Hydrogen evolution efficiency

Abe R, Sayama K, Arakawa H (2002) Efficient hydrogen evolution from aqueous mixture of E and acetonitrile using a merocyanine dye-sensitized Pt/Ti02 photocatalyst under visible light irradiation. Chem Phys Lett 362 441-444... 

Iron/air Motive power Good energy density Uses readily avriilable materials Low self-dischttfge Low efficiency Hydrogen evolution on charge Poor low-temperature performance Low cell voltage... 

Wang C, deKrafft KE, Lin W (2012) Pt nanoparticles photoactive metal-organic frameworks efficient hydrogen evolution via synergistic photoexcilalion and electron injection. J Am Chem Soc 134 7211-7214... 

Li Z-J, Wang J-J, Li X-B, Fan X-B, Meng Q-Y, Feng K. Chen B. Tung C-H, Wu L-Z (2013) An exceptional artificial photocatalyst, Nih-CdSe/CdS core/shell hybrid, made in situ from CdSe quantum dots and nickel salts for efficient hydrogen evolution. Adv Mater 25 6613-6618... 

The standard electrode potential for zinc reduction (—0.763 V) is much more cathodic than the potential for hydrogen evolution, and the two reactions proceed simultaneously, thereby reducing the electrochemical yield of zinc. Current efficiencies slightly above 90% are achieved in modem plants by careful purification of the electrolyte to bring the concentration of the most harmful impurities, eg, germanium, arsenic, and antimony, down to ca 0.01 mg/L. Addition of organic surfactants (qv) like glue, improves the quaUty of the deposit and the current efficiency. 

Some battery designs have a one-way valve for pressure rehef and operate on an oxygen cycle. In these systems the oxygen gas formed at the positive electrode is transported to the negative electrode where it reacts to reform water. Hydrogen evolution at the negative electrode is normally suppressed by this reaction. The extent to which this process occurs in these valve regulated lead —acid batteries is called the recombination-efficiency. These processes are reviewed in the Hterature (50—52). 

The low current efficiency of this process results from the evolution of hydrogen at the cathode. This occurs because the hydrogen deposition overvoltage on chromium is significantly more positive than that at which chromous ion deposition would be expected to commence. Hydrogen evolution at the cathode surface also increases the pH of the catholyte beyond 4, which may result in the precipitation of Cr(OH)2 and Cr(OH)2, causing a partial passivation of the cathode and a reduction in current efficiency. The latter is also inherently low, as six electrons are required to reduce hexavalent ions to chromium metal. 

The inhibitor should not decompose during the life of the pickle nor decrease the rate of scale removal appreciably. Some highly efficient inhibitors, however, do reduce pickling speed a little. It would be expected that since the hydrogen evolution is reduced the amount of hydrogen absorption and embrittlement would also be reduced. This is not always the case thiocyanate inhibitors, for example, actually increase the absorption of hydrogen. 

More recently, Ikeda et a/.108 have examined C02 reduction in aqueous and nonaqueous solvents using metal-deposited p-GaP and p-InP electrodes under illumination. Metal coatings on these semiconductor electrodes gave much improved faradaic efficiencies for C02 reduction. In an aqueous solution, the products obtained were formic acid and CO with hydrogen evolution at Pb-, Zn-, and In-coated electrodes, while in a nonaqueous PC solution, CO was obtained with faradaic efficiencies of ca. 90% at In-, Zn-, and Au-coated p-GaP and p-InP, and a Pb coating on a p-GaP electrode gave oxalate as the main product with a faradaic efficiency of ca. 50% at -1.2 V versus Ag/AgCl. 

The composition of the codeposition bath is defined not only by the concentration and type of electrolyte used for depositing the matrix metal, but also by the particle loading in suspension, the pH, the temperature, and the additives used. A variety of electrolytes have been used for the electrocodeposition process including simple metal sulfate or acidic metal sulfate baths to form a metal matrix of copper, iron, nickel, cobalt, or chromium, or their alloys. Deposition of a nickel matrix has also been conducted using a Watts bath which consists of nickel sulfate, nickel chloride and boric acid, and electrolyte baths based on nickel fluoborate or nickel sulfamate. Although many of the bath chemistries used provide high current efficiency, the effect of hydrogen evolution on electrocodeposition is not discussed in the literature. 

The quantum efficiency of photo electrochemical reactions may vary from 2 to 4 the effective dissolution valence may vary from 2 to 4 and the efficiency of hydrogen evolution may vary from zero to near 1 depending on light intensity and potential. 

Ge, h. and C. Han, Synthesis ofMWNTs/g-C3N4 composite photocatalysts with efficient visible tight photocatalytic hydrogen evolution activity. Applied Catalysis B Environmental, 2012. 117-118(0) p. 268-274. 

Ge, L. Han, C., 160. Synthesis of MWNTs/g-C3N4 composite photocatalysts with efficient visible light photocatalytic hydrogen evolution activity. Appl. Catal., B Env. 2012,117-118 268-274. 

If there are no detrimental organic side reactions, a cell current density in excess of the limiting current density - and as result a loss of current efficiency - may be acceptable for laboratory scale experiments. For example, a hydrogen evolution parallel to an electroorganic cathodic reduction can even be advantageous as it improves the mass transfer by moving gas bubbles and thus enhances the organic cathodic reduction. 

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