N. Arjona

Electrocatalytic activity of well-defined and homogeneous cubic-shaped Pd nanoparticles

Well-ordered, homogenous cube-shaped highly ordered Pd, without other geometries, was obtained by chemical reduction in aqueous media by employing ascorbic acid, polyvinylpyrrolidone and sodium bromide as the reducing agent, surfactant and additive, respectively. The X-ray diffraction (XRD) patterns exhibited a face-centred cubic structure with an average crystallite size of 11.5 nm. Transmission electron microscopy (TEM) images showed a homogeneous distribution of cubic Pd nanoparticles (namely Pd nanocubes) with a preferential (200) crystallographic plane and a particle size of approximately 10.6 ± 1.2 nm. The electrocatalytic activity of the Pd nanocubes was evaluated in terms of the current density attributed to the oxidation reaction of the following three common fuels: methanol, ethanol and formic acid. The results obtained showed that the current density achieved with the Pd nanocube electrocatalyst is 4, 3 and 3.5 (12, 10 and 8.7 mA mg−1 cm−2, respectively) times higher than that reached with commercial Pd at similar oxidation potential values. Furthermore, the Pd nanocube catalyst exhibited higher catalytic activity for formic acid oxidation than that reported for Pd-based materials. The oxygen reduction reaction using the Pd nanocubes in basic media was also tested.

Electrochemical synthesis of flower-like Pd nanoparticles with high tolerance toward formic acid electrooxidation

Flower-like nanoparticles exhibit unique properties due to the presence of edge and corner atoms, ad-atoms, pits and other collection of defects. In this work, flower-like Pd nanoparticles were obtained by surfactantless and additiveless square wave voltammetry as an electrochemical method of synthesis. Furthermore, other well-defined Pd shapes with dendritic growth such as spinous flower-like, cone and coral reef-like shapes were obtained by electrochemical methods using cyclic voltammetry, differential pulse amperometry and second harmonic AC voltammetry. Crystal sizes were calculated through the X-ray diffraction patterns for the spinous flower-like, cone, flower and coral reef-like palladium architectures resulting in sizes of 33, 56, 44 and 47 nm, respectively. SEM and cross-section TEM images confirmed that the architectures were composed of micro and nanostructures with dendritic growths. Cyclic voltammetry in acidic medium confirmed the presence of certain facets and terraces in both, flower-like Pd and the dendritic growths. The electrocatalytic properties of the palladium architectures were evaluated to the formic acid electrooxidation at 0.1, 0.5 and 1 M. The flower-like Pd architectures exhibited the highest tolerance to CO poisoning due to the process being carried out by a direct pathway and can be related to the effect of the unique flower-like shape which due to its nature exhibits a high presence of terraces and defects.

Electrochemical growth of Au architectures on glassy carbon and their evaluation toward glucose oxidation reaction

Arrow-like, flower, splintery flower-like and pin wheel Au architectures were obtained using cyclic voltammetry (I), differential pulse amperometry (II), square wave voltammetry (III) and second harmonic AC voltammetry (IV) as electrochemical methods of synthesis, named systems I, II, III and IV respectively. Architecture sizes were 610, 380, 590 and 480 nm, meanwhile crystal sizes were 48, 76, 120 and 210 nm, respectively. XRD patterns showed that systems I, III and IV preferentially exhibited the (111) plane and system II preferentially exhibited the (200) plane. The catalytic properties of the architectures were tested employing D-(+)-glucose at 10, 50 and 100 mM, where the results showed that two processes occur in the glucose electrooxidation: at ∼−0.2 V vs. NHE, glucose is oxidized to gluconolactone, and after at ∼0.3 V vs. NHE the successive oxidation of the gluconolactone by-product were carried out. Systems with the (111) preferential plane favor gluconolactone oxidation; while system II with the (200) plane enhanced glucose oxidation toward the gluconolactone by-product. Also, system II showed a higher current density with 26.4 mA cm−2 at 100 mM glucose.

