Solar hydrogen production, considered a key solution for harnessing solar energy, faces challenges. Water oxidation, a crucial half-reaction, is slow and energy-intensive, requiring a high overpotential even with expensive catalysts. Oxygen production also introduces safety and separation issues. Alternatives to water as an electron source have been explored, with biomass presenting an attractive option due to abundance and cost-effectiveness. However, using highly processed, expensive biomass-derived chemicals raises ethical and economic concerns. Lignocellulosic (LC) biomass offers a practical alternative, but its low solubility and complexity have hindered its use in solar hydrogen production. Conventional inorganic photocatalysts are limited by their large bandgap and poor optoelectronic properties, restricting efficient solar flux utilization and often requiring external bias. Lead halide perovskites, with their tunable bandgap and high absorption coefficient, present advantages, but their vulnerability to water has limited their application in biomass-coupled solar hydrogen production. This study addresses these limitations by combining a perovskite-based photocathode with LC biomass to achieve high-efficiency, bias-free solar hydrogen generation.

Extensive research has been dedicated to improving solar hydrogen production. The slow kinetics of the water oxidation half-reaction and the high overpotentials required are frequently cited challenges. Researchers have explored various photocatalysts such as TiO₂, WO₃, and BiVO₄, but their limitations in light absorption and charge transport have hampered progress. The use of biomass as an electron source has gained attention, with studies exploring glucose and other alcohols. However, these studies often utilize costly and processed biomass components, leading to questions about economic feasibility and environmental impact. While perovskite materials have emerged as promising candidates for solar energy applications due to their tunable bandgap and high absorption coefficient, their use in photoelectrochemical cells for hydrogen production, particularly in conjunction with biomass, has been limited due to their water sensitivity. This research aims to overcome these limitations using lignocellulosic biomass and perovskite photocathodes.

The study employed a photoelectrochemical (PEC) cell combining a perovskite-based photocathode and LC biomass. Phosphomolybdic acid (PMA) acted as a soluble catalyst and electron/proton mediator to extract electrons from the solid LC biomass. The perovskite photocathode utilized Cs₀.₀₅(FA₀.₈₃MA₀.₁₇)₀.₉₅(Pb₀.₈₃Br₀.₁₇)₃ with a bandgap of 1.61 eV. Field's metal (FM) and Ti foil were used for electrical connection and stability in acidic conditions. Pt nanoparticles facilitated hydrogen reduction. The electron extraction from biomass was investigated using various temperatures and biomass components (lignin, cellulose, hemicellulose). The selective depolymerization of lignin, producing value-added compounds like vanillin and acetovanillone, was confirmed through spectroscopic analysis and chromatography. Electrochemical characterization included linear sweep voltammetry (LSV) and chronoamperometry (CA) to evaluate electron extraction efficiency. The PEC cell performance was evaluated under simulated AM 1.5 G one-sun illumination, measuring current density, Faradaic efficiency, and stability over time. Incident photon-to-electron conversion efficiency (IPCE) was determined to assess the spectral response of the system. Various analytical techniques such as UV-Vis spectroscopy, FTIR, NMR, GC-MS, SEM, and TEM were utilized to characterize the materials and reaction products.

The researchers achieved a record-high photocurrent density of 19.8 mA cm⁻² for hydrogen production in their bias-free PEC cell. This was significantly higher than previously reported values for single-photoelectrode systems. Near-unity Faradaic efficiency for hydrogen generation was observed, indicating minimal energy loss. The system demonstrated excellent stability, operating for over 20 hours with only a minor performance degradation. Lignin was selectively depolymerized to produce vanillin and acetovanillone, adding economic value to the process and validating the strategy's potential for both solar hydrogen production and biomass valorization. The activation energy for PMA reduction by lignin was determined to be significantly lower than that of cellulose and hemicellulose, confirming the preferential oxidation of lignin in the LC biomass. Electrochemical analysis showed that reduced PMA facilitated efficient electron extraction at a potential significantly lower than water oxidation. The perovskite photocathode design, incorporating FM and Ti layers, significantly enhanced stability. The IPCE spectrum showed panchromatic solar hydrogen production across the visible light spectrum.

The high photocurrent density and near-unity Faradaic efficiency achieved in this study significantly advance the field of solar hydrogen production. The bias-free operation eliminates the need for external voltage, leading to increased energy efficiency. The use of readily available and inexpensive LC biomass as a sustainable electron source makes the system more economically viable. The simultaneous production of value-added chemicals from lignin further enhances the economic attractiveness and environmental sustainability. The strategy of using PMA as an electron mediator enables the use of various forms of solid biomass, expanding its applicability. The excellent stability of the PEC cell under continuous illumination underscores the system's practical potential. Further research could focus on optimizing the perovskite material and exploring other types of biomass for broader applicability and improved performance.

This research demonstrates a highly efficient and stable bias-free PEC cell for solar hydrogen production using a perovskite photocathode and lignocellulosic biomass. The record-high photocurrent density of 19.8 mA cm⁻², coupled with near-unity Faradaic efficiency and long-term stability, showcases the system's potential. The simultaneous production of valuable chemicals from lignin depolymerization adds significant economic and environmental benefits. Future research could explore different perovskite compositions, investigate alternative biomass sources, and optimize the system for larger-scale applications.

While the system demonstrated remarkable performance and stability, further improvements could be pursued. The long-term stability of the perovskite photocathode, although improved in this study, could still be further enhanced. Exploring alternative protective layers and catalyst materials may extend the operational lifetime. The selective depolymerization of lignin, while efficient, may not be entirely complete, suggesting further optimization of the PMA-mediated process could increase the yield of value-added chemicals. The scalability of the system for large-scale hydrogen production requires further investigation.