Coal Electrolysis to Produce Hydrogen at Intermediate Temperatures

As an alternative technique for hydrogen production, coal electrolysis was evaluated at intermediate temperatures (80 °C-108 °C).

First, an electrochemical technique was developed to determine the concentrations of Fe (II) and Fe (III) ions simultaneously using a rotating disk electrode, which is based on the relationship between the concentration of iron ions and the limiting current. The detection limit of this technique is 1 ppm with 1 M H₂SO₄ as electrolyte and a rotation rate of 3000 rpm. A mathematical model was successfully established to describe the iron redox reactions on the surface of the rotating disk electrode. The model significantly saved the experimental time and cost for calibration procedure, and impvoed the accuracy of this technique. This technique is important to study the kinetics of coal electrolysis and was applied for the whole research.

Second, the temperature effect on coal electrolysis was studied, where galvanostatic experiments were carried out on a coal electrolytic cell, which was designed and constructed to operate at intermediate temperatures with concentrated H₂SO₄ as the electrolyte. Experimental results show that increasing temperature improved the kinetics of coal electrolysis with the CO₂ efficiency increasing from 2.1% at 40 °C to 57.3% at 108 °C.

Finally, kinetics of coal electrolysis was studied on a bench continuous coal electrolytic cell setup at intermediate temperatures by applying galvanostatic technique. The results showed that coal oxidation is directly related with the characterisitcis of coal particles, electrode catalysts and the presence of Fe (III) ions. Coal electro-oxidation by Fe (III) is the limiting step of electrolyzing of a 0.02 g mL^-1 coal slurry containing 100 mM Fe (II)/Fe (III) under 100 mA applied current at operating temperature 108 °C and 65 mL min^-1 flow rate. The study of coal oxidation with trace iron ions under lower current (from 35 mA to 5 mA) at the same operating conditions shows that coal electro-oxidation by Fe (III) is not the limiting step any more, and the film formed on the surface of the coal particles prevent the further oxidation of coal particles. Based on those experimental results, a mechanism is proposed to describe the coal oxidation process.