Pt Nanocatalyst in Low Temperature Fuel Cells: Comparison
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Please note this is a comparison between Version 1 by Nguyen Viet Long and Version 3 by Lindsay Dong.

Fuel cells (FCs) provide electricity via the generation of ion carriers by electrocatalysis at the electrodes as well as a positive or negative ion transport mechanism and direction of motion through electrolyte membranes. Pt nanomaterials are used in the catalytic layer components of low-temperature FCs associated with the clean H2 fuel industry, which are the most successful and typical examples of generating clean electric energy and power.

  • Pt nanocatalysts
  • Low Temperature Fuel Cells
  • The sol-gel process
  • The polyol process
  • The combined sol-gel and polyol processes
  • Electronics
  • Photonics
  • Optoelectronics
  • Energy & environment
  • Electronics and telecommunication

1. Introduction

At present, fuel cells (FCs), proton exchange membrane fuel cells (PEMFCs), and direct methanol FCs (DMFCs) using excellent Pt electrocatalysts have played an increasing role for engineering, science, technology, and industry [1][2][1,2]. An FC provides electricity via the generation of ion carriers by electrocatalysis at the electrodes as well as a positive or negative ion transport mechanism and direction of motion through electrolyte membranes. In many recent years, modified polyol methods have played an important role in the controlled synthesis of various kinds of crystal nanoparticles used as the nanostructured catalysts applied in energy and environment [3][4][5][6][7][8][9][3,4,5,6,7,8,9]. An FC is a power generation system used to produce electricity using hydrogen fuel with an electrode membrane assembly, which is considered an ion conductor. The electrocatalyst layer involved in the purely so-called standard Pt nanocatalyst, or the special nanocatalyst layer was equivalent relatively to Pt nanocatalyst standard [10][11][12][13][14][15][16][17][10,11,12,13,14,15,16,17]. It is explained that their catalytic and electrocatalytic characterizations originated from high surface-to-volume ratio and quantum size [3]. The various types of Pt-based, Pd-based, Pd-free, Pt-free multimetal nanocatalysts have been being studied as promising candidates to replace the standard Pt catalyst because of its very high cost for low temperature FCs. Here, Pt-group metals (PGM) consist of Ru, Rh, Pd, Os, Ir, and Pt, which means that Pt-M bimetal catalysts for FCs can be synthesized by modified polyol methods. It is known that Pt electrocatalysts are widely used for studying hydrogen evolution reaction (HER), oxygen reduction reaction (ORR), and oxygen evolution reaction (OER) processes in cyclic voltammogram (CV) cycles. In the key points, ORR/OER and HER/OER of Pt- and Pd-based alloy and core-shell nanoparticles electrocatalysts are crucial in order to improve catalytic materials for low temperature FCs. The most important advantages of Pt-based core-shell nanoparticles are applied for reducing the high cost of FCs, DMFCs, and PEMFCs using Nafion® membranes or the hydrophobic perfluorocarbon backbone of -(CF2)n-groups and the chains (–SO3H) [1][2][1,2]. In various works, Ni-, Co-, and Fe-based oxide micro/nanosized particles with grain and grain boundaries were prepared because they showed high structural durability and stability [18][19][20][21][22][23][18,19,20,21,22,23]. In particular significance, they can be used as the oxide supports for noble metal and multimetal nanocatalysts. They are very promising candidates for FCs, DMFCs, PEMFCs, and high-temperature solid oxide FCs (SOFCs) as well as batteries and capacitors [1][2][1,2]. However, the inexpensive cost and long lifetime of PEMFCs and DMFCs are very importantly required [10][11][12][13][14][15][16][17][10,11,12,13,14,15,16,17].

