(13c) Co-Electrolysis of Water and Organic Molecules: Thermo-Economically Promising?

Due to the global aim of transitioning to a green economy, the general interest in electrochemical production
processes has grown in the past few years, especially in the development of novel process concepts
[1, 2]. Commonly, the design of a novel electrochemical process starts with experimental investigations
of the feasibility of a desired reaction in laboratory scales, thereby focusing on improving the reaction
kinetics by applying different catalysts, designs, and operating conditions [3, 4]. However, the long-term
view of such novel process concepts is often insufficiently evaluated at an early stage, leading to high cost
and effort in pursuing experimental work on concepts that are later evaluated as economically unviable,
e.g., due to thermodynamic limitations.

The particular novel process concepts we evaluate are couplings of the electrochemical production of
hydrogen at the cathode with alternative oxidation reactions of organic molecules at the anode. Such
coupling has the potential to produce both hydrogen and valuable side-products while additionally lowering the overall energy consumption compared to pure water electrolysis, since the common oxygen evolution reaction at the anode suffers from slow kinetics and high thermodynamic voltages [5]. The organic molecules for such coupling should not only have the potential to reduce the overall energy demand,
but also need to fulfill further requirements: The need to be sufficiently abundant, ecologically friendly,
and available at a comparatively low market price. One example of such an organic molecule is glycerol,
which, as a byproduct from biodiesel production, is available in large amounts (ca. 4 ·10⁹ kg/a), green
and cheap, and therefore particularly promising [6].

To address the gap of insufficient early-stage evaluation of such co-electrolysis processes, we present
an early-stage analysis of several alternative oxidation reactions coupled with electrochemical hydrogen
production. This early-stage analysis forms part of a screening problem for which, as a first step, the
complexity needs to be reduced. Therefore, we compare the reactions in terms of their (i) lower bounds
to the energy demand and (ii) upper bounds to the economic potential. These two criteria have the
advantage of requiring only minimal data yet still showing the bounds of the practicability of a particular
process concept [7]. Therefore, they are especially applicable to novel (electrochemical) process concepts
for which many design variables, such as electrolyzer design, operating conditions, or catalyst choice, are
still very uncertain [8]. With the results from this early-stage analysis, we evaluated whether some of
the reactions can be eliminated from further screening and if some of the reactions are significantly more
promising than others to focus on in further research.

For electrochemical glycerol oxidation coupled with cathodic hydrogen production, we ascertain that all
feasible reactions reported in literature, which are 14 in total, have a significantly lower thermodynamic
energy demand compared to water electrolysis. Additionally, some of the generated products have the
potential to add value by sale, even with the comparatively high uncertainty in market prices. The
supply of glycerol currently only suffices to cover 0.4% of the global hydrogen demand. Nevertheless, our
results show that coupled glycerol oxidation and hydrogen production may still offer an alternative for
transforming glycerol waste from biodiesel plants into higher-value products. [9]

Acknowledgements We acknowledge funding by the project Hydrogen4Tomorrow (project number
KICH2.V4P.DUI21.004), of the research program Electrochemical Materials and Processes for Green Hydrogen and Green Chemistry (ECCM), which is financed by the Dutch Research Council (NWO) and the Federal Ministry of Education and Research of Germany (BMBF). Part of this work was also supported by the Cluster of Excellence Fuel Science Center (EXC 2186, 21 ID: 390919832) funded by the Excellence Initiative by the German federal and state governments to promote science and research at German universities. We also acknowledge funding by the Federal Ministry of Food and Agriculture granted by the Agency for Renewable Resources (FNR, 2220NR101X). Sonja Mürtz thanks Cusanuswerk e.V. for funding.

References
[1] C. A. Malapit, M. B. Prater, J. R. Cabrera-Pardo, M. Li, T. D. Pham, T. P. McFadden,
S. Blank, S. D. Minteer, Chemical reviews 2022, 122, 3180–3218.
[2] C. Gütz, B. Klöckner, S. R. Waldvogel, Organic Process Research & Development 2016,
20, 26–32.
[3] N. Zhang, B. Yang, K. Liu, H. Li, G. Chen, X. Qiu, W. Li, J. Hu, J. Fu, Y. Jiang, M. Liu,
J. Ye, Small methods 2021, 5, e2100987.
[4] R. Ding, Y. Chen, Z. Rui, K. Hua, Y. Wu, X. Li, X. Duan, J. Li, X. Wang, J. Liu, Journal
of Power Sources 2023, 556, 232389.
[5] J. Brauns, T. Turek, Processes 2020, 8, 248.
[6] A. Behr, J. Eilting, K. Irawadi, J. Leschinski, F. Lindner, Green Chemistry 2008, 10, 13–30.
[7] A. Mitsos, I. Palou-Rivera, P. I. Barton, Industrial & Engineering Chemistry Research
2004, 43, 74–84.
[8] L. C. Brée, M. Wessling, A. Mitsos, Computers & Chemical Engineering 2020, 139, 106890.
[9] K. M. Ebeling, D. Bongartz, S. Mürtz, R. Palkovits, A. Mitsos, 2024, Thermodynamic and
Economic Potential of Glycerol Oxidation to Replace Oxygen Evolution in Water Electrolysis,
submitted.