Near-neutral Media Research Articles

Defective Ru-doped α-MnO2 nanorods enabling efficient hydrazine oxidation for energy-saving hydrogen production via proton exchange membranes at near-neutral pH

Proton exchange membrane water electrolysis (PEMWE) showes substantial advantages over the conventional alkaline water electrolysis (AWE) for power-to-hydrogen (PtH) conversion, given the faster response and wider dynamic current range of the PEMWE technology. However, PEMWE is currently still expensive due partly to the high voltage needed to operate at high current densities and inevitable usage of precious iridium/ruthenium-based catalysts to expedite the slow kinetics of the oxygen evolution reaction (OER) and to ensure sufficient durability under strongly acidic conditions. Herein, we report that ruthenium doped α-manganese oxide (Ru/α-MnO2) nanorods show outstanding electrocatalytic performance toward the hydrazine (N2H4) oxidation reaction (HzOR) in near-neutral media (weak alkaline and weak acid), which can be used to replace the energy-demanding OER for PEMWE. The as-prepared Ru/α-MnO2 is found to comprise abundant defects. When used to catalyze HzOR in the acid-hydrazine electrolyte (0.05 M H2SO4 + 0.5 M N2H4), it can deliver an anodic current density of 10 mA cm−2 at a potential as low as 0.166 V vs. reversible hydrogen electrode (RHE). Moreover, Ru/α-MnO2 exhibits remarkable corrosion/oxidation resistance and remains electrochemically stable during HzOR for at least 1000 h. Theoretical calculations and experimental studies prove that Ru doping elongates the Mn–O bond and produces abundant cationic defects, which induces charge delocalization and significantly lowers material’s electrical resistance and overpotential, resulting in excellent HzOR catalytic activity and stability. The introduction of N2H4 significantly reduces the energy demand for hydrogen production, so that PEMWE can be accomplished under remarkably low voltages of 0.254 V at 10 mA cm−2 and 0.935 V at 100 mA cm−2 for a long term without notable degradation. This work opens a new avenue toward energy-saving PEMWE with earth-abundant OER catalysts.

Vacancy engineered BiVO4 photoanode realizes efficient photoelectrochemical CH3OH oxidation in near-neutral media: Active site regulation improves HCHO selectivity

Photoelectrochemical (PEC) CH3OH oxidation holds great promise for sustainably obtaining high value-added HCHO. Low HCHO yield and selectivity are two major constraints to the development of PEC CH3OH oxidation. Herein, we deliver the first report of efficient PEC CH3OH oxidation catalyzed by vacant BiVO4 (BVO) photoanode derived from electrolyte photooxidation and disclose the oxygen vacancy (OV)-induced catalytic sites shift for HCHO selectivity modulation. The near-neutral electrolyte solution provides an excellent environment for alcohol-aldehyde conversion and stable performance of BVO photoanode. OVs promote photogenerated carrier separation, thus ensuring a high photocurrent density and efficient HCHO production for the BVO photoanode. More importantly, the introduction of OV changes the adsorption site for CH3OH molecules from the Bi site to V site of BVO, which enhances the adsorption of CH3OH molecules, and reduces the key C-H activation energy barrier and the energy required for HCHO desorption, collectively boosting the evolution rate and selectivity of HCHO. Thus, OV engineered BVO photoanode exhibits tunable Faraday efficiencies for CH3OH oxidation of 88.4%, 94.7% and 85.6% at 0.8 VRHE, 1.0 VRHE and 1.2 VRHE, respectively, and achieves a prominent HCHO production rate of 579.9 mmol m-2h−1 and a stable photocurrent density of 3.4 mA cm−2 at 1.2 VRHE in near-neutral electrolyte. This work may inspire the future work for PEC chemicals high value-added conversion by using vacancy engineering and matching suitable electrolyte solution.

Interfacial pH measurements during CO2 reduction on gold using a rotating ring-disk electrode.

