Koutecky-Levich Analysis Research Articles

Lignin-Derived Carbons for Electrocatalytic Oxygen Reduction Reaction in Alkaline Media

With the Paris agreement, many countries around the world have set ambitious decarbonization goals. Due to high abundance and greenhouse gas emitting index, there is a growing interest toward conversion of lignocellulosic biomass into materials used in the production of electrodes for batteries, fuel cells and supercapacitors. Lignin-derived carbons could substitute fossil fuel-based acetylene black. However, there are many challenges facing production of electrode-grade carbon from lignin. They include, control of inorganic impurities such as SiO2, porosity and conductivity. Lignin source and extraction processes also lead to many undesired impurities and batch to batch variability.Carbon black alone is known to catalyze the oxygen reduction reaction (ORR), a key catalytic reaction controlling performance of metal-O2 batteries and fuel cells. The ORR can proceed via sequential 2-electron (2e) or a more desirable direct 4-electrons (4e) reduction pathway depending on the chemical identity of the active material.1-2) In this presentation, we will report on the physio-chemical properties of kraft lignin-derived carbons and their application in formulation of catalytic inks for electrocatalytic applications. In particular, we will discuss the effect of lignin source and pyrolysis parameters on the ORR performance in alkaline media.Lignin-derived carbons were characterized by powder X-ray diffraction, SEM-EDX, Raman spectroscopy and electrochemical methods. The catalytic activities of the electrodes prepared using lignin-derived carbons were studied by linear swept voltammetry (LSV) and rotating disk electrode (RDE) methods. Fig. 1 shows LSV of commercial acetylene black and pine-wood derived carbon, after three-steps pyrolysis. The ORR onset potential and a half-wave potential is 0.81 and 0.72 V vs. RHE, for the lignin carbon, which is closer to the half-wave potential of the widely-used 40% Pt/C catalyst. The electron transfer number, obtained from the Koutecky-Levich analysis, was 1.5, suggesting 2e reduction process.References1) Donghui Guo et al., Active Sites of Nitrogen-doped Carbon Materials for Oxygen Reduction Reaction Clarified using Model Catalysts.Science351,361-365 (2016).2) R.P. Putra, Y. Samejima, S. Nakabayashi, H. Horino, I.I. Rzeznicka, Copper-based Electrocatalyst Derived from a Copper Chelate Polymer for Oxygen Reduction Reaction in Alkaline Solutions, Catalysis Today, (2020).AcknowledgementsThis work was supported by JST Strategic International Collaborative Research Program (SICORP), Grant Number JPMJSC2302, Japan. Fig 1. LSV in the ORR region in 1 M KOH for acetylene black and lignin-derived carbon. Figure 1

Analytical Electrochemistry of Nickel and Platinum Electrolytes

Pt-Ni alloys possess unique catalytic and magnetic properties, and it is highly desirable to synthesize conformal thin films of these materials for integration into magnetic devices and energy conversion systems. Electrodeposition provides large scale and complex form plating without the need for additional equipment such as a vacuum system or furnace used in traditional Pt-Ni synthesis. While nickel and platinum electrodepositions can individually plate effectively, co-deposition provides its own challenges. Large differences in deposition potentials between the two metals and platinum’s reduced overpotential for undesirable hydrogen evolution are necessary challenges that need to be addressed. Investigation into various nickel and platinum bath chemistries over a range of pH’s will help develop a Pt-Ni co-deposition bath that will produce a uniform deposition with desired morphologies.A rotating disk electrode experiment was conducted on each electrodeposition bath along with a Koutecky-Levich analysis to determine the baths diffusion coefficient and rate constant. Nickel baths include boric acid, acetic acid, citric acid, hydrochloric acid, and phosphoric acid. Platinum baths include hydrochloric acid and acetic acid. Hydrogen suppression techniques were also studied for both nickel and platinum baths to determine the effect on hydrogen generation and metal deposition. Successful suppression of the hydrogen generation reaction would result in greater control of nickel deposition onto platinum as well as thicker platinum films.

