Glassy Carbon Rotating Disk Electrode Research Articles

La0.8Sr0.2CoO3 Electrocatalyst for Oxygen Evolution Reaction in Alkaline Electrolysis: From Powder to the Fuel Cell Application

In recent years, there has been a growing need for new renewable energy sources due to the rapid depletion of conventional energy sources and an escalating energy demand. Since environmental protection concerns are increasing, electrochemical processes are becoming increasingly essential [1]. Specifically, the focus has shifted towards storing energy in hydrogen and its subsequent conversion into on-demand electricity [2]. Here, hydrogen, generated via water electrolysis, serves as either an intermediary for generating energy carriers like liquid fuels or plays a key role as an energy carrier [3]. Despite significant efforts focused on enhancing electrolyzers for energy conversion, the progression towards practical application has encountered challenges stemming from differing catalyst development requisites in academic and industrial research. This study proposes a systematic approach for efficiently transitioning electrocatalysts from fundamental research to application readiness for alkaline oxygen evolution reactions.Herein, La0.8Sr0.2CoO3 was chosen as a catalyst and compared with the benchmark material NiFe2O4. Initially, the La0.8Sr0.2CoO3 catalyst was successfully synthesized by scalable spray-flame synthesis. Following this, various inks were formulated utilizing different binders (Nafion®, Naf; Sustainion®, Sus). The dispersion stability of La0.8Sr0.2CoO3 and commercial NiFe2O4 inks was investigated in the presence of Nafion and Sustainion using analytical centrifugation and transmittograms. Subsequently, these selected dispersions were applied onto nickel substrates via spray coating techniques, ensuring a homogeneous catalyst distribution. Finally, the La0.8Sr0.2CoO3 and NiFe2O4 catalysts were subjected to extensive electrochemical evaluations, including glassy carbon rotating disk electrode, scanning droplet cell (SDC), compression cell, flow cell, and zero-gap cell, to evaluate the material performance. SDC findings highlight the excellent uniformity in La0.8Sr0.2CoO3 electrodes and NiFe2O4-Sustainion (standard deviation < 11%). However, the NiFe2O4-Sustainion film experienced delamination, exhibiting a standard deviation of 28%. SDC experiments offer advantages over various half-cell techniques by enabling the detection of any inhomogeneities or coating defects within the catalytic film. Additionally, complementary compression and flow cell experiments provide detailed information on the chemical and mechanical stress parameters resembling those in a full cell configuration. In assessing industrial applicability, the catalytic materials were undergone examination within a scalable full cell under industrial conditions (500 mA/cm² and 60 °C) utilizing a zero-gap cell setup. The observed trend in the full cell for the coated electrodes is as follows: La0.8Sr0.2CoO3-Naf > La0.8Sr0.2CoO3-Sus > NiFe2O4-Sus > NiFe2O4-Naf.These findings achieved through a well-structured coherent workflow, enhance our knowledge of the electrocatalytic system and offer essential insights for implementing novel materials in large-scale industrial applications. REFERENCES: [1] M.-S. Park, J. Kim, K.J. Kim, J.-W. Lee, J.H. Kim, Y. Yamauchi, Porous nanoarchitectures of spinel-type transition metal oxides for electrochemical energy storage systems, Phys. Chem. Chem. Phys. 17 (2015) 30963–30977. https://doi.org/10.1039/C5CP05936D.[2] J.R. Varcoe, P. Atanassov, D.R. Dekel, A.M. Herring, M.A. Hickner, P.A. Kohl, A.R. Kucernak, W.E. Mustain, K. Nijmeijer, K. Scott, T. Xu, L. Zhuang, Anion-exchange membranes in electrochemical energy systems, Energy Environ. Sci. 7 (2014) 3135–3191. https://doi.org/10.1039/C4EE01303D.[3] C. Van Pham, D. Escalera-López, K. Mayrhofer, S. Cherevko, S. Thiele, Essentials of High Performance Water Electrolyzers – From Catalyst Layer Materials to Electrode Engineering, Adv. Energy Mater. 11 (2021) 2101998. https://doi.org/https://doi.org/10.1002/aenm.202101998.

Catalytic Activities of Ir-Based Pyrochlore-Type Oxides Doped with Divalent Cations for Oxygen Evolution Reaction

Alternative energy sources to fossil fuels are essential for a sustainable society, and hydrogen energy is expected to be a solution to environmental and energy problems. Currently, proton exchange membrane water electrolyzers are becoming widely used for the electrochemical production of hydrogen due to their advantages such as high efficiency. However, the highly acidic conditions and the large overpotentials of oxygen evolution reaction (OER) at the anode are problems, and the development of catalysts that are stable and highly active in acidic aqueous solutions is important for further popularization.Ir-based oxides with pyrochlore structure (A2B2O7) are known to exhibit high catalytic activities in acidic aqueous solutions. Recently, it has been reported that the improved catalytic activities of Pr2Ir2O7 by Zn doping, which controlled the composition of A-site elements in the pyrochlore structure [1]. In this study, we synthesized Pr2-x MgxIr2O7 (x = 0, 0.1, 0.2, 0.3, 0.4, and 0.5), in which a part of trivalent cation Pr3+ was replaced by divalent cation Mg2+ and improved the OER catalytic activity in acidic aqueous solutions.Pr2-x MgxIr2O7 was synthesized by the hydrothermal method. The catalyst was mixed with a conductivity aid carbon (Vulcan XC-72) and a cationic conductive ionomer (Nafion) at a weight ratio of 5:1:1 and dispersed in 2-propanol to form a catalyst ink. The catalytic activity was evaluated by cyclic voltammetry (CV) using a glassy carbon rotating disk electrode (RDE) as a working electrode, a reversible hydrogen electrode (RHE) as a reference electrode, and a platinum wire as a counter electrode. The electrolyte was a 0.1 mol dm-3 HClO4 solution saturated with oxygen, and the CV scanning range was 1.1 to 1.7 V with respect to the RHE, scanning speed was 20 mV s-1, and the RDE rotation speed was 1200 rpm. The stability of the catalyst in acidic aqueous solutions was evaluated by chronopotentiometry (CP) using the same cell.X-ray diffraction and energy-dispersive X-ray analysis measurements of the synthesized materials confirmed that the desired compounds were obtained, and CV measurements showed that the OER activity of Pr2-x MgxIr2O7 increased in the first 20 cycles for all compositions, and then stabilized over several tens of cycles. The cycle change was similar to that of Pr2Ir2O7, suggesting that the surface rearrangement proceeds in acidic aqueous solutions [2]. Linear sweep voltammogram (LSV) and OER activity normalized by Brunauer-Emmett-Teller (BET) specific surface area were measured at the 20th cycle, and the catalytic activity stabilized at a high value. The OER activity was enhanced by Mg substitution in the catalysts of Pr2-x Mg x Ir2O7.References L. Zhu, C. Ma, Z. Wang, X. Gong, L. Cao, and J. Yang, Appl. Surf. Sci., 31, 151840 (2022).C. Shang, C. Cao, D. Yu, Y. Yan, Y. Lin, H. Li, T. Zheng, X. Yan, W. Yu, S. Zhou, and J. Zeng, Adv. Mater., 31, 1805104 (2018).

