High Performance Electrocatalysts Based on Pt Nanoarchitecture for Fuel Cell Applications
Fuel cells, converting chemical energy from fuels into electricity directly without the need for combustion, are promising energy conversion devices for their potential applications as environmentally friendly, energy efficient power sources. However, to take fuel cell technology forward towards commercialization, we need to achieve further improvements in electrocatalyst technology, which can play an extremely important role in essentially determining cost-effectiveness, performance, and durability. In particular, platinum- (Pt-) based electrocatalyst approaches have been extensively investigated and actively pursued to meet those demands as an ideal fuel cell catalyst due to their most outstanding activity for both cathode oxygen reduction reactions and anode fuel oxidation reactions. In this review, we will address important issues and recent progress in the development of Pt-based catalysts, their synthesis, and characterization. We will also review snapshots of research that are focused on essential dynamics aspects of electrocatalytic reactions, such as the shape effects on the catalytic activity of Pt-based nanostructures, the relationships between structural morphology of Pt-based nanostructures and electrochemical reactions on both cathode and anode electrodes, and the effects of composition and electronic structure of Pt-based catalysts on electrochemical reaction properties of fuel cells.
Low temperature fuel cells based on the proton exchange membrane fuel cells (PEMFCs) and direct alcohol fuel cells (DAFCs) technology continue to be of considerable interest as one of the most promising electrochemical conversion devices for widespread use as clean, renewable, and nonpolluting power sources in electrical vehicles and portable electronic devices as well as various stationary power systems as they offer high efficiency, modularity, low operating temperature, and low pollutant emissions [1, 2]. However, despite these attractive features and environmentally favorable energy sources with a variety of practical applications, there are challenging important issues to be addressed in order to take fuel cell technology forward towards successful commercialization. In particular, an electrocatalyst, serving as a basis for operation of fuel cells, is extremely important as a crucial component, essentially determining cost-effectiveness, performance, and durability .
Among various electrocatalysts studied so far, platinum- (Pt-) based electrocatalyst materials have attracted extensive attention over the past decade as an ideal fuel cell catalyst due to their most outstanding activity for both cathode oxygen reduction reactions (ORRs) and anode fuel oxidation reactions [4–6]. However, Pt catalysts still suffer from several problems, which need to be solved, such as their high cost, catalyst deactivation through active site blocking caused by CO poisoning generated as reaction by-products, and the kinetic limitation of the ORR as well as high overpotential and low long-term stability of oxygen electroreduction reactions [7–10]. To date there have been significant worldwide efforts to address such problems by introducing various architectures of Pt catalysts: (1) the shape-controlled Pt-based nanostructure [11, 12], (2) the dimension-controlled Pt-based nanostructures [13, 14], (3) the simply alloying of Pt crystal structure with a second transition metal (Co, Fe, Ni, etc.) [15–17], and (4) the bimetallic Pt-based nanostructures with core-shell structure [18, 19]. Recently, most researches have mainly focused on control over the morphology and composition of Pt-based nanostructures to enhance their electrocatalytic activity and stability as genuinely practical approaches for fuel cell electrocatalysts [20–22]. To this end, various strategies have been developed to control the shape of Pt-based nanostructures using different synthetic routes, for example, use of polyol in aqueous or organic solution, thermal-decomposition process, and electrochemical and photochemical reduction techniques [23–27]. Furthermore, the synthesis of diverse Pt-based nanostructures has also been demonstrated by controlling the morphology and composition of catalytic nanostructures to fundamentally understand their catalytic activity and performance with their dimensional (D) versatility, such as 0D (cube, octahedron, truncated cube, and icosahedron) [28–31], 1D (wire and rod) [32, 33], 2D (plate and disk) [34, 35], and 3D nanostructures (star, flower, and dendrite) [36, 37]. In this paper, therefore, we aim to review the practical issues and recent research progress in the development of synthetic methods for high efficient, stable electrocatalysts based on nanoarchitectured Pt with various shapes and faceted morphologies. In addition, in order to understand fundamental electrochemical reaction features in low temperature fuel cells, we will focus on the following essential dynamics aspects of electrocatalytic reactions: (1) shape effects on the catalytic activity of Pt-based nanostructures, (2) relationships between structural morphology of Pt-based nanostructures and electrochemical reactions on both electrodes, and (3) effects of composition and electronic structure of Pt-based catalysts on electrochemical reaction properties of fuel cells. This review will provide insights not only into comprehensively understanding electrochemical mechanism and characteristics in fuel cell reactions, but also into developing novel and practical electrocatalysts useful for both electrode reactions in low temperature fuel cells.
