Browsing by Subject "Oxygen reduction reaction"

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Addition of platinum to palladium-cobalt nanoalloy catalyst by direct alloying and galvanic displacement
(2010-12) Wise, Brent; Manthiram, Arumugam; Ferreira, Paulo
Direct methanol fuel cells (DMFC) are being investigated as a portable energy conversion device for military and commercial applications. DMFCs offer the potential to efficiently extract electricity from a dense liquid fuel. However, improvements in materials properties and lowering the cost of the electrocatalysts used in a DMFC are necessary for commercialization of the technology. The cathode electrocatalyst is a critical issue in DMFC because the state-of-the-art catalyst, platinum, is very expensive and rare, and its performance is diminished by methanol that crosses over from the anode to the cathode through the Nafion membrane. This thesis investigates the addition of platinum to a palladium-cobalt nanoalloy electrocatalyst supported on carbon black in order to improve catalyst activity for the oxygen reduction reaction (ORR) and catalyst stability against dissolution in acidic environment without significantly reducing the methanol-tolerance of the catalyst. Platinum was added to the palladium-cobalt nanoalloy catalyst using two synthesis methods. In the first method, platinum was directly alloyed with palladium and cobalt using a polyol reduction method, followed by heat treatment in a reducing atmosphere to form catalysts with 11 and 22 atom % platinum. In the second method, platinum was added to a palladium-cobalt alloy by galvanic displacement reaction to form catalysts with 10 and 22 atom % platinum. The palladium cobalt alloy was synthesized using a polyol method, followed by heat treatment in a reducing atmosphere to alloy the nanoparticles before the Pt displacement. It was found that both methods significantly improve catalyst activity and stability, with the displaced catalysts showing a higher activity than the corresponding alloy catalyst. However the alloy catalysts showed similar resistance to dissolution as the displaced catalysts, and the alloyed catalysts were more tolerant to methanol. The displaced catalyst with 22 atom % platinum (8 wt. % Pt overall) performed similar to a 20 wt. % commercial platinum catalyst in both RDE and single cell DMFC tests. The 10 and 22 atom % Pt displaced catalysts and 22 atom % Pt alloyed all showed higher Pt mass specific activities than a commercial Pt catalyst.
Advanced characterization of 1-2 nm Au, Pd, and AuPd nanoparticles for applications in electrocatalysis
(2022-12-02) Strasser, Juliette Wells; Crooks, Richard M. (Richard McConnell); Mullins, Charles B.; Warner, Jamie H.; Henkelman, Graeme
Nanoparticles (NPs) synthesized within and stabilized by a dendrimer template are known as dendrimer-encapsulated nanoparticles (DENs). Because DENs are small and generally monodisperse, they have been used as model catalysts, with structure/function relationships that can be directly compared to first principles theory. It is becoming increasingly clear, however, that electrochemical experiments may change the size or structure of NPs, which is further exacerbated for NPs < ~5 nm in diameter due to their inherent instability. The work in this dissertation emphasizes the importance of NP characterization before, during, and after electrochemical measurements, for accurate correlation of NP structure and function. First, we show that three electrochemical scans to modest positive potentials results in substantial growth of 1-2 nm Au DENS (Chapter 3). The observed growth of the DENs directly correlates to changes in their electrocatalytic oxygen reduction reaction (ORR) activity. The key point of Chapter 3 is that after just three electrochemical cleaning scans, the G6-NH₂(Au₁₄₇) and G6-NH₂(Au₅₅) DENs are essentially indistinguishable in terms of both physical and electrocatalytic properties. Second, we report the presence of small clusters of atoms (< 1 nm) (SCs) and single atoms (SAs) in solutions containing 1-2 nm DENs (Chapter 4). We have found that the presence or absence of SAs/SCs depend on both the terminal functional group of the dendrimer (-NH₂ or -OH) and the elemental composition of the DENs (Au or Pd). The observations discussed in Chapter 4 provide insights into the mechanisms for Au and Pd DEN synthesis and stability and demonstrate the need for careful characterization of systems containing NPs. Third, we report a systematic study of the electrocatalytic properties and stability of a series of 1-2 nm Au, Pd, and AuPd alloy DENs for the ethanol oxidation reaction (EOR) (Chapter 5). NP sizes and compositions were characterized both before and after EOR electrocatalysis using aberration-corrected scanning transmission electron microscopy (ac-STEM) and energy dispersive spectroscopy (EDS). The results discussed in Chapter 5 demonstrate the importance of post-catalytic ac-STEM/EDS characterization for fully evaluating NP activity and stability, especially for 1-2 nm NPs that may change in size or structure during electrocatalysis.
