The global mean CO2 concentration has increased from 280 parts per million (ppm) in the mid-18th century to 406 ppm, as measured at the Mauna Loa Observatory (MLO) in Hawaii in February 2017. This increase of CO2 is mainly due to anthropogenic activities, especially the burning of fossil fuels such as coal, petroleum and natural gas. This has been causing a series of problems, including global warming, desertification, and ocean acidification.
Meanwhile, renewable electricity has seen tremendous growth in recent years, including larger capacity growth than conventional fossil fuel sources. This has led to significant reductions in price. Due to the intermittency of sources such as solar and wind, excess generation of electricity can further lead to instantaneous prices near zero. For these reasons, there is tremendous opportunity to use renewable electricity for the clean and economical generation of products such as fuels and chemical feedstocks. The penetration of renewables into industries to replace current fossil-fuel-based power sources would lead to the net reduction of CO2 emissions.
In our group, we aim to address the energy and environmental challenges by electrochemical conversion of CO2 to valuable fuels and chemicals powered using renewable electricity through working on (1) catalyst design, (2) mechanism understanding, and (3) system and devices.
1. Catalyst design
The electrochemical CO2 reduction reaction (CO2RR) involves multiple proton/electron transfer steps to active and convert inert CO2 molecules that impose high thermodynamic and kinetic barriers. Efficient catalyst is required to overcome these barriers. CO2RR can proceed through a two-, four-, six-, eight-, 12-, or an 18-electron reduction pathway to produce various products (CO, formic acid, methanol, methane, ethylene, ethanol, propanol, etc.). Selective production of one specific product at high activity is central yet challenging.
We are particularly interested in carbon upgraded C2+ products (i.e., ethylene, ethanol, propanol and beyond). Current catalyst design strategies include: oxidation state tuning and stabilisation, nanostructuring, crystal phase tuning, alloying, and molecular co-catalyst.
Learn more through recent publications:
Despite progress over the past decade, there remains big challenges: it is still far away to simultaneously achieve high activity, selectivity, and stability for a single C2+ product. For example, the selectivity of alcohol products (ethanol + propanol) is still ~ 30% and higher FEs are only achieved at very low current densities (below 10 mA cm-2). This under-developed status is largely due to the lack of mechanism understanding of the CO2RR. Especially, experimental characterisations that are able to capture the dynamic environment (catalytically active site, diffusion and adsorption of reactants, identity of intermediates, desorption of products, ions, water, electric field) at the catalyst surface with high spatial and temporal resolutions are highly desirable.
We are currently working on the utlisation of in situ /operando spectroscopies (including operando X-ray absorption spectroscopy and operando Raman spectroscopy), coupled with computational modelling through collaborations with theoretical chemists, to elucidate the mechanism of this reaction and to guide the design of next-generation electrocatalysts.
Learn more through recent publications:
At present, most of the CO2RR studies were performed in an H-cell with aqueous KHCO3 being supporting electrolyte. However, the solubility of CO2 in water is only 33 mM at 1 atm and 25 °C, resulting in current densities well below 100 mA cm-2 due to mass transport limitation. This is too low for the commercialisation of CO2RR from a technoeconomic point of view.
The flow cell systems incorporating gas diffusion electrodes (GDEs) can overcome the CO2 mass transport issue and hence achieve a significantly increased current density. This is one of the most promising avenues to the commercialisation of CO2RR. The system is much more complicated than the H-cell and great challenges remain to be solved.
We are currently working on integrating CO2RR into a full flow cell system consisting of both cathode and anode. To achieve so with high energy efficiency, I am working on the ion exchange membrane, anode catalyst, cell configuration, and water management of the cathodic GDEs.
Learn more through recent publications:
