57th Annual Report on Research 2012 Under Sponsorship of the ACS Petroleum Research Fund

Executive Summary: In the funding period of 2011-2012 for ACS-PRF, two major advances have been developed which have opened up a new avenue of research in sulfur utilization.  The surfeit of elemental sulfur, creates an opportunity to use sulfur as a new feedstock for polymers and materials.  But there have been limited synthetic methods to directly utilized sulfur to prepare polymeric materials.  The key breakthrough that has been developed is the use of molten liquid sulfur as a solvent and reaction medium for the preparation of nanoparticles and polymeric materials.  We have prepared gold nanoparticles directly in liquid sulfur, along with direct bulk polymerization of sulfur copolymers in liquid sulfur copolymerization with a divinylic styrenic comonomer.  We have termed the bulk copolymerization of liquid sulfur, inverse vulcanization, which forms copolymers with a very high content of sulfur (upto 90-wt% sulfur).  These sulfur copolymers were further demonstrated to be electrochemically active as high capacity cathode electrodes for Li-S batteries (specific capacity = 823 mAh/g at 100 cycles)

Background on sulfur utilization:The utilization of elemental sulfur as an alternative feedstock for polymers and nanocomposites is an area of increasing interest for emerging areas in materials chemistry and energy technologies.  Current global production of elemental sulfur is on the order of 70 million tons annually, the majority of which is produced from refining of petroleum products via hydrodesulfurization.^[1a]  Traditional utilization of elemental sulfur is directed toward the production of commodity chemicals, such as, sulfuric acid, and phosphates as fertilizers for agrochemicals.  Smaller niche markets for specialty chemicals, such as, vulcanization processes for rubber (e.g., tires) also directly utilize elemental sulfur.  Despite these existing technologies, nearly 7 million tons of sulfur is produced in excess, the majority of which is stored in powder form, or as compressed bricks in exposed, above ground mega-ton deposits (Figure 1).^[1b] Hence, the sheer abundance of elemental sulfur offers opportunities to develop new chemistry and processing methods to utilize sulfur as a novel feedstock for synthetic advanced materials.

Liquid sulfur as reaction medium for AuNPs:  Elemental sulfur as a novel medium for the formation of AuNPs and for the in situ crosslinking of these colloidal dispersions to form vulcanized nanocomposites (Fig. 2).  We report for the first time the direct dissolution of organometallic Au(I) complexes in liquid sulfur for the formation of discrete, dispersed metallic AuNPs.  To the best of our knowledge this is first example of the direct utilization of elemental sulfur as a solvent and reactive medium for the preparation of nanomaterials, which is analogous to related reports using unconventional media, such as, ionic liquids, for the formation of nanomaterials.  In this system, sulphur serves multiple functions, as the solvent, reducing agent and ligand for colloidal stabilization to form AuNPs (5-7 nm).  This was the first report to use liquid sulphur as an unconventional medium for materials chemistry synthesis (Pyun et al., Angew. Chem. Int. Ed. 2011, 50, 11409; featured as Cover Article)

“Plastic Sulfur:” Synthesis of sulphur copolymers via inverse vulcanization:  The utilization of elemental sulfur as an unconventional medium for the synthesis of novel sulfur copolymers is reported along with processing and electrochemical characterization of novel sulfur copolymers into films and electrode materials for Li-S batteries.  In this system, molten liquid sulfur is directly copolymerized with 1,3-diisopropenylbenzene using a method termed, inverse vulcanization, to form chemically stable copolymers that do not undergo depolymerization.  As a consequence of the inverse vulcanization, elemental sulfur was modified into processable copolymer forms with tunable thermomechanical properties, leading to well-defined sulfur-rich micropatterned films created by the unconventional imprint lithography.  (Pyun et al., Nature Chemistry 2012, under revision).

Sulfur Copolymers as active cathode materials for Li-S Batteries:  Considerable interest in elemental sulfur and modified sulfur materials has been generated because of the use as the positive electrode in Li-S batteries. A central motivation of the inverse vulcanization chemistry was to enable modification of elemental sulfur into a processable polymeric form that still retained the electrochemical activity of elemental sulfur.  We demonstrate that this sulfur copolymer can be used as the electroactive material in cathodes for Li-S batteries which was observed to exhibit the highest reported specific capacity & energy density to date for a polymeric material.  These sulfur copolymers exhibit high specific capacity (823 mAh/g at 100 cycles) and enhanced capacity retention relative to elemental sulfur batteries that rapidly loose capacity upon cycling (Fig. 3)  While other nanocomposite materials composed of nanocarbons (graphenes, carbon nanotubes, template nanocarbons) have exhibited comparable, or improved electrochemical performance in Li-S batteries, these sulfur copolymers are prepared using inexpensive starting materials in a single step process that is amenable to multi-gram scale up.   This general approach is anticipated to open a new avenue of research for sulfur utilization and the creation of novel electroactive polymers. (Pyun et al., Nature Chemistry 2012, under revision)

Broader Impact of ACS-PRF ND Support:  This support enabled the development of a seminal effort on sulfur utilization for polymers and nanocomposite materials.  The concept of using liquid sulfur as the medium for chemistry opened a broad range of possibilities to create new and useful materials that harness the useful properties of elemental sulfur.  These properties include high charge capacity in Li-batteries and high refractive index for optical devices.  This is anticipated to create a new field of “Yellow Chemistry.”