Hydrogen-producing artificial leaves derive significant potential from their capacity to use sunlight to produce clean energy
Recent research at the Helmholtz-Zentrum Berlin für Materialien und Energie (HZB) has shown that operating photoelectrochemical cells (PEC cells) at elevated pressures can significantly enhance their efficiency.
The efficiency of a PEC cell, which uses artificial photoelectrodes for the electrolytic splitting of water, depends on numerous factors. One critical aspect is the formation of gas bubbles during the process. Bubbles can have a substantial impact on the PEC cell’s performance:
- Larger bubbles scatter light
- Bubbles can block the electrolyte’s access to the electrode
- These effects hamper electrochemical reactions
Traditionally, PEC cells have been operated at atmospheric pressure (one bar). However, researchers have now studied the effects of higher pressures.
Elevated Pressure Breakthrough
The team from the Institute for Solar Fuels at HZB explored water splitting at pressures ranging from 1 to 10 bar. They observed and recorded various parameters during electrolysis, comparing experimental data with a specially developed multiphysics model under both normal and elevated pressures.
The Results
Experiments and modelling indicated that PEC cells operated at 8 bar halved the total energy loss, leading to a 5-10{c431b1036349617aea55b35aa92592c3cb3fecc7f94273a754a3b674e9a603ce} relative increase in overall efficiency. The high pressure effectively minimizes the formation of large bubbles, reducing optical scattering losses.
“We also saw a significant reduction in product cross-over, especially the transfer of oxygen to the counter electrode,” noted Dr. Feng Liang, the first author of the research paper.
The reduction in product crossover is crucial as it enhances the purity and efficiency of the generated hydrogen. By operating within the optimal pressure range of 6–8 bar, PEC cells can achieve higher overall efficiency without encountering diminishing returns that higher pressures might incur.
Practical Implications
The findings are directly applicable to the design and operation of more efficient PEC cells. By adjusting operating pressures, scientists can now better control the bubble dynamics and enhance the performance of these artificial leaves.
“These findings, and in particular the multiphysics model, can be extended to other systems and will help us to increase the efficiencies of both electrochemical and photocatalytic devices,” remarked Prof. Dr Roel van de Krol, head of the Institute for Solar Fuels at HZB.
Challenges in Current PEC Cell Efficiency
Despite the promising potential of artificial leaves, there are considerable challenges in achieving optimal efficiency in PEC cells. One of the most prevalent limitations is bubble formation during the hydrogen production process. These bubbles can:
- Scatter incoming light, reducing absorption by the photoelectrode
- Impede the photovoltaic reaction
- Block electrolyte contact with the electrode
- Hinder essential electrochemical reactions for hydrogen production
Currently, PEC cells operating at atmospheric pressure (1 bar) face noteworthy inefficiencies due to these bubble-induced losses. Even the highest performing devices, which achieve energy conversion efficiencies up to 19{c431b1036349617aea55b35aa92592c3cb3fecc7f94273a754a3b674e9a603ce}, are notably affected by bubble formation.1
The research conducted by the HZB team demonstrates that increasing the operating pressure to 8 bar can substantially reduce these losses. This higher pressure:
- Reduces the size of the bubbles
- Optimizes their behavior at the electrodes
This dual effect ensures that light scattering is minimized, thus maintaining optimal illumination of the electrode surface and uninterrupted electrolyte contact.
Artificial leaves have been utilized for diverse applications beyond hydrogen production, including the creation of synthetic gas and pharmaceutical drugs. This adaptability highlights their potential across multiple industries. However, hydrogen production continues to be one of the most promising uses due to its relevance for the energy transition.
The HZB study brings to the forefront the importance of elevating operating pressures in PEC cells, thereby advancing the efficiency of artificial leaves in hydrogen production. By considering the pressure dynamics and further refining the bubble behavior, PEC cells can achieve superior hydrogen production efficiency, paving the way for more sustainable and economically viable energy solutions.