Calculations of oxygen reduction reaction on nanoparticles
(2010-05) Tang, Wenjie, 1982-; Henkelman, Graeme; Crooks, Richard M.; Rossky, Peter J.; Makarov, Dmitrii E.; Mullins, C. B.
Proton exchange membrane fuel cells are attractive power sources because they are highly efficient and do not pollute the environment. However, the use of Pt-based catalysts in present fuel cell technologies is not optimal: Pt is rare and expensive, and even the best commercial Pt cathodes have high overpotentials due to slow oxygen reduction kinetics. As a result, much effort has gone toward developing cheaper, more effective catalysts. Nanoparticles are attractive because they have different catalytic properties than analogous bulk systems, require less material, and have tunable reactivities based on their composition and size. It is important to perform detailed studies of nanoparticle catalysts since composition and size effects are poorly understood. Computational simulations of such materials can provide useful insights and potentially aid in the design of new catalysts. Here, I examine composition and size effects in nanoparticle catalysts using computational methods. Two bimetallic systems are investigated to explore composition effects: Pd-shell particles with several different core metals, and Pd/Cu random alloy particles. Depending on how the two metals are mixed (core-shell or random alloy), charge transfer and strain due to alloying are found to have different contributions to the catalytic activity. Size effects are studied for pure Pt particles, where corner and edge sites are found to play an important role. The binding geometries of molecular oxygen to corner and edge sites lead to peroxide formation instead of water on small Pt particles. Results form these calculations can provide useful information for designing novel catalysts in the future. By changing the composition and/or size of nanoparticles in the proper way, the interaction between the adsorbate and catalyst can be optimized, and better catalysts can be obtained.
Development and understanding of Pd-based nanoalloys as cathode electrocatalysts for PEMFC
(2010-08) Zhao, Juan, 1981-; Manthiram, Arumugam; Ferreira, Paulo; Meyers, Jeremy P.; Stevenson, Keith J.; Wheat, Harovel G.
Proton exchange membrane fuel cells (PEMFC) are attractive power sources as they offer high conversion efficiencies with low or no pollution. However, several challenges, especially the sluggish oxygen reduction reaction (ORR) and the high cost of Pt catalysts, impede their commercialization. With an aim to search for more active, less expensive, and more stable ORR catalysts than Pt, this dissertation focuses on the development of non-platinum or low-platinum Pd-based nanostructured electrocatalysts and a fundamental understanding of their structure-property-performance relationships. Carbon-supported Pd–Ni nanoalloy electrocatalysts with different Pd/Ni atomic ratios have been synthesized by a modified polyol reduction method, followed by heat treatment in a reducing atmosphere at 500–900 oC. The Pd–Ni sample with a Pd:Ni atomic ratio of 4:1 after heat treatment at 500 °C exhibits the highest electrochemical surface area and catalytic activity. The enhanced activity of Pd80Ni20 compared to that of Pd is attributed to Pd enrichment on the surface and the consequent lattice-strain effects. To improve the catalytic activity and long-term durability of the Pd–Ni catalysts, Pd–Pt–Ni nanoalloys have been synthesized by the same method and evaluated in PEMFC. The Pt-based mass activity of the Pd–Pt–Ni catalysts exceeds that of commercial Pt by a factor of 2, and its long-term durability is comparable to commercial Pt within the testing duration of 180 h. Both the favorable and detrimental effects of Pd and Ni dissolution on the performance of the membrane-electrode assembly (MEA) have been investigated by compositional analysis by transmission electron microscopy (TEM) of the MEAs before and after the fuel cell test. The MEAs of the Pd–Pt–Ni catalyst have then been characterized in-situ by electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) to better understand the