Rotating Disk Electrode Experiments Research Articles

Relating Solvent Parameters to Electrochemical Properties to Predict the Electrochemical Performance of Vanadium Acetylacetonate for Non-Aqueous Redox Flow Batteries

Non-aqueous redox flow batteries have shown promise for applications in grid energy storage. Increasing the efficiency of these batteries by developing the electrolyte chemistries is needed. Herein, we investigate the correlation between solvent properties and the electrochemical parameters of vanadium acetylacetonate V(acac)3. Using cyclic voltammetry (CV) and rotating disk electrode experiments (RDE), we show that trends in the performance of the V(acac)3 kinetics are directly related to solvent properties. We found strong relationships between the solvents polarity, viscosity, and donor number with the electrochemical behavior of V(acac)3 in terms of the electrochemical working widow, electron kinetics and stability towards cycling. Based on these finding, we also demonstrate how solvent selection can be improved with limited a priori knowledge.

Biomimetic nucleotide-graphene hybrids for electrocatalytic oxygen conversion: Quantifying biomolecule mass loading

Metal-free electrocatalysts for the electrochemical conversion of gases constitute an important asset for a sustainable energy transition. Nucleotides act as redox mediators in the electron transport chain to reduce oxygen in cellular respiration. The biomimicry of such an efficient natural mechanism could be utilized to address the challenges associated with electrochemical gas conversion technologies, such as sluggish kinetics and high overpotentials. Multiple descriptors are generally reported to benchmark the activity of electrocatalysts where the turnover frequency (TOF) is claimed to be the most accurate criterion. Here, a library of graphene nanosheets-nucleotide hybrid materials was prepared, and the electrocatalytic performance towards ORR/OER reactions of a graphene-flavin mononucleotide hybrid was evaluated by rotating disc electrode experiments and TOF estimation. The determination of catalyst loading and dispersion is especially relevant when assessing the intrinsic activity of a catalyst and, therefore, the amount of nucleotide electrocatalyst loaded into the graphene support was thoroughly quantified by a combination of characterization techniques. Density functional theory calculations supported the observed experimental trends, both on the adsorption rate of a given nucleotide on graphene and the catalytic activity of a specific hybrid material. This work constitutes an avenue to predict nature-mimicking electrocatalysts for efficient energy storage.

Fe-N-C Oxygen Reduction Catalysts Via Chemical Vapor Deposition in Fluidized Bed Reactor

Metal nitrogen carbon catalysts (MNC) are an attractive substitute for platinum-based catalysts in the oxygen reduction reaction (ORR) in PEM fuel cells. However, state-of-the-art MNC catalysts still suffer from low activity and instability. Li et al. [1] demonstrated the use of zinc imidazolate frameworks (ZIF-8) in chemical vapor deposition (CVD) to form active sites without unwanted particle formation or additional treatment steps. In this method, gaseous iron chloride exchanges the zinc in the precatalyst to form active sites.In this work, we introduce a vertical fluidized bed reactor (FBR) for high-scale synthesis of FeNC via CVD, allowing for uniform dispersion of the precatalyst and better zinc-iron exchange. Furthermore, we investigated the influence of differently sized ZIF-8 on the CVD and the formation of the FeNC catalyst.The final catalysts were characterized for their electrochemical activity in a rotating ring disc setup, their site density via CO-chemisorption, and their respective turnover frequency. The physicochemical characterization includes the analysis of surface area and pore structure via nitrogen physisorption, SEM, and TEM, as well as surface structure and compositional analysis via elemental analysis, ICP-OES, and XPS.The exchange of Zn was in the range of 90%, resulting in an iron content of 3.8 wt% with only the formation of desired Fe-Nx active sites. STEM-EDX imaging shows an even distribution of iron and nitrogen in the catalyst, with mean particle sizes of 110 nm. In rotating disc electrode experiments with 0.5 M sulfuric acid, mass activities of 0.23 A/g at 0.9 V vs RHE were achieved.Commercially nano-scaled ZIF-8 with mean particle sizes of 500 nm, synthesized particles with mean sizes of 110 nm were tested, compared to the commercial ZIF-8 Basolite Z1200 with mean particle sizes of 4.9 µm. We could show improved activity in RRDE of the nano-scaled ZIF-8 materials compared to the macro-scaled Basolite.Applying this highly scalable synthesis method for MNC catalysts could be a viable way to achieve high-performance catalysts for ORR.[1] Li Jiao, Jingkun Li, Lynne Larochelle Richard, Qiang Sun, Thomas Stracensky, et al.. Chemical vapour deposition of Fe–N–C oxygen reduction catalysts with full utilization of dense Fe–N4 sites. Nature Materials, 2021, 20, pp.1385-1391.