2. Pt Nanocatalyst in Low Temperature FCs

In summary, low temperature FCs using the preferred Pt- or Pt-free based electrocatalysts include the typical types as follows. FCs use polymer electrolyte membranes, which are PEFCs or PEMFCs, phosphoric acid fuel cells (PAFCs), carbonate fuel cells (MCFCs), SOFCs using yttria-stabilized zirconia (YSZ) or ABO3 (A: La; B: Mn, Fe, Co, and Ni) that can be synthesized by the polyol or sol-gel processes [1][2][1,2]. A typical PEMFC has bipolar plates, gas diffusion layers, polymer memberanes as Nafion®, and the two electrodes with anode catalyst layers and cathode catalyst layers. On the other hand, in PEMFCs running on H2/O2 fuel, chemical reaction of hydrogen and oxygen occurs and produces electricity directly. This is a new, effective, and interesting way of producing electricity. Their principle and operation are illustrated with promisingly potential applications on land, in air, and at sea according to technological convergence (Figure 1).
In the operation of PEMFC and DMFC, the catalytic mechanisms of ORR and MOR are the most important keys for their high performance. At the anode catalyst layer, 2H2→4H++4e−, and at the cathode catalyst layer, O2+4H++4e−→2H2O, electricity is generated with clean water and heat via 2H2+O2→2H2O. In its operation, the common appearance of CO-poisoning or catalyst degradation at the catalytic Pt layer reducing PEMFC performance can be reduced by a Pt-based bimetal and multimetal catalyst [10][11][12][13][14][15][16][17][10,11,12,13,14,15,16,17]. The operation of PEMFC takes place at low temperature (70~90 °C), and the fuel is hydrogen, with a power generation efficiency of about 30–40% [1]. These PEMFCs can be very suitable for compact power supplies, power chargers used for mobile phones (new generation smart phones), military applications, FC vehicles, and new and future generation FC bicycles and motorcycles [16]. It is known that FC vehicles use 0.4 mg of Pt per square centimeter (mg Pt cm−2), enough for a period of about 6–7 months in their operation or more on the cathode [1]. Clean water and heat are generated as by-products of the process of generating electricity in FC motorcycles, and FC vehicles in the protection of environment and nature. There are various kinds of FCs, classifying into PEMFCs, PAFCs, MCFCs, SOFCs, DMFCs, and AFCs according to their operation principle and ion carriers. The catalytic layers consisting of Pt catalysts are used in both the electrodes listed in Table 1 [1][2][18][19][20][21][1,2,18,19,20,21]. The low Pt loading leads to reduce the high cost of the FCs. Therefore, non-noble metal catalysts using the Pt metal group or alternate electrocatalysts free from PMG have been recently developed. In short, the key is the catalytic layers of Pt nanocatalyst for the high-performance operation of FCs using Pt according to working temperatures. The global large-scale commercialization of FCs has high scientific and practical significance, which meets the huge needs of clean energy for our lives. FCs can be potentially used to run various kinds of stationary plants, unmanned aerial vehicles (UAV), large trucks, household power sources, and charge power for portable laptops and mobile phones, which are the typical kinds of PEMFCs [16][17][16,17]. Particularly, PEMFCs offer significant energy efficiency and decarbonization benefits to a wide range of industries and technologies—including automotive and heavy transport. This is reason why large companies are investing in mature FC propulsion systems for the aviation market [16][17][16,17]. In the future, AFCs may potentially be used for practical applications in submarines and spaceships. By replacing hydrogen with methanol, i.e., CH3OH, a direct methanol-based liquid FC, i.e., a DMFC, was formed. At present, the hydrogen and methanol production industries are fully developed. In general, the components and operation of a low-temperature FC using hydrogen and liquid fuels, such as alcohol, are based on three main components: the anode, the cathode, and the membrane electrode assembly (MEA) [16]. Table 1 shows the most typical FCs with the use of negative and positive ions for their potential applications in electronics and telecommunications [1][2][1,2]. In this context, the Pt catalyst layer is used at both the anode and cathode of a FC. These are the components that use fuel between the two electrodes and the polymer electrolyte membrane or ion conductor. The Pt nanocatalyst layer is the most important kind, and is the most expensive core catalyst layer used on the electrodes. Thus, both researchers and manufacturers want to reduce the cost of the most expensive fuel cell system, i.e., the high cost of Pt catalyst nanomaterial. Therefore, the research and synthesis of Pt-based catalytic nanomaterial systems with low cost and applications for the catalytic layer in electrodes are important for the development of the next FCs. From applications as a source of electricity for smart residential areas, green-energy FCs, and FC trucks to the most compact FC charging applications, FC’s operating principle is proven to be relatively simple. It is really simple, but it has great feasibility and commercialization, and has a strong impact and influence on people’s lives. The exhaust gas of the FC vehicle is water (H2O) that is completely