Structural and Reactivity Effects of Secondary Metal Doping into Iron-Nitrogen-Carbon Catalysts for Oxygen Electroreduction.

While improved activity was recently reported for bimetallic iron-metal-nitrogen-carbon (FeMNC) catalysts for the oxygen reduction reaction (ORR) in acid medium, the nature of active sites and interactions between the two metals are poorly understood. Here, FeSnNC and FeCoNC catalysts were structurally and catalytically compared to their parent FeNC and SnNC catalysts. While CO cryo-chemisorption revealed a twice lower site density of M-Nx sites for FeSnNC and FeCoNC relative to FeNC and SnNC, the mass activity of both bimetallic catalysts is 50-100% higher than that of FeNC due to a larger turnover frequency in the bimetallic catalysts. Electron microscopy and X-ray absorption spectroscopy identified the coexistence of Fe-Nx and Sn-Nx or Co-Nx sites, while no evidence was found for binuclear Fe-M-Nx sites. 57Fe Mössbauer spectroscopy revealed that the bimetallic catalysts feature a higher D1/D2 ratio of the spectral signatures assigned to two distinct Fe-Nx sites, relative to the FeNC parent catalyst. Thus, the addition of the secondary metal favored the formation of D1 sites, associated with the higher turnover frequency.

Regulating Catalytic Properties and Thermal Stability of Pt and PtCo Intermetallic Fuel-Cell Catalysts via Strong Coupling Effects between Single-Metal Site-Rich Carbon and Pt.

Developing low platinum-group-metal (PGM) catalysts for the oxygen reduction reaction (ORR) in proton-exchange membrane fuel cells (PEMFCs) for heavy-duty vehicles (HDVs) remains a great challenge due to the highly demanded power density and long-term durability. This work explores the possible synergistic effect between single Mn site-rich carbon (MnSA-NC) and Pt nanoparticles, aiming to improve intrinsic activity and stability of PGM catalysts. Density functional theory (DFT) calculations predicted a strong coupling effect between Pt and MnN4 sites in the carbon support, strengthening their interactions to immobilize Pt nanoparticles during the ORR. The adjacent MnN4 sites weaken oxygen adsorption at Pt to enhance intrinsic activity. Well-dispersed Pt (2.1 nm) and ordered L12-Pt3Co nanoparticles (3.3 nm) were retained on the MnSA-NC support after indispensable high-temperature annealing up to 800 °C, suggesting enhanced thermal stability. Both PGM catalysts were thoroughly studied in membrane electrode assemblies (MEAs), showing compelling performance and durability. The Pt@MnSA-NC catalyst achieved a mass activity (MA) of 0.63 A mgPt-1 at 0.9 ViR-free and maintained 78% of its initial performance after a 30,000-cycle accelerated stress test (AST). The L12-Pt3Co@MnSA-NC catalyst accomplished a much higher MA of 0.91 A mgPt-1 and a current density of 1.63 A cm-2 at 0.7 V under traditional light-duty vehicle (LDV) H2-air conditions (150 kPaabs and 0.10 mgPt cm-2). Furthermore, the same catalyst in an HDV MEA (250 kPaabs and 0.20 mgPt cm-2) delivered 1.75 A cm-2 at 0.7 V, only losing 18% performance after 90,000 cycles of the AST, demonstrating great potential to meet the DOE targets.

Pt Particle Size Affects Both the Charge Separation and Water Reduction Efficiencies of CdS-Pt Nanorod Photocatalysts for Light Driven H2 Generation.

Decreasing the metal catalyst size into nanoclusters or even single atom is an emerging direction of developing more efficient and cost-effective photocatalytic systems. Because the catalyst particle size affects both the catalyst activity and light driven charge separation efficiency, their effects on the overall photocatalytic efficiency are still poorly understood. Herein, using a well-defined semiconductor-metal heterostructure with Pt nanoparticle catalysts selectively grown on the apexes of CdS nanorods (NRs), we study the effect of the Pt catalyst size on light driven H2 generation quantum efficiency (QEH2). With the increase of the Pt catalyst size from 0.7 ± 0.3 to 3.0 ± 0.8 nm, the QEH2 of CdS-Pt increases from 0.5 ± 0.2% to 38.3 ± 5.1%, by nearly 2 orders of magnitude. Transient absorption spectroscopy measurement reveals that the electron transfer rate from the CdS NR to the Pt tip increases with the Pt diameter following a scaling law of d5.6, giving rise to the increase of electron transfer efficiency at larger Pt sizes. The observed trend can be understood by a simplified kinetic model that assumes the overall efficiency is the product of the quantum efficiencies of charge separation (including hole transfer, electron transfer, and hole scavenging) and water reduction steps, and for CdS-Pt NRs, the quantum efficiencies of electron transfer and water reduction steps increase with the Pt sizes. Our findings suggest the importance of improving the quantum efficiencies of both charge separation and catalysis in designing efficient semiconductor-metal hybrid photocatalysts, especially in the regime of small metal particle sizes.

