ReviewRecent progress in the development of anode and cathode catalysts for direct methanol fuel cells
Graphical abstract
Introduction
Direct methanol fuel cells (DMFCs) have attracted considerable recent interest because of various advantages, including high power density, zero or low exhaust, ease of recharging, simple structure, and quick startup at low temperature [1], [2], [3]. However, one of the major drawbacks of the DMFC is its high manufacturing cost, which prevents their successful commercialization [3]. By initiating cost-effective steps from prototypes to mass production, manufacturing expenses can be reduced; however, the expensive materials required for the production of DMFCs remain a challenge because DMFCs operating at low temperature use platinum (Pt) and its alloys for the conversion of fuel at the anode and reduction of oxygen at the cathode. The high cost of Pt significantly increases the total price of the fuel cell devices. The problem can be solved in the short term by using single-, double-, and multiple-component catalysts and new catalyst supports. Therefore, the study of single-, double-, and multiple-component catalysts and new catalyst supports not only has emerged as a significant area of research and development but also has greatly influenced everyday life, as more products based on single-, double-, and multiple-component catalysts and new catalyst supports are increasingly introduced to the market. Extensive research in the past two decades has established that the physical and chemical properties of materials show significant changes if their dimensions are on the nanometer scale, which opens new avenues for a wide range of future applications [3], [4]. Especially, physical properties of nanostructures, such as large surface area and novel size effects, markedly improve the efficiency of DMFCs [3], [4]. Therefore, single-, double-, and multiple-component catalysts and new catalyst supports have become increasingly important in the development of DMFCs in recent years. In addition, there are two main types of effects that result from single-, double-, and multiple-component catalysts and new catalyst supports: (1) ‘trivial size effects’, which rely solely on the increased surface-to-volume ratio and decreased layer thickness and volume of the nanoparticles and (2) ‘true size effects’, which also involve changes in local material properties [5], [6]. Therefore, nanoscale engineering of the material appears to be critical in the next stage of advancement of DMFCs. Development in recent years has shown that single-, double-, and multiple-component catalysts and new catalyst supports have great potential for innovative new technology for the recurring energy demand.
In this regard, the review article is mainly focused on recent developments in the field of DMFC anode and cathode catalysts; the role of single-, double-, and multiple-component catalysts and new catalyst supports for non-alloy and alloy nanoparticles is particularly discussed in more detail. In addition, challenges involved in the development of DMFCs and the reaction mechanism for methanol oxidation and oxygen reduction are subsequently discussed in more detail.
The review article briefly describes the principle of DMFC operation. Subsequently, challenges involved in DMFC anode and cathode electrocatalysts are described. In Section 3, a brief summary of the reaction mechanism for methanol oxidation is reported. Section 4 describes the reaction mechanism for oxygen reduction. Anode catalysts of DMFCs, Cathode catalysts of DMFCs address recent progress in the development of anode and cathode electrocatalysts for DMFCs. Anode catalysts of DMFCs, Cathode catalysts of DMFCs particularly focus on the role of single-, double-, and multiple-component catalysts and new catalyst supports with non-alloy and alloy nanoparticles in the development of highly efficient DMFCs. The final section will describe the conclusions. Some open problems and continuing challenges are also highlighted in the final section.
