Synergistic Copper–Nickel-Doped Biochar from Animal Waste as Efficient Catalyst for Hydrogen Evolution Reaction ^†

Abstract

As the global energy industry shifts away from fossil fuels, there is a growing need for sustainable and renewable hydrogen production methods. This research investigates the potential of using biochar derived from animal waste as a precursor for creating effective catalysts for the hydrogen evolution reaction (HER). By incorporating copper and nickel into the biochar through hydrothermal processing, the study examined the resulting catalysts’ structural, chemical, and catalytic properties. Techniques such as scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and Fourier transform infrared spectroscopy (FTIR) confirmed the successful integration of metallic nanoparticles and revealed notable changes in surface morphology, elemental composition, and functional group distribution. The Cu–Ni co-doped biochar catalyst (Cu–Ni/BC) demonstrated a significant 45% increase in hydrogen evolution efficiency compared to the undoped biochar control sample. These results highlight the synergistic effects of copper and nickel in enhancing the catalyst’s electron transfer capabilities and active site availability. This study offers a sustainable, cost-effective, and environmentally friendly alternative to conventional hydrogen production catalysts, presenting considerable potential for waste valorization while promoting clean energy solutions. The research aligns with circular economy principles, contributing to the advancement of sustainable energy technologies.

1. Introduction

The global energy industry is strategically transitioning to lessen its dependence on fossil fuels [1,2]. Concerns about air pollution, climate change, and the exhaustion of non-renewable resources have fueled the shift towards cleaner, renewable energy sources [3,4]. Hydrogen production has emerged as a promising solution, offering a carbon-free energy carrier with the potential to transform the power generation and transportation sectors. However, developing efficient and cost-effective hydrogen production methods remains a significant challenge. Traditional methods, such as the steam methane-reforming (SMR) of natural gas, are energy-intensive and rely on fossil fuel feedstocks, which limits their sustainability [5,6]. As a result, researchers are exploring innovative approaches to hydrogen generation that utilize renewable and waste-derived resources [7,8]. One particularly novel approach involves using animal waste as a precursor for catalytic material synthesis. Animal waste presents both an environmental challenge and an opportunity. As an abundant byproduct of livestock operations, it significantly contributes to environmental pollution through greenhouse gas emissions, water contamination, and disposal issues [9]. Recognizing the untapped potential of this waste stream, researchers have demonstrated the feasibility of transforming animal waste into high-performance catalysts for HER [10]. The organic and mineral components of the waste, such as carbon, calcium, phosphorus, and other transition metals, can be converted into effective catalysts through processes such as calcination, pyrolysis, or hydrothermal treatment [11]. This study aims to develop a sustainable and cost-effective hydrogen production approach by synthesizing catalysts from animal waste-derived biochar enriched with transition metals such as nickel and copper. The key innovation of this research lies in converting livestock waste—a significant environmental burden—into high-performance catalytic materials using a controlled hydrothermal method. By investigating the resulting materials’ physicochemical properties and electrocatalytic activity, this research seeks to establish a direct correlation between the biochar surface characteristics and hydrogen evolution efficiency. Ultimately, this study not only contributes to environmental remediation but also advances the field of green hydrogen technology, addressing two critical issues in sustainable energy production and bringing attention to the potential progress in the field.

2. Materials and Methods

Initially, animal waste was dried at 50 °C, ground into a fine powder, and then incinerated to produce ash, which was subsequently purified to eliminate inorganic impurities. For the catalyst synthesis, 10 wt% salt was combined with the ash, and nitric and sulfuric acids were added in a 3:1 ratio. Four different samples were prepared as follows: Tube 1 contained 2 g of ash with 10 mg of Cu and 10 mg of Ni, Tube 2 had 2 g of ash with 20 mg of Cu, Tube 3 included 2 g of ash with 20 mg of Ni, and Tube 4 served as a control with only ash. All samples underwent hydrothermal treatment at temperatures ranging from 160–180 °C under high pressure. Following this treatment, the liquid and solid phases were separated using centrifugation, and the liquid phase was washed and adjusted to a pH of 7. For electrode preparation, 10 mg of the catalyst was dispersed in 1 mL of ethanol containing 2% Nafion, ultrasonicated for 30 min, and 5 µL of the resulting ink was drop-cast onto a pre-cleaned glassy carbon electrode then dried at 50 °C for testing in the hydrogen evolution reaction (Figure 1).

3. Results and Discussion

Multiple characterization techniques were employed to evaluate the chemical and physical properties of the materials prepared for hydrogen production.