Revealing cellular (poly)sulphide storage in electrochemically active sulphide oxidising bacteria using rotating disc electrodes

Sulphide oxidising bacteria (SOB) have the potential to be used for bioelectrochemical removal, i.e. oxidation, of sulphide from waste streams. In anaerobic conditions, SOB are able to spatially separate sulphide removal and terminal electron transfer to an electrode and act as a sulphide shuttle. However, it is not fully understood how SOB anaerobically remove sulphide and store charge equivalents, and where in this process sulphur is formed. Therefore, the redox behaviour of sulphide shuttling SOB was investigated at haloalkaline conditions using a glassy carbon rotating disc electrode (RDE) and cyclic voltammetry. Voltammograms of SOB in the absence and presence of sulphide were compared to voltammograms of abiotic sulphur species solutions. Polysulphide and sulphide showed different redox behaviour, with distinct potentials for oxidation of > −0.3 V (vs. Ag/AgCl) for polysulphide and > −0.1 V for sulphide. Comparing biotic to abiotic experiments lead to the hypothesis that SOB formed polysulphides during anaerobic sulphide removal, which stayed sorbed to the cells. With this study, further steps were taken in elucidating the mechanisms of sulphide shuttling by SOB.

Effect of Glycine on a Fe-Based Electrolyte for Redox Flow Batteries

Among the rising stationary energy storage technologies, Redox Flow Batteries (RFBs) emerge as a promising electrochemical device capable of high flexibility in terms of power and energy combined with potential low cost and low environmental impact. Alternative chemistries can employ cheap and earth-abundant materials, possibly alleviating the demand for critical raw materials, which are extensively employed in the field of energy production and storage. Iron-based chemistries are among the best candidates for aqueous pH-neutral catholytes, offering fast reaction kinetics and a well-placed standard reduction potential of 0.771 V, that, coupled with a Zinc-based anolyte, would give a nominal cell voltage of 1.53 V [1]. Unfortunately, the dissolution of Iron salts in a neutral aqueous solution triggers hydrolysis, resulting in the formation of Iron hydroxides in the form of insoluble deposits. This causes the loss of electroactive species from the electrolyte, and consequently decreases the battery capacity. Thus far, the conventional solution is the electrolyte acidification, which, however, significantly increases the costs of materials for stacks and electrolyte handling components and aggravates the already critical problem of Hydrogen evolution at the Zinc side. As an alternative, several complexing agents have been proposed in the literature, with preliminary studies on their complexation mechanisms [2]. Glycine is often considered as one of the most promising candidates, even though its suppressing effect on the Fe(II)/Fe(III) kinetics at high concentrations is widely reported [3].This research investigates an optimized pH-neutral catholyte for Zinc-Iron RFBs, focusing on the use of Glycine to avoid Iron-induced hydrolysis. The study is carried out on highly concentrated solutions of Iron chlorides, with the addition of Glycine as ligand at various concentrations. The aim is to preserve the performances of Fe(II)/Fe(III) kinetics and to avoid insoluble hydroxides formation by an accurate choice of the Gly-Fe ratio. The produced electrolytes have been characterized by measuring the solutions conductivity, kinematic viscosity and density. Then, an electrochemical characterization is performed evaluating the diffusion coefficients (𝐷) of the dissolved species, the equilibrium potential (𝐸0) and the exchange current density (𝑗0) as well as the rate constant of the reactions (𝑘0) by means of cyclic voltammetries with a glassy carbon rotating disc electrode. The diffusion coefficients are obtained from Randles-Sevcik equation while the equilibrium potential, the exchange current density and the reaction rate constants are evaluated with a Koutecky-Levich analysis. An optimization of the electrolyte preparation method was also performed, evaluating the effects of pH and temperature and monitoring the complexation time, with the aim of accelerating the Gly-Fe complexation reaction. The effects of KCl and CaCl2 on the electrolyte conductivity are also investigated, to identify suitable supporting electrolytes.The research results show how the addition of Glycine successfully prevents the formation of precipitates in equimolar solutions even at low Gly-Fe ratios. However, Glycine suppresses Fe(II)/Fe(III) kinetics and lowers the electrolyte equilibrium potential, consequently reducing both energy density and the maximum current for its use in RFBs. It is thus possible to identify an optimum for the Gly-Fe ratio by setting a compromise between chemical stability and electrochemical performances. We propose the range 0.5:1 to 1:1 as a convenient Gly-Fe ratio for the 1M FeCl2 and 1M FeCl3 system, reaching a potential higher than 0.721 V vs. SHE, making it a very attractive catholyte with excellent suppression of Iron hydrolysis as no precipitation is observed after more than three months.This work is funded by the Italian Ministry of Enterprises and Made in Italy in the framework of the Important Project of Common European Interest (IPCEI) European Battery Innovation (project IPCEI Batterie 2 - CUP: B62C22000010001). The IPCEI European Battery Innovation is also funded by public authorities from Austria, Belgium, Croatia, Finland, France, Germany, Greece, Poland, Slovakia, Spain and Sweden.[1] Zhang, Huan, Chuanyu Sun, and Mingming Ge. "Review of the research status of cost-effective zinc–iron redox flow batteries." Batteries 8.11 (2022): 202.[2] Hawthorne, Krista L., Jesse S. Wainright, and Robert F. Savinell. "Studies of iron-ligand complexes for an all-iron flow battery application." Journal of The Electrochemical Society 161.10 (2014): A1662.[3] Xie, Congxin, et al. "A Low‐Cost Neutral Zinc–Iron Flow Battery with High Energy Density for Stationary Energy Storage." Angewandte Chemie International Edition 56.47 (2017): 14953-14957. Figure 1

