Enhanced Proton Transfer in Proton-Exchange Membranes with Interconnected and Zwitterion-Functionalized Covalent Porous Material Structures.

Excellent proton-conductive accelerators are indispensable for efficient proton-exchange membranes (PEMs). Covalent porous materials (CPMs), with adjustable functionalities and well-ordered porosities, show much promise as effective proton-conductive accelerators. In this study, an interconnected and zwitterion-functionalized CPM structure based on carbon nanotubes and a Schiff-base network (CNT@ZSNW-1) is constructed as a highly efficient proton-conducting accelerator by in situ growth of SNW-1 onto carbon nanotubes (CNTs) and subsequent zwitterion functionalization. A composite PEM with enhanced proton conduction is acquired by integrating CNT@ZSNW-1 with Nafion. Zwitterion functionalization offers additional proton-conducting sites and promotes the water retention capacity. Moreover, the interconnected structure of CNT@ZSNW-1 induces a more consecutive arrangement of ionic clusters, which significantly relieves the proton transfer barrier of the composite PEM and increases its proton conductivity to 0.287 S cm-1 under 95 % RH at 90 °C (about 2.2 times that of the recast Nafion, 0.131 S cm-1 ). Furthermore, the composite PEM displays a peak power density of 39.6 mW cm-2 in a direct methanol fuel cell, which is significantly higher than that of the recast Nafion (19.9 mW cm-2 ). This study affords a potential reference for devising and preparing functionalized CPMs with optimized structures to expedite proton transfer in PEMs.

Continuous On-Site H2O2 Electrosynthesis via Two-Electron Oxygen Reduction Enabled by an Oxygen-Doped Single-Cobalt Atom Catalyst with Nitrogen Coordination.

Single-Co atom catalysts are suggested as an efficient platinum metal group-free catalyst for promoting the oxygen reduction into water or hydrogen peroxide, while the relevance of the catalyst structure and selectivity is still ambiguous. Here, we propose a thermal evaporation method for modulating the chemical environment of single-Co atom catalysts and unveil the effect on the selectivity and activity. It discloses that nitrogen functional groups prefer to proceed the oxygen reduction via a 4e- pathway and notably improve the intrinsic activity, especially when being coordinated with the Co center, while oxygen doping tempts the electron delocalization around cobalt sites and decreases the binding force toward HOO* intermediates, thereby increasing the 2e- selectivity. Consequently, the well-designed oxygen-doped single-Co atom catalysts with nitrogen coordination deliver an impressive 2e- oxygen reduction performance, approaching the onset potential of 0.78 V vs RHE and selectivity of >90%. As an impressive cathode catalyst of an electrochemical flow cell, it generates H2O2 at a rate of 880 mmol gcat-1 h-1 and faradaic efficiency of 95.2%, in combination with an efficient nickel-iron oxygen evolution anode.

Rational Design of Trimetallic Sulfide Electrodes for Alkaline Water Electrolysis with Ampere-Level Current Density.

Electrochemical water splitting is considered an environmentally friendly approach to hydrogen generation. However, it is difficult to achieve high current density and stability. Herein, we design an amorphous/crystalline heterostructure electrode based on trimetallic sulfide over nickel mesh substrate (NiFeMoS/NM), which only needs low overpotentials of 352 mV, 249 mV, and 360 mV to achieve an anodic oxygen evolution reaction (OER) current density of 1 A cm-2 in 1 M KOH, strong alkaline electrolyte (7.6 M KOH), and alkaline-simulated seawater, respectively. More importantly, it also shows superior stability with negligible decay after continuous work for 120 h at 1 A cm-2 in the strong alkaline electrolyte. The excellent OER performance of the as-obtained electrode can be attributed to the strong electronic interactions between different metal atoms, abundant amorphous/crystalline hetero-interfaces, and 3D porous nickel mesh structure. Finally, we coupled NiFeMoS/NM as both the anode and cathode in the anion exchange membrane electrolyzer, which can achieve low cell voltage and high stability at ampere-level current density, demonstrating the great potential of practicability.

Two-Dimensional Cobalt Sulfide/Iron-Nitrogen-Carbon Holey Sheets with Improved Durability for Oxygen Electrocatalysis.

Transition-metal sulfide as a promising bifunctional oxygen electrocatalyst alternative to scarce platinum-group metals has attracted much attention, but it suffers activity loss over time owing to poor structural/compositional stability during catalysis. Herein, we report a self-template method for preparing a two-dimensional cobalt sulfide holey sheet superstructure with hierarchical porosity followed by the encapsulation of thin iron-nitrogen-carbon as a protective layer. The iron-nitrogen-carbon layer to some degree precludes the phase transition of cobalt sulfide underneath and preserves the structural integrity during catalysis, therefore rendering an exceptional durability in terms of no obvious activity loss after 10,000 cycles of the accelerated durability test. It also noticeably enhances the intrinsic activity of cobalt sulfide and does not influence its exposure into the electrolyte, resulting in showing an extraordinary electrochemical performance in terms of a potential difference of 0.69 V for the overall oxygen redox. A rechargeable zinc-air battery assembled by a cobalt sulfide/iron-nitrogen-carbon air cathode delivers approximately 4.2 times higher power density than that without an iron-nitrogen-carbon layer and stably operates for 300 h with a high voltaic efficiency. This work gives a facile and effective strategy for improving the long-term durability of transition-metal sulfide electrocatalysts.

Cation Modulation of Cobalt Sulfide Supported by Mesopore-Rich Hydrangea-Like Carbon Nanoflower for Oxygen Electrocatalysis.

Transition-metal sulfide is pursued for replacing scare platinum-group metals for oxygen electrocatalysis and is of great importance in developing low-cost, high-performance rechargeable metal-air batteries. We report herein a facile cationic-doping strategy for preparing nickel-doped cobalt sulfide embedded into a mesopore-rich hydrangea-like carbon nanoflower. Nickel cations are introduced to induce the formation of Co3+-active species and more oxygen vacancies due to higher electronegativity and smaller ionic radius, thereby strengthening the intrinsic activity for oxygen electrocatalysis. Moreover, hydrangea-like superstructure composed of interconnected carbon cages provides abundant accessible active sites and hierarchical porosity. As a result, it shows excellent catalytic performance with a superior mass activity for the oxygen reduction reaction to the state-of-the-art Pt/C catalyst and a low overpotential of 314 mV at 10 mA cm-2 for the oxygen evolution reaction. When used as an air cathode for the rechargeable Zn-air battery, it delivers a peak power density of 96.3 mW cm-2 and stably operates over 214 h. This work highlights the importance of cationic doping in strengthening the electrocatalytic performance of 3d-transition-metal chalcogenides.