Noble Metal-based Electrocatalysts Research Articles

3D Noble Metal-Based Porous Materials Self-Assembled By Atomically Modified Building Blocks As Electrocatalysts Boosting Small Molecule Electro-Oxidation or Reduction

Three-dimensional (3D) noble metal-based electrocatalysts are currently being intensively studied due to their unique hybrid porous architectures that could effectively enlarge their active sites, favor the mass and electron transfer rate, avoid the corrosion issues of carbon support and so on. Modifying building blocks is also of great importance for constructing the self-assembled 3D porous electrocatalysts with significantly improved electrochemical performances. However, the synthesis strategies for tailoring the structures and morphologies from atomic scale are still challenging and the mass activity (MA) and stability are still far from sufficient toward small molecule (i.e., ethanol, oxygen) electro-oxidation or reduction. Here, we rationally designed a series of 3D architectures by finely tailoring the morphologies, compositions, and surface atomic configurations etc. of building blocks for boosting their electrocatalytic performances. Core-shell structures are very popular because of their ligands effects, ensemble effects and electronic effects, which are related to charge transfer and structure constraints that could favor desorption of oxygen-containing intermediates from the Pt active sites. Therefore, we firstly synthesized Au@Pt3Pd ternary metallic aerogels via a one-pot gelation strategy employing the dendritic core-shell structured Au@Pt3Pd nanoparticles (NPs) with size of about 24.5 nm as the building block. The as-obtained Au@Pt3Pd metallic hydrogels exhibited an improved MA of 0.812 A/mgPt+Pd and enhanced stability of showing 11.8 mV of half-potential shift after ADT test compared to commercial Pt/C toward oxygen reduction reaction (ORR). Both macro- and micro-structures contributed to the enhanced catalytic performances, like the high porosity for fast mass diffusion, cross-linked nanowires for rapid electron transfer, dendritic surface for providing more active sites, core-shell structure for offering synergistic effect and so on.Compared to disordered alloyed structures, intermetallic nanocrystals (IM-NCs) possesses specific atomic structures with predictable electronic and geometric effect, high selectivity achieved through reducing the homoatomic coordination number of active sites, and stable chemical structures realized by the more negative enthalpy for generation of intermetallics compared to that of alloys. In order to improve the MA, selectivity and durability of the electrocatalysts toward ORR, for the first time, we successfully synthesized 3D IM-Pd3Pb nanowire networks (NNs) using ethylene glycol as the solvent and PVP as the surfactant at 170 ℃ for 1 h. The MA of IM-Pd3Pb NNs increased to be 1.06 A/mg and showed a half-potential shift of only 8 mV after 10000 potential cycles test. Besides, IM-Pd3Pb NNs displayed negligible current changes with addition of methanol while commercial Pt/C showed significant oxidation peak of methanol, indicating a high selectivity of oxygen as the cathode electrocatalysts. What’s more, IM-Pd3Pb NNs also shows a MA of 3.2 A/mg, which was two times higher than commercial Pd black toward ethanol oxidation.Well established is the knowledge that catalytic activities leapt via downsizing the NPs to nanoclusters or single atoms, which is attributed to the remarkably increased active sites along with the altered surface defects, atomic and electronic structure. The as-resulted geometric and electronic/synergistic effects are all beneficial for boosting catalytic activities in electrochemical reactions. As for the ethanol oxidation, it had been revealed that Pd ensembles rather than isolated Pd atoms that serve as highly activated sites toward ethanol oxidation. Based on this principle, we successfully anchored Pd ensembles on the surface of the core-shell structured Au2Cu metallic hydrogels (denoted as Au2Cu@Pd) via galvanic displacement reaction under ice bath and achieved a MA of 22.1 A/mg toward ethanol oxidation, which is 15.8 times higher than Pd black. Also it suffers only 10 % degradation after 300 potential cycle test. The MA and long-term durability of Au2Cu@Pd exhibited remarkable improvement due to the modified surface composition and atomic configuration, core-shell structured NNs, 3D porous structures and so on. All in all, the high porosity and interconnected architecture of 3D porous materials are significant for electrocatalysts because they provide free paths and open channels for electron and mass transfer. However, the modification of the building blocks by optimizing the compositions and structures, tailoring the surface atomic configurations also hold a key for boosting the electrocatalytic performances due to the as-resulted electronic and geometric effect from the atomic scale.

Hierarchical Co-FeS2/CoS2 heterostructures as a superior bifunctional electrocatalyst.

