Recent Advances in Solar Thermal Electrochemical Process (STEP) for Carbon Neutral Products and High Value Nanocarbons (2019)

Climate change represents one of the most important environmental issues of our time. Due to high levels of anthropogenic CO<sub>2</sub> emissions, atmospheric CO<sub>2</sub> has for the first time ever exceeded 415 ppm and has increased from 315 ppm in 1950. An annual increase in atmospheric CO<sub>2</sub> of ∼2 ppm is equal to a net increase of ∼15.6 billion tons of CO<sub>2</sub>. The combustion of fossil fuels for electricity and transportation is still the main reason accounting for the CO<sub>2</sub> accumulation. On the top of that, fossil fuels are widely used in our modern industry for the productions of indispensable social staples. For instance, the millennia old thermal reduction of iron ore by charcoal or baked coal (3C + 2Fe<sub>2</sub>O<sub>3</sub> → 4Fe + 3CO<sub>2</sub>) continues as the main method for the production of iron. The artificial fertilizer ammonia boosts the global population and is mainly produced from the Haber-Bosch process, in which hydrogen is generated via steam reforming of methane (CH<sub>4</sub> + 2H<sub>2</sub>O → 4H<sub>2</sub> + CO<sub>2</sub>). Sequestration and diminution of CO<sub>2</sub> require the development of a portfolio of technologies on (1) efficient and long-term harvesting of renewable energy, that is, solar, not only for electricity but also directly as the energy force in vital chemical processes, wherever possible, (2) carbon-neutral processes to replace current industrial processes that emit vast amounts of CO<sub>2</sub>, such as iron and ammonia production, and (3) new, low-cost technologies for CO<sub>2</sub> capture and conversion with particular interests in the exploration of CO<sub>2</sub> as the feedstock for fuels or other valuable chemicals and materials. To this end, we conducted some studies on the sustainable synthesis of ammonia and iron with net-zero CO<sub>2</sub> emissions and large-scale CO<sub>2</sub> capture and conversion into fuels and high value nanocarbon products via electrolysis in molten salt(s) with the introduction of the Solar Thermal Electrochemical Process (STEP). In STEP, solar UV-visible energy is focused on a photovoltaic device that generates the electricity to drive the electrolysis, while concurrently the solar thermal energy is focused on a second system to generate heat for the electrolysis cell. The utilization of the full spectrum of sunlight in STEP results in a higher solar energy efficiency than other solar conversion processes. STEP has been applied to conduct (1) CO<sub>2</sub>-free ammonia synthesis from nitrogen and water with the aid of nano-Fe<sub>2</sub>O<sub>3</sub> in a molten hydroxide electrolyte, (2) CO<sub>2</sub>-free production of iron via electrochemical reduction of iron ore in molten carbonate, (3) CO<sub>2</sub> capture and conversion into nanostructured carbon products as well as fuels in molten or mixed molten electrolytes, and (4) organic electrosynthesis of benzoic acid from benzene without overoxidizing into CO<sub>2</sub>. In this Account, we highlight some recent achievements in these topics and propose that using STEP is a highly efficient strategy for saving energy and, consequently, the environment. STEP is an ideal tool that can theoretically be applied to all endothermic reactions.