Research progress on anode anti- CO catalysts for direct methanol fuel cells
Suo Chunguang  Zhao Xiaoguang  Zhang Peng  Liu Xiaowei  Zhang Yufeng
( Harbin Institute of Technology MEMS Center  Harbin 150001, China)
Abstract: From the aspects of binary and multi-component platinum-based alloy electrocatalysts and non-precious metal catalysts , the research work of anode anti- CO electrocatalysts for direct methanol fuel cells in recent years is reviewed . At the same time, the electrocatalytic reaction mechanism of methanol was also discussed , which provided a new direction for further development of new anode electrocatalysts.
Key words: physical chemistry ; direct methanol fuel cell ; anode electrocatalyst ; platinum-based alloy catalyst ; non-platinum-based catalyst
CLC number : TM911.4 ; O646 Document code : A Article ID : 1004-0676 (2008) 03-0066-05
Directmethanol fuel cell (DMFC) is an electrochemical reaction device that directly converts chemical energy stored in methanol fuel into electrical energy. Since the DMFC has a high theoretical specific energy density, compact, rich fuel sources, infrared radiation and low, and has broad application prospects in terms of the portable power supply. Technically speaking , the research and development of DMFC still faces the following challenges : that is, the rate of electrocatalytic oxidation of fuel methanol is slow at normal temperature ; the noble metal electrocatalyst is easily poisoned by CO- based intermediates ; in the long-term use , methanol is easily permeable. through the proton exchange membrane to the cathode, such that the cathode of the oxygen reduction catalyst is reduced, the battery performance. At present , the first method to solve the above problems is to develop an anode electrocatalyst with high activity and anti- CO poisoning ; secondly, it is necessary to develop a new proton exchange membrane to effectively reduce the penetration of methanol. With the direct methanol fuel cell stack is applied to accelerate the process of portable products, which requires the use of DMFC at room temperature and pressure, the electrical properties of the catalyst and higher requirements.
Advances in DMFC anode catalyst of platinum-based catalyst alloy, platinum-based perovskite-based catalysts and non-precious metal catalyst to develop these three areas, we summarize the author highlights, with a view to the development of highly active anodic oxidation of methanol electrocatalysts help.
Methanol oxidation anode electrically Mechanism
The electrochemical oxidation of methanol involving six electrons and six protons release and transfer, which makes the oxidation reaction of methanol on even the best electrocatalyst is not easy to reach equilibrium. The main processes are as follows: ( 1) methanol adsorption and gradual deprotonation to form carbon-containing intermediates ; 2 oxygen-containing species participate in the reaction , oxidative removal of the carbon-containing intermediate product ; 3 product transfer , including proton transfer to the catalyst / electrolyte interface, electron transfer to the outside Circuit and carbon dioxide discharge.
There during the electro-oxidation of methanol may be generated in a variety of intermediates and byproducts, such as methanol deprotonated form CO various species, the catalyst can easily be adsorbed and difficult to desorption, the gradual accumulation on the catalyst surface, the catalyst activity occupies position, reduces the utilization efficiency of the catalyst, the catalyst severe poisoning even cause failure, further hindering the adsorption of methanol and the deprotonation reaction, the reaction off continuity. However, such intermediates can be further oxidized to CO2 on highly active catalysts without degrading fuel utilization.
The type and amount of the intermediate product , that is, the reaction mechanism , are related to the type of the catalyst and the reaction conditions ( methanol concentration, reaction temperature, electrolyte, catalyst, etc. ) . At present, a widely accepted model of methanol electrocatalysis is a bifunctional model [10], that is, two active centers are required on the surface of the catalyst : adsorption of methanol and activation of CH bonds, and deprotonation process is mainly in platinum active sites. carried on, and the water adsorption is activated dissociation on the platinum or other components, the final adsorbed carbonaceous intermediates and -OHads interaction, complete the anodic reaction. Other components may be added to promote the adsorption of the solution on the one hand from the water; electron impact platinum Adsorption of methanol and deprotonated process, the intermediate product is decreased adsorption strength of the metal surface while modified by electronic effect. The addition of different components can achieve the different purposes described above separately or simultaneously.
2 platinum-based alloy electrocatalyst
Improve the catalyst activity is one of the key electrode DMFC promote the development, there are three basic requirements for the material of the anode catalyst: activity, stability, proton and electron conductivity. More current research is based on platinum-based binary or multi-catalyst, some catalysts of development in recent years has made some progress, which has to Pt-Ru represented by binary catalyst to Pt-Ru-W represented A three-way catalyst and a four-way catalyst typified by Pt-Ru-Os-Ir .
2.1 binary catalyst
Although platinum can be used as an electrocatalyst for the oxidation of methanol , its activity is not very high , especially its anti-toxicity is poor. Is generally believed that addition of the second metal, such as Ti group, platinum-based catalyst activity after a slight increase in the group V, and Fe, Cu, Co, Ni no promoting effect, while the platinum-based catalyst activity Group plus Mn, Cr Group highest.
