Recent advances in cathode electrocatalysts for PEM fuel cells

Abstract

Great progress has been made in the past two decades in the development of the electrocatalysts for proton exchange membrane fuel cells (PEMFCs). This review article is focused on recent advances made in the kinetic-activity improvement on platinum- (Pt-) based cathode electrocatalysts for the oxygen reduction reaction (ORR). The origin of the limited ORR activity of Pt catalysts is discussed, followed by a review on the development of Pt alloy catalysts, Pt monolayer catalysts, and shape- and facet-controlled Pt-alloy nanocrystal catalysts. Mechanistic understanding is reviewed as well on the factors contributing to the enhanced ORR activity of these catalysts. Finally, future directions for PEMFC catalyst research are proposed.

Keywords

proton exchange membrane fuel cells (PEMFCs) / cathode electrocatalysts / platinum / oxygen reduction reaction (ORR)

Introduction

The proton exchange membrane fuel cell (PEMFC), also known as polymer electrolyte membrane fuel cell, has emerged in the last two decades as one of the most promising clean energy technologies for the application in mobile and stationary power sources in the 21st century. The advantages of a PEMFC include low operating temperature (approximately 80°C), high power density, quick start-up, and quick response to shifting demands for power. While the PEMFC, among other technologies, shows the most advantages for the application in automotive propulsion, the high Pt loading and possible issues with the low durability of the Pt based cathode catalyst in the PEMFC are two of the greatest barriers hindering the mass production of fuel cell vehicles.

In a PEMFC, if pure hydrogen is used as the fuel, the anode reaction is the hydrogen oxidation reaction (HOR) at the surface of the anode platinum electrocatalyst. The HOR and hydrogen evolution reaction (HER) are by far the most thoroughly investigated electrochemical reaction systems [1]. Due to the fast electrode kinetics of hydrogen oxidation at platinum surface [1-3], the anode platinum loading can be reduced down to no more than 0.05 mgPt/cm² without significant performance loss [4]. The cathode reaction in a PEM fuel cell is the oxygen reduction reaction (ORR) at platinum surface in an acidic electrolyte. In contrast to HOR at the anode, the cathode ORR is a highly irreversible reaction even at temperatures above 100°C, even for the best existing catalysts, which contain Pt [5-8]. Gasteiger et al. [9] found that 0.4 mgPt/cm² was close to the optimal platinum loading for the air electrode using the state-of-the-art Pt/C catalyst and an optimized electrode structure. Further reduction of the cathode platinum loading will result in cell voltage loss at low current densities that follows the ORR kinetic loss. This high platinum loading (albeit an order of magnitude lower than the loadings used in previous decades) at the cathode is made necessary by the slow kinetics of the ORR at the platinum surface. To make the fuel cell vehicles commercially viable on the market, the platinum loading on the cathode has to be reduced significantly. Recent increases in Pt prices suggest that at least an 8-fold improvement in the Pt-mass-specific activity (denoted hereafter as Pt-mass activity) should be strived for in order to meet the Pt loading target set for mass production of fuel cell vehicles.

This brief review is focused on the recent progress in the search for more active and more durable platinum-based ORR catalysts. The mechanism of the ORR on platinum surface is discussed, followed by a review of recent advances in the development of platinum-alloy catalysts, platinum monolayer catalysts, and the shape- and facet-controlled platinum-alloy nanocrystal catalysts. Mechanistic understanding of the factors contributing to the enhanced ORR activity of those catalysts is reviewed as well. Finally, future directions for the PEMFC catalyst research are proposed.

