Oxygen evolution in spin-sensitive pathways

The overall performance of water electrolysis suffers from the high kinetic barrier in the oxygen evolution reaction (OER) at the anode. Considerable effort has been made on the fundamental understandings of the reaction mechanisms of OER. Recently, the attention has been given to the OER on magnetic catalysts, which is believed being able to promote the kinetics of an OER process from singlet reactants to triplet oxygen. The process in principle involves spin selective electron transfer. Here, we discuss the effects of spin in OER based on the recent advances and summarize our recently proposed mechanisms of the OER in spin-sensitive pathways under the lattice oxygen oxidation mechanism, the interaction of two M−O entity mechanism, and the adsorbate evolution mechanism.Ministry of Education (MOE)National Research Foundation (NRF)Submitted/Accepted versionThe authors thank the support from the Singapore Ministry of Education Tier 2 Grant (MOE2018-T2-2-027) and this research was also supported by the National Research Foundation, Prime Minister’s Office, Singapore, under its Campus for Research Excellence and Technological Enterprise (CREATE) program

Mass loading optimization for ethylene glycol oxidation at different potential regions

Designing and fabricating the electrocatalysts is attracting more and more attention in recent years due to a global interest in developing techniques for electrochemical energy conversion and storage, as well as elelectro-synthesis of valuable chemicals. The activity is one of the key performance parameters for electrocatalysts, while the observed activity can be affected by mass loading of electrocatalysts. Here, we take cobalt oxide (Co3O4)/graphite paper electrode (Co3O4/GPE) as a model electrode to demonstrate how the mass loading of Co3O4 catalyst influences ethylene glycol (EG) oxidation in alkaline (KOH) by cyclic votammetry (CV) and chronopentiometry (CP) approaches. Analyses from redox peaks and double layer capacitances reveal that increasing the mass loading provided more electrochemical active sites. Increasing loading made a positive contribution to EG oxidation at the low oxidation potential, while less significant improvement at the high oxidation potential. The results will provide some insight for optimzing the mass loading of electrocatalysts for electrocatalysis of small organic molecules.Ministry of Education (MOE)Submitted/Accepted versionThe work was partially supported by the Singapore Ministry of Education Tier 1 Grant (2019-T1-002-125). Dr. S. Sun thanks the funding support from Beijing Natural Science Foundation Program 2212029 and National Natural Science Foundation of China-Youth Science Fund (Grant No. 52001009)

Searching and designing metastable spinel oxides for highly active water oxidation in alkaline solution

Developing highly active electrocatalysts for oxygen evolution reaction (OER) is critical for the commercial effectiveness of water splitting to produce hydrogen fuels. Low-cost spinel oxides have attracted increasing interest as alternatives to noble-metal-based OER catalysts. A rational design of spinel catalysts can be guided by studying the structural/elemental properties which determine the reaction mechanism and activity. Here, using density functional theory (OFT) calculations, we find that the relative position of 0 p-band and Moh (Co and Ni in octahedron) d-band center in ZnC02-x Nix04 (x=0-2) correlates with its stability as well as the possibility for lattice oxygen to participate in OER. We therefore testified it by synthesizing ZnC02-xNix04 spinel oxides, investigating on their OER performance and surface evolution. Stable ZnC02-xNix04 (x=0-0.4) follows adsorbates evolving mechanism (AEM) under OER conditions. Lattice oxygen participate in the OER of metastable ZnC02-xNix04 (x=0.6, 0.8) which gives rise to continuously formed oxyhydroxide as surface-active species and consequently enhanced OER activity. ZnC01.2Nio.a04 exhibits performance superior to the benchmarked lr02. Our work illuminates the design of highly active metastable spinel electrocatalysts through the prediction of the reaction mechanism and OER activity by determining the relative positions of the 0 p-band and Moh d-band center.Published versio

Yolk-shell Fe2O3 ⊙ C composites anchored on MWNTs with enhanced lithium and sodium storage

A unique architecture with yolk–shell Fe2O3 ⊙ C composites attached to the surface of MWNTs is designed. Benefiting from the good electrical conductivity of MWNTs and carbon layers, as well as the large void space to accommodate the volume expansion/extraction of Fe2O3 during battery cycling, the obtained MWNT@Fe2O3 ⊙ C exhibited outstanding lithium and sodium storage performance.Accepted versio