harmless to the environment, animals, and people. The basic research on this material system has been fully invested and developed [11][12][13][14][15][16][17] [11,12,13,14,15,16,17]. Clearly, batteries, FCs, capacitors, supercapacitors, and their common uses and combination in conventional electricity can evolute energy science and technology in our life. On the other hand, the present technologies to produce oxygen, hydrogen, alcohol, ethanol, and methanol fuels are fully developed. Thus, the current catalytic layers of FCs are described in a combination of electrocatalytic Pt nanomaterials with commercial carbon nanomaterials, acting as an ion conductor, which has a large catalytic area to enhance electrocatalytic efficiency for the Pt-free catalytic material catalysts on their surfaces. The uniform distribution of the Pt nanoparticles on the designed catalytic layer is very essential to obtain the highest catalytic properties. In our current review, Pt-based nanomaterials prepared by improved polyol processes are attractive to prepare electrocatalytic nanomaterials used in FCs using methanol, alcohol, and different liquid fuels. Although the polyol method has been applied recently, Pt and Pt-based electrocatalytic bimetallic nanomaterials, typically such as PtPd, PtCu, PtNi, PtCo, FePt, and binary and ternary Pt-based nanocatalysts fabricated by conventional nanochemistry or the improved polyol methods [11][12][13][14][11,12,13,14], which have produced the electrocatalytic layers that are important for practical application for use in low-temperature FCs, i.e., DMFCs and PEMFCs using methanol or other industrial liquid fuels. The use of a second metal with the precious metal Pt to reduce the cost of FCs in the catalytic layer is a very hard problem predicted by scientists. Instead of using a standard Pt catalyst, it is clear that a Pt3Ni alloy can be used, wherein the cost will be possibly reduced by about 1/3 to the alloy catalyst layer in the FC system [23]. In the preferred case of using an electrocatalyst as bimetallic shell-core Ni3Pt nanoparticles, the cost can be significantly reduced around 2/3, which shows the outstanding advantages of core-shell structure with a very thin Pt shell of few atomic monolayers of about 1–3 nm, which is a challenge to science and a new catalyst. The CO-poisoning reducing mechanism on the electrodes containing the nanostructured catalysts has also been introduced by using other metal atoms (second metal atoms) in the preferred Pt-M (M: Ru or Pd) electrocatalysts [13][14] [13,14], increasing the efficiency and stability of PEMFCs and DMFCs, and reducing the very high cost of the whole system of FCs. Due to crises and disasters from atomic energy sources in nuclear power plants, deadly heavy pollution generated from fuel combustion processes in heat engines, internal combustion engines from petroleum-based energy sources, coal fossils, the generation of greenhouse gases such as CO2, CH4, N2O, O3, chlorofluorocarbons (CFCs), or refrigerants, we have to develop new energy sources that do not pollute the environment in developed countries, and especially in developing countries [16][17] [16,17]. To meet the large demand for clean and safe energy, it is very necessary to develop single metal, bimetallic, and multimetal alloy nanomaterial systems based on the various types of single-metal Pt nanoparticles, and combine these with carbon nanomaterials for FC applications. On this topic, the particle size of Pt nanoparticles should be controlled in the range of 10 nm in size for a high quantum effect. Scientists have proposed various methods of chemical synthesis for Pt catalyst materials. The resolution allows using low Pt material as a cost-effective way of creating the electronic catalyst layer in the electrodes of FCs, PEMFCs, and DMFCs, reducing the total cost of the system by about 30–40%, and shows high economic significance [1][2][16][17][1,2,16,17]. It is found that the practical application of fuel cell use is the practice of providing clean energy as well as taking advantage of clean energy sources from solar cells, biomass energy, ocean energy such as tidal energy, solar energy, ocean wave energy, and wind energy, are very meaningful in terms of clean and green living environment when fossil energy sources are gradually depleted as well as polluting environment. For PEMFCs, their electrodes normally consist of anode and cathode containing an electrocatalytic layer with the required Pt-based nanocatalyst, the principle of operation is mainly based on the electrochemical reactions on the electrode surfaces.
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Figure 1. Operation of PEMFCs, and promisingly potential applications for electrical charger in mobile phones, FC cars, FC boats, FC ships, FC airplanes, FC homes and offices, and FC plants in integration with electricity from electric power sources, batteries, and capacitors.
Table 1. FCs, working temperature (WT), and their catalytic mechanisms at two electrodes, both anode and cathode. In addition to the use of Pt catalyst, FCs of high costs must be decreased in the optimization of whole systems and components. Pt-based catalysts or alternative catalysts can be used in both the anode and the cathode of PEMFC, DMFC, and PAFC.
FCs Input/Output Anode Electrolyte Ion Cathode
PEMFC