High-Entropy Alloys as Oxygen Reduction Reaction Electrocatalysts for Proton Exchange Membrane Fuel Cells Application

High entropy alloys (HEA) are complex multi-element materials. Owing to their multi-elemental nature, they can finetune surface adsorption energy and active sites which make them appealing to various catalytic reactions. HEI is a subclass of HEA that consist of an ordered alloy structure rather than a random alloy structure. The order-ness may induce more stable structures and uniform active sites, which in turn enhance the durability and activity under catalytic conditions. Here, we have synthesized a HEI(Pt40Ni20Co20Mn10Ir10) supported on porous carbon via a seed-mediated impregnation method. The intermetallic characteristics with periodic trends can be seen in the HAADF-STEM image (Fig. 1a). Based on the atomic arrangement, the HEI resembled a L12 structure. The corresponding EDX mapping the catalyst also showed the presence of all alloying species in the nanoparticles. The catalyst performance of the catalysts was evaluated in a 3-electrode liquid cell set-up with catalyst coated glassy carbon rotating disk electrode, Ag/AgCl and Pt mesh as working, reference and counter electrode, respectively. The catalyst’s mass activity (MA) was calculated based on the result obtained from ORR polarization test conducted under O2 saturated 0.1M HClO4. The initial mass activities for PtCo and HEA in ORR were 0.81 and 1.23 A mg-1 Pt, respectively (Fig. 1b), while that of Pt/C was only 0.26 A mg-1 Pt. A subsequent accelerated degradation test (ADT) was conducted under Ar-saturated 0.1M HClO4 with 0.6-1.0 V vs RHE square-wave cycling (3s hold at each potential) for 30k cycles. MA was measured after 10k and 30k cycles. HEA retained a MA of 0.48 A mg-1 Pt after the ADT, above the DOE target (0.44 A mg-1 Pt). We also compared our HEI samples synthesized with different annealing temperatures and duration (Fig. 1c), and it showed that 800◦C with 20 mins temperature hold showed the highest MA and electrochemical surface area (ECSA).Extended X-ray absorption spectroscopy (EXAFS) for the catalyst has also been conducted to further elucidate the structural-performance relationship of HEI (Fig.1d). From Pt L3 edge EXAFS fitting, the fitted R space showed that the Pt-Pt bond length was around 2.704 ± 0.004 Å, which was same as that of the PtCo (Fig. 1e). Therefore, it is believed that both HEI and PtCo has the same strain effect but HEI possessed stronger ligand effect than PtCo due to its unique Pt-M coordination, hence resulting in superior ORR performance over PtCo and Pt. Figure 1

Exploring Key Factors for Oxygen Evolution Reaction on La2-X Sr x NiO4+Δ by Oxygen Defect Engineering