2. Synthesis of Pt-Based Nanostructures as Electrocatalysts
2.1. Polyol Method in an Aqueous or Organic Solution
A polyol approach, which is an effective technique to synthesize particles with a wide range of sizes and morphologies, has attracted attention for the induced reduction of a metal precursor due to the easy and cost-effective synthetic method. This method is based on the decomposition of alcohol containing hydroxyl (–OH) functional groups and the proposed basic reduction mechanism in ethylene glycol as a reducing agent is as follows :
Here it is important to note that the kinetic rates of reduction of metal ions, affecting the morphology and shape of particles, can be strongly influenced by alky chain lengths, which are different depending on employed reducing agents, such as ethylene glycol, L-ascorbic acid, citric acid, and poly(vinyl pyrrolidone) (PVP) . Xia group reported that truncated octahedrons (or the so-called Wulff polyhedron) enclosed by eight and six facets of Pd can be obtained from the L-ascorbic acid, exhibiting a fast kinetic rate (Figure 1(b)) . Also, when the citric acid with a moderate kinetic rate of reduction was used as a reducing agent, the icosahedrons, octahedrons, and decahedrons of Pd enclosed by the facets were obtained, as shown Figure 1(c). In contrast, the PVP with a long alkyl chain and two hydroxyl groups at end group of polymer exhibits much low kinetic rate of reduction. As a result, the Pd nanostructures with the shape of hexagonal and triangular nanoplates were synthesized by using PVP because their nuclei with a metastable hexagonal structure could remain small for a long period of time due to the slow addition of atoms, gradually evolving into plate-like seeds in Figure 1(d).
In recent, we synthesized the Pt-Pd nanostructure with an octahedral shape enclosed by facets using glycerol as a new reducing agent in aqueous solutions . It was found that the glycerol can lead to the faster reduction reaction of metal ions because it has a relatively short alkyl chain and three hydroxyl groups. Moreover, it is believed the fast reduction reaction might tend to thermodynamically minimize crystalline surface energy and hence resulting in the formation of crystal facets with the lower surface energy in the structure. Thus, this implies that Pt-Pd nanostructure can be synthesized with side facets, forming the octahedral shape by the rapid reduction reaction due to the relative surface energy of planes in fcc structures . Furthermore, to confirm reduction mechanism by glycerol, we performed Fourier transform infrared (FT-IR) measurements. In general, according to the basic mechanism of metal reduction induced by the polyol, metal ions are reduced to metal atoms by an aldehyde (CHO) functional group, which is transformed into a carboxyl (COOH) group with a ketone (C=O) group. Thus, as a strong evidence of oxidized glycerol, the IR absorption band of the C=O stretch was clearly observed between 1720 and 1740 cm−1 in Figure 2(a). This finding clearly confirmed that the glycerol was changed into glyceraldehydes and glyceric acid during the reduction of metal ions as shown in Figure 2(b).
In addition, it has been also reported that additive agents, such as organic materials like polymer and small molecules acting as capping agents, metal ions with redox potential, and halogen ions, play an important role in controlling the shape of Pt-based nanostructures. For example, organic materials, such as PVP, cetyltrimethylammonium-bromide (CTAB) and chloride (CTAC), citrate, sodium polyacrylate, tetradecyltrimethylammonium bromide (TTAB), effectively act as both a surface adsorption agent for shape-controlled nanoparticles and a surfactant agent for prevention of aggregation among metal nanoparticles during synthesis process [43–46]. However, although organic materials are beneficial for the shape control of the metal, they often exhibit a reduced or poisoning phenomenon of active sites in the electrochemical reactions due to the strong chemical adsorption between capping agents and catalyst surface. Thus, for electrochemical applications in fuel cells, it is essentially imperative to remove organic capping agents from catalyst surface. For this, various strategies have been developed by using different techniques, such as acid , thermal , UV-ozone , electrochemical , plasma , or chemical treatment . However, whereas these approaches have advantages of a relatively high removal rate for organic capping agents, these cause the formation of secondary defects on catalyst surface and the variation of an as-prepared Pt-based nanostructure shape. Accordingly, the advanced development of effective surface treatment process and synthesis methods based on nonorganic capping agents is required for enhanced electrochemical properties in low temperature fuel cells.