Study of Alternative Binders for Battery Electrode in Aqueous System

Materials used to prepare energy storage devices need to be affordable, durable, and safe for environment. PVDF, which is the most popular polymer used as a binder in battery electrodes, requires the use of and toxic solvents for processing, such as N-methyl-pyrrolidone (NMP).1 Water insoluble organic polymers could replace PVDF as the binder in aqueous batteries or redox flow batteries with solid boosters.2 For example, ethyl cellulose is a biodegradable and water-insoluble polymer. Ethyl cellulose-based slurry can be prepared using ethanol as solvent, which makes the process cheaper and more sustainable. Also cross-linked gluten could be used as a binder. In that case water-based slurry could be used to prepare the electrode. Replacing PVDF with more environmentally friendly binder would be beneficial in many ways. It would shorten the assembly procedure due to high speed of solvent evaporation, there are no strict requirements for humidity control and the recycling procedure is easier as there are no fluorinated compounds.3 Therefore, this replacement would enable to produce safer and cheaper energy storage devices.In this work positive electrodes with different composition have been described using galvanostatic charging-discharging and cyclic voltammetry. Various cell setups have been tested. LiMn2O4 (LMO) has been used as active material for cathode and carbon black Super P as conductive additive. PDVF has been compared with various organic biodegradable polymers as the binder. Using ethyl cellulose as a binder, discharge capacity 100 mAhg−1 was achieved at 0.2C rate for LMO cathode which is comparable to the results obtained with PVDF as a binder. 500 cycles at the rate of 1C showed somewhat better capacity retention for electrodes prepared using PVDF. Also rotating disk electrode experiments were conducted to study the effect of binder to the kinetics of the reaction. The results show that some biodegradable and cheaper binders could be promising option to replace PVDF as binder in aqueous systems. This is highly important in aqueous redox flow battery with solid boosters, where large amount of solid charge storage material is used in tanks to increase the capacity of the system.2 Acknowledgements: The research was supported by the Estonian Research Council grant MOBTP1023, Archimedes Foundation project SLTKT16432T and by the EU through the European Regional Development Fund under project TK141 (2014–2020.4.01.15–0011).

The Role of Oxygen Reduction Reaction in Determining Pit Stability of FeCr Alloys

Pitting corrosion is one of the most dangerous forms of corrosion, leading to sudden catastrophic failures in engineering system. Once initiated, pit propagation rates are largely dependent on the magnitude of the supporting cathodic current available from the oxygen reduction reaction (ORR) occurring on the external passive film. Here the ORR kinetics on conventional and high Cr content FeCr alloys made by arc-melter were investigated by experimental and computational methods. DFT calculations suggested that the overpotentials for the ORR on FeCr oxides are in the order Cr2O3 > Fe3O4 > Fe2O3 > FeCr2O4, which was experimentally confirmed on the FeCr alloys by rotating disc electrode experiments in both O2 saturated 0.1 M NaOH and 3.5 wt% NaCl. Koutecký-Levich analysis demonstrated that ORR follows a 4-electron pathway on all surfaces investigated. Moreover, the investigation of the Pourbaix diagrams of FeCr revealed that at potentials where pitting corrosion initiates the main constituents of the passive films should be a mixture of Fe2O3 and Cr2O3, rather than FeCr2O4, which is proved to be a good ORR catalyst. The results support the postulation that one role Cr additions play in the prevention of pitting corrosion is to suppress the ORR reaction.

Valorization of Naturally Abundant Low-Value Peat into Highly Active Non-Platinum Group Metal Oxygen Reduction Catalysts

This work explores the feasibility of synthesizing active non-platinum group metal oxygen reduction catalysts from naturally abundant peat through a facile one-pot method. The procedure yielded highly porous materials (specific surface area ~1000 m2 g-1) with promising oxygen reduction activity. Onset and half-wave potential values of 0.93 and 0.83 V vs RHE, respectively, were attained in rotating disc electrode experiments for the most active catalyst synthesized using dicyandiamide. Notably, the choice of nitrogen precursor influenced both the porosity and the electrochemical activity. Finally, the best-performing catalyst was analyzed in a rotating ring disc electrode setup, where the results confirmed those obtained in rotating disc electrode measurements.

