Darren A. Walsh

Permselective nanostructured membranes based on cellulose nanowhiskers

Nanostructured thin films of cellulose nanowhiskers derived from cotton were formed using a simple drop-coating procedure. The hydrogen-bonded cellulose films were stable in aqueous solutions and their permselective properties were probed using voltammetric techniques. The nanowhisker extraction procedure produces cellulose nanowhiskers with negatively-charged sulfate surface groups that inhibit the transfer of negatively-charged species through the nanowhisker membrane, while the diffusion of neutral species is only slightly hindered. Using rotating-disk electrode measurements, the diffusion of various species within the film was studied and it was shown that the positively-charged species, Ru(NH3)63+, was adsorbed by the film, whereas the negatively-charged species, IrCl63−, was excluded by the film. The thermodynamics of adsorption of the positively-charged species by the cellulose nanoparticles were then studied using isotherm data. These observations open up new possibilities in electrochemical sensor development using renewable cellulosic materials as building blocks. Furthermore, charge-based permselective membranes can also be formed using free standing cellulose nanowhisker films, which offer the promise of renewable, selective membranes for separation technologies.

Synthesis of platinum nanoparticles using cellulosic reducing agents

Platinum nanoparticles were formed by reduction of H2PtCl6 using nanocrystalline cellulose from cotton as the reducing agent.

Biomass-derived activated carbon with simultaneously enhanced CO2 uptake for both pre and post combustion capture applications

We report on the synthesis and CO2 uptake capabilities of a series of activated carbons derived from the biomass raw materials Jujun grass and Camellia japonica. The carbons were prepared via hydrothermal carbonization of the raw materials, which yielded hydrochars that were activated with KOH at temperature between 600 and 800 °C. Carbons activated at KOH/hydrochar ratio of 2 have moderate to high surface area (1050–2750 m2 g−1), are highly microporous (95% of surface area arises from micropores, and 84% of pore volume from micropores of size between 5 and 7 A), and exhibit excellent CO2 uptake capacity at 25 °C of up to 1.5 mmol g−1 at 0.15 bar and 5.0 mmol g−1 at 1 bar, which is amongst the highest reported so far for biomass-derived carbons. On the other hand, activation at KOH/hydrochar ratio of 4 generates carbons with surface area and pore volume of up to 3537 m2 g−1 and 1.85 cm3 g−1, and which, depending on level of activation, simultaneously exhibit high CO2 uptake at both 1 bar (4.1 mmol g−1) and 20 bar (21.1 mmol g−1), i.e. under conditions that mimic, respectively, post combustion and pre combustion CO2 capture from flue gas streams. The present carbons are the first examples of biomass derived porous materials with such all-round CO2 uptake performance, which arises due to the pore size distribution of the carbons being shifted towards small micropores even for samples with very high surface area. Thus the carbons satisfy the requirements for both low pressure (presence of small micropores) and high pressure (high surface area) CO2 uptake.

Synthesis of carbon-supported Pt nanoparticle electrocatalysts using nanocrystalline cellulose as reducing agent

Pt nanoparticles have been synthesized at relatively low temperatures in aqueous solution from hexachloroplatinic acid using cellulose nanocrystals (CNXLs) from cotton as reducing agents. The Pt nanoparticles were characterised using X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy and thermogravimetric analysis. X-ray diffraction and X-ray photoelectron spectroscopy showed that the particles have a metallic Pt core and an oxidised surface layer. TEM analysis showed that the nanoparticles have an average diameter of approximately 2 nm, which is independent of the reactant concentrations. By performing the reduction reaction in the presence of a carbon-black support (Vulcan XC-72R), and removing the cellulosic material by heating in air, it was possible to produce carbon black supported Pt nanoparticles. Electrochemical analysis revealed that this Pt/C was highly active towards electrocatalysis of the oxygen reduction reaction, suggesting that this method may be very useful for fabricating Pt/C electrocatalysts.

