Optimizing the Size of Platinum Nanoparticles for Enhanced Mass Activity in the Electrochemical Oxygen Reduction Reaction

High oxygen reduction (ORR) activity has been for many years considered as the key to many energy applications. Herein, by combining theory and experiment we prepare Pt nanoparticles with optimal size for the efficient ORR in proton‐exchange‐membrane fuel cells. Optimal nanoparticle sizes are predicted near 1, 2, and 3 nm by computational screening. To corroborate our computational results, we have addressed the challenge of approximately 1 nm sized Pt nanoparticle synthesis with a metal–organic framework (MOF) template approach. The electrocatalyst was characterized by HR‐TEM, XPS, and its ORR activity was measured using a rotating disk electrode setup. The observed mass activities (0.87±0.14 A mg_Pt^?1) are close to the computational prediction (0.99 A mg_Pt^?1). We report the highest to date mass activity among pure Pt catalysts for the ORR within similar size range. The specific and mass activities are twice as high as the Tanaka commercial Pt/C catalysis.     

Porous carbon as electrode material in direct ethanol fuel cells (DEFCs) synthesized by the direct carbonization of MOF-5

Porous carbon (PC-900) was prepared by direct carbonization of porous metal-organic framework (MOF)-5 (Zn₄O(bdc)₃, bdc?=?1,4-benzenedicarboxylate) at 900 °C. The carbon material was deposited with PtM (M?=?Fe, Ni, Co, and Cu (20 %) metal loading) nanoparticles using the polyol reduction method, and catalysts PtM/PC-900 were designed for direct ethanol fuel cells (DEFCs). However, herein, we are reporting PtFe/PC-900 catalyst combination which has exhibited superior performance among other options. This catalyst was characterized by powder XRD, high-resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and selected area electron diffraction (SAED) technique. The electrocatalytic capability of the catalyst for ethanol electrooxidation was investigated using cyclic voltammetry and direct ethanol single cell testing. The results were compared with those of PtFe and Pt supported on Vulcan XC72 carbon catalysts (PFe/CX-72 and Pt/XC-72) prepared via the same method. It has been observed that the catalyst PtFe/PC-900 developed in this work showed an outstanding normalized activity per gram of Pt (6.8 mA/g Pt) and superior power density (121 mW/cm² at 90 °C) compared to commercially available carbon-supported catalysts.     

铁氮掺杂对石墨烯担载铂催化剂氧还原反应活性的影响

高氧还原活性担载铂催化剂的研发是加快质子交换膜燃料电池商业化进程的主要手段之一。以石墨烯为碳源,1,10-菲啰啉为氮源,FeCl3为铁源,用浸渍法制备铁氮掺杂石墨烯(Fe/N-G)载体,并通过乙二醇还原法获得PtFe/N-G催化剂,探究铁氮原子的引入对石墨烯担载铂催化剂氧还原反应催化活性的影响。采用X射线衍射、比表面积和孔径分布测试、X射线光电子能谱等表征手段对载体及催化剂结构进行表征,使用电化学方法对载体和催化剂的氧还原反应活性进行测试。结果表明,PtFe/N-G催化剂的氧还原反应起始电位及半波电位分别为0.96 V、0.83 V,优于相同Pt担载量的商业20%Pt/C催化剂。铁氮掺杂后,石墨烯载体具有较大的孔径更有利于氧还原反应过程中生成物与反应物的传递,PtFe/N-G催化剂中存在吡啶氮和Fe-N型氮与铂纳米颗粒的协同催化,以及铂纳米颗粒与铁氮掺杂石墨烯载体间的相互作用,是PtFe/N-G催化剂具有优异的氧还原催化活性的可能原因。     

Carbon black supported PtCu nanoparticles derived from mix-phased Cu precursor for high-performanced oxygen reduction reaction in acid medium

Alloying high-cost Pt with transition metals has been considered as an effective route to synthesize the electrocatalysts with low Pt loading and excellent activity towards oxygen reduction reaction (ORR) under acid solution. The galvanic replacement method, as featured with efficiency and simplicity, is widely reported to produce Pt-based bimetallic alloys and thereby declare the significance of reductive transition metal precursor on the enhancement of ORR performance. Herein, mix-phased Cu−Cu₂O precursor was applied to prepare carbon black supported highly dispersed PtCu alloy nanoparticles (PtCu/C). The proper Cu−Cu₂O ratios can exactly facilitate the generation of small sized PtCu alloy nanoparticles with regulated bimetallic content. Meanwhile, the Cu₂O phase is revealed to benefit the electron transfer from Pt to Cu and thus improve the intrinsic activity of Pt active sites. And the metallic Cu can favor the promotion of electrochemical active surface area. Consequently, the as-prepared PtCu/C behaves impressive ORR activity with half-wave potential of 0.88 V (vs. RHE) and mass activity of 0.49 A cm⁻² mg_Pt⁻¹ at 0.8 V, which is 9.8 times of commercial Pt/C catalysts. Our work will offer helpful advices for the development and regulation of novel Pt-based alloy materials towards diverse electrocatalysis.     