In addition to introduction of various reducing agents with different alky chain lengths and organic capping agents for the control of Pt-based nanostructure shapes, utilizing metal cations, such as Fe2+/Fe3+ and Ag+ ions, and even nonmetal anions, such as , can be considered as another strategy to adjust the shape of Pt-based nanostructures. As shown in Figure 3, Song and coworkers demonstrated the synthesis of Pt nanostructures with the controlled shape by the selective adsorption of Ag ions as an additive agent . As the concentration of Ag ions in reaction solution is continuously increased, the shape of Pt nanostructures is changed from cube to octahedron. We also demonstrated the growth of cubic Pt nanostructures using Fe2+/Fe3+ ions having redox potential as an additive agent . Fe3+ ions could lead to slow nucleation and growth of Pt, leaving behind thermodynamically stable planes. That is, Fe3+ ions can greatly reduce the supersaturation of Pt atoms, resulting in formation of a stable crystal plane. The growth dynamics of Pt nanocubes can be described by the following equation:
Similar to roles of Ag+ ions in nanoparticle growth, Xia and coworkers reported that a variety of shapes in Pt-based nanostructures can be obtained through the control of kinetic reduction rates by ions as an additive agent . It was found that Pt nanoparticles exhibited an irregular shape and their sizes varied in the range of 3–5 nm without the introduction of ions. Noticeably, as the molar concentration of ions was increased, the geometrical morphology of Pt nanoparticles was transformed from an irregular shape with smaller size, through a polyhedral shape with uniform size until 3.3 molar ratios of ion to Pt salt, to the uniform octapod and tetrapod at high concentration of ions over 3.3 molar ratios. These results indicate that ions could hinder the reduction of Pt(IV) ions because the reduction potential of ion is higher than that of Pt(IV) ions as follows:They also confirmed the formation of intermediate ions. This finding suggests that ions can play a role in reducing the rate of reduction during the synthesis process.
2.2. Thermal-Decomposition Method
Compared to a polyol method, a thermal-decomposition approach is beneficial for the production of shape-controlled Pt-based nanostructures with high yield, uniform shape, and high reproducibility because of protection from oxidative etching and subreaction [56, 57]. Furthermore, it is recognized that this is a very effective method for designing core@shell structures and controlling alloy compositions.
Recently, we reported that Pt nanocubes were successfully synthesized by a thermal-decomposition method using PVP as an additive agent, 1-octadecene as a solvent, and oleylamine as a capping agent for enhanced electrochemical properties in alcohol electrooxidation reactions . It is worth noting that the shape controlled Pt-based nanostructure with not only unique exposed surface, but also large surface area is important to enhance the electrochemical properties. However, it has been mostly reported that Pt-based nanostructures with shape control have lager particle size of over 10 nm, hence exhibiting smaller electrochemical active surface areas (EASAs). Accordingly, for enhanced electrochemical properties, the ability to tailor facets and size of Pt-based nanomaterials is tremendously important, allowing favorably exposed surface structures of nanoparticles with smaller size comparable to commercial Pt electrocatalysts. It was clearly observed that our Pt nanocubes have much smaller particle size of average 4.5 nm compared to previously reported nanocubes with particle size of over 10 nm [58, 59]. In particular, we found that PVP can play an important role in forming Pt cubes enclosed by facets and monodispersed nanoparticles in Figures 4(a) and 4(b) due to its attractive feature to bind with Pt facets. On the contrary, Pt nanoparticles synthesized in the absence of PVP showed the polydispersed nanoparticles and slight aggregation between nanoparticles as shown in Figure 4(c). Moreover, it has been shown that amine functional groups could be important key factors to determine the shape and size of Pt-based nanocubes through assistance of W metal ions controlling kinetic reduction rate or/and carbon monoxide (CO) gas acting as a selective adsorption material. For example, Fang and coworkers demonstrated the synthesis of PtM alloy (M is Co, Fe, and Ni) nanocubes by introducing in oleylamine and oleic acid solutions . They suggested that the use of is crucial for controlling nucleation process and that an optimized ratio of oleylamine and oleic acid pair is the key to enable us to obtain the