Operando analysis reveals potential-driven in situ formation of single-Fe-atom electrocatalysts for green production of ammonia.

As a sustainable approach for N2 fixation, electrocatalytic N2 reduction reaction (N2RR) to produce ammonia (NH3) is highly desirable with a precise understanding to the structure-activity relationship of electrocatalysts. Here, firstly, we obtain a novel carbon-supported oxygen-coordinated single-Fe-atom catalyst for highly efficient production of ammonia from electrocatalytic N2RR. Based on such new type of N2RR electrocatalyst, by combining operando X-ray absorption spectra (XAS) with density function theory calculation, we reveal significantly that the as-prepared active coordination structure undergoes a potential-driven two-step restructuring, firstly from FeSAO4(OH)1a to FeSAO4(OH)1a'(OH)1b with the adsorption of another -OH on FeSA at open-circuit potential (OCP) of 0.58 VRHE, and subsequently restructuring from FeSAO4(OH)1a'(OH)1b to FeSAO3(OH)1a″ due to the breaking of one Fe-O bond and the dissociation of one -OH at working potentials for final electrocatalytic process of N2RR, thus revealing the first potential-induced in situ formation of the real electrocatalytic active sites to boost the conversion of N2 to NH3. Moreover, the key intermediate of Fe-NNHx was detected experimentally by both operando XAS and in situ attenuated total reflection-surface-enhanced infrared absorption spectra (ATR-SEIRAS), indicating the alternating mechanism followed by N2RR on such catalyst. The results indicate the necessity of considering the potential-induced restructuring of the active sites on all kinds of electrocatalysts for such as highly efficient ammonia production from N2RR. It also paves a new way for a precise understanding to the structure-activity relationship of a catalyst and helps the design of highly efficient catalysts.

Feed gas exchange (startup/shutdown) effects on Pt/C cathode electrocatalysis and surface Pt-oxide behavior in polymer electrolyte fuel cells as revealed using in situ real-time XAFS and high-resolution STEM measurements.

The synchronizing measurements of both cyclic voltammograms (CVs) and real-time quick XAFSs (QXAFSs) for Pt/C cathode electrocatalysts in a membrane electrode assembly (MEA) of polymer electrolyte fuel cells (PEFCs) treated by anode-gas exchange (AGEX) and cathode-gas exchange (CGEX) cycles (startup/shutdown conditions of FC vehicles) were performed for the first time to understand the opposite effects of the AGEX and CGEX treatments on the Pt/C performance and durability and also the contradiction between the electrochemical active surface area (ECSA) decrease and the performance increase by CGEX treatment. While the AGEX treatment decreased both the ECSA and performance of MEA Pt/C due to carbon corrosion, it was found that the CGEX treatment decreased the ECSA but increased the Pt/C performance significantly due to high-index (331) facet formation (high-resolution STEM) and hence the suppression of strongly bound Pt-oxide formation at cathode Pt nanoparticle surfaces. Transient QXAFS time-profile analysis for the MEA Pt/C also revealed a direct relationship between the electrochemical performance or durability and transient kinetics of the Pt/C cathode.

An Integrated Single-Electrode Method Reveals the Template Roles of Atomic Steps: Disturb Interfacial Water Networks and Thus Affect the Reactivity of Electrocatalysts.

A method enabling the accurate and precise correlation between structures and properties is critical to the development of efficient electrocatalysts. To this end, we developed an integrated single-electrode method (ISM) that intimately couples electrochemical rotating disk electrodes, in situ/operando X-ray absorption fine structures, and aberration-corrected transmission electron microscopy on identical electrodes. This all-in-one method allows for the one-to-one, in situ/operando, and atomic-scale correlation between structures of electrocatalysts with their electrochemical reactivities, distinct from common methods that adopt multisamples separately for electrochemical and physical characterizations. Because the atomic step is one of the most fundamentally structural elements in electrocatalysts, we demonstrated the feasibility of ISM by exploring the roles of atomic steps in the reactivity of electrocatalysts. In situ and atomic-scale evidence shows that low-coordinated atomic steps not only generate reactive species at low potentials and strengthen surface contraction but also act as templates to disturb interfacial water networks and thus affect the reactivity of electrocatalysts. This template role interprets the long-standing puzzle regarding why high-index facets are active for the oxygen reduction reaction in acidic media. The ISM as a fundamentally new method for workflows should aid the study of many other electrocatalysts regarding their nature of active sites and operative mechanisms.

The effect of cooling process on the structure and charge/discharge capacities of Li-rich solid-solution layered oxide cathode materials for the Li-ion battery.

The effect of cooling process after calcination at 900 °C in the preparation of cathode materials, on the crystal structure and charging/discharging capacities of Li2MnO3-LiNi1/2Mn1/2O2-LiNi1/3Mn1/3Co1/3O2 Li-rich solid-solution layered oxide (LLO) cathode materials for the lithium ion battery was examined in twenty-one LLO samples having different compositions. This was achieved by applying two types of cooling processes: (i) quenching the calcinated LLO samples with liquid nitrogen (quenched cooling), and (ii) slow cooling of LLO samples in the furnace at a controlled decreasing rate of the temperature (slow cooling). The results of the comparison between discharging capacities observed with LLO samples prepared with two types of cooling processes indicated that the cooling process for LLO samples to exhibit high discharge capacity was not limited to either one. The process that can be more effective for LLO samples to exhibit the high discharge capacity depended on the composition of LLO samples. LLO samples containing Li2MnO3 of over 60% exhibited higher discharge capacity when samples were quenched with liquid nitrogen than those prepared with the slow cooling process. Among LLOs examined, the effect of quenching was maximum when the Li2MnO3 content was 60%. As the LLO composition deviated from the line of 60% Li2MnO3 in the Li[Li0.20Mn0.58Ni0.18Co0.04]O2 sample compositions, the effect of quenching became smaller and the slow cooling process was superior to the quenching process. A connection was thus made between the structural difference of LLO samples prepared with the two types of cooling processes and the cathode performance was observed.