Rotating Disk Electrode Studies of Benzaldehyde Oxidation on Group Ib Metals

Water electrolysis is a green means of producing hydrogen gas to be used as a fuel or reactant in key industrial processes such as ammonia production. The conventional system of oxygen evolution (OER) and hydrogen evolution (HER) suffers from slow kinetics at the anode and a high cell voltage limited by thermodynamics. One solution is to replace OER with another reaction such as an organic oxidation reaction that has a lower equilibrium potential and produces a valuable organic byproduct at the anode. Recent work has demonstrated that biomass-derived aldehydes such as furfural and 5-hydroxymethylfurfural can be selectively oxidized to their corresponding carboxylates and H2 on Cu-based anodes at low potentials (~0.1 VRHE) in alkaline media. In alkaline media, a hydroxide ion adds to the carbonyl carbon of an aldehyde molecule forming a gem diolate in solution phase. It is argued that dehydrogenation becomes more facile due to the weaker C-H bond of the gem diolate compared to the aldehyde form of the molecule. This may enable oxidation without the need for coadsorbed-OH. Additionally, aldehyde oxidation is only active on Au, Ag, and Cu electrodes at higher pHs.The aim of this work is to identify the conditions under which low potential oxidation of aldehydes and high anodic Faradaic efficiency to H2 occurs, namely, electrode material and applied potential. To avoid polymerization side reactions of furans in alkaline media, we studied benzaldehyde as a model compound which has also been shown to exhibit low potential oxidation and anodic H2. The activity (rate of aldehyde consumption) and selectivity (to anodic H2 versus H2O) were compared for the group Ib metals (Au, Ag, Cu), each of which demonstrates a relatively early onset potential (<0.4 VRHE) and generates anodic H2 at certain potentials. Rotating disk electrode studies with Koutecky-Levich analysis were conducted to separate mass transfer effects from kinetic current to compare the intrinsic activity of each electrode. Additionally, the average number electrons transferred per aldehyde molecule consumed was estimated from Koutecky-Levich analysis; the one-electron pathway combines the aldehyde hydrogens to H2 while the two-electron pathway requires an additional electron per aldehyde molecule to oxidize the hydrogen from the surface to form H2O. Rotating ring disk electrode studies were used to further validate the anodic H2 generation as a Pt ring was still able to oxidize H2 despite the presence of benzaldehyde. This was used as an additional means of estimating the selectivity to H2 as a function of potential.CVs for benzaldehyde oxidation on each metal are shown in Figure a. Peak current followed the order of Au > Ag >> Cu. Au reached a mass transfer limiting current, and Ag also showed an RPM dependence but did not reach full mass transfer limitations. Cu was the least active and did not show any mass transfer dependence. On the other hand, Cu has the earliest onset potential followed by Ag then Au. The average number of electrons transferred per aldehyde molecule according to RRDE measurements is shown in Figure b, where one electron represents H2 and two electrons represents H-oxidation to H2O. Cu had 100% selectivity to H2 over its entire potential window of activity (up to 0.45 VRHE). Au and Ag have high selectivity to H2 at low potentials but decreases to near zero H2 production in the range of 0.6-0.8 VRHE; the decrease in selectivity to H2 on Ag occurs slightly earlier than Au (~0.1 V). H2 selectivity is related to the inability of each metal to oxidize H to H2O at low potentials. Figure 1

Exploring the Kinetics of Oxygen Reduction Reaction in Relation to Pitting Corrosion Resistance in Fe-Cr Alloys

Pitting corrosion, a major concern in materials science and engineering, threatens critical metallic structures and components. Its implications reach across daily life and industries such as energy, transportation, and infrastructure, potentially causing environmental harm, economic losses, and even loss of life. This complex process is influenced by factors including material composition, local environment, and mechanical stresses.1 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.2 In Fe-Cr alloys, the ORR occurs on this film, undergoing a mixed diffusion and kinetic-controlled process. The number of electrons involved and the corrosion rate are determined by the thermodynamically stable state of the passive film.This research studies the ORR kinetics on FeCr alloys fabricated by arc-melting, using both computational and experimental methods. The rotating disc electrode technique was employed across various Fe:Cr alloys in alkaline and neutral electrolytes. The Koutecky-Levich analysis showed a dominant four-electron ORR pathway in all tested Fe-Cr alloys, with the ORR significantly inhibited by higher proportions of Cr2O3 in the film, suggesting a correlation with high chromium content. Experimental ORR overpotentials, when combined with theoretical potential-pH (Pourbaix) diagrams, helped discern the likely characteristics of the passive films on the alloys. It was deduced that the catalytic properties of potential passive films increased in the order Cr2O3 < Fe3O4 < Fe2O3 < FeCr2O4. These findings, excellently aligned with density functional theory (DFT) modelling, underscore the role of increased chromium levels in slowing pitting progression by suppressing the ORR.In summary, this research offers distinctive insights into the correlation between the kinetics of the ORR and the resistance to localized corrosion in Fe-Cr alloys, which enhances our understanding of the underlying mechanisms of pitting corrosion. Keywords FeCr alloy, pitting corrosion, ORR, rotating disc electrode, DFT Reference [1] B. Zhang, J. Wang, B. Wu, X. W. Guo, Y. J. Wang, D. Chen, Y. C. Zhang, K. Du, E. E. Oguzie, and X. L. Ma, Nat. Commun. (2018) 9, 2559.[2] G. T. Burstein, P. C. Pistorius, and S. P. Mattin, Corros. Sci. (1993) 35, 57.

