Oxygen Evolution Overpotential Research Articles

Low Polarization Air Electrode Prepared By Pdl Method for Air Rechargeable Battery

A metal hydride/air secondary battery (HAB) consisting of an air electrode as the positive electrode, a hydrogen storage alloy as the negative electrode, and a KOH solution as the electrolyte has been developed by our group, in which the bi-functional oxygen electrode with a low overpotential for oxygen evolution during charge and oxygen reduction during discharge is the key component to improve the battery performance [1]. Since oxygen is evolved at the electrolyte/catalyst interface and oxygen reduction occurs at the electrolyte/catalyst/air interface, the electrode’s structure needs to be porous and be optimized for oxygen permeability for both the reactions, for which the inside should provide the active sites to O2 and OH-. The polarization behaviors of the bi-functional oxygen electrode strongly depend on not only the materials but also the internal structure. The internal structure is related to many factors in the preparation method and conditions of the air electrode that is mostly prepared by using roll press of the precursor paste comprising the components, although the method is difficult to control the internal structure. In this work, we aimed to develop a novel method to prepare the air electrode, which is PDL (painting, drying, and layering) method. The air electrode used Bi1.87Ru2O6.903 as the catalyst and PTFE as the binder, and some electrodes used carbon as a conductive material. In PDL method, a porous PTFE sheet was pressed onto Ni mesh, and a precursor solution was prepared by dispersing the components in distilled water under ultrasonic agitation. Then, painting the solution onto Ni mesh followed by drying was repeated. The surface structure of the air electrode was observed by SEM. The oxygen evolution and reduction behaviors of the obtained electrodes were examined by cyclic voltammetry with a three-electrode cell with 6 mol/L KOH solutions and compared to those of the previous ones prepared by roll press method. The PDL method was able to prepare the air electrode which worked for both oxygen evolution and oxygen reduction. From SEM images, the PDL-electrode using the precursor containing the catalyst and PTFE had large pores and the catalyst worked as the conductive material and the active site. As results of the optimizing of the number of painting, PTFE ratio, ultrasonic condition, the number of layering, and the presence or absence of conductive material, a high specific activity for oxygen reduction was achieved using carbon as the conductive material. The specific activity for oxygen reduction of the PDL-electrode was much higher than that of the roll press-electrode. The SEM images revealed that the difference was because the PDL method was better in dispersibility of the catalyst on the conductive material and oxygen permeability than the roll press method. The results also revealed that the activity per unit weight of the catalyst of the PDL-electrode was unchanged with and without oxygen flow to the electrode, indicating that the inside pore structure is optimized and the catalyst dispersion is good to maximize the active site of the air electrode. This work was supported by “Advanced Low Carbon Technology Research and Development Program (ALCA), Grant No. JPMJAL1204” of Japan Science and Technology Agency (JST). Reference [1] S. Kino, K. Kawaguchi, and M. Morimitsu, AiMES2018, Abs#A01-0008, Cancun (2018). [2] S. Kino, K. Kawaguchi, and M. Morimitsu, The 59th Battery Symposium in Japan, Abs#3G19, Osaka (2018).

FexNiyOOH/etched stainless steel mesh with different morphology for water electrolysis

Water electrolysis was recognized as an effective method for new energy sources. In this work, an effective FexNiyOOH/ESS with high surface area and good catalytic activity was prepared for water electrolysis. Its morphologies at different ratio of Fe/Ni atoms were diverse, such as flower ball, flake, and block, etc. Test results showed that at 10 mA cm−2 and 300 mA cm−2, the hydrogen evolution overpotentials of Fe1.5Ni0.5OOH/ESS were 172 mV and 258 mV. The oxygen evolution overpotentials of Fe0.2Ni1.8OOH/ESS were 208 mV and 308 mV at 10 mA cm−2 and 600 mA cm−2. During the 136-h stability test, the oxygen evolution activity of Fe0.2Ni1.8OOH/ESS was basically unchanged. The Fe1.5Ni0.5OOH//Fe0.2Ni1.8OOH in alkaline conditions presented good performance for water electrolysis. Water electrolysis experiment was carried out at 1.58 V, and the current density reached to 10 mA cm−2. FexNiyOOH/ESS exhibited higher electrocatalytic activity and superior stability at high current density.