ORR on platinum surfaces

While the detailed mechanism of the ORR still remains elusive [6], it is widely accepted that the ORR on platinum surfaces is dominantly a multi-step 4-electron-reduction process with H₂O being the final product. The overall four electron reduction of O₂ in acid aqueous solutions is

$$\begin{gathered} {\mathrm{O}}_{\mathrm{2}}+{\mathrm{4H}}^{+}+\mathrm{4}{\mathrm{e}}^{-}\to \mathrm{2}{\mathrm{H}}_{\mathrm{2}}\mathrm{O} \\ {\mathrm{E}}_{\mathrm{0}}=\mathrm{1.229}\mathrm{V}\mathrm{v}\mathrm{s}.\mathrm{N}\mathrm{H}\mathrm{E}\mathrm{a}\mathrm{t}\mathrm{298}\mathrm{K}, \end{gathered}$$

where E₀ denotes the electrode potential for the reaction under standard conditions and NHE is the normal hydrogen electrode. Since the four-electron reduction of oxygen is highly irreversible, the experimental verification of the thermodynamic reversible potential of this reaction is very difficult. The irreversibility of the ORR imposes serious voltage losses in fuel cells. In most instances the current densities practical for kinetic studies are much larger than the exchange current density of ORR; therefore, the information obtained from current-potential data is confined only to the rate-determining step. On the other hand, in the ORR kinetic potential region, the electrode surface structure and its chemical properties strongly depend on the applied potential and the time held at that potential, which makes the reaction more complicated. While the relationship between the overall kinetics and the Pt-surface electronic properties is not well understood, it is widely accepted that in the multi-step reaction, the first electron transfer is the rate determining step, which is accompanied by or followed by a fast proton transfer [5-7]. Two Tafel slopes are usually observed for the ORR on Pt in RDE tests in perchloric acid, from -60 mV/decade at low current density, transitioning to -120 mV/decade at high current density. The lower Tafel slope of the ORR in perchloric acid at low current density has been attributed to the potential dependent Pt oxide/hydroxide coverage at high potentials [10-14].

By using DFT models, Norskov and coworkers [15] recently calculated the Gibbs free energy of the ORR intermediates as a function of the cathode potential based on a simple dissociative mechanism, i.e., assuming the adsorbed oxygen atoms and the hydroxyl groups being the only intermediates. They found that the oxygen or the hydroxyl is so strongly bound to the platinum surface at the thermodynamic equilibrium potential that the proton and the electron transfer become exceedingly small. By lowering the potential, the stability of adsorbed oxygen decreases and the reaction may proceed at a measurable rate. They suggested that the overpotential of the ORR on platinum surfaces lie in these effects.

As shown in Fig. 1(a), the model predicts a volcano-shaped relationship between the rate of the ORR and the oxygen adsorption energy, with platinum and palladium being among the best metals for electrocatalysis of the ORR. Figure 1(b) indicates that the bonding energy of OH is roughly linearly correlated to that of O, indicating both are nearly equivalent parameters in determining the ORR activity. More recently, Wang et al. [16] derived an intrinsic kinetic equation for the four-electron (4e^-) oxygen reduction reaction (ORR) in acidic media, by using free energies of activation and adsorption as the kinetic parameters. The values of these parameters were obtained through fitting experimental ORR data from a Pt(111) rotating disk electrode (RDE). In contrast to more conventional ORR kinetic models, in this work there is no assumption of a single particular rate-determining step (RDS). The results indicate that the first electron transfer is not the RDS for the ORR on Pt at high potentials, because dissociative adsorption provides a more active adsorption pathway. However, the reaction intermediates, O and OH, are strongly trapped on the Pt surface. Thus, the ORR on Pt is desorption-limited at high potentials, exhibiting a low apparent Tafel slope at those potentials. Wang et al. [17] further used this kinetic model to fit a typical iR-free polarization curve of a PEMFC by adjusting the parameters to reflect the fuel cell operating conditions at 80°C. The results showed that the transition of the Tafel slope occurs at approximately the same 0.77 V that is the equilibrium potential for the transition between adsorbed O and OH on a Pt surface with low adsorbed total coverages of oxygen species [15].

