OPTIMIZATION OF ANODIZED ALUMINUM OXIDE TEMPLATES FOR ENHANCED HYDROGEN EVOLUTION REACTION EFFICIENCY

ABSTRACT

Anodized Aluminum Oxide (AAO) has emerged as a multifunctional material with significant potential in energy conversion applications, particularly in enhancing the efficiency of hydrogen evolution reactions (HER). This study explores the fabrication and optimization of AAO templates through a two-step electrochemical anodization process, highlighting the impact of barrier layer modification on catalytic performance. The initial anodization in 0.3 M oxalic acid at 50 V resulted in a porous alumina structure, which was subsequently refined through chemical etching using a phosphoric and chromic acid solution to remove the barrier layer. Our findings reveal that the barrier layer obstructs ion and gas diffusion, thereby hindering HER efficiency. Techniques including chemical etching and laser ablation were examined for barrier layer removal, with chemical etching demonstrating effective results. The optimized AAO templates, characterized by an average pore diameter of approximately 100 nm, facilitate the potential creation of high-performance 3D metal nanowires for HER applications. Scanning Electron Microscopy (SEM) analysis confirmed the successful removal of the barrier layer, showing clean hexagonal cell boundaries after 45 minutes of treatment. This research underscores the importance of barrier layer optimization in AAO templates as a crucial step toward the development of efficient catalysts for sustainable hydrogen production technologies.

АННОТАЦИЯ

Анодированный оксид алюминия (АОА) стал многофункциональным материалом, обладающим значительным потенциалом в применении при преобразовании энергии, в частности, в повышении эффективности реакций водородной эволюции (ВЭР). В данном исследовании исследуется изготовление и оптимизация шаблонов ААО путем двухэтапного процесса электрохимической анодизации, выделяется влияние модификации барьерного слоя на каталитические показатели. Первоначальная анодизация в 0,3 М щавелевой кислоте при 50 В привела к образованию пористой алюминиевой структуры, которая в последующем была отшлифована путем химического выщелачивания с использованием раствора фосфорной и хромовой кислот для удаления барьерного слоя. Наши результаты показывают, что барьерный слой препятствует диффузии ионов и газов, тем самым препятствуя их эффективности. Для удаления барьерного слоя были изучены методы, включающие химическую резьбу и лазерную резьбу, при этом химическая резьба показала эффективные результаты. Оптимизированные шаблоны ААО, характеризующиеся средним диаметром пор порядка 100 нм, способствуют созданию потенциальных высокопроизводительных 3D-металлических нанопроводов для применения HER. Анализ сканирующей электронной микроскопии (СЭМ) подтвердил успешное удаление барьерного слоя, который после 45 минут обработки показал чистые гексагональные границы клеток. В данном исследовании подчеркивается важность оптимизации барьерного слоя в образцах ААО как важного шага к разработке эффективных катализаторов для устойчивых технологий производства водорода.

Keywords: AAO template, hydrogen catalyst, barrier layer removal, hydrogen evolution reaction, nanoscale fabrication.

Ключевые слова: шаблон ААО, катализатор водорода, удаление барьерного слоя, реакция водородной эволюции, наномасштабное изготовление.

Introduction. Anodized Aluminum Oxide (AAO) has garnered significant attention as a multifunctional material due to its highly ordered, porous structure and its versatile applications across various fields, including nanofabrication, sensing, filtration, and energy storage. Over the years, researchers have exploited the unique properties of AAO in numerous applications, taking advantage of its tunable pore size, high surface area, and chemical stability [1]. More recently, AAO has emerged as a promising platform for applications in energy conversion, particularly in enhancing the efficiency of hydrogen evolution reactions (HER). The precise control over AAO’s porous structure makes it highly suitable as a template for catalysts, a critical feature for advancing energy-related technologies like hydrogen generation [2]. One of the most critical factors that determine the performance of AAO as a catalyst support for HER is the modification and optimization of its structure. In its unmodified form, AAO possesses a barrier layer that is a byproduct of the anodization process. This barrier layer, while structurally important in certain applications, acts as a significant obstacle in hydrogen evolution reactions [3]. The presence of this insulating layer hampers the free diffusion of ions and gases through the AAO’s pores, limiting the active surface area available for catalysis. Consequently, the removal or thinning of the barrier layer has become a key focus for researchers seeking to maximize the performance of AAO as a catalytic template [4]. The process of forming AAO involves the anodization of aluminum in acidic electrolytes, resulting in a porous structure with a hexagonal arrangement of nanopores [5]. The top layer, known as the porous layer, is essential for applications that rely on the AAO’s high surface area and permeability. However, beneath this porous layer, a barrier layer is formed, consisting of a non-porous, dielectric alumina layer that is several nanometers thick. This barrier layer poses a challenge in applications involving electrochemical reactions, such as HER, where the free flow of ions is crucial for the efficient generation of hydrogen [6]. Therefore, removing or sufficiently thinning this barrier layer is essential to optimize AAO’s performance as a template for hydrogen catalysts [7]. Several methods have been explored to address this challenge. Techniques such as chemical etching, voltage-controlled anodization, and stepwise anodization have been employed to remove or reduce the thickness of the barrier layer [8]. The goal is to produce an AAO template with an open, continuous channel structure that facilitates ion and gas diffusion, enhancing the overall catalytic activity for HER. Moreover, the ease of functionalizing AAO surfaces with various catalytic materials, such as platinum, nickel, or molybdenum, further underscores its potential as a versatile material in hydrogen energy applications [9]. In addition to improving the hydrogen evolution reaction, AAO’s unique properties enable the development of low-cost, scalable methods for producing high-performance catalysts. Its high thermal and mechanical stability, combined with the tunability of its pore structure, allows researchers to design AAO-based systems tailored for specific electrochemical processes [10]. By focusing on optimizing the barrier layer removal, researchers are working to unlock the full potential of AAO as a key component in the drive toward more efficient and sustainable hydrogen production technologies [11,12,13,14,15]. In this study, we synthesized an anodized aluminum oxide (AAO) template to create three-dimensional metal nanowires for hydrogen evolution reactions (HERs). AAO has gained significant attention in recent years as a versatile template for various nanoscale applications. A critical aspect of optimizing AAO templates for HER is the removal of the barrier layer, which is essential for developing efficient hydrogen catalysts. This article explores the key methods and techniques used to remove the barrier layer, emphasizing its significance and discussing the implications for catalytic applications.

