Ultra-small intermetallic NiZn nanoparticles: a non-precious metal catalyst for efficient electrocatalysis

Ruthenium nanoclusters modified by zinc species towards enhanced electrochemical hydrogen evolution reaction

Ruthenium (Ru) has been considered a promising electrocatalyst for electrochemical hydrogen evolution reaction (HER) while its performance is limited due to the problems of particle aggregation and competitive adsorption of the reaction intermediates. Herein, we reported the synthesis of a zinc (Zn) modified Ru nanocluster electrocatalyst anchored on multiwalled carbon nanotubes (Ru-Zn/MWCNTs). The Ru-Zn catalysts were found to be highly dispersed on the MWCNTs substrate. Moreover, the Ru-Zn/MWCNTs exhibited low overpotentials of 26 and 119 mV for achieving current intensities of 10 and 100 mA cm−2 under alkaline conditions, respectively, surpassing Ru/MWCNTs with the same Ru loading and the commercial 5 wt% Pt/C (47 and 270 mV). Moreover, the Ru-Zn/MWCNTs showed greatly enhanced stability compared to Ru/MWCNTs with no significant decay after 10,000 cycles of CV sweeps and long-term operation for 90 h. The incorporation of Zn species was found to modify the electronic structure of the Ru active species and thus modulate the adsorption energy of the Had and OHad intermediates, which could be the main reason for the enhanced HER performance. This study provides a strategy to develop efficient and stable electrocatalysts towards the clean energy conversion field.

Rational Design of NiZnx@CuO Nanoarray Architectures for Electrocatalytic Oxidation of Methanol

Methanol oxidation reaction (MOR) in anodes is one of the significant aspects of direct methanol fuel cells (DMFCs), which also plays a critical role in achieving a carbon-neutral economy. Designing and developing efficient, cost-effective, and durable non-Pt group metal-based methanol oxidation catalysts are highly desired, but a gap still remains. Herein, we report well-defined hierarchical NiZnx@CuO nanoarray architectures as active electrocatalysts for MOR, synthesized by combining thermal oxidation treatment and magnetron sputtering deposition through a brass mesh precursor. After systematically evaluating the electrocatalytic performance of NiZnx@CuO nanoarray catalysts with different preparation conditions, we found that the NiZn1000@CuO (thermally oxidized at 500 °C for 2 h, nominal thickness of the NiZn alloy film is 1000 nm) electrode delivers a high current density of 449.3 mA cm-2 at 0.8 V for MOR in alkaline media as well as excellent operation stability (92% retention after 12 h). These outstanding MOR performances can be attributed to the hierarchical well-defined structure that can not only render abundant active sites and a synergistic effect to enhance the electrocatalytic activity but also can effectively facilitate mass and electron transport. More importantly, we found that partial Zn atoms could leach from the NiZn alloy, resulting in rough surface nanorods, which would further increase the specific surface area. These results indicate that the NiZn1000@CuO nanoarray architecture could be a promising Pt group metal alternative as an efficient, cost-effective, and durable anode catalyst for DMFCs.

Promoting effect of Zn in high-loading Zn/Ni-SiO2 catalysts for selective hydrogen evolution from methylcyclohexane

The dehydrogenation of methylcyclohexane to toluene was investigated over high-loading monometallic Ni-SiO₂ and bimetallic Zn/Ni-SiO₂ catalysts. The catalysts were prepared by the impregnation coupled with the advantageous heterophase sol-gel technique. Their performance was tested in a fixed-bed flow reactor at 250-350 °C, 0.1 MPa pressure, equimolar ratio H₂/Ar (24 nL h^-1 in total), and a methylcyclohexane feed rate of 12 mL h^-1. Information regarding the structure of Ni-Zn catalysts was obtained by N₂ and CO adsorption, temperature-programmed reduction, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, in situ X-ray diffraction, and in situ X-ray absorption spectroscopy. The results have shown that the addition of zinc leads to the hindrance of Ni reducibility along with weakening the Ni interaction with the silica matrix. This behavior particularly indicated the formation of solid oxide nickel-zinc solutions. The catalytic properties of Zn-modified catalysts in the dehydrogenation of methylcyclohexane appeared significantly superior to their Ni-Cu counterparts. For example, the selectivity of Zn/Ni-SiO₂ catalysts toward toluene formation increased markedly with a decrease in the Ni : Zn mass ratio, achieving 97% at 350 °C over the sample with Ni : Zn = 80 : 20. This is attributed to the promoting geometric and electronic effects arising from the formation of bimetallic Ni-Zn solid solutions. Moreover, a deeper reduction of zinc and a more efficient formation of solid bimetallic solutions are observed after the catalytic tests.

