Research

Hybrid Interfaces and Nanomaterials for Clean Energy Applications

As the global climate change becomes more and more evident with each passing decade, cutting greenhouse gas emissions from energy production, storage and utilization must be our top priority in order to minimize future climate changes. From the viewpoint of materials science, the research in energy is closely related to the interfacial transfers of electron/hole, ions and/or atoms, to facilitating the wanted transfers and to suppressing the unwanted transfers. Therefore, the study of physical and chemical properties at interfaces becomes exceptionally desirable in order to explore and fundamentally understand the electrical, optical and chemical phenomena occurring at various hybrid interfaces at nanometer scale. Research in our group is focused on the syntheses, characterizations, modifications and applications of a variety of hybrid interfaces for clean energy applications including gas sensors, hydrogen storage, solar cells, catalysis, Li‐ion battery and molecular electronics.

For electrochemical conversion of CO2 to value-added organic Molecules. The conversion of carbon dioxide (CO2) to value-added multi-carbon (with two or more than two C-C bonds) organic compounds by electrocatalytic reduction of CO2 provides a promising and value-added route to mitigate the greenhouse gas emission. We have developed a series of electrocatalysts using our long-established metal-in-lithium method that converts carbon dioxide to various value-added organic chemicals. Our family of carbon-supported transition and main group metals based electrocatalysts vary their sizes from single atoms to few atom clusters that can electrochemically reduce CO2 to C2 and C3 liquid organic compounds with high single-product selectivity at relatively low potentials. The reaction mechanism is extensively studied by Operando synchrotron X-ray absorption techniques to identify the dynamic evolution of the catalytic center. It is stunning to reveal that atomically dispersed catalytic metal atoms aggregate to few-atom clusters under reductive potentials and depending on the redox potentials of the metals, oxygen atoms can also involve in the catalytic center. In addition, the supporting substrates exhibit a profound nanoscale confinement effect that also impacts the reaction pathways towards C-C coupling. Our work renders atomistic and quantum mechanical level understanding in the reaction coordination for electrochemical CO2 reduction reaction to a versatility of value-added organic compounds. Nature Energy, 2020, 5, 623-632 

Catalyst synthesis. Step-by-step preparation of the carbon-supported Cu SA catalyst using an amalgamated Cu–Li method. 

Electrocatalytic CO2 reduction reaction (CO2RR) Performance. (a) Faradaic Efficient (FE) indicates over 90% of electrical current is used to convert CO2 to ethanol at a very low cathodic potential of -0.6 V vs reversible hydrogen electrode (RHE). (b) High durability of the electrocatalysts evidenced by the FE and current density as the function of time during chronoamperometric electrolysis at -0.6 V vs RHE. 

For perovskite solar cells. Stability, scalability and toxicity are the triathlon challenge impeding the commercialization of perovskite solar cells (PSC) despite their high power conversion efficiency. Uncertainty exists in the mutual compatibility among the reported formulations for improving stability. In addition, >50% infrastructure investment of future PSC production line will be on the expensive, time-consuming and energy-costly vacuum deposition equipment. Lead safety is another public concern bothering the acceptance of PSC by general public. These challenges requires in-depth understanding of the light-matter interaction and charge-charge interactions in the perovskite solar cells. Below is our recent progress in tackling all these challenges. 

Stability. We show (see figure below) that replacing the agile hydrogen in A-site cation with its non-radioactive but twice heavier isotope deuterium offers a low-cost approach to retard the degradation reaction kinetics of hybrid perovskites, thus to improve stability of PSC as an independent add-on feature that is adaptable to any existing PSC formula. PNAS Nexus, 2023, 2(5), pgad160. 

Stability enhancement by replacing H with D in the A-Site cation of PSC. (a) The Photovoltaic performance are the same for H-based and D-based PSC. (b) The stability of D-based PSC outperforms the H-based PSC. 

