Yu-Yun Chen

Pomegranate-like N,P-Doped Mo2C@C Nanospheres as Highly Active Electrocatalysts for Alkaline Hydrogen Evolution.

Well-defined pomegranate-like N,P-doped Mo2C@C nanospheres were prepared by simply using phosphomolybdic acid (PMo12) to initiate the polymerization of polypyrrole (PPy) and as a single source for Mo and P to produce N,P-doped Mo2C nanocrystals. The existence of PMo12 at the molecular scale in the polymer network allows the formation of pomegranate-like Mo2C@C nanospheres with a porous carbon shell as peel and Mo2C nanocrystals well-dispersed in the N-doped carbon matrix as seeds. This nanostructure provides several favorable features for hydrogen evolution application: (1) the conductive carbon shell and matrix effectively prevent the aggregation of Mo2C nanocrystals and facilitate electron transportation; (2) the uniform N,P-doping in the carbon shell/matrix and plenty of Mo2C nanocrystals provide abundant catalytically highly active sites; and (3) nanoporous structure allows the effective exposure of active sites and mass transfer. Moreover, the uniform distribution of P and Mo from the single source of PMo12 and N from PPy in the polymeric PPy-PMo12 precursor guarantees the uniform N- and P-co-doping in both the graphitic carbon matrix and Mo2C nanocrystals, which contributes to the enhancement of electrocatalytic performance. As a result, the pomegranate-like Mo2C@C nanospheres exhibit extraordinary electrocatalytic activity for the hydrogen evolution reaction (HER) in terms of an extremely low overpotential of 47 mV at 10 mA cm(-2) in 1 M KOH, which is one of the best Mo-based HER catalysts. The strategy for preparing such nanostructures may open up opportunities for exploring low-cost high-performance electrocatalysts for various applications.

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.

Sodium chloride-assisted green synthesis of a 3D Fe–N–C hybrid as a highly active electrocatalyst for the oxygen reduction reaction

To promote the oxygen reduction reaction (ORR) on a non-precious-metal catalyst, integrating two-dimensional (2D) nanosheets and one-dimensional (1D) nanotubes in one catalyst is considered as one of the desirable approaches since this hybrid architecture can host more useful active sites and enhance mass/electron transfer. Herein, we demonstrated a sodium chloride-assisted strategy for the in situ synthesis of a three-dimensional (3D) hybrid of carbon nanosheets and nanotubes. The micrometer-scale sodium chloride (NaCl) crystal acted as a recyclable skeleton to adsorb the precursors on its surfaces, which assisted the formation of micrometer-sized graphitic carbon nanosheets with nanometer thickness by the template effect during the pyrolysis, and iron-based nanocrystals with a size of tens of nanometers by helping the distribution of iron sources and preventing their aggregation. The small iron-based nanocrystals favored the growth of long CNTs connected to carbon nanosheets and the outmigration of carbon atoms during the cooling process, which led to the formation of carbon-layer encapsulated metallic iron nanoparticles between the carbon nanosheets or inside the carbon nanotubes. Benefiting from these features, the developed hybrid exhibited a significantly enhanced electrocatalytic activity and durability for the ORR. The results may open up opportunities for exploring cost-effective high-performance electrocatalysts for energy applications.

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.

In situ transformation of Cu2O@MnO2 to Cu@Mn(OH)2 nanosheet-on-nanowire arrays for efficient hydrogen evolution

The development of new non-precious metal catalysts and understanding the origin of their activity for the hydrogen evolution reaction (HER) are essential for rationally designing highly active low-cost catalysts as alternatives to state-of-the-art precious metal catalysts. Herein, manganese oxide/hydroxide was demonstrated as a highly active electrocatalysts for the HER by fabricating MnO2 nanosheets coated with Cu2O nanowire arrays (Cu2O@MnO2 NW@NS) on Cu foam followed by an in situ chronopotentiometry (CP) treatment. It was discovered that the in situ transformation of Cu2O@MnO2 into Cu@Mn(OH)2 NW@NS by the CP treatment drastically boosted the catalytic activity for the HER due to an enhancement of its intrinsic activity. Together with the benefits from such three-dimensional (3D) core–shell arrays for exposing more accessible active sites and efficient mass and electron transfers, the resulting Cu@Mn(OH)2 NW@NS exhibited excellent HER activity and outstanding durability in terms of a low overpotential of 132 mV vs. RHE at 10 mA/cm2. Overall, we expect these findings to generate new opportunities for the exploration of other Mn-based nanomaterials as efficient electrocatalysts and enable further understanding of their catalytic processes.

Self-supported 3D Nanoporous Ni/V2O3 Hybrid Nanoplate Assemblies for Highly Efficient Electrochemical Hydrogen Evolution

Nickel-based non-noble-metal materials have emerged as promising catalysts for electrochemical hydrogen production in view of their attractive intrinsic activities, electrical properties and low cost. Exploring new candidates for further improving the performances of nickel-based catalysts and understanding the structure–activity relationship are still necessary to reduce the overpotential of the hydrogen evolution reaction (HER) thus advancing their application in electrochemical water splitting. Herein, we developed a facile two-step self-templated strategy for fabricating a three-dimensional (3D) nanoporous nickel/vanadium oxide (Ni/V2O3) nanoplate assembly as a new efficient catalyst for alkaline HER. It is found that by controllably annealing the Ni–V–O assembly as a single precursor, Ni and V2O3 components are uniformly integrated in the nanoporous composite, showing a synergistically enhancing effect on the HER. The resulting 3D nanoporous structure not only creates numerous active sites accessible for the HER but also provides a conductive open network towards efficient electron/mass transport. Consequently, the nanoporous Ni/V2O3 nanoplate assembly exhibits excellent catalytic performance for alkaline HER in terms of a low overpotential of 61 mV at 10 mA cm−2 and a small Tafel slope of 79.7 mV dec−1 together with excellent long-term durability. These findings provide new insights into good design and construction of other highly active catalysts for diverse applications.