O2 Reduction Research Articles

Fe‐N‐C in Proton Exchange Membrane Fuel Cells: Impact of Ionomer Loading on Degradation and Stability

AbstractFe single atoms in N‐doped C (Fe‐N‐C) present the most promising replacement for carbon‐supported Pt‐based catalysts for the O2 reduction reaction at the cathode of proton exchange membrane fuel cells (PEMFCs). However, it remains unclear how the I/C ratio affects Fe‐N‐C degradation and the stability of single Fe atom active sites (FeNx). Here, an accelerated stress test (AST) protocol is combined with emerging electrochemical techniques for a porous Fe‐N‐C in PEMFC with a range of I/C ratios. The PEMFC current density degradation rates are found to be comparable; however, with increased I/C ratio the additional FeNx sites accessed are more stable, as shown by their higher active site stability number (electrons passed per FeNx lost) at the end of the AST protocol. Meanwhile, the initial rate of TOF decay is suppressed with increasing I/C. Electrochemical process changes are studied via distribution of relaxation times analysis. Minor changes in H+ and O2 transport resistance at low current density prove kinetic degradation dominants at high potentials. These findings demonstrate how electrochemical techniques can be combined with stability metrics to determine and deconvolute changes from the active site to device level electrochemical processes in PEMFCs.

Kinetic study on the degradation of Acid Red 88 azo dye in a constructed wetland-microbial fuel cell inoculated with Shewanella oneidensis MR-1.

Removal of Acid Red 88 (AR88) as an azo dye from the synthetic type of wastewater was studied in a laboratory-made constructed wetland microbial fuel cell (CW-MFC) inoculated with Shewanella oneidensis MR-1 (SOMR-1). Plant cultivation was implemented using a typical CW plant known as Cyperus alternifolius. The complexity of the SOMR-1 cell membrane having different carriers of electrons and H+ ions gives the microbe special enzymatic ability to participate in the AR88 oxidation link to the O2 reduction. Nernst equation developed based on analyzing the involved redox potential values in these electron exchanges is describable quantitatively in terms of the spontaneity of the catalyzed reaction. Power density (PD) at 100mg/L of the AR88 under closed-circuit conditions in the presence of the plant was 11.83 mW/m2. Reduction of internal resistance positively affected the PD value. In determining degradation kinetics, two approaches were undertaken: chemically in terms of first- and second-order reactions and biochemically in terms of the mathematical equations for rate determination developed on the basis of substrate inhibitory concept. The first-order rate constant was 0.263h-1 without plant cultivation and 0.324h-1 with plant cultivation. The Haldane kinetic model revealed low ks and ki values indicating effective degradation of the AR88. Moreover, the importance of acclimatization in terms of the crucial role of lactate was discussed. These findings suggest that integrating the SOMR-1 electrochemical role with CW-MFC could be a promising approach for the efficient degradation of azo dyes in wastewater treatment.

Pairing Oxygen Reduction and Water Oxidation for Dual‐Pathway H2O2 Production

AbstractHydrogen peroxide (H2O2) is a crucial chemical applied in various industry sectors. However, the current industrial anthraquinone process for H2O2 synthesis is carbon‐intensive. With sunlight and renewable electricity as energy inputs, photocatalysis and electrocatalysis have great potential for green H2O2 production from oxygen (O2) and water (H2O). Herein, we review the advances in pairing two‐electron O2 reduction and two‐electron H2O oxidation reactions for dual‐pathway H2O2 synthesis. The basic principles, paired redox reactions, and catalytic device configurations are introduced initially. Aligning with the energy input, the latest photocatalysts, electrocatalysts, and photo‐electrocatalysts for dual‐pathway H2O2 production are discussed afterward. Finally, we outlook the research opportunities in the future. This minireview aims to provide insights and guidelines for the broad community who are interested in catalyst design and innovative technology for on‐site H2O2 synthesis.