WT: 60–120°C

H+ transport		 In: H2, O2 (Air)

Out: H2O, heat, extra gases		 H2 → 2H+ + 2e−

Pt catalyst		 Membrane

H+ → Membrane → Cathode		 H+ ½O2 + 2H+ + 2e− → H2O; Pt catalyst; Air as oxidant
DMFC

WT: 60–120 °C

H+ transport		 In: CH3OH, O2 (Air)

Out: CO2, H2O, heat, extra gases		 CH3OH + H2O → CO2 + 6H+ + 6e−

Pt catalyst		 Membrane

H+ → Membrane → Cathode		 H+ 3/2O2 + 6H+ + 6e− → 3H2O; Pt catalyst; Air as oxidant
AFC

WT < 100 °C

OH− transport		 In: H2, O2 (Air)

Out: H2O		 H2 + 2OH− → 2H2O + 2e−

Electrode material		 KOH

Anode ← KOH ← OH−		 OH− ½O2 + 2H2O + 2e− → 2OH−

Electrode material
PAFC

WT: 160–220°C

H+ transport		 In: H2, O2 (Air)

Out: H2O, heat, extra gases		 H2 → 2H+ + 2e−

Pt catalyst		 H3PO4

H+ → H3PO4 →

Cathode		 H+ ½O2 + 2H+ + 2e− → H2O; Pt catalyst; Air as oxidant
MCFC

WT: 600–800°C

CO32− transport		 In: CHx, CO, H2

Out: H2O, heat, extra gases		 H2 + CO32− → 2H2O + 2e−

Electrode material		 Molten carbonate

Anode ← Molten carbonate ← CO32−		 CO32− ½O2 + CO2 + 2e− → CO32−; Electrode material; Air as oxidant
SOFC

WT: 800–1000°C

O2− transport		 In: CHx, CO, H2

Out: H2O, heat, extra gases		 H2 + O2− → H2O + 2e−

Electrode material		 Ceramics

Anode ← Ceramics ← O2−		 O2− ½O2 + 2e− → O−2

Electrode material; Air as oxidant
DCFC

WT: 500–1000°C

O2− transport		 In: Carbon, CO

Out: CO, CO2, heat, extra gases		 C + 2CO32− → 3CO2 + 4e−

Electrode material		 Ceramics

Anode ← Ceramics ← O2−		 O2− C + CO2 → 2CO

Electrode material; Air as oxidant

For the use of hydrogen fuel, ORR at the cathode: ½O2+2H++2e−→H2O, hydrogen oxidation reaction (HOR) at the anode: H2→2H++2e−, and the whole reaction of PEMFC using H2: ½O2+H2→ H2O or O2+2H2→2H2O. For DMFCs, under the effect of electronic catalytic layer on the basis of Pt electrocatalyst on the electrodes, the principle of its operation is based on the chemical reaction on the electrode surface. In acidic solutions, the catalytic processes and mechanisms occurred at the electrode surface of the Pt-based electrocatalysts exhibiting the seven specific regions of HOR and ORR in CVs in the kinetics of electrochemical reactions [1][2][1,2], and information [24]and information reviewed [24]: (1) Pt−Hads→Pt+H++e−; (2) QDL(Charge)↔QDL(Discharge); (3) Pt+H2O→Pt−OH+H++e−; (4) PtOH+H2O→Pt(OH)2+H++e−; (5) Pt−(OH)2→PtO+H2O; (6) 2PtO+4H++4e−→Pt−Pt+2H2O; (7) Pt+H++e−→Pt−Hads. For methanol fuel, ORR at the cathode as 3/2O2+6H++6e−→3H2O, methanol oxidation reaction (MOR) at the anode: CH3OH+H2O→CO2+6H++6e−, and the whole reaction of DMFC as CH3OH+3/2O2→ CO2+2H2O [24]. In the most effective MOR in acidic solutions, researchers show that electrocatalytic activity of Pt catalysts to methanol oxidation occurs at (111), (110), (100) and (hkl) low-index crystal planes of Pt nanoparticles as follows: (1) Pt+CH3OH→Pt−(COH)ads+3H++3e−; (2) Pt−(COH)ads+H2O→Pt+CO2+3H++3e−. The catalytic mechanisms of both ORR and MOR are also presented in Scheme 1, which only leads to show CH3OH oxidation into CO2 experimentally. In recent years, PtRu-based electrocatalytic bimetal nanomaterials have been studied in the effective reduction of CO poisoning by Ru according to bifunctional catalytic mechanism, i.e., Ru+H2O→Ru−OH+H++e−, and Ru−OH+Pt−CO→Pt+Ru+CO2+H++e−.

