Making Copper and Alloyed Single Crystal Beads By Oxygen-Hydrogen Flame in Air and Boosting the Electrocatalytic Activity Toward Formaldehyde Oxidation By the Factor of 12 with 1.5 % of Ni

Abstract

This work shows that it is feasible to prepare chemically pure or bimetallic, non-precious metal (Cu, Ni, Co) single crystal electrodes by melting the end of a polycrystalline metal wire in the air by a tactfully designed oxygen-hydrogen torch. The structures and chemical compositions of the as-prepared Cu and CuNi beads were characterized by X-ray photoelectron spectroscopy (XPS). Obtained results show that Cu and the CuNi single crystal beads are free of oxide in the bulk and Cu to Ni molar ratio is 98.5:1.5 in the CuNi bead. After flame annealing, the thickness of cuprous oxide on the (111) surface is less than 10 Å. The (111) planes of Cu and Cu/Ni electrodes are exposed to study their electrocatalysis toward the oxidation of formaldehyde (HCHO) in 0.1 M KOH, which is relevant to the development of fuel cells, hydrogen carrier and the removal of HCHO in waste water. The HCHO fuel cell employs Cu/Ni anode can have multiple advantages. First, the cell potential for HCHO/CuNi anode is ~ 0.5V more greater than a typical CH3OH/Pt anode if the same cathode is used. Second, Cu and Ni metals are the inexpensive and abundant. Third, the aqueous HCHO has a relatively lower vapor pressure than methanol. Fourth, paraformaldehyde, the source of HCHO, is a solid at room temperature, which makes it easy to be transported and stored. Although bulk Ni is inactive toward HCHO oxidation, a tiny Ni in the Cu electrode increases the exchange current density, due to HCHO oxidation, by 12 times. The durability of Cu(111) and Cu98.5Ni1.5(111) electrode in catalyzing HCHO oxidation is researched by stepping the potential from their OCP (-0.9 and -1V vs. Ag/AgCl) to -0.4 and -0.54V. For Cu(111), the current surges after the switch of potential, followed by an exponential drop from 0.9 to 0.06 mA/cm2 within 100 s. For Cu98.5Ni1.5, the current decrease to 1/4 (1mA/cm2) of its original value in 300 sec and then fluctuates between 0.8 and 0.7 mA/cm2. The activity is restored after the interface is refreshed by removing and re-immersing the electrode in the electrolyte. The kinetics of HCHO oxidation at Cu98.5Ni1.5 and Cu(111) electrodes are scrutinized by conducting voltammetry in 0.1 M KOD/KOH + 0.1 M HCHO. The current observed at Cu98.5Ni1.5 in KOD is markedly lower than that recorded in KOH; whereas no effect is seen with Cu(111). This contrast implies that a cleavage of a O-D bond is involved in the rate-determining step of HCHO oxidation at Cu98.5Ni1.5, but not at the Cu(111) electrode. Figure 1