Polarized electric field-mediated graphitic carbon nitride-based S-scheme heterostructure for efficient photocatalytic removal of bisphenol A

Tuning interface charge transfer is considered as a feasible way to promote electron migration and separation in heterojunction system, but the role of in-plane electron behavior in single phase has received little attention. Here, a carboxyl functionalized graphitic carbon nitride (CCN) coupled PbBiO2Br (PBOB) S-scheme heterostructure is elaborately designed for investigation. Compared with pristine CN/PBOB, although the existence of carboxyl group weakens the built-in electric field strength of CCN/PBOB (ΔΦ = 0.44 eV), the formed polarized electric field (µ = 3.18 Debye) is conducive to the pre-separation of photogenerated electrons and holes in CCN, thereby improving interface charge transfer. In addition, the introduced carboxyl group can also serve as an O2 adsorption site to enhance its activation, and generate reactive oxygen species (ROS) through a two-step single-electron O2 reduction route. Thus, the optimized CCN/PBOB exhibits excellent photocatalytic ROS generation ability, which can remove 88.7 % of bisphenol A and significantly reduce the acute toxicity of the formed intermediates. This work provides a new perspective for the development of advanced heterojunction photocatalyst by regulating in-plane/interface charge dynamics, and helps to fully understand the pollutant detoxification/degradation process initiated by molecular oxygen activation.

Role of membrane H+ transport and plasmalemma excitability in pattern formation, long-distance transport and photosynthesis of Characean algae

Illuminated giant cells of Characeae comprise alternating areas with H+ pump activity and zones with high conductivity for H+/OH–, which create counter-directed H+ flows between the medium and the cytoplasm. In areas where H+ enters the cell, the pH on the surface (pHo) increases to pH 10, while the cytoplasmic pH (pHc) decreases. The lack of the permeant substrate of photosynthesis (CO2) and the acidic pHc shift in the region of external alkaline zones redirect electron transport in chloroplasts from CO2-dependent (assimilatory) pathway to O2 reduction. This electron transport route is associated with an increase in thylakoid membrane ΔpH and an enhanced nonphotochemical quenching (NPQ) of chlorophyll excitations, which underlies strict coordination between nonuniform distributions of pHo and photosynthetic activity in resting cells. When the action potential (AP) is generated, the longitudinal pH profile is temporarily smoothed out, while the heterogeneity of the distribution of NPQ and PSII photochemical activity (YII) sharply increases. The damping of the pHo profile is due to the suppression of the H+ pump and passive H+ conductance under the influence of an almost 100-fold increase in the cytoplasmic of Ca2+ level ([Ca2+]c) during AP. The increase in [Ca2+]c stimulates photoreduction of O2 in chloroplasts under external alkaline zones and, at the same time, arrests the cytoplasmic streaming, which causes the accumulation of excess amounts of H2O2 in the cytoplasm in areas of intense production of this metabolite, with a weak effect on areas of CO2 assimilation. These changes enhance the nonuniform distribution of cell photosynthesis and account for the long-term oscillations of chlorophyll fluorescence Fm' and the quantum efficiency of linear electron flow in microscopic cell areas after the AP generation.

Visible light-driven aerobic oxidation of amines coupling with H2O2 production catalyzed by CoPc/alkali metal-doped g-C3N4 Z-scheme heterojunction

Three environmentally friendly Z-scheme CoPc/alkali metal-doped g-C3N4 heterojunction composites were fabricated via in-situ immobilizing cobalt phthalocyanine (CoPc) on the surface of alkali metal-doped g-C3N4 nanosheet by simple hydrothermal method. These composites acted as efficient and recyclable heterogeneous photocatalysts realized that the highly selective aerobic oxidation of amines to the corresponding imines coupling of hydrogen peroxide generation at room temperature. Under irradiation by LED lamp, the imines and H2O2 were obtained with conversion ranges from 87.9 % to 92.0 %, selectivity more than 99 %, and production rate ranges from 3.27 to 3.40 mmol g−1 h−1, respectively. Based on the mechanistic analysis and characterization, the excellent catalytic activity derived from the Z-scheme mechanic and synergistic effect originated from O2 reduction and amine oxidation. On completion, after centrifugation, washing with solvent and drying under vacuum, the catalysts can be recovered and reused for five successive cycles of testing with a slight decrease in reactivity. For this work, a promising photo synthesis pathway with merits of mild, low-cost, pollution-free, and energy-saving was developed to produce imines coupling with H2O2 generation.