However, the cost of Pd and Ru is relatively high to PtRu electrocatalyst. We need to select other inexpensive, non-noble, non-rare metals, such as Au, Ag, Cu, Fe, Co, Ni, Sn, Mo, Pb, W, etc., rather than Pt metal group (PMG) bimetal catalysts, such as PtPd, PtRh, PtRu, PtIr, PtOs [11][12][13] [11,12,13]. There have been published works related to the synthesis of single metal Pt and bimetallic Pt-based nanomaterial catalysts so that they are elements of PMG, such as Ru, Rh, Pd, Ir, and Os, but their cost is high. There are Pt-based nanoparticles with other inexpensive metals, such as Cu, Ni, Co, and Fe, and their Pt-based or Pt-free multi-component, alloy, multimetal electrocatalysts by improved polyol processes with a strong reducing agent (NaBH4 or KBH4) [13] or other strong reducing solid compounds (CaH2) or reducing gases (H2) in heat treatment [24]reviewed [24]. In future, the application of the improved polyol method is suitable for all the popular laboratories. This is a chemical process popular in laboratory that can be easily applied to create electrocatalytic Pt-based nanomaterials, which is a very necessary composite material in the electrocatalytic layers of PEMFCs and DMFCs today, with increasingly scientific and practical significance. Therefore, the polyol process is one of the focuses discussed in order to address the synthesis of single metal, bimetal, and multimetal nanoparticles, especially for shell-core bimetallic nanostructures. In the synthesis of metal, oxide, and alloy nanostructures, especially instead of using inexpensive precious metals, bimetallic alloys, multimetal alloys, or multi-component materials for catalyst of FCs, and magnetic nanoparticles for practical applications in medicine and biology, issues of size, shape, structure and composition are of great importance. Therefore, these parameters must be studied and controlled. In order to confirm that Pt-based nanocatalyst materials can be applied to FCs (PEMFCs and DMFCs), such nanomaterials must be intensively studied for the electrochemical properties of the used catalytic materials on the surfaces of the electrodes. The important electrochemical reactions of using oxygen, methanol, ethanol, or other fuels are ORR, MOR, and ethanol oxidation reaction (EOR) [10][11][12][13][14][10,11,12,13,14]. It is certain that Pt-based catalysts are used in the anode and cathode of low-temperature fuel cell systems. For catalytic applications, other nanoparticles (Au, Ag, Cu, and their related oxides), iron oxide particles (iron and their compounds), spinel oxide particles, and ABO3-type perovskite oxide particles could potentially be used in the future [1][2][11][12][13][14][15][16][17][1,2,11,12,13,14,15,16,17]. In addition, multi-component, multi-metallic particle catalysts, or functional catalytic oxide particles need to be studied with regard to their practical applications and commercialized products [16]. Scientific research methodology and theoretical and experimental research methods of other nanomaterials are applied as for the special case of Pt nanoparticle materials. The polyol method is a good solution for the comprehensive fabrication of platinum nanoparticles. Over the past ten years, there have been the intensive studies on the successful synthesis of Pt-based catalysts by polyol method by researchers in laboratories which have been presented, reported and published. Therefore, it is believed that the nanomaterials capable of replacing Pt catalysts, such as bimetal catalysts, i.e., PtCu, PtAg, PtAu, PtFe, PtNi, PtCo, and other catalytic alloys that are much cheaper in order to replace expensive Pt that can be used for applications of low-temperature FCs [13]. The two kinds of FePt and CoPt magnetic nanomaterials have been also used in hard disk drives. The deep discussion of research results on the successful synthesis of Pt nanoparticles by nanochemistry has been carried out on published works, typically for modified polyol methods or nanochemistry [3][4][5][6][7][8][9][3,4,5,6,7,8,9]. It is known that Pt nanoparticles have been successfully fabricated by the chemical methods. Given the scientific implications of current research on Pt nanoparticles, precious metals, inexpensive metals, oxide materials, and alloys nanoparticles are very necessary to be mainly focused on