Dunfeng Gao

Size-Dependent Electrocatalytic Reduction of CO2 over Pd Nanoparticles

Size effect has been regularly utilized to tune the catalytic activity and selectivity of metal nanoparticles (NPs). Yet, there is a lack of understanding of the size effect in the electrocatalytic reduction of CO2, an important reaction that couples with intermittent renewable energy storage and carbon cycle utilization. We report here a prominent size-dependent activity/selectivity in the electrocatalytic reduction of CO2 over differently sized Pd NPs, ranging from 2.4 to 10.3 nm. The Faradaic efficiency for CO production varies from 5.8% at -0.89 V (vs reversible hydrogen electrode) over 10.3 nm NPs to 91.2% over 3.7 nm NPs, along with an 18.4-fold increase in current density. Based on the Gibbs free energy diagrams from density functional theory calculations, the adsorption of CO2 and the formation of key reaction intermediate COOH* are much easier on edge and corner sites than on terrace sites of Pd NPs. In contrast, the formation of H* for competitive hydrogen evolution reaction is similar on all three sites. A volcano-like curve of the turnover frequency for CO production within the size range suggests that CO2 adsorption, COOH* formation, and CO* removal during CO2 reduction can be tuned by varying the size of Pd NPs due to the changing ratio of corner, edge, and terrace sites.

Cobalt nanoparticles encapsulated in nitrogen-doped carbon as a bifunctional catalyst for water electrolysis

The hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are important electrocatalytic processes in water electrolyzers. Identifying efficient non-precious metal catalysts for HER and OER remains a great challenge for applications in different kinds of electrolyzers. Herein, we report that cobalt nanoparticles encapsulated in nitrogen-doped carbon (Co@N–C) show high activity and durability for HER in a wide pH range and for OER in alkaline medium as a bifunctional catalyst. The HER and OER activities of Co@N–C are higher than those of multiwall carbon nanotube and iron nanoparticles encapsulated in nitrogen-doped carbon with a similar content of nitrogen. Electrolyzer prototypes using Nafion NRE-212 as electrolyte membrane and Co@N–C as cathode or anode catalyst are constructed, showing potential practical applications in water splitting.

Enhancing CO2 Electroreduction with the Metal–Oxide Interface

The electrochemical CO2 reduction reaction (CO2RR) typically uses transition metals as the catalysts. To improve the efficiency, tremendous efforts have been dedicated to tuning the morphology, size, and structure of metal catalysts and employing electrolytes that enhance the adsorption of CO2. We report here a strategy to enhance CO2RR by constructing the metal-oxide interface. We demonstrate that Au-CeOx shows much higher activity and Faradaic efficiency than Au or CeOx alone for CO2RR. In situ scanning tunneling microscopy and synchrotron-radiation photoemission spectroscopy show that the Au-CeOx interface is dominant in enhancing CO2 adsorption and activation, which can be further promoted by the presence of hydroxyl groups. Density functional theory calculations indicate that the Au-CeOx interface is the active site for CO2 activation and the reduction to CO, where the synergy between Au and CeOx promotes the stability of key carboxyl intermediate (*COOH) and thus facilitates CO2RR. Similar interface-enhanced CO2RR is further observed on Ag-CeOx, demonstrating the generality of the strategy for enhancing CO2RR.

Plasma-Activated Copper Nanocube Catalysts for Efficient Carbon Dioxide Electroreduction to Hydrocarbons and Alcohols

Carbon dioxide electroreduction to chemicals and fuels powered by renewable energy sources is considered a promising path to address climate change and energy storage needs. We have developed highly active and selective copper (Cu) nanocube catalysts with tunable Cu(100) facet and oxygen/chlorine ion content by low-pressure plasma pretreatments. These catalysts display lower overpotentials and higher ethylene, ethanol, and n-propanol selectivity, resulting in a maximum Faradaic efficiency (FE) of ∼73% for C2 and C3 products. Scanning electron microscopy and energy-dispersive X-ray spectroscopy in combination with quasi-in situ X-ray photoelectron spectroscopy revealed that the catalyst shape, ion content, and ion stability under electrochemical reaction conditions can be systematically tuned through plasma treatments. Our results demonstrate that the presence of oxygen species in surface and subsurface regions of the nanocube catalysts is key for achieving high activity and hydrocarbon/alcohol selectivity, even more important than the presence of Cu(100) facets.

