Preparation Of Ag/C Catalysts By [Ag(L)(bbi)] Coordination Polymer And Reduction Reaction Of Oxygen On Ag/C

Silver is one of the typical catalysts of oxygen reduction. Ag has a reasonably high catalytic activity for O2 reduction in alkaline electrolyte. Furthermore, it is a good catalyst for perhydroxyl ion decomposition. Therefore, Ag is known as the ideal catalyst in the fuel cell and metal-air battery. Ag particles are easy to accumulate into larger particles in the process of preparation and usage, and therefore it is difficult to prepare finer and well-dispersed Ag particles on the carrier of catalysts. So the key to Ag as the catalysts lies in raising the specific surface area of Ag as far as possible and making Ag particles dispersed evenly onto the carrier of catalysts. In the present study, the Ag(â… ) coordination polymer reduction method was introduced to prepare Ag/C catalysts, and the preparing condition of the catalysts was examined. The Ag/C catalytic activity and the reduction reaction of O2 on Ag/C were explored. The performance of the oxygen electrode was measured using Ag/C as catalysts for the direct sodium borohydride fuel cell and the zinc-air batteryBased on Ag(â… ) sulfonate complexes ([Ag(L)]), the coordination polymers of [Ag(L)(bbi)] and [Ag(tpp)₂(L)]·C₂H₃N have been synthetized using 1,1′-(butane-1,4-diyl)- diimidazole(bbi) and triphenylphosphine(tpp) as ligands by the conventional solution method. The crystal structures of [Ag(L)(bbi)] and [Ag(tpp)₂(L)]·C₂H₃N coordination polymers were obtained on a Rigaku RAXIS-RAPID diffractometer. The literature retrieval proved that the [Ag(L)(bbi)] and [Ag(tpp)₂(L)]·C₂H₃N coordination polymers were the new single crystals. The effects of acidity, solvent and time on the growth of the two coordination polymers were examined in the exploration of synthesis conditions. The tests for acid- and base-resistance and temperature-resistance were carried out on the properties of [Ag(L)(bbi)] and [Ag(tpp)₂(L)]·C₂H₃N coordination polymers.Ag/C catalysts were prepared by the Ag(â… ) coordination polymer reduction method through [Ag(L)(bbi)] and [Ag(tpp)₂(L)]·C₂H₃N coordination polymers using NaBH4 as the reductant. The coordination polymer of [Ag(L)(bbi)] was selected as the object of study by comparing the electrocatalytic activity of Ag/C catalysts. The effects of the particle sizes of [Ag(L)(bbi)] powders on the electrocatalytic activity of Ag/C catalysts and the ultrasonic vibration time on the dispersity on Ag particles on the carrier were examined. By characterizing Ag/C catalysts throuhg X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and scanning electron microscopy (SEM), the result showed that the reduction degree of Ag(â… ) ions in [Ag(L)(bbi)] coordination polymer was high preparing Ag/C catalysts by the Ag(â… ) coordination polymer reduction method, and the average particle size of Ag in Ag/C catalysts prepared by the Ag(â… ) coordination polymer reduction method was smaller than that of Ag in Ag/C catalysts prepared by the Ag2O reduction method, and that the size of Ag particle in Ag/C catalysts prepared by the Ag(â… ) coordination polymer reduction method was smaller and regular in shape compared with Ag/C catalysts prepared by the Ag2O reduction method. The cause of the significant enhancement of electrocatalytic activity was discussed for Ag/C catalysts prepared by the Ag(â… ) coordination polymer reduction method compared with Ag/C catalysts prepared by the Ag2O reduction method.The effect of the particle size of Ag in Ag/C catalysts was investigated on the pathway of oxygen reduction in alkaline electrolyte using the cyclic voltammetry. The relationship was examined between the Ag particle size and the overpotential of the oxygen reduction reaction. The result indicated that the oxygen reduction reaction proceeded by the four-electron pathway mechanism on larger Ag particles, and proceeded by the four-electron pathway and the two-electron pathway, simultaneously, on finer Ag particles. The kinetics of oxygen reduction reaction was studied on Ag/C catalysts in alkaline electrolyte through the rotating disk electrode method. The experimental results showed that the finer Ag particles were beneficial to the oxygen reduction reaction proceeding by the two-electron pathway, and the larger Ag particles were beneficial to the oxygen reduction reaction proceeding by the four-electron pathway in alkaline electrolyte. According to the Koutecky-Levich equation, the electron number and the rate constant of the oxygen reduction reaction were determined, and the ratios of the oxygen reduction reaction of the four-electron pathway and the two-electron pathway were obtained on Ag/C catalysts.The direct sodium borohydride fuel cell and zinc-air battery with flowing alkaline electrolyte were assembled using 10wt%Ag/C prepared by the Ag(â… ) coordination polymer reduction method as catalysts of the oxygen electrodes. The morphology of the catalyst layer of the oxygen electrode was observed by a VEGA51365B scanning electron microscopy(SEM), and the surface morphologies of the catalyst layers of the oxygen electrode and the Au/C electrode were observed by a HIROMX—5040RZ stereomicroscopy. The effects of the concentration and flow rate of NaBH4 solution were examined for the sodium borohydride fuel cell, and the performances of the oxygen electrode and Au/C electrode were discussed through the polarization curves of electrodes and electrochemical impedance spectroscopies(EIS). The performances of the oxygen electrode and zinc electrode were discussed for the zinc-air battery through the polarization curves of electrodes. The results indicated that the polarizations of the anodes are smaller and less effected by the temperature, and the polarizations of the oxygen electrode are larger and declines with rising temperature in the sodium borohydride fuel cell and zinc-air battery. The power densities of the sodium borohydride fuel cell and zinc-air battery were measured at different temperatures, and the electro-oxidation efficiency of the NaBH4 fuel was determined.