Low Anodic Potentials Research Articles

Electrodeposition and Characterization of Platinum Coatings on Titanium Porous Transport Layers for PEM Electrolyzers

Hydrogen production by electrolysis offers significant advantages over established and more common processes such as steam reforming with regard to carbon dioxide emissions, in particular through the use of electrical energy from renewable sources and the establishment of a decentralized supply structure. The advantages of PEM (proton exchange membrane) electrolysis include high power density, scalability, and the high purity of the produced hydrogen. However, so far the manufacturing of PEM electrolyzers is time consuming, which makes their widespread use difficult and is a significant cost driver for this technology. A promising strategy for the cost-effective production of the cell components is functionalization of low-cost materials with protective coatings. In the framework of the publically funded flagship project H2Giga, coating systems for the bipolar plates (BPP) and porous transport layers (PTL) are developed and tested under real conditions in an electrolyzer stack.The anode PTL is facing very low pH and high anodic potentials as it is in close proximity to the anode catalyst layer. Therefore, the PTL is made of titanium which is proven to be long term stable under these conditions. However, titanium forms a passive layer under anodic polarization leading to an increase of contact resistance and ohmic drop in the cell. The passivation can be suppressed by electrodeposition of platinum at the interface to the catalyst layer. To ensure a good adhesion of the platinum and low material loss during electrolyzer operation, a pretreatment of the titanium surface is crucial. A state-of-the-art fluoride-containing etching solution was compared to fluoride-free processes in terms of stability of the subsequent platinum coating. Investigations on the morphology by SEM and FIB show a good correlation to polarization experiments. The results are further validated by operando testing in a five cell test stack. The fluoride-free solutions are very promising for industrial applications, but so far only provide limited activation of the titanium surface and need to be further optimized to ensure a reliable coating adhesion.

Comparison of O2 and H2 Crossover and Concurrent Faradaic Efficiency Loss of AEM and PEM Water Electrolysis Cells

PEM water electrolyzers (PEMWEs) are able to store fluctuating renewable energy as chemical energy in the form of hydrogen gas. Due to the low pH and high anode potentials, however, only scarce iridium-based catalysts are considered to be reasonably stable to be employed commercially. This significant drawback for large scale application encourages research into AEM water electrolyzers (AEMWEs). They are ought to combine the advantages of polymer electrolyte based water electrolysis technologies with operation at elevated pH that allows usage of non-PGM based OER catalysts.1 Though the overall cell reaction is the same whether run in acidic or alkaline environment, the water-splitting electrode switches from the anode to the cathode, which necessarily changes the direction of osmotic drag within the polymer electrolyte. Additionally, AEM cells are commonly operated in a liquid-liquid set-up where both sides are actively flushed with an alkaline electrolyte solution, which further complicates the understanding of interfacial effects, product formation and removal as well as limiting factors for the overall electrical efficiency (i.e., overpotential contributions) of the system.With AEMWE research still being at an infant state compared to PEMWE it is important to learn as much as possible form the established system to boost the understanding and therewith performance and lifetime of the new technology. Especially gas crossover has been identified as a significant driver for PEMWE degradation2-3 and is limiting the operating range due to the formation of explosive mixtures at the anode. Also, the faradaic efficiency loss stemming from gas crossover should not be neglected. With the extent of gas crossover being strongly dependent on a multitude of experimental parameters, it is difficult to quantitatively compare data from different research groups.In this study we show a 1:1 comparison of AEMWE and PEMWE single cell measurements with special focus on the gas crossover characteristics and the concurrent faradaic efficiency achieved for both technologies. We fixed the experimental parameters to be as close as possible (PTL structure, flow field design, compression etc.) in order for this in-house comparison to serve as a basis for a better understanding of crossover phenomena both in AEMWE and PEMWE cells. By thoroughly analyzing similarities and differences regarding crossover dependency on the operating range the most important mechanisms and therewith influencing factors for both technologies are discussed. Miller, H. A.; Bouzek, K.; Hnat, J.; Loos, S.; Bernäcker, C. I.; Weißgärber, T.; Röntzsch, L.; Meier-Haack, J., Green hydrogen from anion exchange membrane water electrolysis: a review of recent developments in critical materials and operating conditions. Sustainable Energy & Fuels 2020, 4, 2114-2133.Kuhnert, E.; Heidinger, M.; Sandu, D.; Hacker, V.; Bodner, M., Analysis of PEM Water Electrolyzer Failure Due to Induced Hydrogen Crossover in Catalyst-Coated PFSA Membranes. Membranes (Basel) 2023, 13.Weiß, A.; Siebel, A.; Bernt, M.; Shen, T. H.; Tileli, V.; Gasteiger, H. A., Impact of Intermittent Operation on Lifetime and Performance of a PEM Water Electrolyzer. J. Electrochem. Soc. 2019, 166, F487-F497.

