Enhanced Oxygen Evolution and Zinc-Air Battery Performance via Electronic Spin Modulation in Heterostructured Catalysts.

Beyond optimizing electronic energy levels, the modulation of the electronic spin configuration is an effective strategy, often overlooked, to boost activity and selectivity in a range of catalytic reactions, including the oxygen evolution reaction (OER). This electronic spin modulation is frequently accomplished using external magnetic fields, which makes it impractical for real applications. Herein, spin modulation is achieved by engineering Ni/MnFe2O4 heterojunctions, whose surface is reconstructed into NiOOH/MnFeOOH during the OER. NiOOH/MnFeOOH shows a high spin state of Ni, which regulates the OH- and O2 adsorption energy and enables spin alignment of oxygen intermediates. As a result, NiOOH/MnFeOOH electrocatalysts provide excellent OER performance with an overpotential of 261 mV at 10 mA cm-2. Besides, rechargeable zinc-air batteries based on Ni/MnFe2O4 show a high open circuit potential of 1.56 V and excellent stability for more than 1000 cycles. This outstanding performance is rationalized using density functional theory calculations, which show that the optimal spin state of both Ni active sites and oxygen intermediates facilitates spin-selected charge transport, optimizes the reaction kinetics, and decreases the energy barrier to the evolution of oxygen. This study provides valuable insight into spin polarization modulation by heterojunctions enabling the design of next-generation OER catalysts with boosted performance.

Alkylamine-Confined Thickness-Tunable Synthesis of Co(OH)2-CoO Nanosheets toward Oxygen Evolution Catalysis.

Thickness regulation of transition metal hydroxides/oxides nanosheets with superior catalytic properties represents a promising strategy to enhance catalytic performance, but it remains an enormous challenge to achieve precise control, especially when it comes to the ultrathin limit (several atomic layers). In this work, a facile strategy of alkylamine-confined growth is proposed for the synthesis of thickness-tunable metal hydroxide/oxide nanosheets. Specifically, ultrathin cobalt hydroxide and cobaltous oxide hybrid (Co(OH)2-CoO) nanosheets (Co-O NSs) with a thickness in the range of 2-6 nm (5-13 atomic layers) are synthesized by using alkylamines with different carbon chain lengths as the ligand to modulate vertical coordination ability. Co-O NSs with a thickness of 2 nm (Co-O NSs-2 nm) exhibit excellent oxygen evolution reaction (OER) performance with an overpotential of 278 mV at 10 mA/cm2. The maximized number of active sites including oxygen vacancies, optimal adsorption strength, and the highest electrical conductivity are considered as the potential factors contributing to the excellent OER performance of Co-O NSs-2 nm. This work holds great significance for the precise thickness-tunable synthesis of transition metal layered hydroxide nanosheets with modulated and improved catalytic performance.

Synergistic tuning of oxygen vacancies and d-band centers of ultrathin cobaltous dihydroxycarbonate nanowires for enhanced electrocatalytic oxygen evolution.

Simple and controllable synthesis of efficient and robust non-noble metal electrocatalysts towards the oxygen evolution reaction (OER) is highly desired and challenging in the development of sustainable energy conversion technologies. Herein, we report a facile one-step solvothermal synthesis of cobaltous dihydroxycarbonate nanowires (Co-OCH NWs) with a tunable diameter ranging from 8.7 to 16.7 nm, which were able to exhibit an interesting diameter-dependent catalytic activity towards the OER. It should be highlighted that the thinnest nanowires (8.7 nm) demonstrated the best OER catalytic activity among the as-prepared nanowires, showing an overpotential of only 307 mV at 10 mA cm-2 and a Tafel slope of 75 mV dec-1 in 1.0 M KOH solution. Based on comprehensive analysis, the excellent electrocatalytic activity of Co-OCH NWs was ascribed to the simultaneous achievement of an enlarged specific surface area, increased oxygen vacancy concentration and favorable position of the 3d-band center for the Co-OCH NWs with the continuous decrease of their diameters. More importantly, this work has emphasized that synergistic tuning of the oxygen vacancy concentration and d-band center position of nanomaterials via facile size control enables boosting their electrocatalytic performance substantially, thereby opening a simple route to design and prepare Earth-abundant electrocatalysts with higher efficiency and lower cost.

Co3O4 nanoparticles anchored on nitrogen-doped reduced graphene oxide as a multifunctional catalyst for H2O2 reduction, oxygen reduction and evolution reaction.

This study describes a facile and effective route to synthesize hybrid material consisting of Co3O4 nanoparticles anchored on nitrogen-doped reduced graphene oxide (Co3O4/N-rGO) as a high-performance tri-functional catalyst for oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and H2O2 sensing. Electrocatalytic activity of Co3O4/N-rGO to hydrogen peroxide reduction was tested by cyclic voltammetry (CV), linear sweep voltammetry (LSV) and chronoamperometry. Under a reduction potential at -0.6 V to H2O2, this constructing H2O2 sensor exhibits a linear response ranging from 0.2 to 17.5 mM with a detection limit to be 0.1 mM. Although Co3O4/rGO or nitrogen-doped reduced graphene oxide (N-rGO) alone has little catalytic activity, the Co3O4/N-rGO exhibits high ORR activity. The Co3O4/N-rGO hybrid demonstrates satisfied catalytic activity with ORR peak potential to be -0.26 V (vs. Ag/AgCl) and the number of electron transfer number is 3.4, but superior stability to Pt/C in alkaline solutions. The same hybrid is also highly active for OER with the onset potential, current density and Tafel slope to be better than Pt/C. The unusual catalytic activity of Co3O4/N-rGO for hydrogen peroxide reduction, ORR and OER may be ascribed to synergetic chemical coupling effects between Co3O4, nitrogen and graphene.