IntroductionDevelopment of Oxygen Evolution Reaction (OER) catalyst is an important research issue for green hydrogen production by water splitting. Perovskite oxides have attracted attention as OER catalysts which can achieve both low cost and high catalytic activity. So far, some hypotheses have been proposed to understand key factors of catalytic activity of perovskite catalyst. Suntivich et al. reported volcano-like relationship between the OER activity and the number of the e g-electron of the B-site transition metal and the highest catalytic activity is achieved when the number of the e g-electron is close to 1.21. Besides, oxygen vacancies act as electrochemically active sites by incorporating reaction intermediates directly2,3. However, it is difficult to distinguish the contribution of electric state and oxygen defects on OER activity because both electronic state and oxygen defect concentration change simultaneously. For instance, the oxygen vacancy formation creates two electrons to maintain charge neutrality (2TMTM ×+OO ×↔2TMTM '+V O ∙∙+1/2 O2(g)).The aim of this study is to clarify the key factors improving OER catalytic activity. For the aim, we controlled the Ni valence state and oxygen defect concentration in layered perovskite oxide La2-x Sr x NiO4+δ by tuning Sr content (x) and the amount of oxygen defect (δ) independently (Fig. 1-a), to reveal the influence of transition metal electronic state and oxygen defects on OER activity. La2-x Sr x NiO4+δ has two types of oxygen defects: oxygen vacancy (V O ∙∙) and unoccupied interstitial site (V i ×). Comparing OER activity of oxygen stoichiometric La2NiO4 (Ni2+), La1.8Sr0.2NiO4 (Ni2.2+) and La1.6Sr0.4NiO4 (Ni2.4+) allowed us to evaluate the influence of Ni valence state on OER activity while no oxygen vacancy and fully unoccupied interstitial sites ([V O ∙∙]=0,[V i ×]=2.0). Additionally, comparing OER activity of La2NiO4.1 (Ni2.2+,[V O ∙∙]=0,[V i ×]=1.9), La1.8Sr0.2NiO4 (Ni2.2+,[V O ∙∙]=0,[V i ×]=2.0) and La1.6Sr0.4NiO3.9 (Ni2.2+,[V O ∙∙]=0.1,[V i ×]=2.0) allowed us to evaluate the influence of oxygen defects on OER activity with negligible effect of Ni valence state.ExperimentalLa2-x Sr x NiO4+δ (x=0, 0.2, 0.4) were synthesized by Pechini method. The oxygen content of La2-x Sr x NiO4+δ was controlled by annealing them under suitable oxygen partial pressure and temperature and then quenched based on oxygen nonstoichiometric data4. Obtained samples were characterized by XRD, XAS and SEM.For the OER test, the catalyst ink was prepared by mixing the catalyst powder, acetylene black, K+-exchanged Nafion and tetrahydrofuran. 6.4 μl of the catalyst ink was sonicated and dropped onto the glassy-carbon rotating disk electrode. The electrochemical measurements were performed in O2-saturated 0.1 M KOH. The disk electrode potential was controlled between 0.3 and 0.9 V versus Hg/HgO reference electrode filled with 0.1 M KOH at a scan rate of 10 mVs-1.Result and DiscussionOxygen stoichiometric La2NiO4 (Ni2+), La1.8Sr0.2NiO4 (Ni2.2+) and La1.6Sr0.4NiO4 (Ni2.4+) showed similar catalytic activity regardless of Ni valence (Fig. 1-b). This implies that the electronic state of Ni has insignificant influence on catalytic activity when the sample has almost no oxygen vacancy. The observed tendency is inconsistent with the Suntivich’s result that the relationship between OER activity and e g filling1.La2NiO4.1 (Ni2.2+,[V O ∙∙]=0,[V i ×]=1.9) and La1.8Sr0.2NiO4 (Ni2.2+,[V O ∙∙]=0,[V i ×]=2.0) which have different amount of unoccupied interstitial sites and almost no oxygen vacancy showed similar OER activity (Fig. 1-c). In contrast, La1.6Sr0.4NiO3.9 (Ni2.2+,[V O ∙∙]=0.1,[V i ×]=2.0) which has oxygen vacancy and fully unoccupied interstitial sites showed much better OER activity than La1.8Sr0.2NiO4 (Ni2.2+,[V O ∙∙]=0,[V i ×]=2.0) which has almost no oxygen vacancy and fully unoccupied interstitial sites (Fig. 1-c). These results indicate that oxygen vacancy has high OER activity as suggested in earlier works2.3, while unoccupied interstitial site which has available space for oxygen intercalation has negligibly low catalytic activity.The difference between oxygen vacancy and interstitial site is the presence of direct bonding with B-site cation. Reaction intermediates in oxygen vacancies can form direct bonding with Ni, while that in the interstitial site cannot. As suggested in the previous studies, a higher hybridization promotes charge transfer between the transition metal and reaction intermediate and effective charge transfer improves OER activity1,5.SummeryIn this work, to reveal the influence of transition metal electronic state and oxygen defects on OER activity, we have synthesized La2-x Sr x NiO4+δ , and controlled Ni valence state and oxygen defect concentration independently by tuning x and δ. It was revealed that oxygen vacancy boosts OER activity while influence of Ni valence state and unoccupied interstitial sites is insignificant.

Microdroplet anodic stripping voltammetry at the in situ preparing antimony‐modified rotating disk electrode for determination of Cd(II) and Pb(II)

AbstractA simple electroanalytical method for Cd(II) and Pb(II) detection based on differential pulse anodic stripping voltammetry (DPSV) with in situ prepared antimony‐modified glassy carbon rotating disk electrode (in situ Sb‐GC‐RDE) was developed. The electrochemical detection was performed in a microdroplet (50 μL) of 0.01 M hydrochloric acid that is placed between the electrode surface (top) and a Parafilm®‐covered glass slide to maintain a hydrophobic surface (bottom). This method includes a preconcentration process using a membrane filter (MF). The target metal ions were complexed with 1‐(2‐pyridylazo)‐2‐naphthol (PAN) as a chelating agent, which was accumulated on the MF via filtration. The RDE microdroplet anodic stripping voltammetry was suitable for the elution and determination of metal ions accumulated on the MF. The in situ preparation of antimony‐modified electrode allows the use of common GC electrode with high performance. The detection limits for Cd(II) and Pb(II) were 1.4 and 1.1 μg/L, respectively. The proposed method was successfully used in natural water samples for the simultaneous determination of Cd(II) and Pb(II).

The electrochemical behaviour of suspended Prussian Blue nanoparticles in forced convection conditions

• Electroreduction of Prussian Blue nanoparticles at RDE is described by Levich equation. • Minor fraction of suspended Prussian Blue nanoparticle is reduced at RDE. • Electrochemical detection of suspended Prussian Blue nanoparticles in flow. The electrochemical studies of suspended nanoparticles became an important method of their characterisation. In reported research, Prussian Blue nanoparticles served as a model redox active nanoobjects for studies in forced convection conditions. Recently, (M. Holdynski et al. Electrochem. Commmun. 86 (2018) 130–134), we have demonstrated that injection of these nanoparticles suspension into electrochemical cell results in a sharp decrease or increase of the current at glassy carbon rotating disc electrode set at potential corresponding to their electroreduction or electrooxidation respectively. Here, on the example of the electroreduction of Prussian Blue nanoparticles, we show that the shape of potential vs time dependence is independent of the electrode rotation rate. Analysis of the current–time dependences based on the Levich equation indicates that only an average 0.12 % of Fe 3+ ions present at the surface of the given nanoparticle undergoes electroreduction in the elementary reaction step. Injection of Prussian Blue nanoparticles into flowing aqueous electrolyte produces a sharp increase of the current recorded at glassy carbon electrode of the electrochemical wall-jet detector, followed by its gradual decrease. Such behaviour is generally observed for redox-active molecules. The ratio of the peak currents corresponding to electrooxidation and electroreduction of Prussian Blue nanoparticles colliding with the electrode differs from the expected from the stoichiometry of reactions.

