[en] Graphical abstract: In-situ soft-templated LFP nanocrystals on interconnected carbon nanotubes/mesoporous carbon nanosheets (designated as LFP@CNTs/CNSs), exhibited superior electrochemical performance due to the synergetic effect between CNTs and CNSs, which form interconnected conductive network for fast transport of both electrons and lithium ions. - Highlights: • LFP nanocrystals were in-situ synthesized on interconnected CNTs/CNSs framework with an in-situ soft-templated method. • LFP@CNTs/CNSs exhibited superior rate capability and cycling stability, due to interconnected conductive network for fast transport of both electrons and lithium ions. • The synergetic effect between CNTs and CNSs on the electrochemical performance of LFP electrode was demonstrated by a systematically electrochemical study compared with LFP/CNSs and LFP/CNTs. - Abstract: Lithium ion phosphate (LiFePO_4) nanocrystals are successfully in-situ grown on interconnected carbon nanotubes/mesoporous carbon nanosheets (designated as LFP@CNTs/CNSs) with a soft-templated method, which involves the multi-constituent co-assembly of a triblock copolymer, CNTs, resol and precursors of LFP followed by thermal treatment. X-ray diffraction, scanning electron microscopy, high resolution transmission electron microscopy and N_2 adsorption-desorption techniques are used to characterize the structure and morphology of the as-synthesized materials. When used as the cathode of lithium ion batteries, the LFP@CNTs/CNSs composite exhibits superior rate capability and cycling stability, compared with the samples modified only with CNSs (designated as LFP/CNSs) or with CNTs (designated as LFP/CNTs). This is mainly attributed to the synergetic effect between CNTs and CNSs caused by their unique structure, which forms interconnected conductive network for fast transport of both electrons and lithium ions, and thus remarkably improves the electrode kinetics. Firstly, nano-sized LFP are in-situ grown on the CNTs/CNSs framework, which can serve as an effective matrix to (i) restrain the size growth of LFP during the thermal treatment process and (ii) prevent them from aggregating during cycling. Secondly, the incorporation of CNTs/CNSs framework into the LFP electrode significantly increases the electronic conductivity of the composite, and thus allow for improved high-rate charge-discharge performance. Finally, open mesoporosity in the CNTs/CNSs framework also provides an efficient transport pathway for lithium ions diffusion to LFP

[en] Highlights: •Lamellar Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ hollow nanospheres serve as a cathode for LIBs. •Unique lamella and hollow structures benefit the enhanced electrochemical performance. •Lamellar shells can provide a short lithium-ion diffusion pathway. •The sufficient void space can accommodate volumetric expansion and contraction. -- Abstract: Although very appealing in developing hollow structured lithium-rich layered transition-metal oxides as cathodes for lithium-ion batteries (LIBs), a great challenge lies in controlling the growth of transition metal elements with desired molar ratios while maintaining intact hollow structures during synthesis. Herein, we propose a scalable strategy to successfully synthesize novel lamellar Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ hollow (L-LMOH) nanosphere cathode for advanced lithium-ion batteries (LIBs). It is proved that the employment of sulfonated polystyrene (SPS) gel nanospheres as the template plays a key role in the formation of flower-like SPS@ Ni-Co-Mn-precursor nanospheres with desired molar ratios, and a subsequently delicate control in the heating rate leads to the intact L-LMOH nanospheres. It is demonstrated that the use of L-LMOH nanosphere cathode not only delivers outstanding reversible discharge capacities of 281.7 mAh g⁻¹ at a current density of 20 mA g⁻¹ and 136.6 mAh g⁻¹ at 2000 mA g⁻¹, but also possess superior cycling stability with a capacity reservation of 80% at 2000 mA g⁻¹ after 200 continuous cycles. It is well analyzed that the ingenious design of both unique lamella and hollow architectures synergistically benefits the significantly enhanced rate capability and cycling stability.

[en] Highlights: • A novel three-dimensional waved flow field is proposed and investigated. • The waved flow filed can introduce forced convection at the through-plane direction. • The waved flow filed significantly improves the water management and PEMFC net power. - Abstract: It has been well recognized that both the performance and operation stability of proton exchange membrane fuel cells (PEMFCs) are closely associated with water transport and accumulation behaviors in the membrane electrode assembly. Therefore, an optimal water management is highly desired. Conventional serpentine flow field (CSFF) can effectively facilitate water removal and prevent water flooding. However, CSFF would cause a high pressure drop from the inlet to outlet, thus resulting in large parasitic power loss. In this study, a novel three-dimensional flow field (WSFF), patterned with waved serpentine flow channels, is designed and analyzed by combing the simulating method with experimental method. A three-dimensional, multi-phase, steady, isothermal, laminar simulation model is firstly established based on FLUENT PEM fuel cell module, and this model reveals that WSFF is overall better than CSFF in promoting oxygen transport though the diffusion layer and removing liquid water accumulated in microstructure. Its periodic waved structure introduces cyclical variation of local flow direction, local flow velocity and local pressure, thus leading to enhanced forced-convection. The superior performance of WSFF has also been experimentally verified, proving that WSFF not only enables a lower pressure drop over the entire current density range, but also improves the cell performance in comparison to CSFF at high current density region. Specifically, there is a 17.8% increment in the peak power density due to the use of WSFF.