Electrochemical corrosion behaviors of Pb-Ag anodes by electric current pulse assisted casting

Electric current pulse was for the first time applied during the casting of PbAg (0.6wt%) anode to have successfully improved its anti-corrosion property and lowered the overpotential of oxygen evolution. It was proven that the electric current pulse applied during the casting has promoted the grain growth of formed equiaxed crystals. Moreover, this treatment contributed to the formation of compact oxide layer which was the reason of the improved corrosion resistance. It was also found that Ag was concentrated at the boundary regions, which well explained the phenomenon that corrosion happened faster along the grain boundaries.

Controllable preparation of nitrogen-doped graphitized carbon from molecular precursor as non-metal oxygen evolution reaction electrocatalyst

Abstract Nitrogen-doped graphitized carbon materials have been proposed as promising catalytic materials for oxygen evolution reaction in alkaline water-electrolysis hydrogen production. In this study, newly graphene-like graphitized carbon nanocages and nitrogen-doped graphitized carbon nanotubes are controllably prepared by using Tween-20 as molecular precursor, Ni as graphitization catalyst and urea as nitrogen source. The as-prepared materials are characterized by X-Ray Diffraction, scanning electron microscope, transmission electron microscopy and Raman spectroscopy X-ray photoelectron spectroscopy. The OER electrochemical performances are evaluated by linear sweep voltammetry and chronoamperometry. It is found that the nitrogen-doped graphitized carbon nanotubes have higher oxygen evolution activity, lower oxygen evolution overpotential and desirable electrochemical stability relative to graphitized carbon nanocages, demonstrating it is a promising catalytic material for OER in alkaline water-electrolysis hydrogen production.

Electrochemical removal of metribuzin in aqueous solution by a novel PbO2/WO3 composite anode: Characterization, influencing parameters and degradation pathways

Abstract In this work, we fabricated PbO2/WO3 composite electrode through composite electrodeposition method and employed it in electrocatalytic degradation of metribuzin. The effects of WO3 particles on surface morphology, structure and electrochemical characters of PbO2 electrodes were investigated. Different from PbO2 electrodes, the PbO2/WO3 composite electrodes owned higher oxygen evolution overpotential, longer service life and higher hydroxyl radicals generation performance. The effect of key operating parameters, for instance, current density, initial metribuzin concentration, initial pH values, and supporting electrolyte concentration, upon electrochemical removal process of metribuzin were studied. The current density and initial metribuzin concentration exerted prominent effects on electrochemical degradation of metribuzin. Kinetic analysis indicated that the electrochemical decomposition of metribuzin was a pseudo-first-order reaction. The byproducts during metribuzin degradation process were identified, several aromatic products and inorganic ions were generated along with the reaction time. Finally, the electrochemical pathways of metribuzin degradation at the PbO2/WO3 composite electrode were proposed based on the identified reaction byproducts, which were separated into three parallel sub-pathways according to the different degradation reactions by hydroxyl radicals, and quantities of hydroxylation, oxidation, demethylation, deamination, and addition reactions’ products were generated. All the byproducts were finally decomposed into inorganic H2O, CO2, NO3−, and NH4+.

Evaluation of the effect of additive group five elements on the properties of Pb-Ca-Sn-Al alloy as the positive grid for lead-acid batteries