Surface coating of a LiNi x Co y Al1-x-y O2 (x > 0.85) cathode with Li3PO4 for applying a water-based hybrid polymer binder during Li-ion battery preparation.

To produce water-stable Ni-rich lithium nickel cobalt aluminum oxides (LiNi x Co y Al1-x-y O2, x > 0.85, NCAs), the formation of trilithium phosphate (Li3PO4)-coated layers on the NCA surfaces was attempted through the use of a surface reaction in a mixture of ethanol and water and a post-heat treatment at 350 and 400 °C. Based on the results of X-ray photoelectron spectroscopy (XPS), the coated layers consisted of nickel phosphate (Ni3(PO4)2) and Li3PO4. The coated NCA surface could have sufficient water stability to maintain the cathode performance in a water slurry for 1 day. In addition, the coated layers formed on the NCA surfaces did not block Li+-ion transfer through the Ni3(PO4)2/Li3PO4-coating layers and enhanced the high-rate discharge performance.

Surface double coating of a LiNi a Co b Al1-a-b O2 (a > 0.85) cathode with TiO x and Li2CO3 to apply a water-based hybrid polymer binder to Li-ion batteries.

Recently a water-based polymer binder has been getting much attention because it simplifies the production process of lithium ion batteries (LIBs) and reduce their cost. The surface of LiNi a Co b Al1-a-b O2 (a > 0.85, NCA) cathode with a high voltage and high capacity was coated doubly with water-insoluble titanium oxide (TiO x ) and Li2CO3 layers to protect the NCA surface from the damage caused by contacting with water during its production process. The TiO x layer was at first coated on the NCA particle surface with a tumbling fluidized-bed granulating/coating machine for producing TiO x -coated NCA. However, the TiO x layer could not coat the NCA surface completely. In the next place, the coating of the TiO x -uncoated NCA surface with Li2CO3 layer was conducted by bubbling CO2 gas in the TiO x -coated NCA aqueous slurry on the grounds that Li2CO3 is formed through the reaction between CO3 2- ions and residual LiOH on the TiO x -uncoated NCA surface, resulting in the doubly coated NCA particles (TiO x /Li2CO3-coated NCA particles). The Li2CO3 coating is considered to take place on the TiO x layer as well as the TiO x -uncoated NCA surface. The results demonstrate that the double coating of the NCA surface with TiO x and Li2CO3 allows for a high water-resistance of the NCA surface and consequently the TiO x /Li2CO3-coated NCA particle cathode prepared with a water-based binder possesses the same charge/discharge performance as that obtained with a "water-uncontacted" NCA particle cathode prepared using the conventional organic solvent-based polyvinylidene difluoride binder.

Observation of Degradation of Pt and Carbon Support in Polymer Electrolyte Fuel Cell Using Combined Nano-X-ray Absorption Fine Structure and Transmission Electron Microscopy Techniques.

It is hard to directly visualize spectroscopic and atomic-nanoscopic information on the degraded Pt/C cathode layer inside polymer electrolyte fuel cell (PEFC). However, it is mandatory to understand the preferential area, sequence, and relationship of the degradations of Pt nanoparticles and carbon support in the Pt/C cathode layer by directly observing the Pt/C cathode catalyst for the development of next-generation PEFC cathode catalysts. Here, the spectroscopic, chemical, and morphological visualization of the degradation of Pt/C cathode electrocatalysts in PEFC was performed successfully by a same-view combination technique of nano-X-ray absorption fine structure (XAFS) and transmission electron microscopy (TEM)/scanning TEM-energy-dispersive spectrometry (EDS) under a humid N2 atmosphere. The same-view nano-XAFS and TEM/STEM-EDS imaging of the Pt/C cathode of PEFC after triangular-wave 1.0-1.5 VRHE (startup/shutdown) accelerated durability test (tri-ADT) cycles elucidated the site-selective area, sequence, and relationship of the degradations of Pt nanoparticles and carbon support in the Pt/C cathode layer. The 10 tri-ADT cycles caused a carbon corrosion to reduce the carbon size preferentially in the boundary regions of the cathode layer with both electrolyte and holes/cracks, accompanied with detachment of Pt nanoparticles from the degraded carbon. After the decrease in the carbon size to less than 8 nm by the 20 tri-ADT cycles, Pt nanoparticles around the extremely corroded carbon areas were found to transform and dissolve into oxidized Pt2+-O4 species.

Visible-light photodecomposition of acetaldehyde by TiO2-coated gold nanocages: plasmon-mediated hot electron transport via defect states.

Skeletal gold nanocages (Au NCs) are synthesized and coated with TiO2 layers (TiO2-Au NCs). The TiO2-Au NCs exhibit enhanced photodecomposition activity toward acetaldehyde under visible light (>400 nm) illumination because hot electrons are generated over the Au NCs by local surface plasmon resonance (LSPR) and efficiently transported across the metal/semiconductor interface via the defect states of TiO2.