Inducing Covalent Atomic Interaction in Intermetallic Pt Alloy Nanocatalysts for High-Performance Fuel Cells

The harsh working environments of proton exchange membrane fuel cells (PEMFCs) pose huge challenges to the stability of Pt-based alloy catalysts. The widespread presence of metallic bonds with significantly delocalized electron distribution often lead to component segregation and rapid performance decay. Here we report L1₀−Pt₂CuGa intermetallic nanoparticles with a unique covalent atomic interaction between Pt−Ga as high-performance PEMFC cathode catalysts. The L1₀−Pt₂CuGa/C catalyst shows superb oxygen reduction reaction (ORR) activity and stability in fuel cell cathode (mass activity=0.57 A mg_Pt⁻¹ at 0.9 V, peak power density=2.60/1.24 W cm⁻² in H₂-O₂/air, 28 mV voltage loss at 0.8 A cm⁻² after 30 000 cycles). Theoretical calculations reveal the optimized adsorption of oxygen intermediates via the formed biaxial strain on L1₀−Pt₂CuGa surface, and the durability enhancement stems from the stronger Pt−M bonds than those in L1₁−PtCu resulted from Pt−Ga covalent interactions.     

Bulk-like Pt(100)-oriented Ultrathin Surface: Combining the Merits of Single Crystals and Nanoparticles to Boost Oxygen Reduction Reaction

Single crystal surfaces with highly coordinated sites very often hold high specific activities toward oxygen reduction reaction (ORR) and others. Transposing their high specific activity to practical high-surface-area electrocatalysts remains challenging. Here, ultrathin Pt(100) alloy surface is constructed via epitaxial growth. The surface shows 3.1–6.9 % compressive strain and bulk-like characteristics as demonstrated by site-probe reactions and different spectroscopies. Its ORR activity exceeds that of bulk Pt₃Ni(100) and Pt(111) and presents a 19-fold increase in specific activity and a 13-fold increase in mass activity relative to commercial Pt/C. Moreover, the electrochemically active surface area (ECSA) is increased by 4-fold compared to traditional thin films (e.g. NSTF), which makes the catalyst more tolerant to voltage loss at high current densities under fuel cell operation. This work broadens the family of extended surface catalysts and highlights the knowledge-driven approach in the development of advanced electrocatalysts.     

Hollow-Structure Pt-Ni Nanoparticle Electrocatalysts for Oxygen Reduction Reaction

An electrocatalyst with high oxygen reduction reaction (ORR) activity and high stability during start–stop operation is necessary. In this paper, hollow-structure Pt-Ni electrocatalysts are investigated as ORR catalysts. After synthesis via sacrificial SiO₂ template method, the electrocatalyst exhibits much higher specific activity (1.88 mA/cm²) than a commercial Pt/C catalyst. The mass activity (0.49 A/mg) is 7 times higher than the commercial Pt/C catalyst. The kinetics of the ORR is evaluated using Tafel and K-L plots. It also exhibits a higher durability than commercial Pt/C catalyst during accelerated durability test (ADT). Moreover, the electrocatalyst shows good resistance against accelerated durability test for start–stop, the specific activity and mass activity drops 34.6% and 40.8%, respectively, far better than the commercial catalyst.     