As an important part of lead-acid batteries, the grid is mainly used to support active substances and conduct current. Currently, Pb-Ca-Sn-Al alloys are widely used as materials for valve-regulated lead-acid battery grids. The influence of bismuth, barium, strontium, and germanium as alloying additives on the physical and electrochemical behaviors of Pb-Ca-Sn-Al alloys was studied by the methods of metallographic, cyclic voltammetry, linear scanning voltammetry, AC impedance, and scanning electron microscopy. The results show that the addition of Bi and Ba can increase the grain size of the alloy and reduce the intergranular corrosion and corrosion rate of Pb-Ca-Sn-Al alloy. Furthermore, the addition of Bi and Ba could inhibit the growth of Pb (II) and PbO2 in corrosion layer and improve the corrosion resistance of the alloy. Significantly, Bi and Ba will reduce the oxygen evolution overpotential of the alloy by about 30 mV, which will play important roles on the maintenance-free performance of lead-acid batteries. Both Sr and Ge promote the grain corrosion and intergranular corrosion of the alloy, reducing the corrosion resistance of the alloy. Therefore, improving the performance and corrosion life of the grid by using additives is an important approach to further extending the service life of lead-acid batteries.

Fabrication and nucleation study of β-PbO2–Co3O4 OER energy-saving electrode

In this study, β-PbO2–Co3O4 composite coatings were synthesized on a substrate using the electrodeposition method in an electrolyte containing Pb2+ and Co3O4 particles. The preparation process study shows that the β-PbO2 bath containing Co3O4 particles becomes more stable for an ultrasonic dispersion time within 30 min, with the stability beginning to decrease after the ultrasonic time exceeds 30 min. Co3O4 particles in the β-PbO2 bath are positively charged, with anions being adsorbed onto the particles. The nucleation behavior study shows that the electric field force plays a dominant role in the adsorption process for the Co3O4 particles. The nucleation and growth of β-PbO2 on the Co3O4 particles started for the particles closer to the substrate. In addition, the thickness of the β-PbO2 layer on the Co3O4 particles was inversely proportional to the distance between the Co3O4 particles and the substrate. Compared to the pure PbO2 electrode, the oxygen evolution overpotentials for the β-PbO2–Co3O4 electrode are decreased significantly, which demonstrates an energy-saving effect.

A novel layer-by-layer CNT/PbO2 anode for high-efficiency removal of PCP-Na through combining adsorption/electrosorption and electrocatalysis

Electrochemical oxidation is an effective mean to degrade non-biodegradable organics from the contaminated waters. However, its current efficiency is very low for the low-concentration organic pollutants treatment. In this work, combining with the outstanding adsorption property of CNT and the effective electrocatalytic oxidation performance of β-PbO2, we constructed novel layer-by-layer carbon nanotube/PbO2 anodes (CNT/PbO2-3, CNT/PbO2-4 and CNT/PbO2-5) with high adsorbability to enhance the electrochemical degradation efficiency of low-concentration sodium pentachlorophenate (PCPNa). In electrochemical oxidation tests, the PCP-Na was rapidly enriched in the CNT/PbO2 film through adsorption/electrosorption and then efficiently degraded in site. The removal efficiency of low-concentration PCP-Na (5 mg/L) on CNT/PbO2-4 reached 73.8% after only 5 min of electrolysis, whereas that on pure PbO2 was only 9.9%. To further clarify the mechanism of improvement of PCP-Na removal on CNT/PbO2-4 anode, the generation capacity of·OH radicals, oxygen evolution overpotential, voltammetric charge quantity, interface resistance, and specific surface area of CNT/PbO2-4 anode were also assessed and compared with those of other three anodes. Results showed that, in addition to the synergistic effect of adsorption/electrosorption and electrocatalysis, the more active sites, smaller electron transfer resistance and higher direct oxidation capacity of CNT/PbO2-4 anode also contributed to improving degradation rate of PCP-Na on CNT/PbO2-4 anode. Additionally the electrochemical degradation pathway of PCP-Na was also proposed based on the gas chromatography-mass spectrometry analysis. Finally, CNT/PbO2-4 electrode exhibited longer accelerated lifetime of 116 h than pure PbO2 electrode (91 h). Thus, these findings provide a new anode to improve the removal of low-concentration organic pollutants through combining electrochemical oxidation and adsorption/electrosorption processes.

In situ confined vertical growth of a 1D-CuCo2S4 nanoarray on Ni foam covered by a 3D-PANI mesh layer to form a self-supporting hierarchical structure for high-efficiency oxygen evolution catalysis.