Switchable CO2 electroreduction via engineering active phases of Pd nanoparticles

Active-phase engineering is regularly utilized to tune the selectivity of metal nanoparticles (NPs) in heterogeneous catalysis. However, the lack of understanding of the active phase in electrocatalysis has hampered the development of efficient catalysts for CO2 electroreduction. Herein, we report the systematic engineering of active phases of Pd NPs, which are exploited to select reaction pathways for CO2 electroreduction. In situ X-ray absorption spectroscopy, in situ attenuated total reflection-infrared spectroscopy, and density functional theory calculations suggest that the formation of a hydrogen-adsorbed Pd surface on a mixture of the α- and β-phases of a palladium-hydride core (α+β PdHx@PdHx) above −0.2 V (vs. a reversible hydrogen electrode) facilitates formate production via the HCOO* intermediate, whereas the formation of a metallic Pd surface on the β-phase Pd hydride core (β PdHx@Pd) below −0.5 V promotes CO production via the COOH* intermediate. The main product, which is either formate or CO, can be selectively produced with high Faradaic efficiencies (>90%) and mass activities in the potential window of 0.05 to −0.9 V with scalable application demonstration.

Gas-phase electrocatalytic reduction of carbon dioxide using electrolytic cell based on phosphoric acid-doped polybenzimidazole membrane

Abstract Carbon dioxide transformation to fuels or chemicals provides an attractive approach for its utilization as feedstock and its emission reduction. Herein, we report a gas-phase electrocatalytic reduction of CO 2 in an electrolytic cell, constructed using phosphoric acid-doped polybenzimidazole (PBI) membrane, which allowed operation at 170 °C. Pt/C and PtMo/C with variable ratio of Pt/Mo were studied as the cathode catalysts. The results showed that PtMo/C catalysts significantly enhanced CO formation and inhibited CH 4 formation compared with Pt/C catalyst. Characterization by X-ray diffraction, X-ray photoelectron spectroscopy and transmission electron microscopy revealed that most Mo species existed as MoO 3 in PtMo/C catalysts and the interaction between Pt and MoO x was likely responsible for the enhanced CO formation rate although these bicomponent catalysts in general had a larger particle size than Pt/C catalyst.

Silicon carbide-supported iron nanoparticles encapsulated in nitrogen-doped carbon for oxygen reduction reaction

Silicon carbide (SiC) was extracted using CCl4 and NH3 at 800 °C to form a SiC core with a derived nitrogen-doped carbon shell (SiC@N–C), which is explored as a supporting material for iron nanoparticles encapsulated in nitrogen-doped carbon (Fe@N–C) due to its excellent corrosion resistance. The carbon shell around SiC is essential to successfully grow Fe@N–C around SiC@N–C during pyrolysis of cyanamide and iron acetate. In sharp contrast, Fe3Si supported on SiC was obtained using pristine SiC as the supporting material. Fe@N–C/SiC@N–C showed much higher activity for oxygen reduction reaction than SiC@N–C and Fe3Si/SiC, even exceeding that of a commercial Pt/C catalyst in alkaline medium. Furthermore, Fe@N–C/SiC@N–C also demonstrated higher durability and methanol resistance than the Pt/C catalyst.

Dynamic Changes in the Structure, Chemical State and Catalytic Selectivity of Cu Nanocubes during CO2 Electroreduction: Size and Support Effects

In situ and operando spectroscopic and microscopic methods were used to gain insight into the correlation between the structure, chemical state, and reactivity of size- and shape-controlled ligand-free Cu nanocubes during CO2 electroreduction (CO2 RR). Dynamic changes in the morphology and composition of Cu cubes supported on carbon were monitored under potential control through electrochemical atomic force microscopy, X-ray absorption fine-structure spectroscopy and X-ray photoelectron spectroscopy. Under reaction conditions, the roughening of the nanocube surface, disappearance of the (100) facets, formation of pores, loss of Cu and reduction of CuOx species observed were found to lead to a suppression of the selectivity for multi-carbon products (i.e. C2 H4 and ethanol) versus CH4 . A comparison with Cu cubes supported on Cu foils revealed an enhanced morphological stability and persistence of CuI species under CO2 RR in the former samples. Both factors are held responsible for the higher C2 /C1 product ratio observed for the Cu cubes/Cu as compared to Cu cubes/C. Our findings highlight the importance of the structure of the active nanocatalyst but also its interaction with the underlying substrate in CO2 RR selectivity.