Scalability. Most reported champion PSC use vacuum deposited gold as the back contact electrode for its high work function for charge extraction and high conductivity for charge transport to external circuit. However, vacuum deposition equipment accounts for >50% of the capital equipment cost and 50% manufacturing energy cost for PSC. Meanwhile, gold is too expensive to use at scale (8 tons of gold is needed for 1GW of PSC, not to mention the wasted gold during vacuum deposition). We show that by allocating the high work function and high conductivity to two separate materials, a bilayer electrode consisting of nature graphite (for its high work function) and low-melting point metal alloy (for its high conductivity) can replace the role of gold in PSC with PV performance compatible with that of gold-based PSC. More strikingly, both nature graphite and metal alloy can be laminated under ambient pressure, thus circumventing the expensive vacuum coating process. 

Bilayer back electrode laminated at ambient pressure on n-i-p type of PSC showing power conversion efficiency of 21% without using any costly vacuum coating process. 

Toxicity. We developed an on-device lead sequestering solar ethylene vinyl acetate (EVA) encapsulating film to mitigate the potential leakage of toxic lead from PSC to environment. Most high-performance perovskite solar cells contains water-soluble lead contents and non-tempered transparent conducting oxide (TCO) glass. Thus, future commercialization of PSC is subject to tight environmental and safety regulations. Based on the charge-ligand interaction, we developed a lead-absorbing solar EVA film technology, offering an excellent solution to form laminated safety TCO glass to avoid glass shards for building integrated photovoltaics (BIPV) and to minimize lead leakage to environment or to residential area. See more details at Nature, 2020, 578, 555 and Nature Sustainability 2021, 4, 1038. 

Process of a lead-absorbing EVA film by laminating P,P’-di(2-ethylhexyl) methanediphosphonic acid (DMDP) on EVA film that is used to encapsulate the PSC devices. 

Water soaking tests show that after soaking the damaged samples in water for even 7 days, 99.9% Pb2+ ions that would otherwise leak to environment can be captured by our leading-absorbing EVA films demonstrated in both (a) n-i-p and (b)p-i-n PSC devices. 

For gas sensor research, as an example illustrated in Figure 1, we used gas‐induced changes in the electrical transport properties of a percolating nanocluster‐like palladium thin film on a self‐assembled organic monolayer to rapidly detect hydrogen and other gaseous species. Appl. Phys. Lett. 2005, 86, 203104.

Figure 1. Schematic drawing of a nanopalladium thin film/organic monolayer assembly used in the rapid detection of hydrogen. Conductivity is low in the absence of hydrogen, but higher when hydrogen is present.

For hydrogen storage materials, one of our research interests is to fundamentally understand the kinetics and thermodynamics associated with the diffusion of hydrogen adatoms across the nanoscale interfaces between nanocatalysts and storage materials. This phenomenon is often termed as hydrogen spillover and is believed to be an effective approach to suppress the activation energy during hydriding and dehydriding processes. We have used an in‐situ electrical method to rapidly probe hydrogen spillover from nanocatalyst to amorphous carbon. We also established a piezoelectric nanogravimetric system for measuring hydrogen mass uptake in thin film materials. Different from any existing bulk gravimetric or volumetric system, our thin film nanogravimetric system allows us to conveniently establish a well‐controlled interface between catalyst and storing material. Meanwhile, we are also working on improving the hydriding kinetics of bulk storage materials including metal hydrides and adsorbents with large surface area through hydrogen spillover. For example, we demonstrated the enhanced hydriding kinetics in palladium‐doped lithium nitride as illustrated in Figure 2. J. Phys. Chem. C, 2009, 113, 8513.

Figure 2. Schematic elucidation of hydrogen spillover-enhanced hydriding in complex metal hydride-based hydrogen storage materials.

