Development of an improved oxygen electrode for use in alkaline H2-O2 fuel cells Quarterly report, 1 Jul. - 30 Sep. 1966
Catalysts prepared and tested for improving oxygen electrode in alkaline hydrox fuel cells
ELECTRODE↗
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High Temperature Sorbents for Oxygen
A sorbent capable of removing trace amounts of oxygen (ppt) from a gas stream at a high temperature above 200 C is introduced. The sorbent comprises a porous alumina silicate support such as zeolite containing from 1 to 10 percent by weight of ion exchanged transition metal such as copper or cobalt ions and 0.05 to 1.0 percent by weight of an activator selected from a platinum group metal such as platinum. The activation temperature, oxygen sorption and reducibility are all improved by the presence of the platinum activator.
Sharma, Pramod K.↗
Development of cathodic electrocatalysts for use in low temperature H2/O2 fuel cells with an alkaline electrolyte Quarterly report 1 Oct. - 31 Dec. 1966
Carbides, nitrides, nitrocarbides, and carbonitrides of iron examined for catalytic activity in oxygen reduction reaction in potassium hydroxide - peroxide fuel cells
CATALYTIC ACTIVITY↗
The Impact of Membrane Inactive Area on the Durability of Pt-Co Catalyst
Platinum (Pt) is the most active catalyst for oxygen reduction reaction; however, its activity still requires a significant increase to meet the demands of practical applications. To enhance the catalytic activity, alloy catalysts like platinum-cobalt (Pt-Co), are being extensively investigated. However, Co leaching from the Pt-Co alloy remains a significant concern. Evaluating the durability of Pt-Co alloy catalyst is further complicated by variations in Co leaching, which affects both the observed performance and durability. This variability often arises from an overlooked factor: choice of the inactive area of the membrane electrode assembly (MEA) used during the evaluation. This study examines the critical role of membrane inactive area on the performance loss observed during durability testing of Pt-Co alloys. Our findings indicate that a large membrane inactive area reduces the impact of Co leaching on performance and durability, up to 200 mA cm −2 difference in performance is observed between large and small inactive area MEA at 0.7 V for dry conditions, and more Co is retained in the active area of MEA for smaller inactive areas which is responsible for larger performance losses.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Oxygen reduction reaction catalyzed by carbon composites with ruthenium-doped iron oxide nanoparticles
Carbon nanocomposites with Ru-doped Fe 2 O 3 nanoparticles are derived from a Fe–Ru–Fe trinuclear complex and zeolitic imidazolate framework 8 and exhibit excellent electrocatalytic activity towards oxygen reduction reaction, comparable to that of Pt/C.
Liu, Qiming↗
Machine-learning-assisted material discovery of oxygen-rich highly porous carbon active materials for aqueous supercapacitors
Abstract Porous carbons are the active materials of choice for supercapacitor applications because of their power capability, long-term cycle stability, and wide operating temperatures. However, the development of carbon active materials with improved physicochemical and electrochemical properties is generally carried out via time-consuming and cost-ineffective experimental processes. In this regard, machine-learning technology provides a data-driven approach to examine previously reported research works to find the critical features for developing ideal carbon materials for supercapacitors. Here, we report the design of a machine-learning-derived activation strategy that uses sodium amide and cross-linked polymer precursors to synthesize highly porous carbons (i.e., with specific surface areas > 4000 m 2 /g). Tuning the pore size and oxygen content of the carbonaceous materials, we report a highly porous carbon-base electrode with 0.7 mg/cm 2 of electrode mass loading that exhibits a high specific capacitance of 610 F/g in 1 M H 2 SO 4 . This result approaches the specific capacitance of a porous carbon electrode predicted by the machine learning approach. We also investigate the charge storage mechanism and electrolyte transport properties via step potential electrochemical spectroscopy and quasielastic neutron scattering measurements.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Promotion of oxygen reduction reaction on a double perovskite electrode by a water-induced surface modification
Highly efficient air electrodes are a key component of reversible fuel cells for energy storage and conversion; however, the development of efficient electrodes that are stable against water vapor remains a grand challenge. Here we report an air–electrode, composed of double perovskite material PrBa 0.8 Ca 0.2 Co 2 O 5+δ (PBCC) backbone coated with nanoparticles (NPs) of BaCoO 3-δ (BCO), that exhibits remarkable electrocatalytic activity for oxygen reduction reaction (ORR) while maintaining excellent tolerance to water vapor. When tested in a symmetrical cell exposed to wet air with 3 vol% H 2 O at 750 °C, the electrode shows an area specific resistance of ~0.03 Ω cm 2 in an extended period of time. The performance enhancement is attributed mainly to the electrocatalytic activity of the BCO NPs dispersed on the surface of the porous PBCC electrode. Moreover, in situ Raman spectroscopy is used to probe reaction intermediates (e.g., oxygen species) on electrode surfaces, as the electrochemical properties of the electrodes are characterized under the same conditions. The direct correlation between surface chemistry and electrochemical behavior of an electrode is vital to gaining insight into the mechanisms of the electrocatalytic processes in fuel cells and electrolysers.