Bulky Thiolate-Protected Silver Nanocluster Ag 213 (Adm-S) 44 Cl 33 with Excellent Electrocatalytic Performance toward Oxygen Reduction

Bulky Thiolate-Protected Silver Nanocluster Ag ₂₁₃ (Adm-S) ₄₄ Cl ₃₃ with Excellent Electrocatalytic Performance toward Oxygen Reduction

Communication—Demonstrating the Role of Mass Transport in Double Layer Formation

Herein we present insight into the structure and behaviour of the electrified interface between a planar non-porous glassy carbon electrode and an aqueous solution of 0.5 M Na2SO4. Specifically, a glassy carbon rotating disk electrode was used to show correspondence between increasing rotation rates, decreasing boundary layer thicknesses, and decreasing interfacial capacitance. The implication is that electrolyte counter charge is being dissipated by convective flow outside of the macroscopically thick boundary layer, indicating that electrolyte counter-charge extends substantially into the electrolyte.

Electrodeposition of Nanostructured Chromium Conglomerates from Cr(III) Dissolved in a Deep Eutectic Solvent: Influence of Forced Convection

In this work, Cr electrodeposition onto a glassy carbon rotating disk electrode (GCRDE) was studied from Cr(III) ions dissolved in the choline chloride and ethylene glycol eutectic mixture (ethaline) under different GCRDE angular speeds, ω, at 343 K. The ethaline kinematic viscosity, = 0.17 was estimated, for the first time, from potentiodynamic plots under forced convection conditions, using the Levich equation. Cr electrodeposition was studied from the analysis of potentiostatic current density transients, j-t plots, recorded at different ω. This study was based on the model proposed by Hyde et al. (J. Electroanal. Chem., 534, 13 (2002)) for analysis of j-t plots due to 3D nucleation and diffusion-controlled growth under forced convection, j dc-fcon(t), complemented by a previous adsorption step, j ad(t), and the residual water reduction, jWR (t), taking place on the growing surfaces of the Cr nuclei; thus, j total (t) = j ad(t) + j dc-fcon(t) + j WR(t). The kinetic parameters, namely: nucleation frequency and number density of active sites for chromium nucleation onto the GCRDE showed small dependence on ω; however, the term j WR (t) clearly depends on the ω value. Scanning Electron Microscopy, Energy Dispersive Spectroscopy and X-ray Photoelectron Spectroscopy techniques were used to characterize the Cr electrodeposited on the GCRDE surfaces. It was observed that the electrodeposits were formed by quasi-spherical conglomerates (ca. 50 and 200 nm diameter for 0 and 1500 rpm, respectively) of nanoparticles (sized less than 30 nm diameter, in both cases). Moreover, these electrodeposits were formed by chromium and oxygen and its content increases with ω. These nanoparticles exhibit a core–shell structure, where the core was formed by metallic, zero valence chromium, and the shell of Cr(III) (as Cr(OH)3 and Cr2O3), which was consistent with the theoretical mechanism used for the analysis of the j-t plots.

Viologen Derivatives in Deep Eutectic Solvents for Energy Storage

Redox flow batteries (RFBs) serve as one of the most promising battery technologies to meet the grid-scale energy storage requirements, owing to their scalable nature and acceptable redox chemistry. To achieve economical and sustainable energy storage, there has been a rapid transition from redox-active inorganic materials to the earth-abundant and tunable redox-active organic compounds for RFBs. Among explored redox-active organic materials, viologen derivatives show promise for battery applications due to their two successive redox reactions and low cost. Taking advantage of both redox reactions, however, may not be possible in aqueous electrolytes due to the insolubility of the charge-neutral viologen in aqueous solutions.1 Deep eutectic solvents (DESs) can offer an inexpensive electrolyte for energy storage which could potentially solve the solubility issues associated with these materials.2 Owing to their low vapor pressure, DESs can be also utilized at elevated temperatures for lower viscosity and enhanced conductivity.3 In an attempt to identify a high-concentrated and reversible redox-active DES, the electrochemical properties and solubility of methyl, ethyl, and benzyl viologens in choline chloride (ChCl) and ethylene glycol (EG) (1:4 molar ratio) mixture were investigated. To better qualify these redox-active molecules as an anolyte material for RFBs, the redox reactions were studied to assess the redox reversibility, the potential separation between the consecutive electron transfer reactions, and the redox kinetics. The electron-transfer rate constants were estimated using linear sweep voltammetry (LSV) at a glassy carbon rotating-disk electrode in a three-electrodes configuration cell. Diffusion coefficients were also calculated from the steady-state diffusion-limited current using the rotating-disk electrode as well as a microelectrode. Elucidating the kinetic parameters, diffusion coefficients, and transport properties of the viologen derivatives in DESs will bring insight into designing a concentrated redox-active DES for energy storage. B. Hu and T. L. Liu, J. Energy Chem., 27, 1326 (2018).B. Chen, S. Mitchell, N. Sinclair, J. Wainright, E. Pentzer, and B. Gurkan, Mol. Syst. Des. Eng., 5, 1147 (2020).N. S. Sinclair, D. Poe, R. F. Savinell, E. J. Maginn, and J. S. Wainright, J. Electrochem. Soc., 168, 020527 (2021).

