ABSTRACT
Atomic arrangement in Cu-Pd alloy nanoparticles (NPs) has been reported to influence the catalytic activity, but they have yet to be studied in detail. Unlike previous studies, where the B2 structure Cu-Pd NPs are obtained by heat treating the A1 structure, this study reports the one-pot direct syntheses of A1- and B2-structured Cu-Pd NPs using an alcohol reduction method. The alcohol reduction technique facilitates the kinetic control of the reduction reaction by selecting the appropriate alcohol type and complexing agent to delay the reduction of easily reducible metallic elements to realize control over the reduction kinetics for coreduction. Different formation mechanisms for A1- and B2-structured CuPd NPs were confirmed by in situ ultraviolet-visible (UV-vis) measurements and morphological and structural analyses of samples withdrawn during the reaction. Finally, the direct formation of single-phase B2-structured Cu-Pd NPs with an average diameter of 18.6 ± 7.6 nm was realized using tri-n-octyl phosphine as a complexing agent. The noticeable crystal structural dependence of the electrocatalytic CO2 reduction reaction properties of A1- and B2-structured CuPd NPs was demonstrated.
ABSTRACT
High-entropy alloys (HEAs) have attracted considerable attention to improve performance of various electrocatalyst materials. A comprehensive understanding of the relationship between surface atomic-level structures and catalytic properties is essential to boost the development of novel catalysts. In this study, we propose an experimental study platform that enables the vacuum synthesis of atomic-level-controlled single-crystal high-entropy alloy surfaces and evaluates their catalytic properties. The platform provides essential information that is crucial for the microstructural fundamentals of electrocatalysis, i.e., the detailed relationship between multi-component alloy surface microstructures and their catalytic properties. Nanometre-thick epitaxially stacking layers of Pt and equi-atomic-ratio Cr-Mn-Fe-Co-Ni, the so-called Cantor alloy, were synthesised on low-index single-crystal Pt substrates (Pt/Cr-Mn-Fe-Co-Ni/Pt(hkl)) as a Pt-based single-crystal alloy surface model for oxygen reduction reaction (ORR) electrocatalysis. The usefulness of the platform was demonstrated by showing the outperforming oxygen reduction reaction properties of high-entropy alloy surfaces when compared to Pt-Co binary surfaces.
ABSTRACT
Alkaline water electrolysis (AWE) is a large-scale hydrogen production technology. A major degradation mode of AWE when using fluctuating power derived from renewable energies is the detachment of the catalyst layer (CL). Here, this study investigates the CL detachment mechanism of NiCo2O4-CL-coated Ni (NCO/Ni) electrodes under an accelerated durability test (ADT) simulating a fluctuating power and the effect of post-annealing on detachment behavior. Microstructural analysis reveals that detachment begins at the nanoscale gaps between the stacked CLs and between CL and the substrate. Post-annealing at 400 °C removes the degradation starting point in CL, and a composition gradient Co-doped NiO interlayer and NiO(111)/Ni(111) epitaxial interface form between CL and the Ni substrate, nearly suppressing CL detachment. Although the electrode performance of the annealed sample is initially lower than that of the as-prepared sample, the overpotential is significantly reduced during ADT due to the formation of the NiCo hydroxide active surface layer. These results demonstrate that interfacial microstructural modulation by post-annealing is a powerful approach to realizing durable electrodes for green hydrogen production by renewable energy-powered AWE.
ABSTRACT
Four types of carbazole dendrimers were applied as modification molecules of Au surfaces to improve carbon dioxide electroreduction. The reduction properties depended on the molecular structures: the highest activity and selectivity to CO was achieved by 9-phenylcarbazole, probably caused by the charge transfer from the molecule to Au.
ABSTRACT
Simultaneous accomplishment of high hydrogen oxidation reaction (HOR) activity and suppressed hydrogen peroxide (H2O2) generation is desired for anode catalysts of polymer electrolyte fuel cells. 0.3 monolayer-thick-Pt-deposited Ir(111) showed three-fold higher HOR activity than Pt(111) and suppressed H2O2 generation under the detection limit, providing insights for effective catalyst development.