lowest total surface energy. They believed that the preferential chemisorption and monolayer adsorption of oleylamine on facets of PtM can lead to the lowering of total surface energy of the facet of PtM, resulting in the cube shape. Fu and coworkers also reported that Pt nanocubes can be synthesized using CO in a mixed solvent system of oleylamine and oleic. Similar to the oleylamine, it is revealed that the CO gas strongly binds with Pt facets, affecting the morphology of Pt nanoparticles . To further understand the role of CO in determining the shape of Pt nanoparticles, they performed calculations for change in the surface energy () between the and facets of Pt before and after adsorption of amine and/or CO molecule using spin-polarized density functional theory (DFT). It is found that the surface energy of (0.90 eV/atom) is higher than that of (0.64 eV/atom) before adsorption of amine and CO molecule. Interestingly, however, the change of surface energy is significantly altered after adsorption of amine and CO molecule; that is, is −0.02 eV/atom. This is because adsorption energy of amine and CO on Pt facets is highly increased compared to Pt facets. This indicates that the coadsorption of CO and amine on Pt results in the formation of Pt nanocubes.
2.3. Electrochemical and Photochemical Reduction Methods
Electrochemical and photochemical reduction approaches, needing external energy for reduction reactions, have the enormous potential for the production of shape-controlled Pt-based nanostructures as they have the advantages of high yield of shape and size of Pt-based nanostructures, reproducibility, and nontoxic reaction as well as detailed understanding of formation mechanism in Pt-based nanostructures. However, there still remain difficulties in large scale production to move towards a genuinely practical technology for commercialization. In this regard, it is highly necessary to seek for a novel and simple strategy to solve such a problem.
On the basis of the electrochemical reduction technique, many studies have been performed to achieve shape-controlled Pt-based nanostructures using different electrochemical techniques, such as square-wave potential (pulse potential), and constant reduction current/potential. Typically, in the electrochemical reduction method, for the synthesis of shape-controlled Pt nanostructures the experimental setup consists of electrolyte and three electrodes, namely, working, reference, and counter electrodes. Furthermore, negative potential or current is applied for the reduction reaction from metal ions to metal atoms. In particular, Wang and coworkers reported tetrahexahedral (THH) Pt nanostructures enclosed by high-index facets with almost facets and some , , or facets, as shown in Figure 5(b) . It was found that the particle size of THH Pt nanostructures was increased with the reaction time, from 53 to 144 nm. In particular, they suggested that formation mechanism of THH Pt nanostructures based on a square-wave potential technique as follows (in Figure 5(a)). (1) At high potential (1.20 V), Pt atoms on surface of as-formed Pt nanospheres can be oxidized and partially dissolved into electrolyte, causing defects. (2) At low potential (−0.20~−0.10 V), dissolved Pt ions diffuse to the surface of Pt crystal and then are reduced to Pt atoms. Thus, Pt atoms on the surface of low-index planes like a facet with the high coordination number (CN) are readily oxidized/dissolved by the oxygen atom, which results in a structural change of the Pt surface. In contrast, in the case of high index planes with the low CN, the oxygen atoms preferentially adsorb on the high index surface sites of Pt, hence maintaining the unchanged surface structures. Also, Raoof and coworkers showed that novel Pt nanostructures can be successfully synthesized using a constant potential deposition method. As-prepared Pt nanostructures exhibited a rod-like shape with 1D structure due to the selectivity adsorption of oxygen atoms of dextrin on the Pt surface .
Alternatively, the photochemical reduction method was developed as a way to form noble metal nanoparticles by reducing noble metal ions in the presence of semiconducting metal oxides (SMOs) such as TiO2 and ZnO. The proposed reduction mechanism of metal (M) ions in the photochemical reduction process in the presence of the SMO materials is as follows:
As shown in the above reaction equations, when light (energy, hv) is irradiated into a reactor, electron, and hole pairs are generated in SMOs (equation (6)). The generated electrons participate in the reduction of M ions (equation (7)), while the holes are used to oxidize ethanol, which is an additive agent (equation (8)).
