A Study Of Non-Pt Catalysts For Oxygen Electrode In Alkaline Medium

As high-efficient and clean energy conversion devices, fuel cells and water electrolysis can solve the two biggest challenges facing human society, namely energy shortage and environment protection. As for fuel cells system, in order to maximize its power output and energy efficiency, it is necessary to speed up the reaction rate of the anodic fuel and the cathodic oxidant in the electrochemical processes, which can be achieved by developing high efficient catalysts. Despite intense research on the catalysts for fuel cells and water electrolysis, the further development of electrochemical energy conversion technology is restricted due to the excessive reliance on rare noble metal catalysts and the corrosion issue of carbon support materials under operating conditions. This work is carried out without the use of carbon substrates, and studied non-Pt group metal catalysts and complex metal oxides catalysts in oxygen reduction reactions (ORR) and oxygen evolution reactions (OER). The main research progresses are listed below.1. Manipulation of the surface reactivity of Ag and its application in ORRCombined computational and experimental studies, the surface reactivit of Ag was successfully regulated. Density functional theory (DFT) calculation points to a peculiar result:by covering a monolayer of Ag onto metals more active than Ag, the surface reactivity of Ag decreases; only when covering Ag onto even more inert Au(l 11), can the surface become more active. To validate this theoretical prediction, we obtained Au electrodes with monolayer of Ag on top, via the method of Pb underpotential deposition and subsequent replacement reactions. The Au@Ag electrode turns out to be superior to both Au and Ag electrodes in terms of catalytic activity toward the ORR in alkaline media. ORR takes place via the 4-electron pathway on the Au@Ag electrode and the half-wave potential turns out to be 50 mV more positive than that of usual Ag electrode. The kinetic current density of Au@Ag is 7 times higher than Ag bulk electrode. SEM characterizations of the electrodes show that both the Au@Ag electrode and the original Au electrode take on a particle morphology with essentially unchanged particle size, which is consistent with the observation of unchanged Pb UPD charge upon Ag coating. The electronic property of Ag, however, has evidently been altered. As revealed by the XPS signal, the binding energy of Ag 3d electron shifts from the standard value by-0.28 eV. Therefore, the weakened electron binding energy explains the much enhanced reactivity of such a Ag surface layer to oxygenate species acting as electron acceptor.2. The synthesis of PbOx-MOx complex metal oxides and their applications in electrocatalysisA series of PbOx-MOx complex metal oxides were synthesized by electrochemical model evaluation and the controllable synthesis of powder, and their electrocatalytic activities were then studied. We found that the ORR catalytic activity of PbOx-MOx prepared by electrochemical methods is improved greatly, compared to either MnO2 or PbO2. It is estimated from XPS results that the Pb mainly exists in the form of PbO2 for PbOx-MnOx oxide, which has a higher electrical conductivity favorable for electron transfers. More Mn3+/Mn4+ redox couple are thus available with the oxidation states of Mn lowered, which increases the catalytic active sites for ORR. When the oxides were synthesized by the oxidative calcination method and the freeze-drying method, it was found that the PbOx-MOx oxides show higher ORR catalytic activity as the ratios of Pb to Mn are 1:1 and 1:10, respectively. Meanwhile, the PbOx-MOx oxides synthesized by chemical replacement method have superior catalytic activity for OER. In addition, because of the complete absence of the carbon supports, the PbOx-MOx oxides showed significant advantages in terms of corrosion resistance and volumetrice activity.3. Synthesis of SnOx-MOx complex metal oxides and their applications in electrocatalysisSnOx-MOx complex metal oxides were synthesized by powder synthesis methods, e.g. oxidation, hydrother-mal and etc., and their electrocatalytic activities were then studied. It was found out that the complex metal oxides have bi-functional activities and the ORR and OER activities maximized when the ratio of Sn to Mn is 1:4.5, with a half-wave potential of ORR only 15mV more negative than commercial Pt/C catalysts, and the potential at 10mA cm-2 in OER reactions is only 100mV more positive than Ir black catalyst which is the best OER catalyst. The catalyst also demonstrated very high stability in the catalytic reactions. It is estimated from XPS that the presence of SnO2 powders changes the oxidation states of Mn oxides during the synthesis process and the best catalytic activity was reached when the ratio of MnO2 to Mn2O3 in SnOx-MOx is 1:0.372. UPS results also show that the decrease of the electron binding energy in SnOx-MOx improves the surface catalytic activity. The implementation of the SnOx-MOx catalysts in alkaline solid state fuel cells demonstrated a peak power density of 145 mW/cm2, which is one of the highest values in all alkaline solid state fuel cells using non-noble metal catalysts.4. Synthesis of perovskite oxides and their applications in electrocatalysisA series of perovskite-type oxides are synthetized by citric acid chelating-combustion method, and their electrocatalytic activities were then investigated. Firstly, the optimal conditions for the preparation of perovskite oxides are obtained through adjusting the synthetic procedures, changing the calcination temperature and the weight ratio of citric acid. Secondly, the catalytic activity is tuned by changing the elements and their ratios in perovskite structures. The experimental results show that the chemical environment of the B-site element in perovskite can be adjusted by the A-site element, through which the catalytic activity of the perovskite can be changed. In the absence of carbon supports, the activity of double perovskite LaBaMn2O6 for ORR is close to that of Pt and is very stable. Besides, layered perovskites, such as LaSr3MmO10 LaSr2MmO7 and LaBa3Mn3O10, are also found to exhibit high catalytic activity toward the ORR.