ABSTRACT
From the viewpoint of the application of Ir-Ru alloys for the anode of proton exchange membrane fuel cells (PEMFCs), hydrogen peroxide (H2O2) generation and the hydrogen oxidation reaction (HOR) properties of well-defined Ir-Ru bimetallic surfaces (Ru/Ir(111)) have been investigated using scanning electrochemical microscopy (SECM). Using thermal inter-diffusion of vacuum-deposited Ru and substrate Ir atoms, the topmost surface atomic ratios of Ru/Ir(111) were controlled via changing the substrate temperature (x) during the deposition of 1 monolayer (ML)-thick Ru. Low-energy ion scattering spectroscopy (LE-ISS) estimated the Ru/Ir ratio to be 1 : 1 (x = 673 K), 1 : 2 (x = 773 K), and 1 : 4 (x = 873 K). The H2O2 generation property of Ru/Ir(111) was similar to that of clean Ir(111) and under the detection limit in the potential region of 0.06-0.3 V, while clean Ru(0001) generated H2O2 in this potential region. The results suggest that the Ir sites contribute to the reduction of H2O2 intermediates generated at neighboring Ru sites. In contrast, the HOR activity of Ru/Ir(111) correlated with the probabilities of ensembles, such as Ir2 dimers and Ir3 trimers: the ensemble probabilities were calculated under the assumption of random solute Ir and Ru atoms at the topmost surfaces. Such a close correlation suggests that the Ir ensemble sites strongly contribute to the HOR. In conclusion, the Ir sites play a key role in the suppression of H2O2 generation and high HOR activity, which is essential for next-generation PEMFC anode catalysts.
ABSTRACT
The electrochemical CO2 reduction reaction (CO2RR) activity of Ni2MnIn and Ni2MnSn Heusler alloys was investigated. Although pure In, Sn and Ni2MnIn generated formate as the major product, Ni2MnSn generated H2 as the major product. The CO2RR selectivity could be controlled by selecting the constituent elements of the intermetallic catalysts.
ABSTRACT
Extended X-ray absorption fine structure (EXAFS) is a powerful tool to determine the local structure in Pt nanoparticles (NP) on carbon supports, active catalysts for fuel cells. Highly oriented pyrolytic graphite (HOPG) covered with Pt NP gives samples with flat surfaces that allow application of surface science techniques. However, the low concentration of Pt makes it difficult to obtain good quality EXAFS data. We have performed in situ highly sensitive BCLA-empowered Back Illuminated EXAFS (BCLA + BI-EXAFS) measurements on Pt alloy nanoparticles. We obtained high quality Pt L3-edge data. We have devised a novel analytical method (model building analysis) to determine the structure of multi-component nanoparticles from just a single absorption edge. The generation of large numbers of structural models and their comparison with EXAFS fits allows us to determine the structures of Pt-containing nanoparticles, catalysts for the oxygen reduction reaction. Our results show that PtCo, PtCoN and AuPtCoN form a Pt-shell during electrochemical dealloying and that the ORR activity is directly proportional to the Pt-Pt bond length.
ABSTRACT
Highly active and inexpensive anode materials are required for large-scale hydrogen production using alkaline water electrolysis (AWE). Here, heterolayered nanostructures of Ni-Fe hydroxides/oxides with high activity for the oxygen evolution reaction (OER) were synthesized on a 316 stainless steel (SS) substrate through constant current density electrolysis. The thicknesses, morphologies, and compositions of the nanostructures, generated through dealloying and surface oxidation of the SS elements with severe oxygen microbubble evolution, were dependent on the electrolysis time. Nanostructural analyses showed that the heterolayered Ni-Fe hydroxide/oxide nanostructures were generated during the initial stage of electrolysis, growing nanofiberlike Ni-Fe hydroxide layers with increasing electrolysis time of up to 5 h. The prolonged electrolysis resulted in densification of the nanofiber structures. The OER overpotential at 10 mA/cm2 was estimated to be 254 mV at 20 °C, demonstrating better performance than a standard OER catalyst, for example, Ir oxide, and obtaining the value of the Ni-Fe layered double hydroxide (LDH). Furthermore, the OER property surpassed the Ni-Fe LDH catalysts at high current density regions greater than 100 mA/cm2. Moreover, stable electrolysis was achieved for 20 h under conditions similar to that of the practical AWE of 400 mA/cm2 in 20 and 75 °C solution. Therefore, the simple surface modification method could synthesize highly active nanostructures for alkaline water splitting anodes.
