Abstract
In the present study, highly homogeneous platinum nanocatalysts with enhanced electrocatalytic activity were uniformly deposited on carbon nanotubes directly grown on a silicon plate (Pt/CNTs/Si) as the electrodes for direct methanol fuel cells (DMFCs) by a novel homemade open-loop reduction system (OLRS). Compared with a traditional reflux system that maintains the ratio of water to ethylene glycol (EG) at ~160 °C for ~4 h, the gradual concentration increase of EG in the precursor solution can be accomplished by distilling off water in the OLRS while increasing the temperature to 130 °C. This process with simultaneous increases in precursor concentration and in reaction temperature rendered high-quality Pt nanoparticles to precipitate with high-density dispersion on the pretreated CNTs. The OLRS is not only able to shorten the reduction time (<1.5 h) but also able to enhance the electrocatalytic activity of the electrodes by creating a preferential orientation of Pt (111) facets for the methanol oxidation reaction (MOR). Cyclic voltammetry and electrochemical impedance spectroscopy were conducted to evaluate the mass activity (MA) and charge transfer resistance (Rct) of the fabricated electrodes for the MOR. Compared with the electrodes prepared by traditional Pt reductions (MA: 100-360 A gPt-1 and Rct: 40-80 Ω-cm2), the Pt/CNTs/Si-based electrodes prepared at 130 °C in the OLRS exhibited superior electrocatalytic properties, including an MA of 435 A gPt-1 and an Rct of ~30 Ω-cm2. By the potential cycling technique of startup and shutdown cycles under strongly oxidizing conditions, the electrochemical stability of the above-prepared CNTs and Pt/CNTs electrodes was evaluated to mimic the real electrode operating environment in DMFCs. The cyclic voltammetry (CV) curves for MOR revealed that the performance degradation of the electrochemically-treated electrodes at 60 °C was 1.7 times higher than those at 25 °C after the electrochemical oxidation tests for 5 h, resulting from the loss of electrochemical surface area (ESA) of Pt catalysts during the electrode operation. This is mainly due to carbon corrosion or rearrangement of Pt catalysts on CNTs, resulting in the Pt agglomeration (or growth of Pt particles) on and Pt detachment from the surface of the CNTs during the electrochemical oxidation process. With regard to the membrane electrode assembly (MEA), a Pt/CNTs electrode with a thin and uniform ionomer layer as the proton-conducting electrolyte on a nano patterned three-phase zone (TPZ) was fabricated in this study, aiming at high electrocatalytic activity and high charge transfer rate for MOR. Unlike conventional paste or spray methods that produced thick and non-uniform ionomer layers to form TPZs within the catalyst layers (50-100 um) of electrodes, thin and uniform ionomer layers (5-10 nm) on the hydrophilic-treated Pt/CNTs electrodes were harvested by spin-coating. The thickness of the ionomer layer was controlled by altering the spin-coating speed, and the effect of the ionomer thickness on the surface of the catalyst layer and on the electrochemical properties of the electrodes for MOR was studied. Compared to the electrode fabricated by spraying (MA: 355 A gPt-1, Rct: 48 Ω-cm2), the ionomer-coated electrode spin-coated at 4000 rpm exhibited superior properties for MOR (MA: 381 A gPt-1, Rct: 15 Ω-cm2). The outcome renders this new electrode to embrace potential applications in micro DMFCs with the design of a thin and uniform TPZ.