Xunyu Lu

Electrodeposition of hierarchically structured three-dimensional nickel–iron electrodes for efficient oxygen evolution at high current densities

Large-scale industrial application of electrolytic splitting of water has called for the development of oxygen evolution electrodes that are inexpensive, robust and can deliver large current density (>500 mA cm−2) at low applied potentials. Here we show that an efficient oxygen electrode can be developed by electrodepositing amorphous mesoporous nickel–iron composite nanosheets directly onto macroporous nickel foam substrates. The as-prepared oxygen electrode exhibits high catalytic activity towards water oxidation in alkaline solutions, which only requires an overpotential of 200 mV to initiate the reaction, and is capable of delivering current densities of 500 and 1,000 mA cm−2 at overpotentials of 240 and 270 mV, respectively. The electrode also shows prolonged stability against bulk water electrolysis at large current. Collectively, the as-prepared three-dimensional structured electrode is the most efficient oxygen evolution electrode in alkaline electrolytes reported to the best of our knowledge, and can potentially be applied for industrial scale water electrolysis.

Electrocatalytic Oxygen Evolution at Surface-Oxidized Multiwall Carbon Nanotubes

Large-scale storage of renewable energy in the form of hydrogen (H2) fuel via electrolytic water splitting requires the development of water oxidation catalysts that are efficient and abundant. Carbon-based nanomaterials such as carbon nanotubes have attracted significant applications for use as substrates for anchoring metal-based nanoparticles. We show that, upon mild surface oxidation, hydrothermal annealing and electrochemical activation, multiwall carbon nanotubes (MWCNTs) themselves are effective water oxidation catalysts, which can initiate the oxygen evolution reaction (OER) at overpotentials of 0.3 V in alkaline media. Oxygen-containing functional groups such as ketonic C═O generated on the outer wall of MWCNTs are found to play crucial roles in catalyzing OER by altering the electronic structures of the adjacent carbon atoms and facilitates the adsorption of OER intermediates. The well-preserved microscopic structures and highly conductive inner walls of MWCNTs enable efficient transport of the electrons generated during OER.

Highly efficient and robust oxygen evolution catalysts achieved by anchoring nanocrystalline cobalt oxides onto mildly oxidized multiwalled carbon nanotubes

Crystalline cobalt oxide nanoparticles have been densely and strongly anchored onto mildly oxidized multiwalled carbon nanotubes (Co3O4/mMWCNT) and applied as electrocatalysts for highly efficient water oxidation. The hybrid catalyses oxygen evolution reactions (OER) with an onset potential of 1.51 V vs. RHE and an overpotential only of 390 mV to achieve a current density of 10 mA cm−2. The Co3O4/mMWCNT catalyst exhibits high Faraday efficiency (>99%) and long-term stability (>25 h) during bulk electrolysis of water. A range of carbon nanostructures including MWCNTs, single-walled CNTs (SWCNTs), and graphene, and CNTs of different oxidation states have been prepared and applied as substrates for Co3O4 nanocrystals and their performance is compared to reveal the interrelationship between the nanocarbon structure, surface functionalization and charge transport for rational design of OER catalysts. Furthermore, a range of techniques has been utilized to characterize the stability of MWCNT substrates during water oxidation. Importantly, it is found that the MWCNTs in the composite can sustain the harsh oxidative environment of water oxidation, with no carbon corrosion detected.

Electrochemistry of Room Temperature Protic Ionic Liquids: A Critical Assessment for Use as Electrolytes in Electrochemical Applications

Ten room temperature protic ionic liquids (RTPILs) have been prepared from low-molecular-weight Brønsted acids and amines with high purity and minimal water content, and their electrochemical characteristics determined using cyclic, microelectrode, and rotating disk electrode voltammetries. Potential windows of the 10 RTPILs were established at glassy carbon, gold, and platinum electrodes, where the largest potential window is generally observed with glassy carbon electrodes. The two IUPAC recommended internal potential reference systems, ferrocene/ferrocenium and cobaltocenium/cobaltocene, were determined for the 10 RTPILs, and their merits as well as limitations are discussed. Other electrochemical properties such as mass transport and double layer capacitances were also investigated. The potential applications of these RTPILs as electrolytes for electrochemical energy devices were discussed, and two novel applications using PILs for metal deposition and water electrolysis were demonstrated.

Gold Nanoparticles Embedded within Mesoporous Cobalt Oxide Enhance Electrochemical Oxygen Evolution

Gold nanoparticles incorporated in mesoporous cobalt oxides (Au/mCo3 O4 ) are fabricated by a nanocasting method using porous silica as the hard template. The Au/mCo3 O4 material exhibits enhanced catalytic activity towards water oxidation compared to bulk mCo3 O4 in both alkaline and neutral solutions. The superior catalytic performance is ascribed to the synergistic effect of electronegative metal gold, which facilitates the generation of active Co(IV) sites, as well as the large specific surface area and the preferential exposure of catalytic active crystalline lattice.

