A Study on Electrochemical Reaction of Bituminous Coal Treated Hydrochloric Acid and Nitric Acid

The DCFC used solid carbon as fuel has been studied about the correlation between the fuel properties and the performances. Through many researches, the performance depends on the characteristics of surface properties of carbonaceous fuel. The ratio of oxygen to carbon of fuel surface has especially an influence on electrochemical reaction known from our previous research. To investigate the effect of surface properties and oxygen functional groups in more detail, a bituminous coal was selected and treated with hydrochloric acid (HCl) and nitric acid (HNO₃).

The physicochemical characteristics and surface properties of raw coal and treated coal were measured by the following analysis. The thermogravimetric analysis was carried out to analyze thermal reactivity with increasing temperature. The pore structure and specific surface area of raw coal and treated coal were estimated by gas adsorption. Through X-ray photoelectron spectroscopy (XPS) and Fourier Transform Infrared Spectroscopy (FT-IR) analysis, the changes of composition and functional groups on fuel surface, that caused by the acid treatment, were analyzed. The correlation between fuel properties and the characteristic of electrochemical reactions was discussed by i-V, i-P, and EIS.

The Glencore coal, which is one of bituminous coals, was used in this research. The coal particle was immersed in 4M acid solution at room temperature for 1 day. After immersion, it was washed with distilled water. These treated coals were subsequently dried at 80 °C for 1day.

The DCFC experimental system is given a detailed account in previous papers¹.  The electrolyte mixed with 62:38 mole ratios of Li₂CO₃ and K₂CO₃. For each condition, the electrolyte and fuel were used 460 g and 46 g, respectively. Three electrodes used as the working electrode, the counter electrode, and the reference electrode were made of silver. The current density and the power density were calculated on considering the effective area of working electrode (1.34 cm² in the standard cell). The performance of fuels was measured by potentiostatic method, and it was used for electrochemical measurement with a Versastat3 (Princeton Applied Research). The performance of DCFC was indicated as current density and power density, and the current was estimated from open circuit voltage (OCV) to 0 V with scan rate of 0.001 V/s by using linear sweep potentiometry. EIS was measured from 0.1 Hz to 100 kHz. The voltage amplitude was 10 mV.

In the cases of HCl treated coal and HNO3 treated coal, they showed similar polarization curve of raw coal in the whole current density region. However, the OCV of HNO₃ treated coal was higher than other fuels. HCl treated coal and HNO₃ treated coal were improved 6.1 % and 30.5 % measured against the maximum power density of raw coal. This is because changing coal properties caused by acid treatment. HCl, non-oxidant acid, removes the mineral component². By comparison, HNO₃, oxidant acid, change the surface composition and surface functional groups³. Through the maximum power density order (raw coal < HCl treated coal < HNO₃ treated coal) results, both acid treatments increased the cell performance. The oxygen functional groups of fuel surface were more dominant.