Wen-Jie Jiang

Understanding the High Activity of Fe–N–C Electrocatalysts in Oxygen Reduction: Fe/Fe3C Nanoparticles Boost the Activity of Fe–Nx

Understanding the origin of high activity of Fe-N-C electrocatalysts in oxygen reduction reaction (ORR) is critical but still challenging for developing efficient sustainable nonprecious metal catalysts in fuel cells and metal-air batteries. Herein, we developed a new highly active Fe-N-C ORR catalyst containing Fe-N(x) coordination sites and Fe/Fe3C nanocrystals (Fe@C-FeNC), and revealed the origin of its activity by intensively investigating the composition and the structure of the catalyst and their correlations with the electrochemical performance. The detailed analyses unambiguously confirmed the coexistence of Fe/Fe3C nanocrystals and Fe-N(x) in the best catalyst. A series of designed experiments disclosed that (1) N-doped carbon substrate, Fe/Fe3C nanocrystals or Fe-N(x) themselves did not deliver the high activity; (2) the catalysts with both Fe/Fe3C nanocrystals and Fe-N(x) exhibited the high activity; (3) the higher content of Fe-N(x) gave the higher activity; (4) the removal of Fe/Fe3C nanocrystals severely degraded the activity; (5) the blocking of Fe-N(x) downgraded the activity and the recovery of the blocked Fe-N(x) recovered the activity. These facts supported that the high ORR activity of the Fe@C-FeNC electrocatalysts should be ascribed to that Fe/Fe3C nanocrystals boost the activity of Fe-N(x). The coexistence of high content of Fe-N(x) and sufficient metallic iron nanoparticles is essential for the high ORR activity. DFT calculation corroborated this conclusion by indicating that the interaction between metallic iron and Fe-N4 coordination structure favored the adsorption of oxygen molecule. These new findings open an avenue for the rational design and bottom-up synthesis of low-cost highly active ORR electrocatalysts.

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.

ITO@Cu2S Tunnel Junction Nanowire Arrays as Efficient Counter Electrode for Quantum-Dot-Sensitized Solar Cells

Quantum-dot-sensitized solar cell (QDSSC) has been considered as an alternative to new generation photovoltaics, but it still presents very low power conversion efficiency. Besides the continuous effort on improving photoanodes and electrolytes, the focused investigation on charge transfer at interfaces and the rational design for counter electrodes (CEs) are recently receiving much attention. Herein, core-shell nanowire arrays with tin-doped indium oxide (ITO) nanowire core and Cu2S nanocrystal shell (ITO@Cu2S) were dedicatedly designed and fabricated as new efficient CEs for QDSSCs in order to improve charge collection and transport and to avoid the intrinsic issue of copper dissolution in popular and most efficient Cu/Cu2S CEs. The high-quality tunnel junctions formed between n-type ITO nanowires and p-type Cu2S nanocrystals led to the considerable decrease in sheet resistance and charge transfer resistance and thus facilitated the electron transport during the operation of QDSSCs. The three-dimensional structure of nanowire arrays provided high surface area for more active catalytic sites and easy accessibility for an electrolyte. As a result, the power conversion efficiency of QDSSCs with the designed ITO@Cu2S CEs increased by 84.5 and 33.5% compared to that with planar Au and Cu2S CEs, respectively.

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.

Confining Iron Carbide Nanocrystals inside CNx@CNT toward an Efficient Electrocatalyst for Oxygen Reduction Reaction

The development of low-cost electrocatalysts with comparable activity for oxygen reduction reaction (ORR) to substitute platinum-based catalysts is imperative but still challenging for the commercialization of fuel cells. Herein, we reported a strategy to effectively confine iron carbide nanocrystals in N-doped carbon coated on carbon nanotubes (CNx@CNT), which prevented the agglomeration of iron carbide during pyrolysis and thus provided the sufficient highly active catalytic sites. Together with the benefit from three-dimensional conductive network of CNT-based core-shell structure for fast electron transfer and rapid mass transfer, the developed nanocatalyst exhibited the significantly enhanced electrocatalytic activity for ORR, as well as high durability and methanol tolerance. Moreover, it was interestingly found that the types of the confined iron compounds appreciably affected the performance of the catalysts, and Fe3C might be most effective on improving ORR activity in this case.

In-situ nitrogen-doped nanoporous carbon nanocables as an efficient metal-free catalyst for oxygen reduction reaction

A new N-doped carbon nanomaterial with nanoporous coaxial nanocable structure was designed for achieving the requirements of high nitrogen content, proper nitrogen bonding state, and sufficient electron and mass transportation for an oxygen reduction reaction (ORR) catalyst. The nanoporous sheaths provided more catalytic sites and allowed oxygen and reactants to easily access them for fast mass transfer, whereas carbon nanotube cores provided a three-dimensional conductive network and guaranteed fast electron transfer. As a result, the designed low-cost catalyst exhibited excellent electrocatalytic performance and is one of the most active metal-free ORR catalyst.

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.

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.

Physical vapor deposition of amorphous MoS2 nanosheet arrays on carbon cloth for highly reproducible large-area electrocatalysts for the hydrogen evolution reaction

Molybdenum sulfide materials have been shown to be promising non-precious metal catalysts for the hydrogen evolution reaction (HER). This work reports a facile and scalable preparation method for amorphous MoS2 nanosheet arrays directly deposited on carbon cloth (a-MoS2 NA/CC) using a highly reproducible physical vapor deposition (PVD) approach. As a result of the three-dimensional nanostructure of the catalyst, the amorphous nature and the abundant exposed edge sites of MoS2, the a-MoS2 NA/CC composite exhibited superior catalytic activity and stability for the HER in acidic solutions.

Self-deposition of Pt nanocrystals on Mn3O4 coated carbon nanotubes for enhanced oxygen reduction electrocatalysis

Developing catalysts with high electrocatalytic activity for oxygen reduction reaction (ORR) has recently attracted much attention because the sluggish ORR limits the performance and commercialization of current PEMFCs and metal–air batteries as well. Herein, a facile approach was reported to synthesize Mn3O4 nanoparticle coated carbon nanotubes (Mn3O4/CNTs) and self-deposit well-dispersed Pt nanocrystals on Mn3O4/CNTs to obtain Pt/Mn3O4/CNTs hybrid catalysts via in situ reduction of support matrix with no need of any capping agent and additional reducing agent. As a result of good dispersion of uncapped Pt nanocrystals, interconnected carbon nanotube conductive network, and possible synergetic co-catalytic effect from heterojunction interfaces of Pt nanocrystals and Mn3O4, the as-prepared Pt/Mn3O4/CNTs hybrid catalysts demonstrated much enhanced electrocatalytic activity for ORR. The reported strategy may inspire the development of new high efficient hybrid electrocatalysts in a cost-effective way with the potential to harness the metal/oxide interaction to improve the performance of catalysts.