In Situ Transformation of Hydrogen-Evolving CoP Nanoparticles: Toward Efficient Oxygen Evolution Catalysts Bearing Dispersed Morphologies with Co-oxo/hydroxo Molecular Units

Reported herein is elucidation of a novel Co-based oxygen evolution catalyst generated in situ from cobalt phosphide (CoP) nanoparticles. The present CoP nanoparticles, efficient alkaline hydrogen-evolving materials at the cathode, are revealed to experience unique metamorphosis upon anodic potential cycling in an alkaline electrolyte, engendering efficient and robust catalytic environments toward the oxygen evolution reaction (OER). Our extensive ex situ characterization shows that the transformed catalyst bears porous and nanoweb-like dispersed morphologies along with unique microscopic environments mainly consisting of discrete cobalt-oxo/hydroxo molecular units within a phosphate-enriched amorphous network. Outstanding OER efficiency is achievable with the activated catalyst, which is favorably comparable to even a precious iridium catalyst. A more remarkable feature is its outstanding long-term stability, superior to iridium and conventional cobalt oxide-based materials. Twelve-hour bulk electrolysis continuously operating at high current density is completely tolerable with the present catalyst

High-Performance Electroactive Polymer Actuators Based on Ultrathick Ionic Polymer–Metal Composites with Nanodispersed Metal Electrodes

Ionic polymer–metal composites (IPMCs) have been proposed as biomimetic actuators that are operable at low applied voltages. However, the bending strain and generating force of the IPMC actuators have generally exhibited a trade-off relationship, whereas simultaneous enhancement of both the qualities is required for their practical applications. Herein, a significant improvement in both the strain and force of the IPMC actuators is achieved by a facile approach, exploiting thickness-controlled ion-exchange membranes and nanodispersed metal electrodes. To guarantee a large generating force of the IPMC actuators, ultrathick ion-exchange membranes are prepared by stacking pre-extruded Nafion films. Metal electrodes with a nanodispersed structure are formed on the membranes via alcohol-assisted electroless plating, which allows increased capacitance and facilitated ion transport. The resulting actuators exhibit greatly enhanced electromechanical properties, including an approximately four times larger strain and two times larger force compared to those of actuators having the conventional structure. Moreover, the ability to lift 16 coins (a weight of 124 g) has been successfully demonstrated using ultrathick IPMC actuators, which shows great promise in realizing artificial muscles

Design of an Advanced Membrane Electrode Assembly Employing a Double-Layered Cathode for a PEM Fuel Cell

The membrane electrolyte assembly (MEA) designed in this study utilizes a double-layered cathode: an inner catalyst layer prepared by a conventional decal transfer method and an outer catalyst layer directly coated on a gas diffusion layer. The double-layered structure was used to improve the interfacial contact between the catalyst layer and membrane, to increase catalyst utilization and to modify the removal of product water from the cathode. Based on a series of MEAs with double-layered cathodes with an overall Pt loading fixed at 0.4 mg cm^–2 and different ratios of inner-to-outer Pt loading, the MEA with an inner layer of 0.3 mg Pt cm^–2 and an outer layer of 0.1 mg Pt cm^–2 exhibited the best performance. This performance was better than that of the conventional single-layered electrode by 13.5% at a current density of 1.4 A cm^–2

Origin of the Enhanced Electrocatalysis for Thermally Controlled Nanostructure of Bimetallic Nanoparticles

The thermal annealing process is a common treatment used after the preparation step to enhance the electrocatalytic properties of the oxygen reduction reaction (ORR). The structure of a Pt-based bimetallic nanoparticle, which is significantly affected by the catalytic properties, is reconstructed by thermal energy. We investigated the effect of structural reconstruction induced by thermal annealing on the improvement of the ORR using various physical and electrochemical methods. We found that the structural evolution of PtNi nanoparticles, i.e., the Pt–Ni ordering with the Pt shell and the surface reorientation into the (111) facet, is the source of the enhanced ORR activity as well as electrochemical stability through the thermal annealing. This result confirms the crucial factors for the ORR properties by the thermal annealing process and proposes a way to design advanced electrocatalysts

