric potential strongly promotes 4-electron reaction, forming water, over 2-electron reaction required for production of hydrogen peroxide [2]. The conditions in the anode are reversed, low electric potential is well suited for peroxide production, but the concentration of oxygen, present in the anode due to diffusion through the membrane, is in general expected to be quite low, making it a rate limiting factor for the peroxide production in the anode. The last possible source of hydrogen peroxide is closely related to the degradation of platinum (Pt) catalyst in the cathode. When Pt catalyst nanoparticles dissolve due to high local electric potential on the cathode, part of the dissolved ions diffuses into the membrane, where it is reduced by counter-diffusing hydrogen from the anode. These Pt particles in the membrane, the so-called Pt band, serve as a catalyst for the reaction between diffusing oxygen from cathode and hydrogen in anode. Since these particles are not electrically connected to either anode or cathode, their electric potential is determined by the rates of oxygen reduction and hydrogen oxidation on their surface, which result in production of water and potentially also hydrogen peroxide.The complex interplay between the processes determining the peroxide production, explained above, suggest that different sources of peroxide might be relevant in different fuel cell operation conditions and at different states of health of the fuel cell. To explore this question quantitatively, we propose a mathematical model of peroxide production which describes physical and electrochemical processes for hydrogen peroxide production in fuel cell catalysts and the membrane, relevant for hydrogen peroxide production: oxygen and hydrogen diffusion in the membrane, electrochemical oxygen reduction in the cathode, anode and on Pt band inside membrane, hydrogen oxidation on Pt band and electric charging of Pt particles in the band due to electrochemical reactions on their surface. To provide realistic internal states of the fuel cell during operation, the model is coupled with advanced spatially and temporally resolved model of the fuel cell operation. [3]Preliminary results of the model indicate that main source of peroxide production depends on the fuel cell operating conditions. In fresh fuel cell, where Pt band in membrane is not yet formed, low current densities and high fuel cell voltages promote the peroxide formation mainly on the anode from the diffused oxygen, while at high current densities the electric potential on the cathode is low enough to allow for significant peroxide production there, outweighing the production on the anode. The formation of the Pt band shifts the production of peroxide from anode to the Pt particles in the band, since large amount of the oxygen is consumed there and therefore its diffusion to the anode is reduced.The results indicate that the question of where the peroxide is formed cannot be resolved by a single answer and that the use of sufficiently complex models, coupling physical and electrochemical processes in different fuel cell components, are required to properly manage fuel cell operation in order to avoid or at least mitigate its detrimental effects of chemical membrane degradation.[1] Frühwirt, P., Kregar, A., Törring, J. T., Katrašnik, T., Gescheidt, G. (2020). Physical Chemistry Chemical Physics, 22(10), 5647–5666.[2] Sethuraman, V. A., Weidner, J. W., Haug, A. T., Motupally, S., Protsailo, L. V. (2008). Journal of The Electrochemical Society, 155(1), B50.[3] Kregar, A., Tavčar, G., Kravos, A., Katrašnik, T. (2020). Applied Energy, 263(March), 114547.

(Digital Presentation) A Mathematical Model for Localized Corrosion of Copper Under a Droplet

The Nuclear Waste Management Organization (NWMO) is responsible for the implementation of Adaptive Phased Management (APM), the federally-approved plan for the safe long-term management of Canada’s used nuclear fuel.1 Under APM, used nuclear fuel will ultimately be placed within a deep geological repository (DGR) in a suitable host rock formation. Part of evaluating the long-term performance and safety of the repository system is understanding the behavior of the copper coated container with respect to localized corrosion. Even though localized corrosion is not expected in the DGR design system, there is still a need to develop a mathematical model for localized corrosion of copper.The presentation will describe the development of a 2-D axisymmetric time-dependent model for the localized corrosion of copper under a drop of electrolyte using the concept of an Evans droplet.2 The model accounts for two reversible electrochemical reactions on the metal surface (copper dissolution/plating and hydrogen (H2) evolution/reduction) and two irreversible electrochemical reactions (cupric ion (Cu2+) reduction and oxygen (O2) reduction). Dissolved cuprous ion (Cu+) reacts in a second chemical step to form the cuprous chloride (CuCl) salt film treated as a diffusion barrier, or it reacts to form the cuprous oxide (Cu2O) film that partially blocks the active area on the metal surface. The model also accounts for a series of homogeneous reactions involving copper chloride complex, copper hydroxide complex, copper carbonate complex, and superoxide species. The associated equilibrium constants were obtained from PHREEQC thermodynamic software.3 The model includes a total of 23 dependent spatial-temporal variables.The mathematical model is developed using the finite-element method (COMSOL Multiphysics). The model includes coupled nonlinear conservation equations for ionic species, which include the contribution of diffusion, migration, local electroneutrality, and homogeneous reactions. The anodic and cathodic regions are not predefined but are rather determined by values of local concentration and potential from the simulation results. The surface coverage of film was calculated implicitly and expressed in terms of the thickness in units of monolayers. The model also accounted for the effect of porous CuCl film thickness on the surface oxygen concentration and potential applied on electrochemical reactions.In this preliminary study, two extreme conditions were chosen such that the droplet boundary could either have an infinite supply of oxygen or it could have no external supply of oxygen for an initially air-saturated droplet. The results show a 3-year time-dependent radial distribution of anodic and cathodic current density, surface coverage of CuCl film and Cu2O film, and localized corrosion rates and depths. The results include a distribution of pH, potential, and concentrations of dissolved gaseous and ionic species through the entire droplet. The potential change as the system transitions from aerobic to anaerobic conditions matches the expected equilibrium potential difference.4 The model could be used to explore the influence of different parameters on the localized corrosion rate and depth, film thickness and growth, and each individual current profiles. The parameters may be adjusted to conform to the DGR environment.References NWMO, “Choosing a Way Forward. The Future Management of Canada’s Used Nuclear Fuel. Final Study,” Nuclear Waste Management Organization, Toronto, Ontario, 2005.R. Evans, The Corrosion of Metals, E. Arnold & Company, London, 1926.PHREEQC Version 3, United States Geological Survey, 202, https://www.usgs.gov/software/phreeqc-version-3Cleveland, M. E. Orazem, and S. Moghaddam, “Response to “Comment on “Nanometer-scale corrosion of copper in de-aerated deionized water” [J. Electrochem. Soc., 161, C107 (2014)],” Journal of the Electrochemical Society, 163 (2016), Y5-Y11. AcknowledgementThis work was supported by the Nuclear Waste Management Organization, Canada, under project 2000904.