A Case for Electrofuels
T
and the absence of a direct large positive effect on the CO2 atmospheric level. However, there is intensive ongoing research to produce carbon-based renewable fuels even if scalable, efficient, and direct routes have yet to be invented. Nitrogen reduction is also under scrutiny even if it remains a long-term scientific challenge. In this Viewpoint, we will advocate the reasons why there is no “hydrogen dream” and “CO 2 nightmare”1 or, in other words, why generation of electrofuels from CO2 (or N2) is an alternative to H2 production as a clean energy vector. In a recent Viewpoint published in this journal,1 it is suggested that hydrogen shall be the main energy vector, acting as the mandatory intermediate in both energy storage and conversion processes, being a reductant in CO2 converting processes instead of the direct electrochemical reduction of CO2. First, it is difficult to imagine any objective reason why, from a general perspective, hydrogen should be the preferred, unique route for either step in the storage and conversion of renewable energy, especially in the current context where new, promising processes and more robust catalysts are emerging, making CO2 conversion more efficient and versatile. From a chemist’s point of view, energy consumption (in terms of cell voltage) for CO2 reduction and hydrogen production are quite similar. Note also that if current CO2 electroreduction processes can not match with the GW-scale release from fossil fuel burning, the same argument applies to H2-based routes. Current density limitation is another commonly argued drawback, but gaseous CO2 feeding through gas diffusion electrodes may bypass such difficulty. Supercritical CO2 may even be employed if current density limitations were the actual bottleneck, and the intense research and progress currently made for the oxygen evolution reaction impact water-, CO2-, and N2-based electrolyzers equally. Finally, it should be pointed out that H2 storage is by far much more difficult and hazardous than the storage of CO2 itself and of most products issued from its reduction (e.g., formic acid, methanol or methane). In advocating for the use of renewable hydrogen to reduce CO2 with current processes rather than further investigating the direct electrochemical reduction of CO2, it was claimed that the costs related to the handling and purification of gas flows in a CO2 electrolyzer are higher than the costs of integrating a water electrolysis AND a secondary reactor to perform the target reaction.1 However, no evidence was provided to support such a decisive assumption. From an engineer’s point of view, the fewer reactors involved, the better. Besides, typical processes in oil refinery and gas processing are designed for large flows (GW thermal power) that are several orders of magnitude higher than the output flow of current electrolysis stacks (∼1−10 MW thermal output). As a consequence, small-scale and intermittent
he United Nations conference on climate change (COP21) held last December in Paris illustrated public awareness regarding our current climate change and future energy issues. Solving these global challenges will involve various actions, from basic science to governmental regulations. In terms of technological development, sustainable scenarios aim at progressively replacing the fossil fuel economy that thrived during the 20th century. Yet, the lack of incentives and low price of oil are leaving the consumer with cheap and convenient technology solutions based on the combustion of fossil fuels. In this context, public research agencies should provide a long-term vision and endorse the funding for new, alternative routes. Figure 1 depicts several current and upcoming pathways that would supplement the integration of renewable energy sources in the electrical production mix. It aims at clearing up the confusion between energy production, conversion, and storage. In these processes, input electrical energy (issued from renewable sources) is stored in chemical bonds thanks to an appropriate electrolyzer. The electrofuels are compatible with the current industrial infrastructure and supply chain, while they can be stored easily, that is, with no particular technical problems and at low cost. As shown below, typical reactions include (eq 1) hydrogen generation, (eq 2) the production of hydrocarbons and oxygenates, and (eq 3) ammonia synthesis. H 2O = H 2 + 0.5O2
(1)
α‐CO2 + β‐H 2O = Cx HyOz + γ‐O2
(2)
N2 + 3H 2O = 2NH3 + 1.5O2
(3)
The case of electrofuels is of particular interest. They may be used either as a means to store the electricity in the chemical bonds of high-energy-content molecules (for later reconversion to electricity or other applications decoupled from the electricity network such as the supply of liquid fuels for transportation) or as various feedstocks to manufacture highvalue compounds. Their production is obviously energyconsuming because the reverse reactions are combustion processes that release energy. A large amount of clean electrical energy is thus required to achieve sufficient production of the clean feedstocks. As a consequence, such strategies are better suited to countries featuring a high percentage of renewable energy in their electricity production mix. However, it will likely concern more and more countries as the energy transition, now backed up by large fractions of citizens, will spread out. Among these fuels, hydrogen production is the most advanced one; commercial electrolysis stacks provide up to 1000 m3 H2/h, corresponding roughly to 5 MW input electric power. Hydrogen is considered by many as the clean energy vector of the future. CO2 conversion is often claimed to be a “much more” difficult and thus unlikely route due to tedious catalysis (CO2 reduction is a multielectron, multiproton process), the lack of a pure source of CO2 (industrial smokes contain many potential poisoners for the chemical conversion), © XXXX American Chemical Society
Received: October 7, 2016 Accepted: October 24, 2016
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DOI: 10.1021/acsenergylett.6b00510 ACS Energy Lett. 2016, 1, 1062−1064
Viewpoint
http://pubs.acs.org/journal/aelccp
Viewpoint
ACS Energy Letters
Figure 1. Pathways from renewable energy generation toward electrofuel production and chemical synthesis.