Molecular Catalyst Synthesis Strategies to Prepare Atomically Dispersed Fe-N-C Heterogeneous Catalysts.

We report a strategy to integrate atomically dispersed iron within a heterogeneous nitrogen-doped carbon (N-C) support, inspired by routes for metalation of molecular macrocyclic iron complexes. The N-C support, derived from pyrolysis of a ZIF-8 metal-organic framework, is metalated via solution-phase reaction with FeCl2 and tributyl amine, as a Brønsted base, at 150 °C. Fe active sites are characterized by 57Fe Mössbauer spectroscopy and aberration-corrected scanning transmission electron microscopy. The site density can be increased by selective removal of Zn2+ ions from the N-C support prior to metalation, resembling the transmetalation strategy commonly employed for the preparation of molecular Fe-macrocycles. The utility of this approach is validated by the higher catalytic rates (per total Fe) of these materials relative to established Fe-N-C catalysts, benchmarked using an aerobic oxidation reaction.

Tuning Catalyst Activation and Utilization Via Controlled Electrode Patterning for Low-Loading and High-Efficiency Water Electrolyzers.

An anode electrode concept of thin catalyst-coated liquid/gas diffusion layers (CCLGDLs), by integrating Ir catalysts with Ti thin tunable LGDLs with facile electroplating in proton exchange membrane electrolyzer cells (PEMECs), is proposed. The CCLGDL design with only 0.08 mgIr cm-2 can achieve comparative cell performances to the conventional commercial electrode design, saving ≈97% Ir catalyst and augmenting a catalyst utilization to ≈24 times. CCLGDLs with regulated patterns enable insight into how pattern morphology impacts reaction kinetics and catalyst utilization in PEMECs. A specially designed two-sided transparent reaction-visible cell assists the in situ visualization of the PEM/electrode reaction interface for the first time. Oxygen gas is observed accumulating at the reaction interface, limiting the active area and increasing the cell impedances. It is demonstrated that mass transport in PEMECs can be modified by tuning CCLGDL patterns, thus improving the catalyst activation and utilization. The CCLGDL concept promises a future electrode design strategy with a simplified fabrication process and enhanced catalyst utilization. Furthermore, the CCLGDL concept also shows great potential in being a powerful tool for in situ reaction interface research in PEMECs and other energy conversion devices with solid polymer electrolytes.

Chemical vapour deposition of Fe-N-C oxygen reduction catalysts with full utilization of dense Fe-N4 sites.

Replacing scarce and expensive platinum (Pt) with metal-nitrogen-carbon (M-N-C) catalysts for the oxygen reduction reaction in proton exchange membrane fuel cells has largely been impeded by the low oxygen reduction reaction activity of M-N-C due to low active site density and site utilization. Herein, we overcome these limits by implementing chemical vapour deposition to synthesize Fe-N-C by flowing iron chloride vapour over a Zn-N-C substrate at 750 °C, leading to high-temperature trans-metalation of Zn-N4 sites into Fe-N4 sites. Characterization by multiple techniques shows that all Fe-N4 sites formed via this approach are gas-phase and electrochemically accessible. As a result, the Fe-N-C catalyst has an active site density of 1.92 × 1020 sites per gram with 100% site utilization. This catalyst delivers an unprecedented oxygen reduction reaction activity of 33 mA cm-2 at 0.90 V (iR-corrected; i, current; R, resistance) in a H2-O2 proton exchange membrane fuel cell at 1.0 bar and 80 °C.

Harvesting Sub-Bandgap IR Photons by Photothermionic Hot Electron Transfer in a Plasmonic p-n Junction.