25 ENERGY STORAGE↗
Chemical and structural evolutions of Li–Mn-rich layered electrodes at different current densities
Although the two active redox centers in Li-rich cathodes, including the anionic and cationic contributions, can enable Li-ion batteries to achieve outstanding specific energy, their behaviors at different current densities have not been clarified. In this report we provide a comparative study of transition metals (TMs) and oxygen redox activities by directly accessing their oxidation states in Li-rich materials operated at very different current rates. Our data reveal that the oxidation of oxygen in the near-surface region is at the same level for electrodes cycled with a wide range of current rates, indicating a reaction gradient of lattice oxygen redox reactions. The oxidation process of lattice oxygen is found to be dynamically compatible with that of the TMs. Combining the results of first principles calculations and complementary experimental findings, we propose a detailed mechanism of structural distortion from octahedral Li to tetrahedral Li and the role of oxygen vacancy in Li + diffusion. It is found that fast delithiation occurring at high current densities can easily cause local structural transformation, leading to a limited Li + diffusion rate and consequently suppressing rate capability.
25 ENERGY STORAGE↗
Surface enhanced performance of La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ cathodes by infiltration Pr-Ni-Mn-O progress
Herein the present study reports the enhancement of electrochemical oxygen reduction activity of porous La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3–δ (LSCF) cathodes by coating a thin film of Pr-Ni-Mn oxide (PNM5) using a multi-step infiltration process. XRD examination reveals that PNM5 mainly contains a multiphase mixture of Pr 6 O 11 , PrNiO 3 , MnO and NiO. SEM morphology shows a thin PNM5 film and small particles are formed on the surface of LSCF backbone particles. Impedance spectrum analysis indicates that PNM5 infiltrated LSCF exhibits dramatically reduced polarization resistance (R p ), reaching R p of 0.244 Ω cm 2 at 973 K, which is one-half of the baseline LSCF cathode. The activation energy of LSCF cathodes infiltrated with PNM5 is 1.45 eV, slightly lower than the baseline LSCF cathode (1.77 eV). Distribution of relaxation time (DRT) function analysis shows PNM5 infiltration layer significantly promotes the oxygen reduction reaction (ORR) of cathode surface. With the increase in firing temperature, the total resistance increases and R p changes from ion transport to oxygen reduction reaction. Degradation rate of the PNM5-infiltrated LSCF is also lower, 0.02168% vs 0.07093% for the baseline LSCF over a 200-h period. A single cell testing indicated that the peak power density of the PNM5-infiltrated cell was increased by 140.66%. Overall, PNM5 could be a potential catalyst for boosting the performance of a commercial LSCF cathode for solid oxide fuel cells (SOFC).
36 MATERIALS SCIENCE↗
Understanding the Active Site Structures and Achieving Catalytic Activity Tuning of Atomically Dispersed FeN 4 Sites for Oxygen Reduction Reaction
Atomically dispersed Fe–—N—C catalysts with high oxygen reduction reaction (ORR) activity have attracted great attention since the last decade. Due to comparable ORR activity and low material cost, they are promising platinum group metal (PGM)-free catalysts that can replace the commercialized Pt/C materials; furthermore, it can facilitate the efficiency of the fuel cell technologies and mitigate dependence on fossil fuels. Great advancements have been made to experimentally optimize the synthesis approach of the Fe–—N—C catalysts, enhance the ORR activity, and improve the catalyst stability. Similarly, recent theoretical studies also provide enriched understanding of the active site structures, properties, and reaction mechanisms. In this review, discussions are made upon utilizing combined experimental and computational spectroscopy to reveal the active site structures, employing mechanistic studies to investigate reaction thermodynamics and kinetics, as well as developing scaling relationships to assist the design and development of future PGM-free catalyst materials. Furthermore, recent advances in studying Fe–—N—C catalysts utilizing electrified surface models and explicit solvation models are also discussed. Not only can these aspects improve the accuracy of theoretical simulation and predictions but also deepen the understanding of the catalyst properties and reaction mechanisms under the effect of surface charges and solvent molecules.
