Shuai Niu

Electronic and Morphological Dual Modulation of Cobalt Carbonate Hydroxides by Mn Doping toward Highly Efficient and Stable Bifunctional Electrocatalysts for Overall Water Splitting

Developing bifunctional efficient and durable non-noble electrocatalysts for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is highly desirable and challenging for overall water splitting. Herein, Co-Mn carbonate hydroxide (CoMnCH) nanosheet arrays with controllable morphology and composition were developed on nickel foam (NF) as such a bifunctional electrocatalyst. It is discovered that Mn doping in CoCH can simultaneously modulate the nanosheet morphology to significantly increase the electrochemical active surface area for exposing more accessible active sites and tune the electronic structure of Co center to effectively boost its intrinsic activity. As a result, the optimized Co1Mn1CH/NF electrode exhibits unprecedented OER activity with an ultralow overpotential of 294 mV at 30 mA cm-2, compared with all reported metal carbonate hydroxides. Benefited from 3D open nanosheet array topographic structure with tight contact between nanosheets and NF, it is able to deliver a high and stable current density of 1000 mA cm-2 at only an overpotential of 462 mV with no interference from high-flux oxygen evolution. Despite no reports about effective HER on metal carbonate hydroxides yet, the small overpotential of 180 mV at 10 mA cm-2 for HER can be also achieved on Co1Mn1CH/NF by the dual modulation of Mn doping. This offers a two-electrode electrolyzer using bifunctional Co1Mn1CH/NF as both anode and cathode to perform stable overall water splitting with a cell voltage of only 1.68 V at 10 mA cm-2. These findings may open up opportunities to explore other multimetal carbonate hydroxides as practical bifunctional electrocatalysts for scale-up water electrolysis.

Self‐Templated Fabrication of MoNi4/MoO3‐x Nanorod Arrays with Dual Active Components for Highly Efficient Hydrogen Evolution

A binder-free efficient MoNi4 /MoO3-x nanorod array electrode with 3D open structure is developed by using Ni foam as both scaffold and Ni source to form NiMoO4 precursor, followed by subsequent annealing in a reduction atmosphere. It is discovered that the self-templated conversion of NiMoO4 into MoNi4 nanocrystals and MoO3-x as dual active components dramatically boosts the hydrogen evolution reaction (HER) performance. Benefiting from high intrinsic activity, high electrochemical surface area, 3D open network, and improved electron transport, the resulting MoNi4 /MoO3-x electrode exhibits a remarkable HER activity with extremely low overpotentials of 17 mV at 10 mA cm-2 and 114 mV at 500 mA cm-2 , as well as a superior durability in alkaline medium. The water-alkali electrolyzer using MoNi4 /MoO3-x as cathode achieves stable overall water splitting with a small cell voltage of 1.6 V at 30 mA cm-2 . These findings may inspire the exploration of cost-effective and efficient electrodes by in situ integrating multiple highly active components on 3D platform with open conductive network for practical hydrogen production.

Crystallinity‐Modulated Electrocatalytic Activity of a Nickel(II) Borate Thin Layer on Ni3B for Efficient Water Oxidation

The exploration of new efficient OER electrocatalysts based on nonprecious metals and the understanding of the relationship between activity and structure of electrocatalysts are important to advance electrochemical water oxidation. Herein, we developed an efficient OER electrocatalyst with nickel boride (Ni3 B) nanoparticles as cores and nickel(II) borate (Ni-Bi ) as shells (Ni-Bi @NB) via a very simple and facile aqueous reaction. This electrocatalyst exhibited a small overpotential of 302 mV at 10 mA cm-2 and Tafel slope of 52 mV dec-1 . More interestingly, it was found that the OER activity of Ni-Bi @NB was closely dependent on the crystallinity of the Ni-Bi shells. The partially crystalline Ni-Bi catalyst exhibited much higher activity than the amorphous or crystalline analogues; this higher activity originated from the enhanced intrinsic activity of the catalytic sites. These findings open up opportunities to explore nickel(II) borates as a new class of efficient nonprecious metal OER electrocatalysts, and to improve the electrocatalyst performance by modulating their crystallinity.

