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Green hydrogen, produced from water electrolysis using renewable energy sources, is being explored as a strategy to reduce the dependence on fossil fuels and decarbonize chemical processes. From an environmental standpoint, this approach is extremely attractive given that mild conditions are used during electrolysis and there are no greenhouse gases produced when using the hydrogen in a fuel cell.

However, the economics of electrolysis and fuel cell systems for energy conversion relies heavily on the costs of electricity and of metals like nickel, platinum, iridium, and titanium. Electrolyzer operating expenses must be minimized for green hydrogen to become an economically viable option. The electricity input contributes heavily to cost. Thus, decreasing the cost of renewable energy is a necessary step. Solar panels becoming more efficient and affordable within the past decades is cause for optimism in this regard [1], but there is much more that can be done to increase the success of green hydrogen. More efficient electrolyzers could make better use of the input electricity and the development of cheaper and more durable components can reduce both the capital and operational costs.

Check out our other blog articles about green hydrogen and decarbonization of chemical processes below!

Cross-disciplinary interest in green hydrogen

Electrolyzers are primarily electrochemical devices with electrocatalysts responsible for water splitting (Figure 1). The scientific challenges related to optimizing electrolyzers are attracting the attention of researchers that are not traditionally trained in electrochemistry. The search for efficient HER (Hydrogen Evolution Reaction) and OER (Oxygen Evolution Reaction) electrocatalysts also piques the interest of inorganic chemists and physicists. Development of better membranes calls for expertise in organic and polymer chemistry. Optimization of catalyst inks and their interaction with substrates requires the know-how of a materials scientist. Heat and mass flow management within the fuel cell stack and balance of plant are engineering endeavors. Clearly, the ongoing development of green hydrogen technologies has encouraged the collaboration of scientists and engineers across many disciplines. The result is an influx of creativity and insight, as well as development of exciting new materials and techniques.

Figure 1. Diagram of the electrolysis of water (water splitting) with respective half reactions at the cathode and anode in alkaline and acidic media.

Back to basics

Working in an unfamiliar domain means there is a need for quickly getting up to speed with best practices and learning a new scientific vocabulary. For many institutions, education on electrochemical principles and laboratory skills was not a key focus area until recent years.

In some cases, the deficiency of fundamental electrochemical training has led to inconsistencies in the reporting of important performance indicators. The electrochemical community has taken note of this and called for a more rigorous approach. As a result, experts have stepped up and provided practical guidance for quantifying and reporting in this domain.

When investigating electrocatalyst materials it is necessary to have benchmarks and well-defined performance indicators. In 2013, a comprehensive benchmarking protocol for evaluating and reporting figures of merit for OER electrocatalysts was published.

This JACS article [2] provides practical advice on how to interpret the catalyst surface in terms of roughness and geometric surface area and how to perform and analyze measurements for valid comparisons of electrocatalytic performance.

A common source of confusion and inconsistency in electrochemical measurements is the use of various reference electrodes (RE). Electrocatalytic activity is judged by the overpotential needed for a specified production rate (i.e., the current density for the HER or OER process, Figure 1). A three-electrode setup is needed to measure the potential, and the RE is crucial for situating this potential on a relative scale, allowing comparison of measurements carried out by different groups and in various conditions.

Find out more about reference electrodes and their usage in our free Application Note.

A 2020 Viewpoint article in ACS Energy Letters [3] provides a detailed explanation of how to report the overpotential of an electrocatalyst, focusing on commonly used reference electrodes like Hg/HgO, Hg/Hg2Cl2 (SCE), and Ag/AgCl.

The reversible hydrogen electrode (RHE) is another commonly used RE that is extremely well-suited for HER and OER studies. A recent ACS Catalysis article [4] explains why the RHE is the ideal reference electrode for electrolysis research and explains how to prepare and work with an RHE. By convention, all standard redox potentials are reported versus the standard hydrogen electrode (SHE). The RHE is a pH-dependent extension of the SHE and refers to the reduction of a proton under non-standard conditions as described by the Nernst equation.

Electrolyzers operate under both acidic and alkaline conditions, thus, the HER and OER are studied across the pH scale (Figure 1). The RHE is suitable for use at any pH and it shares the same dependency on pH as the HER and OER.

A common ground to stand on

Finding common language and understanding between these different fields is vital. This JOC synopsis article [5] clarifies electrochemical concepts for organic chemists. The article is highly visual, providing schematics that link concepts like free energy, redox potential, and overpotential. Equilibrium thermodynamics helps to provide a common point of reference that all chemists can relate to.

Thermodynamic analysis is often applied to quantify the energy efficiency of electrolysis cells and stacks. A recent review article in the Journal of Power Sources [6] highlights diverging definitions for the energy efficiency coefficient from academic and industrial literature. The article provides derivations in various conditions and reminds readers that both electricity and heat must be accounted for in the analysis.

Summary

The articles highlighted in this blog post represent just a small fraction of the many resources available for building a common understanding and better collaboration among all researchers working on the improvement of green hydrogen technologies. When the COVID pandemic shut down laboratory work and travel for many people, the research community carried on with enthusiasm.

Online seminars and working groups held openly and without cost have brought scientists together across disciplines and from around the world. For example, the Electrochemical Online Colloquium was started in 2021. This ongoing series of lectures addresses essential topics in electrochemistry by providing educational content alongside the personal perspective of expert speakers.

The electrochemical community is acutely aware of the importance of transitioning to sustainable and climate-safe energy and chemical processes. Energy storage and conversion through green hydrogen is a promising strategy that requires scientific advancement to thrive. Thankfully, researchers from across many disciplines are bringing their skills and creativity to this topic while the electrochemical community continues to drive collaborative efforts and share their core knowledge.

To find out more about associated topics, download these free Application Notes from Metrohm Autolab.

References

[1] Green Hydrogen Cost Reduction: Scaling up Electrolysers to Meet the 1.5°C Climate Goal; International Renewable Energy Agency: Abu Dhabi, 2020.

[2] McCrory, C. C. L.; Jung, S.; Peters, J. C.; et al. Benchmarking Heterogeneous Electrocatalysts for the Oxygen Evolution Reaction. J. Am. Chem. Soc. 2013, 135 (45), 16977–16987. doi:10.1021/ja407115p

[3] Niu, S.; Li, S.; Du, Y.; et al. How to Reliably Report the Overpotential of an Electrocatalyst. ACS Energy Lett. 2020, 5 (4), 1083–1087. doi:10.1021/acsenergylett.0c00321

[4] Jerkiewicz, G. Standard and Reversible Hydrogen Electrodes: Theory, Design, Operation, and Applications. ACS Catal. 2020, 10 (15), 8409–8417. doi:10.1021/acscatal.0c02046

[5] Nutting, J. E.; Gerken, J. B.; Stamoulis, A. G.; et al. “How Should I Think about Voltage? What Is Overpotential?”: Establishing an Organic Chemistry Intuition for Electrochemistry. J. Org. Chem. 2021. doi:10.1021/acs.joc.1c01520

[6] Lamy, C.; Millet, P. A Critical Review on the Definitions Used to Calculate the Energy Efficiency Coefficients of Water Electrolysis Cells Working under near Ambient Temperature Conditions. J. Power Sources 2020, 447, 227350. doi:10.1016/j.jpowsour.2019.227350

Post written by members of the Metrohm Autolab group, Utrecht, The Netherlands.
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