Recently, Gu and coworkers developed an alternative method for the synthesis of ZnO@Pt nanoparticles using the UV irradiation of an ethanolic solution containing H2PtCl6 and ZnO deposited on graphene sheet as shown in Figure 6 . Also, it has been reported that various M@TiO2 nanoparticles (M = Ag, Pd, Au, Pt) can be synthesized using the photochemical reduction process [65, 66]. Furthermore, Huang and coworkers demonstrated the synthesis of shape-controlled Au nanoparticles with six star using photoirradiation of an ethanolic solution containing HAuCl4 and TiO2 sols without the addition of any other surface capping molecules . Thus, these findings suggest that a photochemical reduction method based on the reduction mechanism of metal ions can be one of the viable ways to achieve Pt nanostructures with controlled size and shape.
For the further comparison of three different synthetic methods, we summarize the advantages and disadvantages of each synthesis approach for the shape-controlled Pt-based nanostructures in Table 1.
3. Electrochemical Reactions of Pt-Based Nanostructures in Low-Temperature Fuel Cells
In general, low-temperature fuel cells, such as PEMFCs and DAFCs, consist of two electrodes with one anode and the other cathode separated by a membrane (so-called MEA: membrane electrode assembly), gas diffusion layer, and bipolar plate in an unit cell as shown in Figure 7. In particular, DAFCs are also classified as direct formic acid, direct methanol, and direct ethanol fuel cells according the type of fuels. Their basic electrochemical reactions involve the electrooxidation reaction of fuels taking place at the anode and the electroreduction reaction of oxygen occurring at the cathode. Therefore, to enhance electrochemical properties in both electrodes, there have been many efforts to manipulate the shape and composition of Pt-based nanostructures as electrocatalysts for fuel cells. In this section, we will review the fuel electrooxidation and oxygen electroreduction reactions and will discuss the detailed electrochemical correlation between each reactant and Pt-based nanostructures associated with their morphology and composition.
3.1. Fuel Electrooxidation Reaction
In direct formic acid fuel cells (DFAFCs), the anodic reaction considered as the most important reaction is the formic acid oxidation reaction (FAOR). It has been well known that Pt-based materials are the most effective electrocatalyst for FAOR. However, Pt and Pt-based catalysts suffer from the catalyst deactivation process caused by CO poisoning, one of the most crucial issues, as CO-species generated as a by-product of the formic acid electrooxidation reaction disturb electrochemical reactions by blocking active sites. There are two possible routes for formic acid electrooxidation reactions as follows:
Here it is noted that, in the direct oxidation pathway, the formic acid is perfectly oxidized to carbon dioxide without reaction intermediates, while, in the indirect pathway, the CO-species are generated as intermediate species, which strongly adsorb onto the Pt catalyst and hence resulting in reducing the active surface area. Thus, recently, there have been enormous efforts to overcome the CO poisoning-induced catalyst deactivation process and to enhance the oxidation rate of formic acid on metallic catalysts by introducing alloying Pt with other highly oxophilic metals and/or shape-controlled Pt nanostructures to induce either electrooxidation of adsorbed intermediate species or the direct oxidation pathway [18, 22, 93].
Recently, it has been demonstrated that Pt- or Pd-based catalysts for FAOR exhibit the enhanced electrochemical properties such as lower on-set potential and higher current density than other catalysts and that surface structure or lattice parameter of catalysts plays a major role for the enhanced electrocatalytic activity due to the bridge-bonded adsorbed formate on catalyst surface in the formic acid electrocatalytic oxidation . Based on these findings, we developed Pt-Ni alloy nanostructures for formic acid electrooxidation by using a thermal-decomposition method . Note that as the lattice parameter of Pt (3.939 Å) is larger than that of pure Pd (3.893 Å), we selected Ni (3.523 Å) to modulate the lattice parameter of Pt-based crystal similar to that of pure Pd as shown in Figure 8(c). It was observed that the Pt-Ni alloy nanostructures show dendritic shapes (Figure 8(a)) and that the lattice parameter of Pt3Ni1 alloy (3.898 Å) is similar to that of pure Pd. Thus, as expected, the Pt3Ni1 alloy nanodendrites exhibited higher current density than commercial Pt at first anodic peak, which means the complete oxidation of formic acid, because of a well-defined alloy formation between Pt and Ni as well as high surface area of dendritic shapes, as shown in Figure 8(b).