Oxygen Electrocatalysis on Mixed-Anion Perovskite Compounds in Alkaline Media

Enhancement of oxygen reduction and evolution reaction (ORR and OER) performance has been considered to play a key role for the development of next-generation energy storage devices such as metal-air batteries and water electrolyzers. Recently, mixed-anion compounds have been found to be highly efficient bifunctional electrocatalysts for ORR and OER[1,2]. Our group has been also clarified that layered perovskite oxychloride Sr2Co1 − x Fe x O3Cl serves as a good bifunctional catalyst, whose OER activity outperformed that of Ba0.5Sr0.5Co0.8Fe0.2O3 −δ [3]. While detailed mechanism of its enhanced OER activity has been discussed previously[4], that of ORR has not been evaluated in detail, even though ORR pathway is generally considered to be much more complicated. Moreover, further exploration of highly active bifunctional mixed-anion perovskites has to be continued for better understanding of oxygen electrocatalysis on mixed-anion perovskite compounds. In the present research, therefore, we focused on the evaluation of ORR pathway for Sr2Co1 − x Fe x O3Cl, and oxygen electrocatalysis on other mixed-anion perovskite compounds in alkaline media.Solid-state method was applied to the synthesis of Sr2Co1 − x Fe x O3Cl and other mixed-anion perovskites. Electrochemical measurements were conducted by a three-electrode cell with a rotating disk electrode (RDE). Glassy carbon RDEs coated by a catalyst layer consisting of catalysts, Vulcan XC-72 (Cabot), and AS-4 (Tokuyama) were used for the working electrode. Pt wire and reversible hydrogen electrode were used for the counter and reference electrodes, respectively. Oxygen-saturated 1 mol dm− 3 KOH solution was used for ORR activity tests, and Argon-saturated 1 mol dm− 3 KOH solution containing 5 mmol dm− 3 H2O2 was used for the activity tests of peroxide reduction reaction (PRR). Peroxide decomposition rates of Sr2Co1 − x Fe x O3Cl were performed by measuring evolved oxygen gas in the same manner as previously described[5].While the highest OER activity was observed at x = 0.2, ORR activity of Sr2Co1 − x Fe x O3Cl was monotonously decreased with increase of Fe doping. Figure 1 shows decomposition rates of Sr2Co1 − x Fe x O3Cl in 1 mol dm− 3 KOH. Surprisingly, their HO2 − decomposition rate was higher than those of cobalt-based perovskite oxides such as LaCoO3, whose ORR activity was comparable to those of oxychlorides. Further discussion about ORR pathway of Sr2Co1 − x Fe x O3Cl, and oxygen electrocatalysis of other mixed-anion perovskites will be provided at the conference.Figure 1. H2O2 decomposition rates of Sr2Co1 − x Fe x O3Cl powders.

The medium effect on electrodissolution of adsorbed or suspended Ag nanoparticles

Electrochemical behaviour of six types of Ag nanoparticles (three stabilised by citrate and three others stabilised by polymers) was studied on carbon screen-printed electrode and glassy carbon rotating disc electrode. A stationary electrode was used as a support for adsorbed nanoparticles, whereas the rotating disc electrode was applied for experiments in nanoparticles suspension. Peak shaped cyclic voltammograms obtained for immobilised nanoparticles indicate their electro-dissolution and electrodeposition of silver in the backward scan. The given peak shape and its position on potential scale depend both on the type of nanoparticles and the supporting electrolyte. When aggregation of Ag nanoparticles was detected by UV–vis spectroscopy, splitting of an anodic peak corresponding to electrodissolution of silver in neutral phosphate buffer or NaOH solution is seen. Injection of a fraction of microgram of Ag nanoparticles was detected in chronoamperometric experiments on rotating disc electrode. However, in the presence of H2O2, the effect of nanograms of silver was seen on voltammetric curve indicating electrocatalytic amplification.

