Factorial optimisation of CoCuFe- LDH/graphene composites for water splitting

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The increasing energy demand, driven by industrialization and global population growth, has increased the focus on developing clean and sustainable energy sources [1]. Hydrogen (H2) has emerged as a promising alternative to carbon-based fuels due to its low cost, higher calorific value, and absence of pollution emissions [2]. Electrochemical water splitting has recently gained prominence as a highly attractive technique for efficient hydrogen production [3]. Therefore, the development of efficient and cost-effective catalysts for the hydrogen evolution reaction (HER) is crucial for advancing energy conversion and storage technologies [2]. The layered double hydroxides (LDHs) have emerged as promising non-noble metal catalysts for HER due to their unique composition and structural features, making them efficient and stable catalysts [4]. Indeed, copper (Cu)-based materials exhibit high electrical conductivity, the CuFe-LDH has been reported as an interesting electrocatalyst since iron (Fe) can enhance the reactivity in the HER process [5]. Likewise, cobalt (Co)-based catalysts emerge as potential alternatives to noble-metal catalysts in water-splitting applications, attributed to the high redox potential of Co species, cost-effectiveness, and stability in both acidic and basic environments [6]. Moreover, in binary LDHs, introducing a third metal ion has been shown to change the electronic structure and improve conductivity; hence, providing more active sites and facilitating a fast electron transfer process [7]. However, the LDHs as electrocatalysts are limited by their poor electronic conductivity and tendency for agglomeration. On the other hand, graphene (G), a single layer of carbon atoms arranged in a hexagonal lattice, serves as an excellent supporting matrix for LDHs due to its exceptional electronic conductivity and high surface area, facilitating efficient electron transfer during the water-splitting reaction [8]. Therefore, in this work, CoCuFe-LDH composites were synthesised and grown on graphene through a cost-effective and straightforward one-step hydrothermal process. Experiments were conducted to assess the electrocatalytic properties of the trimetallic CoCuFe-LDH/G and its binary counterparts to investigate how each component affected the electrochemical performance and HER activity. The onset potential and Tafel slope were selected as the basis for characterising the catalytic performance of the materials. The linear sweep voltammetry (LSV) curves of CoCuFe-LDH/G composites for HER are shown in Figure 1. It can be observed that the trimetallic Co[1.5]Cu[3]Fe[3]-LDH/G[10] shows the lowest onset potential at -0.39 V with a Tafel slope of 76.58 mV dec-1, compared to its binary counterparts. While CuFe-LDH/G shows the highest onset potential (-0.52 V) and Tafel slope (115.56 mVdec-1), indicating that Co plays an important role in the HER performance. Furthermore, the trimetallic composite demonstrated favourable electronic properties, with a charge transfer resistance (RCT) of 486.6 Ω, and exhibited good stability without significant loss of catalytic activity over 24 h. Therefore, this study provides a facile and efficient strategy to design a trimetallic LDH electrocatalyst combined with graphene, demonstrating that the conductive nanoflake structure established by graphene provides a sufficient electron supply to the composite during the electrocatalytic process, enhancing the HER activity.