Enhancement of the Hydrogen Oxidation Reaction in Alkaline Media via a Bi-functional Effect Using Tantalum as an Oxophilic Agent on NiMo/C

To avoid the increase of cost owing to the use of Pt group metal (PGM) catalysts, anion-exchange membrane fuel cells (AEMFCs) operating in an alkaline electrolyte environment where non-precious metals can be used are promising. However, the performance limitations of PGM-free catalysts still need to be overcome. In this study, Ta-on-NiMo/C fabricated by adding Ta on the surface of commercial NiMo/C via physical vapor deposition was developed as an anodic catalyst for AEMFCs with advanced performance. The high oxophilicity of Ta ensures a continuous OH– supply, and the bifunctional effect between Ta and Ni accelerates the hydrogen oxidation reaction (HOR). Therefore, Ta-on-NiMo/C exhibited 1.5 times more HOR activity than bare NiMo/C in rotating disk electrode experiments. Furthermore, the effect of Ta as the oxophilic agent was demonstrated by verifying the enhanced performance in a membrane electrode assembly in which Ta-on-NiMo/C was applied as an anodic catalyst.

Tailoring MOF structure via iron decoration to enhance ORR in alkaline polymer electrolyte membrane fuel cells

Fe-N-C catalysts were synthesized by combining a Zn-based zeolitic imidazolate framework (ZIF-8) structure, adopted as a nitrogen-carbon template, with an iron salt and conductive carbon support followed by a thermal treatment. The effect of three different pyrolysis temperatures (700, 900, and 1000 °C) on Zn removal from ZIF-8 was investigated to enhance the formation of Fe-based moieties in the Nx-C groups during carbonization. Electrochemical characterization using a rotating ring disk electrode in an alkaline electrolyte demonstrated that ORR activity increased as the pyrolysis temperature increased. This trend can be ascribed to a more effective Zn removal and formation of high-active iron- and nitrogen-based catalytic sites, as pointed out by the Fe-N-C materials' chemical surface analysis after the pyrolysis step. The sample Fe-N-C-1000 demonstrated a remarkable ORR activity, even higher than Pt/C taken as reference.When subjected to accelerated stress tests, the Fe-N-C-1000 sample displayed higher performance durability over a long cycling duration (30,000 cycles) compared to Pt/C taken as control. Tests in the AEMFC fed with H2 showed that the performance of the Fe-N-C-1000 catalyst was competitive (OCV = 0.98 vs. 1.05 V, 149 vs. 148 mW cm−2) compared to the state-of-the-art Pt/C electrode, using a FUMASEP® FAA-3-50 membrane. The material found an application also in alkaline direct methanol fuel cell (ADMFC) fed with methanol solutions at high concentrations (up to 10 M) due to a high methanol tolerance, as pointed out by rotating disk electrode experiments.

Influence of Ion-Exchange Capacity on the Solubility, Mechanical Properties, and Mass Transport of Anion-Exchange Ionomers for Alkaline Fuel Cells

We have studied the solubility, mechanical properties, and mass transport behavior of a series of methylpiperidinium-functionalized polyethylene (PEPM) ionomers with different ion-exchange capacities (IECs) for anion-exchange membrane fuel cells (AEMFCs). The influence of IEC on the ionomer viscoelasticity in water and the transport/permeability of charged and uncharged species were investigated by acoustic impedance tests and electrochemical measurements, respectively. We found that the solubility of the PEPM ionomers increased with increasing IEC, while the rigidity of the ionomer films in water was reduced due to the higher content of the hydrophilic cations in the ionomers. In electrochemical tests, N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD) and 1,4-benzoquinone (BQ) were used as redox probes with ionomers serving as inert layers coating the electrodes. Analysis of the cyclic voltammetric profiles indicated that the electrostatic effects from the ionomers exclude cations, such as TMPD+, from the ionomer-coated electrode and attract anions like BQ2– into the ionomer films. Such effects intensified with an increase in the IEC, which could be screened by increasing the ionic strength of the electrolyte. Rotating disk electrode experiments indicated that the increased IEC had a negative impact on the permeability of neutral species. In addition, MEA studies with symmetric and asymmetric ionomers provided preliminary results on the impact of the IEC on fuel cell performance. This work provides insights into the design of ionomers and membranes with deliberately controlled IEC for different applications in AEMFCs from the stand points of solubility, mechanical properties, and mass transport.