2.1.1 　 Pt-Ru binary catalyst : Among many platinum-based binary catalysts , Pt-Ru catalyst is the most representative, and has the best electrocatalytic effect on methanol oxidation , mainly divided into Pt-Ru supported on activated carbon. /C and unsupported, highly dispersed Pt-Ru catalyst. It is generally believed that the methanol oxidation potential between the platinum surface of 0.2 ~ 0.25V, but at less than 0.5V, pure platinum surface will not form an oxygen-containing species. Compared with the pure platinum surface , Ru atoms can adsorb oxygen-containing species at a potential lower than 0.2 to 0.3 , while Pt-Ru alloys can form oxygen-containing species at a potential of 025 V , which is more favorable for oxidation of toxic intermediates. At a potential of 0.5 ~ 0.7V, the rate of oxidation of methanol is Pt90Ru10 surface 30 times pure platinum surface. The addition of Ru may have two effects : on the one hand , Ru transfers part of the electrons to Pt, weakening the interaction between Pt and CO ; on the other hand , Ru can increase the coverage of oxygen species on the surface of the catalyst , and Ru-O species exist. From the dual function model view angle of the methanol oxidation, it is desirable PtRu catalyst Pt is reduced state and Ru is the oxidation state. Microstructural analysis also found that part of Ru in the alloy catalyst and the active oxygen-containing groups formed amorphous RuOx oxide. Experimental results show that when the oxide and Ru-O bond is weak catalyst contains less when Ru, Pt-Ru catalyst exhibits higher catalytic activity in methanol. The catalytic action of Ru may not be limited to various oxide forms , and even other compounds of ruthenium exhibit a certain catalytic effect on platinum.
2.1.2 　 Pt-Mo binary catalyst: Shropshire first reported Mo adsorbed on the surface of Pt facilitate Electrocatalytic Oxidation of Methanol on Pt electrode. Samjeske [17] Studies have shown that in the process of catalytic oxidation of methanol to CO adsorbed Mo components do not contribute, but it can promote the oxidation of CO adsorbed, thereby compensating for the insufficient content of Ru. Mo component may also pass molybdenum bronze (HxMoO3, 0
2.1.3 　 Pt-W binary catalyst: Pt-W catalyst with a Pt-Mo system, there are many similarities, can quickly change oxidation state during the reaction, has good resistance to poisoning. It was found that only the mechanical mixing of tungsten oxide powder into the platinum black catalyst can significantly improve its anti- CO poisoning ability , and the catalyst has a significant improvement in the catalytic activity of methanol. This may be due to the fact that W has more oxidized valence states and is easily changed between W( IV ) , W( V ) and W( VI ) , contributing to water dissociation. Shukla et Pt WO3-x Different studies show that Pt-W ratio, Pt-W atomic ratio of 3: 1 at the highest activity, when the Pt-W atomic ratio was changed to 3: Catalytic activity at least 2. Mainly the content of WO3-x is too high , covering the Pt active site , greatly reducing the adsorption rate of methanol. However, due to the poor stability of the PtWOx system in acidic media , it has not been practically applied in the industry.
2.1.4 　 Pt-Sn binary catalyst : Pt-Sn is also a promising anode catalyst for electrocatalytic oxidation of alcohols. However , the cocatalytic effect of Sn may be different from that of Ru. Ru is obviously promoted by electrochemical deposition on the surface of Pt or alloy structure with Pt , and the effect of Sn may be different depending on the mode of addition. Has not supported type reports prepared Pt-Sn catalysts were prepared for non-supported Pt-Sn catalysts have some research. Pt-Sn exhibits good activity in the catalytic oxidation of CO , but the catalytic activity of methanol is still controversial . Some people think that the activity is very good . For example , studies by Janssen and Motoo show that electrodeposited Sn on the surface of smooth Pt electrode. After that, the activity of the electrocatalyst to oxidize methanol is increased by 50 to 100 times. It has also been suggested that the addition of Sn has no activity for the oxidation of methanol [12] . Therefore , for Pt-Sn alloy electrodes , the results of different researchers are quite different , which may be related to different electrode treatment methods and different experimental conditions.
In addition to the above binary alloy catalysts , other binary alloys such as Pt-Re , Pt-Pb , Pt-Au , Pt-Rh, etc. also exhibit some activity in the oxidation of methanol , but the most successful binary catalysts studied at present are still Pt-Ru catalyst.
2.2 three-way catalyst
It has been found that the stability and activity of binary catalysts such as PtRu cannot meet the requirements for long-term operation of methanol fuel cells. This may be a battery for a long time operation, the accumulated intermediate product, the loss of noble metal, the metal particles becomes large to reduce active sites of the catalyst caused by other reasons, otherwise alter the chemical state of Ru may result in a reduction in activity of the catalyst Pt-Ru For reasons , the formation of surface oxides may be detrimental to the increase in catalyst activity. Other components, based on the Pt-Ru to increase the activity and stability of the catalyst is the focus of current research. More successful researches include Pt-Ru-W , Pt-Ru-Os , and Pt-Ru-Mo . Gotz et 4BEt3H had a metal salt solution and the reaction to form a metal sol of Pt-Ru-M / C ( M is a metal element) catalyst prepared by N (C8H17), wherein the effect of Pt-Ru-Mo / C ratio of the catalyst Pt- Ru/C is good , and Pt-Ru-Mo/C prepared by the method of sol is better than the catalyst prepared by the stepwise impregnation method using ammonium molybdate as raw material ; Lima investigated the electrodeposition of mixed metal salt in conductive polyaniline Catalytic action of Pt-Ru-M three - way catalyst on the substrate , wherein Pt-Ru-Mo has the best catalytic activity. Pt-Ru-Os is another successful ternary alloy catalyst system. By arc melting method or a chemical reduction of Pt-Ru-Os catalyst prepared by having a face-centered cubic crystal structure, the catalyst surface contains more Pt (111) crystal plane, and Os in the presence of the catalyst surface -OH groups richer , active oxygen is supplied more quickly, and thus the catalyst exhibits a relatively Pt-Ru or Pt-Os higher catalytic activity. The ternary alloy catalyst can significantly reduce the adsorption region of intermediate products such as CO on the catalyst surface , and the single cell effect obtained by using the catalyst is much higher than that of the Pt-Ru alloy catalyst under the same conditions . In addition , it has also been reported that a ternary system such as Pt-Ru-Ni exhibits better catalytic activity than Pt-Ru .
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