Revealing the Effect of Surface Composition on Multiwalled Carbon Nanotubes Supported Pt-Fe Alloy Electrocatalysts for Methanol Oxidation Performance

The designs of efficient and inexpensive Pt-based catalysts for methanol oxidation reaction (MOR) are essential to boost the commercialization of direct methanol fuel cells. Here, the highly catalytic performance PtFe alloys supported on multiwalled carbon nanotubes (MWCNTs) decorating nitrogen-doped carbon (NC) have been successfully prepared via co-engineering of the surface composition and electronic structure. The Pt₁Fe₃@NC/MWCNTs catalyst with moderate Fe³⁺ feeding content (0.86 mA/mg_Pt) exhibits 2.26-fold enhancement in MOR mass activity compared to pristine Pt/C catalyst (0.38 mA/mg_Pt). Furthermore, the CO oxidation initial potential of Pt₁Fe₃@NC/MWCNTs catalyst is lower relative to Pt/C catalyst (0.71 V and 0.80 V). Benefited from the optimal surface compositions, the anti-corrosion ability of MWCNT, strong electron interaction between PtFe alloys and MWCNTs and the N-doped carbon (NC) layer, the Pt₁Fe₃@NC/MWCNTs catalyst presents an improved MOR performance and anti-CO poisoning ability. This study would open up new perspective for designing efficient electrocatalysts for the DMFCs field.     

A Nanoporous PdCo Alloy as a Highly Active Electrocatalyst for the Oxygen‐Reduction Reaction and Formic Acid Electrooxidation

A nanoporous (NP) PdCo alloy with uniform structure size and controllable bimetallic ratio was fabricated simply by one‐step mild dealloying of a PdCoAl precursor alloy. The as‐made alloy consists of a nanoscaled bicontinuous network skeleton with interconnected hollow channels that extend in all three dimensions. With a narrow ligament size distribution around 5 nm, the NP PdCo alloy exhibits much higher electrocatalytic activity towards the oxygen‐reduction reaction (ORR) with enhanced specific and mass activities relative to NP Pd and commercial Pt/C catalysts. A long‐term stability test demonstrated that NP PdCo has comparable catalytic durability with less loss of ORR activity and electrochemical surface area than Pt/C. The NP PdCo alloy also shows dramatically enhanced catalytic activity towards formic acid electrooxidation relative to NP Pd and Pd/C catalysts. The as‐made NP PdCo holds great application potential as a promising cathode as well as an anode electrocatalyst in fuel cells with the advantages of superior catalytic performance and easy preparation.     

Accelerating Oxygen‐Reduction Catalysts through Preventing Poisoning with Non‐Reactive Species by Using Hydrophobic Ionic Liquids

Developing cost‐effective electrocatalysts for the oxygen reduction reaction (ORR) is a prerequisite for broad market penetration of low‐temperature fuel cells. A major barrier stems from the poisoning of surface sites by nonreactive oxygenated species and the sluggish ORR kinetics on the Pt catalysts. Herein we report a facile approach to accelerating ORR kinetics by using a hydrophobic ionic liquid (IL), which protects Pt sites from surface oxidation, making the IL‐modified Pt intrinsically more active than its unmodified counterpart. The mass activity of the catalyst is increased by three times to 1.01 A mg^?1_Pt@0.9 V, representing a new record for pure Pt catalysts. The enhanced performance of the IL‐modified catalyst can be stabilized after 30 000 cycles. We anticipate these results will form the basis for an unprecedented perspective in the development of high‐performing electrocatalysts for fuel‐cell applications.     

Synthesis of Pt3Ni-based functionalized MWCNTs to enhance electrocatalysis for PEM fuel cells

Nanoscale Pt₃Ni/functionalized multiwalled carbon nanotubes (FMWCNTs) catalysts, successfully synthesized by anchoring nickel–platinum alloy nanoparticles on FMWCNTs, are presented in this paper. Compared with conventional commercial Pt/C catalysts, the preliminary results revealed that the Pt₃Ni/FMWCNTs catalysts demonstrated not only higher specific activity for oxygen reduction reaction (ORR) but also outstanding stability. The enhancement in the stability of the Pt₃Ni/FMWCNTs catalysts is believed to be due to the anchor effects in Pt₃Ni alloy structure, the stronger interaction between Pt₃Ni alloy nanoparticles and FMWCNTs, and the “π sites” anchoring centers for metal nanoparticles from CNTs with high graphite.     

Tuning Nanoparticle Catalysis for the Oxygen Reduction Reaction

Advances in chemical syntheses have led to the formation of various kinds of nanoparticles (NPs) with more rational control of size, shape, composition, structure and catalysis. This review highlights recent efforts in the development of Pt and non‐Pt based NPs into advanced nanocatalysts for efficient oxygen reduction reaction (ORR) under fuel‐cell reaction conditions. It first outlines the shape controlled synthesis of Pt NPs and their shape‐dependent ORR. Then it summarizes the studies of alloy and core–shell NPs with controlled electronic (alloying) and strain (geometric) effects for tuning ORR catalysis. It further provides a brief overview of ORR catalytic enhancement with Pt‐based NPs supported on graphene and coated with an ionic liquid. The review finally introduces some non‐Pt NPs as a new generation of catalysts for ORR. The reported new syntheses with NP parameter‐tuning capability should pave the way for future development of highly efficient catalysts for applications in fuel cells, metal‐air batteries, and even in other important chemical reactions.     

