Bryan H. R. Suryanto

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.

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.

Hydrothermally driven transformation of oxygen functional groups at multiwall carbon nanotubes for improved electrocatalytic applications

The role of carbon nanotubes in the advancement of energy conversion and storage technologies is undeniable. In particular, carbon nanotubes have attracted significant applications for electrocatalysis. However, one central issue related to the use of carbon nanotubes is the required oxidative pretreatment that often leads to significant damage of graphitic structures which deteriorates their electrochemical properties. Traditionally, the oxidized carbon nanomaterials are treated at high temperature under an inert atmosphere to repair the oxidation-induced defect sites, which simultaneously removes a significant number of oxygen functional groups. Nevertheless, recent studies have shown that oxygen functional groups on the surface of MWCNT are the essential active centers for a number of important electrocatalytic reactions such as hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR). Herein we first show that hydrothermal treatment as a mild method to improve the electrochemical properties and activities of surface-oxidized MWCNT for OER, HER, and ORR without significantly altering the oxygen content. The results indicate that hydrothermal treatment could potentially repair the defects without significantly reducing the pre-existing oxygen content, which has never been achieved before with conventional high-temperature annealing treatment.

Direct Hydrothermal Synthesis of Carbonaceous Silver Nanocables for Electrocatalytic Applications.

This study demonstrates a facile but efficient hydrothermal method for the direct synthesis of both carbonaceous silver (Ag@C core-shell) nanocables and carbonaceous nanotubes under mild conditions (<180 °C). The carbonaceous tubes can be formed by removal of the silver cores via an etching process under temperature control (60-140 °C). The structure and composition are characterized using various advanced microscopic and spectroscopic techniques. The pertinent variables such as temperature, reaction time, and surfactants that can affect the formation and growth of the nanocables and nanotubes are investigated and optimized. It is found that cetyltrimethylammonium bromide plays multiple roles in the formation of Ag@C nanocables and carbonaceous nanotubes including: a shape controller for metallic Ag wires and Ag@C cables, a source of Br(-) ions to form insoluble AgBr and then Ag crystals, an etching agent of silver cores to form carbonaceous tubes, and an inducer to refill silver particles into the carbonaceous tubes to form core-shell structures. The formation mechanism of carbonaceous silver nanostructures depending upon temperature is also discussed. Finally, the electrocatalytic performance of the as-prepared Ag@C nanocables is assessed for the oxidation reduction reaction and found to be very active but much less costly than the commonly used platinum catalysts. The findings should be useful for designing and constructing carbonaceous-metal nanostructures with potential applications in conductive materials, catalysts, and biosensors.

Surface-oxidized carbon black as a catalyst for the water oxidation and alcohol oxidation reactions

Carbon black (CB) is popularly used as a catalyst support for metal/metal oxide nanoparticles due to its large surface area, excellent conductivity and stability. Herein, we show that surface oxidized CB itself, after acidic treatment and electrochemical oxidation, exhibits significant catalytic activity for the electrochemical oxidation of water and alcohols.

Controlled electrodeposition of nanostructured Pd thin films from protic ionic liquids for electrocatalytic oxygen reduction reactions

Palladium (Pd) has been widely used as electrocatalysts for a number of important electrochemical reactions. Herein, we report a facile electrodeposition method for fabrication of nanostructured Pd thin films from protic ionic liquids. The electrochemical behavior and electrodeposition of Pd were studied in a protic ionic liquid, ethylammonium nitrate (EAN), compared with aqueous solution and aprotic ionic liquids, 1-ethyl-3-methylimidazolium chloride-tetrafluoroborate (EMIM-Cl-BF4), using cyclic voltammetry and chronoamperometry. The electrodeposition of Pd in protic ionic liquids is found to proceed via an instantaneous nucleation and diffusion-controlled 3D growth mechanism, and accompanied with hydrogen co-evolution at more negative deposition potentials. By controlling the electrodeposition media, electrodeposition potential and time, we show that Pd thin films can be electrodeposited from protic ionic liquids with finely tuned nanostructures and large surface area. The prepared Pd thin films were employed as electrocatalysts for oxygen reduction reactions in alkaline media with an onset potential of 0.95 V (RHE). It was found that Pd films prepared from protic ionic liquids exhibit larger electrochemically active surface area and higher catalytic activity for oxygen reduction reactions than aqueous and aprotic ionic liquid electrolytes under similar electrodeposition conditions.

Effect of oxygen functionalisation on the electrochemical behaviour of multiwall carbon nanotubes for alcohol oxidation reactions

Multiwall carbon nanotubes (MWCNTs) have been popularly used as catalyst supports for various electrochemical devices and reactions. In their preparation, surface oxidation by chemical oxidants is often necessary to purify MWNCTs, which also results in the formation of oxygen functional groups. However, the effect of these functionalities on electrochemical behavior of MWCNTs for alcohol oxidations remain largely unknown. In this study, we show that surface oxidation activates MWCNTs for electrochemical oxidation of alcohols in alkaline media (0.1 M KOH). Significantly enhanced catalytic activity in terms of higher current density (j) as well as lower alcohol oxidation onset potentials was observed following controlled oxidations via chemical or electrochemical methods. High-resolution XPS analysis suggests that the surface bound oxygen functionalities e.g. ketonic group (CO) contribute primarily to the observed increase in catalytic performance. Moreover, to further increase the activity of MWCNTs, hydrothermal treatment was applied to repair the structural damage induced by the harsh oxidation treatment without the sacrifice of oxygen functional groups. Using the hydrothermally treated, surface-oxidized MWCNTs, EtOH undergoes oxidation into acetic acid with ∼99% faradaic efficiency. This study reveals the unique role of oxygen functional groups on MWCNTs towards catalytic alcohol oxidations for possible applications in direct alcohol fuel cells and alcohol sensors.

Energy-Efficient Nitrogen Reduction to Ammonia at Low Overpotential in Aqueous Electrolyte under Ambient Conditions

The electrochemical nitrogen reduction reaction (NRR) under ambient conditions is a promising alternative to the traditional energy-intensive Haber-Bosch process to produce NH3 . The challenge is to achieve a sufficient energy efficiency, yield rate, and selectivity to make the process practical. Here, we demonstrate that Ru nanoparticles (NPs) enable NRR in 0.01 m HCl aqueous solution at very high energy efficiency, that is, very low overpotentials. Remarkably, the NRR occurs at a potential close to or even above the H+ /H2 reversible potential, significantly enhancing the NRR selectivity versus the production of H2 . NH3 yield rates as high as ≈5.5 mg h-1  m-2 at 20 °C and 21.4 mg h-1  m-2 at 60 °C were achieved at a redox potential (E) of -100 mV versus the reversible hydrogen electrode (RHE), whereas a highest Faradaic efficiency (FE) of ≈5.4 % is achievable at E=+10 mV vs. RHE. This work demonstrates the potential use of Ru NPs as an efficient catalyst for NRR at ambient conditions. This ability to catalyze NRR at potentials near or above RHE is imperative in improving the NRR selectivity towards a practical process as well as rendering the H2 viable as byproduct. Density functional theory calculations of the mechanism suggest that the efficient NRR process occurring on these predominantly Ru (0 0 1) surfaces is catalyzed by a dissociative mechanism.