Effect of Plating Variables on Oxygen Evolution Reaction of Ni–Zn–Fe Electrodes for Alkaline Water Electrolysis

In this study, we investigated the oxygen evolution reaction (OER) characteristics of Ni–Zn–Fe electrodes by varying plating current density and Ni:Fe ratio in a plating bath. The activity of the OER increased up to the plating current density of 160 mA/cm2, as the Fe content of the deposited electrode increased and electrochemical surface area (ECSA) increased after Zn dealloying. However, for the plated electrode with higher than 160 mA/cm2 of current density, the change in composition caused by underpotential deposition led to decreased activity due to decreasing Fe content and diminishing Zn dealloying. Moreover, when the Ni:Fe ratio in the plating bath was varied, outstanding OER activity was observed at Ni:Fe = 2:1. When the Fe content of the bath increased beyond this ratio, Fe could not restrain Ni oxidation and formed Fe oxides in OER reaction, and oxygen vacancy decreased. These caused a degradation of the OER activity.

Ni Nanoparticles on Reducible Metal Oxides (Sm2O3, CeO2, ZnO) as Catalysts for CO2 Methanation

The activity of reducible metal oxide Sm2O3, CeO2, and ZnO as Ni nanoparticles support was investigated for CO2 methanation reaction. CO2 methanation was carried out between 200 °C to 450 °C with the optimum catalytic activity was observed at 450 °C. The reducibility of the catalysts has been comparatively studied using H2-Temperature Reduction Temperature (TPR) method. The H2-TPR analysis also elucidated the formation of surface oxygen vacancies at temperature above 600 °C for 5Ni/Sm2O3 and 5Ni/CeO2. The Sm2O3 showed superior activity than CeO2 presumably due to the transition of the crystalline phases under reducing environment. However, the formation of NiZn alloy in 5Ni/ZnO reduced the ability of Ni to catalyze methanation reaction. A highly dispersed Ni on Sm2O3 created a large metal/support interfacial interaction to give 69% of CO2 conversion with 100% selectivity at 450 °C. The 5Ni/Sm2O3 exhibited superior catalytic performances with an apparent phase transition from cubic to a mixture of cubic and monoclinic phases over a long reaction, presumably responsible for the enhanced conversion after 10 h of reaction. Copyright © 2021 by Authors, Published by BCREC Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0).

Perspective on intermetallics towards efficient electrocatalytic water-splitting

Intermetallic compounds exhibit attractive electronic, physical, and chemical properties, especially in terms of a high density of active sites and enhanced conductivity, making them an ideal class of materials for electrocatalytic applications. Nevertheless, widespread use of intermetallics for such applications is often limited by the complex energy-intensive processes yielding larger particles with decreased surface areas. In this regard, alternative synthetic strategies are now being explored to realize intermetallics with distinct crystal structures, morphology, and chemical composition to achieve high performance and as robust electrode materials. In this perspective, we focus on the recent advances and progress of intermetallics for the reaction of electrochemical water-splitting. We first introduce fundamental principles and the evaluation parameters of water-splitting. Then, we emphasize the various synthetic methodologies adapted for intermetallics and subsequently, discuss their catalytic activities for water-splitting. In particular, importance has been paid to the chemical stability and the structural transformation of the intermetallics as well as their active structure determination under operating water-splitting conditions. Finally, we describe the challenges and future opportunities to develop novel high-performance and stable intermetallic compounds that can hold the key to more green and sustainable economy and rise beyond the horizon of water-splitting application.