The Pt size effect on ORR plays an important role in determining the minimum loading of Pt catalyst required for PEMFCs, not only through altering the fraction of Pt atoms on the surface vs. total Pt atoms, but also through changing the ORR kinetics per surface Pt atom (i.e., the Pt (area-)specific activity). Earlier results by Blurton et al. [18] and Peuckert et al. [19] suggested that the optimal Pt size for the maximum Pt mass activity is approximately 3 nm. Kinoshita [20] reviewed and analyzed the particle size effect for ORR on Pt/C catalysts, and proposed that the decrease of Pt specific activity with the decrease of Pt particle size is a consequence of the changing distribution of surface atoms at the (100) and (111) crystal faces, although recent reliable measurements of ORR activities on large single crystals in non-adsorbing electrolytes show little difference in activity between these two surfaces of pure Pt [72]. Recent data in Ref. [7] in H₃PO₄ reported by Kinoshita showed that when Pt particle size increases from 2.5 nm to 12 nm, there is approximately 3 fold of increase in Pt specific activity. The results also confirmed that the optimal Pt particle size for the maximum Pt-mass activity is around 3nm. Gasteiger et al. [21] investigated Pt/C and Pt black catalysts for ORR in HClO₄ solution at 60°C, with the Pt particle size ranging from 2nm to over 10nm, and found that the magnitude of activity improvement is comparable to those reported in Refs. [22,23]. Norskov and other researchers [24-26], based on their DFT calculations, recently proposed using the concept of averaged d-band center energy to explain the reactivity of metal surface atoms, which was supported by numerous, but not all, experimental data. They reported that, when the Pt-particle size decreases, the average coordination number of surface Pt atoms decreases, causing the d-band center of those atoms to move closer to the Fermi level. As a result of that, the Pt atoms bind oxygen and/or hydroxide more strongly, and therefore they have a lower ORR activity. A stronger adsorption of OH species on Pt surface when the particle size is reduced to below 5 nm was reported in Ref. [27].

ORR on Pt-alloy catalysts

Great progress has been made in the past decades in developing more active and durable Pt-alloy catalysts and in understanding the factors contributing to their activity enhancements. Two- to three-fold specific activity enhancements vs. pure Pt have typically been reported [21,28-30]. As far as which alloy and what alloy compositions confer the highest ORR activity, there seems to be lack of general agreement. This is probably because the measured activity depends highly on the catalyst surface and near-surface atomic composition and structure, on impurities on the surface, and on particle size and shape, all of which could be affected by the preparation method, heat treatment protocol, and testing conditions. For example, to achieve the optimal alloy structure for maximum activity, different Pt alloy particles may require different annealing-temperature protocols to accommodate the distinctions between metal melting points and particle sizes [31,32]. Several representative mechanisms have been proposed to explain the enhanced activities observed on Pt alloy catalysts: a surface roughening effect due to leaching of the alloy base metal [33,34]; decreased lattice spacing of Pt atoms due to alloying [28,35,36]; electronic effects of the neighboring atoms on Pt, such as increased Pt d-band vacancy [28,37] or depressed d-band center energy upon alloying [38-40]; and/or decreased Pt oxide/hydroxide formation at high potential [28,41]. The increased Pt surface roughness alone may help increase the Pt mass activity but will not increase the Pt specific activity. The other mechanisms are at least partly coupled with each other. For example, the decreased lattice spacing may affect the electronic structure of Pt atoms, which in turn may inhibit the Pt oxide/hydroxide formation. A more detailed discussion follows.

Earlier results on ORR in phosphoric acid by Jalan and Taloy [35] suggested that the nearest-neighbor distance between Pt atoms played an important role in the ORR, based on the reaction model proposed by Yeager et al. [5], i.e., the rate determining step being the rupture of O-O bond via various dual-site mechanisms. They proposed that the distance between the nearest-neighbor atoms on the surface of pure Pt is not ideal for dual site adsorption of O₂ or “HO₂”, and that the introduction of foreign atoms which reduce the Pt nearest-neighbor spacing would result in a higher ORR activity. By testing a number of carbon-supported Pt-M alloy catalysts, a linear relationship was obtained between the activity and the nearest-neighbor atomic distance as illustrated in Fig. 2. While the geometric distance between the neighboring Pt atoms is shorter in alloys, the surface electronic structure of Pt alloys is different from that of pure Pt as well, so it is difficult to separate these two factors. Other studies [33,34,42] claimed no activity enhancement was observed on Pt-Cr alloy over pure Pt other than that due to an increased surface roughness [33,34], but this conclusion has not been supported by more recent studies [28,36].