Selective CO2 Electroreduction over Oxide-Derived Gallium Catalyst

The electrochemical CO2 reduction reaction (CO2RR) powered by renewable electricity has emerged as a promising approach to alleviate global warming and energy depletion simultaneously. Notably, efficient catalysts containing Earth-abundant elements to achieve high CO2RR performance are in great demand for future applications. Herein, carbon-supported gallia gel nanoparticles were synthesized by precipitating gallium nitrate on carbon black in an ethanolic ammonia solution. Nano-sized gallia nanoparticles uniformly dispersed on the carbon support achieved a maximum CO faradaic efficiency of 77.0% at −0.71 V vs. the reversible hydrogen electrode (RHE) in CO2-saturated 0.1 M KHCO3 solution, showing a dramatic improvement compared to a bulk Ga electrode with only 24.2% CO faradaic efficiency at −0.80 V vs. RHE. X-ray photoelectron spectroscopy measurements revealed that surface Ga3+ species were reduced to metallic Ga when subjected to a negative potential during the CO2RR, indicative of the formation of oxide-derived active gallium sites. Control experiments further highlighted the necessity of close coalescence between the nano-sized gallia particles and the conductive carbon support. The present study underscores the feasibility of improving the CO2RR performance of Ga-related materials through nanostructuring of oxide-derived gallium catalysts.

Structure- and Electrolyte-Sensitivity in CO2 Electroreduction

The utilization of fossil fuels (i.e., coal, petroleum, and natural gas) as the main energy source gives rise to serious environmental issues, including global warming caused by the continuously increasing level of atmospheric CO2. To deal with this challenge, fossil fuels are being partially replaced by renewable energy such as solar and wind. However, such energy sources are usually intermittent and currently constitute a very low portion of the overall energy consumption. Recently, the electrochemical conversion of CO2 to chemicals and fuels with high energy density driven by electricity derived from renewable energy has been recognized as a promising strategy toward sustainable energy. The activation and reduction of CO2, which is a thermodynamically stable and kinetically inert molecule, is extremely challenging. Although the participation of protons in the CO2 electroreduction reaction (CO2RR) helps lower the energy barrier, high overpotentials are still needed to efficiently drive the process. On the other hand, the concurrent hydrogen evolution reaction (HER) under CO2RR conditions leads to lower selectivity toward CO2RR products. Electrocatalysts that are highly active and selective for multicarbon products are urgently needed to improve the energy efficiency of CO2RR. The reduction of CO2 involves multiple proton-electron transfers and has many complex intermediates. Recent reports have shown that the relative stability of the intermediates on the surface of catalysts determines final reaction pathways as well as the product selectivity. Furthermore, this reaction displays a strong structure-sensitivity. The atomic arrangement, electronic structure, chemical composition, and oxidation state of the catalysts significantly influence catalyst performance. Fundamental understanding of the dependence of the reaction mechanisms on the catalyst structure would guide the rational design of new nanostructured CO2RR catalysts. As a reaction proceeding in a complex environment containing gas/liquid/solid interfaces, CO2RR is also intensively affected by the electrolyte. The electrolyte composition in the near surface region of the electrode where the reaction takes place plays a vital role in the reactivity. However, the former might also be indirectly determined by the bulk electrolyte composition via diffusion. Adding to the complexity, the structure, chemical state and surface composition of the catalysts under reaction conditions usually undergo dynamic changes, especially when adsorbed ions are considered. Therefore, in addition to tuning the structure of the electrocatalysts, being able to also modify the electrolyte provides an alternative method to tune the activity and selectivity of CO2RR. In situ and operando characterization methods must be employed to gain in depth understanding on the structure- and electrolyte-sensitivity of real CO2RR catalysts under working conditions. This Account provides examples of recent advances in the development of nanostructured catalysts and mechanistic understanding of CO2RR. It discusses how the structure of a catalyst (crystal orientation, oxidation state, atomic arrangement, defects, size, surface composition, segregation, etc.) influences the activity and selectivity, and how the electrolyte also plays a determining role in the reaction activity and selectivity. Finally, the importance of in situ and operando characterization methods to understand the structure- and electrolyte-sensitivity of the CO2RR is discussed.