Recent Patents on Materials Science 2012, 5, 175-190
175
Pt and Pd Based Catalysts with Novel Alloy and Core-Shell Nanostructures for Practical Applications in Next Fuel Cells: Patents and Highlights Nguyen Viet Long1,2,3,4*, Cao Minh Thi5, Masayuki Nogami4 and Michitaka Ohtaki1 1
Department of Molecular and Material Sciences, Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, 6-1 Kasugakouen, Kasuga, Fukuoka 816-8580, Japan; 2Department of Education and Training, Posts and Telecommunications Institute of Technology, Nguyen-Trai, Ha-Dong, Hanoi, Vietnam; 3Department of Materials Science and Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan; 4Laboratory for Nanotechnology, Ho-Chi-Minh Vietnam National University, Linh-Trung, Thu-Duc, Ho-Chi-Minh, Vietnam; 5HoChi-Minh City University of Technology (HUTECH),144/24 Dien Bien Phu, Ward 25, Binh Thach, Ho-Chi-Minh City, Vietnam Received: January 24, 2012; Accepted: February 9, 2012; Revised: February 14, 2012
Abstract: In this review, we have investigated research results of recent patents of various kinds of Pt or Pd based nanoparticles for catalysis. It includes aspects of practical applications of metal, bimetal or multi-metal based nanoparticles in catalysis and fuel cells. The synthetic methods and catalysts engineering are comprehensively presented in their excellent applications for fuel cells. The aims of this review are to provide achievements and highlights of patents of recent applications of Pt or Pd based material catalysts for various fuel cells. In particular, the Pt or Pd based nanoparticles of certain size, shape, structure, composition show great and promising applications in fuel cells and energy issues. The new and modified catalysts associated with the Pt or Pd based nanoparticles can improve future fuel cells with very high and robust performance. Our ideas and proposals of using a very low weight of Pt metal in novel robust and efficiently designed catalysts are one of the best ways for the large-scale commercialization of fuel cells technology. In addition, the characterization and controlled synthesis of metal, bimetal, multi-metal, and multi-component nanoparticles are discussed in potential applications for fuel cells. Finally, we think that greatly novel discoveries in science and technology through successful synthesis of novel alloy or core-shell nanoparticles and their excellent applications in catalysis, medicine and biology can be clearly predicted.
Keywords: Alloy, alloy and core-shell catalyst, bimetal nanoparticles, carbon nanomaterials, chemical synthesis, core-shell, core-shell nanostructure, durability, electrocatalysts, electrocatalytic activity, fuel cells, hydrogen, metal nanoparticles, multicomponent catalyst, nanostructured catalyst, noble metals, oxygen reduction reaction (ORR), Pd alloyed catalyst, Pt alloyed catalyst, Pd nanoparticles, Pt nanoparticles, size and morphology, stability, surface. 1. INTRODUCTION At present, various kinds of various Pt nanostructures or Pt catalysts are synthesized in the developments for fuel cell technologies for energy and environment technologies [1-6]. Therefore, scientists are trying to explain critical questions, and deal with critical issues of catalytic materials and technologies for fuel cells. Traditionally, the Pt or Pd catalysts have been used in the anodes, the cathodes in fuel cells integrated with membranes technologies with a very high cost [7, 8]. The main reasons for their uses in fuel cells are that Pt or Pd based catalysts exhibit very good catalytic properties of hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR) in polymer electrolyte fuel cells (PEFCs) [919]. Therefore, the developments of fuel cell technology with low cost mainly depend on the catalytic properties, stability, and durability of various Pt or Pd-based catalysts. In *Address correspondence to this author at the Department of Molecular and Material Sciences, Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, 6-1 Kasugakouen, Kasuga, Fukuoka 816-8580, Japan; Tel: +81-(0)92-583-8835; Fax: +81-(0)92-583-8835; E-mail:
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recent years, various new Pt or Pd based catalysts have been developed for those goals of the realization of various fuel cells (FCs), phosphoric acid fuel cells (PAFC), polymer electrolyte membrane cells (PEMFC), and especially direct methanol fuel cell (DMFC) for portable devices. Here, we have suggested that novel Pt or Pd based nanostructures show interesting catalytic properties and selectivity depending on surface, size, morphology, composition, structure in various nanosized ranges of 1-10nm, 1-50nm, … 1-1000nm for great applications in catalysis, medicine and biology. In particular, they exhibited large surface-to-volume and size quantum effects as well as surface, flat or rough surface structure, shape and morphology effects in the nanosized range of 10nm observed in electro-catalysis [3]. Therefore, various kinds of Pt or Pd based nanostructures or nanoparticles via chemistry or nano-chemistry can be controllably synthesized with target applications. In this critical review of recent patents, research results, and scientific discoveries, we have presented the controlled synthesis of Pt or Pd based nanoparticles via nano-chemistry for practical applications in various fuel cell technologies. The issues of the controlled synthesis of Pt or Pd based © 2012 Bentham Science Publishers
176 Recent Patents on Materials Science 2012, Vol. 5, No. 3
Long et al.