Plasmonic semiconductors are an emerging class of low-cost plasmonic materials, and the presence of a bandgap and band-bending in these materials offer new opportunities to overcome some of the limitations of plasmonic metals. Here, we demonstrate that in a plasmonic p-n heterojunction (Cu2-xSe-CdSe) the near-IR excitation (1.1 eV) of the hole plasmon in the p-Cu2-xSe phase results in rapid hot electron transfer to n-CdSe, with an energy 2.2 eV above the Fermi level. This hot electron generation and energy upconversion process can be well-described by a photothermionic mechanism, where the presence of a bandgap in p-Cu2-xSe facilitates the generation of energetic photothermal electrons. The lifetime of the transferred electrons in Cu2-xSe-CdSe can reach ∼130 ps, which is nearly 100× longer than that of its metal-semiconductor counterpart. This result demonstrates a novel approach for harvesting the sub-bandgap near IR photons using plasmonic p-n junctions and the potential advantages of plasmonic semiconductors for hot carrier-based devices.

Synthesis of Novel Phases in Si Nanowires Using Diamond Anvil Cells at High Pressures and Temperatures.

Silicon has several technologically promising allotropes that are formed via high-pressure synthesis. One of these phases (hd) has been predicted to have a direct band gap under tensile strain, whereas other (r8 and bc8) phases are predicted to have narrow band gaps and good absorption across the solar spectrum. Pure volumes of these phases cannot be made using conventional nanowire growth techniques. In this work, Si nanowires were compressed up to ∼20 GPa and then decompressed using a diamond anvil cell in the temperature range of 25-165 °C. It was found that at intermediate temperatures, near-phase-pure bc8-Si nanowires were produced, whereas amorphous Si (a-Si) dominated at lower temperatures, and a direct transformation to the diamond cubic phase (dc-Si) occurred at higher temperatures under compression. Thus this study has opened up a new pressure-temperature pathway for the synthesis of novel Si nanowires consisting of designed phase components with transformative properties.

Covalent Organic Framework (COF) Derived Ni-N-C Catalysts for Electrochemical CO2 Reduction: Unraveling Fundamental Kinetic and Structural Parameters of the Active Sites.

Electrochemical CO2 reduction is a potential approach to convert CO2 into valuable chemicals using electricity as feedstock. Abundant and affordable catalyst materials are needed to upscale this process in a sustainable manner. Nickel-nitrogen-doped carbon (Ni-N-C) is an efficient catalyst for CO2 reduction to CO, and the single-site Ni-Nx motif is believed to be the active site. However, critical metrics for its catalytic activity, such as active site density and intrinsic turnover frequency, so far lack systematic discussion. In this work, we prepared a set of covalent organic framework (COF)-derived Ni-N-C catalysts, for which the Ni-Nx content could be adjusted by the pyrolysis temperature. The combination of high-angle annular dark-field scanning transmission electron microscopy and extended X-ray absorption fine structure evidenced the presence of Ni single-sites, and quantitative X-ray photoemission addressed the relation between active site density and turnover frequency.

Slow Auger Recombination of Trapped Excitons Enables Efficient Multiple Electron Transfer in CdS-Pt Nanorod Heterostructures.

Solar-to-fuel conversion reaction often requires multiple proton-coupled electron transfer (PCET) processes powered by the energetic electrons and/or holes generated by the absorption of multiple photons. The effective coupling of multiple electron transfer from the light absorber with the multiple PCET reactions at the catalytic center is one of the key challenges in efficient and selective conversion of solar energy to chemical fuels. In this paper, we examine the dynamics of multiple electron transfer in quantum confined CdS nanorods with a Pt tip, in which the CdS rod functions as the light absorber and the Pt tip the catalytic center. By excitation-fluence-dependent transient absorption spectroscopic measurements, we show that the multiexciton Auger recombination rate in CdS rods follows a carrier-collision model, knA = n2(n - 1)/4k2A, with a biexciton lifetime (1/k2A) of 2.0 ± 0.2 ns. In CdS-Pt nanorods, electron transfer kinetics from the CdS conduction band edge to the Pt show negligible dependence on the excitation fluence, occurring with a half-life time of 5.6 ± 0.6 ps. The efficiency of multiple exciton dissociation by multiple electron transfer to Pt decreases from 100% in biexciton states to ∼41% at 22 exciton state due to the competition with Auger recombination. The half-lifetime of the n-charge separated state recombination (with n electrons in the Pt and n holes in the CdS) decreases from 10 µs in the single charge separated state to 42 ns in nine charge separated states. Our findings suggest the possibility of driving multielectron photocatalytic reactions under intense illumination and controlling product selectivity through multielectron transfer.

P-block single-metal-site tin/nitrogen-doped carbon fuel cell cathode catalyst for oxygen reduction reaction.

