(224b) Hydrogen Generation by Plasma-Assisted Electrolysis of H2O/SO2 Gas Mixture

We demonstrated that the reaction between water and sulfur dioxide (SO2) can be catalyzed plasma-chemically and hydrogen species, products of the reaction, can be pumped out electrochemically as hydrogen molecules (H2) with help of palladium bipolar electrode. The major byproduct of the reaction of H2O/SO2 mixture in the plasma reactor is sulfuric acid (H2SO4) which can be decomposed back to SO2 gas using thermal energy source like nuclear heat.

Most widely known method for decomposing water is electrolysis. The thermodynamic voltage of water electrolysis is 1.23 volt, but by adding to water some chemical species such as Ti3+, Sb, Re, SO2, Sb, As, Re, and Bi or organic fuels such as methanol the thermodynamic voltage can be reduced substantially. For example, at the anode of the electrolyzer the mixture of water and SO2 are oxidized to generate hydrogen ion (proton) and H2SO4 at 0.18 V. However, the SO2 gas often crosses over to cathode, the opposite electrode, poisoning it and the electrolysis of the mixture is kinetically sluggish requiring high over potential even when platinum-family catalysts are used. To solve these problems we developed a novel scheme in which conventional electrolysis cell was replaced by an electrochemical hydrogen pump in contact with a plasma reactor. These two units, plasma reactor and electrochemical pump, share a common electrode made of hydrogen-absorbing metal. Microplasma can substitute for the expensive Pt catalyst. The reacting gas contacting the anode is catalyzed by non-thermal electrons generated in the plasma reactor. Hydrogen atoms, products of the plasma-assisted reaction, are dissolved in the hydrogen absorbing metal like palladium (Pd). The hydrogen absorbing metal acts a barrier against transfer of SO2. The dissolved hydrogen atoms are oxidized at the interface between Pd, now the anode of the pump, and the proton conducting membrane, the electrolyte of the pump and reduced into H2 gas at the cathode of the pump. For low temperature operation below 180oC a polymer membrane like Nafion was used as the electrolyte of the pump while for high temperature operation between 600 and 850oC solid oxide membrane like BaCe0.8Y0.2O3-ä oxide was used as the proton-conducting electrolyte.

The production rate of the hydrogen gas out of the pump for the low temperature operation was about 0.5 ml per minute when the voltage and the current were adjusted at 0.1 V and 60 mA∙cm-2, respectively. The energy efficiency defined as the volume of the hydrogen produced per total consumed electrical energy was 6 ml per 100 J for the low temperature operation while the efficiency jumped to 16 ml per 100 J for the high temperature operation. The efficiency further increased when pulse generating power source was used instead of a DC supply to save the electrical energy for the plasma generation. The efficiency was near 50 ml per 100 J of the electrical energy input. For the comparison, the efficiency of the conventional electrolyzer utilizing the H2O/SO2 gas mixture is about 25 ml per 100 J of the electrical energy input.