For solar cell research, one of our research interests is in the field of dye‐sensitized solar cells (DSSCs). We explore new nanoarchitectured electrode designs that can bring new basic sciences to enhance the interfacial electron transport in DSSCs. For example, we recently demonstrated that using the fill factor and open circuit voltage of ZnO‐based DSSCs can be significantly improved by using a Zn‐microtip|ZnO‐nanotip core‐shell hierarchy nano‐architecture (shown in Figure 3) as the anode in a DSSC. Unlike a planar electron‐collecting anode, the rough Zn microtip anode provides more surface area to accommodate more semiconductors without significantly increasing the electron diffusion length in the semiconductor layer. J. Phys. Chem. C, 2009, 113, 20521.

Figure 3. Zn-microtip|ZnO-nanotip core–shell hierarchy nano-architecture as anode in dye-sensitized solar cells.

For nanocatalysis, we are currently focusing on the novel synthesis of precious metal‐based nanocatalysts. Precious metal catalysts such as Pt and Pd are essential for achieving energy‐efficient and greener chemical processes that involve breaking or establishing of H‐H, C‐H, or O‐H bonds. Because of their low natural abundance and high cost, precious metal‐based catalysts in most applications use finely divided metal particles of a few nanometers to maximize the surface‐to‐bulk atomic ratio for atom‐economy. In general, nearly all synthetic methods to prepare precious metal nanoparticles follow a bottom‐up protocol, in which the precursory molecules (precatalysts) of precious metals are first reduced into metal atoms, which then grow into polyatomic particles. However, the use of precursory molecules inevitably lead to various complications, including the energy‐costly and tedious syntheses of precursory molecules and pretreatment of support materials, the possible contamination resulting from the hetero atoms in the precursory molecules, which will likely cause the loss of catalytic activity; the removal of stabilizing agents in many precursory‐based solution methods in order to free the catalytic sites etc.

In contrast, the counter‐strategy in nanomaterials synthesis, namely a top‐down strategy, has been largely overlooked for preparation of precious metal‐based nanoparticles. We recently demonstrated an innovative precursor‐free synthetic strategy to prepare the supported precious metal nanoparticles, starting directly from the corresponding bulk metals or metal alloys to bypass the complications associated with the synthetic methods using precursory molecules. As illustrated in Figure 4, bulk precious metals such as Pt, Pd, Ru, Rh, Ir, Au, Ag can be ruptured into nanonucleis in liquid Li at relatively low temperatures according to their phase diagrams with Li. The metal‐Li liquid alloys can be quenched into a metal‐Li solid solution. Next, Li in the Li‐Pt solid solution can be converted into LiOH powder via a controlled gas‐solid reaction 2Li (s) + 2H₂O (g) → 2LiOH (s) + H₂ (g) at 100°C, which prevents the aggregation of the as‐formed Pt nanoparticles. Furthermore, the metal nanoparticles in LiOH powder can then be mixed with any water‐insoluble support materials. The LiOH in the mixture can be selectively leached off with copious amounts of water under ambient condition, allowing Pt nanoparticles to be adsorbed on the neighboring support materials. This precursor‐free process avoids the harsh pre‐treatments of the support materials that are often required in precursor‐based syntheses. As a result, many soft materials including polymer materials can be used as support, provided they are not water soluble. In addition, the trapping of nanocatalysts in the deep cracks of solid support materials can be mitigated since the entering of particles into the deep cracks would be much harder than the precursory molecules. J. Am. Chem. Soc., 2010, 132, 2151.

Figure 4. Top-down synthesis of supported Pt nanoparticles from the Li–Pt solid solution.

Through our close and active collaborations with researchers at Argonne National Laboratory and National Renewable Energy Laboratory, students in our group will use a variety of cutting‐edge techniques, including atomic force microscopy, scanning electron microscopy, UHV thin‐film sputter systems, thermal evaporation, photo/electron‐beam lithography, electrical transport measurement systems, electrochemical measurement system, advanced photon source (APS at Argonne) to explore the fundamental science at nanoscale hybrid interfaces and to synthesize the corresponding materials for clean energy applications.