Preparation And Electrocatalytic Performance Of Nanoporous Metal Membranes

Nanoporous metal is one of the most important nanomaterials and widely used in electronics, optic, catalysis, due to its unique structure and property. Exploiting new method to fabricate it, further studying its structure, and exploring its applications are of special significance for nanomaterial science and modern industry. On the other hand, the novel material for energy conversion, such as effective fuel cell catalysts, recently is highly desirable for industry. Here we focus on fabricating nanoporous Au, Pt/Au, Pd/Au membranes with ultra-low metal loading. We also investigated their structure and electrocatalytic activity and explored their applications for the electrochemical detection and fuel cells. The results are as follows:1. Nanoporous Au (NP-Au) membrane was made by dealloying Au/Ag alloy in concentrated HNO3. Upon silver dissolution, gold atoms left behind will self-organize into an interconnected network of pores and ligaments. The as-prepared (NP-Au) membrane is about 100 nm in thickness, with a metal loading about 0.1 mg/cm2. On the basis of the NP-Au, we prepared nanoporous Pt/Au (NP-Pt/Au) membrane by plating an atomically thin layer of Pt over NP-Au substrate. Two routes were developed to fabricate NP-Pt/Au membrane, i.e. interface electroless plating technique and electrochemical under potential deposition combining redox replacement method. By controlling the reaction process, the deposited Pt overlayers can be tuned from sub-monolayer to several monolayers. Composition analysis suggests the Pt loadings in NP-Pt/Au membrane are in the range of 1 to 25μg/cm2. The structure of NP-Au and NP-Pt/Au membranes were studied via scanning electron microscope (SEM) and high-resolution transmission electron microscope (HR-TEM), which exhibits a three-dimensional continuous porous structure, exposing a large number of active Pt atoms on the surface. The HR-TEM and electrochemical study suggest that Pt overlayers epitaxially grow on NP-Au surfaces, adopting an islanding growth mode. Cyclic voltammetry (CV) was also performed to study their electrochemical behaviors in acidic medium. As an electrode material, the electrocatalytic activity of NP-Au towards NO2- oxidation was evaluated. NP-Au exhibits sensitive responses to this reaction. Amperometric study showed a linear relationship for NO2- determination in a concentration range from 1 uM to 1 mM. These results suggest that NP-Au has potential applications in electrochemical sensor.2. We systematically studied the electrocatalytic activity of NP-Pt/Au membrane towards a series of important fuel cell reactions, including methanol, formic acid oxidation and oxygen reduction. CV was earried out to evaluate the activity of a series of NP-Pt/Au membranes with the various Pt loadings. While the heavily plated samples (high Pt loadings) were found to display an similar electrocatalysis behavior and better activity with that of commercial Pt/C electrocatalyst, the slightly plated samples (low Pt loadings) display an enhanced mass-normalized activity towards these reactions. To reveal the origin of the observed activities, SEM, HR-TEM, X-ray photoelectron spectroscopy (XPS), and electrochemical CO stripping were combined to characterize the surface structure and property of NP-Pt/Au membrane. Using the NP-Pt/Au membrane as an anodic catalyst, we also performed the H2/air, direct methanol, formic acid fuel cells test and optimized the work parameter.3. We studied the stability and structure evolution of NP-Pt/Au membrane during thermal annealing at relatively low temperatures. A series of NP-Pt/Au samples with various Pt loadings were annealed in an electronic oven under various temperatures ranging from 100 to 400℃. The annealing time, according to the experimental requirement, varies from 2 to 48 h. SEM, HR-TEM, XPS, and electrochemical techniques were combined to characterize the surface structures and chemical state of the annealed NP-Pt/Au membranes. The results suggest that the NP-Pt/Au membranes preserve initial nanoporous structure at the temperature as high as 300℃. But the surface Pt nanoislands smoothed out and alloyed with the Au substrate to form a thin alloy layer coating on NP-Au, resulting in obvious change of surface atom arrangement. The effect from this change on the electrocatalytic property was evaluated. The rearrangement Pt atoms were found to show an enhanced activity for formic acid oxidation.4. By developing the fabrication method, we succeed depositing Pd on NP-Au surface to form nanoporous Pd/Au (NP-Pd/Au) membrane. XPS study suggests that the surface Pd atoms are metallic and HR-TEM observation demonstrates the Pd layer epitaxially deposits on NP-Au surfaces with a layer growth mode. The electrochemical behavior and the electrocatalytic property of the as prepared NP-Pd/Au membrane were characterized, which exhibits a more than two times activity towards formic acid oxidation in acidic medium than that of commercial Pd/C catalyst. Further test was performed on direct formic acid fuel cell using NP-Pd/Au membrane as anodic catalyst.