The large-scale practical application of fuel cells will be difficult to realize if the expensive platinum-based electrocatalysts for oxygen reduction reactions (ORRs) cannot be replaced by other ...efficient, low-cost, and stable electrodes. Here, we report that vertically aligned nitrogen-containing carbon nanotubes (VA-NCNTs) can act as a metal-free electrode with a much better electrocatalytic activity, long-term operation stability, and tolerance to crossover effect than platinum for oxygen reduction in alkaline fuel cells. In air-saturated 0.1 molar potassium hydroxide, we observed a steady-state output potential of -80 millivolts and a current density of 4.1 milliamps per square centimeter at -0.22 volts, compared with -85 millivolts and 1.1 milliamps per square centimeter at -0.20 volts for a platinum-carbon electrode. The incorporation of electron-accepting nitrogen atoms in the conjugated nanotube carbon plane appears to impart a relatively high positive charge density on adjacent carbon atoms. This effect, coupled with aligning the NCNTs, provides a four-electron pathway for the ORR on VA-NCNTs with a superb performance.
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•Carbon nanotubes are vertically-aligned on a carbon fiber to form a biosensing scaffold.•Prussian blue and/or GOx is well-organized along the nanotubes’ length.•The ordering ...electrode structure facilitates the enzymatic electrochemical reactions.•The biosensors feature significantly improved performance in signaling H2O2 and glucose.
We describe our assembly and the analytical performance of a glucose biosensor consisting of an array of carbon nanotubes (CNTs) that perpendicularly fall on a 7-μm-diameter carbon fiber and are modified by a “dual” enzymatic system—viz. glucose oxidase (GOx) and Prussian blue (PB, an artificial peroxidase). We chose to use the PB-catalyzed reduction reaction of hydrogen peroxide, an end-product of the GOx-catalyzed oxidation of glucose, to “relay” electrons from GOx to the substrate electrode. We highlight that the electrode-structural alignment of this novel biosensing system plays a crucial role in optimizing the electrochemical- and catalytic-reactions of the enzymes with their substrates. The vertical alignment of enzyme-modified CNTs with the pores located between neighboring individual CNTs creates the simplest optimized pathways for substrates to diffuse to the enzymes and for the generated electrical signals to transport along the nanotube’s length to an electronic analyzer. Consequently, the glucose biosensor thus constructed exhibits a high sensitivity of 4.9μA/mM with a detection limit of 0.05mmol/L and long-term stability in amperometrically detecting glucose. Our long-range-order assembling of electroactive biomolecules and microscale/nanoscale materials into a multifunctional biocomposite accounts for this superb performance of vital importance in their realistic applications in deciphering glucose and hydrogen peroxide.
More than tips: The electrochemical properties of the tips and sidewalls of carbon nanotubes can be compared when specific regions are selectively masked with a nonconducting polymer coating (e.g. ...polystyrene) such that the electrolyte has access to one region only (see scheme). Surprisingly electrochemistry at carbon nanotube electrodes is not always facilitated by the nanotube tip.
We introduce a novel approach for enhancing ion-storage capabilities through the self-assembled formation of ternary nanowires, consisting of a CoO-and-Nafion nanocomposite encasing metallic ...single-walled carbon nanotubes (SWNTs). These nanowires highlight the indispensable role of the conductor-semimetallic SWNT-CoO heterojunction in expediting the redox reaction kinetics of the CoO while the Nafion component facilitates the transport of hydroxide ions. Consequently, supercapacitors comprising the ternary nanowires deliver the specific capacitance of calculated 18.84 mF cm−2, five times higher than those obtained at the counterpart devices that lack Nafion and/or CoO. This self-assembly method precisely organizes functional components within the nanowires, aligning with electrochemical principles to optimize charge transfer and minimize concentration gradients for improved electrochemical polarization. Our function-enhanced ternary nanowires, together with their straightforward synthetic process, promise an alternative opportunity for formulating high-performance ion-storage electrodes and thus pave the way for their application in next-generation supercapacitor technologies.
A SWNT-CoO conductor-semimetallic heterojunction is engineered through electrostatic adsorption, optimizing the CoO layer's energy band for improved electron transfer in its redox reactions. Nafion's inclusion enhances hydroxide ion diffusion, collectively boosting the material's electrochemical performance. Display omitted
We report on the synthesis, characterization, and electrochemical performance of novel, ultrathin Pt monolayer shell–Pd nanowire core catalysts. Initially, ultrathin Pd nanowires with diameters of ...2.0 ± 0.5 nm were generated, and a method has been developed to achieve highly uniform distributions of these catalysts onto the Vulcan XC-72 carbon support. As-prepared wires are activated by the use of two distinctive treatment protocols followed by selective CO adsorption in order to selectively remove undesirable organic residues. Subsequently, the desired nanowire core–Pt monolayer shell motif was reliably achieved by Cu underpotential deposition followed by galvanic displacement of the Cu adatoms. The surface area and mass activity of the acid and ozone-treated nanowires were assessed, and the ozone-treated nanowires were found to maintain outstanding area and mass specific activities of 0.77 mA/cm2 and 1.83 A/mgPt, respectively, which were significantly enhanced as compared with conventional commercial Pt nanoparticles, core–shell nanoparticles, and acid-treated nanowires. The ozone-treated nanowires also maintained excellent electrochemical durability under accelerated half-cell testing, and it was found that the area-specific activity increased by ∼1.5 fold after a simulated catalyst lifetime.