[en] Highlights: • The present model able to predict the degradation of core-shell Pt₃Co in PEMFCs. • More severe particles corrosion plays a major role in ECA loss near the membrane. • Structural/compositional changes of Pt₃Co catalysts are firstly captured by modeling. • The model can be revised for evaluating the degradation of other Pt-based catalysts. -- Abstract: Understanding the degradation process of Pt-based catalysts is a crucial step to improve the durability of PEMFCs. A mathematical model is established to study the electrochemical surface area (ECA) loss and structural/compositional evolutions of core-shell Pt₃Co catalysts along the cathode under a certain circumstance. Three major processes are included in this model: Pt dissolution and re-precipitation on the Pt shell, Co leaching from the bulk and deposition of Pt ions in the membrane due to crossover hydrogen. The results show that the significant ECA loss next to the membrane is mainly attributed to the severe particle corrosion rather than variation of the particle size distribution. The Pt shell thickness and Pt/Co atomic ratio decrease from gas diffusion layer (GDL)/CCL interface to the cathode catalysts layer (CCL)/membrane interface, while the Co mass loss shows an inverse trend. This work demonstrates that mathematical modeling is effective to predict the structural and compositional evolutions of the catalyst particles across the CCL, and therefore is useful to evaluate the complete performance degradation of Pt₃M catalysts for PEMFCs in the future.

[en] A novel nitrogen and iodine co-doped porous graphene with well-defined hierarchical microstructure is synthesized via a hydrothermal self-assembly strategy followed by heat treatment at elevated temperature. Its electrochemical properties as the cathode catalyst for Li–O₂ batteries are investigated. Compared to un-doped, nitrogen or iodine single doped porous graphene, nitrogen and iodine co-doped porous graphene shows much higher activity for oxygen reduction reaction and oxygen evolution reaction. As oxygen cathode for Li–O₂ batteries, nitrogen and iodine co-doped porous graphene exhibits an outstanding specific reversible capacity up to 14000 mAh g⁻¹ at 200 mA g⁻¹, a superior high-rate capability (5000 mAh g⁻¹ at 1000 mA g⁻¹), and an excellent cycling stability enduring over 225 cycles with a limited capacity of 1000 mA h g⁻¹ at a current density of 500 mA g⁻¹. The fantastic cycling performance can be attributed to a combination of porous structure and dual-doping, which not only provides a large number of channels for oxygen diffusion and electrolyte infiltration, but also supplies abundant active catalytic sites to affect the formation and decomposition of the discharge products with various morphologies. This study provides an effective approach to develop highly-efficient carbon-based catalysts with optimized pore structure and tunable surface chemistry.

[en] Highlights: • Orthogonal method is utilized to optimize the configurations of fuel cell stack. • Effects of configurations and operating parameters are numerically investigated. • Operating parameters affect power density and system efficiency in different way. • Counter-flow of both hydrogen and coolant can improve water and thermal managements. -- Abstract: Water and thermal managements are critical for the performance and operation stability of proton exchange membrane fuel cell (PEMFC) stacks, which are highly associated with the stack configurations and cathode operating parameters that need to be well optimized. In this work, a numerical study is conducted with orthogonal analysis method to investigate the effect of stack configurations and cathode operating parameters on stack performance including power density, system efficiency and stack uniformity. An orthogonal array (OA) with three levels and six factors is designed to determine the interaction of each parameter as well as the optimal combination of configurations and operating parameters. The results indicate that the optimal combination with respect to power density and system efficiency is not consistent due to the associated parasitic loads. Moreover, counter-flow configuration of hydrogen channel is able to improve the water management and counter-flow configuration of coolant channel is beneficial for thermal management, both of which can further improve the stack uniformity that is desired in real application.

[en] Highlights: • Two type of flow fields with three-dimensional geometry are proposed. • The 3D channel geometry can induce local convection at through-plane direction. • The 3D geometry enables improved water management and oxygen transport. -- Abstract: It has been well recognized that the power density of fuel cells is limited by two key issues known as water flooding and oxygen starvation. Since flow field plays a critical role on the mass transport in fuel cells, a flow field design enabling improved water management and enhanced oxygen transport is highly desired to address these problems. In this work, two types of flow fields with three-dimensional channel geometry are proposed and developed. One flow field is designed to own waved channels to induce local oxygen convection flux from flow channel/diffusion layer interface to catalyst layer in order to enhance the oxygen supply. The other one owns the waved channels with gradient channel depth that results in increasing flow velocity at both in-plane and through-plane directions from upstream region to downstream region, accommodating the uneven distribution of oxygen concentration. The experimental results clearly demonstrate that the 3D channel geometry is capable of improving cell performance especially at high current densities, which can be attributed to the enhanced oxygen transport and water removal as illumined by a numerical simulation.

[en] Highlights: • ENRR potential evaluated for 58 types of SACs on nitrogen-doped graphene. • Scaling relations exist in protonation steps related to N₂H and NH_2. • 2D-volcanic plot established between *N₂ to *N₂H and *NH₂ to *NH₃. • 6 candidates identified where V-N₄/graphene shows best ENRR activity and stability. In this study, we performed the first-principles density functional theory calculations to systematically predict the activity of fifty-eight types of different single atom electrocatalysts on nitrogen doped graphene for electrochemical ammonia synthesis. Two strong linear relations were revealed among the reaction energies on these single atom structures, including positive correlation between the adsorption energy of N₂H and the free energy change for *N₂ transition to *N₂H, as well as negative correlation between the adsorption energy of NH₂ and the free energy change for *NH₂ transition to *NH₃. Using the developed scaling relations and some additional factors including nitrogen adsorption, hydrogen evolution reaction, water adsorption, ammonia desorption and structure stability, we have computationally identified six candidate structures as promising active sites for ammonia synthesis. Especially, V-N₄/graphene was predicted to exhibit the best stability, the highest activity with the limiting potential of −0.71 V, and suppression of hydrogen evolution reaction.