[Abstract] This paper introduces the advantages of direct alcohol fuel batterys (DAFCs) with low operating temperature, electrolyte solid state, high energy conversion rate and high specific energy density. The existing anode catalysts (platinum) of DAFCs and their improvement schemes are discussed. The preparation method and catalytic performance of Pt-based catalysts with high catalytic activity, non-Pt-based precious metal catalysts and other catalysts are pointed out. It is pointed out that improving the catalytic performance of anode catalysts will be the research focus of DAFCs.
Key words: direct alcohol fuel battery, anode catalyst, catalytic performance
CLC number: TM911.4 Document code: A Article ID:1009-914X(2016)30-0321-03\u003cbr \u003e Direct alcohol fuel batterys (DAFCs) are one type of alkaline fuel batterys. Directly use methanol, ethanol, ethylene glycol, glycerol, etc. as fuel [1, 2]. For portable commercial applications, performance metrics such as size, volume, energy density, and power density of fuel batterys are particularly important. As a kind of proton exchange membrane fuel battery, DAFCs have attracted the attention of scholars because of their low operating temperature, solid electrolyte and high energy conversion rate compared with other fuel batterys, especially in mobile applications  , 4]. In addition, DAFCs have the advantages of easy transportation and handling, no complicated steam reforming unit, traditional fuel infrastructure, and simple start-up.
The structure and nature of electrode materials play an important role in organic fuel batterys. In the past, DAFC electrodes used nickel-based catalysts sometimes using active platinum. Pt/C gas diffusion electrodes are now commonly used as cathodes and anodes . However, the following main problems exist: Pt-based catalysts are susceptible to CO poisoning in alcohol oxidation and cathode side oxygen reduction reactions have low electrocatalytic activity, and Pt is expensive and non-renewable . Therefore, the design and development of new catalysts to meet the requirements of reducing the Pt loading while increasing the performance and stability of the catalyst is imminent .
The article reviews the research progress of anode catalysts for direct alcohol fuel batterys.
1 Pt-based catalysts
At present, Pt and its alloys are still the most practical catalysts in DAFCs. Although non-Pt-based catalysts have been developed, there are no alternatives for commercial mass production. In the past few decades, the main research directions for Pt-based catalyst improvement have two points: (1) to avoid their common shortcomings, such as rapid aging in fuel battery operation. (2) Reduce the amount of Pt at a constant efficiency. The specific measures are:
(1) Control the shape of Pt nanomaterials. The synthesis of new nanostructured materials is the main development direction for optimizing existing Pt-based catalysts. For example, Liang et al. used ultrathin Te nanowires as a reducing agent and a sacrificial template to synthesize Pt nanotubes and nanowires in ethylene glycol ; Sun et al. passed at room temperature without using surfactants or templates. The chemical process synthesizes Pt nanoflowers ; Ren et al. synthesizes Pt nanocubes by decomposing Pt precursors in a hydrogen atmosphere ; Bi et al. directly add Ag nanowires and H2PtCl6 in standard NaI solution to synthesize Pt nanotubes [ 11]. These various shapes of Pt nanomaterials have strong structure, spacious depressions, continuous flow channels, large surface area, high electrical conductivity, high electrocatalytic performance, and the ability to eliminate or mitigate catalyst polymerization and recombination problems  ], exhibits exbatteryent catalytic performance in direct fuel batterys.
(2) Reduce the load of Pt. The formation of alloys of Pt with suitable noble metals such as Ru, Ir, Pd, Rh, Os, Ag, Au or with pro-transition metals such as Cu, Fe, Co, and Ni as electrocatalysts has attracted widespread interest among researchers [12, 13 ]. For example, Xia et al. used a one-pot synthesis of PtCu nanocage in an organic solution system. Compared with a single-component Pt electrocatalyst, the synthetic material has more available surface area and a special hollow structure to catalyze methanol oxidation. It shows better catalytic performance . Lim et al. used ascorbic acid to reduce K2PtCl4 to synthesize Pd-Pt bimetallic nano-branches in the presence of aqueous solution and Pd nanocrystal seeds. The synthetic material showed better catalysis in oxygen reduction than Pt black and Pt-C. Effect . Nassr et al. used microwave-assisted polyol reaction to synthesize PtNi nanoparticles supported on carbon nanotubes. Compared with commercial Pt-C, the synthetic material has higher activity per unit mass in methanol oxidation reaction and stronger anti-CO poisoning ability. . These Pt-based bimetallic catalysts exhibit enhanced electrocatalytic performance in fuel batterys, primarily due to bifunctional mechanisms and synergistic or electronic effects .
2 Non-Pt-based precious metal catalysts
Although Pd is about five times lower in mass activity than Pt, it is the second active metal in fuel batterys [17, 18]. The valence electron structure and lattice constant of Pd are similar to Pt, but the price is relatively cheap and the amount on the earth is relatively abundant. At present, Pd nanomaterials with various morphologies have been successfully synthesized. For example, Nadagouda uses vitamin B2 as a reducing agent and a scavenger at room temperature. When the sample is prepared using acetone and acetonitrile as a solvent, Pd nanorods are synthesized, and isopropanol is used. Pd nanowires were synthesized in solvent ; ??Cui et al. used a one-step electrochemical method to synthesize Pd nanotubes in anhydrous dimethyl sulfoxide solution ; Jin et al. used seed-mediated method (Pd nanocube as seed, formaldehyde) As a reducing agent), polyhedral Pd nanocrystals were synthesized ; Ksar et al. slowly irradiated divalent Pd particles to synthesize urchin-like Pd nanostructures in a cetylpyridinium chloride mibatterye solution. In addition, Pd-based catalysts exhibit extremely high activity in the oxidation of alcohols and reduction of oxygen in alkaline solutions , making them promising catalysts for DAFCs.