Alternatively, it has been often reported that Pt-based nanostructures with high-index facets can be considered as efficient electrocatalysts for FAOR due to their exposed surface structure with a large amount of atomic steps. Huang and coworkers reported that concave polyhedral Pt nanocrystals exhibit 5.57 times higher current density than that in commercial Pt due to higher density of atomic step on a high-index facet as shown in Figure 9 . Furthermore, Solla-Gullón and coworkers demonstrated comparison of electrochemical properties between modulated Pt nanostructures with different index facets . It has been clearly found that the FAOR activity is significantly dependent on facets of Pt nanostructures, exhibiting quite different FAOR activity as follows: facets.
Similar to FAOR, CO-poisoning, primarily for Pt catalyst deactivation, is a crucial problem in the methanol oxidation reaction (MOR). Thus, many recent efforts have been focused on the deterioration of CO poisoning to enhance the oxidation rate of methanol by alloying Pt with 2nd metallic elements () based on high oxophilic metals, such as Ru, Pd, Ni, and Cu, favorably adsorbing hydroxyl species and then facilitating to oxide the adsorbed CO. Thus, it is expected that Pt-based alloy nanostructures with 2nd metallic elements can excellently enhance the electrocatalytic activity due to a bifunctional effect between both Pt and 2nd metal forming an alloy and the downshift of the d-band center of the pure Pt electronic structure, resulting in oxidizing the CO-species during the reactions. The bifunctional electrocatalytic mechanism for CO-species oxidation is proposed as follows:
Alternatively, on the basis of bifunctional electrocatalyst techniques, the development of shape-controlled nanoparticles with high specific catalyst surface (i.e., dendritic, star, and multipod shape) and/or exposed high active facets is also of particular interest for the enhanced electrochemical activity in MOR. Huang and coworker developed the synthesis of hyperbranched PtRu nanostructures using seed-mediated process at low-temperature for MOR . It has been shown that hyperbranched PtRu nanostructures exhibit a high current density and fast on-set potential compared to pure Pt and PtRu black due to their high surface area and presence of Ru metals for bifunctional action. It was also clearly observed that carbonaceous intermediates, such as CO, HCOO−, and HCO−, were completely removed, indicating that methanol can be perfectly oxidized during the reaction. Moreover, Yang and coworkers developed Pt-Cu alloy concave nanocubes with high-index facets for MOR . They observed that the Pt-Cu alloy concave nanocubes exhibit the enhanced electrochemical properties, that is, 4.7 times higher current density, compared to commercial Pt, because the Cu atoms in Pt-Cu alloy nanostructures might induce the downshift of the d-band center of the pure Pt electronic structure and bifunctional effects, causing the oxidation of CO-species. Additionally, it is noteworthy that Pt facets exhibit a lower on-set potential and a higher current density compared to other low-index facets in MOR. Thus, we investigated octahedral Pt-Pd alloy nanostructures with dominantly exposed facets as shown in Figure 10(b) . The octahedral Pt-Pd alloy nanostructures with the elemental composition of 47.8 at% of Pt and 52.2 at% of Pd exhibited the enhanced electrocatalytic activity, that is, higher maximum current density and lower reverse current density, in comparison with commercial Pt as shown in Figure 10(c), which is believed to be attributed to the bifunctional effect based on Pd and the octahedral shape enclosed by facets.
In addition to methanol, ethanol as another candidate source suitable for fuel cells has attracted attention, because it can offer relatively high theoretical mass energy density (8 kWh kg−1). However, the strong C-C bonds in ethanol cause a high overpotential at an anode, resulting in cell performance deterioration. Thus, researches have been focused on the development of an effective catalyst with high electrocatalytic activity in an ethanol oxidation reaction (EOR) by modifying catalyst surface or employing other 2nd atoms to break strong C-C bonds. For example, Rao and coworkers showed that the cubic Pt-Rh alloy nanostructures have higher ethanol electrocatalytic activity as indicated in Figure 10(a) . Based on electrochemical in situ FT-IR spectroscopic measurements, they found that Pt9Rh1 nanocubes showed higher electrooxidation current density in EOR compared to other nanocubes in Figure 11(b). However, interestingly, the perfect oxidation reaction rate of the Pt9Rh1 nanocubes in ethanol was 0.5 times lower than that of Pt1Rh1 nanocubes (Figure 11(c)), because the Pt9Rh1 nanocubes have not sufficient active sites for breaking strong C-C bonds. On the other hand, Pt1Rh1 nanocubes with relatively large amounts of Rh exhibited the improved conversion selectivity from ethanol to CO2 because Rh can help easily oxidize intermediate species and break C-C bond of ethanol. This finding is consistent with results of other studies where Rh nanostructures can significantly enhance the ethanol electrooxidation activity .