Fe single-atom catalysts↗
Could Irradiation Introduce Oxidized Oxygen Signals in Resonant Inelastic X-ray Scattering of Battery Electrodes?
The characterization of oxidized oxygen states through high-efficiency mapping of resonant inelastic X-ray scattering (mRIXS) has become a crucial approach for studying the oxygen redox activities in high-energy battery cathodes. However, this approach has been recently challenged due to the concern of irradiation damage. In this work, we revisited a typical Li-rich electrode, Li 1.144 Ni 0.136 Mn 0.544 Co 0.136 O 2 , in both lithiated and delithiated states and evaluated the X-ray irradiation effect in the lengthy mRIXS experiments. Our results show that irradiation cannot introduce any oxidized oxygen feature, and the features of oxidized oxygen are weakened with a high X-ray dose. The results confirm again that mRIXS detects the intrinsic oxidized oxygen state in battery electrodes. However, the distinct irradiation effects in different systems imply that irradiation could selectively target the least stable elemental or chemical states, which should be analyzed with caution in the study of active chemical states.
25 ENERGY STORAGE↗
Chemical Vapor Deposition for Atomically Dispersed and Nitrogen Coordinated Single Metal Site Catalysts
Abstract 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 FeN 4 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.
Liu, Shengwen↗
Pyrolysis in Molten Salts Converts Plastics into a Mesoporous Electrocatalyst with a High Density of Atomic Fe–N–C Active Site
Upcycling plastic waste into value-added products is a promising strategy for both economic and environmental sustainability. However, achieving control over structure–property correlations during the upcycling process remains challenging. Here, in this study, we investigate the modulation of active site formation during pyrolytic conversion of plastics to carbon by composition-controlled molten salts. Using melamine formaldehyde (MF) foams as a precursor, we demonstrate that their pyrolysis in eutectic chloride molten salts with Fe 2+ , Na + , and K + ions directs the carbonization process toward the formation of mesoporous functional carbon enriched with atomically dispersed Fe–N–C sites, which are well-known for their catalytic activity in oxygen reduction reaction (ORR). Compared to pyrolysis in the FeCl 2 -only salt, the incorporation of alkali metal ions (Na + , K + ) in the eutectic mixture facilitates nitrogen retention in the carbon matrix and modulates iron speciation. This synergistic environment promotes a higher density and more uniform dispersion of Fe–N–C moieties within the carbon matrix. Consequently, the resulting catalyst exhibits a high surface area, hierarchical porosity, and improved electrochemical properties. When tested as an ORR catalyst, the Fe–N–C-enriched catalyst delivers a high half-wave potential (E 1/2 = 0.875 V vs RHE) with nearly exclusive 4e – pathway. These properties are comparable to those of commercial Pt/C catalysts, along with improved stability.
Fe−N−C active sites↗
Polymer Entrapment Flash Pyrolysis for the Preparation of Nanoscale Iridium-Free Oxygen Evolution Electrocatalysts
This paper describes the use of a new polymer entrapment flash pyrolysis (PEFP) method for making nanoscale yttrium ruthenate (Y 2 Ru 2 O 7-δ ) electrocatalysts. Here, this approach effectively reduced the synthesis temperature of phase-pure pyrochlore catalysts from 1000 °C to 550 °C, and greatly suppressed the sintering of catalyst particles. The supported nanocrystalline Y 2 Ru 2 O 7-δ catalysts showed enhanced activity towards oxygen evolution reaction (OER) in acidic electrolyte and were stable at 1.50 V for the comparative study (>20 h) under the current density of 10 mA/cm 2 geo in chronopotentiometry testing. This is equivalent to an overpotential value of 270 mV, about half of that of the reference IrO 2 catalyst. X-ray absorption spectroscopy (XAS) and transmission electron microscopy (TEM) analysis showed that the high-surface-area Y 2 Ru 2 O 7-δ catalyst had an oxygen-deficient structure. This study provides a route to the synthesis of fine ceramic (or oxide)-based electrocatalysts for making high-performing electrocatalysts.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Microdistribution of oxygen in silicon
The microdistribution of oxygen in Czochralskii-grown, p-type silicon crystals was determined by using the SEM in the EBIC mode in conjunction with spreading resistance measurements. When the conductivity remained p-type, bands of contrast were observed in the EBIC image which corresponded to maxima in resistivity. When at the oxygen concentration maxima the oxygen donor concentration exceeded the p-type dopant concentration, an inversion of the conductivity occurred. It resulted in the formation of p-n junctions in a striated configuration and the local inversion of the EBIC image contrast. By heat-treating silicon at 1000 C prior to the activation of oxygen donors, some silicon-oxygen micro-precipitates were observed in the EBIC image within the striated oxygen concentration maxima.