Insights into how to control the activity and selectivity of the electrochemical CO2 reduction reaction are still limited because of insufficient knowledge of the reaction mechanism and kinetics, which is partially due to the lack of information on the interfacial pH, an important parameter for proton-coupled reactions like CO2 reduction. Here, we used a reliable and sensitive pH sensor combined with the rotating ring-disk electrode technique, in which a functionalized Au ring electrode works as a real-time detector of the OH- generated during the CO2 reduction reaction at a gold disk electrode. Variations of the interfacial pH due to both electrochemical and homogeneous reactions are mapped and the correlation of the interfacial pH with these reactions is inferred. The interfacial pH near the disk electrode increases from 7 to 12 with increasing current density, with a sharp increase at around -0.5 V vs. RHE, which indicates a change of the dominant buffering species. Through scan rate-dependent voltammetry and chronopotentiometry experiments, the homogenous reactions are shown to reach equilibrium within the time scale of the pH measurements, so that the interfacial concentrations of different carbonaceous species can be calculated using equilibrium constants. Furthermore, pH measurements were also performed under different conditions to disentangle the relationship between the interfacial pH and other electrolyte effects. The buffer effect of alkali metal cations is confirmed, showing that weakly hydrated cations lead to less pronounced pH gradients. Finally, we probe to which extent increasing mass transport and the electrolyte buffer capacity can aid in suppressing the increase of the interfacial pH, showing that the buffer capacity is the dominant factor in suppressing interfacial pH variations.

Enzyme-based biofuel cells for biosensors and in vivo power supply

Biofuel cells (BFCs) are energy conversion devices using biocatalysts to convert biochemical energy into electric energy. According to different catalysts, they can be divided into microbial fuel cells (MBFCs), enzyme biofuel cells (EBFCs), organelle biofuel cells (OBFCs), and photocatalytic fuel cells (PFCs). Compared with traditional fuel cells, EBFCs can provide sustainable energy under mild conditions such as physiological temperatures and near-neutral media, which have attracted extensive attention in the field of medical implants, drug pumps, biosensors and other devices. Enzyme load capacity and electron transfer rate are the key issues affecting the performance of EBFCs. Immobilization of enzyme on conductive materials with high specific surface area and good biocompatibility can effectively solve the bottleneck problems of EBFCs in actual applications such as low power density and short service life. This paper summarizes common biological enzymes of EBFCs and their immobilization methods, and emphasizes the breakthrough of nanomaterials, such as carbon nanomaterials, metal nanomaterials and composites in the fields of EBFCs. In addition, the application, challenges, and prospects of the EBFCs in the future development direction are also proposed.

Tungsten doped manganese silicate films as stable and efficient oxygen evolution catalysts in near-neutral media

We designed novel manganese silicate-based film materials on FTO which can act as potential oxygen evolution reaction (OER) catalysts in near-neutral media. W doping not only induces better apparent activity, but also results in much improved stability of manganese silicate.

Cubic Nanostructures of Nickel-Cobalt Carbonate Hydroxide Hydrate as a High-Performance Oxygen Evolution Reaction Electrocatalyst in Alkaline and Near-Neutral Media.

Catalyst development for the efficient direction of electrocatalytic water splitting with much less overpotential is crucial for meeting large-scale hydrogen generation. Being highly abundant and cost-effective, 3d transition-metal-based catalysts show promising activities in alkaline conditions. In this work, bimetallic nickel-cobalt carbonate hydroxide hydrate (NiCoCHH) was prepared by a co-precipitation method with varying molar ratios of Ni/Co of 0.5:1, 1:1, and 1.5:1, which were analyzed for oxygen evolution reaction (OER) study in both alkaline (1 M KOH) and near-neutral (1 M NaHCO3) media. For OER in 1 M KOH, NiCoCHH 1:1 required overpotential of just 238 mV at 10 mA cm-2 current density compared to other ratios of 0.5:1 and 1.5:1, which required 290 and 308 mV, respectively. Similarly, in 1 M NaHCO3, NiCoCHH 1:1 required an overpotential of 623 mV to reach 10 mA cm-2. A post-OER study confirmed the formation of NiOOH during OER. The observed faradaic efficiency was nearly 95.21% for the NiCoCHH 1:1 catalyst. A two-electrode setup with NiCoCHH 1:1∥Pt required just 312 mV as an overpotential at 10 mA cm-2. These kinds of comparative studies can be used in other 3d transition-metal-based catalysts for OER in the future.

