Control Of Porous Anodic Aluminum Oxide Pore Structure And Fabrication Of Electrode For Efficient Oxygen Evolution

Electrochemical water splitting as an ideal method to gain hydrogen is promising to cope with the growing energy demands,global climate change and environmental issues,but high overpotential,especially for oxygen evolution reaction?OER?,makes it high cost for hydrogen production.In order to enhance the energy efficiency,developing catalyst and optimizing the electrode structure have been hot research topics over decades.An excellent electrode should provide large specific surface area for catalyst load,have good wettability and expedite bubble departure.In light of past experiences on bionic structures,using well-arranged micro structures can be a reliable strategy to improve the electrode structure.Porous anodic aluminum oxide?AAO?is a typical self-organized porous material,whose structural parameters,such as pore diameter,thickness and interpore distance,can be precisely controlled by adjusting anodizing or annealing conditons.Therefore,AAO is flexible to design and fabricate micro structures,especially for nanowire array.In this paper,the properties and anodization behavior of AAO annealed at various temperatures were studied systematically based on the previous work of our group.A series of AAO with structured pores were designed and dual-diameter nanowire arrays were fabricated for studying OER.The details are as follows:?1?For AAO formed in oxalic and phosphoric acid,though the changes of amorphous feature and pores arranged in hexagon are hardly to be observed after annealed below 630°C,their pore widening rate can decrease significantly to less than 50%with temperature increasing.Anodizing the annealed AAO,the morphological investigation reveals a critical temperature.Below 550°C,AAO grows orderly,however,above 550°C,AAO formed in oxalic acid grows disorderly and AAO grown in phosphoric acid stop to thicken.The sever morphology change of the barrier layer and Al substrate caused by interfacial reaction between them should account for the disordered growth and shutoff of anodization.The critical temperature can be well distinguished from the curve of reciprocal of capacitor vs time?1/C_s-t?acquired via in situ monitoring the capacitor during etching the barrier layer.?2?In terms of fabricating AAO with structured pores and through holes via combination of annealing,anodization and pore widening,the possibility and boundary conditions have been discussed based on time nodes of pore collapse and AAO disappearance during etching an as-prepared AAO.A new method for high efficient fabrication of through-hole AAO is developed,in which the AAO's area can be larger than 29 cm² and Al substrate can be used repeatedly.Various AAO with structured pores were designed and ultilized to fabricate hollow nanocones,core-shell nanocones and multi-diameter namowires.?3?Diameter-adjustable,length-controllable and period-defined Ni nanowire arrays were fabricated with AAO formed in phosphoric acid.At small aspect ratio,nanowires stand upright and their spacing is uniform.At large aspect ratio,nanowires bend and form hierarchical morphology featuring nanowire clusters and microvalleys.The hydrophilicity of Ni nanowire array can be largely improved by increasing the aspect ratio of nanowire or reducing the area percentage of nanowire.The nanowire array with NiFe-based catalysts loaded is able to deliver current density of 500 mA cm^-2 at overpential of 297 mV,far surpassing the Ni foam treated with the same conditions.The OER performance could survive after 24 h test under 500 mA cm^-2.The excellent OER performance of Ni nanowire array not only depends on lager available surface area,but the unique structure of nanowire clusters and microvalleys also contributes a lot,which provides separate bubble departure and electrolyte supply pathways and expels bubble quickly.