The traditional method of preparing hydrogen and oxygen as efficient clean energy sources mainly relies on the use of platinum, palladium, and other precious metals. However, the high cost and low abundance limit wide application of such metals. As such, one challenging issue is the development of low-cost and high-efficiency electrocatalysts for such purposes. In this study, we synthesized Co–FeS2/CoS2 heterostructures via a hydrothermal method for efficient hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Benefitting from their unique three-dimensional hierarchical nanostructures, Co-doped FeS2, and CoS2 formed heterostructures on Co–FeS2 petals, which bestowed remarkable electrocatalytic properties upon Co–FeS2/CoS2 nanostructures. Co–FeS2/CoS2 effectively catalyzed the OER with an overpotential of 278 mV at a current density of 10 mA cm−2 in 1 M KOH solution, and also is capable of driving a current density −10 mA cm−2 at an overpotential of −103 mV in 0.5 M H2SO4 solution. The overpotential of the OER and HER only decreased by 5 mV and 3 mV after 1000 cycles. Our Co–FeS2/CoS2 materials may offer a promising alternative to noble metal-based electrocatalysts for water splitting.

Thermally oxidized porous NiO as an efficient oxygen evolution reaction (OER) electrocatalyst for electrochemical water splitting application

Low-cost and competent electrocatalysts play a key role in an electrocatalytic water oxidation reaction. Herein, we report that readily available bare nickel foam (NF) can be used as conductive substrate and precursor to grow a porous nickel oxide (NiO) using a simple and scalable thermal oxidation method. The obtained NiO supported on NF is used as binder-free electrocatalyst for the oxygen evolution reaction (OER) and its electrochemical properties are evaluated by linear sweep voltammetry (LSV) in 1M KOH. The porous NiO thin film acts as an efficient electrocatalyst for the OER and achieves a catalytic current density of 10mAcm−2 at an overpotential of 310mV with a smaller Tafel slope of 54mVdec−1. The electrode also shows good durability over 24h with negligible degradation. This durable and high-performance electrocatalyst can be a competitor to electrocatalysts that consist of costly elements and, require advanced synthesis; the NiO electrocatalyst shows progress towards the replacement of noble metal-based electrocatalysts for the OER.

One-Step Growth of Iron–Nickel Bimetallic Nanoparticles on FeNi Alloy Foils: Highly Efficient Advanced Electrodes for the Oxygen Evolution Reaction

Electrochemical water splitting is an important process to produce hydrogen and oxygen for energy storage and conversion devices. However, it is often restricted by the oxygen evolution reaction (OER) due to its sluggish kinetics. To overcome the problem, precious metal oxide-based electrocatalysts, such as RuO2 and IrO2, are widely used. The lack of availability and the high cost of precious metals compel researchers to find other resources for the development of cost-effective, environmentally friendly, earth-abundant, nonprecious electrocatalysts for OER. Such catalysts should have high OER performance and good stability in comparison to those of available commercial precious metal-based electrocatalysts. Herein, we report an inexpensive fabrication of bimetallic iron-nickel nanoparticles on FeNi-foil (FeNi4.34@FeNi-foil) as an integrated OER electrode using a one-step calcination process. FeNi4.34@FeNi-foil obtained at 900 °C shows superior OER activity in alkaline solution with an overpotential as low as 283 mV to achieve a current density of 10 mA cm-2 and a small Tafel slope of 53 mV dec-1. The high performance and durability of the as-prepared nonprecious metal electrode even exceeds those of the available commercial RuO2 and IrO2 catalysts, showing great potential in replacing the expensive noble metal-based electrocatalysts for OER.

Oxygen Vacancies Enhancing Electrocatalysis Performance of Porous Copper Oxide

Porous CuO particles with oxygen vacancies are first synthetized by a simple calcination of Cu2C2O4 particles. Oxygen vacancies are confirmed by high resolution transmission electron microscopy, electron paramagnetic resonance measurements, and X-ray photoelectron spectroscopy. When porous CuO particles with oxygen vacancies are as-assembled as nonenzymatic glucose sensors, they reveal high sensitivity and good anti-interference ability. The existing oxygen vacancies can help to increase the rate that electron reaches to the surface and accelerate the kinetics of the surface redox reactions to enhance the electrochemical performances of materials. In this work, the response time of porous CuO particles with oxygen vacancies' modified electrode for the electrocatalysis reaction is less than 3 s. Meanwhile, the calibration plot is linear over the wide concentration range of 0.5 × 10−6-6.32 × 10−3m and a detection limit of 50 × 10−9m. The developed sensor displays high sensitivity of 10 490.45 µA mm−1 cm−2 and good anti-interference ability, which is prior than many previous noble metal-based and Cu-based electrocatalysts.