Surface Structures and Electrochemical Activities of PtRu Overlayers on Ir Nanoparticles

PtRu overlayers were deposited on carbon-supported Ir nanoparticles with various Pt:Ru compositions. Structural and electrochemical characterizations were performed using transmission electron microscopy (TEM), X-ray diffraction, high-resolution powder diffraction (HRPD), X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), and CO stripping voltammetry. The PtRu overlayers were selectively deposited on the Ir nanoparticles with good uniformity of distribution. As a result, the PtRu utilization of the present samples was higher than that of PtRu/C. The mass-specific activities for methanol oxidation were also significantly higher. Single-cell performance using the Pt₂Ru₁ overlayer sample as an anode catalyst was slightly higher than that obtained using commercial PtRu/C despite the fact that the PtRu anode loading for Pt₂Ru₁/Ir/C was only 42% of that of PtRu/C

Nickel Nanofoam/Different Phases of Ordered Mesoporous Carbon Composite Electrodes for Superior Capacitive Energy Storage

Electrochemical energy storage devices based on electric double layer capacitors (EDLCs) have received considerable attention due to their high power density and potential for obtaining improved energy density in comparison to the lithium ion battery. Ordered mesoporous carbon (OMC) is a promising candidate for use as an EDLC electrode because it has a high specific surface area (SSA), providing a wider charge storage space and size-controllable mesopore structure with a long-range order, suppling high accessibility to the electrolyte ions. However, OMCs fabricated using conventional methods have several drawbacks including low electronic conductivity and long ionic diffusion paths in mesopores. We used nickel nanofoam, which has a relatively small pore (sub-100 nm to subμm) network structure, as a current collector. This provides a significantly shortened electronic/ionic current paths and plentiful surface area, enabling stable and close attachment of OMCs without the use of binders. Thus, we present hierarchical binder-free electrode structures based on OMC/Ni nanofoams. These structures give rise to enhanced specific capacitance and a superior rate capability. We also investigated the mesopore structural effect of OMCs on electrolyte transport by comparing the capacitive performances of collapsed lamellar, cylindrical, and spherical mesopore electrodes. The highly ordered and straightly aligned cylindrical OMCs exhibited the highest specific capacitance and the best rate capability

Reversible Surface Segregation of Pt in a Pt₃Au/C Catalyst and Its Effect on the Oxygen Reduction Reaction

Reversible surface segregation of Pt in Pt₃Au/C catalysts was accomplished through a heat treatment under a CO or Ar atmosphere, which resulted in surface Pt segregation and reversed segregation, respectively. The Pt-segregated Pt₃Au/C exhibited a significantly improved oxygen reduction reaction (ORR) activity (227 mA/mg_metal) compared to that of commercial Pt/C (59 mA/mg_metal). For the Pt-segregated Pt₃Au/C, the increased OH-repulsive properties were validated by a CO bulk oxidation analysis and also by density functional theory (DFT) calculations. Interestingly, the DFT calculations revealed that the binding energy for Pt-segregated Pt₃Au (111) surfaces was 0.1 eV lower than that for Pt (111) surfaces, which has been previously reported to exhibit the optimum OH binding energy for the ORR. Therefore, the reversible surface segregation is expected to provide a practical way to control the surface states of Pt–Au bimetallic catalysts to enhance ORR activity. In addition, the Pt-segregated Pt₃Au/C showed excellent electrochemical stability, as evidenced by its high-performance retention (96.4%) after 10 000 potential cycles, in comparison to that of Pt/C (55.3%)

Impact of d‑Band Occupancy and Lattice Contraction on Selective Hydrogen Production from Formic Acid in the Bimetallic Pd₃M (M = Early Transition 3d Metals) Catalysts

