Synthesis and Characterization of PdCoNi Nanocomposites Supported on Graphene as Anodic Electrocatalysts for Methanol Oxidation in Direct Methanol Fuel Cell

Abstract

PdCoNi nanocomposites supported on graphene (PdCoNi/G) have been obtained from chemical reduction of metal catalysts and graphite oxide (GO) with a strong reducing agent, followed by calcination at high temperature under N2 condition, and used for electrooxidation of methanol in direct methanol fuel cell. The morphologies and structural properties of electrocatalysts were examined by scanning electron microscopy (SEM) and X-ray diffraction (XRD). X-ray spectroscopy techniques (X-ray photoelectron spectroscopy XPS) was used to investigate the chemical state of the synthesized catalysts. The results of Pd XPS spectra showed the metallic Pd and PdO phases for precalcined and calcined PdCoNi/G nanocomposite, respectively. The X-ray measurement of Co and Ni displayed the various metallic oxides in synthesized electrocatalysts. For electrochemical analysis, cyclic voltammetry (CV) and chronoamperometry (CA) indicated that the PdCoNi/G nanocomposites enhanced the methanol oxidation, compared to the lower activity in the calcined electrocatalysts.

Pro‐catalytic Role of Transition Metal/Metal Oxide Blend Low Pd Catalysts for Ethanol Oxidation: Diminution of Activation Energy and Promoting Electrode Kinetics at Low Temperature

“…At elevated temperatures, the dissociative adsorption of ethanol become increasingly favored on the multi-metallic matrix which offers abundant electrochemically active reaction centers facilitated by the (OH) ads coverage, oxide and per-oxo linkage formation as evident from the existence of Co 3 O 4 and Ni(OH) 2 /NiOOH species. [20,[31][32][33] The concurrent promotion of oxidation kinetics on Pd 35 Co 40 Ni 25 /C leads to accumulation of surface intermediates which are however removed by the assistance of transition metal/metal oxides as revealed from the TMOR and AOR features of the surface sites discussed earlier (Figure 3a) and manifested by the sharp and intense renewed oxidation peaks in the cathodic sweep (inset of Figure 3 d). At 80 °C the onset for ethanol oxidation (taken at 2 mA cm À 2 ) was driven to a lower potential for the single and ternary catalysts as demonstrated in Figure 3c 3d), even though the loading of the precious Pd metal was reduced to 35 % and the major share of co-metals is supplemented by cheap transition metals Co and Ni.…”

“…The voltammetric features also reveal much higher peak current density for the ternary catalyst compared to the single Pd catalyst and at the same time generate ~ three times greater current output at 80 °C to that obtained at 20 °C. At elevated temperatures, the dissociative adsorption of ethanol become increasingly favored on the multi‐metallic matrix which offers abundant electrochemically active reaction centers facilitated by the (OH) ads coverage, oxide and per‐oxo linkage formation as evident from the existence of Co 3 O 4 and Ni(OH) 2 /NiOOH species [20,31–33] . The concurrent promotion of oxidation kinetics on Pd 35 Co 40 Ni 25 /C leads to accumulation of surface intermediates which are however removed by the assistance of transition metal/metal oxides as revealed from the TMOR and AOR features of the surface sites discussed earlier (Figure 3a) and manifested by the sharp and intense renewed oxidation peaks in the cathodic sweep (inset of Figure 3 d).…”

“…A rapid equilibrium is attained between the Ni(OH) 2 /NiOOH species in alkaline medium, through the spill over of hydrogen [ Ni(OH) 2 +OH − ↔ NiOOH+H 2 O+e − ] which ultimately gears up the oxidation of ethanol on the multi‐metallic matrix without any bias temperature and potential, while improving the proton abstraction and electron transport in the oxidation sequences. On the other hand, [−O−Co−O−Co−O−] type of network formed by the Co‐oxides present in the matrix, effectively take part in the EOR sequences and drive the reaction toward final release of CO 2 , even at lower potentials [29,31] . Extensive coverage (65 %) of Co, Ni and their oxides with respect to the entire matrix creates a synergy in terms of the co metal partnership of Co, Ni and the base Pd, rendering faster electrode kinetics with low activation energy throughout the temperature range.…”

“…On the other hand, [À OÀ CoÀ OÀ CoÀ OÀ ] type of network formed by the Co-oxides present in the matrix, effectively take part in the EOR sequences and drive the reaction toward final release of CO 2 , even at lower potentials. [29,31] Extensive coverage (65 %) of Co, Ni and their oxides with respect to the entire matrix creates a synergy in terms of the co metal partnership of Co, Ni and the base Pd, rendering faster electrode kinetics with low activation energy throughout the temperature range. Further considering the d band characteristics of the constituent metal NPs, it may be understood that Ni and Co incorporation into Pd matrix presumably downshift the Pd energy centre so that the anti-bonding molecular orbital gets filled up.…”