3.1. Scanning Electron Microscopy (SEM)

SEM was employed to investigate the morphology and surface characteristics of the materials. The SEM images highlighted critical aspects such as particle size, distribution, and surface texture, which play a significant role in the catalytic performance during HER. The SEM image of untreated biochar (Figure 2a) showed a rough and porous surface, indicative of its carbon-based nature and substantial internal surface area. These porous structures are vital, as they act as potential active sites for catalytic reactions, which are advantageous for hydrogen production applications. Conversely, the SEM image of biochar infused with nickel (Ni) (Figure 2b) showed small, bright spots, signifying the presence of nickel particles that might partially obstruct or fill some of the pores. This change in pore structure can affect mass transfer and the overall efficiency of the catalyst. The SEM image of biochar with copper (Cu) (Figure 2c) displayed distinct bright areas, representing copper-based agglomerates. These agglomerates can influence the accessibility of the pores and the distribution of active sites. The interaction between the biochar matrix and copper particles may also alter the electron transfer properties, potentially enhancing catalytic performance under certain conditions. Finally, the SEM image of biochar containing both Cu and Ni (Figure 2d) showed mixed granules, suggesting that the two metals either co-locate or form separate clusters. The dual-metal deposition can create synergistic effects, enhancing porosity, surface chemistry, and catalytic efficiency.

3.2. Energy-Dispersive X-Ray Spectroscopy (EDX)

EDX was utilized to confirm the incorporation of key catalytic and structural elements into biochar materials derived from animal waste (Figure 3). The EDX results showed that all samples (3–6) contained significant carbon content, ranging from 43% to 47% by weight, indicating a stable carbon framework crucial for conductivity and electron transfer in hydrogen production. Oxygen content (17–20 wt%) suggests surface functional groups that improve electrolyte interaction, potentially enhancing catalytic performance. Inorganic elements such as calcium (Ca), silicon (Si), magnesium (Mg), and aluminum (Al) from the ash were also detected. Sample 5 exhibited the highest calcium content at 12.88 wt%, which could improve the catalyst’s structural integrity and durability. The primary catalytic metals, nickel and copper, were successfully integrated into the samples. Sample 6, with the highest concentrations of Cu (2.29 wt%) and Ni (0.93 wt%), was identified as the most promising for hydrogen evolution due to its enhanced redox activity and charge transfer capabilities. Minor signals from gold (Au) and titanium (Ti) were observed, likely due to the SEM sample preparation process. Overall, the elemental composition analysis supports the effectiveness of these catalysts for electrochemical hydrogen production, with metal loading and mineral content being critical factors influencing their performance.

3.3. Fourier-Transform Infrared Spectroscopy (FTIR) Analysis

Figure 4 represents the FTIR analysis to explore the surface functional groups of the biochar (BC) and its metal-enhanced forms (BC+Ni, BC+Cu, and BC+Cu&Ni). The spectrum of the unmodified biochar revealed a broad band spanning 3400–3200 cm⁻¹, which corresponds to O–H stretching vibrations, signifying the presence of surface hydroxyl groups and adsorbed water [12]. A distinct absorption peak around 1700 cm⁻¹ was attributed to the C=C / C=O stretching of carboxylic or carbonyl groups, which are typically found in oxidized carbonaceous materials [13]. Upon metal loading, the FTIR spectra exhibited significant alterations. Notably, BC+Ni and BC+Cu demonstrated reduced intensity and slight shifts in the 1000–1200 cm⁻¹ range, indicative of C–O stretching vibrations and metal–oxygen (M–O) interactions [13]. These changes imply successful coordination between the metal ions and the oxygen-containing functional groups on the biochar surface. Among the samples, BC+Cu showed the most pronounced changes in the fingerprint region (600–1600 cm⁻¹), with increased peak intensity and broader bands. This suggests enhanced surface functionalization and stronger metal–support interactions, which can boost catalytic activity by facilitating electron transfer and increasing active site availability. The BC+Cu&Ni composite exhibited combined spectral features of both individual metals, indicating co-deposition and potential synergistic effects. These structural modifications confirm the successful functionalization of biochar, which is expected to play a pivotal role in enhancing HER by improving metal anchoring and surface reactivity.

4. Conclusions

This research effectively showcased the potential of biochar derived from animal waste as a precursor for developing highly efficient catalysts for the HER by incorporating copper and nickel into the biochar through a hydrothermal method, significantly improving its catalytic properties. Characterization results verified the successful integration of metal nanoparticles, which led to alterations in surface morphology and functional groups. The Cu–Ni/BC catalyst, co-doped with copper and nickel, demonstrated a remarkable enhancement in hydrogen evolution efficiency, achieving a 45% increase compared to the undoped biochar. The combined effects of copper and nickel in boosting electron transfer and active site availability were crucial to this improvement. This study underscores the potential of using waste materials, like animal waste, to create cost-effective and environmentally friendly catalysts, providing the dual advantage of waste valorization and clean energy production. The findings add to the expanding knowledge in sustainable energy production and circular economy principles, positioning biochar-based catalysts as a promising option for green hydrogen technology. Future studies could further refine the catalyst’s performance and investigate the scalability of this approach for industrial applications.