Effect of Viscosity on Steady-State Voltammetry and Scanning Electrochemical Microscopy in Room Temperature Ionic Liquids

The electrochemical properties of a series of room temperature ionic liquids (RTILs) were studied using voltammetric methods and scanning electrochemical microscopy (SECM). The RTILs consisted of 1-alkyl-3-methylimidazolium cations, [C(n)C(1)Im](+), and either bis[(trifluoromethyl)sulfonyl]imide anions, [Tf(2)N](-), or hexafluorophosphate anions, [PF(6)](-). The effect of RTIL viscosity on mass transfer dynamics within each RTIL was studied electrochemically using ferrocene as a redox probe. In the case of the [C(n)C(1)Im][Tf(2)N] RTILs, the viscosity was altered by changing the alkyl chain length. [C(4)C(1)Im][PF(6)] was used for comparison as its viscosity is significantly higher than that of the [C(n)C(1)Im][Tf(2)N] RTILs. The RTIL viscosity affected the ability to record steady-state voltammograms at ultramicroelectrodes (UMEs). For example, it was possible to record steady-state voltammograms at scan rates up to 10 mV s(-1) in [C(2)C(1)Im][Tf(2)N] using 1.5 mum radius disk UMEs, but non-steady-state behavior was observed at 50 mV s(-1). However, at 12.5 microm radius UMEs, steady-state voltammetry was only observed at 1 mV s(-1) in [C(2)C(1)Im][Tf(2)N]. The RTIL viscosity also affected the ability to record SECM feedback approach curves that agreed with conventional SECM theory. In the most viscous [C(n)C(1)Im][Tf(2)N] RTILs, feedback approach curves agreed with conventional theory only when very slow tip approach speeds were used (0.1 microm s(-1)). These observations were interpreted using the Peclet number, which describes the relative contributions of convective and diffusive mass transfer to the tip surface. By recording feedback approach curves in each RTIL at a range of tip approach speeds, we describe the experimental conditions that must be met to perform SECM in imidazolium-based RTILs. The rate of heterogeneous electron transfer across the RTIL/electrode interface was also studied using SECM and the standard heterogeneous electron transfer rate constant, k(0), for ferrocene oxidation recorded in each RTIL was higher than that determined previously using voltammetric methods.

Heterogeneous electron transfer kinetics at the ionic liquid/metal interface studied using cyclic voltammetry and scanning electrochemical microscopy.

The electrochemical behavior of a redox-active, ferrocene-modified ionic liquid (1-ferrocenylmethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) in acetonitrile and in an ionic liquid electrolyte (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) is reported. Reversible electrochemical behavior was observed in each electrolyte with responses typical of those for unmodified ferrocene observed in each medium. In the ionic liquid electrolyte, the diffusion coefficient of the redox-active ionic liquid increased by a factor of 5 upon increasing the temperature from 27 to 90 degrees C. The kinetics of electron transfer across the ionic liquid/electrode interface were studied using cyclic voltammetry, and the standard heterogeneous electron transfer rate constant, k (0) was determined to be 4.25 x 10 (-3) cm s (-1). Scanning electrochemical microscopy was then also used to probe the heterogeneous kinetics at the interface between the ionic liquid and the solid electrode and conventional kinetic SECM theory was used to determine k (0). The k (0) value obtained using SECM was higher than that determined using cyclic voltammetry. These results indicate that SECM is a very useful technique for studying electron transfer dynamics in ionic liquids.

