One-Step Synthesis of a Binder-Free, Stable, and High-Performance Electrode; Cu-O|Cu3P Heterostructure for the Electrocatalytic Methanol Oxidation Reaction (MOR)

Abstract

Although direct methanol fuel cells (DMFCs) have been spotlighted in the past decade, their commercialization has been hampered by the poor efficiency of the methanol oxidation reaction (MOR) due to the unsatisfactory performance of currently available electrocatalysts. Herein, we developed a binder-free, copper-based, self-supported electrode consisting of a heterostructure of Cu₃P and mixed copper oxides, i.e., cuprous-cupric oxide (Cu-O), as a high-performance catalyst for the electro-oxidation of methanol. We synthesized a self-supported electrode composed of Cu-O|Cu₃P using a two-furnace atmospheric pressure-chemical vapor deposition (AP-CVD) process. High-resolution transmission electron microscopy analysis revealed the formation of 3D nanocrystals with defects and pores. Cu-O|Cu₃P outperformed the MOR activity of individual Cu₃P and Cu-O owing to the synergistic interaction between them. Cu₃P|Cu-O exhibited a highest anodic current density of 232.5 mAcm^-2 at the low potential of 0.65 V vs. Hg/HgO, which is impressive and superior to the electrocatalytic activity of its individual counterparts. The formation of defects, 3D morphology, and the synergistic effect between Cu₃P and Cu-O play a crucial role in facilitating the electron transport between electrode and electrolyte to obtain the optimal MOR activity. Cu-O|Cu₃P shows outstanding MOR stability for about 3600 s with 100% retention of the current density, which proves its robustness alongside CO intermediate.

Keywords: chemical vapor deposition; heterostructure; methanol oxidation reaction; self-standing electrode; synergistic effect.

Scheme 1

The drawing demonstrates the synthesis method of Cu-O|Cu₃P.

Figure 1

The powder XRD of Cu-O, Cu₃P, and Cu₃P|Cu-O.

Figure 2

Core-level high-resolution XPS spectra of (a) Cu 2p; (b) O 1s due to the existence of CuO; and (c) P 2p due to the presence of Cu₃P.

Figure 3

HR-SEM of the formed heterostructure: (a) low-resolution image, (b) high-resolution image; (c) HR-TEM image of Cu-O|Cu₃P (inset; SAED pattern); and (d) lattice distance measurement from high-resolution image.

Figure 4

STEM color mapping image of the heterostructure to differentiate the elements (a) copper, (b) oxygen, and (c) phosphorous.

Figure 5

(a) Cyclic voltammetry showing MOR of three catalysts carried out in 1 M methanol and 1 M KOH; (b) comparative OER and MOR results of Cu-O|Cu₃P (the inset presents OER result); (c) comparison of CA results carried out at fixed faradic potentials; and (d) Nyquist impedance plots of Cu-O, Cu₃P, and Cu-O|Cu₃P.