Photoinduced Electrochemiluminescence Immunoassays.

Optimization of electrochemiluminescence (ECL) immunoassays is highly beneficial for enhancing clinical diagnostics. A major challenge is the improvement of the operation conditions required for the bead-based immunoassays using the typical [Ru(bpy)3]2+/tri-n-propylamine (TPrA) system. In this study, we report a heterogeneous immunoassay based on near-infrared photoinduced ECL, which facilitates the imaging and quantitative analysis of [Ru(bpy)3]2+-modified immunobeads at low anodic potential. The photovoltage generated by the photoanode under near-infrared light promotes oxidation processes at the electrode/electrolyte interface, thus considerably lowering the onset potential for both TPrA oxidation and ECL emission. The anti-Stokes shift between the excitation light (invisible to the human eyes) and the visible emitted light results in a clear and stable signal from the immunobeads. In addition, it offers the possibility of site-selective photoexcitation of the ECL process. This approach not only meets the performance of traditional ECL immunoassays in accuracy but also offers the additional benefits of lower potential requirements and enhanced stability, providing a new perspective for the optimization of commercial immunoassays.

Analyzing the Temperature Dependence of Titania Photocatalysis: Kinetic Competition between Water Oxidation Catalysis and Back Electron-Hole Recombination.

This study examines the kinetic origins of the temperature dependence of photoelectrochemical water oxidation on nanostructured titania photoanodes. We observe that the photocurrent is enhanced at 50 °C relative to 20 °C, with this enhancement being most pronounced (by up to 70%) at low anodic potentials (<+0.6 V vs RHE). Over this low potential range, the photocurrent magnitude is largely determined by kinetic competition between water oxidation catalysis (WOC) and recombination between surface holes and bulk electrons (back electron-hole recombination, BER). We quantify the BER process by transient photocurrent analyses under pulsed irradiation. Remarkably, we find that the kinetics of BER (∼90 ms half-time) are independent of temperature. In contrast, the kinetics of WOC, determined from the analysis of the photoinduced absorption of accumulated surface holes, are found to accelerate up to 2-fold at 50 °C relative to 20 °C. We conclude that the enhanced photocurrent densities observed in the low-applied potential region result primarily from the accelerated WOC, reducing losses due to the competing BER pathway. At higher applied potentials (>+0.6 V vs RHE), a smaller (∼10%) enhancement in photocurrent density is observed at 50 °C relative to 20 °C. Photoinduced absorption studies, correlated with studies using triethanolamine as a hole scavenger, indicate that this more modest enhancement at anodic potentials primarily results from an enhanced charge separation efficiency. We conclude by discussing the implications of these results for the practical application of photoanodic WOC under solar irradiation, influenced by these temperature-independent and -dependent underlying kinetic processes.

Anion intercalation of NiMn-LDH accelerating urea electrooxidation on trivalent nickel

Reducing the urea oxidation reaction (UOR) barriers is a key knot for accelerating its practical applications. Here, we demonstrate that NiMn-LDH with sulfate anion interaction can enable urea electrooxidation with a low anodic potential of 1.36 V at 100 mA cm−2. We find that the UOR on NiMn-LDH is driven by Ni3+ species and the Ni3+ generation is the rate-determining step of UOR. Both the Mn doping and sulfate anion interaction contribute to the low-barrier phase transformation from Ni(OH)2 to NiOOH to produce the Ni3+ state with high activity in UOR, owing to that Mn doping optimizes the electronic states and intercalation of guest anions weakens the interlayer interactions, which ultimately tunes Ni3+ generation kinetics toward the superior UOR activity. Our findings provide new insights into the design of the highly active UOR catalysts.

Insight into the nonradical selective oxidation mechanism for versatile organic pollutants removal by perovskite activated peroxymonosulfate system

