Siew Hwa Chan

Poly(vinyl alcohol) Nanocomposites Filled with Poly(vinyl alcohol)-Grafted Graphene Oxide

We present a novel approach to the fabrication of advanced polymeric nanocomposites from poly(vinyl alcohol) (PVA) by incorporation of PVA-grafted graphene oxide. In this work, we have synthesized PVA-grafted graphene oxide (PVA-g-GO) for the strong interfacial adhesion of graphene oxide (GO) to the PVA matrix. It was found that the mechanical properties of PVA were greatly improved by incorporating PVA-g-GO. For example, the tensile strength and Youngs modulus of the PVA nanocomposite films containing 1 wt % net GO in the PVA-g-GO significantly increased by 88 and 150%, respectively, as compared to unfilled PVA. The elongation at break was also increased by 22%, whereas the GO/PVA nanocomposite containing 1 wt % pristine GO was decreased by 15%. Therefore, the presence of the PVA-g-GO in the PVA matrix could make the PVA not only stronger but also tougher. The strong interfacial adhesion between PVA-g-GO and the PVA matrix was attributed to the good compatibility between PVA-g-GO and the matrix PVA as well as the hydrogen-bonding between them.

Micromachined polymer electrolyte membrane and direct methanol fuel cells—a review

This review reports recent progress of the development of micromachined membrane-based fuel cells. The review first discusses the scaling law applied to this type of fuel cell. Impacts of miniaturization on the performance of membrane-based fuel cells are highlighted. This review includes only the two most common micro fuel cell types: proton exchange membrane micro fuel cells (PEMµFC) and direct methanol micro fuel cells (DMµFC). Furthermore, we only consider fuel cells with the active area of a single cell less than 1 square inch. Since the working principles of these fuel cell types are well known, the review only focuses on the choice of material and the design consideration of the components in the miniature fuel cell. Next, we compare and discuss the performance of different micro fuel cells published recently in the literature. Finally, this review gives an outlook on possible future development of micro fuel cell research.

Polarization effects in electrolyte/electrode-supported solid oxide fuel cells

The effects of activation, ohmic and concentration polarization on the overall polarization in solid oxide fuel cells are presented. A complete analysis was conducted based on thermodynamic principles for the calculation of cell voltage. Treating the fuel cell as a control volume, the irreversibility term in a steady flow thermodynamic system was related to the overall polarization. The entropy production was calculated and related to the lost work of the fuel cell, while the heat loss from the cell was determined from the entropy balance. To generalize the cell voltage–current density expression, the Butler–Volmer model was used in the calculation of activation polarization and both ordinary and Knudsen diffusions were considered in the calculation of concentration polarization. The overall cell resistance was deduced from the generalized cell voltage–current density expression. The concentration resistance at the anode can be minimized by humidifying the hydrogen with an appropriate amount of water, depending on the thickness of the anode used. Comparison of polarization effects on the cell performance between the electrolyte-supported and anode-supported cells showed that the latter would give a better cell performance.

A membraneless hydrogen peroxide fuel cell using Prussian Blue as cathode material

This communication describes the exploitation of Prussian Blue, ferric ferrocyanide (Fe4III[FeII(CN)6]3), for the cathode side in a single-chamber membraneless fuel cell running on hydrogen peroxide (H2O2) as both fuel and oxidant. An open-circuit voltage (OCV) of 0.6 V has been obtained, which could be the highest OCV with H2O2 ever reported. The maximum power density was 1.55 mW cm−2 which showed a stable long-term operation in acidic media.

Reliability and accuracy of measured overpotential in a three-electrode fuel cell system

Numerical simulation was conducted to study the potential and current density distributions at the active electrode surface of a solid oxide fuel cell. The effects of electrode deviation, electrolyte thickness and electrode polarization resistance on the measurement error were investigated. For a coaxial anode/electrolyte/cathode system where the radius of the anode is greater than that of cathode, the cathode overpotential is overestimated while the anode overpotential is underestimated. Although the current interruption method or impedance spectroscopy can be employed to compensate/correct the error for a symmetric electrode configuration, it is not useful when dealing with the asymmetric electrode system. For the purpose of characterizing the respective overpotentials in a fuel cell, the cell configuration has to be carefully designed to minimize the measurement error, in particular the selection of the electrolyte thickness, which may cause significant error. For the anode-support single fuel cell, it is difficult to distinguish the polarization between the anode and cathode with reference to a reference electrode. However, numerical results can offer an approximate idea about the source/cause of the measurement error and provide design criteria for the fuel cell to improve the reliability and accuracy of the measurement technique.