Carbon defect induced electron transfer promotes electrocatalytic activation of molecular oxygen to selectively generate singlet oxygen for pollutants removal

This study explored the design of heterojunctions incorporating defective graphene and boron nitride (BN) to activate molecular oxygen (O₂) and degrade sulfamethoxazole (SMX) by establishing efficient electron transport channels. These heterojunction catalysts, designed using theoretical predictions, were synthesized by controlling carbon defect concentration and pyrolysis temperature. The optimized GBN cathode achieved a maximum SMX degradation rate of 0.0252min⁻¹, with a total organic carbon (TOC) removal efficiency of 47.31%. Singlet oxygen (¹O₂) was identified as the primary reactive oxygen species, exhibiting a generation rate of 18.66μMmin⁻¹ and contributing 93.5% to SMX degradation. Results demonstrated that an optimal defect concentration enhances electron transfer, promoting the 2-electron reduction of O₂ to H₂O₂ and facilitating further H₂O₂ activation, thereby accelerating SMX degradation. This work advances the development of non-metallic cathodic catalysts and provides valuable insights into electrochemical degradation mechanisms for organic pollutants.

Engineering of Single-Atomic Sites for Electro- and Photo-Catalytic H2O2 Production.

Direct electro- and photo-synthesis of H2O2 through the 2e- O2 reduction reaction (ORR) and H2O oxidation reaction (WOR) offer promising alternatives for on-demand and on-site production of this chemical. Exploring robust and selective active sites is crucial for enhancing H2O2 production through these pathways. Single-atom catalysts (SACs), featuring isolated active sites on supports, possess attractive properties for promoting catalysis and unraveling catalytic mechanisms. This review first systematically summarizes significant advancements in atomic engineering of both metal and nonmetal single-atom sites for electro- and photo-catalytic 2e- ORR to H2O2, as well as the dynamic behaviors of active sites during catalytic processes. Next, the progress of single-atom sites in H2O2 production through 2e- WOR is overviewed. The effects of the local physicochemical environments on the electronic structures and catalytic behaviors of isolated sites, along with the atomic catalytic mechanism involved in these H2O2 production pathways, are discussed in detail. This work also discusses the recent applications of H2O2 in advanced chemical transformations. Finally, a perspective on the development of single-atom catalysis is highlighted, aiming to provide insights into future research on SACs for electro- and photo-synthesis of H2O2 and other advanced catalytic applications.

Undervalued role of metal-carbon junction in selective generation of H2O2: An example of the zinc-carbon junction edge providing asymmetric active sites for efficient oxygen reduction

The spontaneous oxygen reduction is a potential technology for in-situ H2O2 production, which gets rid of the dependence on electricity and solar energy. Previous some metal–carbon compounds reported are feasible to trigger oxygen reduction. These reports always claim the high H2O2 generation ability of metal–carbon composites, while rarely discuss the contribution of carbon and the carbon–metal junction to the oxygen reduction process. Inspired by the dual active sites, we infer that the role of interfacial carbon in metal–carbon composites is underestimated. By a heat treatment of the co-precipitate of zinc and glucose, the Zn-C junction is constructed in Zn-gc to achieve a spontaneous selective reduction of O2 to H2O2. The Zn-C interface edge is active site, where an asymmetric activation is achieved by bonding two O atoms from O2 to C and Zn, respectively. The Zn traction to one of the O atoms is mainly in xy plane, while the C traction to the other O atom is parallel to z-axis. The yield of H2O2 produced per gram of Zn increases from 27.89 mg to 122.21 mg, an improvement of 4.38 times. And the inertness of Zn and its oxygen-containing compounds towards H2O2 enables Zn-gc/O2 system to stabilize H2O2 over a long period of time. This work reveals the crucial role of neglected junction interfaces in oxygen reduction reactions in metal–carbon composites.

Regulating the Electronic Structure of Cu Single-Atom Catalysts toward Enhanced Electro-Fenton Degradation of Organic Contaminants via 1O2 and •OH.