In a related study, Mukerjee et al. [28] investigated five binary Pt alloys (PtCr/C, PtMn/C, PtFe/C, PtCo/C, and PtNi/C) supported on high-surface-area carbon for ORR in a proton exchange membrane fuel cell. The electrode kinetic studies on the Pt alloys showed a two- to threefold increase in ORR activity relative to a reference Pt/C electrocatalyst, with the PtCr/C alloy exhibiting the best performance. Contractions in the Pt-Pt bond distances were observed by both extended X-ray absorption fine structure (EXAFS) and X-ray diffraction (XRD). In addition, they found that in the double-layer potential region (0.54V vs. RHE), the alloys possess higher Pt d-band vacancies than Pt/C, while in the high potential region (0.84V vs. RHE), Pt/C shows higher d-band vacancy relative to alloys. This was interpreted as a consequence of the significant adsorption of OH species at high potential on Pt/C but to a lesser extent on Pt alloys. They rationalized the enhanced activity on alloys on the basis of electronic and geometric effects, and of inhibition of OH adsorption.

It has been reported that the topmost atomic layer of an annealed Pt-alloy catalyst is composed of pure Pt while the second layer is enriched in the transition metal M, at least for Pt-rich Fe, Co and Ni alloys [39,43,44], This segregation is produced during annealing by displacement of Pt and M atoms in the first several layers to minimize the total free energy. By using DFT calculations, Xu et al. [45] studied the adsorption of O and O₂ and the dissociation of O₂ on the (111) faces of ordered Pt₃Co and Pt₃Fe alloys and on monolayer Pt skins covering these two alloys. The results were compared with those calculated for two Pt(111) surfaces, one at the equilibrium lattice constant and the other laterally compressed by 2% to match the strain in the Pt alloys. The absolute magnitudes of the binding energies of O and O₂ followed the same order in the two alloy systems: Pt skin<compressed Pt(111)<Pt(111)<unsegregated Pt₃Co(111) or unsegregated Pt₃Fe(111). The reduced bonding strength of the compressed Pt(111) and Pt skins for oxygen was rationalized as being caused by the shifting of the d-band center increasingly below from the Fermi level. They proposed that an alleviation of poisoning by O and enhanced rates for reactions involving O could be some of the reasons why Pt skins are more active for the ORR.

For acid-treated Pt₃Co nanoparticles, Chen et al. [43,46] observed the formation of percolated Pt-rich and Pt-poor regions within individual nanoparticles, analogous to the skeleton structure proposed for sputtered polycrystalline Pt alloy surfaces after acid leaching [47]. The acid-treated alloy nanoparticles yielded approximately two times the specific activity of pure-Pt nanoparticles. After annealing of the acid treated particles, sandwich-segregated surfaces of ordered Pt₃Co nanoparticles were directly observed, with the topmost layer being pure Pt atoms. The specific activity of annealed nanoparticles was approximately 4 times that of pure Pt nanoparticles. The enhanced Pt specific activity toward ORR was attributed to the reduced binding energy of oxygenated species, owing to a combination of two effects, 1) the increased compressive strain in Pt atoms, and 2) the ligand effect from underlying Co atoms. Strasser and coworkers [31,48,49] recently applied a freeze-drying technique in the synthesis of Pt alloy nanoparticle precursors from which most of the alloying-element atoms were subsequently removed by electrochemical voltage cycling. These dealloyed catalysts were reported to have both mass and specific activities about 4-6 times those of a standard commercial Pt/C catalyst, in both RDE and MEA tests. Bulk and surface structural and compositional characterization suggested that the most common dealloyed active catalyst phase consists of a core-shell structure in which a Pt-rich shell of several atomic layers thick surrounds a Pt-poor alloy core. This work constitutes significant progress on initial activitiy, since a 4-fold of increase of Pt mass activity has been the performance target for commercially viable fuel cell cathode catalysts [21]. Figure 3 represents the schematic of the Pt-Cu-particle dealloying process and the electrochemical testing results of the dealloyed catalysts.. Figure 3(a) shows a schematic of the dealloying process, and Fig. 3(b) is the cyclic voltammetry and the ORR polarization curves of Pt-Cu/C synthesized at different temperatures, compared with the Pt/C catalyst. Strasser’s group concluded that geometric effects play a key role, because the low residual Cu near-surface concentrations suggested by XPS measurements make significant direct electronic interactions between surface Pt atoms and Cu atoms unlikely. A 4-fold of enhancement in Pt mass activity on monodispersed Pt₃Co nanoparticles with a particle size of 4.5 nm was also reported recently [50]. While Pt-alloy catalysts with 4-fold enhancement in both Pt mass activity and specific activity relative to standard Pt/C catalyst seem achievable, as has been shown above, the long-term durability of the alloy catalysts is still a concern, due to the possibility of dissolution of the base metal from the alloys [21,51].

