NiFe-based electrocatalyst anchored on nanocone with tip-field effect for improved oxygen evolution reaction.

The development of high efficiency oxygen evolution electrocatalyst is of great significance for water splitting reaction. Herein, an efficient cone-structured NiFe-LDH/Nicone/Ti catalyst is fabricated by electrodeposition method towards enhanced oxygen evolution reaction (OER). The featured tip curvature of nanocone structure can accelerate the reaction kinetics of OER by offering a field-enhanced aggregation of local hydroxide ion reactant on the catalyst surface, and thus improves the performance of the NiFe catalyst. Accordingly, NiFe-LDH/Nicone/Ti requires only a low overpotential of 292 mV to achieve 50 mA cm-2, and with high stability under continuous high-current operations. In addition, the alkali-electrolyzer using NiFe-LDH/Nicone/Ti electrode exhibits good performance with a voltage of 1.73 V at 50 mA cm-2and displays excellent stability in long-term stability test. This cone-structured catalyst design with field-enhanced local hydroxide ion aggregation effect provides a promising method for the development of highly active OER electrocatalysts.

Mass Transfer in a Co/N/C Catalyst Layer for the Anion Exchange Membrane Fuel Cell.

Developing highly efficient non-noble metal catalysts for the cathode of fuel cells is an urgent requirement for reducing the cost. Although the intrinsic activity of non-noble metal materials has been greatly improved, the fuel cell performance is also determined by the mass transfer within the catalyst layer (CL), particularly at high current density. Electrochemical impedance spectroscopy (EIS) combined with rotating disk electrode (RDE) analysis is a powerful tool to quantitatively analyze the influence of the structural properties on CL performance. Here, Co/N/C CLs with gradient pore structures are constructed based on the controllable synthesis of zeolitic imidazolate framework (ZIF)-derived catalyst. The influences of the carbon support, active site, and catalyst loading are comprehensively studied by EIS in different regions (kinetic and mixed-diffusion). The results indicate that a high micro-/mesopore ratio is beneficial to increasing the density of active sites while reducing the mass-transfer efficiency. Inversely, abundant mesopores promote mass transfer, but they result in low active site density. By carefully adjusting the pore structure and chemical composition of the ZIF-derived catalyst, the Co/N/C CL shows a low mass-transfer resistance (95.5 Ω at 0.75 V vs RHE). This work demonstrates the importance of mass transfer within the fuel cell CL, beyond seeking only high activity.

Contact angle hysteresis of a water droplet on a hydrophobic fuel cell surface.

The contact angle hysteresis is an important parameter influencing the micro-size droplet wetting on the solid surface. In this study, a theoretical method for calculating the contact angle hysteresis of a moving water droplet on the gas diffusion layer (GDL) surface in a proton exchange membrane (PEM) fuel cell flow channel is developed through the force analysis. Correlations for the contact angle hysteresis are investigated. It is found that the contact angle hysteresis increases with the sliding angle, the static contact angle and the water droplet size. The contact angle hysteresis is also influenced by the water spreading shape and the distribution of the dynamic contact angle.

Hierarchically Porous Co-N-C Cathode Catalyst Layers for Anion Exchange Membrane Fuel Cells.

As a new class of metal-nitrogen-carbon (M-N-C) material with 3 D microstructure, zeolitic imidazolate frameworks (ZIFs) are used to synthesize highly active electrocatalysts for the oxygen reduction reaction, as substitutes for commercial Pt/C in anion exchange membrane fuel cells. However, to form an effective catalyst layer (CL), the relationship between the microstructure of the ZIF-derived catalyst and the fuel cell performance must be investigated. In this work, a hierarchically porous CL based on the carbon black (CB)-controlled synthesis of a Co-based ZIF (denoted as ZIF-CB-700) is constructed to optimize the triple-phase boundary (TPB) and mass transfer. The power density at 40 °C of ZIF-CB-700 (95.4 mW cm-2 ) as cathode catalyst is about 4 times higher than that of the catalyst synthesized in the absence of CB and is comparable to that of the commercial 60 % Pt/C catalyst (112.0 mW cm-2 ). Both online and offline measurements suggest that the morphology and microstructure of the CL is crucial to form an active TPB region, dominating the fuel cell performance rather than only the high catalyst activity.

Magnetic field alignment of stable proton-conducting channels in an electrolyte membrane.

Proton exchange membranes with short-pathway through-plane orientated proton conductivity are highly desirable for use in proton exchange membrane fuel cells. Magnetic field is utilized to create oriented structure in proton exchange membranes. Previously, this has only been carried out by proton nonconductive metal oxide-based fillers. Here, under a strong magnetic field, a proton-conducting paramagnetic complex based on ferrocyanide-coordinated polymer and phosphotungstic acid is used to prepare composite membranes with highly conductive through-plane-aligned proton channels. Gratifyingly, this strategy simultaneously overcomes the high water-solubility of phosphotungstic acid in composite membranes, thereby preventing its leaching and the subsequent loss of membrane conductivity. The ferrocyanide groups in the coordinated polymer, via redox cycle, can continuously consume free radicals, thus helping to improve the long-term in situ membrane durability. The composite membranes exhibit outstanding proton conductivity, fuel cell performance and durability, compared with other types of hydrocarbon membranes and industry standard Nafion® 212.