Core–Shell-Structured Low-Platinum Electrocatalysts for Fuel Cell Applications

Theoretical Investigation of Single-Atom Catalysts for Hydrogen Evolution Reaction Based on Two-Dimensional Tetragonal V2C2 and V3C3

Developing stable and effective catalysts for the hydrogen evolution reaction (HER) has been a long-standing pursuit. In this work, we propose a series of single-atom catalysts (SACs) by importing transition-metal atoms into the carbon and vanadium vacancies of tetragonal V2C2 and V3C3 slabs, where the transition-metal atoms refer to Ti, V, Cr, Mn, Fe, Co, Ni, and Cu. By means of first-principles computations, the possibility of applying these SACs in HER catalysis was investigated. All the SACs are conductive, which is favorable to charge transfer during HER. The Gibbs free energy change (ΔGH*) during hydrogen adsorption was adopted to assess their catalytic ability. For the V2C2-based SACs with V, Cr, Mn, Fe, Ni, and Cu located at the carbon vacancy, excellent HER catalytic performance was achieved, with a |ΔGH*| smaller than 0.2 eV. Among the V3C3-based SACs, apart from the SAC with Mn located at the carbon vacancy, all the SACs can act as outstanding HER catalysts. According to the ΔGH*, these excellent V2C2- and V3C3-based SACs are comparable to the best-known Pt-based HER catalysts. However, it should be noted that the V2C2 and V3C3 slabs have not been successfully synthesized in the laboratory, leading to a pure investigation without practical application in this work.

Exploring the Potential of Bimetallic PtPd/C Cathode Catalysts to Enhance the Performance of PEM Fuel Cells

Bimetallic platinum-containing catalysts are deemed promising for electrolyzers and proton-exchange membrane fuel cells (PEMFCs). A significant number of laboratory studies and commercial offers are related to PtNi/C and PtCo/C electrocatalysts. The behavior of PtPd/C catalysts has been studied much less, although palladium itself is the metal closest to platinum in its properties. Using a series of characterization methods, this paper presents a comparative study of structural characteristics of the commercial PtPd/C catalysts containing 38% wt. of precious metals and the well-known HiSpec4000 Pt/C catalyst. The electrochemical behavior of the catalysts was studied both in a three-electrode electrochemical cell and in the membrane electrode assemblies (MEAs) of hydrogen-air PEMFCs. Both PtPd/C samples demonstrated higher values of the electrochemically active surface area, as well as greater specific and mass activity in the oxygen reduction reaction in comparison with conventional Pt/C, while not being inferior to the latter in durability. The MEA based on the best of the PtPd/C catalysts also exhibited higher performance in single tests and long-term durability testing. The results of this study conducted indicate the prospects of using bimetallic PtPd/C materials for cathode catalysts in PEMFCs.

Synergistic Ni-W Dimer Sites Induced Stable Compressive Strain for Boosting the Performance of Pt as Electrocatalyst for the Oxygen Reduction Reaction

Alloying Pt catalysts with transition metal elements is an effective pathway to enhance the performance of oxygen reduction reaction (ORR), but often accompanied with severe metal dissolution issue, resulting in poor stability of alloy catalysts. Here, instead of forming traditional alloy structure, we modify Pt surface with a novel Ni-W dimer structure by the atomic layer deposition (ALD) technique. The obtained NiW@PtC catalyst exhibits superior ORR performance both in liquid half-cell and practical fuel cell compared with initial Pt/C. It is discovered that strong synergistic Ni-W dimer structure arising from short atomic distance induced a stable compressive strain on the Pt surface, thus boosting Pt catalytic performance. This surface modification by synergistic dimer sites offers an effective strategy in tailoring Pt with excellent activity and stability, which provides a significant perspective in boosting the performance of commercial Pt catalyst modified with polymetallic atom sites.