This contribution reports the discovery and analysis of a p-block Sn-based catalyst for the electroreduction of molecular oxygen in acidic conditions at fuel cell cathodes; the catalyst is free of platinum-group metals and contains single-metal-atom actives sites coordinated by nitrogen. The prepared SnNC catalysts meet and exceed state-of-the-art FeNC catalysts in terms of intrinsic catalytic turn-over frequency and hydrogen-air fuel cell power density. The SnNC-NH3 catalysts displayed a 40-50% higher current density than FeNC-NH3 at cell voltages below 0.7 V. Additional benefits include a highly favourable selectivity for the four-electron reduction pathway and a Fenton-inactive character of Sn. A range of analytical techniques combined with density functional theory calculations indicate that stannic Sn(IV)Nx single-metal sites with moderate oxygen chemisorption properties and low pyridinic N coordination numbers act as catalytically active moieties. The superior proton-exchange membrane fuel cell performance of SnNC cathode catalysts under realistic, hydrogen-air fuel cell conditions, particularly after NH3 activation treatment, makes them a promising alternative to today's state-of-the-art Fe-based catalysts.

Efficient Hot Electron Transfer from Small Au Nanoparticles.

Many important chemical transformations enabled by plasmonic hot carrier photocatalysis have been reported, although their efficiencies are often too low for practical applications. We examine how the efficiency of plasmon-induced hot electron transfer depends on the Au particle size in Au-tipped CdS nanorods. We show that with decreasing Au size, the plasmon width increases due to enhanced surface damping contributions. The excitation of Au nanoparticles leads to an instrument response time-limited ultrafast hot electron transfer process to CdS (≪140 fs). The quantum efficiency of this process increases from ∼1% to ∼18% as the particle size decreases from 5.5 ± 1.1 to 1.6 ± 0.5 nm due to both enhanced hot electron generation and transfer efficiencies in small Au particles. Our finding suggests that decreasing plasmonic particle size is an effective approach for improving plasmon-induced hot carrier transfer efficiency and provides important insight for the rational improvement of plasmonic hot carrier-based devices.

Dynamically Unveiling Metal-Nitrogen Coordination during Thermal Activation to Design High-Efficient Atomically Dispersed CoN4 Active Sites.

We elucidate the structural evolution of CoN4 sites during thermal activation by developing a zeolitic imidazolate framework (ZIF)-8-derived carbon host as an ideal model for Co2+ ion adsorption. Subsequent in situ X-ray absorption spectroscopy analysis can dynamically track the conversion from inactive Co-OH and Co-O species into active CoN4 sites. The critical transition occurs at 700 °C and becomes optimal at 900 °C, generating the highest intrinsic activity and four-electron selectivity for the oxygen reduction reaction (ORR). DFT calculations elucidate that the ORR is kinetically favored by the thermal-induced compressive strain of Co-N bonds in CoN4 active sites formed at 900 °C. Further, we developed a two-step (i.e., Co ion doping and adsorption) Co-N-C catalyst with increased CoN4 site density and optimized porosity for mass transport, and demonstrated its outstanding fuel cell performance and durability.

Bridging Thermal Catalysis and Electrocatalysis: Catalyzing CO2 Conversion with Carbon-Based Materials.

Understanding the differences between reactions driven by elevated temperature or electric potential remains challenging, largely due to materials incompatibilities between thermal catalytic and electrocatalytic environments. We show that Ni, N-doped carbon (NiPACN), an electrocatalyst for the reduction of CO2 to CO (CO2 R), can also selectively catalyze thermal CO2 to CO via the reverse water gas shift (RWGS) representing a direct analogy between catalytic phenomena across the two reaction environments. Advanced characterization techniques reveal that NiPACN likely facilitates RWGS on dispersed Ni sites in agreement with CO2 R active site studies. Finally, we construct a generalized reaction driving-force that includes temperature and potential and suggest that NiPACN could facilitate faster kinetics in CO2 R relative to RWGS due to lower intrinsic barriers. This report motivates further studies that quantitatively link catalytic phenomena across disparate reaction environments.

Engineering Atomically Dispersed FeN4 Active Sites for CO2 Electroreduction.

Atomically dispersed FeN4 active sites have exhibited exceptional catalytic activity and selectivity for the electrochemical CO2 reduction reaction (CO2RR) to CO. However, the understanding behind the intrinsic and morphological factors contributing to the catalytic properties of FeN4 sites is still lacking. By using a Fe-N-C model catalyst derived from the ZIF-8, we deconvoluted three key morphological and structural elements of FeN4 sites, including particle sizes of catalysts, Fe content, and Fe-N bond structures. Their respective impacts on the CO2RR were comprehensively elucidated. Engineering the particle size and Fe doping is critical to control extrinsic morphological factors of FeN4 sites for optimal porosity, electrochemically active surface areas, and the graphitization of the carbon support. In contrast, the intrinsic activity of FeN4 sites was only tunable by varying thermal activation temperatures during the formation of FeN4 sites, which impacted the length of the Fe-N bonds and the local strains. The structural evolution of Fe-N bonds was examined at the atomic level. First-principles calculations further elucidated the origin of intrinsic activity improvement associated with the optimal local strain of the Fe-N bond.