Inspired by the patchwork of artificial turf, where planting in a smaller area can result in a more uniform lawn that grows in one direction, here, we defined the growth position and orientation of a CuCo2S4 nanoarray for the first time by electroplating a PANI mesh layer onto a Ni foam to obtain a self-supporting hierarchical electrode material. The nitrogen species derived from the PANI building blocks act as bridging sites to bind with metal ions, which provides a strong coupling effect for the in situ growth of CuCo2S4. At the same time, the mesh structure of PANI divides the growable location into smaller blocks. Compared with a mesh plane with uniformly distributed nitrogen sites, only a small portion of the nitrogen sites are located on the narrow-width fence structure, which may make it difficult for CuCo2S4 to grow onto the fence structure, thereby limiting the self-growth space and confining CuCo2S4. The uniformly distributed growth sites direct CuCo2S4 to grow perpendicular to the plane while limiting their growth size. The excellent structural features further enhance the electrochemical oxygen evolution activity, and the oxygen evolution overpotential at a current density of 100 mA cm-2 is only 291 mV, which is superior to that of the currently known cobalt-copper-based catalyst materials. In addition, the stable structure provides excellent electrode cyclic stability. The preparation of hierarchical self-supporting cobalt-copper bimetallic sulfide nanoarrays provided a reference direction for other transition metal catalytic materials and provided a basis for industrial applications.

Zinc-telluride nanospheres as an efficient water oxidation electrocatalyst displaying a low overpotential for oxygen evolution

Electrochemical water splitting is economically unviable due to the sluggish kinetics of the anodically uphill oxygen evolution reaction (OER).

Effect of Molar Ratio of Ruthenium and Antimony on Corrosion Mechanism of Ti/Sn-Sb-RuOx Electrode for Zinc Electrowinning

In this study, a Ti/Sn-Sb-RuOx electrode was prepared by thermal decomposition technique. Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were used to analyze the microstructure and composition of the prepared coating. The composition of the prepared electrode was analyzed by X-ray diffractometry (XRD). The electrochemically active area can be measured by the number of reactive sites in the electrochemical reaction. Electrochemical workstations were used to test the anodic polarization curve (LSV), AC impedance test (EIS), and cyclic voltammetry (CV). The electrochemical properties of the electrode could be obtained. It was found that the oxygen evolution overpotentials of the electrode prepared at a Ru:Sb molar ratio of 2:1 are the lowest and is 0.3058 V under the conditions of 500 A/m2. The surface morphology is a block-like crack structure. The electrode prepared at a Ru:Sb molar ratio of 2:1 obtained a larger electrochemical active area. With the increase of Ru, the life is gradually increased to 103 h.

A Comprehensive Study on Nano-Diamond Doped β-PbO2 Electrode: Preparation, Properties and Electrocatalytic Performance

In this study, a nano-diamond modified β-PbO2 electrode with the efficiently electrochemical performance was successfully prepared by electrodeposition method (marked as Ti/SnO2-Sb/PbO2-ND). Chronoamperometric curves for PbO2 deposition reveals that the addition of nano-diamond particles could promote the growth of β-PbO2 crystallites. This can be ascribed to the enhancement in the electrochemical generation of •OH in the presence of nano-diamond. Additionally, the presence of nano-diamond could effectively improve the electromigration of Pb2+ toward the anode surface by the formation of negatively charged [Pb2+-nano-diamond] complex. The optimum doping concentration of nano-diamond for β-PbO2 preparation was 1.5 g/L. As compared with blank β-PbO2 electrode, Ti/SnO2-Sb/PbO2-1.5ND electrode has smaller grain size, more compact film and higher ratio of Oad/(Oad+OL). The electrochemical active area for Ti/SnO2-Sb/PbO2-1.5ND electrode was increased to 4.55 cm2, while it was only 2.73 cm2 for β-PbO2 electrode. Notably, the oxygen evolution overpotential of Ti/SnO2-Sb/PbO2-1.5ND electrode was approximately 2.1 V/SCE, which was higher than blank β-PbO2 electrode (1.81 V/SCE). The almost complete removal of methylene blue could be attained within 60 min at a current density of 30 mA cm−2. Generally, the Ti/SnO2-Sb/PbO2-1.5ND shows promise as an efficient and low-cost electrode for electrochemical treatment of industrial wastewater.