ORR on Pt monolayer electrocatalysts

The Pt monolayer electrocatalyst has been one of the key concepts towards reducing the Pt loading of PEM fuel cells in recent years. Pt submonolayers deposited on Ru nanoparticles had earlier been demonstrated to give superior performance with ultra-low Pt loading compared to commercial Pt/C or Pt-Ru alloy catalysts for the anode CO-tolerant HOR [53-57]. More recently, Adzic and coworkers applied this concept in making novel Pt monolayer catalysts for the cathode ORR, which is the focus of the review in this section. In general, their method of synthesizing Pt monolayer catalysts involves underpotential deposition (UPD), a technique well known to produce an ordered atomic monolayer metal deposition onto a foreign metal substrate [58-60]. Specifically, the method consists of two steps [61,62]: First, a monolayer of a sacrificial less-noble metal is deposited on a more noble metal substrate by UPD, such as Cu UPD on Au or Pd; second, the sacrificial metal is spontaneously and irreversibly oxidized and dissolved by a noble metal cation, such as a Pt cation, which is simultaneously reduced and deposited onto the foreign metal substrate. The whole procedure can be repeated in order to deposit multilayers of Pt (or another noble metal) on the foreign metal.

The advantages of Pt monolayer catalysts include full utilization of the Pt atoms which are all on the surface, and tailoring of the Pt activity and stability by the selection of the substrate metals. As an example [63], when a Pt monolayer is deposited onto different substrate metals, as shown in Fig. 4, it can experience either compressive or tensile stress due to the lattice mismatch between the metals, which is known to affect the Pt activity by adjusting its d-band center energy [2445] and consequently its ORR activity.

Zhang et al. investigated Pt monolayer deposits on Pd(111) single crystals (Pt/Pd(111)) and on Pd/C nanoparticles (Pt/Pd/C) for ORR [62]. The ORR reaction mechanism of the monolayer catalysts was found to be the same as that on pure Pt surface. Pt/Pd(111) was found to have a 20mV improvement in half-wave potential vs. Pt(111), and the Pt/Pd/C had a Pt-mass activity 5-8 times higher than that of Pt/C catalyst. If the total noble metal amount (Pt+ Pd) is counted, the mass activity is approximately 1.8 times that of Pt/C catalyst [62]. The enhanced ORR activity is attributed to the inhibited OH formation at high potential, as evidenced from XAS measurements. In a real fuel cell test, 0.47 gPt/kW was demonstrated at 0.602 V [64]. Zhang et al. further investigated the ORR on platinum monolayers supported on Au(111), Ir(111), Pd(111), Rh(111), and Ru(0001) single crystal RDE surfaces [65]. A comparison of the polarization curves at 1600rpm is given in Fig. 5. The increase of the ORR activities follows the sequence Pt/Ru(0001)<Pt/Ir(111)<Pt/Rh(111)<Pt/Au(111)<Pt(111)<Pt/Pd(111).