Towards a realistic prediction of catalyst durability from liquid half-cell tests.

Liquid half-cell measurements provide a convenient laboratory method for determining relevant parameters of electro-catalysts applied in e.g. polymer electrolyte membrane fuel cells. While these measurements may be effective in certain contexts, their applicability to real-world systems, such as single-cells in a membrane electrode assembly (MEA) configuration, is not always clear. This is particularly true when assessing the stability of these systems through accelerated stress tests (ASTs). Due to different electrode compositions and operating conditions, nanoscale degradation proceeds differently. Nevertheless, given the high demands of MEA measurements in terms of time, testing equipment complexity, and amount of catalyst material, application-relevant predictions of catalyst durability from liquid half-cell tests are highly desirable. This study combines electrochemical and nanoparticle analysis based on transmission electron microscopy to conduct a typical voltage cycling AST for rotating disc electrode (RDE) measurements, showing that the loss of the electrochemically active surface area (ECSA) of the used Pt/Vulcan catalyst is strongly enhanced at 80 °C compared to room temperature, which goes along with increased nanoparticle coarsening. Additionally, a high ionomer/carbon mass ratio (I/C = 0.7) accelerates the ECSA loss, and further investigations of its influence suggest a combination of several factors, including the high local proton concentration and the presence of adsorbing anions. At the same temperature (80 °C) and I/C ratio (0.7), the ECSA loss vs. AST cycle number of the Pt/Vulcan catalyst is essentially identical for a voltage cycling AST conducted in either an RDE half-cell or an MEA configuration, suggesting that liquid electrolyte half-cell based ASTs can provide application-relevant results. Thus, our study points out a way for predicting the stability of electro-catalysts in MEAs based on RDE experiments that require less specialized equipment and only μg-quantities of catalysts.

Assessment of Conductive Sites on Carbon Fiber Reinforced Polymer Plastics' Surfaces Using Electrochemical Methods

The components of automobile bodies have been manufactured primarily from steel since cars were first made. With the goal of energy savings, light-weighting is being adopted by substituting steel with a variety of light metals and composite materials. A DOE sponsored project in collaboration with PPG and Ford studied the use of automotive closure panels comprising an inner carbon fiber reinforced polymer (CFRP) sheet and an outer aluminum alloy (AA) sheet. The ends of closure panels are sealed with hem flange joints. A major concern is the formation of a galvanic cell within the joint wherein CFRP is the cathode that will accelerate the corrosion of the AA sheet. Various combinations of AA6xxx alloys with CFRP having two different orientations of carbon fiber, i.e., random and twill, are studied in this work. CFRP can only exert a galvanic driving force on a coupled AA panel by electrical connection at cut edges where sheared carbon fibers are exposed, or if carbon fibers are exposed on the CFRP panel surface by lack of coverage of the insulating epoxy matrix. It is believed that processing and curing variables lead to varied epoxy cover on the surfaces of CFRP. Electrochemical measurements such as the measurement of oxygen reduction limiting current density during cathodic polarization show that as-fabricated surfaces of CFRP panels exhibit some extent of electrochemical activity, indicating some lack of coverage of C fibers. It is of interest to characterize the epoxy cover on the CFRP surfaces in terms of the nature of the active sites (e.g., whether C fibers are totally uncovered or covered by a very thin layer of epoxy) and the density and distribution of the sites. A technique was developed to identify the exact locations of active sites on both random and twill CFRP by electrodeposition of small amounts of Cu. If the amount of deposited copper is small, the deposits indicate the location and number of the active sites on the CFRP surface. These deposits were also analyzed by serial sectioning using Focused Ion Beam/Scanning Electron Microscope. High resolution images of the deposit/epoxy interfaces provide a description of the nature of the defective/conductive regions on CFRP surfaces. Rotating disk electrode experiments were also performed to determine the oxygen reduction reaction (ORR) limiting current densities of CFRP at different electrode rotation rates i.e., different diffusion boundary layer thicknesses. These aspects are related to the overall mass transport in the electrolyte, which provides a deeper understanding of the kinetics on CFRP and the nature of active sites behaving as microelectrodes. The observations from the copper electrodeposition study and rotating disk electrode experiments will be correlated and the role of CFRP as a cathode contributing to the corrosion of AA in the galvanic couple will be presented.This work was supported by the U.S. Department of Energy through award DE-EE0007760, in collaboration with PPG Industries and Ford Motor Company.