Doped Tin Oxide Aerogels As Oxygen Evolution Reaction Catalyst Supports

Lowering the noble metal (Ir or Ru) loading of Oxygen Evolution Reaction (OER) catalysts while maintaining both a high activity and a long-term stability for Proton Exchange Membrane Water Electrolysis (PEMWE) cells is a challenging topic for industry and academia. A possible strategy is the use of support materials (Figure 1) that are stable under OER conditions (> 1.4 V vs. the standard hydrogen electrode).Due to its large specific surface area and high electrical conductivity, carbon black is a popular and widely used catalyst support for electrochemical applications.[i] However its use is limited by the high anodic potentials required for the OER which would, due to corrosion, cause detachment of the supported catalyst.[ii] Therefore, non-carbon based supporting materials are required. Titanium dioxide (TiO2)[iii] or tin dioxide (SnO2)[iv] based materials are promising candidates due to their good corrosion resistance and strong interaction with noble metals catalysts. Wang and co-workers, reported enhanced OER activity and stability of antimony doped tin oxide (ATO)[v] aerogels supported Ir oxide nanoparticles.[vi] Uchida et al. also reported improved utilization of Ir deposited on tantalum doped tin oxide (TaTO) catalysts used at a PEMWE anode.[vii] However, Claudel et al. underlined that the stability of the doping element is a key issue for doped tin oxide to be implemented in PEMWE anodes.[viii] In this work[ix], our 3D highly porous aerogel materials were revisited to prepare Sb or Ta-doped tin oxide based catalyst support. IrOx nanoparticles were in-situ deposited over the different doped tin oxide based aerogels. After synthesis and characterization of both the support and the IrOx nanoparticles, the OER activity was measured on glassy carbon rotating disk electrodes under conditions simulating PEMWE anode operation. Investigations were based on N2 sorption, Scanning Electron Microscopy (SEM), X-Ray Photoelectron Spectroscopy (XPS), X-Ray Diffraction (XRD) and electrochemical characterization to analyse the physicochemical properties of the support and their impact on the catalytic activity and the stability of the catalysts.Our results show that supported Ir oxide nanoparticles are both more active and more stable than unsupported ones. Consistently with the results reported by Uchida et al.,[vii] despite very different electronic conductivities of the supports, the OER mass activities of supported IrOx nanoparticles were found similar in thin-film electrode configuration. Figure 1: Graphical representation of the designed anode for a PEWE cell, where the IrOx NPs are supported on a 3D porous aerogel material based on doped SnO2. Acknowledgements The authors wish to thank Pierre Ilbizian for supercritical drying, Frédéric Georgi for XPS analysis, Suzanne Jacomet for SEM/EDX analysis and Gabriel Monge for XRD analysis. This work was funded by the European Union's H2020 Program within the Fuel Cells and Hydrogen 2 Joint Undertaking under grant agreement 779478 (FCH-JU project PRETZEL) and the French National Research Agency (ANR-17-CE05-0033 project MOISE). It was supported by Capenergies and Tenerrdis. [i] (a) A. Dicks, J. Power Sources, 2006, 156, 128; (b) E. Antolini, E. Gonzalez, Appl. Catal., A, 2009, 365, 1; (c) E. Antolini, Appl. Catal., B, 2012, 52,123. [ii] (a) H.-S. Oh, K. H.Lim, B. Roh, I. Hwang, H. Kim, Electrochim. Acta, 2009, 54, 6515; (b) H.-S. Oh, J.-G. Oh, S. Haam, K. Arunabha, B. Roh, I. Hwang, H. Kim, Electrochem. Commun,. 2008, 10, 1048; (c) S. Maass, F. Finsterwalder, G. Frank, R. Hartmann, C. J. Merten, Power Sources, 2008, 176, 444. [iii] G. Chen, S. R. Bare, T. E. Mallouk, J. Electrochem. Soc., 2002, 149, 1092 [iv] H. S. Oh, H. N. Nong, D. Teschner, T. Reier, A. Bergmann, M. Gliech, J. Ferreira de Araujo, E. Willinger, R. Schloegl, P. Strasser, J. Am. Chem. Soc., 2016, 138, 12552. [v] G. Ozouf, Ch. Beauger, J. Mater. Sci., 2016, 51 (11), 5305-5320 [vi] L. Wang, F. Song, G. Ozouf, D. Geiger, T. Morawietz, M. Handl, P. Gazdzicki, Ch. Beauger, U.Kaiser, R. Hiesgen, A. Gago, K. Friedrich, Journal of Materials Chemistry, 2017, 5, 3172 [vii] H. Ohno, S. Nohara, K. Kakinuma, M. Uchida, H. Uchida, Catalysts, 2019, 9, 74 [viii] F. Claudel, L. Dubau, G. Berthomé, L. Sola-Hernandez, C. Beauger, L. Piccolo, F. Maillard, ACS Catal, 2019, 9 (5), 4688-4698 [ix] L. Solà-Hernández, F. Claudel, F. Maillard, C. Beauger, Int. J. Hydrog. Energy, 2019, 44 (45), 24331–24341. Figure 1

X-ray photoelectron spectroscopy and rotating disk electrode measurements of smooth sputtered Fe-N-C films

Electrocatalysts for the oxygen reduction reaction (ORR) based on complexes of iron and nitrogen in a carbon matrix (Fe-N-C) are a promising alternative to platinum group metal (PGM) based catalysts in polymer electrolyte membrane (PEM) fuel cells. Further improvements of Fe-N-C catalysts would benefit from model thin film studies of activity and stability of catalytic sites, but synthesis of Fe-N-C model thin films is challenging. Here we report on synthesis and characterization of Fe-N-C thin films produced by co-sputtering iron and carbon in a reactive nitrogen atmosphere onto removable glassy carbon rotating disk electrode (RDE) tips. Scanning electron microscopy (SEM) measurements indicate that the Fe-N-C films deposited at high temperature are smoother than the films annealed at high temperature. Electrocatalytic activity measured on the thin Fe-N-C films is greater for both high-temperature samples than for the room-temperature sample. From the analysis of X-ray photoelectron spectroscopy (XPS) data, exposure of the films to high temperatures results in increased graphitization of the carbon within the Fe-N-C films, and increased relative amount of graphitic and hydrogenated nitrogen species. Overall, the results of this study demonstrate the feasibility of a thin film model system approach for studying active sites in PGM-free catalysts.

Electrocatalytic Study of Low-Cost Bimetallic Cobalt/Iron Catalyst on Carbon for Non-Enzymatic Glucose Sensor in Human Urine

This study investigated an electrocatalytic behavior of non-enzymatic glucose detection in urine by using low-cost non-precious metal CoFe catalyst on carbon (C) supported. The bimetal catalyst was prepared by the reduction of oleic acid and loaded 10% wt. metal onto the activated carbon. Due to the synergistic effect, CoFe exhibited its intrinsic electrocatalytic property, suitable for the chemisorption of glucose molecule and the d-electron of metal. For morphology and elemental composition, CoFe/C was characterized by Transmission Electron Microscopy (TEM), and X-ray Energy Dispersive Spectroscopy (EDS) and X-ray Diffraction (XRD), exhibiting Co (111) and Fe2O3(104) with the nanocluster average diameter of 25 nm. More importantly, electrocatalytic behaviors of CoFe/C were assessed by cyclic voltammetry (CV) and Differential Pulse Voltammetry (DPV) on the glassy carbon rotating disk electrode for glucose detection (0-3 mM) in modified artificial urine (mAUM), and human urine specimens. In particular, excellent sensitivities from the lower range of glucose level (&lt; 1 mM) and the higher level by DPV in mAUM were estimated to be 318.42 and 82.20 μA.cm-2.mM-1 with the correlation coefficient (R2) values of 0.90 and 0.94, respectively. Furthermore, the as-prepared CoFe/C biosensor also demonstrated practical measurement in human urine sample with the sensitivity of 59.72 μA.cm-2.mM-1 (R2 = 0.99) without any electron facilitators (e.g. sodium hydroxide), thereby providing a promising cost-effective catalyst design for future technology of non-enzymatic glucose sensing applications in urine.