Facile synthesis of porous nitrogen-doped holey graphene as an efficient metal-free catalyst for the oxygen reduction reaction

Nitrogen-doped graphene is a promising candidate for the replacement of noble metal-based electrocatalysts for oxygen reduction reactions (ORRs). The addition of pores and holes into nitrogen-doped graphene enhances the ORR activity by introducing abundant exposed edges, accelerating mass transfer, and impeding aggregation of the graphene sheets. Herein, we present a straightforward but effective strategy for generating porous holey nitrogen-doped graphene (PHNG) via the pyrolysis of urea and magnesium acetate tetrahydrate. Due to the combined effects of the in situ generated gases and MgO nanoparticles, the synthesized PHNGs featured not only numerous out-of-plane pores among the crumpled graphene sheets, but also interpenetrated nanoscale (5–15 nm) holes in the assembled graphene. Moreover, the nitrogen doping configurations of PHNG were optimized by post-thermal treatments at different temperatures. It was found that the overall content of pyridinic and quaternary nitrogen positively correlates with the ORR activity; in particular, pyridinic nitrogen generates the most desirable characteristics for the ORR. This work reveals new routes for the synthesis of PHNG-based materials and elucidates the contributions of various nitrogen species to ORRs.

Metal-Free and Noble Metal-Free Heteroatom-Doped Nanostructured Carbons as Prospective Sustainable Electrocatalysts.

The large-scale deployment of many types of fuel cells and electrolyzers is currently constrained by the lack of sustainable and efficient catalysts that can replace the less earth-abundant, noble metal-based catalysts, which are commonly used in these renewable energy systems. This burgeoning issue has led to explosive research efforts worldwide to find alternative, metal-free and noble metal-free catalysts that are composed of inexpensive and earth-abundant elements. Hence, the recent discoveries that doping carbon nanomaterials with heteroatoms (such as N, S, B, etc.) can give sustainable materials with good electrocatalytic activity for reactions carried out in fuel cells and electrolyzers have been not only quite exciting but also very promising to address these challenging issues. Interestingly, even though they contain no metals or involve only the inexpensive, more earth-abundant ones, the catalytic activity of some of these materials fares well with those of the commercially used noble metal-based electrocatalysts, such as Pt/C. However, research efforts to improve the catalytic activity, selectivity, and stability of some of these materials for various reactions are still necessary and thus continuing. While some of these efforts have focused on finding synthetic methods that can tune the structures and compositions of already known materials and thereby improve their catalytic properties (activity, selectivity, stability, etc.), others have focused on developing entirely new materials that can exhibit better or superior catalytic properties. In these efforts, additional considerations are also being paid to find facile synthetic routes or renewable and inexpensive precursors that can lead to such types of catalysts in order to make the entire process highly sustainable and widely applicable. In this Account, notable heteroatom-doped carbon catalysts that have been developed for reactions in fuel cells and water electrolyzers, the various synthetic procedures employed to make them, and the challenges involved in their synthesis as well as their characterizations are discussed. The methods used to systematically vary the structures and compositions of the precursors of these materials, as well as the materials themselves, and the different experimental, imaging, and spectroscopic methods used to investigate the properties and structure-property relationships of the materials for various energy related reactions are also included. The discussions focus mainly around the recent notable results reported on these materials by the author's and other research groups worldwide, albeit not exhaustively. Finally, the author's perspective about the challenges remaining in the field that need to be addressed, the many existing unanswered questions that beg for more research, and the future prospects for research related to the above topics are also mentioned.

Ultrasmall tungsten carbide catalysts stabilized in graphitic layers for high-performance oxygen reduction reaction

Tungsten carbide (WC) has been proven Pt-like features in many catalytic reactions, and presents promising potentials to substitute noble metal-based electrocatalysts. However, the catalytic performances cannot be improved to the industrial standard due to the low density of catalytically active sites that are relevant to the size and stability of catalysts. Herein, we synthesized WC catalysts with a mean diameter of 1.9±0.9nm, comprising a high density of single W atoms, sub-nanometer and nanometer WC clusters, which are completely encapsulated in high-defective graphitic layers (WC@C). Such high-defective graphitic layers not only improve the overall conductivity for accelerating the penetration and exchange of ions and electrons during the electrocatalytic process of oxygen reduction, but also suppress the chemical/thermal coarsening of WC catalysts, thus providing excellent electrocatalytic performances in four-electron transfer process for oxygen reduction reaction.

A metal–organic framework-derived bifunctional oxygen electrocatalyst

Oxygen electrocatalysis is of great importance for many energy storage and conversion technologies, including fuel cells, metal–air batteries and water electrolysis. Replacing noble metal-based electrocatalysts with highly efficient and inexpensive non-noble metal-based oxygen electrocatalysts is critical for the practical applications of these technologies. Here we report a general approach for the synthesis of hollow frameworks of nitrogen-doped carbon nanotubes derived from metal–organic frameworks, which exhibit higher electrocatalytic activity and stability for oxygen reduction and evolution than commercial Pt/C electrocatalysts. The remarkable electrochemical properties are mainly attributed to the synergistic effect from chemical compositions and the robust hollow structure composed of interconnected crystalline nitrogen-doped carbon nanotubes. The presented strategy for controlled design and synthesis of metal–organic framework-derived functional nanomaterials offers prospects in developing highly active electrocatalysts in electrochemical energy devices. Precious metals are efficient oxygen electrocatalysts but suffer from poor stability and high cost. Now, nitrogen-doped carbon nanotubes derived from metal–organic frameworks are shown to have activity and durability comparable to that of Pt/C catalysts.

