Postdoc, California Institute of Technology
2D Covalent Metals: A New Materials Domain of Electrochemical CO2 Conversion
Electrochemical CO2 conversion into useful chemicals is regarded as a promising technology for a sustainable carbon cycle and easing climate change. Towards a successful deployment of the technology activating thermodynamically stable CO2, it is essential to develop catalysts with high activity, selectivity and stability. However, there has been an intrinsic limitation in the optimization of catalyst performance, a strong interdependence among binding energies of adsorbates involved during the CO2 reduction steps, which is called a scaling relationship. Recently, we have suggested that an extrinsic treatment such as the introduction of a heterogeneous boundary between metallic and non-/semi-metallic elements can be a way to pave a path to overcome the limitation, but the blending of different constituents may invoke some concerns about the catalytic stability and electrical conductivity. In this work, we offer a new idea to break the scaling relationship by using the intrinsic nature of two-dimensional (2D) covalent metals. We find that there exists a vast of 2D covalent metals with zero bandgap (i.e., good electrical conductivity), most of which are covered with non-/semi-metallic elements at the surface. Using high-throughput catalyst screening of such 61 2D covalent metals, we demonstrated an entire disruption of the scaling relation between COOH and CO binding energies, leading to the computational design of new catalysts for CO2 reduction either into CO or CH4. We also investigated the catalytic activity of the hydrogen evolution reaction (HER), a key side reaction of CO2 reduction, which was used to design catalysts not only for CO2 reduction with high selectivity, but also for HER with high activity. It should be emphasized that these finding are important because this provides insight into the way to break the strong scaling relationship limiting the catalyst performance and thereby offers a new list of materials to the field, which can be fully validated by experimental groups in the future.
Abstract: Selective electrochemical reduction of CO2 is one of the most sought-after processes because of the potential to convert a harmful greenhouse gas to a useful chemical. We have discovered that immobilized Ag nanoparticles supported on carbon exhibit enhanced Faradaic efficiency and a lower overpotential for selective reduction of CO2 to CO. These electrocatalysts were synthesized directly on the carbon support by a facile one-pot method using a cysteamine anchoring agent resulting in controlled monodispersed particle sizes. These synthesized Ag/C electrodes showed improved activities, specifically decrease of the overpotential by 300 mV at 1 mA/cm(2), and 4-fold enhanced CO Faradaic efficiency at -0.75 V vs RHE with the optimal particle size of 5 nm compared to polycrystalline Ag foil. DFT calculations enlightened that the specific interaction between Ag nanoparticle and the anchoring agents modified the catalyst surface to have a selectively higher affinity to the intermediate COOH over CO, which effectively lowers the overpotential.
Pub.: 09 Oct '15, Pinned: 30 Jun '17
Abstract: CO2 conversion is an essential technology to develop a sustainable carbon economy for the present and the future. Many studies have focused extensively on the electrochemical conversion of CO2 into various useful chemicals. However, there is not yet a solution of sufficiently high enough efficiency and stability to demonstrate practical applicability. In this work, we use first-principles-based high-throughput screening to propose silver-based catalysts for efficient electrochemical reduction of CO2 to CO while decreasing the overpotential by 0.4-0.5 V. We discovered the covalency-aided electrochemical reaction (CAER) mechanism in which p-block dopants have a major effect on the modulating reaction energetics by imposing partial covalency into the metal catalysts, thereby enhancing their catalytic activity well beyond modulations arising from d-block dopants. In particular, sulfur or arsenic doping can effectively minimize the overpotential with good structural and electrochemical stability. We expect this work to provide useful insights to guide the development of a feasible strategy to overcome the limitations of current technology for electrochemical CO2 conversion.
Pub.: 26 Jul '14, Pinned: 30 Jun '17
Abstract: Towards a sustainable carbon cycle, electrochemical conversion of CO2 into valuable fuels has drawn much attention. However, sluggish kinetics and a substantial overpotential, originating from the strong correlation between the adsorption energies of intermediates and products, are key obstacles of electrochemical CO2 conversion. Here we show that two-dimensional (2D) covalent metals with a zero band-gap can overcome the intrinsic limitation of conventional metals and metal alloys and thereby substantially decrease the overpotential for CO2 reduction because of their covalent characteristics. From first-principles-based high-throughput screening results on 61 2D covalent materials, we find that the strong correlation between the adsorption energies of COOH and CO can be entirely broken. This leads to the computational design of CO2-to-CO and CO2-to-CH4 conversion catalysts in addition to hydrogen-evolution-reaction catalysts. Towards efficient electrochemical catalysts for CO2 reduction, this work suggests a new materials domain having two contradictory properties in single material; covalent nature and electrical conductance.
Pub.: 28 Sep '16, Pinned: 30 Jun '17