Encased Copper Boosts the Electrocatalytic Activity of N-Doped Carbon Nanotubes for Hydrogen Evolution

Nitrogen (N)-doped carbons combined with transition-metal nanoparticles are attractive as alternatives to the state-of-the-art precious metal catalysts for hydrogen evolution reaction (HER). Herein, we demonstrate a strategy for fabricating three-dimensional (3D) Cu-encased N-doped carbon nanotube arrays which are directly grown on Cu foam (Cu@NC NT/CF) as a new efficient HER electrocatalyst. Cu nanoparticles are encased here instead of common transition metals (Fe, Co, or Ni) for pursuing a well-controllable morphology and an excellent activity by taking advantage of its more stable nature at high temperature and in acidic or alkaline electrolyte. It is discovered that metallic Cu exhibits strong electronic modulation on N-doped carbon to boost its electrocatalytic activity for HER. Such a nanostructure not only offers plenty of accessible highly active sites but also provides a 3D conductive open network for fast electron/mass transfer and facilitates gas escape for prompt mass exchange. As a result, the Cu@NC NT/CF electrode exhibits superior HER performance and durability, outperforming most of the reported M@NC materials. Furthermore, the etching experiments together with X-ray photoelectron spectroscopy (XPS) analysis reveal that the electronic modulation from encased Cu significantly enhances the HER activity of N-doped carbon. These findings open up opportunities for exploring other Cu-based nanomaterials as efficient electrocatalysts and understanding their catalytic processes.

Facile and Scalable Synthesis of Robust Ni(OH)2 Nanoplate Arrays on NiAl Foil as Hierarchical Active Scaffold for Highly Efficient Overall Water Splitting

Developing highly efficient low‐cost electrocatalysts for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline electrolyte is essential to advance water electrolysis technology. Herein, Ni(OH)2 nanoplates aligned on NiAl foil (Ni(OH)2/NiAl) are developed by simply dealloying NiAl foil in KOH, which exhibits high electrocatalytic activity for OER with a small overpotential of 289 mV to achieve 10 mA cm−2 and outstanding durability with no detectable degradation during long‐term operation. Furthermore, such Ni(OH)2/NiAl can effectively act as an active and robust hierarchical scaffold to simply electrodeposit other catalysts with intrinsically higher activity such as NiMo and NiFe nanoparticles for highly efficient HER and OER, respectively. The prepared NiFe/Ni(OH)2/NiAl displays superior OER catalytic activity with overpotentials of 246, 315, and 374 mV at 10, 100, and 500 mA cm−2, respectively. While NiMo/Ni(OH)2/NiAl catalyst exhibits remarkable HER performance with a small overpotential of 78 mV to deliver 10 mA cm−2. Consequently, the electrolysis device composed of the above two electrocatalysts demonstrates superb water splitting performance with a cell voltage of 1.59 V at 10 mA cm−2. These results open up opportunities to explore and optimize low‐cost advanced catalysts for energy applications.

Well-Defined Metal–O6 in Metal–Catecholates as a Novel Active Site for Oxygen Electroreduction

Metal-nitrogen coordination sites, M-Nx (M = Fe, Co, Ni, etc.), have shown great potential to replace platinum group materials as electrocatalysts for oxygen reduction reaction (ORR). However, the real active site in M-Nx is still vague to date due to their complicated structure and composition. It is therefore highly desirable but challenging to develop ORR catalysts with novel and clear active sites, which could meet the needs of comprehensive understanding of structure-function relationships and explore new cost-effective and efficient ORR electrocatalysts. Herein, well-defined M-O6 coordination in metal-catecholates (M-CATs, M = Ni or Co) is discovered to be catalytically active for ORR via a four-electron-dominated pathway. In view of no pyrolysis involved and unambiguous crystalline structure of M-CATs, the M-O6 octahedral coordination site with distinct structure is determined as a new type of active site for ORR. These findings extend the scope of metal-nonmetal coordination as an active site for ORR and pave a way for bottom-up design of novel electrocatalysts containing M-O6 coordination.

Bimetal Prussian Blue as a Continuously Variable Platform for Investigating the Composition–Activity Relationship of Phosphides-Based Electrocatalysts for Water Oxidation

Doping unary transition-metal phosphides (TMPs) by secondary metal is a powerful method to improve their catalytic activity for electrochemical oxygen evolution reaction (OER). However, the composition-activity relationship of such doping has not been systematically investigated yet because of the challenge in constructing bimetal TMPs with continuously variable composition while keeping homogenous elemental distribution. Herein, we develop a strategy of using bimetal Prussian blue analogues with homogenous elemental distribution at molecular scale as an ideal platform to achieve bimetal cobalt-iron phosphides (Co1- xFe xP) with a continuously changeable Co/Fe ratio (0 < x < 1) and uniform metal distribution. Such a system allows us to draw out a composition-activity volcano profile of Co1- xFe xP for OER. By optimizing the composition, the best catalytic activity is obtained at the Co/Fe ratio of 1.63 in Co1- xFe xP with small overpotentials of 230 and 268 mV at 10 and 100 mA cm-2, respectively, which outperform most of the reported TMPs. These results may inspire the use of multicomponent molecular platforms to understand composition-dependent performance and explore highly efficient catalysts for diverse applications.