Facile Synthesis of Nitrogen and Sulfur Dual-doped Hierarchical Micro/mesoporous Carbon Foams as Efficient Metal-free Electrocatalysts for Oxygen Reduction Reaction

It’s almost hardly to realize the large-scale practical application of fuel cells if the costly noble metal-based electrocatalysts for oxygen reduction reaction (ORR) cannot be replaced by other efficient, stable, and low-cost non-precious metal or even metal-free ORR electrocatalysts. Herein, we report the novel design and synthesis of N and S dual-doped hierarchical micro/mesoporous carbon (NS-MC) foams via high temperature treatment of polyaniline coated sulfur sphere composite without any side products. The highlight of our method lies in its environmental concept of green synthesis and simplicity for no use of toxic gas (NH3 or H2S) or templates (SiO2 or MgO). The main focus of this research is to develop a high-performance metal-free ORR electrocatalyst as well as to gain insight into the synergistic effects originating from multiple-element doping on ORR performance. It is found that much graphitic-N and thiophene-S binding configurations formed for NS-MC foams at 1000°C, which shows excellent electrocatalytic activity (high onset potential (0.051V vs. Ag/AgCl (sat.KCl)), large diffusion-limiting current density (5.61mAcm−2 at 1600rpm) and kinetic current density (22.22mAcm−2 at −0.25V)), high selectivity towards methanol and long durability compared to commercial Pt/C with a nearly four-electron transfer passway when they are employed as metal-free catalyst for ORR. Besides, the desired NS-MC1000 shows a strong amperometric response towards ethanol indicating its potential application as an anode metal-free electrocatalyst in the alkaline direct ethanol fuel cell.

Additive-Mediated Electrochemical Synthesis of Platelike Copper Crystals for Methanol Electrooxidation

A room-temperature electrochemical approach to synthesizing anisotropic platelike copper microcrystals and nanocrystals in the presence of potassium bromide is presented. Morphological and elemental characterization was performed using SEM, TEM, and XRD to confirm the anisotropic morphology and crystal structure of the synthesized copper particles. A possible mechanism for explaining the anisotropic crystal growth is proposed on the basis of the preferential adsorption of bromide ions to selective crystal faces. The shape-dependent electrocatalytic property of copper particles is demonstrated by its enhanced catalytic activity for methanol oxidation. Further development of such anisotropic copper particles localized on an electrode surface will lead us to find a suitable alternative for noble metal-based electrocatalysts for the methanol oxidation reaction relevant to fuel cells.

Catalytically active single-atom niobium in graphitic layers

Carbides of groups IV through VI (Ti, V and Cr groups) have long been proposed as substitutes for noble metal-based electrocatalysts in polymer electrolyte fuel cells. However, their catalytic activity has been extremely limited because of the low density and stability of catalytically active sites. Here we report the excellent performance of a niobium-carbon structure for catalysing the cathodic oxygen reduction reaction. A large number of single niobium atoms and ultra small clusters trapped in graphitic layers are directly identified using state-of-the-art aberration-corrected scanning transmission electron microscopy. This structure not only enhances the overall conductivity for accelerating the exchange of ions and electrons, but it suppresses the chemical/thermal coarsening of the active particles. Experimental results coupled with theory calculations reveal that the single niobium atoms incorporated within the graphitic layers produce a redistribution of d-band electrons and become surprisingly active for O2 adsorption and dissociation, and also exhibit high stability.

Enhanced Catalytic Performance of Pt-Free Iron Phthalocyanine by Graphene Support for Efficient Oxygen Reduction Reaction

It is commonly accepted that it is almost not possible to realize the large-scale practical application of fuel cells if the expensive noble metal-based electrocatalysts for oxygen reduction reactions (ORR) cannot be replaced by other low-cost, efficient, and stable ones. Herein, our studies demonstrate that iron phthalocyanine (FePc) supported on chemically reduced graphene through π–π interaction can act as a noble metal-free electrocatalyst with a comparable activity, long-term operation stability, and better tolerance to methanol crossover and CO poisoning compared with commercial Pt/C for ORR in alkaline media. The improved electrochemical activity and stability of FePc by graphene is mainly attributed to the inherent properties of graphene and the π-stacking interaction between FePc and planar aromatic structure of graphene. The as-prepared graphene–iron phthalocyanine (g-FePc) composite exhibits an efficient 4-electron pathway and can be used as a promising Pt-free ORR electrocatalyst.