A laser-micromachined polymeric membraneless fuel cell

This paper presents a laser-micromachined polymeric membraneless fuel cell. The membraneless fuel cell, constructed with three polymethyl methacrylate (PMMA) layers, takes advantage of two laminar flows in a single micro channel to keep the fuel and oxidant streams separated yet in diffusional contact. Laser micromachining was employed to make the flow channel and electrode substrate based on PMMA. The anode and cathode electrodes were fabricated by wet-spraying catalyst inks onto the gold-coated PMMA substrate. The packed fuel cell has been electrochemically characterized by an electrochemical analyser. The membraneless fuel cell works stably with Reynolds numbers ranging from 7.65 to 30.6. At room temperature, the laminar-flow-based micro membraneless fuel cell can reach a maximum power density of 0.58 mW cm−2 with 0.5 M HCOOH in 0.1 M H2SO4 solution as fuel and O2 saturated 0.1 M H2SO4 solution as oxidant. When 0.01 M H2O2 in 0.1 M H2SO4 solution is used as oxidant, a maximum power density of 1.98 mW cm−2 is obtained. The paper reports for the first time the use of hydrogen peroxide in sulfuric acid as the oxidant. The new oxidant composition allows a simple recycling process and better fuel utilization.

An air-breathing microfluidic formic acid fuel cell with a porous planar anode: experimental and numerical investigations

This paper reports the fabrication, characterization and numerical simulation of an air-breathing membraneless laminar flow-based fuel cell with carbon-fiber-based paper as an anode. The fuel cell uses 1 M formic acid as the fuel. Parameters from experimental results were used to establish a three-dimensional numerical model with COMSOL Multiphysics. The simulation predicts the mass transport and electrochemical reactions of the tested fuel cell using the same geometry and operating conditions. Simulation results predict that the oxygen concentration over an air-breathing cathode is almost constant for different flow rates of the fuel and electrolyte. In contrast, the growth of a depletion boundary layer of the fuel over the anode can be the major reason for low current density and low fuel utilization. At a low flow rate of 10 µl min−1, simulation results show a severe fuel diffusion to the cathode side, which is the main reason for the degradation of the open-circuit potential from 0.78 V at 500 µl min−1 to 0.58 V at 10 µl min−1 as observed in experiments. Decreasing the total flow rate 50 times from 500 µl min−1 to 10 µl min−1 only reduces the maximum power density approximately two times from 7.9 to 3.9 mW cm−2, while fuel utilization increases from 1.03% to 38.9% indicating a higher fuel utilization at low flow rates. Numerical simulation can be used for further optimization, to find a compromise between power density and fuel utilization.

Amino acid modified copper electrodes for the enhanced selective electroreduction of carbon dioxide towards hydrocarbons

Electroreduction of carbon dioxide to hydrocarbons has been proposed as a promising way to utilize CO2 and maintain carbon balance in the environment. Copper (Cu) is an effective electrocatalyst for such a purpose. However, the overall selectivity towards hydrocarbons on Cu-based electrodes is still very limited. In this work, we develop a general amino acid modification approach on Cu electrodes for the selective electroreduction of CO2 towards hydrocarbons. A remarkable enhancement in hydrocarbon generation is achieved on these modified copper electrodes, regardless of the morphology of the Cu electrodes. A density functional theory calculation reveals that the key intermediate CHO* is stabilized by interacting with –NH3+ of the adsorbed zwitterionic glycine. Our results suggest that amino acids and their derivatives are promising modifiers in improving the selectivity of hydrocarbons in CO2 electroreduction.

Catalysis mechanisms of CO2 and CO methanation

Understanding the reaction mechanisms of CO2 and CO methanation processes is critical towards the successful development of heterogeneous catalysts with better activity, selectivity, and stability. This review provides detailed mechanisms of methanation processes and undesired catalyst deactivation. We characterize the methanation processes into two categories: (1) associative scheme, in which hydrogen atoms are involved in the C–O bond breaking process, and (2) dissociative scheme, where C–O bond breaking takes place before hydrogenation. For the catalyst deactivation mechanisms, we highlight three important factors, i.e. sulfur poisoning, carbon deposition and metal sintering.

Modelling of engine in-cylinder thermodynamics under high values of ignition retard

Abstract This paper presents the work on a carburetted gasoline engine, in particular the complete modelling of an engine in-cylinder thermodynamics under high values of ignition retard (HVIR). The “combustion” is a two-zone burnt/unburned model with the fuel burning rate described by a Wiebe function. Under extreme spark timing retard conditions, the Wiebe function describing the heat release of the fuel–air reactions was modified to account for the critical change in pressure distribution in the cylinder due to the abnormal spark retard. An empirical correlation for cylinder pressure variation during the mass blowdown process, which occurs between the exhaust valve opened and bottom-dead-centre, was included in the simulation to enhance the predictive capability of the engine model. The complicated mass blowdown process across the exhaust valves was simplified by two processes: (i) isentropic expansion from the cylinder pressure to the constant exhaust manifold pressure, and (ii) constant pressure throttling which gives rise to increased exhaust gas temperature due to the recovery of kinetic energy.