An electrolyzer is a reactor that conducts chemical reactions with electricity as the stimulus. Like a battery, it intrinsically has a positive side (anode) that conducts oxidations and a negative side (cathode) that conducts reductions. Currently, most of the electrochemistry research is on the negative side, which enables one to generate precious hydrogen from water or make commodity chemicals from CO2. Research into the positive side focuses on mainly reducing the energy cost of making O2 from water, rather than generating value add products. We started out with the simple idea; what if we can pair meaningful oxidation and reduction reactions in an electrolyzer, such that we are making two products instead of one, while consuming similar amounts of electricity?
It must be noted however, that this is not as easy as combining two random oxidation and reduction reactions. First, the products of both reactions must have comparable market demands, or the market will be faced with a huge over-surplus of one product, which may end up being disposed. Second, we need to identify processes with the highest carbon emissions, so that we can make the largest impact in alleviating climate change.
With this idea in mind, we sought the world’s top oxidation and reduction processes in terms of production volume and carbon footprints. Our life-cycle-assessment shows that 30% of emissions in the chemicals industry arise from the oxidation of hydrocarbons such as ethylene and xylene, which turns them into oxygenates such as ethylene glycol and terephthalic acid (Fig. 1A). These are important plastics precursors that eventually go into the making of our clothes, drinking bottles and other everyday items. Such reactions release large amounts of CO2 as fossil fuels need to be burnt to generate heat needed to facilitate the oxidation reaction, and because the conversion of hydrocarbons to oxygenates is imperfect, with a portion of the hydrocarbons turning into CO2 (Fig. 1B).
The other large contributor of greenhouse gas is ammonia manufacture, due to the reliance of our food supply on ammonia-based fertilizers (Fig. 1A). The current production process starts from methane, which undergoes reforming and water-gas-shift reactions to form hydrogen, which then undergoes the Haber Bosch reaction with nitrogen to yield ammonia (Fig. 1B). Most of the CO2 emissions arise from the hydrogen production step.
Although hydrogen production step can be replaced by water electrolysis to H2 and O2, it is not favoured by industries as a large amount of electricity is consumed per unit of H2. The electricity used needs to be clean and renewable in order to release less CO2 emissions compared to the current industrial process. However, as the amount of CO2 emissions that can be reduced per unit of electricity is not high, such precious and limited renewable energy can find better uses in other avenues such as electric vehicles.
We postulate that that electrification and coupling of hydrocarbon-to-oxygenate conversion with H2 evolution reaction from water would be able to kill two birds with one stone (Fig. 1C). Using such coupled systems, the amount of CO2 emissions that can be saved per unit electricity will be doubled compared to conventional water electrolysis (Fig. 1D). Our calculations show that such coupled electrolyzer systems can potentially reduce 88% of emissions associated with hydrocarbon-to-oxygenate and ammonia manufacture. In comparison, electrical heating and conventional water electrolysis can only reduce up to 11% and 53% of associated emissions respectively. Even if renewable electricity is not available, these emissions can be reduced by up to 39% with the electricity mix in USA and China today.
We note that there is a gap today between our postulated scenario and state-of-art reactions. While the development of electrochemical oxidations as a synthetic toolkit for chemical upgrading has been recently gaining attention, research to date was not focused on the primary chemicals contributing to global CO2, nor the associated markets commensurable with that of NH3. We believe that research efforts can therefore be redirected to the following key scientific targets: (1) expanding the scope of electrochemical oxidations towards hydrocarbons such as methane, ethylene, propylene, benzene and xylenes, (2) enabling such reactions at high current densities in order to minimize the surface area of electrochemical reactors and the resultant capital costs, (3) optimizing the energy efficiencies in order to increase the yield of desired chemical products for a given electrical input. If these can be achieved, we will be one step closer to a green and sustainable economy.
The full paper can be found here: https://www.nature.com/articles/s41467-023-37382-3
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