Enhancement of electrocatalytic oxygen evolution performance for FeCrNiCoTi alloys via powder modification

To address the issues of limited reserves and environmental pollution associated with traditional carbon-based fossil fuels, and in response to the dual carbon goals of "peak carbon" and "carbon neutrality", the search for green, clean, and sustainable energy sources is imperative. Hydrogen energy is considered a viable alternative energy source due to its high calorific value and abundant production capacity. Consequently, extensive scientific research have been focused on the efficient hydrogen production technologies, such as methane catalytic reforming, coal gasification, and electrolysis of water [1], [2]. Among them, the former two methods involve the use of fossil fuels, which do not meet the standards for green hydrogen production. In contrast, electrolysis of water can convert electrical energy into hydrogen energy through wind power, hydropower, and other means of electricity generation, without the involvement of fossil fuels, making it a green method for hydrogen production [3]. The electrolysis of water consists of two half-reactions, i.e., oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), in which OER is a complex four-electron process and constrains the overall rate of the water-splitting reaction. It thus is of necessity for the utilization of highly active catalysts to enhance OER reaction rate and hydrogen production efficiency [4]. Currently, Ir- and Ru-based materials exhibit good catalytic performance in the OER process [5], but their scarcity and high cost limit their own large-scale industrial production [6]. Transition metals possess partially filled d orbitals similar to precious metals, making them ideal candidates as substitutes for precious metal catalysts. In the search for suitable catalysts, precious metals are often combined with transition metals to form binary alloys. However, traditional binary alloys tend to produce brittle intermetallic compounds, resulting in poor corrosion resistance and structural stability [7]. Under this context, there is an urgent need to develop novel, efficient, and low-cost electrocatalysts that are either free of or contain low amounts of precious metals while maintaining high catalytic activity and stability.

Multi-principal entropy alloys (MPEAs), including medium-entropy alloys (MEAs) and high-entropy alloys (HEAs), are promising metallic materials for the novel OER catalysts [8]. Compared to traditional mono or binary alloys, MPEAs exhibit four distinct effects: high-entropy effect, lattice distortion effect, sluggish diffusion effect and cocktail effect. Here, the high-entropy effect facilitates the acquisition of high thermal stability solid solutions and nanostructures, even amorphous structures [9], which are advantageous for catalytic performance and stability. The lattice distortion effect generates more micro-defects, increasing the number of active catalytic sites [10]. Additionally, the inherent surface complexity of MPEAs provides nearly continuous adsorption energy distribution on surface sites, maximizing the proportion of active sites and accordingly enhancing catalytic performance [11]. As a result, MPEAs have emerged as a focal point in the field of electrolytic water catalysis.

Given that the electrocatalysis of water electrolysis occurs on the electrode surface, the specific surface area and surface structure of catalysts greatly influence catalytic activity [12]. Generally, bulk alloys typically possess low specific surface areas, poor pore structures and inert surfaces, failing to meet the requirements of highly active catalysts [13]; whereas nanomaterials have high specific surface area and sufficient active sites, which is suitable for efficient catalysis [14]. The catalyst size modification has proven to be an economically effective approach for enhancing the efficiency of water electrolysis reaction [15]. This is often implemented through mechanical ball milling, which as a low-cost and simple operation technique can reduce particle size, increase specific surface area, adjust crystallinity, and introduce defects, thereby significantly improving the catalytic performance of milled powders [16], [17]. For instance, Dai et al. [18] utilized mechanical ball milling to prepare a MnFeCoNi electrode based on carbon fiber paper for OER, achieving an overpotential of 302 mV at 10 mA/cm² and a tafel slope of 83.7 mV/dec. Similarly, Zhang et al. [19] synthesized a (CoNiMnZnFe)₃O_3.2 high-entropy oxide via mechanical alloying, exhibiting good OER activity with an overpotential of 336 mV at 10 mA/cm² and a Tafel slope of 47.5 mV/dec. After a 20-h chronoamperometry test, it retained 86.9 % of its catalytic activity. Also in both cases, ones can find that transition metals of Fe, Ni and Co are the commonly used components in high-entropy materials, due to their relatively abundant reserves on Earth and similar atomic radius [20]. The latter factor is a prerequisite for the formation of stable MPEAs [21]. Importantly, three metals are easily transformed into their respective hydroxyl oxide under alkaline conditions, providing intermediates and active sites for OER [22], and their catalytic effects are considered equivalent to that of precious metal oxides [23]. Furthermore, such sites and intermediates as well can be optimized by altering electronic structure after the introduction of other metals, as inferred from the facts that the overpotentials of NiFe- or CoNi-based OER catalysts had respectively been reduced by adding Cr and Ti [24], [25]. Specially, the synergistic interactions among different metal elements in MPEAs contribute to the enhancement of electrocatalytic performance [26]. On basis of the aforementioned descriptions, the present work selects Fe, Cr, Ni, Co, and Ti as the elemental composition of MPEAs for OER catalyst prepared by long-term mechanical ball milling technique [27]. The 80 h-milled FeCrNiCoTi MPEAs exhibit high catalytic activity and good durability towards OER, surpassing the catalytic performance of commercially available RuO₂ catalysts.