This paper describes the adsorption of electroactive methylene blue (MB) dye onto single-walled carbon nanotubes (SWNTs) to form an electrochemically functional nanostructure and its layered ...nanocomposite. UV−visible and FT-IR spectroscopy and electrochemistry used for characterization of the MB adsorption onto SWNTs reveal that MB essentially interacts with SWNTs through charge-transfer and hydrophobic interactions, leading to the formation of a MB−SWNT adsorptive nanostructure which exhibits distinct electrochemical properties from those of MB adsorbed onto a glassy carbon (GC) electrode. The interactions between MB and the SWNTs are demonstrated to closely associate with the structural properties of the SWNTs by comparing the electrochemical properties of MB adsorbed onto different substrates, i.e., glassy carbon, SWNTs, and SWNTs intentionally sidewall functionalized with hydroxyl groups (SWNT-OHs). The stable adsorption of water-soluble and positively charged MB molecules onto the SWNTs is further demonstrated to be able to solubilize the formed nanostructure in water quite well and to fabricate a functional nanocomposite by layer-by-layer assembling of the formed nanostructure on a solid substrate.
This review addresses recent developments in electrochemistry and electroanalytical chemistry of carbon nanotubes (CNTs). CNTs have been proved to possess unique electronic, chemical and structural ...features that make them very attractive for electrochemical studies and electrochemical applications. For example, the structural and electronic properties of the CNTs endow them with distinct electrocatalytic activities and capabilities for facilitating direct electrochemistry of proteins and enzymes from other kinds of carbon materials. These striking electrochemical properties of the CNTs pave the way to CNT-based bioelectrochemistry and to bioelectronic nanodevices, such as electrochemical sensors and biosensors. The electrochemistry and bioelectrochemistry of the CNTs are summarized and discussed, along with some common methods for CNT electrode preparation and some recent advances in the rational functionalization of the CNTs for electroanalytical applications.
By performing Raman spectroscopic and electrochemical measurements along the nanotube length, we have demonstrated that there is a concentration gradient of structural defects along the tube length ...for super-long (ca. 5 mm) vertically aligned double-walled carbon nanotubes (SLVA-DWNTs) produced by water-assisted CVD growth. An increase in the structural defect content along the nanotube length from the top to bottom end was observed in the present study due to the “bottom growth” process and the decay of the catalyst reactivity with the growth time. The newly observed defect content gradient facilitated us to tune the nanotube electrochemistry along its length and to deposit metal particles in a length-specific fashion.
We describe a combination of electrochemical superfilling and oxidative cutting approaches to producing multilayer carbon nanocylinders (ML-CNCs) by cutting off a specific length of ...vertically-aligned MWNTs (VA-MWNTs) along their circumference. Superfilling the gaps in VA-MWNTs with copper to produce a void-free Cu/VA-MWNT film is mainly based on the competitive diffusion and adsorption of polyethylene glycol and bis-(3-sulfopropyl) disulfide that leads to a peculiar kinetic profile decreasing from the bottom to topside of the gaps. The subsequent etching of copper in Cu/VA-MWNTs forms a fraction of about 30-nm-high VA-MWNTs protruding out of copper, which is cut off to produce ML-CNCs via the thermal oxidation of MWNTs, catalyzed by an ultrathin layer of copper oxide at the topmost side. The mechanisms for both the superfilling and the length-controlled cutting are systematically studied and discussed. ML-CNCs feature the shortest length among the existing MWNTs, minimal damage in the sidewalls, and high spectroscopic and electrocatalytic activities so that they can be used as multi-responsive probes to decipher biomolecules (e.g., β-nicotinamide adenine dinucleotidedinucleotide).
•FePc/SWNT heterojunction nanowires are prepared via a self-assembly approach.•FePc/SWNT performs much better than Pt/C in the ORR in alkaline media.•DFT and experimental results are combined to ...understand the ORR at FePc/SWNT.•The O2–FePc interaction is facilitated with fast electron transfer to the surface.
A self-assembly approach to preparing iron phthalocyanine/single-walled carbon nanotube (FePc/SWNT) heterojunction nanowires as a new oxygen reduction reaction (ORR) electrocatalyst has been developed by virtue of water-adjusted dispersing in 1-cyclohexyl-pyrrolidone (CHP) of the two components. The FePc/SWNT nanowires have a higher Fermi level compared to pure FePc (d-band center, DFT=−0.69eV versus −0.87eV, respectively). Consequently, an efficient channel for transferring electron to the FePc surface is readily created, facilitating the interaction between FePc and oxygen, so enhancing the ORR kinetics. This heterojunction-determined activity in ORR illustrates a new stratagem to preparing non-noble ORR electrocatalysts of significant importance in constructing real-world fuel cells.
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