Electric Catalyst Qualities And Germanium Metal Film On The Skin Layer Of Chemical Deposition

Proton Exchange Membrane Fuel Cells (PEMFCs) are promising green power sources for portable devices. Commercialization of PEMFCs is largely hindered by the high cost due partly to its great demand for precious Pt-based catalysts used in the cathode and anode. Therefore, investigation on catalysts of higher activities and/or lower Pt contents is of great importance to promote the development of PEMFCs. Decorating Pt (sub)monolayer on other relatively cheaper metal substrate not only enhance the catalytic properties through the interaction between the Pt adatoms and the substrate, but also reduce the usage of this rare and precious metal. Underpotential deposition (UPD) method is a widely used method to prepare such catalysts, but it requires external electricity controls, and thus limits its applicability for scale-up surface modification on powder nanocatalysts. To address this issue, our group initially proposed a mimetic underpotential deposition (MUPD) tactic and obtained preliminary results, further demonstration and development of this MUPD method is highly demanded for its wider applications.On the other hand, the design of new efficient catalysts is largely based on the understanding of the mechanism of corresponding electrocatalytic processes. Nowadays, more efforts should be devoted to the mechanistic study of anodic reactions in alkaline electrolytes in consideration of growing interests in alkaline polymer electrolyte fuel cells. ATR surface-enhanced infrared absorption spectroscopy (SEIRAS) involving a metallic film on Si prism does not work well in alkaline solutions due to the dissolution of Si and the formation of SiOx in the metal/Si interface. Ge is relatively stable in alkaline solutions, and should be tested as a potential window for electrochemical ATR-SEIRAS in alkaline solutions. To this end, a facile fabrication of metallic films on Ge needs developing and their electrochemial responses need clarifying.In the present work, we have successfully extended the MUPD strategy for surface decorations of different kinds of catalysts to attain Pt-submonolayer catalysts towards formic acid electrooxidation. Meanwhile, we have proposed a UPD approach using a rotating electrode to achieve the surface modification of powder catalysts. Furthermore, we have prepared Au and Pd films on the basal plane of a Ge prism and examined its feasibility for electrochemical ATR-SEIRAS in alkaline solutions.The main results of the research work are summarized as follows: 1. Extension of the MUPD tacticMUPD method exploits suitable reductants to adjust the open circuit potential (OCP) of a metallic substrate to a suitable UPD potential to enable metal mono layer deposition. Herein, different reductants to adjust the OCP of the metal substrate to the potential for Cu UPD have been screened and glyoxal and hypophosphite are found to be the appropriate ones for Cu UPD on the Au and Pd substrates, respectively, in terms of the resulting potentials. Indeed, Cu monolayer decoration on Au disk electrode and Pd/C surface were successfully realized with the aid of selective reducing agents. Furthermore, we have proposed a Cu MUPD combined with subsequent redox-replacement reaction to achieve a (sub)monolayer modification of noble metal (Au, Pd or Pt) atoms on desired substrates. Since the skin-layer attained by this technique can be either a pure metal or a mixed metal monolayer, the surface component could also be tailored aiming for different catalytic reactions. As a matter of fact, the MUPD technique was newly extended to achieve surface decorations of Pt (sub)monolayer on Au, and Pt-Pd mixed monolayer on Pd, respectively. The electrocatalysts thus obtained exhibited significantly enhanced catalytic activities for formic acid oxidation. All these above results demonstrate that the extended MUPD approach is promising for the scale-up skin modification on nanocatalysts with foreign adatoms.2. UPD on Powder with a Rotating ElectrodeTo extend the feasibility the conventional UPD tactic, we have used a rotating electrode to assist the UPD of submonolayer of Cu on powder nanocatalysts. Specifically, a specially designed metal foil with a sufficiently large working electrode area is drived to rotate in a Cu UPD bath containing a desired powder nanocatalyst. The catalyst powder suspended in the electrolyte is mechanically stirred to continually hit the rotating working electrode to attain the electrochemical Cu UPD. This Cu UPD combined with subsequent redox-replacements on different catalysts enables the skin modifications of Pd on Pt/C, Pt on Pd black, Pt on Pd/C and Au on Pt/C, respectively. These catalysts showed improved activity towards formic acid oxidation and oxygen reduction. The XRD and CV characterizations further confirm the effectiveness of this method.3. Electroless deposition of Au or Pd film on Ge In this work, we have succeeded in chemical plating of bright, uniform and conductive Au and Pd films on Ge substrates from simple alkaline baths. The thickness of a metallic film can be controlled effectively by adjusting the bath temperature and deposition time. The XPS and XRD measurements indicate that the films are not contaminated by other elements. However, it turns out that the electrochemical responses in the process of positive-going potentials due to a plated film on Ge exhibit abnormally huge oxidation currents, suggesting that Ge is not qualified for an IR window of EC-SEIRAS unless a barrier layer is introduced. The abnormal electrochemical behavior may be explained by assuming a promoted anodic corrosion of Ge in the interface of metal and Ge where alloying or doping with each other is likely.