Here, please note that it is substantially difficult to directly compare the differences in electrochemical performance among previously reported results because shape-modified Pt-based nanostructures for the diverse anodic reactions in PEMFC have been developed by using various synthesis approaches. To this end, in order to further introduce various shape-controlled Pt-based electrocatalysts, we summarize various synthesis approaches applied for enhanced electrochemical properties in typical alcohol electrooxidation reactions in Table 2.
3.2. Oxygen Electroreduction Reaction of Pt-Based Nanostructures
The ORR is a fundamental and pivotal reaction occurring at the cathode in low temperature fuel cells where molecular oxygen adsorbs to the catalyst surface and is reduced to water through the following two pathways:
As shown in Figure 12, the ORR requires a high potential (1.23 V) for the direct four-electron pathway and usually uses Pt-based alloy catalysts as the most effective catalysts for oxygen electroreduction . In particular, however, there are critical several problems associated with the ORR in low temperature fuel cells, which need to be solved, such as kinetic limitations for oxygen diffusion process and its low long-term stability for oxygen electroreduction. Thus, in order to address these issues, the critical factors, affecting the electrocatalytic properties of Pt-based catalysts, should be considered and identified.
For the enhanced electrochemical activity in the ORR, various Pt-based catalysts with alloy, shape-controlled, and core-shell nanostructures have been reported as shown in Figure 13(a) . Mayrhofer and Arenz showed that nanostructured alloy Pt3M1 catalysts can improve the electrocatalytic activity in the ORR due to their modified electronic and surface structures according to the 2nd metallic materials selected in Pt structures in Figure 13(b) . In addition, it has been reported that (, M = Pd, Ir, Co, Fe, Ni, Y, and Sc) alloy catalysts have greater ORR activity and thermodynamically more stable state in comparison with different elemental compositions of () or pure Pt catalysts. Moreover, Stamenkovic and coworkers demonstrated that the surface facet of Pt single crystal shows the improved electrochemical activity for ORR compared to the surface facet in an acid electrolyte by utilizing shape-controlled Pt nanostructures . We also developed the synthesis of octahedral nanostructures with various compositions (i.e., Pt3Pd1, Pt1Pd1, and Pt1Pd3), but having dominantly exposed facets, as catalysts for ORRs as indicated in Figure 14(a) . Interestingly, among them, it was found that the octahedral Pt3Pd1 nanostructure shows the significantly enhanced electrochemical catalytic activity, such as the increased specific area by 2.35 times and enhanced mass by 1.88 times activity at 0.55 V, and the most stable electrochemical properties in comparison with commercial Pt in Figure 14(b) due to the octahedral shape with dominant facets and favorable elemental composition.
Furthermore, Markovic and coworkers have demonstrated that Pt-Ni alloy nanostructures can be an effective electrocatalyst for the ORR. They showed that Pt3Ni1 facet is 10 and 90 times more electrochemically active than Pt facets and commercial Pt in ORRs, respectively, due to their unusual electronic structures for the downshift of d-band center as well as arrangement of surface atoms in the near-surface region in crystal structures . Based on their demonstration of nanostructured Pt-Ni alloy catalysts, many researches have been reported on various nanostructured Pt-Ni alloys. Xia and coworkers demonstrated that octahedral Pt2.5Ni1 nanostructures exhibited highly specific mass activity compared to spherical Pt3Ni and commercial Pt, because they have the clean catalysts surface and the mostly exposed facets . More recently, in order to maximize surface area, Chen and coworker suggested highly crystalline Pt3Ni1 nanoframes with 3D structure . They clearly observed that Pt3Ni1 nanoframes significantly enhanced the electrocatalytic mass activity with a world record of 5.7 A at 0.9 V (versus RHE) in the ORR due to their high surface-to-volume ratio, 3D surface molecular accessibility for frame structure, and the modulated electronic structure for Pt-Ni alloying phase as shown in Figure 15.
