Graphene supported electrocatalysts for alkaline anion-exchange membrane-based direct methanol fuel cell (AAEM-DMFC) applications / Ng Jen Chao

Ng , Jen Chao (2019) Graphene supported electrocatalysts for alkaline anion-exchange membrane-based direct methanol fuel cell (AAEM-DMFC) applications / Ng Jen Chao. PhD thesis, Universiti Malaya.

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Abstract

Fuel cells are potential alternative green energy sources. Alkaline anion-exchange membrane-based direct methanol fuel cell (AAEM-DMFC) emerges as one of the best candidates because of its cheap liquid fuel, low operating temperature, and high efficiency. Nonetheless, AAEM-DMFC has been limited by poor anodic methanol oxidation reaction (MOR). In this regard, catalyst plays an important role by lowering the activation energy of MOR. Palladium (Pd) has recently emerged as potential catalysts owing to its lower cost and abundancy. Pd exhibits superior catalytic activity and stability for the alcohol oxidation in alkaline medium because of its higher tolerance to CO-poisoning. Nevertheless, catalyst nanoparticles (NPs) tend to irreversibly agglomerate, reducing the surface area for the reaction, and thus poor electrochemical performance. Loading catalysts on catalyst supports, especially graphene, is an effective solution to prevent agglomeration. In this study, graphene was produced by reduction of graphene oxide (GO) by microwave reduction method. Our group successfully loaded Pd NPs on the surface of reduced graphene oxide (rGO). This paper started with preliminary studies to optimize the rGO supported Pd (Pd/rGO) from the aspect of: (i) optimum ratio of palladium to graphene and (ii) reduction duration. Results proved that 1:1 and 700 s are the optimum ratio and reduction duration, respectively. On the basis of the preliminary works, the electrocatalytic activity of Pd/rGO was further improved in two main directions: (i) catalyst and (ii) catalyst support (graphene). In the context of catalyst, particle size and distribution of catalyst NPs can influence the catalytic performance. Despite the remarkable performance via the introduction of various reagents, such as surfactants, these reagents tend to cover the catalytic sites and are difficult to be completely removed at the end of the synthesis. This work manipulated the Pd particle size and dispersion by (i) adjusting the concentration of precursor mixture solution (palladium chloride + GO) and (ii) modifying the conventional synthesis method. The concentration of the metal precursor mixture has significant effect to the rate of nucleation and growth. Results discovered performance difference as high as 195 % by simply varying the mixture concentration. The second approach was stemmed from the observation on the colour changes of the mixture solution during the conventional synthesis process. The colour changes reflected multi-stages reduction of the nanocomposite which is unfavourable for the synthesis of small size nanoparticles with uniform distribution. A modified method was proposed to discard the synthesis steps that induce multi-stages mild reduction, and induce rapid nucleation and supersaturation that facilitate small size nanoparticles. As to the catalyst support, rGO, its weak interaction with Pd NPs leads to agglomeration. Guanine, a nucleobases found in deoxyribonucleic acid and ribonucleic acid, was introduced to Pd/rGO to overcome the limitation. The guanine adsorbed to rGO via π-π interaction, whereas its amino, imino, and amide groups serve as anchoring sites for Pd ions and nanoparticles and control their synthesis. The guanine was revealed to exhibit catalytic capability, significantly enhancing the MOR of Pd/rGO compared to its counterpart at the absence of guanine.

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