Electrochemical interfaces for energy, biology, and environment

The Liu Lab is located in the Department of Chemistry and Biochemistry, University of California, Los Angeles. We are an inorganic chemistry lab with specific interests in electrochemical systems for energy, biology, and environments. Combining our expertise in inorganic chemistry, nanomaterials, and electrochemistry, we aim to address some of the challenging questions in catalysis, energy conversion, CO2/N2 fixation, and microbiota.

Perfluorocarbon Nanoemulsions Enhance Hybrid Bio-Inorganic CO2 and N2 Fixation Systems – With the surge of intermittent, renewable electricity, the storage of excessive electricity and reduction of CO2 or N2 into value-added chemicals is of great significance for a sustainable society. One viable route that fulfills such a target is to construct a hybrid inorganic-biological system that converts electricity into chemical energy and reduces CO2/N2 into commodity chemicals. In this general approach, water is split into H2 and O2 by renewable electricity and the yielded H2 is consumed by microbes as a reducing equivalent for CO2/N2 reduction. However, in both the CO­2 and N2 fixation examples, the limited solubility of the reducing equivalent, H­2, contributes to low product yields. Perfluorocarbon (PFC) nanoemulsions are known to have high gas solubilities, and our research demonstrated that they enhance the gas’s transport in solution. Coupling the PFC nanoemulsions with our CO2 fixation system yielded over 90% Faradaic Efficiency (F.E.) at all tested current densities and 3.5 times the H2 transfer rate. When applied to our N2 fixation hybrid system, a 250% increase in efficiency of NH3 production was achieved.  

Microbial-material Hybrid System for Nitrogen Fixation – Synthesis in biology is used to sustain life. Modifying non-photosynthetic bacteria for photosynthesis or optimizing the efficiency of existing synthesis provides promising approaches for sustainable chemical synthesis compared to traditional means. Microbial-material hybrid systems combine inorganic materials to provide/accept energy with biological catalysts namely microbes. The microbial-material hybrid systems possess the merits of both the microbes and the materials. The microbes enable desired chemical reactions with high specificity and activity under mild conditions, while the inorganic materials provide energy more effectively comparing to natural conditions for microbial growth. Here, we innovated a microbial-material hybrid system for nitrogen fixation and also digging the fundamentals of energy transfer at the interface of microbes and materials.

Machine Learning Optimization of Electrodes – Electrode morphology selection is imperative when considering electrode design. The surface area and density of morphology features can greatly alter the current density and efficiency of an electrode. Because the selection of morphology can alter electrode efficacy so much, tools are needed to select morphologies so that design is rationally guided. In order to do this, simulation is combined with Bayesian optimization – a sect of machine learning – to optimize and therefore select for electrode morphologies that favor our desired reaction. We have demonstrated this computationally with the nitrogen reduction reaction and are working to do the same with the carbon dioxide reduction reaction, this time adding experimental validation.

Designing O2 and H2O2 gradient on electrodes – Oxygen and hydrogen peroxide (H2O2) gradients are ubiquitous in bacterial/biological environments, having impacts on behavior – notably in biofilm formation. We have shown the feasibility of generating O2 gradients with four electron oxygen reduction (ORR) practically and in simulation. Similar to the 4 eoxygen reduction reaction, 2 e ORR can be used to generate H2O2 gradients. Currently, we aim to use experimental ORR results to improve finite element method (FEM) simulation precision so that accurate simulation models can be used with machine learning to optimize an ORR electrode. We optimize these electrodes by tuning their morphology (wire array) which in turn tunes their oxygen gradients at various potentials.

Electrochemical Concentration Gradients Promote New Catalytic Routes – The combination of electro, organometallic, and materials chemistries presents novel techniques for catalytic small-molecule functionalization. For example, cathodic potentials in wire-electrodes can generate oxygen (or other reactant) gradients, resulting in anoxic pseudo-glove box-like conditions. This allows for oxygen sensitive species, such as a rhodium porphyrin metalloradical, to activate methane in the anoxic regions of the electrode while being run under ambient conditions. Currently, efforts are underway to exploit these gradients to tune the microstructure of polyketone plastics. In this study, CO2 is reduced to CO – a key reactant in polyketone synthesis – with the aim of changing the amount and order of CO added, disrupting their typically alternating structure thus tuning their physical properties.

Electrocatalytic Activation of Methane – Methane is an abundant gaseous chemical, but its uncontrolled oxidation hinders its effective use for fuels and commodity chemicals. Currently commodity chemicals made from CH4 comes from using high pressures and temperatures, which is an expensive process. The wide geological distribution of natural gas resources leads to an undesirable loss of methane (CH4) especially at remote locations via flaring or direct emission into the atmosphere. One possible strategy to mitigate such an issue is to convert CH4 into liquid chemicals at the source of emission under ambient condition with minimal reliance on an industrial infrastructure, which remains to be studied. We have demonstrated a selective electrochemical functionalization of CH4 and natural gas mixtures at ambient pressure and room temperature with a molecular catalyst of vanadium (V)-oxo dimer.

Metal-Mediated Electrocatalytic Ambient CH4 Functionalization – Inspired by the HSAB theory, we explored the electrocatalytic CH4 functionalization mediated by a soft metal, whose reactivity towards CH4 is unknown. Detailed mechanistic investigation unveils a low activation energy of 13.1 kcal•mol−1 and a high pseudo-first-order rate constant of CH4 activation up to 2800 hr−1 at room temperature. Now we are encouraged to discover more CH4-activating catalysts, hopefully address the engineering constraints and achieve higher selectivity and activity.