Impact of surface area in evaluation of catalyst activity

Electrocatalysis has been one of the hottest research fields in recent years due to the global effort in exploring sustainable energy technologies. The central effort is to develop highly active electrocatalysts for those reactions playing key roles in fuel cells, electrolyzers, and rechargeable metal-air batteries, such as oxygen evolution, oxygen reduction, and hydrogen evolution reactions. A highly active electrocatalyst should be able to promote the reaction rate of the corresponding reaction. In kinetics, the activation energy should be lowered, and, electrochemically, a lowered overpotential should be observed at a certain current density.Ministry of Education (MOE)Accepted versionZ.J.X. acknowledges the funding support from the Singapore Ministry of Education Tier 2 Grants (MOE2017- T2-1-009) and the Singapore National Research Foundation under its Campus for Research Excellence and Technological Enterprise (CREATE) program. Z.J.X. also thanks The First International Symposium on Electrocatalysis and Electrosynthesis, held in Changsha, China, from March 30 to April 1, 2018, in which the issue of activity evaluation in the literature was raised by valued colleagues and students

Electrochemistry in magnetic fields

Developing new strategies to advance the fundamental understanding of electrochemistry is crucial to mitigating multiple contemporary technological challenges. In this regard, magnetoelectrochemistry offers many strategic advantages in controlling and understanding electrochemical reactions that might be tricky to regulate in conventional electrochemical fields. However, the topic is highly interdisciplinary, combining concepts from electrochemistry, hydrodynamics, and magnetism with experimental outcomes that are sometimes unexpected. In this Review, we survey recent advances in using a magnetic field in different electrochemical applications organized by the effect of the generated forces on fundamental electrochemical principles and focus on how the magnetic field leads to the observed results. Finally, we discuss the challenges that remain to be addressed to establish robust applications capable of meeting present needs.Ministry of Education (MOE)National Research Foundation (NRF)Published versionThe authors thank and acknowledge support from the Singapore Ministry of Education Tier 2 Grant (MOE-T2EP10220-0001), Tier 1 Grant (RG62/21). This research was also supported by the National Research Foundation, Prime Minister’s Office, Singapore, under its Campus for Research Excellence and Technological Enterprise (CREATE) program

In situ X-ray absorption spectroscopy studies of nanoscale electrocatalysts

Nanoscale electrocatalysts have exhibited promising activity and stability, improving the kinetics of numerous electrochemical reactions in renewable energy systems such as electrolyzers, fuel cells, and metal-air batteries. Due to the size effect, nano particles with extreme small size have high surface areas, complicated morphology, and various surface terminations, which make them different from their bulk phases and often undergo restructuring during the reactions. These restructured materials are hard to probe by conventional ex-situ characterizations, thus leaving the true reaction centers and/or active sites difficult to determine. Nowadays, in situ techniques, particularly X-ray absorption spectroscopy (XAS), have become an important tool to obtain oxidation states, electronic structure, and local bonding environments, which are critical to investigate the electrocatalysts under real reaction conditions. In this review, we go over the basic principles of XAS and highlight recent applications of in situ XAS in studies of nanoscale electrocatalysts.Published versio

Spinel manganese ferrites for oxygen electrocatalysis : effect of Mn valency and occupation site

Spinel catalysts have been widely explored for the electrochemical oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). To consolidate the understanding on electrocatalysis by spinel family, intermediate spinels should be deliberately examined because most spinel oxides are of intermediate structure. Here, we report an investigation on the ORR and OER performance of intermediate spinel MnFe2O4. The modulation of cation oxidation state and inversion degree of spinel MnFe2O4 were achieved by a simple annealing process. X-ray absorption spectroscopy analysis reveals that the Mn occupancy in octahedral sites varied from 0.25 ~ 0.41 and Mn cations were oxidized from 2+ to 3+ with increasing temperature treatment. Convinced by the leading role of octahedral-geometric, we further reveal the role of Mn oxidation state through normalizing the activity to active Mn[Oh] site number. Our findings clearly indicate that Mn3+ was more catalytically active than Mn2+ in catalyzing ORR and OER.NRF (Natl Research Foundation, S’pore)MOE (Min. of Education, S’pore

Electrochemical sensor highly selective for lindane determination: a comparative study using three different a-MnO2 nanostructures

Here we describe a simple, highly reproducible ultra-sensitive electrochemical sensor for lindane based on a-MnO2 nanostructures. The results showed that the a-MnO2 nanostructures effectively catalyzed the electrochemical reduction of lindane. A good linearity was obtained in the range of 1.1 to 510 mM with a detection limit of 114 nM. The proposed lindane sensor was successfully employed for the determination of lindane in tap water samples with good recoveries. Negligible amperometric currents are observed in the control experiments using triclosan (T), chlorobenzene (CB), benzene (B), 1,3,5-trichlorobenzene (1,3,5-TCB), and 4-chlorobenzaldehyde (4-CBA), suggesting a sensing specificity to lindane. The proposed sensor also exhibited good stability and reproducibility for lindane determination.MOE (Min. of Education, S’pore)Accepted versio