Unsupported Platinum-Based Electrocatalysts for Oxygen Reduction Reaction Xin Long Tian,†,‡ Yang Yang Xu,†,‡ Wenyu Zhang,§ Tian Wu,∥ Bao Yu Xia,*,†,‡ and Xin Wang*,§ †
Key Laboratory of Material Chemistry for Energy Conversion and Storage (Ministry of Education), Hubei Key Laboratory of Material Chemistry and Service Failure, School of Chemistry and Chemical Engineering, Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology (HUST), 1037 Luoyu Road, Wuhan 430074, China ‡ Shenzhen Institute of Huazhong University of Science and Technology, Shenzhen 518000, China § School of Chemical and Biomedical Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798 Singapore ∥ College of Chemistry and Life Science, Institution Hubei University of Education, Wuhan 430205, People’s Republic of China ABSTRACT: Electrocatalyst development has been a cornerstone for fuel cell applications but heavily hindered by several technical challenges, especially the insufficient activity and poor durability of electrocatalysts toward the sluggish kinetics of the oxygen reduction reaction. Here we propose our perspective on the exciting developments in unsupported platinum (Pt)-based nanocatalysts, with an emphasis on the multidimensional Pt architectures as an alternative to carbon-supported Pt electrocatalysts. After reviewing recent developments, we highlight the challenges and opportunities of unsupported Pt-based electrocatalysts, which may offer a broad materials library of noble metal structures for fuel cells and other sustainable energy technologies.
T
he massive consumption of fossil fuels and the consequential ever-growing environmental concerns have stimulated and triggered extensive research efforts in the innovation of clean and sustainable energy technologies. Due to the high energy conversion efficiency, low operation temperature, and low pollution emissions, fuel cell technologies have attracted a great deal of attention from the scientific community, commercial organizations, and governments.1−4 A fuel cell involves the oxidation of chemical fuels on the anode and oxygen reduction reaction (ORR) on the cathode. The cathodic reaction is kinetically slow as it involves complex processes, for example, proton transfer, electron transfer, and oxygen bond breaking. Therefore, highly efficient electrocatalysts are needed in order to lower the reaction overpotential and facilitate oxygen reduction.5−7
Currently, platinum (Pt)-based composites are the most frequently used and effective catalysts to catalyze ORR.8,9 A very high mass activity of 13.6 A mgPt−1, which is 52 times that of the commercial Pt/C electrocatalyst, was reported by Duan’s group very recently. This sheds new hope on the use of Ptbased electrocatalysts.10 Particularly, the first commercialized fuel cell vehicle (FCV), Toyota Mirai, with a power of over 140 kW and a cruising range exceeding 600 km, was launched at the end of 2014, making hydrogen fuel cells approachable to the public. Despite the initial success of fuel cells in portable electronics and electric vehicles, to further extend their commercial applications, several critical technological challenges must be overcome. First, the large amount of Pt in the cathodes for improving the sluggish kinetics of ORR leads to the high cost of the whole fuel cell. Second, the Pt atoms suffer from severe dissolution and migration during the electrochemical process, leading to the loss of electrochemical surface area (ECSA) and performance degradation. Finally, the severe oxidation and corrosion of carbon in electrochemical processes, especially in transient conditions such as start-up/shutdown or local fuel starvation events at the anode, will lead to the detachment and aggregation of Pt particles and accelerate the loss of ECSA. This will lead to a serious durability problem
The severe oxidation and corrosion of carbon in electrochemical processes, especially in transient conditions such as start-up/shutdown or local fuel starvation events at the anode, will lead to the detachment and aggregation of Pt particles and accelerate the loss of ECSA. © 2017 American Chemical Society
Received: July 7, 2017 Accepted: August 8, 2017 Published: August 8, 2017 2035
DOI: 10.1021/acsenergylett.7b00593 ACS Energy Lett. 2017, 2, 2035−2043
Perspective
http://pubs.acs.org/journal/aelccp
ACS Energy Letters
Perspective
Figure 1. (a) SEM image of Pt NTs, (b) stability results of Pt/C (E-TEK), Pt-black (E-TEK), and Pt NTs (reprinted from ref 12; Copyright 2007 John Wiley & Sons). (c) Mass and specific activities shown as a function of durability cycling for the Pd@Pt NWs and Pt/C catalysts at 0.9 V (versus RHE), (d) TEM image, (e) HRTEM image, and HAADF/STEM-EDS mapping analysis showing the elements Pt (f) and Pd (g) dispersed on the Pd@Pt NWs. Scale bar, 20 nm (reprinted from ref 20).
