Hydrogen production from wastewater via Pt-free cathodes: a sustainable paradigm towards carbon-neutral energy

Introduction In modern times, shifting towards carbon-neutral energy production is crucial for tackling climate change and ensuring a reliable energy supply for future generations. Hydrogen (H₂) stands out as a promising energy carrier due to its high energy density and versatility, and its market is projected to expand rapidly. However, actually, only about 4% of produced hydrogen comes from renewable sources such as electrochemical water splitting [1]. This limitation is mainly due to the high overpotential needed for the Oxygen Evolution Reaction (OER), which hampers the efficiency of hydrogen generation, along with the prevalent use of noble metal-based electrodes that are expensive limiting their use. To address this issue, wastewater, typically characterized by high levels of COD (Chemical Oxygen Demand), can act as an effective electron donor during the anodic reaction, significantly lowering the oxidation potential compared to traditional OER methods. In this scenario, process intensification can offer a crucial contribution to overcome these challenges, aligning with the United Nations’ 2030 Agenda for Sustainable Development. By focusing on key objectives, such as clean water and sanitation (Goal 6) and affordable and clean energy (Goal 7), the electrochemical hydrogen production from wastewater, showcases its potential to integrate energy generation with environmental remediation [2]. Building on these concepts, this research explores the use of a noble metal-free cathode made from an electrodeposited composite of cobalt phosphide (CoP) and elemental phosphorus (P) for hydrogen generation from wastewater. This approach exemplifies process intensification by enabling efficient hydrogen production from simulated wastewater, thereby contributing to a sustainable circular economy while promoting advancements in clean energy technologies. Methods The CoP/P electrode was fabricated according to Falletta and Bernasconi et al. [3] and the synthetic process is schematized in Figure 1A. Electrochemical experiments were conducted in a 100 mL single-chamber cell featuring a three-electrode setup using a Pt foil anode, a CoP/P cathode, and a Standard Calomel Electrode (SCE) as the reference. A 60 mL solution of Rhodamine B (5 mg/L) was used as simulated wastewater, adding 0.5 M NaOH or H2SO4 as the supporting electrolyte. To ensure inert environment, nitrogen was bubbled into the cell prior to starting the experiments. The reactions were performed for 2 hours at a constant potential. The Rhodamine B degradation was evaluated using UV-vis spectroscopy selecting the wavelength at 554 nm, while hydrogen production was assessed at the midpoint and conclusion of the test by collecting gas samples for analysis with a GC equipped with aTCD detector. Results and discussion The morphology of the investigated electrode was evaluated through Scanning electron microscopy (SEM) revealing that CoP matrix alone (Figure 1B, CoP/P) is characterized by low-roughness, wherease the addition of red phosphorus particles drastically alters the material's morphology. The presence of P microparticles was further confirmed by the EDS elemental mapping of Co and P. Additionally, XRD analysis (data not shown) revealed that the deposited material, i.e., CoP/P, appears to be amorphous wherease, after annealing the level of crystallinity increases. The tests of hydrogen generation and RhB electrooxidation were performed at a fixed cathodic potential. Preliminary, electrochemical investigation were carried out through Linear Sweep Voltammetry (LSV) in the cathodic region. These studies helped determining the optimal working potential for efficient hydrogen generation showing that in acidic conditions (0.5 M H₂SO₄) a cathodic potential of -1 V vs SCE was sufficient to produce sufficient current, while in an alkaline environment (0.5 M NaOH) a more negative potential (-2 V vs SCE) was required due to the lower mobility of OH⁻ ions. Based on these findings, the electrochemical treatment of the simulated wastewater was carried out at the potentials identified. Figure 1C highlights RhB electrochemical degradation in both acidic and alkaline environments, revealing a higher RhB removal efficiency in the alkaline setting likely due to the higher anodic potential observed in alkaline conditions (+1.9 V compared to +1.6 V in acid), which promotes indirect RhB oxidation via the generation of transient radicals. Additionally, hydrogen generation (Figure 1D) tests indicated that a greater hydrogen yield was achieved in the alkaline environment, compared to the acidic conditions. Further tests are underway in quasi-real conditions to investigate the impact of competing species on both processes in more complex water matrices. Conclusions In the present work it was demonstrated that in the wiev of process intensification, the electrochemical treatment of wastewater using Pt-free electrodes offers a promising and sustainable approach for achieving both clean water recovery and hydrogen production, contributing to a greener and more resource-efficient future.