their structures and properties. Up to the present time in 2021, Pt nanoparticles, and Pt-based shell-core nanoparticles have been applied in energy technologies, typically such as FC technology, allowing the fabrication of creating mobile phones, transport vehicles, and clean energy sources for households in remote places. Many energy projects have mainly focused on Pt nanomaterials as well as PGM-free catalysts and alternative electrocatalysts [1][2][16][17][1,2,16,17]. In a number of present studies, it is possible to synthesize Pt nanoparticles in the range of 10 nm, and Pt-based bimetallic nanoparticles in the range of 30 nm or up to hundreds of nm in size. Thus, the successful synthesis of metal, bimetal, and multimetal nanoparticles has very high scientific and practical significance for potential application in new technologies of electronic catalysis, photocatalysis, energy, medicine, and biology [24]that were reviewed [24]. At present, a large number of Pt single-metal nanostructures are also researched and developed by chemical methods. Through the polyol process, scientists have successfully fabricated Pt simple-metal nanoparticles for catalysis, but Cu, Au, and Ag nanoparticles are commonly applied in medicine and biology [2][3][4][2,3,4]. Accordingly, the research results have only focused on Au and Ag nanoparticles by modified polyol methods for medical and biological applications. It is obvious that the more complex Pt-based metal nanostructures, typically such as bimetallic and multi-component nanoparticles with alloy or mixing structures, Pt bimetallic shell-core nanostructures, and multi-component nanostructures by modified polyol methods, have not been researched yet, due to the use of much more complex synthesis technologies [14][15][14,15]. The catalytic mechanisms and oxidation of methanol by the crystal planes of Pt nanoparticles were revealed in acid and alkaline electrolyte, changing methanol in to CO2 [15] [15]. Thus, the successful synthesis of Pt-based core-shell nanoparticles with Pt shells of 1–10 nm in new promising properties will open up new and excellent applications that are not available to single-metal nanostructures. Therefore, the as-prepared Pt nanostructures and Pt-based core-shell nanostructures are of particular interest because of their very high practical importance. The main reason is that metal, bimetal, and alloy nanoparticles are potentially used to provide a large extent, and have a wide range in interdisciplinary sciences, typically such as physics, chemistry, electronics, biomedicine, pharmaceuticals, optics-photonics, and catalysis. Typically, Pt-Pd core-shell nanostructures are also nanomaterials that exhibit their outstanding properties. The synergistic properties can be discovered from the Pt shell catalytic property, from the core property, or generated from the co-electrocatalytic properties of both the core and the shell when the Pt-based core and shell nanoparticles are the different catalytic nanomaterials. By changing the shape, structure, size, and composition of the metal core or shell, the electrocatalytic properties of Pt-Pd core-shell nanostructure system can be well controlled. The atom-monolayers shell is an alloy of Pt with another element that also reduces the high cost of the FC system, typically such as Pt3Co, Pt3Ni, Pt3Fe, and Pt3Cu [22]. This means that the price of the Pt catalyst material layer has been reduced by one-third compared with only Pt-based nanostructured catalysts. On that basis, the core-shell nanostructures of the different types of Pt atom-monolayers shells can be studied and developed by physical and chemical methods, such as modified polyol methods. For example, expensive metallic nanoparticles (typically such as Au, and critical elements, such as Pt) are used in order to coat with inexpensive nanoparticles (such as Co, Cu, Ni, Fe etc), leading to the amount of Pt being greatly reduced, but the electrocatalytic properties of the Pt-based catalytic nanoparticles are not less, or even much better. To confirm the catalytic activity of Pt catalyst in the CV cycles, the HER involved in the (111), (100), (111) crystal planes, and other crystal planes of the pure Pt catalyst followed the key reactions of Volmer, Tafel, and Heyrovsky that must be clearly measured as follows.