Mass Transfer from Ion-Sensing Component-Loaded Nanoemulsions into Ion-Selective Membranes: An Electrochemical Quartz Crystal Microbalance and Thin-Film Coulometry Study.

Recent work has shown that ion-selective components may be transferred from nanoemulsions (NEs) to endow polymeric membranes with ion-selective sensing properties. This approach has also been used for nanopipette electrodes to achieve single-entity electrochemistry, thereby sensing the ion-selective response of single adhered nanospheres. To this date, however, the mechanism and rate of component transfer remain unclear. We study here the transfer of lipophilic ionic compounds from nanoemulsions into thin plasticized poly(vinyl chloride) (PVC-DOS) films by chronoamperometry and quartz crystal microbalance. Thin-film cyclic coulovoltammetry measurements serve to quantify the uptake of lipophilic species into blank PVC-DOS membranes. Electrochemical quartz crystal microbalance data indicate that the transfer of the emulsion components is insignificant, ruling out simple coalescence with the membrane film. Ionophores and ion-exchangers are shown to transfer into the membrane at rates that correlate with their lipophilicity if mass transport is not rate-limiting, which is the case with more lipophilic compounds (calcium and sodium ionophores). On the other hand, with less lipophilic compounds (valinomycin and cation-exchanger salts), transfer rates are limited by mass transport. This is confirmed with rotating disk electrode experiments in which a linear relationship between the diffusion layer thickness and current is observed. The data suggests that once the nanoemulsion container approaches the membrane surface, transfer of components occur by a three-phase partition mechanism where the aqueous phase serves as a kinetic barrier. The results help better understand and quantify the interaction between nanoemulsions and ion-selective membranes and predict membrane doping rates for a range of components.

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 .

Tuning the course of the oxygen reduction reaction at a carbon electrode using alkaline electrolytes based on binary DMSO–water solvent mixtures

We studied the oxygen reduction reaction (ORR) at a glassy carbon electrode in KOH electrolytes based on DMSO–water solvent mixtures of various compositions (ranging from 0.0 to 93.3 vol% DMSO). The results of RDE (rotating disk electrode) experiments show that by changing the composition ofthe DMSO–water solvent mixture, one can control the course of oxygen electroreduction. Specifically, in the absence of dimethyl sulfoxide, the ORR at −1.5 V vs. Hg/HgO proceeds mainly as a two-electron process (n = 2.28 at 400 rpm), whereas with increasing DMSO content the ORR shifts gradually towards complete four-electron reduction. This tendency continues until the maximum electron transfer number is reached (n = 3.70 at 400 rpm) in an alkaline electrolyte based on a 1:1 H2O:DMSO (v/v) solvent mixture. The effect of DMSO addition on the course of the ORR process at the carbon electrode is thought to arise from the unique physicochemical properties of DMSO–water systems, i.e. their high viscosity and polarity as a result of H-bonded intermolecular complexes formed between water and DMSO molecules.

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.

Designing Robust Two-Electron Storage Extended Bipyridinium Anolytes for pH-Neutral Aqueous Organic Redox Flow Batteries.

Bipyridinium derivatives represent the most extensively explored anolyte materials for pH-neutral aqueous organic redox flow batteries, and most derivatives feature two separate electron-transfer steps that cause a sharp decrease in cell voltage during discharge. Here, we propose a strategy to fulfill the concurrent two-electron electrochemical reaction by designing extended bipyridinium derivatives (exBPs) with a reduced energy difference between the lowest unoccupied molecular orbital of exBPs and the β-highest occupied molecular orbital of the singly reduced form. To demonstrate, a series of exBPs are synthesized and exhibit a single peak at redox potentials of −0.75 to −0.91 V (vs standard hydrogen electrode (SHE)), as opposed to the two peaks of most bipyridinium derivatives. Cyclic voltammetry along with diffusion-ordered spectroscopy and rotating disk electrode experiments confirm that this peak corresponds to a concurrent two-electron transfer. When examined in full-flowing cells, all exBPs demonstrate one charge/discharge plateau and two-electron storage. Continuous galvanostatic cell cycling reveals the side reactions leading to capacity fading, and we disclose the underlying mechanism by identifying the degradation products. By prohibiting the dimerization/β-elimination side reactions, we acquire a 0.5 M (1 M e–) exDMeBP/FcNCl cell with a high capacity of 22.35 Ah L–1 and a capacity retention rate of 99.95% per cycle.

