Kaname Ito

Photoelectrochemical reduction products of carbon dioxide at metal coated p-GaP photocathodes in non-aqueous electrolytes

Photoelectrochemical reduction of CO2 was carried out using metal-coated p-GaP photocathodes in non-aqueous electrolytes prepared from tetraalkylammonium salts and propylene carbonate as an aprotic solvent. In non-aqueous electrolytes, the coating of Au, In and Pb increased the cathodic photocurrents and the stability of the electrodes, while Zn coating did not show such significant effects. Photoelectrochemical reduction products at a bare p-GaP in non-aqueous electrolytes were (COOH)2, HCOOH, CO and H2, and the faradaic efficiency for CO formation became 50%, in contrast to that in the aqueous electrolytes where it was only a few %. The Photoelectrochemical reduction products at metal-coated p-GaP photocathodes depended greatly on the catalytic properties of the coated metal, i.e. the catalytic effect on the electrochemical reduction of CO2. By Pb coating, the faradaic efficiency for (COOH)2 formation became ca. 50%, and by Au, In or Zn coating, that for CO became almost 100%. The water content in non-aqueous electrolytes affected the faradaic efficiency of each product significantly.

Solid electrolytes with multivalent cation conduction. 1. Conducting species in MgZrPO4 system

Abstract Magnesium ion conducting materials have been found in the system MgZrPO4, and the composition of the material with maximum conductivity has been estimated to be MgZr4(PO4)6 having the same crystal structure as that of NaZr2(PO4)3. The conductivities are2.9 × 10−5 and 6.1 × 10−3Scm−1 at 400 and 800 °C, respectively and the activation energy for conduction is 79.5 kJ/mol. The charge carrier of this material has been confirmed to be magnesium ions by the Tubandts method and the electron probe micro analysis.

Ionic conduction behavior in zirconium phosphate framework

Abstract A series of M I Zr 2 (PO 4 ) 3 (M I ZP;M=Li, Na, K, Rb, Cs and Ag) solid electrolytes have been investigated. The crystallographic and electrochemical properties of M I ZP are compared with those of M II Zr 4 (PO 4 ) 6 (M II ZP). M I ZP and M II ZP crystallized in the NASICON- and β-Fe 2 (SO 4 ) 3 -structures: The crystal structure was determined by the size of the guest cation. The ionic conductivity of the NASICON-type compound was highly sensitive to the size of the guest (i.e., mobile) cation, whereas that of the β- Fe 2 (SO 4 ) 3 -type one was slightly size-dependent. This difference is expected to arise from the difference in the nature of site in which mobile cation is accommodated; a six coordination site of the NASICON-type structure and a four coordination one of the β-Fe 2 (SO 4 ) 3 -type structure. M I ZP showed ca. two or three orders higher conductivity than M II ZP, when they have almost the same ionic radius of mobile cation. AgZP, CdZP, and BaZP, in which mobile cations have high polarizability, showed high conductivities for the size of the mobile cations.

Carbon dioxide sensor using solid electrolytes with zirconium phosphate framework

Abstract We adopted divalent cationic conductor, MgZr4(PO4)6(MgZP), for the first time as the electrolyte of EMF type CO2 gas sensors, having the following cell construction: Pt|CO2, O2| metal carbonate | solid electrolyte |O2, Pt. In the case of sodium carbonate (Na2Co3), the sensor showed the Nernstian response to CO2 in pressure range between 1 and 100 KPa at temperatures from 400 to 500°C within a few minutes.

Reduction of carbon dioxide on partially-immersed Au plate electrode and Au-SPE electrode

Abstract The electrocatalytic reduction of carbon dioxide was carried out on two types of Au plate electrode; one was partially immersed in electrolyte solution containing CO2 and the other was completely immersed. Their surface areas in contact with the solution were kept equal, so that the difference in Faradaic efficiency for the reduction of CO2 to CO between the two types of electrodes was examined. The effects of surface treatments of the electrodes and electrolytes on the Faradaic efficiency were also investigated for each type of electrode. When the electrode surface was treated by electroplating Au on the Au plate, the partially-immersed electrode brought about a higher Faradaic efficiency for CO2 reduction than the completely immersed one both in 1 mol dm−3 KHCO3 and in 0.1 mol dm−3 (C2H5)4NClO4 aqueous solution at room temperature. The Au-SPE (SPE: solid polymer electrolyte) electrode was applied as a gas diffusion electrode to the reduction of CO2, and the Faradaic efficiency for CO production was compared with that obtained by the partially-immersed Au plate electrode. The Au-SPE electrode gave rise to a higher Faradaic efficiency than the partially-immersed electrode.

Carbon dioxide sensor using solid electrolytes with zirconium phosphate framework (2). Properties of the CO2 gas sensor using Mg1.15Zr4P5.7Si0.3O24 as electrolyte

Abstract We report EMF type, all solid-state CO 2 gas sensors using Mg 2+ ion conductor in Mg 1.15 Zr 4 P 5.7 Si 0.3 O 24 [MgZrPSiO], having a cell construction of Pt¦CO 2 , O 2 ¦metal carbonate (Na 2 CO 3 or K 2 CO 3 )∥solid electrolyte ¦O 2 , Pt. The sensors showed Nernstian response to CO 2 in pressure range between 0.5 and 60 kPa, at temperatures from 300 to 500 °C. A change in solid electrolyte from MgZr 4 (PO 4 ) 6 to MgZrPSiO brought about a considerable reduction in response time.

