Oxygen Evolution And Reduction On Non-Precious Metal Catalysts

Water electrolysis and PEMFCs have been recognized as the basic and key technology in hydrogen energy field. However, their development are hampered by the excessive reliance on rare noble metals, which have been considered as the best catalyst for the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). The cheap price and high catalytic activity allow the non-noble metal catalysts to be the alternative to noble metals, whereas several scientific and technological problems are still unsolved for these catalysts. For OER catalyst, owing to the effect of analysis technique, prepared method and uncontrolled factors in prepared process, difference and dispute still exist in OER mechanism and the relationship between morphology, surface area, conductivity, catalytic site and catalytic activity. In addition, most OER research has been done in alkaline and acid media, while few reports refer to materials for OER in neutral water. For ORR catalyst, although the reported catalyst, such as transition-metal macro cyclic complexes, transition-metal chalcogenide complexes and so on, exhibited attractive catalytic activity and reliability, they are not to be compared with the noble platinum, hence PEMFCs still faces the difficulty in design and fabrication of non-noble metal cathode catalyst with high activity and reliability. Based upon the above problems, in this dissertation, the OER and ORR catalysts are prepared for using in different electrolytes, while their catalytic performance and mechanism are also studied.Firstly, the Cu-Co complex oxides with different Cu/Co ratios have been prepared onto titanium support by reactive DC magnetron sputtering. By changing the sputtering power of Co target and Cu target, the deposited complex oxides show three crystal structures of monoclinic (Cu_1-xCo_xO), Cu₂CoO₃ and spinel (CuCo₂O₄), respectively. Experimental results reveal the doping of foreign element can obviously enlarge the surface area of the deposited oxides, furthermore, with increasing content of doped element, i.e., with different Cu/Co ratios, the complex oxides exhibit distinctly transformation in the morphology and crystal structure. Based on the electrochemical determination, the increase OER activity could be ascribed to the enlarged surface roughness, nevertheless, further analysis demonstrates that the new active sites formed during doping process are the main factor for the improvement of catalytic activity. In addition, compared with single oxides, although doping can decrease the oxide resistance (R), it seems that R has no obviously effect on the electrochemical performance of these oxides, according to the EIS. By comparison, the Cu_1-xCo_xO and Cu₂CoO₃ with low Co content show better performance in catalysis of the OER than the conventional CuCo₂O₄, moreover, the deposited Cu1.99Co1.01O3 oxide even show catalytic activity corresponded to that of RuO₂.Secondly, the CuO electrodes have been prepared by reactive DC magnetron sputtering. Experimental results show that the deposited CuO, with three low index surfaces of (110), (111) and (-111), is capable of catalyzing OER. Based on the above results, the most stable CuO(111) has been chosen to study the adsorption-desorption behaviors of various oxygen species by density function theory (DFT). The calculation results reveal that H2O, Oads, OHads and OOHads preferentially bind to unsaturated three-coordinate Cu atom, and form strong chemical bond with CuO(111) surface, except O₂, which has weak interaction with the CuO surface. Finally, the OER mechanism on CuO(111) surface has been proposed, according to the adsorption geometries of various oxygen species and reaction energy of two subsequent intermediates.Thirdly, cobalt hydroxide (Co(OH)₂) was synthesized by DC electro-deposition for the first time using an ethanol additive. The presence of ethanol in the preparation process shows obviously tailor-effect on morphology of the deposits and finally causes the cactus-like shape and amorphous structure of the deposits. Based on the analysis of deposits, the synthesis mechanism of Co(OH)₂ is proposed, which indicates the generation of alkoxy under cathode reduction potential may play a key role in the catalyst formation. The catalytic activities have a sharp decline after anneal process, attributing to the crystal transformation and surface shrinking during anneal. In the neutral electrolyte, the amorphous Co(OH)₂ electrode exhibits better catalysis for OER than the commercial Pt/C and nickel foam. The electrochemical results reveal that the catalyst performance is limited by the delayed oxidation of cobalt ion and the lack of OH- radicle in neutral solution. At low overpotential, the lack of Co³⁺/Co⁴⁺ activity sites must control the OER, while, at high overpotential, the electrochemical reaction step must be the control step of OER.Finally, the reactive DC magnetron sputtering is used to prepare C-N and metal doped C-N catalyst on titanium support. The doping of nitrogen (N) element results in an amorphous C-N catalyst with enlarged surface area. The proper doping content of N generates various species of activity sites for ORR, such as graphite-like C=C and pyridine-like C=N, and enhances the catalyst performance, whereas excessive doping content will cause the decline of catalytic activity. The best ratio of C: N obtained in the experiment is 4.1:1. The doping of Fe or Co element can obviously increase the C-N catalysts performance both in onset potential and current density of ORR, but excessive metal content will decrease their catalytic activity due to the inactive matter formed on the surface. Based on the chemical composition and electrochemistry analysis, the enhanced performance of M-C-N (M: Fe or Co) catalyst could be ascribed to three reasons: first of all, the enlarged surface caused by the metal doping; secondly, increased content of graphite-like C=C and pyridine-like C=N; thirdly, the forming of MNx (M: Fe or Co; x: 2 or 4) activity sites which similar to metal phthalocyanine or metal porphyrin. Comparison test of M-C-N and M-C demonstrated the chemical bond of N-M induced by N doping plays a key role for stability of the metal doped catalyst in acid. Among the four amorphous catalysts, metal doped M-C-N shows the best catalysis of ORR; M-C and C-N catalyst exhibits weak catalysis of ORR; M-N is unstable because it easily dissolves into acid solution.