PtFe掺杂石墨烯抗CO中毒性能的理论研究

基于密度泛函理论第一性原理研究了以单空位缺陷(SV)石墨烯为载体的Pt,Fe及PtFe二元金属催化剂的抗CO中毒能力.结果表明,对于单金属原子Pt和Fe,Fe更易吸附在SV石墨烯上;而对于PtFe二元金属催化剂,SV石墨烯对其固定能力明显好于Pt-SV,即Pt催化剂中掺杂Fe大大增加了SV石墨烯对金属催化剂的稳定性.Pt,Fe及PtFe二元金属催化剂抗CO中毒能力的研究结果表明,PtFe-6结构的抗CO中毒能力明显强于Pt-SV,接近于Fe-SV的抗CO中毒能力,在所有二元金属催化剂中PtFe-6的稳定性最好,明显优于Pt在SV石墨烯上的稳定性.通过在Pt中加入非贵金属Fe既可提高DMFC中阳极Pt催化剂的抗CO中毒能力,又可提高其催化活性.     

Tungsten‐Doped L10‐PtCo Ultrasmall Nanoparticles as a High‐Performance Fuel Cell Cathode

The commercialization of proton exchange membrane fuel cells (PEMFCs) relies on highly active and stable electrocatalysts for oxygen reduction reaction (ORR) in acid media. The most successful catalysts for this reaction are nanostructured Pt‐alloy with a Pt‐skin. The synthesis of ultrasmall and ordered L1₀‐PtCo nanoparticle ORR catalysts further doped with a few percent of metals (W, Ga, Zn) is reported. Compared to commercial Pt/C catalyst, the L1₀‐W‐PtCo/C catalyst shows significant improvement in both initial activity and high‐temperature stability. The L1₀‐W‐PtCo/C catalyst achieves high activity and stability in the PEMFC after 50 000 voltage cycles at 80 °C, which is superior to the DOE 2020 targets. EXAFS analysis and density functional theory calculations reveal that W doping not only stabilizes the ordered intermetallic structure, but also tunes the Pt‐Pt distances in such a way to optimize the binding energy between Pt and O intermediates on the surface.     

Efficient Oxygen Reduction Reaction (ORR) Catalysts Based on Single Iron Atoms Dispersed on a Hierarchically Structured Porous Carbon Framework

Single Fe atoms dispersed on hierarchically structured porous carbon (SA‐Fe‐HPC) frameworks are prepared by pyrolysis of unsubstituted phthalocyanine/iron phthalocyanine complexes confined within micropores of the porous carbon support. The single‐atom Fe catalysts have a well‐defined atomic dispersion of Fe atoms coordinated by N ligands on the 3D hierarchically porous carbon support. These SA‐Fe‐HPC catalysts are comparable to the commercial Pt/C electrode even in acidic electrolytes for oxygen reduction reaction (ORR) in terms of the ORR activity (E_1/2=0.81 V), but have better long‐term electrochemical stability (7 mV negative shift after 3000 potential cycles) and fuel selectivity. In alkaline media, the SA‐Fe‐HPC catalysts outperform the commercial Pt/C electrode in ORR activity (E_1/2=0.89 V), fuel selectivity, and long‐term stability (1 mV negative shift after 3000 potential cycles). Thus, these nSA‐Fe‐HPCs are promising non‐platinum‐group metal ORR catalysts for fuel‐cell technologies.     

Boosting Pt oxygen reduction reaction activity by tuning the tin oxide support

The influence of the dopant element on the physicochemical properties of the SnO₂ support and on the oxygen reduction reaction (ORR) activity of the supported Pt nanoparticles was systematically studied. The dopant element and concentration significantly influence the SnO₂ crystal structure and the electrical conductivity, as well as the ORR activity in 0.1 M HClO₄ of the supported Pt catalysts. ORR activities significantly exceeding that of Pt deposited on carbon were observed for Pt nanoparticles supported on Nb- and W-doped SnO₂. No linear correlation between the support conductivity and the ORR activity was observed, suggesting that other factors, such as (electronic) metal–support interactions, could play a role in the ORR activity of Pt catalysts supported on metal oxides.     

