PEC water splitting is a process that uses sunlight and semiconductor materials to split water molecules into hydrogen and oxygen through electrochemical reactions. This technology directly converts solar energy into chemical energy stored in hydrogen, offering a clean and sustainable method for hydrogen production without greenhouse gas emissions.1
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PEC water splitting systems integrate light absorption and electrochemical catalysis in a single device, providing a cost-effective and environmentally friendly alternative to conventional hydrogen production methods like steam methane reforming.
By utilizing abundant solar energy, PEC systems can produce green hydrogen with minimal greenhouse gas emissions, supporting global efforts to transition to a low-carbon energy landscape and establish hydrogen as a versatile energy carrier for power generation, transportation, and industrial processes.1,3
How PEC Water Splitting Works
PEC water splitting combines light absorption, charge generation, and catalytic reactions to drive the water-splitting process.4 The system relies on two key components: a photoanode and a photocathode, both made from semiconducting materials.
The photoanode, typically made of an n-type semiconductor, plays a critical role in the oxygen evolution reaction (OER).4 When sunlight irradiates the photoanode with energy equal to or exceeding its band gap, electrons are excited from the valence band to the conduction band. This excitation creates electron-hole pairs, where the photogenerated holes accumulate on the photoanode's surface and drive the OER. Water molecules react with these holes, producing oxygen gas and protons:
H2O+ + 2h+ → 2H+ + O2
The photocathode, on the other hand, facilitates the hydrogen evolution reaction (HER). The excited electrons from the photoanode are transported to the cathode, where they combine with protons from the electrolyte to produce hydrogen gas:
2H+ + 2e− → H2
This electron transport between the photoelectrodes is essential for completing the circuit and sustaining the redox reactions. The efficiency of PEC water splitting depends on several factors, including the ability of the semiconductor material to absorb sunlight, resist corrosion, and maintain effective charge separation to reduce electron-hole recombination.2,4
A More Efficient Method for Harvesting Hydrogen
Materials Used in PEC Water Splitting
The efficiency and stability of PEC water splitting are strongly influenced by the materials used for photoelectrodes and catalysts.
Photoanodes
Photoanodes are key components in PEC cells, responsible for driving the OER. Traditional materials like planar metal oxides, such as titanium dioxide (TiO₂) and hematite (Fe₂O₃), have been extensively used due to their stability and photochemical properties.5
Recent advances in nanostructured materials, including chalcogenides like CdS and Sb₂S₃, and nitrides, have significantly enhanced the performance of photoanodes. Nanostructures, such as nanorods, nanocrystals, and nanotubes, provide a larger surface area for redox reactions, reduce the recombination of photoexcited electrons and holes, and improve light absorption.5
Photocathodes
Photocathodes facilitate the HER by reducing protons to hydrogen gas. Materials such as silicon and transition metal dichalcogenides, including MoS2, MoSe2, WS2, and WSe2, have emerged as excellent candidates for photocathodes. These materials offer superior charge transfer properties and compatibility with PEC systems.
The use of nanostructures further enhances charge separation and electron transport, thereby reducing losses due to recombination. These advancements have enabled more efficient hydrogen generation under solar irradiation.6
Catalysts
Catalysts play an important role in enhancing the efficiency of both OER and HER by lowering the activation energy required for these reactions. Platinum remains the benchmark catalyst for HER due to its exceptional electrocatalytic activity, while cobalt- and nickel-based materials are commonly used for OER, offering a cost-effective alternative.7
Integration of these catalysts with nanostructured photoelectrodes further improves the overall performance of PEC systems. For example, Popczun et al. demonstrated that Ni2P nanoparticles exhibit high HER activity among non-noble metal electrocatalysts, achieving nearly quantitative faradaic yield for H2 generation in acidic media.8
Advantages of PEC Water Splitting
PEC water splitting offers significant advantages, making it a promising technology for sustainable hydrogen production. One primary benefit is the direct utilization of sunlight as the energy source, allowing the conversion of solar energy into
storable chemical energy in the form of hydrogen. This eliminates the need for external circuits or fuel-based energy sources, making the process simple and highly efficient.3
Another major advantage of PEC water splitting is environmental sustainability. The process generates clean hydrogen fuel without producing greenhouse gases or toxic by-products, positioning it as an eco-friendly alternative to thermocatalysis and high-temperature electrolysis.3 By generating solar fuel through a simple, scalable setup, PEC water splitting contributes to the development of green technologies while addressing the challenges of energy storage and the intermittency of solar power.
Challenges with PEC Water Splitting
Despite its potential, PEC water splitting faces several challenges that must be overcome for widespread adoption.