Alternatively, on the basis of expectations that facets ( and/or is greater than one) can improve the electrochemical ORR activities in comparison with the low-index facets, other strategies based on the surface structure modification of Pt nanostructures have been developed to enhance the electrochemical ORR activity. Wang and coworkers demonstrated the synthesis of Pt high concave cubic nanostructures with high-index facets and their utilization for ORR . They showed that the specific mass activity of the Pt concave cubic nanostructures was about 0.71 A at 0.9 V (versus RHE) in ORR, improved by 2.8 times higher than that of commercial Pt. Additionally, Lim and coworkers also reported the Pd-Pt bimetallic nanodendrites as an ORR electrocatalyst. It was shown that Pd-Pt bimetallic nanodendrites exhibited 2.5 times higher specific mass activity compared to commercial Pt due to their relatively higher surface area resulting from dendritic structures and the exposed facets on branch surface as an active site for ORR .
In addition to the development of nanostructured catalysts based on single metal and alloy, many researches have focused on the development of the Pt-based core-shell nanostructures as attractive structures for electrocatalysts in ORR because these structures with designed compositions and morphologies can efficiently reduce the cost of fuel cell manufacturing by significantly reducing the overall amount of Pt required [111–113]. Specially, it would be desirable to reduce the amount of Pt at the cathode because the cathode requires 2~3 times higher Pt loading than required at the anode due to the low kinetic reduction reaction rate in comparison with fuel oxidation reaction. Furthermore, both the exposed surface area and active site density of Pt-based electrocatalysts are very important since the electrochemical reaction in a fuel cell actually occurs on the surface of catalysts. For this reason, many studies have been focused on the control over the shell thickness and composition of Pt-based core-shell nanostructures with inexpensive metal elements as the core and Pt as the shell to enhance specific mass activity in ORR . Wang and coworkers reported that the Cu@Pt core-shell nanoparticles prepared using a galvanic displacement method have approximately 2 times higher specific mass activity than that of commercial Pt/C at 0.85 V (versus NHE) . Additionally, many researches have been reported on a core@Pt-based alloy shell nanostructure in order to increase the catalytic activity and the specific mass activity [116, 117]. In recent, Choi and coworkers reported that octahedral Pd@Pt-Ni core-shell nanostructures can be synthesized by seeding-mediated and thermal-decomposition methods and that an ORR mass activity (2.5 A mg−1) and a specific activity (2.7 mA cm−2) are 12.5-fold and 14-fold higher than that of commercial Pt/C at 0.9 V (versus RHE), respectively . Mazumder and coworkers also demonstrated the synthesis of the shell thickness-controlled Pd@FePt core-shell nanostructures with 1~3 nm shell thickness for the enhanced electrochemical activity and stability in ORR . They reported that a Pd@FePt nanostructure with 1 nm shell thickness showed 12 times improved specific mass activity compared to commercial Pt due to the thinner alloy FePt shell with higher ORR activity. In order to further provide recent progress in Pt-based electrocatalyst technology, various Pt-based nanostructures with the enhanced catalytic activity and related synthesis approaches are summarized in the Table 3.
In summary, we present snapshots of recent research carried out on Pt-based electrocatalysts technology for high performance low-temperature fuel cells, particularly focusing on the recent issues and progress in the development of various synthetic approaches for Pt-based nanostructures with controlled shapes and their electrochemical characteristics of both electrodes. In addition, we describe fundamental aspects of electrochemical reactions and mechanism associated with the shape, dimension, faceted morphology, and composition of nanostructured Pt-based catalyst materials, significantly affecting their electrocatalytic activity. It is expected that this review will give insights not only into understanding the basic electrochemical mechanism and kinetics of both electrode reactions in fuel cells, but also into developing a genuinely practical electrocatalysts technology for fuel cell commercialization.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was supported by the International Collaborative Energy Technology R&D Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (no. 20138520030800 and 20128510010080).
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