Amphiphilic coatings on cobalt boride nanocatalysts for stability in hydrogen generation applications

Abstract

While the idea of using hydrogen as a fuel has been in circulation for hundreds of years, only in recent decades have viable hydrogen generation and storage mechanism become widely appealing for a future hydrogen economy. In addition, the field of energy research continues to make great strides in the utilization of hydrogen fuel for stationary and mobile power systems. Transport of hydrogen gas itself is inherently bulky and with regards to safety concerns may be preferable in a less flammable formulation, such as stored within an advanced carbon material or more commonly as a chemical hydride. More important than ever is the exploration of an effective catalytic system for on-demand hydrogen fuel generation from such chemical hydride precursors for effective use in applications including proton exchange membrane fuel cells. Among the chemical hydrides, sodium borohydride (NaBH4) has been praised for its hydrogen storage density, stability in air and in basic solutions, and the recyclability of its sideproduct sodium metaborate (NaBO2). In prior works, cobalt has proven to be an effective and low-cost catalyst for the hydrolysis of stable NaBH4 solutions into H2 gas and NaBO2 as an alternative to high priced noble metal based catalysts. However, like many metals, cobalt is highly susceptible to surface oxide layer formation, which when in the form of a nanomaterial catalyst means a dramatic change in catalytic properties. Therefore, a system which can eliminate or slow down the passivation of cobalt to its catalytically inactive form will offer significant benefit for hydrogen generation systems. Strategies that have been applied to improve the stability of Co nanoparticles have implemented protective coatings of aluminum oxide, silica, and gold among others; however, these methods may not offer an effective catalytic surface for the hydrolytic generation of hydrogen from sodium borohydride and moreover the processing is inherently expensive. Therefore, we present in this work our analysis of a technique for generating an inexpensive Co-based catalyst for hydrolysis of sodium borohydride which makes use of stabilizing amphiphilic molecules to hinder formation of a surface oxide layer. Numerous systems have been conceived for the on-demand generation of hydrogen, taking on various forms including cobalt and nickel foam catalysts and cobalt alloy nanoparticle catalysts. Cobalt nanoclusters are often supported on poly(vinyl pyrrolidone) by reductive processes and have yielded catalytic scaffold with high hydrogen generation rates. Other recent works have shown Co2B nanoparticle catalysts to be competitive with noble metals and have revealed that particle size and dispersion characteristics could yield high catalytic rates owing to their increased accessibility to active sites. In order to continue to improve the stability and activity of such nanoscale catalysts with respect to oxidative passivation, we have explored the use of amphiphiilic components (particularly polydiacetylene and polysorbate 20) in order to protect the surface of cobalt nanoparticles. By utilizing the inherent magnetic properties of the colloidal cobalt for separation and collection of cobalt, this approach may also offer controlled magnetic removal of catalyst for on-demand catalytic generation of hydrogen gas from stable sodium borohydride precursor solution. In detail, we explore the use of amphiphilic polysorbate or amphiphilic polydiacetylene for coating of the cobalt particles in order to hinder their transformation to the non-magnetic, poorly catalytic oxidized state (Figure 1). The formation of cobalt boride (Co2B) nanoparticles was carried out by a routine technique involving the reduction of a cobalt salt solution by dropwise addition to a solution of sodium borohydride. Similar metal boride nanoparticles are commonly prepared in such a technique owing to the simple synthesis and low cost. Related reports confirm that shortly after cobalt nanoparticle particle synthesis, there is a gradual, complete loss in magnetization due to the progressing oxidation process. In this work, we proceed to collect the Co2B nanoparticles after synthesis by magnet and subsequently expose the nanoparticles to amphiphilic polysorbate 20 and 10,12-pentacosadiynoic acid (PCDA), referred to in its polymerized form as polydiacetylene (PDA). The role of the polysorbate 20 and liposomal PDA was assessed with respect to their effect on the hydrogen generation rate of the nanoparticle catalysts. In addition, we identified interesting chemisorptions effects upon mixing of Co2B nanoparticles with PDA at particular pH conditions, which was explored by spectrophotometer and surface charge measurements. In the DOI 10.1007/s13233-015-3048-7