One key issue is the degradation of photocatalytic materials under prolonged exposure to sunlight and water. This degradation reduces the durability and efficiency of the system, as the materials cannot maintain their structural integrity and functionality over time. Advanced designs, such as Z-scheme configurations and photocatalytic membrane reactors, aim to address these issues, but they often increase complexity and costs, limiting their scalability.3,9
Another major challenge is the low quantum efficiency of PEC systems. Photogenerated electron-hole pairs often recombine before they can contribute to the redox reactions required for water splitting, resulting in a significant loss of potential efficiency. The timescale for recombination (nanoseconds) typically dominates over the reaction timescale (milliseconds). Strategies to extend the lifetime of charge carriers, such as the addition of co-catalysts, particle size reduction, or the use of 2D materials, have shown promise.3, 10
The high costs of catalysts and advanced materials also hinder the economic viability of PEC water splitting. Precious metals like platinum, often used to enhance HER, are expensive and scarce.
Additionally, fabricating high-performance semiconductor materials at a large scale remains a challenge. Emerging technologies, such as three-dimensional (3D) printing of hierarchical photocatalytic structures, offer potential for scalability but require further optimization to balance cost and efficiency.3
The Future of Solar-Powered Hydrogen Production
Recent research has focused on addressing the challenges of PEC water splitting to improve its viability and efficiency. Nanostructured photocatalysts have transformed PEC water splitting by increasing light absorption, stability, and electron-hole separation while reducing recombination losses. Their increased surface area and tunable band gaps optimize photon capture and align redox potentials for efficient water splitting.3,9
Tandem cells, which combine two semiconductors with complementary absorption ranges, improve solar-to-hydrogen efficiency by utilizing a broader spectrum of sunlight. This design enhances charge separation and enables efficient redox reactions through two-step photon absorption, overcoming the limitations of single-material photocatalysts.3
Integrating PEC systems with renewable energy sources like solar and wind is crucial for large-scale hydrogen production. This allows excess energy to be used for hydrogen generation and grid balancing. This hybrid approach enhances the scalability of PEC systems by utilizing existing renewable infrastructure, advancing the shift to a hydrogen-based economy.9
These advancements mark significant progress in PEC water splitting, addressing both efficiency and environmental challenges.
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References and Further Reading
1. Vilanova, A.; Dias, P.; Lopes, T.; Mendes, A. (2024). The Route for Commercial Photoelectrochemical Water Splitting: A Review of Large-Area Devices and Key Upscaling Challenges. Chemical Society Reviews. https://pubs.rsc.org/en/content/articlelanding/2024/cs/d1cs01069g
2. Bora, LV.; Bora, NV. (2025). Photoelectrocatalytic Water Splitting for Efficient Hydrogen Production: A Strategic Review. Fuel. https://www.sciencedirect.com/science/article/pii/S0016236124027911
3. Villa, K.; Galán-Mascarós, JR.; López, N.; Palomares, E. (2021). Photocatalytic Water Splitting: Advantages and Challenges. Sustainable Energy & Fuels. https://pubs.rsc.org/en/content/articlelanding/2021/se/d1se00808k
4. Hemmerling, JR.; Mathur, A.; Linic, S. (2021). Design Principles for Efficient and Stable Water Splitting Photoelectrocatalysts. Accounts of Chemical Research. https://pubs.acs.org/doi/10.1021/acs.accounts.1c00072
5. Wang, Y.; Zhang, J.; Balogun, M.-S.; Tong, Y.; Huang, Y. (2022). Oxygen Vacancy–Based Metal Oxides Photoanodes in Photoelectrochemical Water Splitting. Materials Today Sustainability. https://www.sciencedirect.com/science/article/pii/S2589234722000100
6. Bozheyev, F.; Ellmer, K. (2022). Thin Film Transition Metal Dichalcogenide Photoelectrodes for Solar Hydrogen Evolution: A Review. Journal of Materials Chemistry A. https://pubs.rsc.org/en/content/articlelanding/2022/ta/d2ta01108e
7. Marwat, MA.; Humayun, M.; Afridi, MW.; Zhang, H.; Abdul Karim, MR.; Ashtar, M.; Usman, M.; Waqar, S.; Ullah, H.; Wang, C. (2021). Advanced Catalysts for Photoelectrochemical Water Splitting. ACS Applied Energy Materials. https://pubs.acs.org/doi/10.1021/acsaem.1c02548
8. Popczun, EJ.; McKone, JR.; Read, CG.; Biacchi, AJ.; Wiltrout, AM.; Lewis, NS.; Schaak, RE. (2013). Nanostructured Nickel Phosphide as an Electrocatalyst for the Hydrogen Evolution Reaction. Journal of the American Chemical Society. https://pubs.acs.org/doi/10.1021/ja403440e
9. Wang, Q.; Domen, K. (2019) Particulate Photocatalysts for Light-Driven Water Splitting: Mechanisms, Challenges, and Design Strategies. Chemical Reviews. https://pubs.acs.org/doi/full/10.1021/acs.chemrev.9b00201
10. Zhang, X.; Zhang, Z.; Wu, D.; Zhang, X.; Zhao, X.; Zhou, Z. (2018). Computational Screening of 2d Materials and Rational Design of Heterojunctions for Water Splitting Photocatalysts. Small Methods. https://onlinelibrary.wiley.com/doi/abs/10.1002/smtd.201700359
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