Development of Porous Pt-based Electrochemical Electrodes and Their Application to Amperometric Hydrogen Sensors

Abstract

Counter and reference electrodes (RE) for solid electrochemical sensors were prepared on a Nafion electrolyte membrane through the platinum nanoparticles (Pt NPs) deposition by the impregnation-reduction (I-R). Various material analysis methods and cyclic voltammogram (CV) measurements were employed in order to characterize microstructures and electrochemical properties of the Pt layer. At the optimized conditions, the porous Pt thin-film, consisting of sphere-like particles formed by the agglomeration of primary polycrystalline Pt NPs with an average crystal size of 13  18 nm, was obtained and confirmed to have a large surface area and strong adhesion due to the formation of interfacial Pt-Nafion composites. Furthermore, this electrode exhibited well-resolved CV peaks for the hydrogen redox reactions in an acid solution, suggesting the existence of different adsorption sites and good electrochemical behaviors. Three different types of Pt-based materials were developed as a sensing electrode (SE) in the electrochemical hydrogen sensors. First, Pt NP-decorated multiwall carbon nanotube (Pt-MWNT) electrode was prepared on Nafion by a hot-pressing at relatively low temperature. This electrode exhibited an intricate entangled, nanoporous structure as a result of gathering highly anisotropic Pt-MWNTs. Individual Pt NPs were confirmed to have a polycrystalline face-centered cubic (FCC) structure with an average crystal size of around 3.5 nm. From the CV for hydrogen redox reactions, the Pt-MWNT electrode was found to have a similar electrochemical behavior to polycrystalline Pt, and a specific electrochemical surface area (ESA) of 2170 cm2 mg−1. Upon exposure to hydrogen analyte, the Pt-MWNT/Nafion electrode demonstrated a very high sensitivity of 3.60 μA ppm−1 and an excellent linear response over the concentration range of 100  1000 ppm. Next, bimetallic core-shell Ag-Pt NPs attached on MWNT (Ag@Pt-MWNT) were synthesized via the formation of Ag NPs on a MWNT surface through chemical reduction and subsequent galvanic replacement reaction (GRR) of Ag with PtCl62−. The bimetallic Ag@Pt particles were confirmed to have a core-shell structure in the TEM image: an Ag core with an average size of approximately 7 nm was enclosed by a shell composed of small Pt NPs. The ESA of the Ag@Pt-MWNT-modified glassy carbon electrode was 896 cm2 mg−1, which was 1.5 times higher than that of the commercial 20 wt% Pt-C. The fabricated electrode displayed a high sensitivity of 1.1 µA ppm−1 in H2 detection and an excellent linear response over the wide concentration range of 5  1000 ppm. Finally, hollow Pt nanostructure-decorated MWNT (h-Pt/MWNTs) were synthesized by two step processes: the GRR between Ag NPs and PtCl62- salts, and the subsequent chemical Ag removal from the resultant core-shell Ag@Pt NPs. The hollow nanoporous agglomerations consisting of very small polycrystalline Pt NPs confirmed. The ESA of h-Pt/MWNTs was determined to be 1180 cm2 mg−1. Furthermore, the amperometric gas-sensing electrode based on the h-Pt/MWNTs exhibited a high sensitivity of 1.48 µA ppm−1 and excellent response linearity (R = 0.9996) in a concentration range of 5  50 ppm. Moreover, these electrodes were also evaluated in terms of response and recovery times, reproducibility, and long-term stability. On the whole, the obtained results revealed excellent sensing performance in hydrogen detection.