Section snippets
Principle of DMFC operation
The contemporary science and technology of DMFCs have already been extensively reported in many review papers and articles; interested readers can refer to references [7], [8], [9] for more details. In short, the main active components of a DMFC are the fuel electrode (anode), oxidant electrode (cathode), and an electrolyte sandwiched between them. Figure 1 shows the basic operational principle of a fuel cell with its reactant or product gases and ion conduction flow directions. A schematic
Reaction mechanism for methanol oxidation
Over the past several decades, materials scientists have sought to improve their knowledge of methanol oxidation mechanisms at different electrocatalysts under perfectly well-controlled conditions, such as different single crystal orientations and foreign metal clusters on polycrystalline or single crystal surfaces [10], [16], [17], [18]. The basic mechanism for the methanol oxidation reaction (MOR) was first reviewed in 1988 [19]. Based on previous reports, the reaction can be summarized as
Reaction mechanism for oxygen reduction
The oxygen reduction reaction (ORR) is another important reaction in energy-converting systems such as DMFCs. However, the mechanism of the electrochemical ORR is very complicated and involves many intermediates. In addition, the ORR is highly dependent on the nature of the electrode material, catalyst, and electrolyte solution. The electrochemical reduction of O2 in solution occurs by two main pathways: one involving the gain of two electrons to produce H2O2 and another producing H2O by a
Anode catalysts of DMFCs
The success of DMFC technology depends on several factors, such as membrane, anode and cathode electrocatalysts. Among these, the anode electrocatalyst suffers from slow reaction kinetics that can only be overcome through developing new electrocatalyst types. With regard to new fuel cell anode electrocatalysts, there are two major concerns: performance, including activity, reliability and durability, and cost reduction. In the following section, recent progress in the development of DMFC anodes
Cathode catalysts of DMFCs
Like the anode electrode, the cathode electrode of DMFCs also lacks an adequate electrocatalyst. Therefore, it is necessary to develop new cathode electrocatalysts (low cost and better durability) that have high electrocatalytic activity for the oxygen reduction reaction at low temperatures. In the following section, recent progress in the development of cathode electrocatalysts for DMFCs is discussed in detail. For a detailed discussion of cathode electrocatalysts, we collected more than 100
Conclusions and future outlook
As mentioned in this review, one of the biggest challenges for our society is providing powerful electrochemical energy devices. DMFCs are amongst the most promising candidates in terms of energy density and power density. Nanostructured materials are currently of interest for such DMFCs because of their high surface areas, novel size effects, significantly enhanced kinetics, and so on. The present review describes some recent progress in the developments of nanostructured electrocatalysts for
Acknowledgments
The authors gratefully acknowledge the corresponding publishers for kind permission to reproduce their materials, especially figures, for use in this review article. This work was supported by the NRF (National Honor Scientist Program: 2010−0020414, WCU: R32−2008−000−10180−0.
Jitendra N. Tiwari received his Ph.D. degree in electrochemistry from the Department of Materials Science and Engineering, National Chiao Tung University, Taiwan in 2009 working on synthesis of highly durable catalysts for electrochemical energy devices. He was a postdoctoral research fellow at the Institute of Nanotechnology, National Chiao Tung University, Taiwan (August 2009-July 2010). Currently, he is a postdoctoral research scientist at the Department of Chemistry, Pohang University of
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Jitendra N. Tiwari received his Ph.D. degree in electrochemistry from the Department of Materials Science and Engineering, National Chiao Tung University, Taiwan in 2009 working on synthesis of highly durable catalysts for electrochemical energy devices. He was a postdoctoral research fellow at the Institute of Nanotechnology, National Chiao Tung University, Taiwan (August 2009-July 2010). Currently, he is a postdoctoral research scientist at the Department of Chemistry, Pohang University of Science and Technology and focuses on graphene-based materials for fuel cell applications.
Rajanish N. Tiwari was born in India. He received his B.S. and M.S. from H. N. B. Garhwal University, India, and a Ph.D. degree in Materials Science and Engineering from National Chiao Tung University, Taiwan in 2010. Currently, he is working in Japan as a postdoctoral fellow at Toyota Technological Institute. His postdoctoral fellowship funded by Toyota Motor Corporation. His current interests include study of the synthesis, characterization, and application of novel carbon materials. He has published many scientific papers in refereed journals and given presentations.
Gyan Singh was born in 1984 in Bihar, India. He earned his B.Sc. (Hons.) in Biochemistry from Allahabad Agricultural Institute –Deemed University, India, in 2006. In the same year he was awarded Taiwan Government Fellowship to pursue Master of Science (MS) in Molecular Medicine and Bioengineering from National Chaio Tung University, Taiwan. Currently he is perusing his Doctoral research under supervision of Prof. Yun-Ming Wang at National Chaio Tung University. His primary research interests include design and development of nanosensors for clinically relevant biomolecules.
Kwang S. Kim received his Ph.D. degree from University of California, Berkeley. He was a postdoctoral fellow at IBM and a visiting professor or scientist at Rutgers University, MIT, and Columbia University. Currently, he is a professor in the Department of Chemistry and the director of the Center for Superfunctional Materials at Pohang University of Science and Technology. His research interests include design and development of novel nanomaterials and molecular devices.