performance changes during cell operation. The surface state change from Pd-enrichment to Pt-enrichment and the consequent decrease in the charge transfer resistance during cell operation is believed to contribute to the activity enhancement. To further improve the MEA performance and durability, the as-synthesized Pd–Pt–Ni catalysts have been pre-leached in acid and Pd–Pt alloy catalysts have been synthesized to alleviate contamination from dissolved metal ions. Compared to the pristine Pd–Pt–Ni catalyst, the preleached catalyst shows improved performance and the Pd–Pt catalyst exhibits similar performance in the entire current density range. Finally, the catalytic activities for ORR obtained from the rotating disk electrode (RDE) and PEMFC single-cell measurements of all the catalysts are compared. The improvement in the activities of the Pd-Pt-based catalysts compared to that of Pt measured by the RDE experiments is much lower than that obtained in single cell test. In other words, RDE tests underestimate the value of the Pd-Pt-based electrocatalysts for real fuel cell applications. Also, based on the RDE data, the Pd–Pt–Cu catalyst exhibits the highest catalytic activity among all the Pd–Pt–M (M = Fe, Ni, Cu) catalysts studied.
Development of electrode materials with matched thermal expansion for solid oxide fuel cells
(2018-07-09) Lai, Ke-Yu; Manthiram, Arumugam; Goodenough, John B.; Kovar, Desiderio; Hwang, Gyeong S.
Solid oxide fuel cells (SOFCs) are electrochemical energy conversion devices with a conversion efficiency of over 50 % from fuel to electricity. Their high operation temperature (600 - 1000 °C) enables SOFCs to directly utilize hydrocarbon fuels without an external fuel reforming system or precious-metal catalyst. However, several critical electrode challenges impede the mass commercialization, such as high thermal stress, electrode material decomposition, unwanted reactions between neighboring components, and impurity poisoning. A rapid SOFC failure during operation is mainly caused by the mismatch of thermal expansion coefficients (TECs) among device components. Unfortunately, few electrode materials with suitable TECs and adequate electrochemical activities have been reported. With the aim of achieving high phase stability and enhancing catalytic activity, new anode and cathode materials with compatible TECs are developed. YBaCo₄O₇-based swedenborgite oxides with Y-site dopants (In³⁺ and Ca²⁺) and Co-site dopants (Ga³⁺, Al³⁺, and Fe³⁺) are investigated as cathode materials in intermediate-temperature SOFCs (600 - 800 °C). The high-spin state of the Co cation in a tetrahedral coordination prevents spin transition at elevated temperatures and makes the TECs of YBaCo₄O₇-based materials much lower than those of Co-containing perovskite oxides. However, YBaCo₄O₇-based materials may decompose at > 600 °C. Hence, the cation doping effect on the long-term phase stability is examined with 50 compositions. The electrical conductivity, TECs, thermal behavior, catalytic activity toward the oxygen reduction reaction, and SOFC performance and stability are comprehensively evaluated. A Co-doped chromite perovskite oxide with self-regenerating Co-Fe nanoparticles is utilized as a catalytically-active anode. The moderate TEC of the chromite perovskite oxide is slightly higher than the TECs of common electrolyte materials. Unlike the conventional Ni - electrolyte cermet anode, the oxide anode exhibits high redox phase stability without irreversible performance degradation during a reduction and oxidation (redox) cycle. The performance is significantly enhanced with exsolved Co-Fe nanocatalysts. The sulfur impurity tolerance and coking resistance are evaluated with an electrolyte-supported single cell by various fuels. Meanwhile, the self-regeneration behavior of exsolved nanoparticles on the oxide surface is described by carefully observing the surface evolution during a redox cycle at 700 and 800 °C.
Dynamical simulation of molecular scale systems : methods and applications
(2010-12) Lu, Chun-Yaung; Henkelman, Graeme; Rossky, Peter J.; Makarov, Dmitrii E.; Vanden Bout, David A.; Truskett, Thomas M.