during the real operation of fuel cells.11,12 Therefore, rational design and convenient fabrication of highly active and durable Pt-based nanocatalysts toward ORR at low cost are very desirable but remain challenging. The most effective method to boost the ORR performance of Pt-based electrocatalysts would be alloying Pt with nonprecious metals to form PtM alloys or core/shell structured M@Pt catalysts (where M normally represents transition metal Fe, Co, Ni, Cu, and so forth or their alloys). The introduction of foreign metals cannot only significantly increase the utilization efficiency of Pt but also remarkably improve its ORR activity by optimizing oxygen adsorption strength on the catalyst surface through synergetic effects, for example, the ligand effect and the strain effect.8,9,13 On the other hand, the incorporation of novel supporting materials, such as metal oxides, nitrides, or carbides, can alleviate the negative effect of carbon corrosion and endow the electrocatalysts much higher stability.14−16 Although multiple methods have been proposed to develop electrocatalysts with both excellent ORR activity and durability at low cost, it is, however, often not possible to independently or simultaneously address the challenges mentioned above. Recently, the development of nanomaterials and nanotechnologies inspired various Pt-based nanostructures. Onedimensional (1D) Pt-based nanorods/wires/nanotubes (NTs), two-dimensional (2D) nanosheets/plates/films/membranes, even three-dimensional (3D) networks and porous superstructures have attracted much attention for their excellent electrocatalytic activity and stability.17−22 Generally, these multidimensional Pt-based architectures are significantly less vulnerable to dissolution, aggregation, and ripening than zerodimensional (0D) isotropic Pt nanoparticles due to the inherent anisotropic structure and relatively low defect density. Moreover, their unique structural advantages including high aspect ratios, excellent porosity coupled with specific surface/ interface atom arrangement, and enhanced electron transfer compared to their nanoparticle counterparts are all desired to achieve the high electrochemical performance of Pt-based nanocatalysts. In fact, unsupported Pt-based nanostructures may provide an alternative solution for an ideal ORR electrocatalyst. It is because they exhibit good performance and inherently do not have the negative issues associated with
the use of carbon, including corrosion and oxidation and possible weak interaction between Pt and supports. In addition, the absence of carbon supports would form a reduced electrocatalyst layer in the membrane electrode assembly (MEA) because the catalyst layer (Pt nanostructures, without carbon supports) could be in direct contact with the reactants at the three-phase zones (the polymer electrolyte, the solid catalysts, and the oxygen gas). A thin catalyst layer offers better heat and water transport, a shorter proton and electron pathway, and better utilization of the catalyst at all current densities.23 All of these attractive advantages corroborate that the unsupported Pt-based nanostructures are indeed promising due to their excellent catalytic performance and durability for practical applications. In this Perspective, we provide an overview of the recent exciting results for unsupported Pt-based electrocatalysts toward ORR. The sufficient large quantities of investigations coupled with their scientific output in this field make this Perspective highly selective. We highlight the representative examples of 1D NTs and nanowires (NWs), 2D nanosheets and membranes, 3D interconnected assembles, mesoporous superstructures, and open frameworks, which would have potential applicaion in unsupported catalysts. Then, we briefly review the various synthesis methods and procedures of unsupported Pt-based architectures and demonstrate electrocatalysis results with the mechanisms of enhanced performance. Finally, we examine previous work as well as upcoming challenges and future pathways to further optimize unsupported Pt-based electrocatalysts for ORR. Some other important topics such as the activity dependence on size, compositions, structures, as well as detailed synthesis procedures of the Pt-based electrocatalysts are beyond the scope of this Perspective. however, those topics have been reported in other excellent reviews.24−27 1D Pt-Based Nanotubes and Nanowires. A prerequisite for the synthesis of unsupported electrocatalysts is to prepare highly dispersed/exposed Pt nanostructures of well-defined composition without any support. 1D Pt nanostructures can meet this fundamental requirement due to their nanometer-sized dimensions and long aspect ratio. In addition, highly anisotropic growth endows these materials many unique 2036
DOI: 10.1021/acsenergylett.7b00593 ACS Energy Lett. 2017, 2, 2035−2043
ACS Energy Letters
Perspective
Figure 2. (a) FESEM, (b) TEM, and (c) HRTEM images of Pt NWs assemblies. The inset in (a) shows an optical photo of a Pt NWs membrane, 2 cm in diameter, fabricated by a simple casting process. (d) Cyclic voltammetry (CV) curves of a Pt NW membrane before and after 3000 cycles of accelerated stability tests (reprinted from ref 32). (e) HAADF-STEM image of Pd@Ptmonolayer and (f) ORR polarization curves for Pd@Ptmonolayer before (dashed line) and after (solid line) 5000 potential sweeps (reprinted from ref 43; Copyright 2015 Royal Society of Chemistry). (g) TEM image of nanoporous Pt1Ni1. (h) Stability test of Pt/C, nanoporous-Pt1Ni1, and Pt6Ni1 catalysts in 0.1 M HClO4 electrolyte (reprinted from ref 45; Copyright 2012 Royal Society of Chemistry).