Oxidation of capsaicin in acetonitrile in dry and wet conditions

• Water greatly affects the formal potential of capsaicin oxidation. • Two anodic processes were observed for capsaicin under dried conditions. • The second anodic processes shifted negatively with increasing water content. • Different numbers of electrons transferred under CV and CPE conditions. • Two to four electrons were transferred during the electrolysis of capsaicin. An electrochemical study of the phenol capsaicin (CAPH), the active ingredient in chilli pepper, was performed in dried and wet acetonitrile on a glassy carbon electrode. Under dried conditions, two oxidation peaks at ca. 0.7 vs. (Fc/Fc + )/V (labelled E 1 ) and 1.0 vs. (Fc/Fc + )/V (labelled E 2 ) and two reduction peaks at ca . 0.1 and −0.5 vs. (Fc/Fc + )/V when the scan direction was reversed were observed. Rotating disk electrode experiments indicated that the two oxidation processes occur by same number of electrons and it is proposed they occur in two one-electron steps. As water was added to the acetonitrile, hydrogen bonding interactions between the water and the phenolic groups led to the E 1 and E 2 potentials shifting progressively more negatively. The shift in E 2 as water was added was greater than the shift in E 1 , so that after the addition of approximately 0.2 M H 2 O, only one voltammetric wave was observed (labelled as E 1 ′) corresponding to a two-electron oxidation, that continued to shift more negatively as more water was added. Under very wet conditions ([H 2 O] > 1 M), only one chemically irreversible oxidation peak was observed ( E 1 ′) at ca . > 0.5 vs. (Fc/Fc + )/V with one reduction peak at ca . −0.1 vs. (Fc/Fc + )/V upon reversal of the scan direction. Controlled-potential electrolysis under wet conditions indicated a total of two-electrons per molecule were transferred with the overall mechanism interpreted as a −2e – /−H + oxidation, followed by a hydrolysis reaction and loss of a methoxy group to form a 1,2-benzoquinone moiety.

Characterizing the Impact of the Oxygen Reduction Reaction Mechanism and Current Distribution on Mineral Deposit Structure and Composition

In marine environments, cathodic protection is applied to metallic structures to prevent corrosion through polarization. The cathodic current causes secondary mineral deposition reactions, whose makeup, phase, and existence are dependent on the pH at the interface. These deposits can cause a significant increase in the interfacial impedance through electrolytic exclusion and obstructions to diffusion. Two factors contributing to the pH at a surface are current distributions across a surface and the reaction mechanism mediating the current. This work looks at the influence of these two factors on the make-up, phase and surface impedance impacts of mineral deposits on electrode surfaces. The role of Oxygen Reduction Reaction (ORR) mechanism and current distribution on the formation of Ca and Mg deposits on platinum and 316L stainless steel were assessed using a combination of rotating disc electrode (RDE), electrochemical impedance spectroscopy (EIS), and surface analysis techniques.In order to verify the material and potential dependent ORR mechanisms on platinum, glassy carbon, and 316L stainless steel, a series of RDE experiments were conducted. The platinum electrode and glassy carbon electrodes were shown to have excellent agreement with the predicted Levich behavior for a 4-electron ORR mechanism and a 2-electron ORR mechanism respectively, for all of the potentials in the mass transport limiting current range for ORR. However, the 316L stainless steels samples exhibited a deviation in the predicted Levich response at lower cathodic overpotentials in the ORR limiting current region. This behavior has been hypothesized to be caused by a reduction in the iron oxide layer present on the stainless-steel surface, which in turn results in an increase in ORR efficiency at more cathodic overpotentials. This transition in ORR efficiency may result in a change in the interfacial pH gradient, and impact the resultant mineral deposit structure and composition. In this work, four different ORR mechanism conditions were evaluated in terms of mineral deposit formation potential: polished platinum electrodes, polished glassy carbon electrodes, polished 316L stainless steel electrodes, and pre-reduced 316L stainless steel electrodes.Current distribution was evaluated in a fabricated, macro scale scanning probe system. These experiments assessed Pt and 316L as a 7 cm diameter disk working electrode, surrounded by a platinized ring counter electrode under quiescent conditions. Current distribution was observed via electric field measurement and surface potential mapping. The electrodes were polarized in ASTM D1141 artificial seawater, and the growth of the deposit was monitored in situ using EIS. The structure of the deposits on each material was evaluated by confocal profilometry and scanning electron microscopy with energy dispersive x-ray spectroscopy to determine structure and chemical composition. Mineral deposits were evaluated spatially and correlated to RDE work via an equipotential model. The work presented here provides insight into the contribution of ORR mechanism and current distribution across several size scales, on mineral deposit formation on materials with both ideal and hybrid ORR activity.