The heterogeneous catalysis of peroxymonosulfate (PMS) through nonradical pathway has shown many advantages in environmental remediation. It is necessary to unveil the factors affecting oxidation routes and selectivity toward different organic pollutants. Herein, a series of ABO3-type BiFexCo1-xO3 perovskite catalysts were fabricated via simple hydrothermal process to realize the changeover of catalytic activity and mechanism in PMS activation and pollutants oxidation. The as-prepared BiFe0.5Co0.5O3 (BFCO-3) achieved a 98 % removal efficiency of SMZ with a high kinetic constant (kobs = 0.334 min−1) in 10 min. The superior catalytic performance was ascribed to the introduction of bimetals at B-site, which enhanced electron donating capacity to PMS, confirming by d-band center calculations. Several parameters such as inorganic anions and humic acids affected the SMZ degradation slightly, owing that singlet oxygen was verified to be the primary reactive species in the BFCO-3/PMS system. Moreover, twelve organic pollutants including amoxicillin crystalline (AMX), cephalexin hydrate (CFX), ciprofloxacin (CIP), levofloxacin (LFX), Sulfadiazine (SDZ), sulfamethoxazole (SMZ), 4-acetamidophenol (ACP), benzoic acid (BA), bisphenol A (BPA), carbamazepine (CBZ), metronidazole (MNZ) and naproxen (NPX) were utilized to investigate the selective oxidation mechanism. A linear correlation between lnkobs and their half-wave potential (φ1/2) values (except for BA and MNZ) suggesting the dominant role of nonradical catalysis where phenolic compounds with lower anodic potential tend to lose electrons are more easily degraded. Furthermore, the reactive sites of representative SMZ for electrophilic species attack were precisely identified based on the Fukui index. And the degradation intermediates of SMZ were further determined by LC-MS/MS technology for proposing their degradation pathway and predicting ecotoxicity. This study not only provides an efficient advanced oxidation system for organic pollutants removal but also advances our understanding of the selective oxidation mechanism in nonradical-dominated catalytic process.

Value-Added Electrolysis Hydrogen Production Via Methanol Oxidation over a Synergetic Pt1-W Dual-Site Catalyst

Electrochemical water splitting is a promising way of generating hydrogen for future sustainable green energy source technology options to replace nonrenewable and environmentally pollutant hydrocarbon energy sources. However, due to its high overpotential in oxygen evolution reaction (OER) and low-value product O2, some candidate anodic reactions with low potential should be proposed to replace OER, such as alcohol oxidation. Unlike fuel cell applications, partial oxidation is preferred to achieve high-value products rather than complete oxidation. For this purpose, we design a synergistic effect Pt1-W site on the dual-doped TiO2 support (Ti0.8W0.2NxOy) to catalyze alkaline methanol oxidation and gain 90% selectivity of formate production. Comparing the case of the Pt nanoparticles (69% selectivity of formaldehyde), the Pt single-atom catalyst controls selectivity and enhances the reactivity and stability attributed to the strong catalyst-support interaction. As a result, this work shows a novel strategy for electrochemical hydrogen production at far lower anodic potential by designing a stable and efficient Pt-based single-atom catalyst.

Advanced Hybrid Polymer/Ceramic PEO-SnF2 Protective Mechanism for Sodium Metal Anodes

In the face of mounting global energy requirements and pressing environmental issues, the scientific community turned to lithium-metal batteries, celebrated for their superior capacity and efficiency. However, challenges arising from the scarcity and increasing costs of lithium have redirected research efforts towards sodium batteries. Sodium metal anodes, with their remarkable theoretical specific capacity of ~1166 mAh/g, low anode potential of -2.714 V vs standard hydrogen electrode, and cost benefits, are increasingly recognized as promising candidates for conventional energy storage systems. Notwithstanding their advantages, the unstable and fragile solid electrolyte interphase (SEI) due to the spontaneous reaction between metallic Na and a liquid electrolyte remains a significant limitation. As this SEI layer expands, it hinders ion transport, causing increased polarization and diminishing the battery's energy efficiency. Specifically, during the plating process, Na ions are deposited beneath the SEI layer, causing significant expansion. This expansion fractures the fragile SEI, allowing dendrites to grow through these defects. As these dendrites grow, they can pierce the separator, posing a risk of short-circuiting the battery. While numerous studies have proposed designs for protecting sodium metal interphase, to overcome these challenges, this study amalgamates mechano-electrochemical protective strategies to enhance sodium metal anode stability, offering groundbreaking solutions to these challenges.Central to our exploration were tin fluoride (SnF2) and silicon nitride (Si3N4), both established as effective in generating robust artificial SEI layers on the sodium metal surface. These layers, rich in NaF and NaxN respectively, demonstrated a notable improvement in cycling performance, effectively suppressing dendrite growth and enhancing the battery's cycling stability by nearly 3.5 times compared to bare sodium anodes.Building upon these foundational insights, a novel approach was conceptualized: a hybrid protective mechanism combining polyethylene oxide (PEO) with the presence of SnF2 as an additive, is particularly efficacious. This combination was specifically chosen to provide chemical stability, morphological conformality, and mechanical flexibility to accommodate the mechano-electrochemical instability upon sodiation/desodiation processes. Through this novel strategy, we aimed to amalgamate the strengths of both components, ensuring optimal outcomes. This composite artificial SEI under a 0.25 mA/cm2 current density, delivers a cycling performance 10 times superior to bare sodium metal and survived up to 1800 hours of cycling. Impressively, at a high current density of 0.5 mA/cm2, the hybrid system still performed much better than its untreated counterpart and continued cycling for up to 800 hours of cycling. The synergy of PEO and SnF2 remarkably minimized electrolyte decomposition, inhibited dendrite formation, and stabilized Na plating/stripping processes, all of which consolidated its promise for sodium metal battery technology.