Rapid Screening of Bimetallic Electrocatalysts for Oxygen Reduction in Acidic Media by Scanning Electrochemical Microscopy

Additional bimetallic electrocatalysts for the oxygen reduction reaction ORR in acidic media were designed using a previously reported thermodynamic selection guide. The electrocatalyst mixtures were prepared in large arrays on glassy carbon substrates and the electrocatalytic activity was screened using scanning electrochemical microscopy SECM. Activities were measured for a range of bimetallic combinations that showed a synergetic electrocatalytic effect during screening, including Au–V, Ag–V, Pd–Mn, and Pd–V. Upon initial screening, a highly active electrocatalytic combination consisting of 60:40 Pd–V was identified. Using rotating disk electrode RDE experiments, the high activity of this combination for the ORR in acidic media was confirmed when the electrocatalysts were supported on Vulcan carbon. The electrocatalytic activity of Pd–V was close to that exhibited by Pt, the electrocatalyst of choice for the ORR in acidic media, and thus is another example of a nonplatinum catalyst with high activity that follows the previous strategy for catalyst design.

High total-electrode and mass-specific capacitance cellulose nanocrystal-polypyrrole nanocomposites for supercapacitors

For practical applications, new supercapacitor electrode materials need to exhibit a high mass-specific capacitance (CM/F g−1), a high total-electrode capacitance (CE/F cm−2), and high stability during charge–discharge cycling. Very often, newly developed materials display high CM values for thin films (nm or μm thickness) but these rapidly drop off in the thicker electrode structures needed for commercial devices. In this work, we describe the fabrication of thick nanocomposites of polypyrrole (PPY) and cellulose nanocrystals (CNXLs) with consistently high capacitance (CM = 240 F g−1) and performance. CE of the PPY-CNXL nanocomposite increased linearly with increasing film thickness up to a value of 1.24 F cm−2 and this increased to a maximum of 1.54 F cm−2 for even thicker films where non-linear CE increases were due to electrolyte diffusion limitations. Testing of a symmetric supercapacitor with these high CE electrodes showed that it retained half of its initial capacity after 50 000 charge–discharge cycles, demonstrating the excellent stability of PPY-CNXL supercapacitor electrode materials.

Nanocomposite oxygen reduction electrocatalysts formed using bioderived reducing agents

Crystalline cellulose nanofibrils from cotton were used as reducing agents for the synthesis of nanostructured silver. The hydrothermal synthesis involved heating an AgNO3 solution containing suspended cellulose nanofibrils at 80 °C for 2 h. The formation of metallic silver was verified using UV/Visible spectroscopy, X-ray diffraction and transmission electron microscopy (TEM). Cellulose/silver nanocomposite films were formed at glassy carbon surfaces by drop coating with the product suspension and scanning electron microscopy (SEM) was used to characterise the modified surfaces. The film morphology depended on the ratio of silver to cellulose in the films. Cyclic voltammetry and rotating-disk electrode voltammetry were used to study the electrochemical and electrocatalytic behavior of these films. The nanocomposite films formed using this approach were highly active electrocatalysts for the reduction of oxygen in alkaline media.

Formation and Growth of Oxide Layers at Platinum and Gold Nano- and Microelectrodes

The construction and characterization of platinum and gold disk electrodes with minimum radii of 7 nm (platinum) and 500 nm (gold) is reported. The electrodes were prepared with a micropipet puller using a two step procedure and have been characterized using scanning electron microscopy, scanning electrochemical microscopy, high speed chronoamperometry, and cyclic voltammetry. The formation and growth of platinum and gold oxide layers, on the electrodes at time scales from microseconds to seconds, is reported. Significantly, the apparent microscopic area as determined by forming and subsequently reducing an oxide layer in acidic electrolyte using cyclic voltammetry depends dramatically on the scan rate. While conventional roughness factors between 1.8 and 3 are observed on average for scan rates above 5 V s(-1), the apparent roughness can exceed 30 for scan rates less than 0.5 V s(-1). Chronoamperometry, conducted on the microsecond to millisecond time scale, is used to probe the dynamics of monolayer and multilayer oxide formation as well as the reversibility of the oxide formation and removal. The latter study suggests that (at least for platinum) the growth of the oxide layer proceeds with a lower constant rate after an oxide monolayer is formed.