Heat-Treated Aerogel as a Catalyst for the Oxygen Reduction Reaction.

Aerogels are fascinating materials that can be used for a wide range of applications, one of which is electrocatalysis of the important oxygen reduction reaction. In their inorganic form, aerogels can have ultrahigh catalytic site density, high surface area, and tunable physical properties and chemical structures-important features in heterogeneous catalysis. Herein, we report on the synthesis and electrocatalytic properties of an iron-porphyrin aerogel. 5,10,15,20-(Tetra-4-aminophenyl)porphyrin (H2 TAPP) and FeII were used as building blocks of the aerogel, which was later heat-treated at 600 °C to enhance electronic conductivity and catalytic activity, while preserving its macrostructure. The resulting material has a very high concentration of atomically dispersed catalytic sites (9.7×1020  sites g-1 ) capable of catalyzing the oxygen reduction reaction in alkaline solution (Eonset =0.92 V vs. RHE, TOF=0.25 e- site-1 s-1 at 0.80 V vs. RHE).

Chemical Vapor Deposition for Atomically Dispersed and Nitrogen Coordinated Single Metal Site Catalysts.

Atomically dispersed and nitrogen coordinated single metal sites (M-N-C, M=Fe, Co, Ni, Mn) are the popular platinum group-metal (PGM)-free catalysts for many electrochemical reactions. Traditional wet-chemistry catalyst synthesis often requires complex procedures with unsatisfied reproducibility and scalability. Here, we report a facile chemical vapor deposition (CVD) strategy to synthesize the promising M-N-C catalysts. The deposition of gaseous 2-methylimidazole onto M-doped ZnO substrates, followed by an in situ thermal activation, effectively generated single metal sites well dispersed into porous carbon. In particular, an optimal CVD-derived Fe-N-C catalyst exclusively contains atomically dispersed FeN4 sites with increased Fe loading relative to other catalysts from wet-chemistry synthesis. The catalyst exhibited outstanding oxygen-reduction activity in acidic electrolytes, which was further studied in proton-exchange membrane fuel cells with encouraging performance.

Thermally Driven Structure and Performance Evolution of Atomically Dispersed FeN4 Sites for Oxygen Reduction.

FeN4 moieties embedded in partially graphitized carbon are the most efficient platinum group metal free active sites for the oxygen reduction reaction in acidic proton-exchange membrane fuel cells. However, their formation mechanisms have remained elusive for decades because the Fe-N bond formation process always convolutes with uncontrolled carbonization and nitrogen doping during high-temperature treatment. Here, we elucidate the FeN4 site formation mechanisms through hosting Fe ions into a nitrogen-doped carbon followed by a controlled thermal activation. Among the studied hosts, the ZIF-8-derived nitrogen-doped carbon is an ideal model with well-defined nitrogen doping and porosity. This approach is able to deconvolute Fe-N bond formation from complex carbonization and nitrogen doping, which correlates Fe-N bond properties with the activity and stability of FeN4 sites as a function of the thermal activation temperature.

Selective and Stable Non-Noble-Metal Intermetallic Compound Catalyst for the Direct Dehydrogenation of Propane to Propylene.

A non-noble intermetallic compound catalyst consisting of Ni3Ga nanoparticles supported on Al2O3 that exhibits high selectivity (∼94%), comparable activity (TOF = 4.7 × 10-2 s-1), good stability (∼94% to 81% over the 82 h test), and regenerability in the direct dehydrogenation of propane to propylene at 600 °C has been developed. Through synthesis techniques that stabilize the Ni3Ga phase, the surface composition of the catalytic nanoparticles could be tuned by Ni and Ga loading such that improved selectivity toward propylene may be achieved. Comparisons with well-defined silica-supported Ni3Ga and NiGa catalysts and Ni3Ga/Al2O3 with a range of Ni:Ga loading suggested that a specific surface composition range was most promising for propylene production. The presence of Ni at the active particle surface was also found to be critical to drive dehydrogenation and enhance conversion, whereas the presence of Ga was necessary to attenuate the reactivity of the surface to improve selectivity and catalyst stability.