Pt1.8Pd0.2CuGa Intermetallic Nanocatalysts with Enhanced Methanol Oxidation Performance for Efficient Hybrid Seawater Electrolysis.

Seawater electrolysis is a potentially cost-effective approach to green hydrogen production, but it currently faces substantial challenges for its high energy consumption and the interference of chlorine evolution reaction (ClER). Replacing the energy-demanding oxygen evolution reaction with methanol oxidation reaction (MOR) represents a promising alternative, as MOR occurs at a significantly low anodic potential, which cannot only reduce the voltage needed for electrolysis but also completely circumvents ClER. To this end, developing high-performance MOR catalysts is a key. Herein, a novel quaternary Pt1.8Pd0.2CuGa/C intermetallic nanoparticle (i-NP) catalyst is reported, which shows a high mass activity (11.13 A mgPGM -1), a large specific activity (18.13mA cmPGM -2), and outstanding stability toward alkaline MOR. Advanced characterization and density functional theory calculations reveal that the introduction of atomically distributed Pd in Pt2CuGa intermetallic markedly promotes the oxidation of key reaction intermediates by enriching electron concentration around Pt sites, resulting in weak adsorption of carbon-containing intermediates and favorable adsorption of synergistic OH- groups near Pd sites. MOR-assisted seawater electrolysis is demonstrated, which continuously operates under 1.23V for 240h in simulated seawater and 120h in natural seawater without notable degradation.

Conversion-type anode chemistry with interfacial compatibility toward Ah-level near-neutral high-voltage zinc ion batteries.

High-voltage aqueous zinc ion batteries (AZIBs) with a high-safety near-neutral electrolyte is of great significance for practical sustainable application; however, they suffer from anode and electrode/electrolyte interfacial incompatibility. Herein, a conversion-type anode chemistry with a low anodic potential, which is guided by the Gibbs free energy change of conversion reaction, was designed for high-voltage near-neutral AZIBs. A reversible conversion reaction between ZnC2O4·2H2O particles and three-dimensional Zn metal networks well-matched in CH3COOLi-based electrolyte was revealed. This mechanism can be universally validated in the battery systems with sodium or iodine ions. More importantly, a cathodic crowded micellar electrolyte with a water confinement effect was proposed in which lies the core for the stability and reversibility of the cathode under an operating platform voltage beyond 2.0V, obtaining a capacity retention of 95% after 100 cycles. Remarkably, the scientific and technological challenges from the coin cell to Ah-scale battery, sluggish kinetics of the solid-solid electrode reaction, capacity excitation under high loading of active material, and preparation complexities associated with large-area quasi-solid electrolytes, were explored, successfully achieving an 88% capacity retention under high loading of more than 20mg cm-2 and particularly a practical 1.1 Ah-level pouch cell. This work provides a path for designing low-cost, eco-friendly and high-voltage aqueous batteries.

Anodic dissolution and passivation of manganese monosilicide in fluoride-containing sulfuric acid solutions