Heparin as a bifunctional biotemplate for Pt nanocluster with exclusively peroxidase mimicking activity at near-neutral pH

Heparin serves as both reductant and stabilizer for biogenic synthesis of ultrasmall Pt nanoclusters. The surface remaining heparin enhances the affinity of Pt nanoclusters with the substrate 3,3′,5,5′-tetramethylbenzidine, thereby contributing to outstanding peroxidase mimicking activity in near-neutral media. Heparin-reduced Pt nanoclusters exclusively exhibit peroxidase mimicking activities without oxidase-like characteristics. The most efficient Pt nanozyme was reduced at 80 °C by adopting the [K2PtCl4]/[heparin] of 1, showing the Km of 165 mM for H2O2 and 0.016 mM for TMB. This developed Pt nanozyme displays a marked superior in both peroxidase-like activity and peroxidase specificity under near-neutral environment over previously reported Pt nanozymes. Based on these two considerable features of heparin-Pt nanozyme, a colorimetric test was developed for directly monitoring glucose at pH 6. This work proposes an efficient way to construct peroxidase mimetics at near-physiological pH without the interference of dissolved oxygen.

Nanosheets of Nickel Iron Hydroxy Carbonate Hydrate with Pronounced OER Activity under Alkaline and Near-Neutral Conditions.

Evaluation of unique catalysts of the iron group metals with activity in the OER region similar to that of scarce metals is of great importance to achieve sustainable production of H2 on a large scale. Herein, we report the unique nanosheets of nickel iron hydroxy carbonate hydrate (NiFeHCH) which were prepared through a wet-chemical route within 1 h of reaction time, acting as an efficient electrocatalyst for the oxygen evolution reaction (OER) in both alkaline and near-neutral media. The NiFeHCH was prepared with different concentrations of Fe in different ratios: 1:0.2, 1:0.4, 1:0.6, 1:0.8, and 1:1. Among them, nanosheets of NiFeHCH (1:0.2)were found to have superior OER activity, which required an overpotential of 250 mV to reach 20 mA cm-2 with a very low Tafel slope value of 39 mV/decade in 1 M KOH. Nanosheets with other ratios also had comparable OER activity with overpotentials ranging from 256 to 290 mV with Tafel slope values from 39 to 49 mV/decade. Nanosheets of NiFeHCH electrocatalysts screened for the OER in 1 M NaHCO3 (pH ∼8.5) required overpotentials for all of the ratios ranging from 389 to 507 mV at 10 mA cm-2 and Tafel slope values from 155 to 205 mV/decade, of which nanosheets of NiFeHCH (1:0.4) showed better activity by requiring an overpotential of 389 mV at 10 mA cm-2 and Tafel slope value of 155 mV/decade. With these fruitful advantages, these prepared nanosheets of NiFeHCH can be a better alternative to scarce metals for industrial water electrolysis.

Homologous Catalysts Based on Fe-Doped CoP Nanoarrays for High-Performance Full Water Splitting under Benign Conditions.

The design and development of earth-abundant electrocatalysts for efficient full water splitting under mild conditions are highly desired, yet remain a challenging task. A homologous Fe-doped Co-based nanoarray incorporating complementary catalysts is shown to effect high-performance and durable water splitting in near-neutral media. Iron-doped cobalt phosphate borate nanoarray on carbon cloth (Fe-Co-Pi-Bi/CC) derived from iron-doped cobalt phosphide on CC (Fe-CoP/CC) through oxidative polarization behaves as a highly active bimetallic electrocatalyst for water oxidation with an overpotential of 382 mV to afford a catalytic current density of 10 mA cm-2 in 0.1 m potassium borate (K-Bi, pH 9.2). Fe-CoP/CC is also highly active for the hydrogen evolution reaction, capable of driving 10 mA cm-2 at an overpotential of only 175 mV in 0.1 m K-Bi. A two-electrode water electrolyzer incorporating Fe-Co-Pi-Bi/CC as anode and Fe-CoP/CC as cathode achieves 10 mA cm-2 water-splitting current at a cell voltage of 1.95 V with strong long-term electrochemical durability.

Amorphous Nickel-Cobalt-Borate Nanosheet Arrays for Efficient and Durable Water Oxidation Electrocatalysis under Near-Neutral Conditions.

