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

Rhodium–Tin Binary NanoparticleA Strategy to Develop an Alternative Electrocatalyst for Oxygen Reduction

A Rh–Sn nanoparticle is achieved by combinatorial approaches for application as an active and stable electrocatalyst in the oxygen reduction reaction. Both metallic Rh and metallic Sn exhibit activities too low to be utilized for electrocatalytic reduction of oxygen. However, a clean and active Rh surface can be activated by incorporation of Sn into a Rh nanoparticle through the combined effects of lateral repulsion, bifunctional mechanism, and electronic modification. The corrosion-resistant property of Rh contributes to the construction of a stable catalyst that can be used under harsh fuel cell conditions. Based on both theoretical and experimental research, Rh–Sn nanoparticle designs with inexpensive materials can be a potential alternative catalyst in terms of the economic feasibility of commercialization and its facile and simple surfactant-free microwave-assisted synthesis

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

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

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

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%)

Electrocatalytic Effects of Carbon Dissolution in Pd Nanoparticles

Highly dispersed Pd nanoparticles were prepared by borohydride reduction of Pd­(acac)₂ in 1,2-propanediol at an elevated temperature. They were uniformly dispersed on carbon black without significant aggregation. X-ray diffraction showed that carbons from the Pd precursor dissolved in Pd, increasing its lattice parameter. A modified reduction process was tested to remove the carbon impurities. Carbon removal greatly enhanced catalytic activity toward the oxygen reduction reaction. It also generated an inconsistency between the electronic modifications obtained from X-ray photoelectron spectroscopy and the electrochemical method. CO displacement measurements showed that the formation of Pd–C bonds decreased the work function of the surface Pd atoms

Understanding Interface between Electrode and Electrolyte: Organic/Inorganic Hybrid Design for Fast Ion Conductivity

Ion transport is an important issue in electrochemical-based energy conversion and storage devices. Ion transport at the interface of the electrode and electrolyte is critical for performance. However, there is little understanding of the interface phenomena based on ion transport properties. Here, the proton transport behavior in a Nafion membrane (electrolyte) and that of an ionomer in the catalyst layer (electrode/electrolyte interface) was investigated simultaneously by electrochemical impedance spectroscopy. Our study indicates that the proton transport behavior in the catalyst layer is different from that in membrane. To elucidate the interface phenomena, we analyzed the Nafion electrolyte and proton behavior by molecular dynamics (MD). On the basis of the MD results, we modified the catalyst with a hybrid of inorganic Pt catalyst and organic 3-mercaptopropionic acid to promote a positive interfacial reaction between the electrolyte and electrode, which resulted in improved proton transport and performance

Highly Durable and Active PtFe Nanocatalyst for Electrochemical Oxygen Reduction Reaction

Demand on the practical synthetic approach to the high performance electrocatalyst is rapidly increasing for fuel cell commercialization. Here we present a synthesis of highly durable and active intermetallic ordered face-centered tetragonal (fct)-PtFe nanoparticles (NPs) coated with a “dual purpose” N-doped carbon shell. Ordered fct-PtFe NPs with the size of only a few nanometers are obtained by thermal annealing of polydopamine-coated PtFe NPs, and the N-doped carbon shell that is <i>in situ</i> formed from dopamine coating could effectively prevent the coalescence of NPs. This carbon shell also protects the NPs from detachment and agglomeration as well as dissolution throughout the harsh fuel cell operating conditions. By controlling the thickness of the shell below 1 nm, we achieved excellent protection of the NPs as well as high catalytic activity, as the thin carbon shell is highly permeable for the reactant molecules. Our ordered fct-PtFe/C nanocatalyst coated with an N-doped carbon shell shows 11.4 times-higher mass activity and 10.5 times-higher specific activity than commercial Pt/C catalyst. Moreover, we accomplished the long-term stability in membrane electrode assembly (MEA) for 100 h without significant activity loss. From <i>in situ</i> XANES, EDS, and first-principles calculations, we confirmed that an ordered fct-PtFe structure is critical for the long-term stability of our nanocatalyst. This strategy utilizing an N-doped carbon shell for obtaining a small ordered-fct PtFe nanocatalyst as well as protecting the catalyst during fuel cell cycling is expected to open a new simple and effective route for the commercialization of fuel cells