Scale Effect on Producing Gaseous and Liquid Chemical Fuels via CO2 Reduction

Producing chemical fuels from sunlight is a sustainable way to utilize solar energy and reduce carbon emissions. Within the current photovoltaic-electrolysis or photoelectrochemical-based solar fuel generation system, electrochemical CO2 reduction is the key step. Although there has been important progress in developing new materials and devices, scaling up electrochemical CO2 reduction is essential to promote the industrial application of this technology. In this work, we use Ag and In as the representative electrocatalyst for producing gas and liquid products in both small and big electrochemical cells. We find that gas production is blocked more easily than liquid products when scaling up the electrochemical cell. Simulation results show that the generated gas product, CO, forms bubbles on the surface of the electrocatalyst, thus blocking the transport of CO2, while there is no such trouble for producing the liquid product such as formate. This work provides methods for studying the mass transfer of CO, and it is also an important reference for scaling up solar fuel generation devices that are constructed based on electrochemical CO2 reduction.

A Mini-Review on Nanostructured g-C3N4 Photocatalysts for Solar Fuel Production

Graphitic carbon nitride (g-C3N4) is an important photocatalytic material that receives a lot of research attention globally. This is because of its favourable thermal and chemical stability as well as electronic band structure. However, the photocatalytic performance of the bulk g-C3N4 is limited by fast recombination of electron-hole pair and poor visible light-harvesting ability. Thus, different strategies, such as heterostructuring, nanotuning, doping, etc., have been adopted to overcome the aforementioned challenges to enhance the photocatalytic performance of g-C3N4. In recent times, various nanostructured g-C3N4 photocatalytic materials with various tuned morphologies have been designed and fabricated in literature for different photocatalytic activities. This mini-review summarized the progress development of nanostructured g-C3N4 photocatalysts with various tuned morphologies for solar fuel generation. The article briefly highlights the research status of various g-C3N4 with tuned morphologies and enhanced solar fuel generation abilities. Finally, a conclusion and future research were also suggested, opening up new areas on g-C3N4 photocatalysis.

Theoretical Thermodynamic Efficiency Limit of Isothermal Solar Fuel Generation from H2O/CO2 Splitting in Membrane Reactors

Solar fuel generation from thermochemical H2O or CO2 splitting is a promising and attractive approach for harvesting fuel without CO2 emissions. Yet, low conversion and high reaction temperature restrict its application. One method of increasing conversion at a lower temperature is to implement oxygen permeable membranes (OPM) into a membrane reactor configuration. This allows for the selective separation of generated oxygen and causes a forward shift in the equilibrium of H2O or CO2 splitting reactions. In this research, solar-driven fuel production via H2O or CO2 splitting with an OPM reactor is modeled in isothermal operation, with an emphasis on the calculation of the theoretical thermodynamic efficiency of the system. In addition to the energy required for the high temperature of the reaction, the energy required for maintaining low oxygen permeate pressure for oxygen removal has a large influence on the overall thermodynamic efficiency. The theoretical first-law thermodynamic efficiency is calculated using separation exergy, an electrochemical O2 pump, and a vacuum pump, which shows a maximum efficiency of 63.8%, 61.7%, and 8.00% for H2O splitting, respectively, and 63.6%, 61.5%, and 16.7% for CO2 splitting, respectively, in a temperature range of 800 °C to 2000 °C. The theoretical second-law thermodynamic efficiency is 55.7% and 65.7% for both H2O splitting and CO2 splitting at 2000 °C. An efficient O2 separation method is extremely crucial to achieve high thermodynamic efficiency, especially in the separation efficiency range of 0–20% and in relatively low reaction temperatures. This research is also applicable in other isothermal H2O or CO2 splitting systems (e.g., chemical cycling) due to similar thermodynamics.