Hierarchical AgAu alloy nanostructures for highly efficient electrocatalytic ethanol oxidation

doi:10.1016/S1872-2067(21)63895-0

Abstract

Abstract:

The ethanol oxidation reaction is a significant anodic reaction for direct alcohol fuel cells. The most commonly used catalysts for this reaction are Pt-based materials; however, Pt-based electrocatalysts cause carbon monoxide poisoning with intermediates before the complete transformation of alcohol to CO₂. Herein, we present hierarchical AgAu bimetallic nanoarchitectures for ethanol electrooxidation, which were fabricated via a partial galvanic reduction reaction between Ag and HAuCl₄. The ethanol electrooxidation performance of the optimal AgAu nanohybrid was increased to 1834 mA mg^‒1, which is almost 10 times higher than that of the pristine Au catalyst (190 mA mg^‒1) in alkaline solutions. This was achieved by introducing Ag into the Au catalyst and controlling the time of the replacement reaction. The heterostructure also presents a higher current density than that of commercial Pt/C (1574 mA mg^‒1). Density functional theory calculations revealed that the enhanced activity and stability may stem from unavoidable defects on the surface of the integrated AgAu nanoarchitectures. Ethanol oxidation reactions over these defects are more energetically favorable, which facilitates the oxidative removal of carbonaceous poison and boosts the combination with radicals on adjacent Au active sites.

Key words: AgAu nanohybrids, Hierarchical nanostructures, Defected surface, DFT calculation, Ethanol electrooxidation

Caiqin Wang, Danil Bukhvalov, M. Cynthia Goh, Yukou Du, Xiaofei Yang. Hierarchical AgAu alloy nanostructures for highly efficient electrocatalytic ethanol oxidation[J]. Chinese Journal of Catalysis, 2022, 43(3): 851-861.

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URL: https://www.cjcatal.com/EN/10.1016/S1872-2067(21)63895-0

https://www.cjcatal.com/EN/Y2022/V43/I3/851

Figures/Tables 9

Fig. 1. (a) SEM image of bare CF and a magnified SEM image of CF (inset); (b) SEM image of Ag/CF; (c,d) Representative TEM image of Ag nanoparticles; (e) HRTEM image of Ag nanoparticles; (f) EDX results of Ag nanoparticles.

Fig. 2. (a) Schematic representation of the preparation process of AgAu catalysts; SEM images of AgAu-10/CF (b), AgAu-20/CF (c), AgAu-40/CF (d), and AgAu-60/CF (e) at high magnification.

Fig. 3. (a) Cyclic voltammograms of AgAu-40/CF, Au/CF, and Ag/CF in a 1.0 mol/L KOH solution at a scan rate of 50 mV s-1; (b) Cyclic voltammograms of AgAu-10/CF, AgAu-20/CF, AgAu-40/CF, AgAu-60/CF, and Au/CF in a 1.0 mol/L C2H5OH/1.0 mol/L KOH solution at a scan rate of 50 mV s-1; (c) Chronoamperograms (CAs) of AgAu-10/CF, AgAu-20/CF, AgAu-40/CF, AgAu-60/CF, and Au/CF at 0.05 V in a 1.0 mol/L C2H5OH/1.0 mol/L KOH solution; (d) Nyquist plots of ethanol electrooxidation using AgAu-10/CF, AgAu-20/CF, AgAu-40/CF, AgAu-60/CF, and Au/CF electrodes in a 1.0 mol/L C2H5OH/1.0 mol/L KOH solution at an electrode potential of 0.1 V.

Table 1 Summary of the electrochemical properties of electrodes.

Electrode	CVs			CAs
Electrode	E_Onset/V (vs. SCE)	j_f/(mA mg^-1)	j_f/j_b	j_i/(mA mg^-1)
AgAu-10/CF	-0.21	938	2.4	364
AgAu-20/CF	-0.26	1240	2.3	597
AgAu-40/CF	-0.40	1834	3.7	935
AgAu-60/CF	-0.29	512	2.4	214
Au/CF	-0.11	190	6.3	67

Scheme 1. Proposed mechanism of ethanol oxidation using Au-based catalysts.

Fig. 4. Cyclic voltammograms of AgAu-40/CF and JM-Pt/C electrodes in a 1.0 mol/L C2H5OH/1.0 mol/L KOH solution. Scan rate: 50 mV s-1.

Fig. 5. (a) Representative TEM image of AgAu-40 nanoparticles; (b) the magnified TEM image of AgAu-40 nanoparticles; EDX line scan (c) and HRTEM image (d) of an AgAu-40 nanoparticle in the red rectangle in Fig. 5(b); the inset is the diffraction pattern calculated using an FFT of the HRTEM images; (e) EDX of the area inside the red rectangle in Fig. 5(b); (f-i) EDX element mapping of AgAu-40 nanoparticles: TEM image of AgAu-40 nanoparticles (f) and its corresponding STEM image (g); the element distribution of Ag (h) and Au (i).

Fig. 6. (a) XRD patterns of a bare CF substrate, Ag/CF, Au/CF, and AgAu-40 electrodes; XPS spectra of the Au 4f region (b) and Ag 3d region (c) in AgAu-40. The spectra were obtained via calibration based on the C 1s peak at 284.5 eV.

Fig. 7. Optimized atomic structure of perfect (a) and defected (b) AuxAgy slab with a gold cover and a quasi-random distribution of gold and silver atoms for AuxAgy with a molar ratio of 1:1 at first (CH3CH2OH + OH- → CH3CH2O-* + H2O, * representing substrate), followed by (CH3CH2O-* + OH- → CH3CHO-* + H2O) the most energetically favorable steps of the EOR. The free energies of those two EOR steps for various AuxAgy ratios in a perfect substrate (c) and a surface with defects (d).

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