Glycerol electro-oxidation in alkaline media using Pt and Pd catalysts electrodeposited on three-dimensional porous carbon electrodes

In this work, Pd and Pt electrocatalysts were electrodeposited on three-dimensional carbon paper and carbon nanofoam with the purpose of increasing the catalytic area to improve the glycerol electro-oxidation. SEM and cross-sectional SEM micrographs showed that Pd and Pt particles were well-distributed over the entire three-dimensional electrode surfaces. Commercial Pd/C and Pt/C catalysts deposited by the spray method were used for comparison, showing lower surface area (SA) utilization than those electrodeposited. The electrodeposition effectiveness to cover the electrode surfaces was evaluated by changes in overall SA and through the calculation of electrochemically active surface area (EASA) and specific surface area (SSA). Despite the larger EASA values found for Pd and Pt on nanofoam, Pt on paper showed the highest utilization of the surface area, obtaining an SSA of 58.1 m2 g−1. Moreover, the electrodeposition of Pd and Pt dramatically increased the EASA versus the geometrical area, improving this ratio 16 (Pd on paper), 151 (Pt on paper), 158 (Pd on nanofoam) and 277-fold (Pt on nanofoam). The electrodeposited porous Pt electrodes showed good activity for glycerol oxidation, exhibiting a more negative potential than Pd-based materials. However, for fuel cell applications operated at intermediate temperatures, Pd on carbon paper is the optimal candidate to be used as an anode because of its high current density and excellent poisoning tolerance.

Formic acid microfluidic fuel cell based on well-defined Pd nanocubes

Microfluidic fuel cells (μFFC) are emerging as a promising solution for small-scale power demands. The T-shaped architecture of the μFFC promotes a laminar flow regimen between the catholyte and anolyte streams excluding the use of a membrane, this property allows a simplest design and the use of several micromachining techniques based on a lab-on-chip technologies. This work presents a combination of new materials and low cost fabrication processes to develop a light, small, flexible and environmental friendly device able to supply the energy demand of some portable devices. Well-defined and homogeneous Pd nanocubes which exhibited the (100) preferential crystallographic plane were supported on Vulcan carbon and used as anodic electrocatalyst in a novel and compact design of a SU-8 μFFC feeded with formic acid as fuel. The SU-8 photoresist properties and the organic microelectronic technology were important factors to reduce the dimensions of the μFFC structure. The results obtained from polarization and power density curves exhibited the highest power density (8.3 mW cm−2) reported in literature for direct formic acid μFFCs.

Evaluation of single and stack membraneless enzymatic fuel cells based on ethanol in simulated body fluids

The purpose of this work is to evaluate single and double-cell membraneless microfluidic fuel cells (MMFCs) that operate in the presence of simulated body fluids SBF, human serum and blood enriched with ethanol as fuels. The study was performed using the alcohol dehydrogenase enzyme immobilised by covalent binding through an array composed of carbon Toray paper as support and a layer of poly(methylene blue)/tetrabutylammonium bromide/Nafion and glutaraldehyde (3D bioanode electrode). The single MMFC was tested in a hybrid microfluidic fuel cell using Pt/C as the cathode. A cell voltage of 1.035V and power density of 3.154mWcm-2 were observed, which is the highest performance reported to date. The stability and durability were tested through chronoamperometry and polarisation/performance curves obtained at different days, which demonstrated a slow decrease in the power density on day 10 (14%) and day 20 (26%). Additionally, the cell was tested for ethanol oxidation in simulated body fluid (SBF) with ionic composition similar to human blood plasma. Those tests resulted in 0.93V of cell voltage and a power density close to 1.237mWcm-2. The double cell MMFC (Stack) was tested using serum and human blood enriched with ethanol. The stack operated with blood in a serial connection showed an excellent cell performance (0.716mWcm-2), demonstrating the feasibility of employing human blood as energy source.

An electrokinetic-combined electrochemical study of the glucose electro-oxidation reaction: effect of gold surface energy

The glucose electro-oxidation reaction typically involves several steps and it is strongly influenced by the crystalline structure. In this paper, gold with typical {111} defects (namely Au{111}) and gold with defects enclosed in the (200) plane (Au{200}) were used to determine the effect of the surface energy in the adsorption and electro-oxidation of D-(+)-glucose. To this end, an electrokinetic analysis of surface species was made by means of zeta potential (ζ) measurements and was correlated with an electrochemical study. At low glucose concentration (0.1 mM), the system Au{200} showed a positive and large ζ value of 261.26 mV related to protons from the glucose dehydrogenation. Au{111} presented a negative ζ value of −98.11 mV associated to the glucose chemisorption plus OH− adsorption from the electrolyte. At a higher concentration (>20 mM) both systems exhibited positive ζ values (from 40 to 60 mV) related to the glucose dehydrogenation because of saturation of the electrical double layer by glucose molecules. Through cyclic voltammetry, it was observed that at low glucose concentration (<20 mM), both materials had preference for oxidation of glucose by-products. However, at higher concentrations, Au{111} favors glucono-lactone oxidation (0.4 V vs. NHE); meanwhile Au{200} favors glucose oxidation (−0.43 V vs. NHE). Through the electrokinetic analysis, the behavior of Au{111} can be related to its affinity toward the chemisorption of glucose molecules, and that of Au{200} to weak glucose chemisorption, which allows the desorption of glucose by-products renewing the gold surface for the further oxidation of glucose molecules.