Electrolytic hydrogen generation needs earth-abundant oxygen evolution reaction electrocatalysts that perform efficiently at mild pH. Here, the development of amorphous nickel-cobalt-borate nanosheet arrays on macroporous nickel foam (NiCo-Bi/NF) as a 3D catalyst electrode for high-performance water oxidation in near-neutral media is reported. To drive a current density of 10 mA cm-2 , the resulting NiCo-Bi/NF demands an overpotential of only 430 mV in 0.1 m potassium borate (K-Bi, pH 9.2). Moreover, it also shows long-term electrochemical durability with maintenance of catalytic activity for 20 h, achieving a high turnover frequency of 0.21 s-1 at an overpotential of 550 mV.

Core-Shell NiFe-LDH@NiFe-Bi Nanoarray: In Situ Electrochemical Surface Derivation Preparation toward Efficient Water Oxidation Electrocatalysis in near-Neutral Media.

The corrosion issue with acidic and alkaline water electrolyzers can be avoided by developing water oxidation catalysts performing efficiently under benign conditions. In this Letter, we report that a NiFe-borate layer can be generated on a NiFe-layered double hydroxide nanosheet array hydrothermally grown on carbon cloth via an in situ electrochemical surface derivation process in potassium borate (K-Bi) solution. The resulting 3D NiFe-LDH@NiFe-Bi nanoarray (NiFe-LDH@NiFe-Bi/CC) demonstrates high activity for water oxidation, demanding overpotentials of 444 and 363 mV to achieve 10 mA cm-2 in 0.1 and 0.5 M K-Bi (pH: 9.2), respectively, rivaling the performances of most reported non-noble-metal catalysts in near-neutral media. Notably, this electrode also shows strong electrochemical durability with a high turnover frequency of 0.54 mol O2 s-1 at overpotential of 600 mV. All these features promise its use as an efficient earth-abundant catalyst material for water oxidation under eco-friendly conditions.

Cobalt–borate nanowire array as a high-performance catalyst for oxygen evolution reaction in near-neutral media

As a durable catalyst, cobalt–borate nanowire array on Ti mesh exhibits high activity for water oxidation in near-neutral media, thereby achieving a geometrical catalytic current density of 10 mA cm−2 at an overpotential of 420 mV.

Water splitting in near-neutral media: using an Mn–Co-based nanowire array as a complementary electrocatalyst

A Mn–Co–borate nanowire array on carbon cloth (Mn–Co–Bi/CC) acts as an efficient electrocatalyst for water oxidation with an overpotential of 366 mV to drive 10 mA cm−2 in 0.5 M K-Bi. The two-electrode electrolyser using Mn–Co–Bi/CC as the anode and MnCo2O4/CC as the cathode drives 10 mA cm−2 at 1.97 V.

Photoassisted mineralization of aromatic and aliphatic N-heterocycles in aqueous titanium dioxide suspensions and the fate of the nitrogen heteroatoms

The photoassisted degradation of aromatic heterocycles (pyrrole, imidazole, pyrazole, isoxazole, oxazole and thiazole) and N-containing alicycles (aliphatic heterocycles: pyrrolidine, 4-butanelactam and 5-pentanelactam) was examined in liquid–solid dispersions. Complete mineralization (TOC) of the aromatic heterocycles was attained within ca. 1h of UV irradiation of the TiO2/heterocycle system in acidic (pH 3), near-neutral (pH 6.0–7.6) and alkaline (pH 11) media. Mineralization kinetics were, in general, not appreciably influenced by the presence of acid but tended to be somewhat slower in alkaline media. N-alicycles were photomineralized more slowly than were the aromatics. The former could be mineralized in acidic and near-neutral media in less than 2h, but not in alkaline media in which pyrrolidine, 4-butanelactam and 5-pentanelactam (and for comparison 4-butanelactone) were not mineralized even after 3h of UV irradiation. Final products were, in all cases, CO2, NH4+ and NO3− ions and SO42− ions in the case of thiazole. Nitrogen (N2) gas was detected only during the photooxidation of pyrazole (contains two adjacent N heteroatoms) but not imidazole in which the N atom are separated by a C atom. The molar ratio NH4+/NO3− depended closely on the chemical structure of the substrates at the longer irradiation times. The site and mode of adsorption of these heterocycles onto the TiO2 particle surface were inferred from point charge calculations and the point of zero charge of the TiO2 (pHpzc ∼6.7); the position of elecrophilic attack by the photogenerated OH radicals was deduced from calculated frontier electron densities.