Layer-by-layer assembly of transparent amorphous Co3O4 nanoparticles/graphene composite electrodes for sustained oxygen evolution reaction

Amorphous Co3O4 nanoparticle/graphene composites (Co3O4/GR) were fabricated via layer-by-layer (LBL) assembly onto indium tin oxide (ITO) coated conductive glass and applied as a catalyst for efficient oxygen evolution reaction in electrolytic water splitting. A thin uniform graphene layer was deposited onto the ITO surface through electrophoretic deposition (EPD), followed by subsequent deposition of a thin layer of Co3O4 nanoparticles onto the graphene surface by chemical bath deposition (CBD) to yield a transparent Co3O4/GR bi-layer. X-ray diffraction spectroscopy (XRD), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) were employed to characterise the fabricated composites. The layered Co3O4/GR composite is applied for electrocatalytic oxygen evolution reaction (OER) in 0.1 M KOH and exhibits remarkable catalytic activity with a high Faradaic efficiency (95%) and excellent long-term stability.

Determination of water in room temperature ionic liquids by cathodic stripping voltammetry at a gold electrode.

An electrochemical method based on cathodic stripping voltammetry at a gold electrode has been developed for the determination of water in ionic liquids. The technique has been applied to two aprotic ionic liquids, (1-butyl-3-ethylimidazolium tetrafluoroborate and 1-butyl-3-methylimidazolium hexafluorophosphate), and two protic ionic liquids, (bis(2-hydroxyethyl)ammonium acetate and triethylammonium acetate). When water is present in an ionic liquid, electrooxidation of a gold electrode forms gold oxides. Thus, application of an anodic potential scan or holding the potential of the electrode at a very positive value leads to accumulation of an oxide film. On applying a cathodic potential scan, a sensitive stripping peak is produced as a result of the reduction of gold oxide back to gold. The magnitude of the peak current generated from the stripping process is a function of the water concentration in an ionic liquid. The method requires no addition of reagents and can be used for the sensitive and in situ determination of water present in small volumes of ionic liquids. Importantly, the method allows the determination of water in the carboxylic acid-based ionic liquids, such as acetate-based protic ionic liquids, where the widely used Karl Fischer titration method suffering from an esterification side reaction which generates water as a side product.

Endosomal pH-activatable magnetic nanoparticle-capped mesoporous silica for intracellular controlled release

Endosomal pH-driven linkage-disintegration is a promising strategy to achieve intracellular delivery and controlled drug release. In this paper, a rapid endosomal pH-sensitive MSNs ensemble (i.e., MCM-TAA-Fe3O4) with magnetic nanoparticle caps was developed by anchoring superparamagnetic Fe3O4 nanoparticles on the pore openings with an acid-labile substituted 1,3,5-triazaadamantane (TAA) group. The functionalized Fe3O4 nanoparticles served as a nanogate to regulate the release pattern and/or dosage of payload. The in vitro release experiment with model dexamethasone showed that the MCM-TAA-Fe3O4 ensembles exhibited quick release at pH 5.0–6.0 and zero release in physiological environment (pH = 7.4). Demonstrated with a MC3T3-E1 model cell line, this hybrid nanomaterial could successfully be endocytosed into cells and then release the encapsulated exogenous cargos into the cytosol. The new rapid endosomal pH-sensitive Fe3O4-capped-MSNs could serve as efficient carriers for intracellular controlled release of therapeutic agents in live cells, and may be potentially applied in clinical disease therapy, especially therapeutics and the metabolic manipulation of cells.

Highly Selective and Stable Reduction of CO2 to CO by a Graphitic Carbon Nitride/Carbon Nanotube Composite Electrocatalyst.

A stable and selective electrocatalyst for CO2 reduction was fabricated by covalently attaching graphitic carbon nitride onto multiwall carbon nanotubes (g-C3 N4 /MWCNTs). The as-prepared composite is able to reduce CO2 exclusively to CO with a maximum Faraday efficiency of 60 %, and no decay in the catalytic activity was observed even after 50 h of reaction. The enhanced catalytic activity towards CO2 reduction is attributed to the formation of active carbon-nitrogen bonds, high specific surface area, and improved material conductivity of the g-C3 N4 /MWCNT composite.

Interconnected core–shell carbon nanotube–graphene nanoribbon scaffolds for anchoring cobalt oxides as bifunctional electrocatalysts for oxygen evolution and reduction

Bifunctional electrocatalysts for oxygen evolution reactions (OER) and oxygen reduction reactions (ORR) are crucial to the development of regenerative fuel cells or rechargeable metal–air batteries. However, the sluggish kinetics of OER and ORR often require the use of precious metal-based catalysts such as iridium, ruthenium, and platinum to lower the energy barriers of OER and ORR. Developing a highly efficient and stable bifunctional catalyst that is made of non-precious elements for ORR and OER still remains a significant challenge. Here, we show a novel catalyst architecture based on coupling non-precious Co3O4 nanocrystals onto nitrogen-doped, core–shell structured carbon nanotube–graphene nanoribbon (N-csCNT–GNR) scaffolds prepared by microwave-assisted, controlled upzipping of multiwall carbon nanotubes. The unzipped graphene nanoribbon shell enables a high surface area for loading of Co3O4 nanocrystals while the intact inner carbon nanotube core facilitates efficient transport of electrons. The as-prepared Co3O4/N-csCNT–GNR composite catalysts exhibit remarkably high activity towards both OER and ORR as a result of synergistic interactions between Co3O4 and the N-csCNT–GNR substrates.