based nanostructures. As mentioned above, in addition to the geometric effect, ligand and strain effects are the two main factors determining catalytic activity. The ligand effect is caused by adjacent atoms with different electronegativity resulting in reinforced electronic charge transfer between/among the atoms and thus affects their electronic d-band structure. On the other hand, the strain effect is attributed to the different atomic arrangement of surface/interface atoms and the defects in crystals, resulting in compressive or tensile strain. The ligand and strain effects, in some cases, are induced by the geometric effect due to the different atoms involved (mostly the transition metals) and the unique crystalline structure. However, both the ligand and the strain effects usually function synergistically. They are present in the electrocatalysts simultaneously and affect the observed catalytic reactivity together.13,35−37 Both of them can optimize the balance between the kinetics of the O− O bond breaking and the ad/desorption of the oxygenated intermediates. Recently, 1D Pt-based nanostructures were synthesized with a wide range of bimetallic or multiple compositions, which decreased the precious metal loading, meanwhile enhancing the inherent activity of the catalysts. Su et al. reported that PtCu NTs were synthesized by the galvanic replacement method using a Cu NW template.17 Detailed analysis of the products demonstrated that the as-synthesized PtCu NTs were composed of a bulk PtCu alloy, which contained a Pt-enriched surface with the d-band center position downshifted compared to that of the pure Pt. As a result, the PtCu NT catalyst exhibited an enhanced specific activity of roughly 10-fold improvement compared to the commercial Pt/ C catalyst. Core/shell structured Pd@Pt NWs with tunable shell thickness via a galvanic replacement reaction were also successfully synthesized (Figure 1c−g), and the mass activity of Pd@Pt NWs with a Pt content of 21.2 wt % was almost 10 times higher than that of the Pt/C catalysts.20 Most importantly, after a durability test with 80 000 cycles, the
properties, such as preferential exposure of highly active/lowenergy facets, higher aspect ratios, and improved electron transport. 1D Pt NTs and Pt NWs were universally synthesized by the galvanic displacement reaction with the assistance of metal Ag, Cu, and Te NW templates. After complete consumption of the metal templates, NW/NT structures were obtained.12,17,18,28,29 Biomaterials such as DNA, insulin amyloid fibrils, and even chain viruses were also often used as the substrates to grow ultrathin Pt NWs as these biomaterial skeletons could be easily removed in post-treatment procedures.30 Another effective strategy is the wet chemistry reduction method without any templates as some reaction system-derived organic dimers or amine molecules would lead nanoparticles and short nanorods to grow into long NWs via the oriented attachment mechanism.31−33 In the subsequent few years since the report of unsupported Pt and Pt−Pd alloy NTs with greatly improved stability (Figure 1a,b),12 the 1D systems have attracted much attention as potential ORR electrocatalysts and consistently showed enhanced electrocatalytic activity and stability compared to conventional Pt nanoparticles/carbon (Pt/C) counterparts.18 Nevertheless, their relatively low ECSA (
