Junjie Ge

Ultrathin cobalt phosphide nanosheets as efficient bifunctional catalysts for a water electrolysis cell and the origin for cell performance degradation

Low-temperature electricity-driven water splitting is an established technology for hydrogen production, yet only few materials are able to catalyze hydrogen and oxygen evolution reactions in the same medium. Herein, ultrathin CoP nanosheets (CoP NS) as durable bifunctional catalysts for electrochemical water splitting are reported. The OER and HER activity for CoP NS/C reaching 10 mA cm−2 needs an overpotential of only 0.277 V and 0.111 V in a basic solution. Whats more, when integrated into a practical anion exchange membrane water electrolysis cell using CoP NS as both anode and cathode catalysts, a current density of 335 mA cm−2 at 1.8 V is achieved, which is rather competitive to the state-of-the-art Pt/IrO2 catalyst. This work would open a new avenue to explore the use of transition metal phosphides as green and attractive bifunctional catalysts toward mass production of hydrogen fuel for applications.

A modified Nafion membrane with extremely low methanol permeability via surface coating of sulfonated organic silica

We developed a method to significantly decrease the methanol permeability of a Nafion membrane that does not require sacrificing its proton conductivity and mechanical stability. The Nafion membrane modified by the coating of a thin layer of sulfonated organic silica on the membrane surface exhibits significantly decreased methanol permeability--the permeability is decreased to an undetectable level--while retaining an acceptable ionic conductivity of 0.029 S cm(-1).

Growth mechanism and active site probing of Fe3C@N-doped carbon nanotubes/C catalysts: guidance for building highly efficient oxygen reduction electrocatalysts

Non-platinum (NP) electrocatalysts with high activity and durability for oxygen reduction reactions (ORR) are required for fuel cells and other renewable systems. To avoid trial-and-error methods and achieve the rational design and synthesis of efficient NP catalysts, in-depth knowledge of the formation/growth mechanism of nanocatalysts and the origin of active sites is highly desirable. Here, we report a new class of NP catalysts with a novel structure of Fe3C encapsulated in N-doped carbon nanotubes/C. We study the formation mechanism of the nanostructure to pave the way for controlled fabrication of high-performance NP catalysts. The encapsulation of iron into carbon occurs during the first step of CNT growth and the surface functional groups on carbon black are identified as being essential for forming CNTs. The catalyst shows ultrahigh catalytic performance in both acid and alkaline media. We also examine the structure–performance dependency. The catalytic performance is highly dependent on the nanostructure and the encapsulation of Fe3C. Fe affects the catalytic performance through electronic effects rather than by directly participating in the active sites. This result is confirmed by DFT calculations, which show an increase in the density of states and a reduction in the local work function, XPS studies, and electrochemical measurements. The likelihood of N participating in the active sites is low because the catalytic performance does not depend on pyridinic and graphitic N.

Significantly enhanced oxygen reduction reaction performance of N-doped carbon by heterogeneous sulfur incorporation: synergistic effect between the two dopants in metal-free catalysts

Developing highly active non-noble metal oxygen reduction reaction (ORR) catalysts is crucial for a variety of renewable energy applications including fuel cells and metal–air batteries. Heteroatom doped carbon materials, known as metal-free catalysts, show potential applications in the ORR, and may be promising replacement candidates for expensive, scarce platinum catalysts. Despite the inspiring progress made, the performance of the current metal-free carbon catalysts is still far from satisfactory for large-scale applications. Herein, we introduce an effective and robust ORR catalyst based on N, S co-doped carbon materials with abundant surface active sites. Electrochemical results indicate that the incorporation of sulfur into nitrogen-doped carbon (S-NCx) can dramatically improve the stability of the catalyst by improving the selectivity of O2 electro-reduction to H2O. Density functional theory calculations reveal that sulfur doping lowers the energy barrier of O2(ads) hydrogenation to form OOH(ads), thus leading to enhanced intrinsic activity. In particular, the correlation between ORR activity and nitrogen and sulfur species in these materials is studied in-depth, and it is found the ORR performance of S-NCx catalysts is significantly affected by pyridinic N and C–S–C contents.