ABSTRACT
The oxygen reduction reaction (ORR) activity and electrochemical stability of well-defined n monolayer (ML)-Pt/PtxNi100-x(111) (n = 2 and 4; x = 75, 50, and 25) model electrocatalyst surfaces were investigated in this study. The initial activity of the as-prepared two-monolayered Pt-covered PtxNi100-x(111) substrates (2ML-Pt/PtxNi100-x(111)) increased with increasing Ni composition in the PtxNi100-x(111) substrate. In particular, 2ML-Pt/Pt25Ni75(111) showed the initial activity that was 25 times higher than that of clean Pt(111) although the higher Ni composition resulted in destabilization of the catalyst upon the application of potential cycles (PCs). As for 4ML-Pt/PtxNi100-x(111), activity enhancements were insensitive to alloy composition and thicker Pt shell layers stabilized the catalyst against PC applications. In particular, the activities of 4ML-Pt/Pt50Ni50(111) and 4ML-Pt/Pt25Ni75(111) gradually increased during 1000 PCs probably because of the PC-induced mono-atomic heights and nanometer-size islands that had (110) and (100) steps introduced into the topmost (111) terraces. Thus, the simultaneous tuning of core-alloy composition and Pt shell thickness is vital for developing practical, highly efficient Pt-based alloy ORR electrocatalysts.
ABSTRACT
In this study, we perform ultrahigh vacuum (UHV) and arc-plasma synthesis of strain-controlled Pt(111) model shells on Pt-Co(111) layers with various atomic ratios of Pt/Co and an oxygen reduction reaction (ORR) activity enhancement trend against the surface strain induced by lattice mismatch between the Pt shell and Pt-Co alloy-core interface structures was observed. The results showed that the Pt(111)-shell with 2.0% compressive surface strain vs intrinsic Pt(111) lattice gave rise to a maximum activity enhancement, ca. 13-fold higher activity than that of clean Pt(111). This study clearly demonstrates that the UHV-synthesized, strain-controlled Pt shells furnish useful surface templates for electrocatalysis.
ABSTRACT
Voltammetric dealloying is a typical method to synthesize Pt-shell/less-noble metal (M) alloy core nanoparticles (NPs) toward the oxygen reduction reaction (ORR). The pristine nanostructures of the Pt-M alloy NPs should determine the ORR activity of the dealloyed NPs. In this study, we investigated the voltammetric dealloying behavior of the Pt-Co and nitrogen-introduced Pt-Co alloy NPs generated by synchronous arc-plasma deposition of Pt and Co. The results showed that the dealloying behavior is sensitive to cobalt nitride in the pristine NPs, leading to the preferential generation of a Pt-rich shell/Pt-Co alloy core architecture having enhanced ORR activity.
ABSTRACT
The oxygen reduction reaction (ORR) activity and durability of various Au(x)/Pt100 nanoparticles (where x is the atomic ratio of Au against Pt) are evaluated herein. The samples were fabricated on a highly-oriented pyrolytic graphite substrate at 773 K through sequential arc-plasma depositions of Pt and Au. The electrochemical hydrogen adsorption charges (electrochemical surface area), particularly the characteristic currents caused by the corner and edge sites of the Pt nanoparticles, decrease with increasing Au atomic ratio (x). In contrast, the specific ORR activities of the Au(x)/Pt100 samples were dependent on the atomic ratios of Pt and Au: the Au28/Pt100 sample showed the highest specific activity among all the investigated samples (x = 0-42). As for ORR durability evaluated by applying potential cycles between 0.6 and 1.0 V in oxygen-saturated 0.1 M HClO4, Au28/Pt100 was the most durable sample against the electrochemical potential cycles. The results clearly showed that the Au atoms located at coordinatively-unsaturated sites, e.g. at the corners or edges of the Pt nanoparticles, can improve the ORR durability by suppressing unsaturated-site-induced degradation of the Pt nanoparticles.
ABSTRACT
Effects of surface Pt monolayer thickness on electrochemical oxygen reduction reaction of molecular-beam-epitaxially-prepared Pt/Ni/Pt(111) were investigated. The effective thickness of Pt for stabilizing the topmost surface can be deduced to be three monolayers.