It is simply emphasized that the electrocatalytic properties of Pt catalyst in acid solution are Pt−Hads→Pt+H++e− (the region is characterized by double–layer charging and discharging), QDL(Charge)↔QDL(Discharge), Pt+H2O→Pt−OH+H++e−, PtOH+H2O→Pt(OH)2+H++e−, Pt−(OH)2→PtO+H2O, 2PtO+4H++4e−→Pt−Pt+2H2O, and Pt+H++e−→Pt−Hads, respectively, [24]that were reviewed [24]. During the catalytic mechanisms and processes, it is confirmed that the Pt catalyst has shown the two peaks of catalytic activity of CH3OH electrooxidation in the CV cycles. Above all, the selectivity, durability, stability, and catalytic activity of multimetal Pt electrocatalysts should need to be certainly verified by a very large number of the CV cycles in order to address the applications of FCs. The high electrochemically active surface area of Pt nanocatalysts, the relationship of high current density vs voltage, the chronoamperometric measurement, or that of current density vs time for a long time must be clearly measured in order to prove in the detail. Similarly, Pt-based multimetal, alloy, and core-shell multimetal nanoparticles need to be intensively confirmed in their high and stable electrocatalytic activity, enough for the applications of FCs.

 
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Scheme 1.
Oxidation of methanol in to CO
2
. (
a
) Simple mechanism. (
b
) Proposed mechanism.
In this context, it is certain that Pt-based multimetal catalysts are promising candidates for electrodes, which significantly reduces the high cost of Pt standard catalysts. It is important that the standard electrolyte solutions for CV measurements to the survey of catalytic activities of Pt-based catalysts are 0.5 M H2SO4 or 0.1 M HClO4, 1.0 M CH3OH, etc., (Figure 2) [24]that were reviewed [24]. Their systematic comparison of catalytic activity of between standard Pt nanocatalysts and Pt-based multimetal nanocatalysts will lead to finding an inexpensive, effective, and highly active catalyst for PEMFC and DMFC [10][11][12][13][14][10,11,12,13,14]. A wide variety of Pt core-shell nanoparticles can be synthesized by modified polyol methods from inorganic core nanoparticles with thin Pt shells on their defined nanostructured cores. Depending on the properties of the as-prepared nanoparticles fabricated by different methods, nanoparticles of core-shell nanostructures are definitely classified into several types, including inorganic-inorganic shell-core nanostructures, organic-inorganic shell-core nanostructures, and shell-core nanostructures (organic shell-organic core nanoparticles for the pharmaceutical-medical industry) [10][11][12][13][14][10,11,12,13,14]. Moreover, the core-shell (inorganic-inorganic) nanostructure is one of the most important nanostructures because it shows great practical applicability that leads to the synthesis of multimetal core-shell and alloy nanoparticles. This capability allows optimal and thorough exploitation of the superior properties of nanostructures in various applications such as catalysis, biomedicine (MRI imaging agent in cancer therapy), and nanomagnetism (hard drives using Fe-Pt and Co-Pt nanomaterials) [24]that were reviewed [24]. The chemical synthesis of multimetal nanomaterials mainly focuses on new research and fabrication technologies that allow the size and shape of the fabricated nanoparticles to be controlled. Therefore, in addition to performing basic studies, the research results of nanochemistry will be very meaningful in practice through the creation of new generations of catalytic nanoparticles by modified polyol methods with promising applications. In particular, the synthesis of metal or bimetallic nanostructures with sizes in the sized ranges of 10 nm, 100 nm, and 1000 nm is of great significance in the field of catalysis and aims to apply for FCs [13][14][13,14]. Specifically, Pt-based bimetallic, Pt-based multi-metallic alloy, or Pt-based core-shell nanostructures are structures of durability and stability with multifunctional new applications. These Pt-based bimetal metal and multi-metal nanoparticle alloy nanostructures and shell core nanostructures in relation to the cheap metal and multimetal cores, the thin Pt or Pd shells are the types currently being researched by the leading research research groups and incorporations. Scientists intensively focus on fabrication research and explore the electrocatalytic properties of new next-generation Pt-based