Toward Electrocatalytic Methanol Oxidation Reaction: Longstanding Debates and Emerging Catalysts

The study of direct methanol fuel cells (DMFCs) has lasted around 70 years, since the first investigation in the early 1950s. Though enormous effort has been devoted in this field, it is still far from commercialization. The methanol oxidation reaction (MOR), as a semi-reaction of DMFCs, is the bottleneck reaction that restricts the overall performance of DMFCs. To date, there has been intense debate on the complex six-electron reaction, but barely any reviews have systematically discussed this topic. To this end, the controversies and progress regarding the electrocatalytic mechanisms, performance evaluations as well as the design science toward MOR electrocatalysts are summarized. This review also provides a comprehensive introduction on the recent development of emerging MOR electrocatalysts with a focus on the innovation of the alloy, core-shell structure, heterostructure, and single-atom catalysts. Finally, perspectives on the future outlook toward study of the mechanisms and design of electrocatalysts are provided.

An Innovative PEMFC Magnetic Field Emulator to Validate the Ability of a Magnetic Field Analyzer to Detect 3D Faults

An original non-invasive methodology of the fuel cell diagnosis is proposed to identify different positions of the faults in Proton Exchange Membrane Fuel Cell (PEMFC) stacks from external magnetic field measurements. The approach is based on computing the external magnetic field difference between normal and faulty PEMFC operating conditions. To evaluate the external magnetic field distribution, in this paper, we propose an improved design of the magnetic field analyzer. This analyzer amplifies the magnetic field around the cell to perform an accurate detection of the fault position. Moreover, the main contribution of this work is represented by conceiving and implementing a 3D multi-physical current distribution emulator of a proton exchange membrane fuel cell. The new concept of a proton exchange membrane fuel cell emulator has been specially designed to emulate the magnetic field of a real fuel cell stack. This emulator concept is also beneficial for a new model of the fuel cell, which implies a multi-physical coupling between electrochemical electric conduction and the generated magnetic field. Finally, finally, the numerical model and the emulator have been involved in the realization of numerical simulations and experimental analysis to prove the ability of the system to detect and localize 3D faults.

Pt-Based Oxygen Reduction Reaction Catalysts in Proton Exchange Membrane Fuel Cells: Controllable Preparation and Structural Design of Catalytic Layer

Proton exchange membrane fuel cells (PEMFCs) have attracted extensive attention because of their high efficiency, environmental friendliness, and lack of noise pollution. However, PEMFCs still face many difficulties in practical application, such as insufficient power density, high cost, and poor durability. The main reason for these difficulties is the slow oxygen reduction reaction (ORR) on the cathode due to the insufficient stability and catalytic activity of the catalyst. Therefore, it is very important to develop advanced platinum (Pt)-based catalysts to realize low Pt loads and long-term operation of membrane electrode assembly (MEA) modules to improve the performance of PEMFC. At present, the research on PEMFC has mainly been focused on two areas: Pt-based catalysts and the structural design of catalytic layers. This review focused on the latest research progress of the controllable preparation of Pt-based ORR catalysts and structural design of catalytic layers in PEMFC. Firstly, the design principle of advanced Pt-based catalysts was introduced. Secondly, the controllable preparation of catalyst structure, morphology, composition and support, and their influence on catalytic activity of ORR and overall performance of PEMFC, were discussed. Thirdly, the effects of optimizing the structure of the catalytic layer (CL) on the performance of MEA were analyzed. Finally, the challenges and prospects of Pt-based catalysts and catalytic layer design were discussed.

Architecture Evolution of Different Nanoparticles Types: Relationship between the Structure and Functional Properties of Catalysts for PEMFC

This review considers the features of the catalysts with different nanoparticle structures architecture transformation under the various pre-treatment types. Based on the results of the publications analysis, it can be concluded that the chemical or electrochemical activation of bimetallic catalysts has a significant effect on their composition, microstructure, and catalytic activity in the oxygen reduction reaction. The stage of electrochemical activation is recommended for use as a mandatory catalyst pre-treatment to obtain highly active de-alloyed materials. The literature is studied, which covers possible variants of the structural modification under the influence of thermal treatment under different processing conditions. Additionally, based on the literature data analysis, recommendations are given for the thermal treatment of catalysts alloyed with various d-metals.

