Shen’s group [6, 22] investigated the ethanol oxidation activity of Pd-based catalysts and observed that Pt had less catalytic activity than Pd in alkaline media. They modified the catalysts with several oxides, i.e., CeO2, NiO, Co3O4 and Mn3O4 and the oxide-promoted catalysts displayed better activities than a commercial E-TEK PtRu/C catalyst. Among them, Pd-NiO (6:1 by weight)/C showed the highest performance. The onset potential for the ethanol oxidation reaction (EOR) on Pd–NiO/C is shifted negatively by 300 mV compared with that of Pt/C. The authors proposed that OHad species could easier form on the surface of oxide at lower potential and helped to transform CO-like poisoning species on Pd to CO2 or other products, releasing the active sites on Pd for further electrochemical reaction [21-22, 39]. Recently, Chu et al. [40] have synthesized carbon nanotube – supported Pd-In2O3 and tested its activity for EOR. It was shown that the catalytic activity of Pd for EOR was promoted by the addition of In2O3 nanoparticles into the catalyst. The catalyst with a mass ratio of Pd to In2O3 of 10:3 showed the highest activity.
The combination of Pd with other metals has been used to create new catalysts for ethanol oxidation. Carbon-supported Pd4Au- and Pd2.5Sn-alloyed nanoparticles were prepared and used for EOR in high pH media [41]. It was found that the Pd alloy nanocatalysts showed higher current density and long term stability than commercial Pt/C.
The alloy catalysts also showed significantly higher current densities in comparison to Pd/C. The Pd-based alloy catalysts showed a higher tolerance to surface poisoning compared with Pt/C. Pd4Au/C displayed the best catalytic activity among the catalysts.
Bagchi’s group [42-43] investigated the electroactivity of binary PdRu catalyst on Ni support for EOR in ADEFCs. The catalyst was seen to improve the current density and reduce the anodic overvoltage compared to pure Pt, Pd and Ni. Its electrocatalytic capability is similar to that of Pt-Ru supported on Ni.
Fundamentals for designing of DEFC electrocatalysts:
Bifunctional mechanism (promoted mechanism) was proposed to explain the promotion effect of Ru or Sn for Pt/C towards the ethanol oxidation in acidic media [6, 35, 44-48]. According to the promoted mechanism, Pt active sites are the place where the adsorption and decomposition of ethanol and its intermediate reaction products happen, while the dissociative adsorption of water to form OH- species takes place on Ru (Sn) sites. These species will help to remove CO-like intermediates out of the catalyst surface. Xu et al. [22]
observed that the addition of oxide (CeO2, NiO) to Pt/C and Pd/C could significantly promote catalytic activity for EOR in alkaline media. It was suggested that the promotion is derived from the easier formation of OHad
species on the surface of oxide. The OHad species formed at lower potential can convert CO-like poisoning species on Pt and Pd into CO2 or other products,
According to d-band center theory proposed by Nứrskov and co-workers [49- 58], the trend of reactivity will follow the trend in d-band center values. When metals with small lattice constants are overlayed or alloyed on metals with larger lattice constants, the d-band center shifts up and vice versa, which subsequently affects the reaction rate. If the d-band center is shifted up, the adsorption ability of the adsorbate onto the metals will be stronger and this may help to improve the electro-oxidation of ethanol on the surface of the metals.
An application of d-band center theory was shown in Lima et al.’s work [56]. Pt monolayers were deposited on different metal surfaces as electrocatalysts for oxygen reduction reaction (ORR) in alkaline media. The lattice mismatch between Pt and the host metals induced either compressive or expansive strain in the Pt monolayers. The Pt monolayer was compressed on Ir(111), Ru(0001) and Rh(111), whereas it was stretched on Au(111), compared to Pt(111) surface [59]. As a result, Pt/Au(111) binds oxygen much more strongly than Pt(111) while Pt/Ru(0001), Pt/Rh(111) and Pt/Ir(111) bind oxygen considerably less strongly than Pt(111) [56]. This causes Pt/Ru(0001), Pt/Rh(111) and Pt/Ir(111) to be less active in ORR than Pt because breaking the O-O bond is more difficult on these surfaces than on Pt(111). On the other hand, the Pt/Au(111) surface, which binds oxygen most strongly, will facilitate the O-O bond breaking but will slow down the O and OH hydrogenation. This slowing down will increase the coverage of these species on the catalyst surface and impede the adsorption of the main reactant, O2 [51, 56, 59-60]. Pd/Pt(111) with d-band
center lying in the middle and therefore forming a moderate bond with the adsorbates has the best activity for ORR.