Heterogeneous electro-Fenton degradation with 1O2 and •OH generated from O2 reduction is cost-effective for the removal of refractory organic pollutants from wastewater. As 1O2 is more tolerant to background constituents such as salt ions and a high pH value than •OH, tuning the production of 1O2 and •OH is important for efficient electro-Fenton degradation. However, it remains a great challenge to selectively produce 1O2 and improve the species yield. Herein, the electronic structure of atomically dispersed Cu-N4 sites was regulated by doping electron-deficient B into porous hollow carbon microspheres (CuBN-HCMs), which improved *O2 adsorption and significantly enhanced 1O2 selectivity in electro-Fenton degradation. Its 1O2 yield was 2.3 times higher than that of a Cu single-atom catalyst without B doping. Meanwhile, •OH was simultaneously generated as a minor species. The CuBN-HCMs were efficient for the electro-Fenton degradation of phenol, sulfamethoxazole, and bisphenol A with a high mineralization efficiency. Its kinetic constants showed insignificant changes under various anions and a wide pH range of 1-9. More importantly, it was energy-efficient for treating actual coking wastewater with a low energy consumption of 19.0 kWh kgCOD-1. The superior performance of the CuBN-HCMs was contributed from 1O2 and •OH and its high 1O2 selectivity.

Imidazolate‐Stabilized Cu(III): Dioxygen to Oxides at Type 3 Copper Sites

Imidazole ligation of metals through histidine is extensive among metalloproteins and enzymes, yet the role of the imidazolate conjugate base is often neglected, despite its potential accessibility when bonded to a highly oxidized metal center. Using synthetic models of oxygenated tyrosinase enzymes with exclusive monodentate imidazole ligation, we find that deprotonation of the μ2‐η2:η2‐peroxidodicopper(II) species triggers redox isomerization to an imidazolate‐ligated bis(μ2‐oxido)dicopper(III) species. Formal two‐electron oxidation to Cu(III) remains unprecedented in biological systems, yet is effected readily by addition of base in these model systems. Spectrophotometric titrations by UV/visible/near‐IR and copper K‐edge X‐ray absorption spectroscopies are interpreted most simply as two cooperative, 2H+ transformations in which the peroxide O‐O is cleaved in the first step. Elaboration from simple imidazoles to a protected histidine extends this isomerization into an amino acid environment. The role of phenolate as a base suggests this four‐electron reduction of O2 is energetically viable in a biological context and requires only two copper centers, which act as two‐electron shuttles when imidazole deprotonation assists. This existential precedent of viable imidazolate intermediates invites speculation into an alternative mechanism for phenol hydroxylation not previously considered at Type 3 copper sites such a tyrosinases.

Imidazolate-Stabilized Cu(III): Dioxygen to Oxides at Type 3 Copper Sites.

Imidazole ligation of metals through histidine is extensive among metalloproteins and enzymes, yet the role of the imidazolate conjugate base is often neglected, despite its potential accessibility when bonded to a highly oxidized metal center. Using synthetic models of oxygenated tyrosinase enzymes with exclusive monodentate imidazole ligation, we find that deprotonation of the μ2-η2:η2-peroxidodicopper(II) species triggers redox isomerization to an imidazolate-ligated bis(μ2-oxido)dicopper(III) species. Formal two-electron oxidation to Cu(III) remains unprecedented in biological systems, yet is effected readily by addition of base in these model systems. Spectrophotometric titrations by UV/visible/near-IR and copper K-edge X-ray absorption spectroscopies are interpreted most simply as two cooperative, 2H+ transformations in which the peroxide O-O is cleaved in the first step. Elaboration from simple imidazoles to a protected histidine extends this isomerization into an amino acid environment. The role of phenolate as a base suggests this four-electron reduction of O2 is energetically viable in a biological context and requires only two copper centers, which act as two-electron shuttles when imidazole deprotonation assists. This existential precedent of viable imidazolate intermediates invites speculation into an alternative mechanism for phenol hydroxylation not previously considered at Type 3 copper sites such a tyrosinases.

CuFe Cooperativity at the Membrane-Electrode Interface Elicits a Tandem 2e-+2e- Mechanism for Exclusive O2-To-H2O Electroreduction.