Electrochemical oxidation of glucose in alkaline environment—A comparative study of Ni and Au electrodes

The glucose oxidation reaction (GOR) was studied on Au and Ni electrodes by cyclic voltammetry (CV), rotating disk electrode (RDE) measurements coupled with Koutecky–Levich analysis, Differential Electrochemical Mass Spectrometry (DEMS), in situ Fourier Transform Infrared spectroscopy (FTIRS) and High Performance Liquid Chromatography (HPLC) analysis of the reaction products after electrolysis in potentiostatic mode under continuous flow conditions. This study allowed to identify the reaction products and propose tentative reaction mechanisms. On gold, glucose is adsorbed on metallic sites through its anomeric function (C1) resulting in the formation of gluconate as the main GOR product at potentials close to 0.6 V vs. RHE, with a selectivity towards gluconate close to 100% and a projected faradaic efficiency of ca. 70%. The conversion rate is rather low, close to 20%, due to poisoning of the surface, leading to a strong deactivation. At potentials above 0.7 V vs. RHE, the selectivity towards gluconate decreases and non-selective GOR oxidation proceeds through CC bond cleavage. On nickel, the GOR occurs at high potentials (close to 1.2 V vs. RHE) on Ni(OH)2 and NiOOH sites, and proceeds through C1C2 bond breaking, resulting in arabinose and formate. At higher potentials, the selectivity towards arabinose decreases, formate being the main reaction product.

Electrochemical Reduction of Ytterbium in Non-Aqueous Solvents - Towards Production of Pure Medical Lutetium-177

In recent years targeted radionuclide therapy has become a topic of great interest as a growing number of cancers can potentially be treated with radionuclides like lutetium-177 and samarium-153. To facilitate this growth and meet the demand, preparation and purification of lutetium-177 is increasing in importance. In the most common production route, enriched ytterbium-176 targets are irradiated and the produced lutetium-177 is separated from the ytterbium target material. This challenging separation is commonly performed using ion exchange resins, a method limited in scalability as the chemical similarity between ytterbium and lutetium results in poor separation at higher concentrations.An alternative method to aid the separation of lutetium from ytterbium is the electrochemical reduction of ytterbium as a change in oxidation states changes chemical properties significantly. Ytterbium has an accessible divalent state, whereas lutetium has not. However, the very negative value of the Yb(III)/Yb(II) standard reduction potential (E0= -1.04 V vs. SHE) does pose some challenges as it becomes a necessity to work in non-aqueous solvents with a very low water content. Similar research has already been performed in our group, looking into the separation of samarium-153 from europium-154, reducing europium in nitrate media, followed by a separation over TEVA resin. In this study, we investigated the reduction of ytterbium in ethylene glycol, dimethyl sulfoxide, tetrahydrofuran and N,N-dimethyl formamide by cyclic voltammetry, linear sweep voltammetry and electronic impedance spectroscopy. The influence of multiple variables such as moisture content on the reduction kinetics and stability of Yb(II) was determined and a Koutecky-Levich analysis was performed. Our results demonstrate that the reduction of ytterbium in non-aqueous solvents is feasible but attempts to perform bulk electrolysis were not successful yet. Ethylene glycol, dimethyl sulfoxide and N,N-dimethyl formamide demonstrate favorable conditions and in each solvents clear reduction and oxidation peaks were observed. It was found that the solvents do affect the peak separation, reduction potential and sensitivity to passivation. Figure 1