Hollow Spheres of Iron Carbide Nanoparticles Encased in Graphitic Layers as Oxygen Reduction Catalysts

Nonprecious metal catalysts for the oxygen reduction reaction are the ultimate materials and the foremost subject for low‐temperature fuel cells. A novel type of catalysts prepared by high‐pressure pyrolysis is reported. The catalyst is featured by hollow spherical morphologies consisting of uniform iron carbide (Fe₃C) nanoparticles encased by graphitic layers, with little surface nitrogen or metallic functionalities. In acidic media the outer graphitic layers stabilize the carbide nanoparticles without depriving them of their catalytic activity towards the oxygen reduction reaction (ORR). As a result the catalyst is highly active and stable in both acid and alkaline electrolytes. The synthetic approach, the carbide‐based catalyst, the structure of the catalysts, and the proposed mechanism open new avenues for the development of ORR catalysts.     

Pt基合金氧还原催化剂设计的密度泛函理论计算研究

本文提出以合金形成能、Pt表面偏析能和氧原子吸附能作为依据筛选具有高活性和高稳定性的表面富Pt氧还原合金催化剂. 利用DFT计算对Pt与各种过渡金属形成的合金的热力学、表面化学和电子性质进行了系统研究,在此基础上预测Pt-V,Pt-Fe,Pt-Co,Pt-Ni,Pt-Cu,Pt-Zn,Pt-Mo,Pt-W等合金可能具有好的氧还原催化活性和稳定性. 所预期的大部分催化剂已有文献研究结果支持. 另外,Pt-Zn和Pt-Mo体系目前报道尚不多,值得进一步的细致研究.     

Catalytic approaches towards highly durable proton exchange membrane fuel cells with minimized Pt use

Proton exchange membrane fuel cells (PEMFCs) produce electricity from H₂ without carbon emission, and they are considered as environmentally benign energy conversion devices. Although PEMFCs are mature enough to find themselves in a few commercial automobiles such as Hyundai Nexo and Toyota Mirai, their durability should be enhanced, especially under transient conditions, and Pt use should be reduced significantly to expand their market. Herein, we introduce examples of how catalysts can contribute to enhancing the durability of PEMFCs while minimizing Pt use. Numerous electrocatalysts have been reported claiming superior activity in a half-cell setup, but they often fail to show the same enhancement in a single cell setup due to various transfer problems, impurity poisoning, etc. This perspective focuses on catalysts tested in a membrane-electrode-assembly (MEA) setup. As examples to obtain durability under transient conditions, catalysts used in reversal-tolerant anodes (RTAs) and selective anodes are explained. RTAs can endure sudden H₂ starvation, and selective anodes can operate properly when O₂ is unexpectedly mixed with H₂ in the anode. As examples with high durability in long-term operation, Pt-based nanoparticle catalysts encapsulated with carbon shells are explained. Interestingly, PtCo nanoparticles supported on Co–N–C or PtFe nanoparticles encapsulated with a carbon shell presented a superior cell performance in spite of <1/10 Pt use in an MEA setup. Non-Pt group metal (PGM) catalysts used in an MEA setup are also briefly explained. With these highly durable catalysts which can respond properly under transient conditions with minimum Pt use, PEMFC technology can bring about a more sustainable society.

Catalytic approaches to enhance PEMFC performances are introduced, especially focusing on the studies reporting MEA cell data.     

Carbon/titanium oxide supported bimetallic platinum/iridium nanocomposites as bifunctional electrocatalysts for lithium-air batteries

Platinum (Pt) and iridium (Ir) catalysts are well known to strongly enhance the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) kinetics, respectively. Pt–Ir-based bimetallic compounds along with carbon-supported titanium oxides (C–TiO₂) have been synthesized for the application as electrocatalysts in lithium oxygen batteries. Transition metal oxide-based bimetallic nanocomposites (Pt–Ir/C–TiO₂) were prepared by an incipient wetness impregnation technique. The as-prepared electrocatalysts were composed of a well-dispersed homogenous alloy of nanoparticles as confirmed by X-ray diffraction patterns and Fourier transform scanning electron microscopy analyses. The electrochemical characterizations reveal that the Pt–Ir/C–TiO₂ electrocatalysts were bifunctional with high activity for both ORR and OER. When applied as an air cathode catalyst in lithium-air batteries, the electrocatalyst improved the battery performance in terms of capacity, reversibility, and cycle life compared to that of cathodes without any catalysts.