High O2 reduction reaction (ORR) kinetics and exclusive 4e- pathway selectivity are keys to realizing a sustainable society. However, nonprecious electrocatalysts at present cannot enhance the ORR turnover frequency and H2O Faradaic efficiency (FE) concurrently. To address these two challenges, hybrid bilayer membrane (HBM) electrodes with earth-abundant metal centers are developed to control proton-coupled electron transfer (PCET) in ORR. Here, an oxidase-inspired CuFe active site is supported on a tris(2-pyridylmethyl)amine HBM and explored as a unique interface for efficient ORR. This bimetallic HBM displayed an ORR activity 1.4 times higher than the monometallic systems and exhibited the highest FE for H2O (∼94%) among Cu-, Fe-, Ni-, and Co-based HBMs. Contrary to previous studies where the ORR current decreases upon embedding the metal center in a hydrophobic lipid environment, here, the incorporation of a nitrile-terminated proton carrier at the HBM interface boosts the ORR current by 1.7 folds relative to the case where the catalytic site is directly exposed to protons in solution. This intriguing dual improvement is supported by density function theory calculations where an additional 2e-+2e- mechanism occurs in parallel to the direct 4e- pathway, highlighting the synergistic effect of the CuFe HBM for facilitating high-performance ORR. A Zn-air battery is constructed using this CuFe HBM for the first time, further demonstrating that the knowledge gained from this HBM technology holds practical values in real-life applications. These findings on interfacial PCET are envisioned to spark new design principles for future catalysts with optimal electrochemical properties for advanced energy conversion schemes.

The effect of glass sealing stabilization on LSM–YSZ cathode poisoning and oxygen reduction reaction processes in solid oxide fuel cell

AbstractThe perovskite structure and electrochemical activity of (La0.80Sr0.20)0.95MnO3−x–(Y2O3)0.08(ZrO2)0.92 (LSM–YSZ) composite cathode can be significantly affected by volatile boron species originated from sealing glass–ceramics. Herein, we investigate the impact of doping the varying Er2O3 content (ranging from 0 to 4%) into an aluminoborosilicate glass to mitigate boron volatilization, its interaction with the LSM–YSZ cathode and consequent effects on its electrochemical performance. The results reveal that the volatility of boron species can be significantly suppressed by introducing Er oxide into the glass network by enhancing the conversion of [BO3] to [BO4] units, thereby increasing the binding energy of boron and strengthening of the glass structure. Moreover, the electrocatalytic performance for the O2 reduction reaction on the LSM–YSZ/8YSZ/LSM–YSZ symmetric cells is studied under open‐circuit conditions in the presence and absence of glass sealants. The analyses utilizing electrochemical impedance spectroscopy (EIS) and distribution of the relaxation times (DRT) analysis indicate that the electrocatalytic performance of LSM–YSZ cathodes can be enhanced in the presence of glass with 4% Er2O3 (GE4). This enhancement in the electrocatalytic activity of the LSM–YSZ electrode for the oxygen reduction reaction (ORR) is attributed to formation of the thin layer on the electrode surface, leading to a notable reduction in the polarization resistance (Rp) from 0.81 Ω cm2 in the glass absence to 0.45 Ω cm2 in the presence of GE4. However, DRT analysis demonstrates that the prolonged exposure of the cathode to GE4 causes a deterioration in cell performance primarily due to the blocking the cathode active sites, which impedes dominant cathode processes of oxygen dissociative adsorption/desorption and charge transfer and consequently reducing kinetics of the ORR.