nanomaterials by modified polyol methods. In this respect, recently, a group of authors has researched and devised a new synthesis process, initially successfully fabricating a bimetallic shell core structure [24]that were reviewed [24]. The thickness of the shell is several nm, consisting of monolayers of Pd or Pt atoms. The main results of our research groups have shown that it is necessary to continue researching and mastering the synthetic technology of nanoparticles with desirable crystal structures systematically, and with high repeatability of new metal nanoparticles, bimetallic alloy particles, and multi-component alloy particles. The designed nanoparticle has a novel structure such as core-shell configuration, or Pt-based alloy configuration with a reduction in the amount of Pt-catalyst loading used on the electrodes [24]. Then, thee are applications for alloy-structured and core-shell-structured nanoparticles, as well as magnetic alloy and oxide nanoparticles, in catalysis for chemical production, catalysis, FCs, capacitors, and batteries for energy and environment, drug carriers, and markers in biomedicine in the integration of technologies. In the polyol process, the fabrication of metal- and Pt-based metal, bimetallic, multimetal, and multicomponent nanoparticles to bring about applications in catalysis, medicine, and biology [24], there are four major problems that need to be solved, which are the characteristics of the size, shape, structure, and composition of the nanostructures. Thus, an important focus of the researchers is the need to develop various kinds of nanocatalysts by modified polyol methods with strong catalytic activity and high strength and stability on the basis of Pt. The key of nanoelectronic catalysis technology applied in PEMFC and DMFC is the polyol process to produce catalytic Pt nanoparticles. On the basis of investigating the pure Pt nanoparticles (single metal particle and its application) leading to Pt-based multimetal alloy and core-shell nanoparticles, it has been seen that the great power of the application of the ultra-narrow size Pt nanoparticle less than 10 nm is very large, and has high value in science. Other types of multimetal nanoparticles replacing of Pt nanoparticles can also be synthesized and fabricated by chemical polyol processes, and also have other special applications in many key areas. Multimetal alloy and core-shell nanocatalysts are also studied for similar applications to the various structural kinds of Pt nanomaterials; when using each expensive metal or cheap metal, their application range will expand. The high cost of the Pt catalyst layer on the two electrodes of low temperature FCs will be significantly reduced.
Crystals 12 00375 g002 550
Figure 2. (a) Cyclic voltammogram of Pt nanoparticles in electrolyte solution of 0.1 M HClO4 (scan rate: 50 mV/s, N2 bubbling time: 30 min). (b) Cyclic voltammogram of Pt nanoparticles the solution of 0.1 M HClO4 and 1 M CH3OH (scan rate: 50 mV/s, N2 bubbling for 30 min prior to catalytic measurement [24]that was reviewed [24]).

3. Prospects

The synthetic methods of Pt- or Pd-based bimetal, multimetal, and alloy nanoparticles, as well as hybrid Pt/AB2O4-type ferrite, Pt/ABO3-type perovskite, Pt/oxide, and Pt/ceramic catalysts by modified sol-gel or polyol processes have been reviewed in electrocatalysis   [24][24].

  • The new Pt-based catalysts in the forms of alloys, metals, multimetal components, oxides, ceramics, and core-shell structures will be prepared by the new, effectively modified polyol and sol-gel process for new FCs, especially for PEMFCs and DMFCs with the use of very low Pt loading.
  • The new core-shell-structure nanomaterials will be prepared with metal atomic monolayers from one layer to several layers about 1 nm to 10 nm. The shells can be used as electrocatalysis for chemical reactions.
  • The broad application of sol-gel and polyol process for materials and components will be predicted in very promising applications for electronics, photonics, optoelectronics, electronics and telecommunication, engineering and integrated technologies as well as related sciences (Figure 3).

Figure 3. The combined sol-gel and polyol processes for the synthesis of materials and components are predicted in our future [24]. This figure is modified in its original form. [24]. This figure is modified in its original form.

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