Method for synthesizing AAO nanopores. The AAO template was fabricated using a two-step electrochemical anodization process on high-purity aluminum foil. The first anodization step was carried out in a 0.3 M oxalic acid electrolyte solution at a temperature of 5°C and a DC voltage of 50 V for 15 minutes. The resulting anodic alumina layer was then removed by chemical etching in a mixture of diluted phosphoric acid (6 wt.% H₃PO₄) and chromic acid solution (1.8 wt.% CrO₃) for 5 hours at 60℃.

The second anodization step was carried out under the same conditions as the first step, with different time durations of 4, 8, 16, and 24 hours. The resulting AAO templates examined using field emission scanning electron microscopy.(Figure 1) It shows the longer anodization time led to the formation of distorted pores in some areas and the creation of new smaller nanopores within the previously formed nanopores. The average pore diameter of the AAO template was observed to be approximately 100 nm. This AAO template can potentially be used for the creation of 3D metal nanowires for HERs by depositing metal ions into the pores of the AAO template and then reducing them to form metallic nanowires. The highly ordered and uniform pore structure of the AAO template can facilitate the creation of nanocatalysts with high catalytic activity and stability. Overall, this study demonstrates a promising method for the synthesis of AAO templates for the creation of 3D metal nanowires as a catalyst for HERs.

Methods of Barrier Layer Removal: Chemical Etching: Chemical etching is one of the most common methods used to remove the barrier layer. Acidic or alkaline solutions are employed to dissolve the aluminum oxide, selectively leaving behind a porous structure without the barrier layer. The choice of etching solution and conditions can be tailored to obtain specific pore sizes and structures, enhancing the AAO's catalytic performance.

Electrochemical Methods: Electrochemical techniques involve applying an electric field to the AAO template, which can facilitate the controlled removal of the barrier layer. Anodic or cathodic dissolution methods can be employed to tailor the pore diameter and distribution in the AAO template, making it suitable for hydrogen catalyst applications.

Laser Ablation: Laser ablation is a precise and efficient technique for removing the barrier layer in AAO templates. High-energy laser pulses are focused on the AAO surface, vaporizing the barrier layer and creating well-defined pores for improved catalytic performance.

Significance in Hydrogen Catalyst Applications: The removal of the barrier layer in AAO templates is crucial for hydrogen catalyst applications due to its direct impact on the efficiency of hydrogen evolution reactions. The absence of the barrier layer allows for enhanced mass transport of ions and gases, improving the kinetics of the catalytic reaction. This results in reduced overpotentials and higher catalytic activity, making AAO templates an attractive choice for green hydrogen production and storage.

In this work, we used a chemical method to remove the barrier layer. It was done by applying 10 wt% H3PO₄ to open the barrier layer. As depicted in the SEM micrographs, the bottom surface of the prepared AAO is characterized by hexagonal close-packed arrays of hemispherical domes. According to the results presented in the paper, the barrier oxide was gradually etched away from the entire surface of each dome, revealing relatively clean hexagonal cell boundaries. The removal of the barrier layer of AAO was carried out at different times. The opening of the barrier layer occurs in the very center of the hemispherical dome. The barrier oxide layer was completely removed after wet chemical treatment for 45 minutes. (Figure 2)

Figure 2. SEM images of remove the barrier layer of AAO. a) Cross-sectional SEM images of AAO, b) bottom view before chemical etching, c) bottom view at 5 minutes, d) bottom view at 30 minutes, e) bottom view at 40 minutes, (process performed in 10% H₃PO₄ solution)

Conclusion. This study demonstrates the significant potential of Anodized Aluminum Oxide (AAO) as a versatile template for enhancing the efficiency of hydrogen evolution reactions (HER). Through a systematic two-step electrochemical anodization process and subsequent barrier layer removal using chemical etching, we have successfully optimized the structure of AAO to maximize its catalytic performance. The removal of the insulating barrier layer is essential for improving ion and gas diffusion, which directly impacts the kinetics of the catalytic reaction and overall hydrogen production efficiency. The resulting AAO templates, characterized by well-defined nanopore structures and high surface areas, present exciting opportunities for the development of 3D metal nanowires as advanced catalysts for HER applications. Furthermore, the findings highlight the importance of tailoring fabrication techniques and conditions to achieve specific pore sizes and distributions, thus enhancing the AAO's applicability in energy-related technologies. Future work should focus on further refining the AAO templates and exploring alternative methods for barrier layer removal to further enhance catalytic performance. Overall, this research contributes to the growing body of knowledge on AAO and its role in advancing sustainable hydrogen production technologies, paving the way for innovative solutions in the field of energy conversion and storage.

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