A single-droplet-based electrochemical fluorescence method for the determination of aluminum at the nanomolar level

This paper combines a single-droplet-based electrochemical system with an optical fiber fluorescence spectrometer to develop an electrochemical fluorescence method for determining the nanomolar level of aluminum. Al3+ ions were preconcentrated on a glassy carbon rotating disk electrode (RDE) at −2.0 V. After preconcentration, the RDE was rinsed and a droplet (80 μL) of solution containing 0.2 mM 8-hydroxyquinoline-5-sulfonic acid (HQS) was sandwiched between the RDE (above) and the reference/counter integrated electrode (below). Subsequently, the RDE was rotated and aluminum was stripped into the droplet at −1.4 V; the fluorescence intensity was then measured using a 395 nm LED excitation source. The fluorescence intensity at 515 nm is positively correlated with Al3+ concentration, allowing the determination of aluminum. This method exhibits high selectivity and sensitivity with a detection limit of 7.6 nM. Trace aluminum in real samples was determined by this method and satisfactory results were obtained.

A comparative study of non-enzymatic glucose detection in artificial human urine and human urine specimens by using mesoporous bimetallic cobalt-iron supported N-doped graphene biosensor based on differential pulse voltammetry

Development of non-invasive glucose measurement allows future advancement of smart sensing platform for diagnostic technology. Especially for endocrine disorders, advanced kidney diseases, and diabetes, monitoring excessive glucose level in urine can provide invaluable information for clinical prognosis and preventive healthcare. Herein, we present a comparative electrochemical study of cobalt/iron (CoFe) catalyst on nitrogen-doped graphene (NG) for non-enzymatic glucose detection, carried out in physiological pH urine including (i) modified artificial urine medium (mAUM), (ii) commercial standard urine (Surine), and (iii) human urine specimens. With no requirement of strong alkaline addition, catalytic properties of CoFe-NG were assessed by using cyclic voltammetry (CV) and differential pulse voltammetry (DPV) on a glassy carbon rotating disk electrode. Upon successive glucose additions from 0 to 3 mM, DPV results revealed two anodic peaks at +0.18 V and +0.42 V versus Ag/AgCl, corresponding to Co3+ and Co4+ as a result of glucose binding in urine. By evaluating at +0.18 V, the sensitivities of CoFe-NG were estimated to be 16.77 (R2 = 0.987), 45.36 (R2 = 0.988), and 20.26 (R2 = 0.991) μA mM−1 cm−2 with the limit of detection of 0.25, 0.07, and 0.19 mM in mAUM, Surine, and human urine specimen with low serum creatinine, respectively. Furthermore, the effects of CoFe on graphene (G) and carbon Vulcan XC-72 (C) were also studied in comparison of NG on the bimetal. Interestingly, CoFe-C showed a good electrochemical trend in glucose detection in urine. However, negligible catalytic activity was presented in CoFe-G. Thus, electrochemical responses of CoFe-C were also further studied in the comparison of CoFe-NG in each type of urine. Overall, CoFe-NG outperformed CoFe-C in all types of urine and exhibited an excellent anti-interference property toward uric acid, thereby suggesting great potential for the next generation of glucose sensing platform in urine.

The role of 3,4-dihydroxyphenylacetic acid adsorption in the oxidation of homovanillic acid at a glassy carbon rotating disc electrode

Homovanillic acid (HVA) is an important biomarker for neurological disorders. Traditional methods for detection of this compound are chromatographic methods coupled with fluorescence, mass spectrometric, or electrochemical detectors. However, voltammetric methods can be useful because of their low cost, easy automation, portability, and sensitivity. 3,4-Dihydroxyphenylacetic acid (DOPAC) is formed during electrochemical oxidation of HVA. The formation of this compound could interfere with the quantification of HVA using electrochemical methods. This work aims at describing the behavior of the intermediate species generated in the oxidation of HVA using cyclic voltammetry (CV), linear staircase voltammetry (LSV), and electrochemical impedance spectroscopy (EIS) with a glassy carbon rotating disc electrode (GC RDE). The importance and desirability of detailed investigation of surface phenomena connected with electrochemical oxidation used for electroanalytical purposes are thus clearly demonstrated. The EIS measurements indicate that the intermediate DOPAC formed by the oxidation of HVA can adsorb on the surface of a glassy carbon electrode (GCE). For the static GCE, a semi-finite linear diffusion was observed and for the GC RDE, a finite-length diffusion was verified. In the case of the semi-finite linear diffusion, EIS measurements revealed that the resistance to charge transfer decreases from 100 to 50Ω, and the capacitance of the double layer increases from 1.44 to 1.66μF with increasing concentration of HVA. However, LSV results showed that Levich equations for electrochemical reduction of K3[Fe(CN)6] probe in basic medium are ilim=−0.157 ω1/2−0.006 (R2=0.9999) in the absence of HVA (i.e. without DOPAC adsorption at GCE) and ilim=−0.154 ω1/2−0.021 (R2=0.9998) in the presence of HVA (i.e. with DOPAC adsorption at GCE). The small difference in these two equations proves a negligible influence of DOPAC adsorption on the quantification of HVA using stationary methods.