hydrogen production does not fit current chemical reactors’ design. Another argument that would disqualify CO2-based electrofuel production is the fact that it could not contribute to mitigate climate changes. It is a triviality just because of the scale issue, and claiming the reverse is unjustifiable. Renewable energy consumption to recycle all CO2 emissions to synthetic natural gas is 10-fold the initial electricity output of the thermal power stationeven larger than what was suggested.1 CO2 reduction is not a way to mitigate large-scale greenhouse gas emissions, especially from large point sources such as coalpowered thermal plants, and neither is hydrogen production. However, large concentrated sources (e.g., cement kilns) will still remain for years and decades an abundant carbon source, so that directly grabbing CO2 from the air is not necessary, contrary to what is sometimes objected (such approaches may be further developed when new technology emerges).2 Mitigating CO2 emissions is a global challenge, involving all actors from our societies, citizens, industry, and policy makers, and no technological “miracle” is going to solve this issue, at least in the short term. The ambition behind solar fuel and electrofuel production does not lie in recycling these emissions but rather stands in accompanying the development of renewable energies so that obsolete, polluting thermal power stations can be progressively shut down without compromising energy access and security. Solar fuels will also strongly contribute to the synthesis of high-value chemicals from a cheap and decentralized renewable energy source. Direct electrochemical valorization of CO2 is a complementary pathway to biological processes that face inevitable CO2 emissions such as fermentation (e.g., bioethanol production) and anaerobic digestion (e.g., biogas production from agricultural residues and municipal waste). CO2 is a byproduct in these processes, and a separation step is necessary, but it also provides a suitable carbon source for small-scale valorization routes (Bio-CCU). Some examples of direct CO2 conversion may even bypass the separation step (such as high-temperature biogas upgrade). Overall, electrofuels fit into a carbon-neutral loop (like photosynthesis) where CO2 is captured and released when the fuel is consumed.
Full electrolysis processes typically consist of coupling water oxidation to either CO2, N2, or H2O reduction. They target the same purpose, that is, renewable electricity conversion and storage into chemical bonds, as depicted in Figure 1, while only the final chemical products may be different. Note that among these chemicals, many involve carbonaceous materials, the production of which requires a carbon source, whether the chemical energy source is hydrogen or advanced electrofuels. So why consider that one of these processes would be intrinsically better than the others while each of them provides remarkable opportunities to manufacture electrofuels? Carbonand nitrogen-based electrofuels may be more difficult to synthesize, but they display a much higher energy density, fit the current infrastructure, and provide key building blocks to the chemical industry. On the other hand, hydrogen is more versatile and therefore serves a larger purpose but at the same time lacks some attractive specific features of carbon-based fuels. In other words, if water splitting is probably the easiest route and thus features an advanced maturity level compared to the other routes, the benefits of other electrofuels make it desirable to strengthen funding and develop new opportunities. Electrofuel production is not (only) “an interesting academic problem”1 but rather a key contributor to technological advances toward a sustainable development. There is in fact not one solution but a diversity of solutions and options depending on the final energy utilization. Eventually, if competition may arise between several pathways that fulfill the same needs, economic actors will then pick up the most appropriate process. However, pointing out potential disadvantages in future applications based on the state-of-the-art of current technologies does not provide a rational basis for longterm socioeconomic investments. Large-scale development of electrofuels is a necessity and implies not focusing on a specific process. We need to address water spitting as well as carbon dioxide and nitrogen reduction. If CO2 conversion, either by electro- or photo(electro)chemical routes remains a challenge, this challenge promises to meet our needs, from energy vectors to fine chemicals.
Arnaud Tatin Julien Bonin Marc Robert* 1063
DOI: 10.1021/acsenergylett.6b00510 ACS Energy Lett. 2016, 1, 1062−1064
Viewpoint
ACS Energy Letters Université Paris Diderot, Sorbonne Paris Cité, Laboratoire d’Electrochimie Moléculaire, UMR 7591 CNRS, 15 rue Jean-Antoine de Baïf, F-75205 Paris Cedex 13, France
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
Views expressed in this viewpoint are those of the authors and not necessarily the views of the ACS. The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We warmly thank Dr. T. Porter for careful reading of the manuscript. REFERENCES
(1) Parkinson, B. Advantages of Solar Hydrogen Compared to Direct Carbon Dioxide Reduction for Solar Fuel Production. ACS Energy Lett. 2016, 5, 1057−1059. (2) Osterloh, F. The Low Concentration of CO2 in the Atmosphere Is an Obstacle to a Sustainable Artificial Photosynthesis Fuel Cycle Based on Carbon. ACS Energy Lett. 2016, 5, 1060−1061.
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DOI: 10.1021/acsenergylett.6b00510 ACS Energy Lett. 2016, 1, 1062−1064