Construction of ternary hierarchical heterostructures with favorable interfacial compatibility for ultrahigh photocatalytic H2O2 production

Sluggish charge transfer dynamics are still the key issue limiting the development of photocatalysis. In this study, CdS/ZnIn2S4/MIL-53-NH2 ternary hierarchical heterostructures were constructed via a building block approach, where ZnIn2S4 lamellae first grow on MIL-53-NH2 followed by CdS nanoparticle deposition. The favorable interfacial compatibility and ternary heterostructures contribute to rapid and multistep spatial charge transfer, which suppresses charge recombination. Therefore, ultrahigh H2O2 production of 1792 μmol·L−1 was obtained over CdS/Znln2S4/MIL-53-NH2 composites with visible light, pure water and ambient air, which is not only higher than that of each monomer and binary heterostructure but also better than those of most reported references. Notably, the apparent quantum yield reached 4.5 % at 400 nm, and the solar-to-H2O2 conversion efficiency was 0.081 % in pure water and ambient air. Additionally, H2O2 generation over CdS/Znln2S4/MIL-53-NH2 composites involves two-electron O2 reduction with ·O2- as an intermediate. This work may encourage heterostructure design for efficient photocatalytic H2O2 production.

Solar‐Assisted CO2 Methanation via Photocatalytic Sabatier Reaction by Calcined Titanium‐based Organic Framework Supported RuOx Nanoparticles

AbstractCO2 reduction by sunlight under mild reaction conditions is a research area of increasing interest expected to favor decarbonization and produce fuels and chemicals in the circular economy. We hereby report on the development of a series of titanium oxide‐based solids produced by calcination of MIL‐125(Ti)‐NH2 decorated with RuOx nanoparticles (1 wt %) material at temperatures from 350 to 650 °C and used as photocatalysts for CO2 methanation under simulated sunlight irradiation (45 mW/cm2) at <200 °C and 1.5 atm total pressure. The material synthesized at 350 °C produced the highest photoactivity of the series (4.73 mmol g−1 CH4 at 22 h and an apparent quantum yield at 400, 500 and 750 nm of 0.76, 0.65 and 0.54 %, respectively), comparing favorably with the activities of other MOF‐based materials reported so far. Insights into the material's photocatalytic performance and a study of the possible reaction pathways during CO2 methanation were obtained by electrochemical impedance, electron spin resonance, photoluminescence and in situ FT‐IR spectroscopies together with transient photocurrent and hydrogen temperature programed desorption measurements. The study showed the possibility of using MOF‐based materials as precursors to develop metal oxide photocatalysts with enhanced activities for solar‐driven gaseous CO2 photomethanation.

Mechanistic Study of the Electrochemical Reduction of CO2 in Aprotic Ionic Liquid in Air.

The capture and electrochemical conversion of dilute CO2 in air is a promising approach to mitigate global warming. Aiming to increase the efficiency of the electrochemical reduction of CO2, we fabricated electrodes and developed a custom-designed sealed electrochemical reaction system to study the mechanism of this conversion. The performance of three metal electrodes, Ag, Cu, and SUS 316L, was compared in an aprotic ionic liquid as the electrolyte to monitor the CO2 concentration and chemical reactions using a CO2 sensor and diffuse reflectance infrared Fourier transform spectroscopy and Raman spectroscopy in CO2/N2 (400 ppm CO2 and 99.96% N2) or synthetic air (400 ppm CO2, 21% O2, and 79% N2). The CO2 concentration decreased at negative potentials and was more drastic in synthetic air than in CO2/N2. At negative potential in synthetic air, IR revealed carbon monoxide, carbonate, or peroxydicarbonate on the Ag, Cu, or SUS 316L electrodes, respectively. Reaction intermediates were identified using Raman spectroscopy. Superoxide (O2•-), produced by the reduction of O2 on each electrode, promotes the electrochemical reduction of CO2 whose reduction potential is higher on the negative side than that of O2. This research deepens our understanding of the electrochemical capture/release and conversion of dilute CO2.

Boosting the bifunctional catalytic activity of Co3O4 on silver and nickel substrates for the alkaline oxygen evolution and reduction reactions