Monocrystalline Ni12P5 hollow spheres with ultrahigh specific surface areas as advanced electrocatalysts for the hydrogen evolution reaction

Monocrystalline Ni12P5 hollow spheres with ultrahigh specific surface areas were prepared by a water-in-oil microemulsion method. The novel structured Ni12P5 catalyst exhibited excellent catalytic activity and stability towards the hydrogen evolution reaction in acidic solutions.

A Nanosheet‐Structured Three‐Dimensional Macroporous Material with High Ionic Conductivity Synthesized Using Glucose as a Transforming Template

The advancements of both top-down and bottom-up synthesis strategies have yielded very interesting findings towards the synthesis of nanosheets. The preparation of nanosheets with top-down methods typically involves the exfoliation of layerstructured materials. Examples include the nanosheet synthesis of transition-metal dichalcogenides (TMDs), transition-metal oxides (TMOs), and compounds such as BN, Bi2Te3, and Bi2Se3. [1a,2] As the parent layered compound is the essential requirement for exfoliations, nanosheet synthesis for the materials without layered structures have been excluded from the top-down method. On the other hand, the bottomup method also presents difficulties in the synthesis of nanosheet materials with structures other than layered compounds, such as cubic structures, because of the lack of intrinsic driving force for two-dimensional (2D) growth. Furthermore, the existing methods are not readily scaled up for large quantity preparations. Clearly, a method that can produce large scale nanosheets for a wide variety of materials is of particular significance. Ceria has found application in catalysis, solid oxide fuel cells, and oxygen sensors. In particular, when ceria is doped with other rare-earth ions, it can significantly increase the oxygen vacancies and ionic conductivities. Zeroand onedimensional ceria-based nanocrystals have been investigated using different approaches. However, it is difficult to achieve 2D structured ceria-based materials owing to their face-centered cubic structure (Figure 1a). Glucose has been used as a source for carbon materials, as it is readily available, environmentally benign, and easy to control. Upon heating in air, glucose undergoes successive reactions, that is, polymerization, carbonization, and combustion. Inspired by this feature, a novel simple one-pot glucose combustion route was devised to synthesize inorganic compounds with Sm0.2Ce0.8O1.9 (SDC) as an example. The glucose acts as both the fuel and a transforming template, leading to the formation of three-dimensional (3D) macroporous SDC with a 2D nanosheet microstructure. To our best knowledge, this microstructure was achieved for the first time. The SDC sample prepared through the combustion process is shown in Figure 1b; free-flowing yellow powders were obtained. The filled density of the SDC powders was as low as 0.020 gcm , which is only 0.28% of the theoretical density (7.15 gcm ) of the bulk SDC. Equivalently, the apparent volume of this SDC powder is about 357 times that of the same amount of bulk material. In general, materials produced through wet chemistry methods are dispersed in solvents; therefore, complicated separation processes are needed to obtain the solid materials, and agglomerations may occur. The combustion method shown herein offers a simple way to produce porous solid powders directly in large scales. The powder X-ray diffraction (XRD) pattern (Figure 1c) confirmed that the synthesized SDC sample exhibits the single-phase cubic fluorite structure (Fm3m ; JCPDS card no. 34-394). The strong peak intensity indicates that the sample Figure 1. a) The unit cell of ceria with a fluorite structure; b) porous SDC powders obtained by the glucose combustion method; c) the XRD pattern of the sample; d), e) SEM images of the sample; f) a nitrogen adsorption/desorption isotherm of the sample.

Discontinuously covered IrO2–RuO2@Ru electrocatalysts for the oxygen evolution reaction: how high activity and long-term durability can be simultaneously realized in the synergistic and hybrid nano-structure