Effect of metal content on the electrocatalytic activity of AuxPdy mixtures and their use in a glucose membraneless microfluidic fuel cell

AuxPdy bimetallic mixtures with different elemental contents were synthesized on glassy carbon electrodes using electrochemical techniques, which are easy, quick, versatile and cheap. Pulse potential and staircase techniques such as cyclic voltammetry (Au60Pd40), square-wave voltammetry (Au50Pd50 and Au35Pd65) and second harmonic AC voltammetry (Au15Pd85) were used to easily change the metal proportion and reduce the Au content in the AuxPdy mixtures. Au60Pd40 exhibited the most negative potential (−0.4 V vs. NHE) towards the glucose electro-oxidation reaction. For this reason, it was used in the anode compartment of a microfluidic fuel cell and compared with single Au and Pd materials by cyclic voltammetry. Au60Pd40 showed a greater negative potential than that of the Au anode; meanwhile, Pd showed no electrocatalytic activity. The lattice parameters were calculated by X-ray diffraction patterns resulting in values of 3.83 and 4.03 A for Au and Pd, respectively, and 3.94 A for Au60Pd40, which provides evidence for the internal structural changes due to the incorporation of Pd to the Au matrix. The maximum power density obtained with a glucose membraneless microfluidic fuel cell (GMMFC) using 10 mM glucose and Au60Pd40 as the anode was 0.28 mW cm−2.

Effect of pH in a Pd-based ethanol membraneless air breathing nanofluidic fuel cell with flow-through electrodes

In this work, a nanofluidic fuel cell (NFC) in which streams flow through electrodes was used to investigate the role of pH in the cell performance using ethanol as fuel and two Pd nanoparticles as electrocatalysts: one commercially available (Pd/C from ETEK) and other synthesized using ionic liquids (Pd/C IL). The cell performances for both electrocatalysts in acid/acid (anodic/cathodic) streams were of 18.05 and 9.55 mW cm-2 for Pd/C ETEK and Pd/C IL. In alkaline/alkaline streams, decrease to 15.94 mW cm-2 for Pd/C ETEK and increase to 15.37 mW cm-2 for Pd/C IL. In alkaline/acidic streams both electrocatalysts showed similar cell voltages (up to 1 V); meanwhile power densities were of 87.6 and 99.4 mW cm-2 for Pd/C ETEK and Pd/C IL. The raise in cell performance can be related to a decrease in activation losses, the combined used of alkaline and acidic streams and these high values compared with flow-over fuel cells can be related to the enhancement of the cathodic mass transport by using three dimensional porous electrodes and two sources of oxygen: from air and from a saturated solution.

Evaluation of alcohol dehydrogenase and aldehyde dehydrogenase enzymes as bi-enzymatic anodes in a membraneless ethanol microfluidic fuel cell

Alcohol dehydrogenase (ADH) and aldehyde dehydrogenase (AldH) enzymes were immobilized by covalent binding and used as the anode in a bi-enzymatic membraneless ethanol hybrid microfluidic fuel cell. The purpose of using both enzymes was to optimize the ethanol electro-oxidation reaction (EOR) by using ADH toward its direct oxidation and AldH for the oxidation of aldehydes as by-products of the EOR. For this reason, three enzymatic bioanode configurations were evaluated according with the location of enzymes: combined, vertical and horizontally separated. In the combined configuration, a current density of 16.3 mA cm-2, a voltage of 1.14 V and a power density of 7.02 mW cm-2 were obtained. When enzymes were separately placed in a horizontal and vertical position the ocp drops to 0.94 V and to 0.68 V, respectively. The current density also falls to values of 13.63 and 5.05 mA cm-2. The decrease of cell performance of bioanodes with separated enzymes compared with the combined bioanode was of 31.7% and 86.87% for the horizontal and the vertical array.