Catalysts that are highly selective and active for H₂ production from HCOOH decomposition are indispensable to realize HCOOH-based hydrogen storage and distribution. In this study, we identify two effective routes to promoting the Pd catalyst for selective H₂ production from HCOOH by investigating the effects of early transition metals (Sc, Ti, V, and Cr) incorporated into the Pd core using density functional theory calculations. First, the asymmetric modification of the Pd surface electronic structure (d_<i>z</i>² vs d_<i>yz</i> + d_<i>zx</i>) can be an effective route to accelerating the H₂ production rate. Significant charge transfer from the subsurface Sc atom to the surface Pd atom and subsequent extremely low level of d band occupancy (<0.1) around the Sc atoms are identified as a key factor in deriving the asymmetric modification of the Pd surface electronic structure. Second, in-plane lattice contraction of the Pd surface can be an effective route to suppressing the CO production. Compressive strain of the Pd surface is maximized as a result of alloying with V and induces subsequent changes in adsorption site preference of the key intermediates for the CO production path, resulting in a significant increase in the activation energy barrier for the CO production path. The unraveled atomic-scale factors underlying the promotion of the Pd surface catalytic properties provide useful insights into the efforts to overcome limitations of current catalyst technologies in making the HCOOH-based H₂ storage and distribution economically feasible

Role of Electronic Perturbation in Stability and Activity of Pt-Based Alloy Nanocatalysts for Oxygen Reduction

The design of electrocatalysts for polymer electrolyte membrane fuel cells must satsify two equally important fundamental principles: optimization of electrocatalytic activity and long-term stability in acid media (pH <1) at high potential (0.8 V). We report here a solution-based approach to the preparation of Pt-based alloy with early transition metals and realistic parameters for the stability and activity of Pt₃M (M = Y, Zr, Ti, Ni, and Co) nanocatalysts for oxygen reduction reaction (ORR). The enhanced stability and activity of Pt-based alloy nanocatalysts in ORR and the relationship between electronic structure modification and stability were studied by experiment and DFT calculations. Stability correlates with the d-band fillings and the heat of alloy formation of Pt₃M alloys, which in turn depends on the degree of the electronic perturbation due to alloying. This concept provides realistic parameters for rational catalyst design in Pt-based alloy systems

Electrochemical Synthesis of NH₃ at Low Temperature and Atmospheric Pressure Using a γ‑Fe₂O₃ Catalyst

The electrochemical synthesis of NH₃ by the nitrogen reduction reaction (NRR) at low temperature (<65 °C) and atmospheric pressure using nanosized γ-Fe₂O₃ electrocatalysts were demonstrated. The activity and selectivity of the catalyst was investigated both in a 0.1 M KOH electrolyte and when incorporated into an anion-exchange membrane electrode assembly (MEA). In a half-reaction experiment conducted in a KOH electrolyte, the γ-Fe₂O₃ electrode presented a faradaic efficiency of 1.9% and a weight-normalized activity of 12.5 nmol h^–1 mg^–1 at 0.0 V_RHE. However, the selectivity toward N₂ reduction decreased at more negative potentials owing to the competing proton reduction reaction. When the γ-Fe₂O₃ nanoparticles were coated onto porous carbon paper to form an electrode for a MEA, their weight-normalized activity for N₂ reduction was found to increase dramatically to 55.9 nmol h^–1 mg^–1. However, the weight- and area-normalized N₂ reduction activities of γ-Fe₂O₃ decreased progressively from 35.9 to 14.8 nmol h^–1 mg^–1 and from 0.105 to 0.043 nmol h^–1 cm^–2_act, respectively, during a 25 h MEA durability test. In summary, a study of the fundamental behavior and catalytic activity of γ-Fe₂O₃ nanoparticles in the electrochemical synthesis of NH₃ under low temperature and pressure is presented