metal, bimetal, mixed and multi-metal catalysts relating to various control method of surface, size, morphology, composition, and structure as well as the issues of structure porosity, self-aggregation and self-assembly are presented. In addition, the issues of crystallization and re-crystallization, the ranges of composition, size and morphology, catalytic sensitivity, activity and selectivity are introduced. Moreover, Pt or Pd based catalysts on carbon support materials for their practical applications in fuel cells are discussed. With these aspects, the discoveries and ideas of novel and modified Pt or Pd based catalysts for fuel cells as well as the stability, durability and utilization of Pt or Pd based catalysts are discussed.
Pt + H+ + e Pt-Hads (hydrogen adsorption)
To evaluate the catalytic activity of the Pt catalysts or Pt based catalysts, the electrochemical active surface area (ECSA) of the Pt catalyst is calculated as ECSA = QH/(0.21LPt) [2, 20, 23]. Now, researchers and scientists have tried to use a low loading containing Pt or Pd based catalysts with a low Pt weight for cost-effective design, but a very high electrochemical active surface area and a acceptable low content of CO intermediates. 2.1.2. Oxygen Reduction Reaction (ORR) The oxygen reduction reaction occur in two main pathways in acidic electrolytes as follows (i) O2 + 4H+ + 4e 2H2O
2. FUNDAMENTAL
Traditionally, Pt catalyst showed the most typical characteristics of hydrogen reactions [20-24]. Hydrogen evolution on the Pt catalyst was explained in the known mechanisms, which are the Volmer, Tafel, and Heyrovsky mechanisms. In addition, the new combinations of Volmer-Tafel and Volmer-Heyrovsky mechanisms were experimentally found [2024]. The surface kinetics and chemical activity occurring at the electrode surface containing Pt catalyst were characterized by Eq. (1)-(7) as follows.
Pt-(OH)2 PtO + H2O (formation of platinum oxide) 2PtO + 4H+ + 4e Pt-Pt + 2H2O
+
(10) -
(ii) Cathode: O2 + 2H + 2e H2O
(11)
(iii) Overall: H2 + O2 H2O
(12)
(i) Anode: CH3OH + H2O CO2 + 6H+ + 6e-
(2)
PtOH + H2O Pt(OH)2 + H + e
The basic fuel cell reactions associated with electrooxidation of hydrogen and electro-reduction of oxygen in respective to the overall combustion reaction of hydrogen in oxygen are shown as follows.
(4)
QDL (Charge) Q DL (Discharge)
In general, fuel cells with alternative-energy functions through environmentally friendly treatment methods can generate electric energy via the chemical reactions between hydrogen and oxygen as shown in Fig. (1a) for PEMFC, and via the chemical reactions between methanol and oxygen as shown in Fig. (1b) for DMFC.
(3)
(1)
Pt + H2O Pt-OH + H + e
2.2. Fuel Cells
The typical chemical reactions at anode and cathode lead to form a current of electrons through an external circuitdirect electrical current [20, 21, 25]. Today, PEMFC is one of the most important hydrogen/air fuel cells operating at relatively low temperatures less than 100°C. In fuel cell technology, DMFC has been intensively studied because of high conversion efficiency at low relatively temperature