Zhang et al. [65] correlated the kinetic activities of the “monolayers” with the Pt d-band center energies (not shown) and Pt-O binding energies, and found a “volcano” relationship, with the Pt/Pd(111) having the optimal d-band center, as well as Pt_ML-O binding energy, for the maximum ORR activity (See Fig. 6). The “volcano” behavior was rationalized as being controlled by the two key steps in ORR: ① the O-O bond dissociation, followed by ② the O-H bond formation. As shown in Fig. 6, the activation energies of the two steps both correlated linearly with the Pt_ML-O binding energies (and with the d-band center energies, not shown), but in the opposite trend, indicating the Pt_ML-O binding can be neither too strong, nor too weak, for the best ORR activity. For further fine tuning of the monolayer Pt/Pd ORR activity, Zhang et al. introduced mixed metal+ Pt monoloayer catalysts [66], which contained 0.2 monolayer of a foreign metal from selection of (Au, Pd, Rh, Ir, Ru, Os, and Re) combined with 0.8 monolayer of Pt co-deposited on Pd(111) or on Pd/C nanoparticles. The foreign metals have either a weaker M-OH bond (for the case of Au-OH), or a stronger M-OH bond (for the rest of the cases) than the Pt-OH bond. DFT calculations [66] showed that, in addition to altering the Pt d-band center energies, the OH_(-M)-OH_(-Pt) (or O_(-M))-OH_(-Pt)) repulsion plays an important role in augmenting the ORR activity, as displayed in Fig. 7. Instead of adjusting the composition of the top-most Pt monolayer, replacing the substrate Pd(111) with Pd₃Fe(111) to generate Pt/Pd₃Fe(111) was recently reported to also yield an enhanced ORR activity [67].

Other Pt monolayer electrocatalysts showing improved ORR activity or durability include Pt deposited on (noble metal)/(non-noble metal) core-shell nanoparticles [68], Au cluster modified Pt/C catalysts [69], and Pt on Pd-alloy catalysts [70,71].

In summary, Pt monolayer catalysts give a promising pathway toward solving one of the major problems facing PEM fuel cells by enhancing the Pt specific activity and the utilization of Pt atoms, thereby reducing the cost of the cathode catalyst. However, more fuel cell tests of durability are needed before monolayer catalysts can be used in fuel cell vehicles, and there is still a need for a reduction in the total noble metal content in these catalysts.

ORR on Facet- and shape-controlled Pt-alloy nanocrystal electrocatalysts

Stamenkovic and coworkers [72] demonstrated that on RDE Pt₃Ni(111) single-crystal surfaces, the specific activity for ORR is approximately an order of magnitude higher than on the Pt(111) surface and is approximately 90 times higher than on Pt/C catalyst, while the other two low-index surfaces, [Pt₃Ni(100) and Pt₃Ni(110)] are much less active than Pt₃Ni(111). This result is very intriguing in that it suggests that if Pt₃Ni nanocrystals with all exposed surfaces having {111} orientations can be made, an enhancement of specific activity by up to two orders of magnitude relative to state-of-the-art Pt/C catalysts can be hopefully gained. Recently, two interesting papers [73,74] have shown progress on synthesizing such Pt alloy nanocrystals.

Wu et al. [73] recently reported an approach to the preparation of truncated-octahedral Pt₃Ni (t,o-Pt₃Ni) catalysts that have dominant exposure of {111} facets. Three sets of Pt₃Ni nanocrystals were generated with various mixture of truncated octahedra (exposing both {111} and {100} facets) and cubes (exposing only {100} facets).

The particle size was approximately 5 to 7 nm. The fraction of the total surface area in {111} facet could be calculated based on particle geometries and population statistics. Figure 8 shows a comparison of polarization curves, cyclic-voltammetry curves, mass activities, and specific activities of the Pt₃Ni nanocrystals and the standard TKK Pt/Vulcan carbon catalyst. As exhibited in Fig. 8, almost-linear correlations were obtained for both mass activities and specific activities versus the fraction of the (111) surface area. While the {111} facets of the nanocrystals showed much higher specific activity than the {100} facets (in agreement with the trend found on bulk Pt₃Ni single crystal disks), the absolute values of the specific activities of the nanocrystals were still far below those observed on bulk single crystal surfaces [72].