A Comparative Study of Equivalent Circuit Models for Electro-Chemical Impedance Spectroscopy Analysis of Proton Exchange Membrane Fuel Cells

Electrochemical impedance spectroscopy is one of the important tools for the performance analysis and diagnosis of proton exchange membrane fuel cells. The equivalent circuit model is an effective method for electrochemical impedance spectroscopy resolution. In this paper, four typical equivalent circuit models are selected to comprehensively compare and analyze the difference in the fitting results of the models for the electrochemical impedance spectroscopy under different working conditions (inlet pressure, stoichiometry, and humidity) from the perspective of the fitting accuracy, change trend of the model parameters, and the goodness of fit. The results show that the fitting accuracy of the model with the Warburg element is the best for all under each working condition. When considering the goodness of fit, the model with constant phase components is the best choice for fitting electrochemical impedance spectroscopy under different inlet pressure and air stoichiometry. However, under different air humidity, the model with the Warburg element is best. This work can help to promote the development of internal state analysis, estimation, and diagnosis of the fuel cell based on the equivalent circuit modeling of electrochemical impedance spectroscopy.

Achievements in Pt nanoalloy oxygen reduction reaction catalysts: strain engineering, stability and atom utilization efficiency

The Pt nanoalloy surfaces often show unique electronic and physicochemical properties that are distinct from those of their parent metals, which provide significant room for manipulating their oxygen reduction reaction (ORR) behaviour. In this Feature Article, we present the progress of our recent research and that of other groups in Pt nanoalloy catalysts for ORR from three aspects, namely, strain engineering, stability and atom utilization efficiency. Some new insights into Pt surface strain engineering will be firstly introduced, with a focus on discussing the effect of compressive and tensile strain on the chemisorption properties. Secondly, the design concepts and synthetic methodologies to intensify the inherent stability of Pt nanoalloys will be summarized. Then, the exciting research push in developing nanostructured alloys with high atom utilization efficiency of Pt will be presented. Finally, a brief illumination of challenges and future developing perspectives of Pt nanoalloy catalysts will be provided.

Catalysts for Oxygen Reduction Reaction in the Polymer Electrolyte Membrane Fuel Cells: A Brief Review

This mini-review presents a short account of materials with exceptional activity towards oxygen reduction reaction. Two main classes of catalytic materials are described, namely platinum group metal (PGM) catalyst and Non-precious metal catalyst. The classes are discussed in terms of possible application in low-temperature hydrogen fuel cells with proton exchange membrane and further commercialization of these devices. A short description of perspective approaches is provided and challenging issues associated with developed catalytic materials are discussed.

Mechanism study of Single-Step synthesis of Fe(core)@Pt(shell) nanoparticles by sonochemistry

Transition metal (TM) core-platinum (Pt) shell nanoparticles (TM@Pt NPs) are attracting a great deal of attention as highly active and durable oxygen reduction reaction (ORR) electrocatalysts of fuel cells and metal-air batteries. However, most of the reported synthesis methods of TM@Pt NPs are multistep in nature, a significant disadvantage for real applications. In this regard, our group has reported a single-step method to synthesize TM@Pt NPs for TM = Mn, Fe, Co, and Ni by using sonochemistry, namely the UPS (ultrasound-assisted polyol synthesis) method. Previously, we proposed the mechanism of the formation of these TM@Pt NPs by UPS method, but rather in a rough sense. Some details are missing and the optimal conditions have not been established. In the present work, we performed detailed studies on the formation mechanism of UPS reaction by using Fe@Pt NPs as the model system. Effects of synthesis parameters such as the nature of metal precursor, conditions of ultrasound, and temperature profile as a function of reaction time were assessed, along with the analyses of intermediates during the UPS reaction. As results, we verified our previously proposed mechanism that, under appropriate conditions, Fe core is formed through the cavitation and implosion of the solvent, induced by the ultrasound, and the Pt shell is formed by the chemical reaction between Fe core and Pt reagent, independent from the direct effect of ultrasound. In addition, we established the optimal conditions to obtain a high purity Fe@Pt NPs in a high yield (>90% based on Pt), which may enable the increase of synthesis scale of Fe@Pt NPs, a necessary step for the real application of TM@Pt NPs.