The purpose of this study was to investigate the anode resistance of manganese monosilicide MnSi in fluoride-containing sulfuric acid solutions and the concentration effect of sodium fluoride on the anodic dissolution and passivation of the silicide. The study was carried out on a single-crystal MnSi sample in 0.5 M H2SO4 + (0.0025−0.05) M NaF solutions. The study presents micrographs and elemental composition of the electrode surface after anodic polarization from E corrosion to E = 3.2 V in 0.5 M H2SO4 and 0.5 M H2SO4 + 0.05 M NaF solutions. A stronger etching of the electrode surface was observed in the presence of fluoride ions; elemental analysis showed an increase in the oxygen content in certain areas of the silicide surface associated with the formation of manganese and silicon oxides and their partial removal at high polarization values. The kinetic regularities of the MnSi-electrode anodic dissolution were studied by the methods of polarization, capacitance, and impedance measurements. It was established that the addition of fluoride ions leads to weaker barrier properties of the silicon dioxide surface film, which determines the high silicide resistance in a fluoride-free medium. The order of the reaction was calculated for the MnSi anodic dissolution for NaF depending on the potential. In the region of low anodic potentials (from Ecor to E ≈ –0.2 V), the reaction order ranged from 1.8 to 1.1, which was due to the high influence of silicon in the composition of the silicide and its oxidation products. With an increase in the polarization value (up to E = 0.9 V), the reaction order decreased to 0.5. An increase in the contribution of manganese ionization and oxidation reactions to the kinetics of the anodic dissolution of the silicide was observed. The silicide passivation in a fluoride-containing electrolytewas characterized by higher values of the dissolution current density (10–4−10–3 A/cm2) as compared to a fluoride-free electrolyte (10–6 A/cm2), the reaction order in region of the passive state was ~1.0. Passivation was due to the formation of MnO2 and SiO2 oxides on the surface. In the transpassivation region (E ≥ 2.0 V), there was a weak dependence of the current density on the concentration of fluoride ions. Oxygen release was observed on the surface of the electrode, and the formation of MnO4 – ions was recorded in the near-electrode layer. The article discusses mechanisms and kinetic regularities of anodic processes on an MnSi-electrode in sulfuric acid solution in the presence of fluoride ions

Material design of biodegradable primary batteries: boosting operating voltage by substituting the hydrogen evolution reaction at the cathode.

Transient primary batteries (TPBs) degrade after use without leaving harmful toxic substances, providing power sources for developing low-invasive and environmentally benign sensing platforms. Magnesium and zinc, both abundant on Earth, possess low anodic potentials and good biodegradability, making them useful as anode materials. However, molybdenum, a biodegradable metal, causes the hydrogen evolution reaction (HER) at the cathode, reducing the operating voltage of cells because of its low cathodic potential. In this review, we examine recent material designs to increase the operating voltage by introducing alternative electrochemical reactions at the cathode, including the oxygen reduction reaction, metal-ion intercalation into transition metal oxides, and halogen ionization, all of which have higher cathodic potentials than the HER. After discussing the characteristics, constituents, and demonstration of TPBs, we conclude by exploring their potential as power sources for implants, wearables, and environmental sensing applications.

Operando and Dynamic XAS Studies of Metal-Support Interactions of Iridium-Based Catalysts for Oxygen Evolution Reaction (OER) in Proton Exchange Membrane Water Electrolysis (PEMWE)