Developing an efficient bifunctional catalyst for oxygen reduction (ORR) and evolution reactions (OER) is a crucial step for the wide commercialization of alkaline water electrolyzers. However, a proper assessment and comparison of various catalysts is still challenging due to the different catalyst substrates and loadings. In this work, Co3O4 spinel nanoparticles (NPs) were investigated as a bifunctional catalyst at different substrates of glassy carbon or Ag bulk substrate or Ni particles and with different loadings. For Co3O4 (800 µg cm-2)/Ag electrode, not only the overpotential for ORR was 50 mV lower than the bare Ag electrode, but also the OER overpotential was 40 mV lower than the sole Co3O4. Besides, less than 1% peroxide intermediate was detected during ORR using the rotating ring disc electrode (RRDE) method, suggesting a 4-electron O2 reduction. Tafel slopes of 70 and 60 mV dec‑1 at Co3O4/Ag were obtained for ORR and OER, respectively. The improved bifunctional activity could be attributed to a synergistic effect between Co3O4 and the Ag support. For the loading effect, it is revealed that the number of accessible sites of Ag surface decreases with increasing the Co3O4 loading, as evaluated from lead-underpotential deposition measurements. Interestingly, after running ORR/OER cycles, the surface area increased by 4–6 times. This surface roughening was also confirmed by SEM and EDX. The study was extended to Co3O4+Ni mixed catalyst to validate the effect of the support on this induced synergism. Hence, this work provides a simple route for designing potential effective catalytic interfaces and contributes to unravelling the influence of the substrate and loading on the catalyst activity for water electrolyzers.

Enzyme-Modified Microelectrode for Simultaneous Local Measurements of O2 and pH.

The use of miniaturized probes opens a new dimension in the analysis of (bio)chemical processes, enabling the possibility to perform measurements with local resolution. In addition, multiparametric measurements are highly valuable for a holistic understanding of the investigated process. Therefore, different strategies have been suggested for simultaneous local measurements of various parameters. Electroanalytical methods are a powerful strategy in this direction. However, they have been mainly restricted to coupling concurrent independent measurements with different miniaturized probes. Here, we present an enzymatic microbiosensor for the simultaneous detection of O2 and pH. The sensing strategy is based on the pH-dependent bioelectrocatalytic process associated with O2 reduction at a gold microelectrode modified with a multicopper oxidase. After initial investigations of the bioelectrocatalytic reaction over gold macroelectrodes, the fabrication and characterization of micrometer-sized probes are presented. The microbioelectrode exhibits a linear current increase with O2 concentration extending to 17.2 mg L-1, with a sensitivity of (5.56 ± 0.13) nA L mg-1 and a limit of detection of (0.5 ± 0.3) mg L-1. Moreover, a linear response allowing pH detection is obtained between pH 5.2 and 7.5 with a slope of -(47 ± 8) mV per pH unit. In addition, two proof-of-concept analytical examples are shown, demonstrating the capability of the developed sensing system for simultaneous local measurements of O2 and pH. Compared with other miniaturized probes reported before for simultaneous detection, our strategy stands out as the two investigated parameters are acquired from the very same measurement. This strategy greatly simplifies the analytical setup and for the first time provides truly simultaneous local detection in the micrometer scale.

3-Electrode Setup for the Operando Detection of Side Reactions in Li-Ion Batteries: The Quantification of Released Lattice Oxygen and Transition Metal Ions from NCA

Detecting parasitic side reactions is paramount for developing stable cathode active materials (CAMs) for Li-ion batteries. This study presents a method for the quantification of released lattice oxygen and transition metal ions (TMII+ ions). It is based on a 3-electrode cell design employing a Vulcan carbon-based sense electrode (SE) that is held at a controlled voltage against a partially delithiated lithium iron phosphate (LFP) counter electrode (CE). At this SE, reductive currents can be measured while polarizing a CAM working electrode (WE), here a LiNi0.80Co0.15Al0.05O2 (NCA), against the same LFP CE. In voltammetric scans, we show how the SE potential can be selected to specifically detect a given side reaction during CAM charge/discharge, allowing, e.g., to discriminate between lattice oxygen and dissolved TMs. Furthermore, it is shown via online electrochemical mass spectrometry (OEMS) that O2 reduction in the here-used LP47 electrolyte consumes ∼2.3 electrons/O2. Using this value, the lattice oxygen release deduced from the 3-electrode setup upon charging of the NCA WE is in good agreement with OEMS measurements up to NCA potentials >4.65 VLi. At higher potentials, the contributions from the reduction of TMII+ ions can be quantified by comparing the integrated SE current with the O2 evolution from OEMS.