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

Variation of graphene/titanium dioxide concentration as electrocatalyst for an oxygen reduction reaction in constructed wetland-microbial fuel cells

Constructed wetlands-microbial fuel cells (CW-MFC) are an innovative technology used for simultaneous bioelectricity generation and wastewater treatment. This is possible due to the installation of macrophytes in an electrode configuration, in which electroactive microorganisms use organic substrates as biofuel. One way to improve the electrochemical performance of CW-MFCs is through the impregnation of cathodic electrocatalysts. Therefore, in this study the bioelectricity production capacity of CW-MFCs was evaluated from the oxygen reduction reaction (ORR). For this study, the concentrations 0 (CW-MFC1), 0.5 (CW-MFC2), and 1 mg/cm2 (CW-MFC3) of graphene/titanium dioxide (G/TiO2) as electrocatalyst on the cathodes were evaluated. Using the Koutecky-Levich analysis, it was determined that the ORR transfer mechanism arises via a 4-electron pathway. The electrokinetic parameters of Tafel slope, charge transfer coefficient, and exchange current density determined the efficiency of the ORR, registering 92 mV/dec, 0.93 (α), and 2.30 x10-3 mA/cm2, respectively for CW-MFC3. The highest electrochemical performance was obtained at a concentration of 1 mg/cm2 (CW-MFC3) of G/TiO2, generating 144 mW/m2 of power density, 157 Ω of internal resistance, −150 mV of anodic potential, and 383 mV of cathodic potential. The surface modification carried out on the cathodes resulted in a catalytic increase in the ORR.

Size dependent electrocatalytic activities of h-BN for oxygen reduction reaction to water.

Electrocatalytic activities for the oxygen reduction reaction (ORR) of Au electrodes modified by as prepared and size selected (0.45-1.0, 0.22-0.45, and 0.1-0.22 µm) h-BN nanosheet (BNNS), which is an insulator, were examined in O2 saturated 0.5M H2SO4 solution. The overpotential was reduced by all the BNNS modifications, and the smaller the size, the smaller the overpotential for ORR, i.e., the larger the ORR activity, in this size range. The overpotential was reduced by as much as ∼330 mV compared to a bare Au electrode by modifying the Au surface by the BNNS of the smallest size range (0.1-0.22 µm). The overpotential at this electrode was only 80 mV more than that at the Pt electrode. Both the rotation disk electrode experiments with Koutecky-Levich analysis and rotating ring disk electrode measurements showed that more than 80% of oxygen is reduced to water via the four-electron process at this electrode. These results strongly suggest and theoretical density functional theory calculations support that the ORR active sites are located at the edges of BNNS islands adsorbed on Au(111). The decrease in size of BNNS islands results in an effective increase in the number of the catalytically active sites and, hence, in the increase in the catalytic activity of the BNNS/Au(111) system for ORR.

Cu2O-Cu@Titanium Surface with Synergistic Performance for Nitrate-to-Ammonia Electrochemical Reduction.

Transition metals, such as titanium (Ti) and copper (Cu) along with their respective metal oxides (TiO2, Cu2O, and CuO), have been widely studied as electrocatalysts for nitrate electrochemical reduction with important outcomes in the fields of denitrification and ammonia generation. Based on this, this work conducted an evaluation of a composite electrode that integrates materials with different intrinsic activities (i.e., Cu and Cu2O for higher activity for nitrate conversion; Ti for higher faradaic efficiency to ammonia) looking for potential synergistic effects in the direction of ammonia generation. The specific performance of single-metal and composite electrodes has shown a strong dependence on pH and nitrate concentration conditions. Faradaic efficiency to ammonia of 92% and productivities of 0.28 mmolNH3 ·cm-2·h-1 at 0.5 V vs reversible hydrogen electrode (RHE) values are achieved, demonstrating the implicit potential of this approach in comparison to direct N2RR with values in the order of μmolNH3 ·h-1·cm-2. Finally, the electrochemical rate constants (k) for Ti, Cu, and Cu2O-Cu/Ti disk electrodes were determined by the Koutecky-Levich analysis with a rotating disk electrode (RDE) in 3.02 × 10-6, 3.88 × 10-4, and 4.77 × 10-4 cm·s-1 demonstrating an apparent synergistic effect for selective NiRR to ammonia with a Cu2O-Cu/Ti electrode.