Iridium (Ir) is an important electrocatalyst material for the oxygen evolution reaction (OER) in acidic media. However, high loadings of very costly and scarce Ir are needed to achieve sufficient performance in proton exchange membrane water electrolysis (PEMWE). [1] To establish a large-scale green hydrogen economy based on PEMWE in the near future, the Ir loading needs to be further reduced. In the last decades, promising Ir-based catalyst strategies has been developed to improve the OER activity by controlling the size of Ir nanoparticles (NPs) [2] and tuning the electronic interactions of the Ir NPs with the catalyst support material, referred to as strong metal support interaction (SMSI). The latter has frequently been observed for heterogeneous catalysts (e.g. Au NPs supported on titanium oxide [3]), while the SMSI for Ir-based OER electrocatalysts is poorly understood to date.This study focuses on operando and dynamic X-ray absorption spectroscopy (XAS) analysis of Ir NPs deposited on three support materials: graphitized carbon (C), titanium oxide (TiO2) and antimony-doped tin oxide (ATO). The surfactant-free Ir NPs with controlled size of around 1 nm were obtained by a colloidal approach in low-boiling point alcohols. [2, 4] Afterwards, the Ir NPs were homogeneously distributed on different support materials with similar loadings and particle sizes confirmed by TEM and XRD. Solute components from the aged catalyst materials (Ir and/or support material) in the electrolyte solution were determined by ICP-OES, while the solid catalyst powders were analysed by TGA and µ-XRF. Our first/initial electrochemical results by recording cyclic voltammetry (CV) profiles show that after the immobilization the Ir NPs obtained from the same batch are strongly oxidized on TiO2, while a metallic character of the Ir is still observed on the C and ATO. The OER mass activity of 120±8 A/gIr for Ir/C is 2-times higher than for Ir/TiO2 (60±6 A/gIr) and 1.2-times higher than for Ir/ATO (95±4 A/gIr) at 1.5 VRHE,iR-free and 25 °C obtained from rotating disc electrode (RDE) setup.Operando and dynamic Ir LIII edge XANES measurements during the cycling from 0.06 to different upper potentials (0.8, 1.0, 1.2, 1.4 and 1.6 VRHE) at 2 mV/s were performed in acidic media at the SuperXAS (X10DA) beamline, Switzerland. All measurements were carried out using a home-made spectro-electrochemical flow-cell. Principal component analysis (PCA) of Quick-XANES data confirmed the tendencies of the formation of irreversible Irz+ species on TiO2 even at low upper potentials, while ATO and C tend to retain the metallic character of the Ir NPs. From the PCA, two independent components were identified to describe the changes in the series of XANES data. The compositions of these components were then analyzed using linear combination fitting (LCF).For 60 wt.% Ir/C, the cycling up to 1.4 VRHE shows two components with 50:25:25 wt.% of Ir0:Ir3+:Ir4+ and 42:0:58 wt.% of Ir0:Ir3+:Ir4+ obtained from the PCA and LCF analysis. During the cycling, the ratios of these components change gradually. In spite of the strong hysteresis behavior, we observed that the ratio between both components reverses to the initial state. In other words, only the component with 42:0:58 wt.% of Ir0:Ir3+:Ir4+ is present at 1.4 VRHE, while the other component is dominant at lower anodic potential.In the same experiment, the analysis of our data for 60 wt% Ir/ATO using PCA indicates two independent components with 74:11:15 wt.% of Ir0:Ir3+:Ir4+ and 70:0:30 wt.% of Ir0:Ir3+:Ir4+. The cycling up to 1.4 VRHE shows similar behavior like 60 wt.% Ir/C.Although the same batch of colloidal Ir NPs were used for the immobilization, the PCA and LCF analysis for 60 wt.% Ir/TiO2 reveal two components with high content of Ir4+ species, more precisely 36:64 wt.% of Ir0:Ir4+ and 2:98 wt.% of Ir0:Ir4+. The cycling up to 1.4 VRHE shows mainly a reversible behavior due to high content of Ir4+ species on TiO2. With increasing anodic potential, the content of metallic Ir further decreases and the main component with almost only Ir4+ species is stable at 1.4 VRHE. In the backward direction, the reduction to metallic Ir takes place to small extent. Therefore, the component with 36:64 wt.% of Ir0:Ir4+ is mainly present at 0.06 VRHE.In summary, our PCA and LCF analysis of the operando and dynamic Quick-XANES data provide further insights into the electronic structure of Ir species and the mechanism of the complex metal-support interaction for Ir NPs immobilized on C, ATO and TiO2.

Enhanced Urea Oxidation Electrocatalytic Activity by Synergistic Cobalt and Nickel Mixed Oxides.

Exploring reactive and selective Ni-based electrocatalysts for the urea oxidation reaction (UOR) is crucial for developing urea-related energy conversion technologies. Herein, synergistic interactions in Ni/Co mixed oxides/hydroxides enhanced the UOR with low onset potential, fast reaction kinetics, and good selectivity against the oxygen evolution reaction (OER). Our electrochemical measurements and theoretical calculations signified the collaborative interaction of Ni/Co mixed oxide/hydroxide heterostructures to enhance UOR activity. Our results showed that Ni3+ species, formed at high anodic potential, produced a high anodic current primarily from unwanted OER. Instead, the Ni/Co heterostructures with dominant Ni2+ and Co3+ species remained stable at low anodic potential and exhibited anodic current exclusively attributed to UOR. This work highlights the importance of tuning valence charges for designing high-performance and selective UOR electrocatalysts to benefit the environmental remediation of urea runoff and enable urea electrolysis for hydrogen production by replacing conventional OER with UOR at the anode.

Unveiling the contribution of catalytic particles to the electrochemical performance of a Pb-dispersed particle composite anode for nonferrous metal electrowinning

Incorporating oxygen evolution catalytic particles into the Pb matrix via a powder mixing-pressing-sintering process is an effective strategy to develop efficient lead-dispersed particle (Pb-DP) composite anodes for nonferrous metal electrowinning. Catalytic particles are demonstrated to significantly reduce the anodic potential of Pb-DP anodes. However, the intrinsic contribution of catalytic particles to the electrochemical performance of Pb-DP anodes remains obscure. Herein, Pb-DP anodes with different dispersed particles (chemically inert Si or C, catalytic Co3O4 or MnCo2O4) were prepared. The anodic potential, microstructure and phase composition of the oxide layer, and oxygen evolution behavior of these Pb-DP anodes were investigated and compared with those of the Pure-Pb anode (prepared without dispersed particles). The results showed that compared with the Pure-Pb anode, Pb-Si and Pb-C anodes present similar microstructures and phase compositions of the oxide layer. However, they exhibit a larger Tafel slope for the oxygen evolution reaction (OER). Although the incorporation of catalytic particles (Co3O4 and MnCo2O4) reduced the thickness and specific surface area of the oxide layer and inhibited PbO2 growth, the presence of Co3O4 and MnCo2O4 significantly reduced the Tafel slope and charge transfer resistance (Rct) of the OER. Consequently, the anodic potentials of Pb-Co3O4 and Pb-MnCo2O4 are approximately 45 mV and 69 mV lower than that of Pure-Pb, respectively. It is demonstrated that the lower anodic potential of the Pb-Co3O4 and Pb-MnCo2O4 composite anodes originates from the catalytic activity of the dispersed particles rather than the physical effect of the dispersed particles.

