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. 2015 Feb 1;6(2):1570-1577.
doi: 10.1039/c4sc03003f. Epub 2014 Dec 22.

Platinum-decorated Carbon Nanotubes for Hydrogen Oxidation and Proton Reduction in Solid Acid Electrochemical Cells

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Free PMC article

Platinum-decorated Carbon Nanotubes for Hydrogen Oxidation and Proton Reduction in Solid Acid Electrochemical Cells

V Sara Thoi et al. Chem Sci. .
Free PMC article

Abstract

Pt-decorated carbon nanotubes (Pt-CNTs) were used to enhance proton reduction and hydrogen evolution in solid acid electrochemical cells based on the proton-conducting electrolyte CsH2PO4. The carbon nanotubes served as interconnects to the current collector and as a platform for interaction between the Pt and CsH2PO4, ensuring minimal catalyst isolation and a large number density of active sites. Particle size matching was achieved by using electrospray deposition to form sub-micron to nanometric CsH2PO4. A porous composite electrode was fabricated from electrospray deposition of a solution of Pt-CNTs and CsH2PO4. Using AC impedance spectroscopy and cyclic voltammetry, the total electrode overpotential corresponding to proton reduction and hydrogen oxidation of the most active electrodes containing just 0.014 mg cm-1 of Pt was found to be 0.1 V (or 0.05 V per electrode) at a current density of 42 mA cm-2 for a measurement temperature of 240 °C and a hydrogen-steam atmosphere. The zero bias electrode impedance was 1.2 Ω cm2, corresponding to a Pt utilization of 61 S mg-1, a 3-fold improvement over state-of-the-art electrodes with a 50× decrease in Pt loading.

Figures

Scheme 1
Scheme 1. Synthetic approach for Pt-decorated carbon nanotubes (Pt-CNTs).
Fig. 1
Fig. 1. (a) X-ray powder diffraction of 46 wt% Pt-CNTs (red) along with a reference trace (black), and (b) Raman spectra of as-received (red) and 46 wt% Pt-decorated (black) commercial CNTs showing that the carbon nanotube structure is largely unchanged after decoration.
Fig. 2
Fig. 2. High resolution SEM images of (a) as received and (b) 46 wt% Pt-decorated commercial CNTs. Pt nanoparticles with diameters of <17 nm are evident in the latter. Bundling and agglomeration of CNTs is evident in both images.
Fig. 3
Fig. 3. TGA profiles of 30 (black) and 46 wt% (red) Pt-CNTs (measured under flowing air at a heating rate of 2 °C min–1).
Fig. 4
Fig. 4. SEM images of (a) drop-cast 30 wt% Pt-CNTs and (b) electrosprayed 30 wt% Pt-CNTs (inset: image of electrosprayed Pt-CNTs on a carbon paper substrate).
Fig. 5
Fig. 5. SEM images of electrosprayed 46 wt% Pt-CNT–CsH2PO4 composites, showing direct contact between the electrolyte particles with the carbon nanotubes: (a) low and (b) high magnification.
Scheme 2
Scheme 2. Two strategies for electrospray deposition of Pt-CNTs.
Fig. 6
Fig. 6. Symmetric cell impedance measurements of 30 wt% Pt-CNT based electrodes prepared by (a) strategy 1 (see Scheme 2) in which Pt-CNT bearing and CDP bearing solutions were electrosprayed sequentially, and (b) strategy 2 in which a single Pt-CNT and CDP bearing solution was sprayed. In (a), the Pt loading and utilization are 4.2 × 10–3 mg cm–2 Pt and 64 S mg–1, respectively, whereas in (b) they are 5.1 × 10–3 mg cm–2 Pt and 86 S mg–1. Measurements are performed at 240 °C in a dynamic atmosphere of 0.4 atm H2O and balance H2 supplied at a gas velocity of 6 cm min–1 and spectra are taken after 4 hours (insets: equivalent circuit used for fitting).
Fig. 7
Fig. 7. Symmetric cell impedance measurements Pt-CNT based electrodes prepared by strategy 2: (a) Nyquist plot for 46 wt% Pt-CNT–CsH2PO4 composite after 4 hours (inset: equivalent circuit used for fitting) and (b) temporal evolution of the electrode resistance values of 30 wt% (black) and 46 wt% (red) Pt-CNT–CsH2PO4 composites, showing higher stability in the electrolyte with higher Pt loading.
Fig. 8
Fig. 8. IR-corrected polarization curve of a 46 wt% Pt-CNT–CsH2PO4 composite electrode at 1 mV s–1, showing high current density at low overpotentials for both proton reduction and hydrogen oxidation.

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