Functional nano-metallic electrodes for applications in electrocatalysis and flexible electronics
thesisposted on 27.02.2017 by Tang, Yue
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The thesis focuses on low-cost bottom-up approach to design nanostructured electrode materials for novel applications in electrocatalysis and stretchable electronics. In particular, the work presented in this thesis were about (1) the fabrication of functional nanostructured electrodes with synthetic meta-atoms from the “artificial periodic tables” (Figure 1.3) using cost-effective “bottom-up” approach; (2) the size- and shape-dependent electrocatalytic properties of the as-fabricated electrodes; and (3) the applications of the as-fabricated electrodes in lightweight and stretchable electronics. The key novel features are summarized as follows: A robust chemical-tethering approach was developed to immobilize AuNPs onto transparent indium tin oxide (ITO) glass electrode surface, and their size- and shape-dependent electrocatalysis towards methanol oxidation reaction (MOR) and oxygen reduction reaction (ORR) was systematically investigated. In particular, mono-dispersed 20 nm nanospheres (AuNS20s), 45 nm nanospheres (AuNS45s) and 20 × 20 × 63 nm3 nanorods (AuNRs) were synthesized and chemically tethered to ITO surface forming submonolayers without any aggregation. These nanoparticle-modified ITO electrodes exhibited strong electrocatalytic activities towards MOR and ORR but their mass activities were highly dependent on particle sizes and shapes. For particle with similar shapes, size determined the mass activities: smaller particle size led to greater catalytic current density per unit mass due its greater surface-to-volume ratio (AuNS20s > AuNS45s). For particles with comparable sizes, shapes or crystalline structures governed selectivity of electrocatalytic reactions: AuNS45 exhibited a higher mass current density in MOR than that for AuNRs due to its dominant (111) facets exposed; whereas AuNRs exhibited a higher mass current density in ORR due to its dominant (100) facets exposed. A low-cost and simple yet efficient drop-casting method was developed to fabricate lightweight and mechanically flexible electrocatalytic electrodes from AuNRs and AuNS45s. Low cost, flexible, and highly porous tissue paper sheets were chosen as the substrates; AuNPs were packed closely on the surface, therefore the electrodes were highly conductive without any requirement of conductive substrates. Moreover, owing to the hierarchical porous structure of paper fibers and the high volume-to-surface ratio of AuNPs, the modified electrode exhibited extremely large electrochemical active surface area (EASA). In comparison with conventional gold disk electrodes, the electrocatalytic activities of AuNP electrodes towards ORR and MOR were greatly improved. Furthermore, AuNP electrode are highly stable; AuNPs kept their well-defined shapes after 1000 cycles scanning in 0.1 M PBS (pH=7.2) or 500 cycles scanning in 0.1 M KOH with 3 M methanol. In addition, the universality of this approach was proved by extending the substrate to commercial available sponges. The ultra-lightweight copper aerogel monoliths were synthesized from copper nanowires (CuNWs) for the first time through an environmentally friendly freeze-drying approach. Their mechanical and electrical properties were finely tunable simply by varying the densities of the aerogels. However, the poor mechanical strength of the aerogels limited their applications in flexible electronics. A trace amount of additive – polyvinyl alcohol (PVA) was found to substantially improve the mechanical strength of CuNWs aerogel monoliths while maintaining high conductivity (~ 0.83 S•cm-1) and ultra-low density (~10 mg•cm-3). The CuNW-PVA composite aerogels exhibited high durability at cyclic loads, enabling their applications as the elastic piezo-resistivity switch. More importantly, the CuNW-PVA aerogels could be embedded into PDMS matrix, leading to electrically conductive rubber ambers without the need of pre-wiring. The rubber ambers could be further manufactured into various one-dimensional (1D), 2D and 3D objects that were all conductive after shaping. We believe that our fabrication strategy represents a new low-cost route to manufacture under mild conditions flexible conductors with versatile shapes for versatile applications in flexible electronics.