Chemivoltaic Effects on Semiconductors for Direct Gas Fuel Energy to Electricity Conversion

A conversion of energy released during heterogeneous chemical reactions on solids into electronic excitations of solids has been studied for decades. When exothermic reactions take place on semiconductor surfaces, chemical energy can be effectively transfer into non-equilibrium electron-hole (e-h) pairs. It was found that in the reactions of recombination of hydrogen atoms (H) on germanium surface (Ge), the probability of e-h pair generation per recombination event, H_g + H_s => H₂, can be reached 60%. The corresponding effects on p-n junction are associated with generation of electric "chemicurrent" in a closed circuit or "chemiEMF" in an open circuit. They both can be metrics of conversion efficiency of chemical energy into electricity.  In our early studies H atoms were generated by ICP plasma source (13.6 MHz) and injected into a vacuum chamber where Ge diode with open planar p-n junction was placed. All surface reaction parameters, as well as heat released, were measured independently. The electrical outputs, chemicurrent and chemiEMF, as functions of atomic H flow are studied. We calculated that energy conversion coefficient K was ~ 4%. We also demonstrated the chemivoltaic effects are direct analogy of photovoltaic effects providing conversion of energy of light into electricity.  

We studied similar phenomena on p-n junctions made from other semiconductor materials such silicon carbide (SiC), selenium (Se), silicon (Si) and others, and in different active gases. The gases were dissociated in RF plasma and injected into a chamber with p-n junction structures. The chamber configuration allowed no ions but atomic and molecular radicals to reach the semiconductor device and react on its front surfaces generating chemivoltaic effects. The gases of interest were H₂, O₂, CO and CH₄, H₂+O₂ and CO+O₂ mixture, and the surface reactions proposed are H + H => H₂, O + O => O₂, CH₃ + H => CH₄,  2H +O => H₂O and 2CO + O₂ => 2CO₂. In recent studies, we applied no plasma but catalyst activated reactions to demonstrate ability of chemivoltaic effects in providing gas fuel energy conversion into electricity through catalytic oxidation reactions.

In the H + H reaction on Se p-n junction, the chemicurrent was stabilized during few seconds at ~100nA. In case of O-atoms, the chemicurrent was ~30% higher. For SiC p-n junction, we studied chemiEMF as a function of time, temperature, type of gaseous radicals and radical flow. The temperature was varied from 310K to 380K. Other reactions on SiC, O + O and H + O, resulted in ~1.5 mV of chemiEMF that is two times higher than chemiEFM  caused by the reactions of H + H and CH₃ + H.

To bring the chemivoltaic technology into practice, an appropriate device should be able to provide a conversion of chemical energy at atmospheric pressure and at ambient temperatures. To support these conditions, a catalyst is required to provide semiconductor surface activation. One of the solution, chemivoltaic effects based on catalytic Schottky diode, was recently proposed and characterized. A thin metal film, a part of Schottky junction, plays also a role of a catalyst. We studied catalytic reaction of molecular H₂ and O₂ on Pd film (~5 nm) deposited on n-type of Si substrate to form a planar Pd/n-Si Schottky junction. A stoichiometric H₂ + O₂ mixture was injected into a chamber containing the described Schottky diode. A chemicurrent in a circuit with the diode was simultaneously observed with H₂O byproduct of the catalytic oxidation reaction monitored by mass-spectrometer. A pressure in the chamber was ~10 Pa, temperature was 310K, and the chemicurrent was sharply stabilized at 40 nA. Per our rough estimation, a coefficient of energy conversion in this system was 10^-3 - 10^-4.

In the alternative solution, we applied Si p-n junction activated by catalyst to realize H₂ + O₂ and CO + O₂ catalytic reactions at ~10Torr pressure and at elevated temperatures (340 – 360K). The appropriate catalyst was precipitated from a liquid solution on a front side of planar Si p-n junction (~4cm²) and modified in plasma in the following treatment. A chemicurrent with initial peak of 0.5mA and steady state of ~ 0.15μA was observed in 2H₂ + O₂ => 2H₂O catalytic reaction at ~5 Torr.  In the reaction of 2CO + O₂ => 2CO₂ at 375K and ~7.5Torr, the chemicurrent was much higher, ~0.5μA. The reaction byproducts, H₂O and CO₂, were monitored by mass-spectrometer. 

The described chemivoltaic effects are considered as the unique technology for direct gas fuel energy to electricity conversion when oxidation reactions of such fuel gases as hydrogen and methane are used.