Exploring Strategies toward Synthetic Precision Control within Core-Shell Nanowires

ConspectusAchieving precision and reproducibility in terms of physical structure and chemical composition within arbitrary nanoscale systems remains a "holy grail" challenge for nanochemistry. Because nanomaterials possess fundamentally distinctive size-dependent electronic, optical, and magnetic properties with wide-ranging applicability, the ability to produce homogeneous and monodisperse nanostructures with precise size and shape control, while maintaining a high degree of sample quality, purity, and crystallinity, remains a key synthetic objective. Moreover, it is anticipated that the methodologies developed to address this challenge ought to be reasonably simple, scalable, mild, nontoxic, high-yield, and cost-effective, while minimizing reagent use, reaction steps, byproduct generation, and energy consumption.The focus of this Account revolves around the study of various types of nanoscale one-dimensional core-shell motifs, prepared by our group. These offer a compact structural design, characterized by atom economy, to bring together two chemically distinctive (and potentially sharply contrasting) material systems into contact within the structural context of an extended, anisotropic configuration. Herein, we describe complementary strategies aimed at resolving the aforementioned concerns about precise structure and compositional control through the infusion of careful "quantification" and systematicity into customized, reasonably sustainable nanoscale synthetic protocols, developed by our group. Our multipronged approach involved the application of (a) electrodeposition, (b) electrospinning, (c) a combination of underpotential deposition and galvanic displacement reactions, and (d) microwave-assisted chemistry to diverse core-shell model systems, such as (i) carbon nanotube-SiO₂ composites, (ii) SnO₂/TiO₂ motifs, (iii) ultrathin Pt-monolayer shell-coated alloyed metal core nanowires, and (iv) Cu@TiO₂ nanowires, for applications spanning optoelectronics, photocatalysis, electrocatalysis, and thermal CO₂ hydrogenation, respectively.In so doing, over the years, we have reported on a number of different characterization tools involving spectroscopy (e.g., extended X-ray absorption fine structure (EXAFS) spectroscopy) and microscopy (e.g., high-resolution transmission electron microscopy (HRTEM) and atomic force microscopy (AFM)) for gaining valuable insights into the qualitative and quantitative nature of not only the inner core and outer shell themselves but also their intervening interface. While probing the functional catalytic behavior of a few of these core-shell structures under realistic operando conditions, using dynamic, in situ characterization techniques, we found that local and subtle changes in chemical composition and physical structure often occur during the reaction process itself. As such, nuanced differences in atomic packing, facet exposure, degree of derivatization, defect content, and/or extent of crystallinity can impact upon observed properties with tangible consequences for performance, mechanism, and durability.

Oleylamine Aging of PtNi Nanoparticles Giving Enhanced Functionality for the Oxygen Reduction Reaction

We report a rapid solution-phase strategy to synthesize alloyed PtNi nanoparticles which demonstrate outstanding functionality for the oxygen reduction reaction (ORR). This one-pot coreduction colloidal synthesis results in a monodisperse population of single-crystal nanoparticles of rhombic dodecahedral morphology with Pt-enriched edges and compositions close to Pt₁Ni₂. We use nanoscale 3D compositional analysis to reveal for the first time that oleylamine (OAm)-aging of the rhombic dodecahedral Pt₁Ni₂ particles results in Ni leaching from surface facets, producing aged particles with concave faceting, an exceptionally high surface area, and a composition of Pt₂Ni₁. We show that the modified atomic nanostructures catalytically outperform the original PtNi rhombic dodecahedral particles by more than two-fold and also yield improved cycling durability. Their functionality for the ORR far exceeds commercially available Pt/C nanoparticle electrocatalysts, both in terms of mass-specific activities (up to a 25-fold increase) and intrinsic area-specific activities (up to a 27-fold increase).

Synthesis of Pd nanonetworks with abundant defects for oxygen reduction electrocatalysis

The Pd nanonetworks with abundant defects were synthesized by a one-pot method for enhanced oxygen reduction reaction performance.