Membrane-free electrochemical deoxygenation of aqueous solutions using symmetric activated carbon electrodes in flow-through cells

An electrochemical cell using symmetric carbon cloth electrodes and flow-through geometry that removes more than 97% of incoming aqueous dissolved oxygen (DO) is presented. The electro-deoxygenation (EDO) cell achieves O2 removal by leveraging the high overpotential of oxygen evolution on activated carbon and its propensity to oxidize under anodic polarization in aqueous solution: oxygen is reduced at the cathode while water is oxidized and incorporated into surface oxide functional groups at the anode, effectively sequestering dissolved oxygen. Polarized electrodes promote the two-step reduction of DO resulting in some residual hydrogen peroxide in the effluent, which may be beneficial for certain applications. A subsequent cell is modified with Ni cathodes downstream to reduce all H2O2 to water for particularly sensitive applications; in this cell >99% of incoming DO could be removed to lower than 10 ppb. EDO cells, which currently employ sacrificial anodes, can deaerate 30 L g−1anode of water at an energy consumption of 1 kWh per 10,000 L; carbon anode replacement can recharge the cell. The technique is versatile, inexpensive, and environmentally friendly, deoxygenating solutions from dilute to seawater concentrations at flow rates beyond 50 ml min−1 (O2 flux = 10−4 mol s−1 m−2), more than 50x faster DO removal than similar technologies.

Surface Modification of Hematite Photoanodes for Improvement of Photoelectrochemical Performance

Solar water splitting is a promising method for producing renewable fuels. Thermodynamically, the overall water splitting reaction is an uphill reaction involving a multiple electron transfer process. The oxygen evolution reaction (OER) has been identified as the bottleneck process. Hematite (α-Fe2O3) is one of the best photoanode material candidates due to its band gap properties and stability in aqueous solution. However, the reported efficiencies of hematite are notoriously lower than the theoretically predicted value mainly due to poor charge transfer and separation ability, short hole diffusion length as well as slow water oxidation kinetics. In this Review Article, several emerging surface modification strategies to reduce the oxygen evolution overpotential and thus to enhance the water oxidation reaction kinetics will be presented. These strategies include co-catalysts loading, photoabsorption enhancing (surface plasmonic metal and rare earth metal decoration), surface passivation layer deposition, surface chemical etching and surface doping. These methods are found to reduce charge recombination happening at surface trapping states, promote charge separation and diffusion, and accelerate water oxidation kinetics. The detailed surface modification methods, surface layer materials, the photoelectrochemical (PEC) performances including photocurrent and onset potential shift as well as the related proposed mechanisms will be reviewed.

Electrochemical oxidation of acetamiprid using Yb-doped PbO2 electrodes: Electrode characterization, influencing factors and degradation pathways

The Yb-doped PbO2 electrodes were fabricated by electrodeposition method. SEM and XRD experiments revealed that the adulteration of Yb can decrease the crystal grain size and make the electrodes more compact. Electrochemical measurements, accelerated service lifetime and bulk electrolysis experiments proved that Yb-doped PbO2 electrodes had higher oxygen evolution overpotential, stability and acetamiprid removal ratio than pure PbO2 electrodes. The effect of operating parameters, such as current density, initial acetamiprid concentration, and pH value, on the electrocatalytic degradation of acetamiprid were systematically studied. The electrocatalytic oxidation reaction of acetamiprid on Yb-doped PbO2 electrodes was highly effective, which obeyed the pseudo-first-order kinetics model. Intermediates analysis and electrocatalytic degradation mechanism of acetamiprid by Yb-doped PbO2 electrodes were implemented, and the degradation pathways were divided into three different sub-routes due to three function groups on acetamiprid initially attacked by hydroxyl radicals. Finally, all the byproducts were decontaminated into CO2 and H2O. These results proved that Yb-doped PbO2 electrodes have more promising application potential for the electrocatalytic degradation of refractory organic pollutants.