Understanding and addressing the current limitations is crucial for stakeholders in the hydrogen market. Enhancing PEC cell efficiency through elevated operating pressures not only counters the challenges of bubble formation but also optimizes the overall energy conversion process. The implications for energy transition projects are significant, offering a pathway to efficient, scalable hydrogen production – a cornerstone for a sustainable future.
Here’s a video about a study in Switzerland and artificial leaves and how they are making hydrogen..
Applications and Future Implications
Given the promising results from the HZB study, translating these findings to other systems could have far-reaching implications in multiple sectors. By strategically increasing operating pressures, the multiphysics model developed can enhance the efficiency of PEC cells and potentially improve other electrochemical and photocatalytic devices.
For instance, in electrochemical systems such as those used in chlorine production through brine electrolysis, similar mechanisms of bubble formation lead to efficiency drops. The pressure-tuning approach could mitigate bubble-related losses in these processes, much like it does in PEC cells.
The multiphysics model’s applicability extends to photocatalytic devices as well. These systems leverage light to catalyze chemical reactions, essential in sectors ranging from environmental purification to synthetic fuel production. By enhancing light utilization and reducing electrochemical deactivation via optimized pressure conditions, we can ensure a consistent and higher yield, critical for industrial scalability.
Versatility of Artificial Leaf Technologies
Artificial leaf technologies are not solely confined to hydrogen production. Their versatility is evident in applications like:
- Synthetic gas production
- Pharmaceutical ingredient synthesis
- Environmental purification
In synthetic gas production, precise control over gas means and chemical reactions is paramount. By stabilizing bubble formation and minimizing optical scattering through elevated pressures, artificial leaves can deliver more efficient synthetic gas production. This efficiency boost can achieve lower production costs and higher output rates, contributing to the commercial viability of artificial leaf technologies in the synthetic gas sector.
In pharmaceutical applications, artificial leaves have been engineered to produce complex organic molecules that can serve as active pharmaceutical ingredients. By incorporating high-pressure operational settings, the consistency and purity of the output can be significantly enhanced, thereby ensuring better compliance with stringent pharmaceutical standards. This breakthrough could improve the production of specific compounds, making it more efficient and cost-effective.2
Impact on Energy Transition
Current hydrogen-producing artificial leaves, known as photoelectrochemical cells (PECs), utilize light-powered electrodes to split water into hydrogen and oxygen. The best of these cells currently achieve an energy conversion rate of 19{c431b1036349617aea55b35aa92592c3cb3fecc7f94273a754a3b674e9a603ce}, slightly lower than the 24{c431b1036349617aea55b35aa92592c3cb3fecc7f94273a754a3b674e9a603ce} rate of the most efficient solar panels. However, with this latest finding, there is potential for a 5-10{c431b1036349617aea55b35aa92592c3cb3fecc7f94273a754a3b674e9a603ce} increase in energy conversion rates over the existing systems.
For stakeholders in energy projects, these insights offer clear pathways to innovation. Participants in hydrogen projects can leverage these strategies to optimize their processes, ensuring higher yields and better cost efficiency. The capability to adapt these advances across varying operational contexts underscores the potential for future innovations.
Enhanced efficiency will not only make hydrogen a more viable option but also ensure its wider adoption in industrial and transportation sectors critical to achieving global carbon reduction targets.
In conclusion, the HZB study’s findings have transformative potential across a multitude of applications. By refining artificial leaf technology, elevating operating pressures, and applying the multiphysics model to diverse systems, we are not just improving hydrogen production but setting the stage for advancements across synthetic gas and pharmaceutical industries. This transition signifies a significant step towards a more sustainable and technologically advanced future.
- Nocera DG. The artificial leaf. Acc Chem Res. 2012;45(5):767-776.
- Liu C, Colón BC, Ziesack M, Silver PA, Nocera DG. Water splitting-biosynthetic system with CO? reduction efficiencies exceeding photosynthesis. Science. 2016;352(6290):1210-1213.