Nitrogen doped carbonaceous materials as platinum free cathode electrocatalysts for oxygen reduction reaction (ORR)

Comparison of physicochemical properties and electrocatalytic behavior of different N-doped carbonaceous materials as potential catalysts for oxygen reduction reaction (ORR) was attended. Ball-milling of graphite with melamine and solvothermal treatment of graphite oxide, graphene nanoplatelets (GNP) with ammonia were used as preparation methods. Elemental analysis and N2 physisorption measurements revealed the synthesis of N-doped materials with strongly different morphological parameters. Contact angle measurements proved that all three samples had good wettability properties. According to analysis of XRD data and Raman spectra a higher nitrogen concentration corresponded to a smaller size of crystallites of the N-doped carbonaceous material. Surface total N content determined by XPS and bulk N content assessed by elemental analysis were close, indicating homogenous inclusion of N in all samples. Rotating disc electrode tests showed that these N-doped materials weremuch less active in acidic medium than in an alkaline environment. Although the presence of in-plane N species is regarded to be advantageous for the ORR activity, no particular correlation was found in these systems with any type of N species. According to Koutecky–Levich analysis, both the N-containing carbonaceous materials and the reference Pt/C catalyst displayed a typical one-step, four-electron ORR route. Both ball-milled sample with high N-content but with low SSA and solvothermally synthesized N-GNP with high SSA but low N content showed significant ORR activity. It could be concluded that beside the total N content other parameters such as SSA, pore structure, structural defects, wettability were also essential for achieving high ORR activity.

Efficient bifunctional cerium-zeolite electrocatalysts for oxygen evolution and oxygen reduction reactions in alkaline media

Four cerium-zeolites were examined as low-cost bifunctional electrocatalysts for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) in an alkaline solution. Cerium-exchanged zeolites, synthetic 13X and natural clinoptilolite were employed as prepared and calcined (cal). Four samples (Ce-13X cal, Ce-13X, Ce-Cli cal, and Ce-Cli) were characterized by X-ray powder diffraction analysis (XRPD), scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS), Fourier transform infrared (FTIR) spectroscopy, fluorescence spectroscopy and Brunner-Emmet-Teller (BET) method. All four zeolites exhibited activity for the two studied reactions. Ce-13X cal gave the lowest onset potential accompanied by the highest OER current density. Furthermore, Ce-13X cal showed the most positive ORR onset potential value of 0.84 V and the most positive ORR half-wave potential of 0.75 V (ca. 50 mV more positive than value for the other three cerium-zeolite electrodes) accompanied by the lowest Tafel slope of 80 mV dec−1. Koutecky-Levich analysis indicated that ORR follows 2-electron pathway. Overall, Ce-13X cal showed the best OER/ORR performances in alkaline solution.

Rapid Measurement of Lactate in the Exhaled Breath Condensate: Biosensor Optimization and In-Human Proof of Concept.

Lactate concentration is of increasing interest as a diagnostic for sepsis, septic shock, and trauma. Compared with the traditional blood sample media, the exhaled breath condensate (EBC) has the advantages of non-invasiveness and higher user acceptance. An amperometric biosensor was developed and its application in EBC lactate detection was investigated in this paper. The sensor was modified with PEDOT:PSS-PB, and two different lactate oxidases (LODs). A rotating disk electrode and Koutecky-Levich analysis were applied for the kinetics analysis and gel optimization. The optimized gel formulation was then tested on disposable screen-printed sensors. The disposable sensors exhibited good performance and presented a high stability for both LOD modifications. Finally, human EBC analysis was conducted from a healthy subject at rest and after 30 min of intense aerobic cycling exercise. The sensor coulometric measurements showed good agreement with fluorometric and triple quadrupole liquid chromatography mass spectrometry reference methods. The EBC lactate concentration increased from 22.5 μM (at rest) to 28.0 μM (after 30 min of cycling) and dropped back to 5.3 μM after 60 min of rest.

Electrocatalytic Behavior of an Amide Functionalized Mn(II) Coordination Polymer on ORR, OER and HER.