Preparation and electrochemical treatment application of Ce-PbO2/ZrO2 composite electrode in the degradation of acridine orange by electrochemical advanced oxidation process.

Novel Ce-PbO2/ZrO2 composite electrodes were successfully fabricated in lead nitrate solution containing cerium ions and ZrO2 particles by composite electrodeposition method. SEM images and XRD results indicated that Ce-PbO2/ZrO2 composite electrodes have compact structure and fine grain size. Ce-PbO2/ZrO2 composite electrode has higher oxygen evolution overpotential and stability than Ce-PbO2 electrode. The service life of Ce-PbO2/ZrO2 composite electrode reaches 318 h, which is 4.2 times as that of Ce-PbO2 electrodes (74 h). The novel Ce-PbO2/ZrO2 composite electrode was employed as anode to decontaminate acridine orange (AO) by electrochemical oxidization methods. The effect of initial concentration of AO, current density, and initial pH values on the removal ratio of AO were analyzed. The results showed that AO could be completely removed after 90 min electrolysis under the optimal condition: initial AO concentration was 30 mg L-1, current density was 50 mA cm-2, and the initial pH value was 5.0. Moreover, the possible degradation pathway of AO was elucidated based on the identification of stable byproducts generated during the electrochemical degradation process by HPLC-MS, which revealed that AO and its byproducts could be effectively eliminated and mineralized into CO2, H2O, ammonium and nitrate ions.

Development of Bi-Functional Oxygen Electrode By Low Temperature Roll Press Method for Aqueous Air Secondary Battery

A bi-functional oxygen electrode is the key component of aqueous air secondary batteries, and a low overpotential for oxygen evolution and reduction and a bi-functionality for a long-term charge-discharge cycles are required. The oxygen electrode often consists of carbon material as the conducting matrix, oxygen catalyst, and PTFE, which are mixed, pressed to form a sheet, and finally heated at the temperature close to the melting point of PTFE, e.g., 370 oC, and oxygen in air is reduced during discharge and oxygen evolution occurs during discharge inside of the electrode. For the reactions, the electrode’s structure needs to be porous and optimized for oxygen permeability, and the inside should provide active sites for O2 and OH-. The polarization behaviors of the bi-functional oxygen electrode strongly depend on not only the materials but also the process and condition of the electrode preparation. The purpose of this study is to control and improve the gas diffusion behaviors of the oxygen electrode prepared by low temperature roll press at 80 oC, which was developed by our group [1]. The electrode used graphite as conductive material, Bi2-xRu2O7-z as bi-functional catalyst, and PTFE as binder, which were mixed and pressed to form a sheet by heated roller at 80 oC, and pressed again onto a nickel mesh. The polarization behaviors of the electrode were examined by cyclic voltammetry using a three-electrode cell, in which one side of the electrode was exposed to air and the other side to 6 mol/L KOH solution. The surface and cross section of the electrodes were examined by SEM. SEM observation of the obtained electrodes at various roll press conditions indicated that the shape of PTFE changed depending on the pressure, temperature, and clearance of the rollers, and PTFE particles were fiberized even at 80 oC when the clearance between the two rollers was reduced at constant pressure. The cathodic polarization curves of the electrodes revealed that the onset potential of oxygen reduction hardly depended on the preparation condition, while the polarization in the potential region under mass transfer control, i.e., oxygen diffusion inside of the electrode, was strongly affected by the preparation condition. Oxygen evolution was also seen differently in polarization depending on the electrode, suggesting that the oxygen permeability seems to affect anodic polarization because evolved oxygen should be discharged from the electrode. In this work, detailed relationship between the internal structure of the electrode and the polarization behaviors for oxygen reactions will be presented. Reference [1] Y. Ujino, M. Morimitsu, TMS 2017, San Diego (2017).

