Hydrogen production in a microbial electrolysis cell with nickel-based gas diffusion cathodes

Gas diffusion cathodes with Ni alloy and Ni catalysts manufactured by chemical deposition were tested for H₂ production in a microbial electrolysis cell (MEC). In a continuous flow MEC, multi-component cathodes containing Ni, Mo, Cr, and Fe, at a total catalyst load of 1 mg cm⁻² on carbon support demonstrated stable H₂ production at rates of with only 5% methane in the gas stream. Furthermore, a Ni-only gas diffusion cathode, with a Ni load of 0.6 mg cm⁻², demonstrated a H₂ production rate of . Overall, H₂ production was found to be proportional to the Ni load implying that inexpensive gas diffusion cathodes prepared by chemical deposition of Ni can be successfully used for continuous production of H₂ in a MEC.     

Photocatalytic hydrogen evolution over CuCrO2

We have been studying the technical feasibility of a photochemical H₂ evolution based on a dispersion of CuCrO₂ powder in aqueous electrolytes containing various reducing agents (S²⁻, and ). The title oxide combines a fair resistance to corrosion with an optimal band gap E_g of 1.32 eV. The intercalation of a small amount of oxygen should be accompanied by a partial oxidation of Cu⁺ into Cu²⁺ implying a p-type semiconductivity. The S²⁻ oxidation inhibits the photocorrosion and the H₂ evolution increases parallel to polysulfides formation. Most of H₂ is produced when p-CuCrO₂ is connected to n-Cu₂O formed in situ. H₂ liberation proceeds mostly on CuCrO₂ while the oxidation of S²⁻ takes place over Cu₂O surface and the hetero system Cu₂O/CuCrO₂ is optimized with respect to some physical parameters. The photoactivity is dependent on preparation conditions and lowering the synthesis temperature through nitrate route leads to an increase in specific surface area S_sp. The photoelectrochemical H₂ production is a multistep process where the rate determining step is the arrival of electrons at the interface because of their low mobility. Prolonged irradiation (>80 min) leads to a pronounced decrease of the photoactivity; the tendency toward saturation is due to the undesired back reduction of polysulfides in a closed system and to their strong absorption in the visible region (λ_max = 520 nm).     

Kinetics of the NH reaction with H2 and reassessment of HNO formation from NH + CO2, H2O

The reaction of ground-state NH with H₂ has been studied in a high-temperature photochemistry (HTP) reactor. The NH(X³Σ) radicals were generated by the 2-photon 193 nm photolysis of NH₃, following the decay of the originally produced NH(A³Π) radicals. Laser-induced fluorescence on the transition at 336 nm was used to monitor the progress of the reaction. We obtained , with ±2σ precision limits varying from 12 to 33% and corresponding accuracy levels from 23 to 39%. This result is in excellent agreement with that of Rohrig and Wagner [Proc. Combust. Inst. 25 (1994) 975] and the data sets can be combined to yield . Starting with this agreement, it is argued that their rate coefficients for NH + CO₂ could not be significantly in error [Proc. Combust. Inst. 25 (1994) 975]. This, combined with models of several combustion systems, indicates that HNO + CO cannot be the products, contrary to their suggestion [Proc. Combust. Inst. 25 (1994) 975]. Ab initio calculations have been performed which confirm this conclusion by showing the barriers leading to these products to be too high compared to the measured activation energies. The calculations indicate the likelihood of formation of adducts, of low stability. These then may undergo further reactions. The NH + H₂O reaction is briefly discussed and it is similarly argued that HNO + H₂ cannot be the products, as had been previously suggested.