Rare-event phenomena are ubiquitous in nature. We propose a new strategy, kappa-dynamics, to model rare event dynamics. In this methodology we only assume that the important rare-event dynamics obey first-order kinetics. Exact rates are not required in the calculation and the reaction path is determined on the fly. kappa-dynamics is highly parallelizable and can be implemented on computer clusters and distributed machines. Theoretical derivations and several examples of atomic scale dynamics are presented. With single-molecule (SM) techniques, the individual molecular process can be resolved without being averaged over the ensemble. However, factors such as apparatus stability, background level, and data quality will limit the amount of information being collected. We found that the correlation function calculated from the finite-size SM rotational diffusion trajectory will deviate from its true value. Therefore, care must be taken not to interpret the difference as the evidence of new dynamics occurred in the system. We also proposed an algorithm of single fluorophore orientation reconstruction which converts three measured intensities {I₀,I₄₅,I₉₀} to the dipole orientation. Fluctuations in the detected signals caused by the shot noise result in a different prediction from the true orientation. This difference should not be interpreted as the evidence of the nonisotropic rotational motion. Catalytic reactions are also governed by the rare-events. Studying the dynamics of catalytic processes is an important subject since the more we learn, the more we can improve current catalysts. Fuel cells have become a promising energy source in the past decade. The key to make a high performance cell while keeping the price low is the choice of a suitable catalyst at the electrodes. Density functional theory calculations are carried out to study the role of geometric relaxation in the oxygen-reduction reaction for nanoparticle of various transition metals. Our calculations of Pt nanoparticles show that the structural deformation induced by atomic oxygen binding can energetically stabilize the oxidized states and thus reduces the catalytic activity. The catalytic performance can be improved by making alloys with less deformable metals.
Electrocatalytic reduction of oxygen on metal nanoparticles in the absence and presence of interactions with metal-oxide supports
(2018-08-16) Ostojic, Nevena; Crooks, Richard M. (Richard McConnell); Mullins , Charles; Henkelman , Graeme; Webb, Lauren; Rose, Michael
A combined experimental and theoretical approach is reported with a goal to develop a deeper understanding of strong metal support interactions (SMSI) and particularly their role in the oxygen reduction reaction (ORR). This was accomplished by developing a well-defined experimental system that can almost perfectly be modeled using first principles theory. First, an electrocatalytic model was constructed based on NP-mediated electron transfer (eT). That is, thin films of Al₂O₃ (2.5-5.7 nm) were deposited onto pyrolyzed photoresist film (PPF) electrodes resulting in passivation of faradaic current. Next, previously passivated eT was recovered by immobilizing dendrimer-encapsulated PtNPs (Pt DENs) (1.3 nm) onto the Al₂O₃ surface. The resulting PPF/Al₂O₃/Pt DENs electrodes were stable under various electrochemical conditions and showed an activity for the ORR. When DENs are immobilized onto solid supports, the dendrimers prevent direct contact between the encapsulated NPs and underlying supports. Consequently, in such systems, SMSI are not observed. Therefore, in the next step of the study we developed an ultraviolet/ozone (UV/O₃)-based procedure that allows for the removal of dendrimers, as confirmed by X-ray photoelectron spectroscopy (XPS), without affecting shape, size, or composition of the encapsulated NPs. Third, electrocatalytic activity of a PPF/Al₂O₃/G6-OH(Pt₅₅) electrode was studied before and after the UV/O₃ treatment using a novel microelectrochemical ﬂow cell. The results indicated that direct interactions between Al₂O₃ and PtNPs do not aﬀect the reaction pathway for the ORR. This was indeed anticipated because Al₂O₃ is a non-reducible oxide and will be used in the future studies as a control metal-oxide support. Finally, we switched to a theory-first approach in which density function theory (DFT) was used to predict catalysts having desired properties based on the previously discussed experimental model. Based on the results of the DFT calculations, a PPF/SnO [subscript x] (x = 1.9 or 2.0)/Au₁₄₇ DEN system was studied before and after the removal of dendrimers. Experimental results indicated that improvements in activity for the ORR were observed when Au₁₄₇ NPs interacted directly with SnO [subscript x] supports. Moreover, XPS studies showed that the observed catalytic enhancements were due to eT from surface oxygen vacancies in SnO [subscript x] to Au₁₄₇ NPs. These experimental results agreed with the theoretical predictions.