The new 3D coordination polymer (CP) [Mn(L)(HCOO)]n (Mn-CP) [L = 4-(pyridin-4-ylcarbamoyl)benzoate] was synthesised via a hydrothermal reaction using the pyridyl amide functionalized benzoic acid HL. It was characterized by elemental, FT-IR spectroscopy, single-crystal and powder X-ray diffraction (PXRD) analyses. Its structural features were disclosed by single-crystal X-ray diffraction analysis, which revealed a 3D structure with the monoclinic space group P21/c. Its performance as an electrocatalyst for oxygen reduction (ORR), oxygen evolution (OER), and hydrogen evolution (HER) reactions was tested in both acidic (0.5 M H2SO4) and alkaline (0.1 M KOH) media. A distinct reduction peak was observed at 0.53 V vs. RHE in 0.1 M KOH, which corresponds to the oxygen reduction, thus clearly demonstrating the material's activity for the ORR. Tafel analysis revealed a Tafel slope of 101 mV dec-1 with mixed kinetics of 2e- and 4e- pathways indicated by the Koutecky-Levich analysis. Conversely, the ORR peak was not present in 0.5 M H2SO4 indicating no activity of Mn-CP for this reaction in acidic media. In addition, Mn-CP demonstrated a noteworthy activity toward OER and HER in acidic media, in contrast to what was observed in 0.1 M KOH.

Diffusion-charge transfer characterization of a rotating cylinder electrode reactor used for the complete electrocatalytic removal of nitrate from water

Groundwater nitrate contamination is an emerging threat in stressed regions under intensive farming although, lately, efforts to valorize such residues are highly encouraged. Here, electrochemical nitrate removal has been investigated as a versatile strategy for this purpose, using a reactor equipped with a cheap central Fe-based rotating cylinder electrode (RCE) as cathode and six concentric Ti|IrO2 plates as anodes. The study of the effect of Ecath and rotational speed (ω) on NO3− electroreduction from a synthetic aqueous solution with high conductivity revealed the feasibility of complete nitrate removal, which only required 100–120 min at Ecath = −1.80 V vs Hg|Hg2SO4|sat. K2SO4 within the ω-range of 100–500 rpm. The concentration decays agreed perfectly with a first-order kinetics. NH3 was accumulated as main product, being partly volatilized due to the quick solution alkalization, whereas NO2− was not found. Linear sweep voltammetries demonstrated the high electrocatalytic activity of carbon steel RCE as compared to inactive stainless steel. Koutecky-Levich analysis showed that the reduction process with carbon steel at Ecath from −1.80 V involved 8 electrons. The participation of H radical in the reduction mechanism was ascertained by electron paramagnetic resonance. The mass transport and charge transfer of the RCE reactor were characterized under turbulent flow by means of the dimensionless Damköhler (Da) number, as well as from the Sherwood-Reynolds-Schmidt (Sh-Re-Sc) analysis. A mixed regime with a prevalence of mass transport control was determined at Ecath from −1.8 V. The Sh = 0.70Re0.46Sc0.356 correlation obtained for this reactor may serve to guide the scale-up of electrochemical NO3− removal as more electrocatalytic cathode materials are developed. Successful NO3− elimination from solutions with low conductivity that mimicked groundwater is finally reported.

Bio-Electrocatalytic Conversion of Food Waste to Ethylene via Succinic Acid as the Central Intermediate.

Ethylene is an important feedstock in the chemical industry, but currently requires production from fossil resources. The electrocatalytic oxidative decarboxylation of succinic acid offers in principle an environmentally friendly route to generate ethylene. Here, a detailed investigation of the role of different carbon electrode materials and characteristics revealed that a flat electrode surface and high ordering of the carbon material are conducive for the reaction. A range of electrochemical and spectroscopic approaches such as Koutecky-Levich analysis, rotating ring-disk electrode (RRDE) studies, and Tafel analysis as well as quantum chemical calculations, electron paramagnetic resonance (EPR), and in situ infrared (IR) spectroscopy generated further insights into the mechanism of the overall process. A distinct reaction intermediate was detected, and the decarboxylation onset potential was determined to be 2.2-2.3 V versus the reversible hydrogen electrode (RHE). Following the mechanistic studies and electrode optimization, a two-step bio-electrochemical process was established for ethylene production using succinic acid sourced from food waste. The initial step of this integrated process involves microbial hydrolysis/fermentation of food waste into aqueous solutions containing succinic acid (0.3 M; 3.75 mmol per g bakery waste). The second step is the electro-oxidation of the obtained intermediate succinic acid to ethylene using a flow setup at room temperature, with a productivity of 0.4-1 μmol ethylene cmelectrode -2 h-1. This approach provides an alternative strategy to produce ethylene from food waste under ambient conditions using renewable energy.