Oxidation mechanism of pyrite in a thiosulfate solution: electrochemical impedance (EIS) and in situ Raman spectroscopy

Ammoniacal thiosulfate has been used lately as an alternative lixiviant for leaching gold from sulfides ores which are not amenable for cyanidation. However, the oxidation of the sulfide minerals generates products that inhibit the dissolution of gold and can promote the degradation of the leaching solution. The complexity of the ammoniacal thiosulfate leaching system has prevented the unification and clarification of the mechanisms of oxidation of sulfide ores used for gold extraction.In this study, a method combining polarization curves, Electrochemical impedance spectroscopy (EIS), and in situ Raman spectroscopy was implemented to investigate the oxidation process of high-purity pyrite. Pyrite samples were dispersed in carbon paste electrode (CPE-Py). The polarization curves of CPE-Py exhibited an increase in current values for overpotentials greater than 0.1 V, indicating the initiation of mineral oxidation processes. Subsequently, a maximum current was observed initially, followed by subsequent decrease, indicating the occurrence of passivation processes on the electrode surface. Hydrodynamic polarization curves demonstrated that the overpotential at which the passivation process occurs is independent of mass transport, suggesting that the passivation products were formed through solid-state transformation.Impedance spectra revealed that at overpotentials below 0.1 V, a partially resolved capacitive semicircle was observed, which was associated with the resistance encountered when charge was transferred between the solution and the surface layer interface. This resistance decreased as the polarization overpotential increased, implying a decrease in charge transfer kinetics. At higher overpotentials (0.3 V–0.4 V), a second capacitive semicircle appeared, linked to the oxidation of one or several species present in the mineral.In situ Raman spectroscopy was utilized to identify the oxidation species of pyrite in ammonia-thiosulfate ((NH4)2S2O3) leaching solution at a pH = 10.2. The composition of the species varied depending on the applied anodic potential. At low anodic potentials (0.1 V), Fe(OH)2 and thiosulfate (S2O32−) were formed, while at high anodic potentials (0.4 V), iron products such as Fe3O4 and γ-FeOOH, as well as sulfide species including thiosulfate, tetrathionates and sulfates (S2O32−, S4O6−2 and SO42−) were formed.

Value-Added Electrolysis Hydrogen Production Via Anodic Oxidation of Ethylene Glycol over an Efficient Pd-Based Single-Atom Catalyst

A promising way of generating hydrogen is electrochemical water splitting for future sustainable green energy source technology options to replace nonrenewable and environmentally pollutant hydrocarbon energy sources. However, the anodic oxygen evolution reaction (OER) is the leading resource of high energy consumption due to its high overpotential. Notably, the generated O2 at the anode side is less needed in practical use. Based on this, why not consider looking for anodic reactions with lower energy consumption and forming value-added products during electrolysis? Ethylene glycol oxidation (EGO) as a small organic molecule is one of the candidates to match the consideration. Therefore, we developed a Pd-based single-atom catalyst prepared via the electrochemical method. The Pd-based single-atom catalyst exhibits excellent EGO activity compared to the commercial Pd/C. In addition, it possesses excellent glycolate selectivity (above 88%) in an alkaline solution with an onset oxidation potential as low as 0.6 V (vs. RHE), much lower than 1.23 V (vs. RHE) of H2O/O2 equilibrium potential. As a result, this work shows a new strategy for electrochemical hydrogen production at far lower anodic potential by designing a stable and efficient Pd-based single-atom catalyst.

Cobalt hydroxide supported nickel nanoparticles for an efficient electrocatalytic urea oxidation reaction

An efficient electrochemical urea oxidation reaction (UOR) could be used in energy production technology. Metallic nickel nano-particles (Ni NPs) are incorporated in cobalt hydroxide layered material to form Ni/Co(OH)2. The synthesized material is utilized as an electrocatalyst in UOR. It has been revealed that the incorporation of Ni NPs into cobalt hydroxide significantly enhances their electrocatalytic activity toward UOR. Ni/Co(OH)2 catalyzes UOR at a very low anodic potential of 1.15 V vs RHE with a peak current density of 14.2 mAcm−2 and Tafel slope of 75 mVdec−1. The current work is a step ahead towards use of transition metal hydroxide (TMH) as support in the electrocatalytic oxidation applications especially UOR.