Perovskite oxide electrode materials for energy conversion and storage
(2016-09-16) Mefford, John Tyler; Rose, Michael J., Ph. D.; Stevenson, Keith J.; Johnston, Keith P; Crooks, Richard M; Henkelman, Graeme
The development of renewable energy conversion and storage technologies has become a leading focus of the fields of Materials Chemistry and Electrochemistry, with the ultimate goal of converting energy generated by renewable sources such as wind and solar into chemical fuels that can be stored and then utilized with clean energy conversion technologies. With energy densities primarily being a factor of fuel weight, metal-air batteries and fuel cell technologies are attractive candidates, having energy densities near gasoline. In addition, storage solutions that are both fast charging/discharging and have extended lifetimes are needed to implement renewable generation technology on a wide scale. In this vain, perovskite oxide catalysts were chosen as a model system to investigate both their applications as pseudocapacitor electrodes for storing energy and as air electrodes for both the reduction of oxygen and water electrolysis during the generation of energy. For energy storage, LaMnO [subscript 3±δ] was shown to be an active pseudocapacitor electrode, taking advantage of oxygen vacancies are charge storage sites that could intercalate oxygen anions from the electrolyte. For energy generation, a number of systems were studied. Firstly, the connection between the element in the active site and the chemical functionalities of the carbon support were identified using a number of perovskite systems, LaBO3 (B = Mn, Co, Ni, Ni₀.₇₅5Fe₀.₂₅), and high surface area carbons that were either unfunctionalized (reduced graphene oxide) or nitrogen-doped. Out of this work came the hypothesis of lattice-oxygen being an intermediate reactant during the OER and that the chemical functionalities of the carbon support were crucial to the ORR. This work was extended further by examining the system La₁₋ [subscript x] Sr [subscript x] CoO [subscript 3-δ] (0 ≤ x ≤ 1). Through a combination of rigorous characterization of the materials coupled with computation modeling, the connection was made between the covalency of the Co-O bond and access to an OER pathway utilizing lattice oxygen. On the ORR side, the connection between support interactions with the catalyst was extended to demonstrate that the ORR followed a 2-site series pathway with O₂ being reduced to HO₂⁻ on the carbon support and HO₂⁻ being further reduced to OH⁻ on the perovskite surface.
Perovskite-related and trigonal RBaCo₄O₇-based oxide cathodes for intermediate temperature solid oxide fuel cells
(2011-12) Kim, Young Nam, 1974-; Manthiram, Arumugam; Kovar, Desiderio; Wheat, Harovel G.; Ferreira, Paulo J.; Mullins, Charles B.
Solid oxide fuel cells (SOFCs) offer the advantages of (i) employing less expensive catalysts compared to the expensive Pt catalyst used in proton exchange membrane fuel cells and (ii) directly using hydrocarbon fuels without requiring external fuel reforming due to the high operating temperature. However, the conventional high operating temperatures of 800 - 1000 °C lead to interfacial reactions and thermal expansion mismatch among the components and limitations in the choice of electrode and interconnect materials. These problems have prompted a lowering of the operating temperature to an intermediate range of 500 - 800 °C, but the poor oxygen reduction reaction kinetics of the conventional La[subscript 1-x]Sr[subscript x]MnO₃ perovskite cathode remains a major obstacle for the intermediate temperature SOFC. In this regard, cobalt-containing oxides with perovskite or perovskite-related structures have been widely investigated, but they suffer from large thermal expansion coefficient (TEC) mismatch with the electrolytes. With an aim to lower the TEC and maximize the electrochemical performance, this dissertation focuses on perovskite-related and trigonal RBaCo₄O₇-based oxide cathode materials. First, the effect of M = Fe and Cu in the perovskite-related layered LnBaCo₂₋xMxO₊[delta] (Ln = Nd and Gd) oxides has been investigated. The Fe and Cu substitutions lower the polarization resistance and offer fuel cell performance comparable to that of La[subscript 1-x]Sr[subscript x]CoO₃₋[delta] perovskite due to improved chemical stability with the electrolyte and a better matching of the TEC with those of standard electrolytes. Second, the perovskite-related intergrowth oxides Ln(Sr,Ca)₃Fe₁.