Internal Mass Transport Induced Voltage Losses during Water Electrolysis on Interconnected Nickel Nanowire Mesh Electrodes

Nickel nanowire mesh electrodes offer great potential for water electrolysis in alkaline environment, given their high internal surface area, mechanical strength, and alkaline resistance. Here we use a nickel nanowire mesh with a specific area of 26 m2/cm3, resulting in ~100-fold area enhancement for a 4µm thick nanowire mesh, compared to planar nickel. This highly benefits geometric current density, potentially into a regime where mass transport is the main limiting factor. Therefore, we performed a systematic study of electrode behaviour under various mass transport conditions to decouple kinetic and mass transfer related contributions to electrode voltage losses.An electrochemical flow cell was designed to measure the nanowire mesh electrode performance under optimal mass transport conditions. The cell provided electrolyte flow through the nanomesh, enabling convective supply of reagent into the nanopores as well as convective removal of reaction products. Facilitated by the inherent strength of the nanowire mesh, superficial flow velocities up to 1 cm/s could be obtained, with which geometric current densities as high as 320 mA/cm2 could be measured in the kinetically limited (flowrate independent) regime.Internal mass transport limitations were isolated by use of an inverted rotating disk electrode (iRDE). Koutecky-Levich analysis was performed to compensate for external mass transfer contributions on both nanowire mesh electrodes and planar electrodes. The nanowire mesh as iRDE resembles operating conditions in an electrolyzer, whereas the planar electrode provides a well-defined benchmark area for the electrode surface activity.For both the oxygen evolution reaction as the hydrogen evolution reaction, mass transport limitations were studies as function of current density and electrolyte (NaOH) concentration. Nanowire mesh electrode operation in the presence of internal mass transport limitations was compared to the non-limited condition and planar benchmark, from which the electrode effectiveness factor could be calculated. A 1-d simplified model of the electrode was used to explain the observed phenomena and to provide guidance for design optimization.

Polythiophene, Polypyrrole and Carbon Nanotube-Based Catalyst for Alkaline Membrane Fuel Cell Cathode

The transformation from hydrocarbons to a cleaner and sustainable H2 economy is considered a favorable approach to meet the increasing energy demand. Among different H2 technologies, fuel cell technology is a viable option for environmentally clean energy conversion (1). Anion-exchange membrane fuel cell (AEMFC) is one of the suitable candidates to meet this demand as they could be more cost-effective than their widely used acidic counterpart, proton-exchange membrane fuel cell. For providing an affordable AEMFC, the non-precious metal catalyst (NPMC) for oxygen reduction reaction (ORR) at the cathode is needed (2).Co-doping (e.g. with N, S and Fe) of the nanocarbon material has been found to be more beneficial for the preparation of highly active ORR electrocatalysts compared to the application of just one dopant atom (e.g. N) (3). Therefore, we have prepared the polypyrrole (PPy), polythiophene (TPh), and multi-walled carbon nanotubes (MWCNT) based composite material that has been further pyrolysed and subjected to the acid treatment procedure to prepare the NPMC for the AEMFC cathode. Versatile optimization procedures were performed to finally obtain the nanocarbon material with the high ORR activity (A-PPy/PTh/MWCNT) (4).According to the scanning electron microscopy images, the composite catalyst exhibited mainly tubular morphology with PPy and PTh wrapped around the MWCNT. The physical characterization of A-PPy/PTh/MWCNT revealed the BET surface area of 379 m2 g−1 and micro-mesoporous tubular structure. The surface composition of the catalyst was found to contain S, C, N, O and Fe, while the latter originates from the FeCl3 polymerization catalyst. The ORR studies with A-PPy/PTh/MWCNT electrocatalyst in 0.1 M KOH showed the ORR half-wave potential of 0.85 V vs. RHE. According to the Koutecky-Levich analysis, the NPMC catalyzed a preferred 4-electron oxygen reduction pathway (4).A single-cell AEMFC experiment with A-PPy/PTh/MWCNT cathode catalyst showed the maximum power density (P max) of 284 mW cm−2, while the P max value of 328 mW cm−2 was recorded for commercial Pt/C in similar AEMFC conditions (Figure 1). The obtained data show that the three-component (PPy, PTh and MWCNT) composite prepared herein is a promising route for the development of AEMFC cathode catalyst (4).References A. Serov, I. V. Zenyuk, C. G. Arges and M. Chatenet, Journal of Power Sources, 375, 149 (2018). T.-W. Chen, P. Kalimuthu, P. Veerakumar, K.-C. Lin, S.-M. Chen, R. Ramachandran, V. Mariyappan and S. Chitra, Molecules, 27, 761 (2022). A. Sarapuu, E. Kibena-Põldsepp, M. Borghei and K. Tammeveski, Journal of Materials Chemistry A, 6, 776 (2018). A. Sokka, M. Mooste, M. Marandi, M. Käärik, J. Kozlova, A. Kikas, V. Kisand, A. Treshchalov, A. Tamm, J. Leis, S. Holdcroft and K. Tammeveski, ChemElectroChem, 9, e202200161 (2022). Figure 1