Solubility of thorium phosphate-diphosphate

The solubility of the pure thorium phosphate-diphosphate, Th4(PO4)4P2O7, has been measured in 0.1 M NaClO4 solutions by PERALS spectrometry (photon electron rejecting alpha liquid scintillation), ICP-MS (inductively coupled plasma-mass spectrometry) and PIXE (particle induced X-ray emission). The results obtained by the three methods are in good agreement and show that the total concentration of thorium in solution is mainly controlled by the precipitation of two compounds: Th(HPO4)2 in acidic media (pH⩽4.5) and Th(OH)4 in basic and near-neutral media.

Excited-state prototropic reactions of tetrahydroharmane

Steady-state and time-resolved fluorescence measurements of THHN (9H-1-methyl-1,2,3,4-tetrahydropyrido[3,4-b]indole), have been carried out as a function of the acidity or basicity of the medium. In these media, four different THHN species have been detected, namely: dication (DC), cation (C), neutral (N) and anion (A). Attempts have been made to determine the excited state pK*a values for the three prototropic processes: N–A, C–N and DC–C reactions. The dicationic and anionic forms, observed outside the 0–14 pH range, are extremely weakly fluorescent species and only the second-order rate constants for the protonation and deprotonation processes, respectively, could be determined. In the case of the C–N process, studied in near-neutral media, excited-state proton exchange is much slower than fluorescence emission and the inflection points in the fluorimetric titration curves correspond quite closely to the ground state pKGa.

Formation of uranium oxide sols in bicarbonate solutions

Colloidal particles of hydrated uranium oxide were formed by reducing a solution of sodium tricarbonatouranate(Vl) with sodium sulfide. These particles were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, and Fourier Transform infrared spectroscopy. Particle size was determined by photon correlation spectroscopy. Depending on conditions, the particles that formed consisted of either a mixed valency (U(IV) and U(VI)) uranium oxide or a complex uranium(VI) hydrous oxide. The mixed valency uranium oxide particles showed nearly spherical morphology, whereas the complex uranium(VI) hydrous oxide consisted of irregularly shaped particles. Electrokinetic measurements showed that both types of particles were negatively charged in near-neutral media.

Effects of Aoueous Phase Composition on the Leach Behavior of Nuclear Waste Glasses

AbstractA systematic matrix of leach tests on a Rest Valley waste glass composition is described. These tests are intended to provide quantitative systematic information on leachant composition effects. and accordingly elucidate the controlling orocesses in hiuh-solute media. e.g. silicate ground water. brine. etc. In order to separate the effects of the nature and concentration of solutes from pH effects. leach tests were first carried out on the glass in de-ionized water under highly interactive conditions (high surface to volume ratio) in a configuration which pre- vented lowering the pH due to the intrusion of CO:. After determination of the resulting pH. the leach behavior of the glass was characterized in a series of high-dilution tests in various leachants buffered at this pH value in order to quantify the effects of leachant composition and concen- trations on the leaching orocess. The high-dilution conditions were used at this stage to ensure that the leachant composition mould not be signifi- cantly affected by accumulation of leached glass components. It was concluded that increased ionic strength appears to depress the leach rates in near-neutral media but enhances the leach rates under moderately basic conditions.

Evidence for a close similarity in the catalytic sites of papain and ficin in near-neutral media despite differences in acidic and alkaline media. Kinetics of the reactions of papain and ficin with chloroacetate.

1. The pH-dependences of the second-order rate constants (k) for the alkylation by chloroacetate of the active-centre thiol groups of papain (EC 3.4.22.2) and ficin (EC 3.4.22.3) were determined over a wide range of pH at 25 degrees C at I 0.1. 2. The main feature of both pH-k profiles is a striking rate maximum at pH6 (characterizing parameters in both cases pKI approx. 3.5, pKII approx. 8.4 and pH-independent rate constant approximately kXH 2.5-3.0 M-1 . s-1). 3. The profile for the ficin reaction contains a plateau at high pH, with approximately kX 0.10 M-1 . s-1; if an analogous plateau exists in the papain reaction, approximately kX ix much lower, less than 0.02 M-1 . s-1. 4. Both enzymes appear to contain closely similar thiolate-imidazolium interactive systems at pH6, but differences in their behaviour in more-acidic media and in alkaline media suggest differences in interaction with the postulated carboxylate component of the putative catalytic triad.