₅Co₁.₅O₀ and La₁.₈₅Sr₁.₁₅Cu[subscript 2-x]Co[subscript x]O[subscript 6 +delta] and their composites with gadolinia-doped ceria (GDC) have been investigated. The electrical conductivity, TEC, and catalytic activity increase with increasing Co content. The composite cathodes exhibit enhanced electrochemical performance due to lower TEC and increased triple-phase boundary. Third, RBa(Co,Zn)₄O₇ (R = Y, Ca, and In) oxides with a trigonal structure and tetrahedral-site Con+ ions have been investigated. The chemical instability normally encountered with this class of oxides has been overcome by appropriate cationic substitutions as in (Y₀.₅Ca₀.₅)Ba(Co₂.₅Zn₁.₅)O₇ and (Y₀.₅In₀.₅)BaCo₃ZnO₇. With an ideal matching of TEC with those of standard electrolytes, the RBa(Co,Zn)₄O₇ (R = Y, Ca, and In) + GDC composite cathodes exhibit low polarization resistance and electrochemical performance comparable to that of perovskite oxides.
Tuning the electrocatalytic activity of perovskites and related oxides and the elucidation of new catalyst design criteria
(2017-05) Hardin, William Guy; Johnston, Keith P., 1955-; Stevenson, Keith J.; Mullins, Charles B; Henkelman, Graeme A
Increasing global energy demand requires greater efficiency in water electrolyzers for low cost hydrogen generation and rechargeable metal-air batteries to enable pragmatic development of these key technologies. Given that the efficiencies of these technologies are limited primarily by the sluggish kinetics of the oxygen evolution reaction (OER) and/or oxygen reduction reaction (ORR), I have made extensive efforts to reduce the overpotential for the OER and ORR in alkaline media by designing advanced non-precious metal electrocatalysts. Alkaline conditions were chosen owing the more facile kinetics of the ORR, as compared to acid, and because alkaline electrolytes enable the use of catalysts containing non-precious metals. Perovskites, represented by ABO₃± [subscript δ], in which A is a rare-earth or alkaline earth element and B is a transition metal, were selected because of their demonstrated ability to promote the OER and the ORR, owing to their high ionic and electronic conductivities, good structural stability, and synthetic versatility. This made perovskites an ideal crystal structure for the development of rational catalyst design criteria. To this goal, I designed a series of lanthanum-based perovskite electrocatalysts LaBO₃, B = Ni, Ni₀.₇₅Fe₀.₂₅, Co, Mn) that are highly active for both the OER and ORR in an aqueous alkaline electrolyte. LaCoO₃ sup7ported on nitrogen-doped carbon is shown to be ~3 times more active for the OER than high surface-area IrO₂, and was demonstrated to be highly bifunctional by having a lower total overpotential between the OER and ORR (ΔE = 1.00 V) than Pt (ΔE = 1.16) and Ru (ΔE = 1.01). I discovered a new catalyst design principle using a series of Ruddlesden-Popper (RP) La₀.₅Sr₁.₅Ni₁- [subscript x] Fe [subscript x] O₄+ [subscript δ] oxides that promote Ni-O-Fe charge transfer interactions which significantly enhance OER catalysis. Using selective substitution of Sr and Fe to control the extent of hybridization between e [subscript g] (Ni), p(O) and e [subscript g] (Fe) bands, I have demonstrated exceptional OER activity of 10 mA cm⁻² at a 360 mV overpotential and mass activity of 1930 mA mg⁻¹ [subscript ox] at 1.63 V, over an order of magnitude more than the leading precious-metal oxide electrocatalyst IrO₂. In the course of this work, I also helped discover reversible charge storage via anion-based intercalation in LaMnO₃+ [subscript δ] electrodes, and a new OER mechanism whereby redox-active lattice oxygen directly participates in the formation of O₂.