Au has been reported as a catalyst to catalyze the oxidation of methanol , ethanol , isopropanol  and glycerol [26, 27] in an alkaline medium. Moreover, the Au catalyst also exhibits high activity in an oxygen reduction reaction in an alkaline medium [28, 29]. For example, Xu et al. use regenerated silk protein as a reducing agent and a binder ("bundle" Au nanoparticles with graphene), and synthesize Au nanoflowers supported on graphene in a one-pot method. Excessive catalytic performance; Feng et al. Electrochemical deposition of dendritic Au nanocrystals with the help of ethylenediamine, the composite material has good catalytic performance for ethanol oxidation .
In addition, precious metal Ag catalysts have also been reported for fuel batterys. According to Guo et al., in the alkaline solution, if the metal loading in the Ag/C catalyst is from 10% to 60%, the number of electron transfer in the oxygen reduction reaction increases from 2e- to 4e-, and the catalytic performance is found to be optimal. The metal loading in Ag/C is 60% . In addition, alloys formed between these metals (Au-Pd, Au-Ag, Pd-Ag) also exhibit exbatteryent electrocatalytic properties in DAFCs [31, 32]. Lv et al. used L-glutamic acid as a growth directing agent to synthesize wheat-like Au-Pd heterostructures by one-step electrochemical co-deposition. The current density of synthetic materials in methanol oxidation was 45.8 mA cm-2, much higher than the same. Conditions of pure Au (5.4 mA cm-2) or pure Pd (25.0 mA cm-2)  catalyst. Oliveira et al. deposited Ag and Pd on a stainless steel disk and synthesized Pd-Ag alloys with different Ag contents (8%, 21%, 34%). It was found that when the Ag content was 8%, the Pd-Ag alloy reacted with oxygen. It has the best catalytic ability and is resistant to ethanol. When the Ag content is 21%, the Pd-Ag alloy has the best catalytic ability for ethylene glycol oxidation . Shim et al. synthesized Ag/Au/AgCl composites by electrical displacement reaction between Ag nanowires and different concentrations of Au precursors. It was found that the morphology of Ag/Au/AgCl composites was improved by the increase of Au precursor concentration. The shell solid line structure becomes porous hollow linear structure, and the Ag/Au/AgCl component ratio is 4:86:10, which has the best catalytic ability for oxygen reduction reaction in alkaline solution .
3 Other Catalysts
In 1964, Jasinski first reported the application of cobalt phthalocyanine to carbon electrodes to form a non-precious metal catalyst capable of catalyzing oxygen reduction in an alkaline solution . Subsequently, Jahnke discovered that this catalyst also has catalytic activity in acidic electrolytes . In the following decades, Co- and FeN4 macrocyclic chelate catalysts have been studied to catalyze oxygen reduction in alkaline or acidic media . Despite its poor stability, subsequent studies have shown that high temperature heat treatment of these compounds in inert gases can improve their stability and activity. In addition, metal oxides, cermet-based compounds, and metal carbonyl compounds have been studied as catalysts for DAFCs.
In recent years, people have been working on the application of metal-free catalysts to fuel batterys. Theoretical calculations and detailed experiments show that the introduction of N (or P, B, S) atoms into the sp2 hybrid carbon framework in graphene (or carbon nanotubes, ordered mesoporous carbon) can improve their electrochemical properties. Recent studies have shown that these carbon materials not only exhibit high catalytic activity, long-term stability and good methanol resistance in alkaline media, but also have low price and environmental protection, and are expected to be substitutes for Pt-based catalysts in fuel batterys [38, 39]. ]. For example, synthetic nitrogen-doped graphene  and Yan et al. synthesized sulfur-doped graphene  in an alkaline fuel battery, the oxygen is directly reduced to water by a four-electron route, and its electrocatalytic performance. Long-term operational stability and resistance to cross-infiltration are better than Pt-C. Dai et al believe that the reason why N-doped carbon materials have high activity may be that the electronegativity of N atoms (electronegativeness is 3.04) is larger than that of C atoms (electronegativeity is 2.55), which is adjacent to C atoms. Produces a positive charge density that is more favorable for adsorption of O2 . However, some doping other atoms with lower electronegativity or equivalent to the electronegativity of C atoms (such as the electronegativity of P is 2.19 , B is 2.04 , S is 2.58 ) carbon materials. It also has significant catalytic activity, so the reason why the carbonaceous material has high activity requires further research.
DAFCs have attracted more and more attention as a new energy source. Compared with other types of fuel batterys, DAFCs have the advantages of low operating temperature, solid electrolyte, high energy conversion rate and high specific energy density. Pt catalysts are the most commonly used catalysts in DAFCs because of their high electrocatalytic activity for oxygen reduction and alcohol oxidation. However, Pt catalysts face some disadvantages such as slow kinetics, low efficiency, high cost and limited supply. In this way, the researchers' current research focuses on the development of new, lower cost catalysts with good catalytic performance, stability and durability. In order to achieve this goal, it is necessary to consider: (1) The size of the Pt-based nanocatalyst needs to be reduced to provide a larger electrochemical active area. (2) Controlling the synthesis of Pt-based catalysts with more complex morphology. (3) The nanocatalyst preferably has a high index crystal face. (4) Designing a Pt-based bimetallic or trimetallic nanocatalyst having a controllable structure. (5) Design efficient non-Pt multi-metal catalysts. (6) Looking for new carrier materials with high electrical conductivity, chemical stability and surface area. (7) Effectively controlling the metal particles to be uniformly dispersed on the surface of the new carrier.
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