Zhang et al. [74] recently reported a success on the separate syntheses of Pt₃Ni nano-octahedra and nano-cubes, with these two shapes of nanocrystals having only {111} facets and {100} facets exposed, respectively. Figure 9 demonstrates a SEM image of those shape- and size-controlled nano-octahedral crystals. The chemical compositions of the crystals were analyzed by using combined ICP-MS and EDS techniques (from both TEM and SEM), and the results suggested that the average molar ratio of Pt to Ni was 3∶1.

Zhang et al. [74] further investigated ORR activities of the shape controlled nanocrystals by using RDE measurements conducted in an O₂-saturated 0.1 M HClO₄ solution at 295 K. Figure 10 is the electrochemical testing results of these faceted Pt₃Ni-nanocrystal catalysts. A characteristic set of polarization curves at 900rpm for the ORR on Pt₃Ni nanoctahedra, Pt₃Ni nanocubes, and Pt nanocubes are displayed in Fig. 10(a). The Pt specific activity and mass activity at 0.9 V are plotted in Fig. 10(b). The significant shape dependence of ORR activity agreed with the observation from the extended Pt₃Ni single crystal surfaces, although (again) the absolute values of specific activities observed on Pt₃Ni nanocubes and nanoctahedra were approximately 4- to 7-fold lower than those reported for large single-crystal surfaces in Ref. [72]. One apparent puzzle in these reported results is that the Pt surface area or ECSA of the nanocrystals derived from the specific activity and mass activity (ECSA per unit mass of Pt= mass activity/specific activity) is 5-10 times lower than would be expected from the size of particles revealed by SEM and TEM images. The discrepancy may come from a low utilization of the surface area because of impurities or from overlap of the unsupported nanocrystals. Another dataset for the octahedral supported on carbon was reported in the online supporting information of the paper, and this showed a better agreement between measured ECSA and diameter to TEM, suggesting that particle aggregation caused the low area observed for the unsupported catalysts.

In summary, the size- and shape-controlled synthesis of nanocrystal Pt-based electrocatalysts has paved a promising path to high Pt specific activity, although the absolute number of the activity for the faceted nanoparticles has still not been comparable to that observed on extended Pt-alloy single crystal surfaces. These discrepancies in specific activity probably arose from either an effect of facet size or residual impurities and defects in the nanocrystal surface and/or incomplete formation of smooth, segregated Pt layers on the facet surfaces. The Pt mass activity achieved for the best case is already about 4 times higher than the state-of-the-art Pt/C catalyst. If the core of the nanocrystals could be replaced with some corrosion-resistant material but keeping the surface of the Pt alloy shell still in {111} facets, a significant further reduction of the Pt loading required for the cathode catalyst could be expected. In addition, increases in size of the core/shell faceted particles might be able to take further advantage of the specific-activity benefits of continuous Pt and Pt-alloy surfaces without wasting Pt in the center of the large particle. The durability of such nanocrystals could be expected to be high because of the lack of low-coordination atoms in their surfaces.

Future directions

Low platinum loadings, high activity and more durable catalysts still remain as critical challenges for PEFCs for automotive applications. Further fundamental understanding of the correlations between activity, stability and structural properties at the atomic level are most desirable from both theoretical and experimental perspectives. Studies of the connections between the activities of controlled-facet-orientation nanoparticles and extended single-crystal surfaces would be helpful. Structure and surface controlled syntheses of catalysts (Pt monolayer catalysts, nano-structured catalysts and electrodes, size- and facet- controlled Pt-alloy nanocrystals, combined with core-shell structures) should provide a practical path to achieving the low fuel cell catalyst loadings required for the large-scale commercialization of fuel cell vehicles.

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