Atomically dispersed scandium Lewis acid sites on carbon nitride for efficient photocatalytic hydrogen peroxide production

Photocatalytic reduction of oxygen represents a promising way to produce hydrogen peroxide (H2O2) owing to the merits of energy saving and environmental benignancy. However, the low activity and selectivity of the photocatalyst impede its practical application. Herein, following the principle of metal ion-coupled electron transfer, we have fabricated a class of graphitic carbon nitride (g-C3N4) decorated with atomically dispersed scandium Lewis acid (Sc/H-CN) using a facile impregnation-calcination method. The as-synthesized Sc/H-CN exhibits excellent H2O2 production performance with a rate of 55 µmol h−1, which is 6.2 times over bare CN. The improved performance is ascribed to the enhancement of O2 electron-accepting capability through binding of Sc Lewis acid sites with intermediate ·O2−, inhibiting its reverse reaction. Moreover, density function theory (DFT) calculation and Koutecky-Levich analysis show that the reduced O−O bond breakage is responsible for the high selectivity in the production of H2O2. This work provides a new strategy for the design of photocatalysts equipped with appropriate active sites towards various applications.

Evidence of cathodic peroxydisulfate activation via electrochemical reduction at Fe(II) sites of magnetite-decorated porous carbon: Application to dye degradation in water

Peroxydisulfate (PDS, S2O82−)-based advanced oxidation processes have been developed as an alternative to those based on OH, as PDS activation yields a much more stable radical like SO4− that can maintain the oxidation ability of water treatment systems for longer time. Here, the electrochemical PDS activation has been investigated using reticulated vitreous carbon (RVC) substrate modified with Fe3O4 nanoparticles (NPs) as cathode. The NPs were exhaustively characterized by different surface analysis techniques (TEM, SEM) and Mössbauer spectroscopy. Cyclic voltammetry and linear sweep voltammetry with a rotating disk electrode allowed concluding that the main electrocatalytic role in the cathodic PDS activation to SO4− corresponded to the Fe(II) active sites continuously promoted upon cathodic polarization. These sites were less catalytic for O2 reduction reaction, although it was still feasible with n = 2.7 electrons as determined from Koutecky-Levich analysis. Both cathodic reactions followed an inner-sphere reaction mechanism. The Fe3O4-modified RVC cathodes were employed to electrolyze Methylene Blue aqueous solutions at pH 3.5, employing different current values and PDS concentrations. Dissolved O2 was purged to impede the competitive cathodic H2O2 production and Fenton’s reaction. The occurrence of dye adsorption/electrosorption on the cathode reduced the mass transport limitations, enhancing the reaction between SO4− and organic molecules. The best operation conditions to reach total and fast color removal at 18 min were 2 mM PDS and 10 mA, yielding > 80% TOC abatement at 45 min. Reproducible degradation profiles were found after 5 runs, thereby ensuring the stability of the Fe3O4-modified RVC, with no iron sludge production.