Self-supported amorphous phosphide catalytic electrodes for electrochemical hydrogen production coupling with methanol upgrading

Efficient catalytic electrodes for cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER) are pivotal for massive production of green hydrogen from water electrolysis, and the further replacement of kinetically sluggish OER by tailored elecrooxidation of certain organics is a promising way to co-produce hydrogen and value-added chemicals via a more energy-saving and safer manner. Herein, amorphous Ni-Co-Fe ternary phosphides (NixCoyFez-Ps) with different Ni:Co:Fe ratios electrodeposited onto Ni foam (NF) substrate were served as self-supported catalytic electrodes for alkaline HER and OER. The Ni4Co4Fe1-P electrode deposited in solution at Ni:Co:Fe ratio of 4:4:1 displayed low overpotential (61 mV at −20 mA cm−2) and acceptable durability for HER, while the Ni2Co2Fe1-P electrode fabricated in deposition solution at Ni:Co:Fe ratio of 2:2:1 showed good OER efficiency (overpotential of 275 mV at 20 mA cm−2) and robust durability, the further replacement of OER by anodic methanol oxidation reaction (MOR) enabled selective production of formate with 110 mV lower anodic potential at 20 mA cm−2. The HER-MOR co-electrolysis system based on Ni4Co4Fe1-P cathode and Ni2Co2Fe1-P anode could save 1.4 kWh of electric energy per cubic meter of H2 relative to mere water electrolysis. The current work offers a feasible approach to co-produce H2 and value-upgraded formate via an energy-saving manner by rational design of catalytic electrodes and construction of co-electrolysis system, and paves the way for cost-effective co-preparation of more value-added organics and green hydrogen via electrolysis.

Nitrogen Fixation and Ammonium Assimilation Pathway Expression of Geobacter sulfurreducens Changes in Response to the Anode Potential in Microbial Electrochemical Cells.

Nitrogen gas (N2) fixation in the anode-respiring bacterium Geobacter sulfurreducens occurs through complex, multistep processes. Optimizing ammonium (NH4+) production from this bacterium in microbial electrochemical technologies (METs) requires an understanding of how those processes are regulated in response to electrical driving forces. In this study, we quantified gene expression levels (via RNA sequencing) of G. sulfurreducens growing on anodes fixed at two different potentials (-0.15 V and +0.15 V versus standard hydrogen electrode). The anode potential had a significant impact on the expression levels of N2 fixation genes. At -0.15 V, the expression of nitrogenase genes, such as nifH, nifD, and nifK, significantly increased relative to that at +0.15 V, as well as genes associated with NH4+ uptake and transformation, such as glutamine and glutamate synthetases. Metabolite analysis confirmed that both of these organic compounds were present in significantly higher intracellular concentrations at -0.15 V. N2 fixation rates (estimated using the acetylene reduction assay and normalized to total protein) were significantly larger at -0.15 V. Genes expressing flavin-based electron bifurcation complexes, such as electron-transferring flavoproteins (EtfAB) and the NADH-dependent ferredoxin:NADP reductase (NfnAB), were also significantly upregulated at -0.15 V, suggesting that these mechanisms may be involved in N2 fixation at that potential. Our results show that in energy-constrained situations (i.e., low anode potential), the cells increase per-cell respiration and N2 fixation rates. We hypothesize that at -0.15 V, they increase N2 fixation activity to help maintain redox homeostasis, and they leverage electron bifurcation as a strategy to optimize energy generation and use. IMPORTANCE Biological nitrogen fixation coupled with ammonium recovery provides a sustainable alternative to the carbon-, water-, and energy-intensive Haber-Bosch process. Aerobic biological nitrogen fixation technologies are hindered by oxygen gas inhibition of the nitrogenase enzyme. Electrically driving biological nitrogen fixation in anaerobic microbial electrochemical technologies overcomes this challenge. Using Geobacter sulfurreducens as a model exoelectrogenic diazotroph, we show that the anode potential in microbial electrochemical technologies has a significant impact on nitrogen gas fixation rates, ammonium assimilation pathways, and expression of genes associated with nitrogen gas fixation. These findings have important implications for understanding regulatory pathways of nitrogen gas fixation and will help identify target genes and operational strategies to enhance ammonium production in microbial electrochemical technologies.