Electrolysis Of Coal And Carbon Slurry Suspensions

Abstract

In this dissertation study, different ranks of coal and carbons were tested as anode depolarizers in a three electrode electrochemical cell designed for hydrogen generation. The focus of this study was mainly Texas lignite coal (TXLC). For comparison purposes, other coals were carefully chosen to cover the range from high-rank, intermediate-rank, and to low-rank (TXLC). Carbon blacks and carbon nanotubes were also studied to gain more insight into the mechanistic aspects of the electrolysis process.The Fe<super>3+/2+</super> redox couple was used as an oxidation mediator throughout the study. It shuttles the electrons between the coal or carbon particles and the anode surface. A standard reduction potential of 0.76 V explains the ability of Fe<super>3+</super> species to (partially) oxidize the bulk carbon phase as well as the surface functional groups of coal and carbons. In addition, the Fe<super>2+</super> species can be anodically regenerated at a low potential (0.8 V), that is much lower than the oxygen evolution potential. Finally, It is recognized that these species exist as aqua complexes in solution, and among the Fe<super>3+</super> species, the dominant photoactive complex is the 6-coordinated Fe (OH) (H₂O)₅ <super>2+</super> complex. The photoactivity of the Fe (OH)(H₂O)₅ <super>2+</super> complex allowed the use of light as a mechanistic probe of photoelectrolysis of coal and carbons. In the photoelectrolysis of aqueous lignite coal and carbon black slurry suspensions, UV irradiation of the anolyte in the presence of iron species, afforded enhanced currents associated with the free radical-induced oxidative attack of the coal (or carbon) surface. Useful mechanistic insights were gleaned into the factors responsible for the anode depolarization by the coal (or carbon) particles in the slurry suspension. According to a photo-Fenton-like mechanism, UV light was used to modulate chemical reactions in the solution phase generating very reactive <super>*</super>OH and other reactive oxygen species (ROS) that oxidatively attack the coal matrix. It was found that the hydroxyl radicals (<super>*</super>OH) and the ROS photogenerated via this mechanism can enhance hydrogen production in the cathode compartment of a coal photoelectrolysis cell. GC analyses of the evolved gases in the anolyte compartment revealed the gradual increase in the amount of CO₂. Infrared (IR) spectrophotometric analysis of the samples before and after UV irradiation (in the presence of Fe<super>3+/2+</super>) showed an overall increase in the surface oxygen groups and a decrease in aromaticity. These data trends are consistent with an attack of the coal matrix by the photogenerated <super>*</super>OH species and other ROS. Two carbon black samples were included in this study for comparative purposes: (a) to assess the effect of oxidizability of the carbon matrix (relative to lignite coal); and (b) to examine the influence of graphitization of the carbon black on its ease of oxidation.The consequences of chemical pre-treatment of coals of varying rank and selected carbon black samples, on their ability to generate hydrogen in an electrolytic environment were explored. Concurrently, thermal analyses (differential scanning calorimetry or DSC and thermogravimetry or TGA) were performed on these pre-treated samples to investigate the consequences in terms of corresponding alterations in thermal reactivity. The chemical pre-treatment consisted of digestion with strong acid (1 M each of HClO₄, H₂SO₄, or HNO₃) or by stirring the coal (or carbon black) sample with 35 % H₂O₂ overnight. The influence of H₂O₂ pre-treatment was shown to be critically dependent on the coal rank. Further, coal samples responded differently relative to carbon black surfaces in terms of how the hydrogen-generating capacity and thermal reactivity were altered by either acid or H₂O₂ pre-treatment. The improvement of the chemical reactivity of coal samples following chemical pre-treatment was attributed to changes in surface area and surface oxygen functional groups. The surface area of coal particles was measured (via nitrogen adsorption and the BET model) before and after treatment. The surface and bulk oxygen functional groups were investigated by X-ray photoelectron spectroscopy (XPS) and IR analysis, respectively. The results showed an appreciable increase in the oxygen functional groups, specifically the carbonyl groups following the acid and H₂O₂ treatments. Multiwalled carbon nanotubes (MWCNTs) were included in the oxidation treatment to assess which oxygen functional group was responsible for the improvement of coal reactivity. Potassium permanganate (KMnO₄), which is a more powerful oxidizing agent than H₂O₂, was used to ensure complete oxidation of the chemically inert MWCNTs. The XPS and IR data showed a specific increase in the hydroxyl rather than the carbonyl groups. The complete absence of any improvement in the chemical and electrochemical reactivity of MWCNTs following the oxidation treatment ruled out any contribution from the hydroxyl groups to the improved reactivity of chemically pretreated coal.Finally, economic analysis of hydrogen production by coal (dark and photo) electrolysis was performed. The analysis aimed at carrying out a sensitivity analysis that addresses the influence of variation of main system components (e.g., electricity price, operating potential, and process efficiency) on the hydrogen production cost. Economic barriers associated with the commercial application of coal electrolysis for hydrogen production were also addressed.