Gas Diffusion Electrode Half Cells – a Powerful Tool for Fuel Cell Electrocatalyst Evaluation in Relevant Conditions

In aqueous rotating disk electrode (RDE) measurements, advanced catalyst materials show tremendous performance improvements in comparison to commercial Pt/C catalysts towards oxygen reduction reaction (ORR). However, these promising improvements cannot (yet) be transferred to realistic membrane electrode assembly (MEA). [1] These discrepancies can be explained by non-ideal catalyst layer composition, leading to mass transport limitations of either oxygen (which is transported through the pores) or protons (via the ionomer) to the active centers of the catalyst material. Mass transport phenomena in real catalyst layers are still under debate and hold huge potential to significantly improve catalyst performance.However, mass transport in realistic fuel cell catalyst layers cannot be assessed with RDE experiments, as they are limited to low current densities and idealized catalyst layers on solid substrate. Therefore, MEA experiments are usually employed to evaluate those phenomena. Yet, those investigations are time consuming, expensive (large quantities of catalyst needed, extended test equipment) and do not allow segregated investigation of either cathode or anode catalyst layer. Furthermore, comparison of different MEA studies can be challenging due to varying operating conditions. Therefore, techniques combining the advantages of RDE, namely simplicity and comparability, with the realistic operating ranges of MEA are urgently required to retrieve the potential of catalyst layer optimization.In this presentation half-cells using gas diffusion electrodes (GDE) are proposed as a new powerful experimental tool to enable high mass transport catalyst screening in relevant potential ranges and realistic electrode structures.On the example of a Pt loading study we show, that current densities of up to 2 A/cm² can be achieved [2]. In contrary to other formerly used methods [3], it is thereby possible to overcome mass transport limitations at relevant fuel cell operating potentials. Additionally, we demonstrate good compliance with MEA experiments, proving the method´s suitability for catalyst evaluation in realistic fuel cell potential ranges.Besides activity, also stability of the electrocatalyst is affected significantly by the catalyst layer structure. We present here a novel method, where a GDE half-cell is coupled to an inductively coupled plasma mass spectrometer (ICP-MS) to gain deeper insights into dissolution of electrocatalysts in realistic electrode structures [4]. With this unique online tool, we could measure Pt dissolution in realistic catalyst layers for the first time reported in literature. We show, how mass transport of dissolved Pt species influences net Pt dissolution and how Pt dissolution can also be detected through Nafion membranes. Besides that, also the mobility of Pt-ions through Nafion membranes is further investigated. Literature [1] Ly, A., et al. J. Power Sources, 2020. 478: 228516[2] Ehelebe, K., et al., J. Electrochem. Soc., 2019. 166(16): F1259-F1268,[3] Inaba, M., et al., Energy Environm. Sci., 2018. 11(4): 988-994,[4] Ehelebe, K. et al. submitted Figure 1

Bridging the gap between highly active oxygen reduction reaction catalysts and effective catalyst layers for proton exchange membrane fuel cells

Ultralow platinum loading and high catalytic performance at the membrane electrode assembly (MEA) level are essential for reducing the cost of proton exchange membrane fuel cells. The past decade has seen substantial progress in developing a variety of highly active platinum-based catalysts for the oxygen reduction reaction. However, these high activities are almost exclusively obtained from rotating disk electrode (RDE) measurements and have rarely translated into MEA performance. In this Review, we elucidate the intrinsic limitations that lead to a persistent failure to transfer catalysts’ high RDE activities into maximized MEA performance. We discuss catalyst-layer engineering strategies for controlling mass transport resistances at local catalyst sites, in the bulk of the catalyst layer and at the interfaces of the MEA to achieve high performance with ultralow platinum loading. We also examine promising intermediate testing methods for closing the gap between RDE and MEA experiments. The promising performance of low-platinum-loading oxygen reduction reaction catalysts in preliminary electrochemical tests is rarely translated into similarly impressive performance in real fuel cells. In this Review, Li and colleagues explore the underlying reasons for this issue and outline strategies to overcome it.