Electrochemical Reduction Of Carbon Dioxide Using Gas Diffusion Electrodes Loaded With Fine Catalysts

Gas diffusion electrodes (GDEs) promising to use electrochemical reaction of gases at higher rates such as in fuel cells were used for CO2 electro‐reduction. Cu powder, Cu and ZnO mixed powder, and CuO/ZnO mixed oxides were examined as the catalysts. The reduction products with Cu‐GDEs in K2SO4 aq. soln. were CH4, C2H4, C2H5OH, CO, HCOO−, etc. The purer and the smaller particle size of the Cu powder, it shows the higher activity and efficiency for CO and hydrocarbons formation. CO2 reduction can be carried out ca. 100 times greater rate using a GDE than that with a Cu‐foil electrode. With GDEs of CuO/ZnO = 3/7, the main product was C2H5OH with an efficiency of 17% with selectivity of 88% at −1.32 V. With GDEs of Cu/ZnO = 3/7 reduced by H2, the selectivity became poorer due to additional production of n‐C3H7OH and C2H4, although the total efficiencies for CO2 reduction became 41% at −1.30 V.

Solid electrolytes with multivalent cation conduction (2) zinc ion conduction in ZnZrPO4 system

Abstract Zinc ion conducting materials have been found in the system Zn-Zr-PO 4 , and the composition of the material with the maximum conductivity has been estimated to be ZnZr 4 (PO 4 ) 6 having the similar crystal structure as that of MgZr 4 (PO 4 ) 6 . The conductivities of this compound are 1.34×10 −6 and 1.57×10 −3 S/cm at 500 and 900°C, respectively, and the activation energies for conduction are 90 kJ/mol between 750 and 900°C and 134 kJ/mol between 500 and 700°C, respectively. The charge carrier of this material was confirmed to be zinc ions by the Tubandts method and the electron probe micro-analysis.

Complex formation of beryllium(II) with salicylate and hydroxide ions in 1 mol dm–3 sodium perchlorate aqueous solution at 25 °C

The complex formation of Be2+ with salicylate (L2–) and hydroxide ions was investigated at 25 °C in 1 mol dm–3 NaClO4 by potentiometry using a glass electrode. The e.m.f. measurements were performed under the conditions 2.7 ⩽–log [H+]⩽ 8, 0.002 ⩽cM⩽ 0.01 mol dm–3, 0.002 ⩽cL⩽ 0.015 mol dm–3, and 1 ⩽cL/cM⩽ 7, where cM, and cL, denote the total concentrations of BeII and salicylic acid, respectively. The e.m.f. data were explained with the three major species [Be(HL)L]–, [BeL], and [BeL2]2– and at least one or even all of the four minor species [Be(HL)]+, [Be(HL)2], [Be(OH)L]–, and [Be3(OH)3L3]3–. The hydrolytic reactions of BeII in 1 mol dm–3 NaClO4, data on which were used for the analysis of the above beryllium(II)–salicylate-hydroxide system, were also studied by separate potentiometric titrations. It was found that [Be2(OH)]3+, [Be3(OH)3]3+ and [Be6(OH)8]4+ occur in 1 mol dm–3 NaClO4 aqueous solution.

Electroreduction of CO2 using Cu/Zn oxides loaded gas diffusion electrodes

Gas diffusion electrodes (GDEs) consist of a gas layer (mixture of hydrophobic carbon black (CB phob ) and PTFE dispersion) and a reaction layer (mixture of catalyst powder, CB phob , hydrophilic CB (CB phil ), and PTFE) laminated on a Cu mesh as a current collector. As the catalyst, CuO/ZnO (3:7 by mole ratio) mixed oxides and a mixture of a Cu powder (4N, -325 mesh) and a ZnO powder were examined. Electroreduction was performed potentiostatically passing in general 200 C using a W -shaped Pyrex cell having one gas and two liquid chambers, with two lines of gas circulating systems. When using a GDE of (CuO / ZnO = 3 / 7 : CB = 6 : 5 [by weight], the reduction products were mainly C 2 H 5 OH with slight amounts of CO and HCOO - , and a comparable amount of H 2 . Faradaic efficiency maximum of 16.7% for C 2 H 5 OH formation with maximum selectivity of 88% was observed at -1.32 V vs. Ag-AgCl, at a partial current density of 4.23 mA/cm 2 , which is about 50 times greater than that obtained on a sintered oxide electrode. In the case of the GDE of (Cu / ZnO = 3 : 7) : CB = 3 : 1 reduced by H 2 , the selectivity of the reduction products became poorer, like in the case of a Cu foil electrode, with lower current density, although the total faradaic efficiencies for CO 2 reduction was 40.5% with additional formation of n-C 3 H 7 OH and C 2 H 4 at -1.30 V.