Platinum nanocluster catalysts supported on Marimo carbon via scalable dry deposition synthesis

The development of efficient fuel cells greatly promotes reducing the consumption of fossil energy, and it is crucial to enhance the platinum (Pt) catalytic activity by optimizing both the nanoparticle size and support effect.

Engineering the Low Coordinated Pt Single Atom to Achieve the Superior Electrocatalytic Performance toward Oxygen Reduction

Configuring metal single-atom catalysts (SACs) with high electrocatalytic activity and stability is one efficient strategy in achieving the cost-competitive catalyst for fuel cells' applications. Herein, the atomic layer deposition (ALD) strategy for synthesis of Pt SACs on the metal-organic framework (MOF)-derived N-doped carbon (NC) is proposed. Through adjusting the ALD exposure time of the Pt precursor, the size-controlled Pt catalysts, from Pt single atoms to subclusters and nanoparticles, are prepared on MOF-NC support. X-ray absorption fine structure spectra determine the increased electron vacancy in Pt SACs and indicate the Pt-N coordination in the as-prepared Pt SACs. Benefiting from the low-coordination environment and anchoring interaction between Pt atoms and nitrogen-doping sites from MOF-NC support, the Pt SACs deliver an enhanced activity and stability with 6.5 times higher mass activity than that of Pt nanoparticle catalysts in boosting the oxygen reduction reaction (ORR). Density functional theory calculations indicate that Pt single atoms prefer to be anchored by the pyridinic N-doped carbon sites. Importantly, it is revealed that the electronic structure of Pt SAs can be adjusted by adsorption of hydroxyl and oxygen, which greatly lowers free energy change for the rate-determining step and enhances the activity of Pt SACs toward the ORR.

Surfactant-Free One-Pot Synthesis of Homogeneous Trimetallic PtNiCu Nanoparticles with Size Control by Using Glycine

Homogeneous platinum alloy nanoparticles (NPs) are of great interest to the electrocatalytic community for potential use in various fuel cell electrodes. Increasing the surface area available per unit mass by decreasing the size of NPs while maintaining or improving activity is one of the key tasks of fuel cell catalysis. Achieving both in a synthesis of multielement NPs is still a challenging workup. In this investigation, we report the use of glycine as a size control agent to make ultrasmall homogeneous trimetallic PtNiCu NPs within 2-5 nm range. The mechanistic roles of dimethyl formamide (DMF), formaldehyde, water, and glycine are explored to understand the formation of these small NPs. Interestingly, it was observed that these PtNiCu NPs exhibited substantially enhanced mass activities toward the electro-oxidation of ethanol in comparison to commercial Pt black.

MXene (Ti3C2Tx) and Carbon Nanotube Hybrid-Supported Platinum Catalysts for the High-Performance Oxygen Reduction Reaction in PEMFC

The metal-support interaction offers electronic, compositional, and geometric effects that could enhance catalytic activity and stability. Herein, a high corrosion resistance and an excellent electrical conductivity MXene (Ti₃C₂T_x) hybrid with a carbon nanotube (CNT) composite material is developed as a support for Pt. Such a composite catalyst enhances durability and improved oxygen reduction reaction activity compared to the commercial Pt/C catalyst. The mass activity of Pt/CNT-MXene demonstrates a 3.4-fold improvement over that of Pt/C. The electrochemical surface area of Pt/CNT-Ti₃C₂T_x (1:1) catalysts shows only 6% drop with respect to that in Pt/C of 27% after 2000 cycle potential sweeping. Furthermore, the Pt/CNT-Ti₃C₂T_x (1:1) is used as a cathode catalyst for single cell and stack, and the maximum power density of the stack reaches 138 W. The structure distortion of the Pt cluster induced by MXene is disadvantageous to the desorption of O atoms. This issue can be solved by adding CNT on MXene to stabilize the Pt cluster. These remarkable catalytic performances could be attributed to the synergistic effect between Pt and CNT-Ti₃C₂T_x.