Effect of Ag Content and β-PbO2 Plating on the Properties of Al/Pb-Ag Alloy

Due to the excellent electrical conductivity and mechanical properties, Al/Pb-Ag alloy has a great potential to be used as an alternative anode for zinc electrowinning. In this work, the effects of Ag content and surface plating with β-PbO2 on the anodic behavior and reaction kinetics were investigated by cyclic voltammetry (CV), anodic polarization curves, electrochemical impedance spectroscopy (EIS) and corrosion rate test. The phase composition and microscopic morphology of the anode oxide layers after electrolysis were observed by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The results indicate that high content of silver and electroplating β-PbO2 layer can increase the oxygen evolution activity, electrocatalytic activity and corrosion resistance of the anodes. Al/Pb-0.75%Ag plating β-PbO2 has the lowest oxygen evolution overpotential followed by Al/Pb-0.3%Ag plating β-PbO2, Al/Pb-0.75%Ag and Al/Pb-0.3%Ag. Besides, high content of silver is more beneficial to improving corrosion resistance compared with electroplating β-PbO2 on anode. In addition, the phase composition of four anodes layer are mainly composed of α-PbO2, β-PbO2, Pb and PbO.

Electrochemical oxidation of 4-chlorophenol for wastewater treatment using highly active UV treated TiO2 nanotubes

In the present work, we report on a facile UV treatment approach for enhancing the electrocatalytic activity of TiO2 nanotubes. The TiO2 nanotubes were prepared using an anodization oxidation method by applying a voltage of 40 V for 8 h in a DMSO + 2% HF solution, and further treated under UV light irradiation. Compared with Pt and untreated TiO2 nanotubes, the UV treated electrode exhibited a superior electrocatalytic activity toward the oxidation of 4-chlorophenol (4-ClPh). The effects of current density and temperature on the electrochemical oxidation of the 4-ClPh were also systematically investigated. The high electrocatalytic activity of the UV treated TiO2 nanotubes was further confirmed by the electrochemical oxidation of other persistent organic pollutants including phenol, 2-, 3-, 4-nitrophenol, and 4-aminophenol. The total organic carbon (TOC) analysis revealed that over 90% 4-ClPh was removed when the UV treated TiO2 electrode was employed and the rate constant was 16 times faster than that of the untreated TiO2 electrode; whereas only 60% 4-ClPh was eliminated at the Pt electrode under the same conditions. This dramatically improved electrocatalytic activity might be attributed to the enhanced donor density, conductivity, and high overpotential for oxygen evolution. Our results demonstrated that the application of the UV treatment to the TiO2 nanotubes enhanced their electrochemical activity and energy consumption efficiency significantly, which is highly desirable for the abatement of persistent organic pollutants.

Electronic coupling induced high performance of N, S-codoped graphene supported CoS2 nanoparticles for catalytic reduction and evolution of oxygen

A simple synthetic method is developed for the synthesis of CoS2/N, S-codoped graphene. The result shows the existence of a strong electronic coupling between CoS2 and N, S-codoped graphene. The pyrrolic and pyridinic type nitrogen and S in the form of C-S-C in N, S-codoped graphene are found to be the anchoring sites of the CoS2 nanoparticles. As a bifunctional catalyst, the CoS2/N, S-codoped graphene exhibits an oxygen reduction onset potential of 0.963 V vs. RHE and delivers an oxygen evolution overpotential of 393 mV at the current density of 10 mA cm−2. Its oxygen reduction and evolution catalytic activities are comparable to those of the Pt/C and the state-of-art RuO2/C, respectively. Most impressively, the CoS2/N, S-codoped graphene exhibits a potential gap of 771 mV. This value is lower than those of most bifuntional catalysts reported, clearly indicating its potential use as the bifunctional catalyst to replace the noble-metal based catalysts for practical applications. Additionally, our results also suggest a great importance to prepare a single pure phase CoS2 in improving the catalytic bifunctionality of the CoS2/N, S-codoped graphene. The primary Zn-air battery with CoS2/N, S-codoped graphene shows a higher discharge peak power density than that with Pt/C.