The potential of solar energy as a renewable energy source to substitute fossil fuels and nuclear power is widely recognized. While photovoltaic (PV) technology is the most common way to convert solar energy into electricity, the intermittent nature of sunlight necessitates the storage of this electricity. One approach is to connect PV systems with rechargeable batteries. Another method is to use (photo)electrochemical cells to convert solar energy into chemical fuels, such as converting CO2 gas emitted by power plants and transportation into hydrocarbons through the CO2 reduction reaction (CO2RR). These resulting products can serve as building blocks for producing valuable chemicals. The research in the CJ group  focuses on the development of semiconductors and nanomaterials for energy conversion and storage, including Solar flow battery (SFB) and Electrocatalysis.

(a) Solar flow battery (SFB)

    A typical solar flow battery (SFB) system is composed of three electrodes: a photoelectrode (photovoltaic, PV), an inert cathode, and an inert anode. Two different electrolyte fluids are circulated through respective compartments that are separated by an ion-exchange membrane. One electrolyte fluid has a relatively positive formal potential and is referred to as the "posolyte," while the other has a relatively negative formal potential and is called the "negolyte."

The photoelectrode in the SFB device captures sunlight, and the resulting photo-induced carriers charge up redox couples, which allows the solar energy to be stored as chemical energy in the electrolytes. If electricity is required, the charged redox species are discharged on the inert electrodes, similar to the discharging process of a regular redox flow battery (RFB).

The performance of an SFB system is evaluated through its solar-to-output electricity efficiency (SOEE), which is calculated as the ratio of the electrical output energy delivered by the device to the total solar energy input. To achieve a high SOEE value, the cell voltage (i.e., the redox potential difference of the redox couples) should be matched as closely as possible with the maximum power point (MPP) of the photoelectrode. The fast kinetics of the redox species in the SFB system result in low overpotential during the charging and discharging processes, leading to low kinetic loss. Additionally, by altering the structures or functionalities of the organic redox species, their redox potential can be finely tuned to be close to the MPP, resulting in low coupling loss and high SOEE performance.

(b) Electrocatalysis

The increasing global demand for energy has led to the overuse of fossil fuels, resulting in significant carbon emissions and negative environmental impacts. To combat this, electrochemical CO2 reduction reaction (CO2RR) has emerged as a promising strategy to convert CO2 molecules into valuable products under mild conditions. These products can be used as green fuels (such as H2, CH4, etc.) or as feedstocks for synthesizing other chemicals (such as CO, ethylene, etc.). However, to achieve the balance of redox stoichiometry in the electrochemical cell, oxygen evolution reaction (OER) is commonly used as the counter reaction, despite the limited practical value of O2 product in the industry. Therefore, novel oxidation reactions must be developed to pair with CO2RR.

Biomass molecules, obtained from crops, plants, or algae, can serve as sustainable precursors for synthesizing high-value oxygen-containing chemicals. Electrochemical biomass oxidation is a potential counter reaction to pair with CO2RR and save energy because the thermodynamic potentials of oxidizing biomass are generally lower than those of OER. In the CJ group, we synthesize nanomaterials as electrocatalysts for performing these target reactions. In-situ synchrotron characterizations are employed to observe material variation during catalysis, and to determine the reaction pathway or deactivation mechanism.