Bimetallic PtAu electrocatalysts for the oxygen reduction reaction: challenges and opportunities

Highly active, durable oxygen reduction reaction (ORR) electrocatalysts have an essential role in promoting the continuous operation of advanced energy technologies such as fuel cells and metal-air batteries. Considering the scarce reserve of Pt and its unsatisfactory overall performance, there is an urgent demand for the development of new generation ORR electrocatalysts that are substantially better than the state-of-the-art supported Pt-based nanocatalysts, such as Pt/C. Among various nanostructures, bimetallic PtAu represents one unique alloy system where highly contradictory performance has been reported. While it is generally accepted that Au may contribute to stabilizing Pt, its role in modulating the intrinsic activity of Pt remains unclear. This perspective will discuss critical structural issues that affect the intrinsic ORR activities of bimetallic PtAu, with an eye on elucidating the origin of seemingly inconsistent experimental results from the literature. As a relatively new class of electrodes, we will also highlight the performance of dealloyed nanoporous gold (NPG) based electrocatalysts, which allow a unique combination of structural properties highly desired for this important reaction. Finally, we will put forward the challenges and opportunities for the incorporation of these advanced electrocatalysts into membrane electrode assemblies (MEA) for actual fuel cells.

Nature-inspired electrocatalysts and devices for energy conversion

A NICE approach for the design of nature-inspired electrocatalysts and electrochemical devices for energy conversion.

Structural and Electrochemical Properties of Nesting and Core/Shell Pt/TiO2 Spherical Particles Synthesized by Ultrasonic Spray Pyrolysis

Pt/TiO2 composites were synthesized by single-step ultrasonic spray pyrolysis (USP) at different temperatures. In an in-situ method, Pt and TiO2 particles were generated from tetra-n-butyl orthotitanate and chloroplatinic acid, and hydrothermally-prepared TiO2 colloidal dispersion served as Pt support in an ex-situ USP approach. USP-synthesized Pt/TiO2 composites were generated in the form of a solid mixture, morphologically organized in nesting huge hollow and small solid spheres, or TiO2 core/Pt shell regular spheroids by in-situ or ex-situ method, respectively. This paper exclusively reports on characteristic mechanisms of the formation of novel two-component solid composites, which are intrinsic from the USP approach and controlled precursor composition. The generation of the two morphological components within the in-situ approach, the hollow spheres and all-solid spheres, was indicated to be caused by characteristic sol-gel/solid phase transition of TiO2. Both the walls of the hollow spheres and the cores of all-solid ones consist of TiO2 matrix populated by 10 nm-sized Pt. On the other hand, spherical, uniformly-sized, Pt particles of a few nanometers in size created a shell uniformly deposited onto TiO2 spheres of ca. 150 nm size. Activities of the prepared samples in an oxygen reduction reaction and combined oxygen reduction and hydrogen evolution reactions were electrochemically tested. The ex-situ synthesized Pt/TiO2 was more active for oxygen reduction and combined oxygen reduction and hydrogen reactions in comparison to the in-situ Pt/TiO2 samples, due to better availability of Pt within a core/shell structure for the reactions.

Recent Progresses in Oxygen Reduction Reaction Electrocatalysts for Electrochemical Energy Applications

Abstract

Electrochemical energy storage systems such as fuel cells and metal–air batteries can be used as clean power sources for electric vehicles. In these systems, one necessary reaction at the cathode is the catalysis of oxygen reduction reaction (ORR), which is the rate-determining factor affecting overall system performance. Therefore, to increase the rate of ORR for enhanced system performances, efficient electrocatalysts are essential. And although ORR electrocatalysts have been intensively explored and developed, significant breakthroughs have yet been achieved in terms of catalytic activity, stability, cost and associated electrochemical system performance. Based on this, this review will comprehensively present the recent progresses of ORR electrocatalysts, including precious metal catalysts, non-precious metal catalysts, single-atom catalysts and metal-free catalysts. In addition, major technical challenges are analyzed and possible future research directions to overcome these challenges are proposed to facilitate further research and development toward practical application.

Graphic Abstract

Core@shell nanostructured Au-d@NimPtm for electrochemical oxygen reduction reaction: effect of the core size and shell thickness

Au-d@Ni_mPt_m nanostructures are studied to address the effects of the Au-core size (d) and NiPt-shell thickness (m) on the electrocatalytic performance of Pt for the ORR.