Murgai, A.↗
The Role of Oxygen Transfer in Oxide Heterostructures on Functional Properties
A variety of mechanisms are reported to play critical roles in contributing to the high carrier/electron mobility in oxide/SrTiO 3 (STO) heterostructures. By using La 0.95 Sr 0.05 TiO 3 (LSTO) epitaxially grown on different single crystal substrates (such as STO, GdScO 3 , LaAlO 3 , (LaAlO 3 ) 0.3 (Sr 2 AlTaO 6 ) 0.7 , and CeO 2 buffered STO) as the model systems, the formation of a conducting substrate surface layer (CSSL) on STO substrate is shown at relatively low growth temperature and high oxygen pressure (725 °C, 5 × 10 –4 Torr), which contributes to the enhanced conductivity of the LSTO/STO heterostructures. Different from the conventional oxygen vacancy model, this work reveals that the formation of the CSSL occurs when growing an oxide layer (LSTO in this case) on STO, while neither annealing nor the growth of an Au layer alone at the exact same growth condition generates the CSSL in STO. It demonstrates that the oxide layer actively pulls oxygen from STO substrate at given growth conditions, leading to the formation of the CSSL. Here, the observations emphasize the oxygen transfer across film/substrate interface during the synthesis of oxide heterostructures playing a critical role in functional properties.
36 MATERIALS SCIENCE↗
Stability and Activity of Cobalt Antimonate for Oxygen Reduction in Strong Acid
Guided by computational Pourbaix screening and high-throughput experiments aimed at the development of precious-metal-free fuel cells, we investigate rutile CoSb 2 O 6 as an electrocatalyst for oxygen reduction in 1 M sulfuric acid. Following 4 h of catalyst conditioning at 0.7 V vs RHE, operation at this potential for 20 h yielded an average current density of –0.17 mA cm –2 with corrosion at a rate of 0.04 nm hour –1 that is stoichiometric with catalyst composition. Surface Pourbaix analysis of the (111) surface identified partial H coverage under operating conditions. The Sb active site has an HO* binding free energy of 0.49 eV, which is near the peak of the kinetic 4e – ORR volcano for transition-metal oxides in acidic conditions. Furthermore, the experimental demonstration of operational stability and computational identification of a reaction pathway with favorable energetics place rutile CoSb 2 O 6 among the most promising precious-metal-free electrocatalysts for oxygen reduction in acidic media.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Using anion photoelectron spectroscopy of cluster models to gain insights into mechanisms of catalyst-mediated H 2 production from water
We report that metal oxide cluster models of catalyst materials offer a powerful platform for probing the molecular-scale features and interactions that govern catalysis. This perspective gives an overview of studies implementing the combination of anion photoelectron (PE) spectroscopy and density functional theory calculations toward exploring cluster models of metal oxides and metal-oxide supported Pt that catalytically drive the hydrogen evolution reaction (HER) or the water–gas shift reaction. The utility in the combination of these experimental and computational techniques lies in our ability to unambiguously determine electronic and molecular structures, which can then connect to results of reactivity studies. In particular, we focus on the activity of oxygen vacancies modeled by suboxide clusters, the critical mechanistic step of forming proximal metal hydride and hydroxide groups as a prerequisite for H2 production, and the structural features that lead to trapped dihydroxide groups. The pronounced asymmetric oxidation found in heterometallic group 6 oxides and near-neighbor group 5/group 6 results in higher activity toward water, while group 7/group 6 oxides form very specific stoichiometries that suggest facile regeneration. Studies on the trans-periodic combination of cerium oxide and platinum as a model for ceria supported Pt atoms and nanoparticles reveal striking negative charge accumulation by Pt, which, combined with the ionic conductivity of ceria, suggests a mechanism for the exceptionally high activity of this system towards the water–gas shift reaction.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