Here, we present our effort in simultaneously enhancing the intrinsic activity and improving the durability of catalysts based on Ir and Ru oxides. Through successful design and fabrication of the discontinuously covered IrO2–RuO2@Ru (3 : 1) supported structure, we are able to combine the high activity of RuO2 and the outstanding stability of IrO2. Specifically, the overpotential of the IrO2–RuO2@Ru (3 : 1) catalyst at 10 mA cm−2 is only 281 mV, and the turnover frequency (TOF) values of IrO2–RuO2@Ru (3 : 1), Ir3RuO2 and IrO2 catalysts are ca. 0.039, 0.023 and 0.017 s−1 at 1.55 V, respectively, indicating that IrO2–RuO2@Ru (3 : 1) exhibits higher intrinsic activity and the active surface sites possess greater ability to generate oxygen. The excellent durability of the IrO2–RuO2@Ru (3 : 1) catalyst is confirmed with the overpotential at 10 mA cm−2 positively shifting by only 16 mV after 3000 cycles through accelerated durability tests. Remarkably, the IrO2–RuO2@Ru (3 : 1) catalyst exhibits a huge advantage in reducing the use of precious metals in water electrolysis. Hence, the supported iridium–ruthenium bimetallic oxides show huge potential towards an active and stable application for the oxygen evolution reaction in acidic media.

Platinum nanoparticles partially-embedded into carbon sphere surfaces: a low metal-loading anode catalyst with superior performance for direct methanol fuel cells

Pt-based catalysts are considered as the most efficient and indispensable catalysts for methanol electro-oxidation reactions (MORs) in acidic media; however, issues linked to cost and stability impede their large-scale application. Here, we present a novel structured catalyst with Pt nanoparticles partially embedded in resorcinol-formaldehyde carbon spheres (Pt@RFC) towards MORs. Pt@RFC exhibits excellent CO-tolerance and MOR activity, and specifically, the CO electro-oxidation peak-potential is negatively shifted by ∼150 mV and the electrocatalytic activity is 2 times that of commercial Pt/C. These enhancements are due to the endowed high Pt utilization (decreased particle size) from strong metal-support interaction and the decorated electronic properties. Moreover, the firmly anchored Pt nanoparticles are prevented from possible dissolution, agglomeration and detachment during long-term use. Remarkably, after an accelerated degradation test through a 3000 cycle cyclic voltammetry test, the mass activity for Pt@RFC is well maintained and 5.8 times that of the commercial Pt/C. Upon integration into a DMFC, Pt@RFC (58.5 mW cm−2) exhibits a competitive power density at 60 °C compared to a commercial PtRu/C catalyst (52.3 mW cm−2) with only 1/3 of the noble metal loading, as well as a slower degradation rate during discharge testing. The present findings indicate that Pt@RFC might be a viable alternative as a commercial catalyst applied in DMFCs.

Cathode catalytic dependency behavior on ionomer content in direct methanol fuel cells

Cathode catalyst layers (CLs) with varying ionomer (Nafion) contents were prepared and the direct methanol fuel cell structure and catalytic behavior were investigated as a function of ionomer content. CL roughness and thickness increased with increasing Nafion content. Contact angle measurements determined that CL hydrophilicity also increased as a function of Nafion content. Poor bonding between the CL, microporous layer, and the proton exchange membrane was obtained when the ionomer content was too low. The electrochemical surface areas (ESAs) were found to increase with increasing Nafion content before reaching an asymptote at elevated loading levels. However, upon increasing the ionomer content above 30 wt.%, the water and oxygen mass transfer properties were difficult to control. Considering the above conditions, N30 (30 wt.% Nafion) was found to be the optimal level to effectively extend the three-phase boundaries and enhance cell performance.

Selectively doping pyridinic and pyrrolic nitrogen into a 3D porous carbon matrix through template-induced edge engineering: enhanced catalytic activity towards the oxygen reduction reaction

Developing cost-effective and highly efficient oxygen reduction electrocatalysts, such as non-precious metal and metal-free catalysts, is undoubtedly crucial for the commercialization of low-temperature fuel cells. Here, edge-rich nitrogen doped porous carbon catalysts for the oxygen reduction reaction (ORR) with a high proportion of pyridinic and pyrrolic N (up to 94%) were synthesized by an in situ released CO2 activation method, using glucose and melamine as precursors and nano-CaCO3 as the template. The catalysts exhibit a three-dimensional structure, hierarchical pores and large pore volumes. Benefiting from the increased active site density and structural advantage, the optimized catalyst shows excellent ORR activity with a half-wave potential of 0.853 V and long-term stability in alkaline media, which is among the best for metal-free catalysts reported to date.