Ethanol Electrooxidation On Rh And Pd-based Catalysts In Alkaline Medium: Reaction Mechanism And Catalytic Performance

Most recently,ethanol oxidation reaction?EOR?,the anodic half-reaction of direct ethanol fuel cells?DEFCs?,has attracted wide attention.In this field,there are two main research hot-points.First,palladium?Pd?might serve as an alternative catalyst for EOR in alkaline media,due to its low-cost and high activity with respect to platinum?Pt?.On the other hand,the poor faraday efficiency of EOR C1 reaction pathway is long-standing,which is expected to be overcome by Rh-contained or Rh-based EOR catalysts.Herein,in this study,several Pd-based EOR catalysts are rationally designed and prepared,and the EOR reaction mechanism on Rh electrode surface is subsquently investigated by in situ infrared spectra,which is briefly summarized as follows.Part ?.highly active Pd-based catalysts toward EOR in aklaline medium.Firstly,a series of Pd-Ni-P alloy electrocatalysts with ultra-low doping amount of Ni and P are prepared,using ca.1.5 mg NaH₂PO₂ as reducing agent.The mass fraction of Ni and P are respectively as low as 0.2%and 0.05%when the pH value of the synthetic solution is 10.The results show that the Pd-Ni-P/C-pH10 catalyst exihibits nearly 2.7-time higher oxidation current than the commercial Pd/C catalyst?JM?.Meanwhile,the durability of Pd-Ni-P/C-pH10 for ethanol oxidation is improved by ca.2.8 times compared to the commercial catalyst.Secondly,the electronic and geometric structures of PdCu/C catalyst were effectively modulated by voltammetric dealloying post-treatment,and Pd₂Cu₁/C-DA15 show the most optimal EOR activity,being 4.6times higher than commercial Pd/C.Thirdly,surface-clean low-doped PdB/C catalysts are successfully prepared through a facial and green routine.Their mass activity of EOR in alkaline media was as high as ca.4018 mA mg_Pd^-1.Note that,although the activity?oxidation current?of the above Pd-based catalysts is significantly promoted,nevertheless,the?pseudo-?quantitative analyses of reaction production distribution shows that the Faraday efficiency of the C1 reation path is only 2%.Therefore,it is imperative to further improve the C1-path efficiency through mechanism investigation and catalysts preparation.Part ?.In situ surface enhanced infrared spectroscopy?SEIRAS?for CO adsorption and oxidation on Rh flim electrodes.Rh based catalysts might resolve the longstanding problem of poor C1 pathway efficiency toward an ethanol oxidation reaction.The mechanism of Rh surface EOR reaction still needs to be clarified.In addition to being an important C1 intermediate for ethanol electrooxidation,CO is also studied as a model molecule.We studied the adsorption and oxidation of CO on Rh film elactrode by in situ ATR-SEIRAS.The results show that the formation of CO_ad layer on Rh is more sluggish in alkaline media,may be due to the high OH-concentration induced bigger electronegativity of Rh electrode.The oxidation of CO_ML on Rh in alkaline media might be divided into two potential sections.One may follow the Eley-Rideal mechanism at 0.45–0.55 V with OH^—ions directly attracting the surface weakly adsorbed CO_L species;the other is the oxidation of CO_ML species through the Langmuir-Hinshelwood mechanism.This work may provide some fundamental supporting for fabricating highly efcient ethanol oxidation catalysts.Part ?.In situ IR study of Rh film electrodes toward EOR in the alkaline conditionTo facilitate the rational design and preparation of Rh-based EOR catalysts,here we fundamentally study ethanol adsorptive dissociation and oxidation on an Rh electrode surface by electrochemical infrared absorption spectroscopy.First,real-time infrared spectral results show that ethanol could be easily split on an Rh surface into COad and CHx intermediates only in alkaline media but not in acidic media.Second,the onset oxidation potential of EOR on Rh is ca.180 mV more negative than that on Pd and Pt electrode in alkaline media.Accordingly,the apparent selectivity efficiency of the C1 pathway???is estimated to be 100%when the potential is at 0.4-0.6 V vs RHE,subsequently?sharply decreases to zero at 0.65-0.8 V vs RHE,and then,?gradually rebounds to ca.16%when the potential moves positively.This work may provide some theoretical support for fabricating highly efficient EOR catalysts.