H-Li Research Group
-Energy Conversion via Catalyst Engineering
Research Focus & Prospective
We are interested in transition metals and their compounds as catalysts for various electrochemical reactions, particularly two-dimensional transition metal compounds such as transition metal chalcogenides, carbides and nitrides. The most appealing reason for choosing two-dimensional catalysts containing transition metals is that (1) transition metals' half-filled d orbital make it easy to accept or donate electrons, and (2) two-dimensionality enables most of their atoms be exposed to the environment. For instance, doped graphene is a good catalyst for oxygen reduction reaction, and tungsten diselenide is a superior catalyst for electrochemical hydrogen evolution and carbon dioxide reduction.
Our group focus on electrochemical reactions that are beneficial for overcoming energy and environment challenges. Specifically, we focused on hydrogen generation from water, and carbon dioxide reduction to energy fuels.
However, pristine materials are usually not optimized for specific reactions, and engineering processes are needed to change these pristine materials to practical catalysts; this is where our research plays a role. For instance, creating sulphur vacancy can activate the inert basal plane of molybdenum disulphide for hydrogen evolution, and doping of cobalt into molybdenum can result in a good catalyst for direct methane to methanol conversion (see figure below). These engineering approaches also represent a unique feature of our research. We strive to discover new catalysts, to optimize existing catalysts, to integrate catalysts into devices, and to demonstrate prototypes of products.
Energy Applications
Our global power consumption is about 18 TW, where renewable energy only contributes to about 4% due to its poor scalability limited by its precious metal catalysts. For instance, hydrogen generation using PEM electrolyzer employs platinum as catalyst with loading about 0.5-1 mg/cm2 for power of 1W/cm2. Thus, 500-1000 tons platinum is needed to produce hydrogen with power of 1 TW, which is about 2.5-5 years platinum production globally. Therefore, replace of platinum is inevitable for hydrogen fuel to make relevant impact for future energy supply.
Problem 3: Global energy consumption increases by 15% every decade while petroleum supply is declining gradually. Traditional energy sources alternative to petroleum such as coal will worsen the climate change by emitting more greenhouse gases. Greenhouse gas emission, mainly carbon dioxide and methane emission, results in global warming that causes about 300,000 casualties every year.
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Our effort: We investigate electrochemical carbon dioxide reduction. We try to increase the efficiency of carbon dioxide reduction efficiency by improving the catalyst via various engineering techniques. We strive to find cost-effective solution for carbon dioxide utilization.
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Waste-to-Value for Circular Economy
The other focus of ours is to couple hydrogen evolution reaction with other reactions to widen its application. For instance, couple hydrogen reduction with natural gas activation. Converting gaseous natural gas to liquid fuels is particularly important for utilizing the so-called stranded natural gas, meaning natural gas reserves that are remote and small in scale. Stranded natural gas actually represents half of entire natural gas reserve.
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One of our focuses is to catalyze hydrogen generation from water using more earth-abundant catalysts such as molybdenum disulphide. We controllably create sulphur vacancies in monolayer molybdenum disulphide to activate its inert basal plane. The sulphur removal exposes the undercoordinated molybdenum atoms that act as new active centers for hydrogen evolution reactions. We further optimize this new catalyst using tensile elastic strain (see figures below). With the combination of vacancy and strain engineering, we are able to obtain the most active molybdenum disulphide based electrocatalyst for hydrogen evolution.
Problem 1: Billion tons of solid waste has been generated annually by our society, including woody biomass and food waste. The solid and gaseous wastes we produce from consumption of plants (biomass) go into the environment as carbon. The carbons in the air and soil are utilized by plants and turn into biomass and fuel. In this carbon cycle, biomass provides renewable fuel that help to mitigate greenhouse problem. It utilizes biodegradable waste to produce fuel resources and energy, which doesn’t result in pollution as burning fossil fuel does. Cellulose, an important structural component of the cell wall of green plants, is the most abundant biopolymer. Therefore, utilization of cellulose is an important part of biomass refinery. Cellulose can be converted to sugars, alcohols or other chemicals. The main challenge of cellulose refinery originates from its stubborn crystalline structure that typically needs harsh process conditions including high temperature, high pressure, and high acid concentration to break it down to useful products. Wood biomass is a major composition of these waste. Wood has been a major source of fuel since the beginning of human history. More than two billion people still rely on wood (or charcoal derived from wood) for heating or cooking. Besides being a source of thermal energy, woody biomass can also provide electrical energy if the steam boiler heated by wood combustion is used to drive an electrical turbine. Wood pyrolysis, which causes thermal destruction of woody biomass in an anaerobic environment, produces bio-oil. A hydrolysis process followed by fermentation can also convert wood to alcohol. Gasification with limited oxygen converts wood to producer gas that can be further upgraded to syngas as a common feedstock for valuable chemicals. Food waste is rich of valuable nitrogen and phosphor that are provide nutrition to animals and human. A method to extract these valuable components is needed.
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Our effort: We are trying to extract value from lignocellusic biomass wastes at low temperature and decentralized systems driven by renewable energies, which is a greener way compared to high-temperature and high-pressure processes.
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Problem 2: Plastic, one of the most incredible inventions, has become more and more problematic for our society. Plastic pollution becomes one of the main environment issues. Our focus on plastic pollution has been long limited to that in the ocean. As we think plastic pollution only threatens marine animals’ lives, microplastics were found in human body very recently. Eight million tons of plastic enter our waterways annually, and more is disposed in landfills. Overall, less than 10% of the plastic produced are recycled. Moreover, the plastic industry is projected to grow with 5% annual rate.
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Our effort: We investigate photoelectrochemical catalysts and processes to breakdown unrecyclable/unsorted plastics into small molecules that could be fuel and commodity chemicals. As most of plastic waste is generated in remote and developing areas, we believe a decentralized system with processes driven by renewable energies is a viable solution.
