Electrochemical reduction of CO2 using solid oxide electrolysis cells (SOECs) has emerged as an attractive approach for converting CO2 to high energy molecules, such as CO, a key precursor for the ...synthesis of fuels and chemicals using the commercially established Fischer–Tropsch process. The in situ generation of syngas (CO and H2) has also been demonstrated in SOECs through the coelectrolysis of CO2 and H2O. However, conventional Ni-based SOEC cathodes exhibit high overpotential losses associated with CO2 activation, leading to the disproportional activation of CO2 and H2O during coelectrolysis, facilitating the equilibrium-limited thermochemical reverse water gas shift (RWGS) reaction. Thus, identification of factors that govern CO2 activation on transition metal electrocatalysts is important toward optimizing the performance of SOEC cathodes for modulated production of syngas. Herein, we experimentally assess the electrocatalytic performance of monometallic transition metal electrocatalysts (Fe, Ni, and Pd) toward electrochemical CO2 reduction in SOECs with the aim of understanding the electrocatalyst characteristics that govern this performance. We report that metal oxophilicity (a property correlated to the strength of metal–oxygen bonding) plays an important role in the energetics associated with electrochemical CO2 reduction and electrocatalyst deactivation via oxidation. We suggest that a compromise in the oxophilicity of the metal is required to achieve optimal electrochemical activity and stability because CO2 activation is facile on highly oxophilic transition metals to the left of Ni (i.e., Fe); however, strong oxygen binding on these metals leads to their deactivation via oxidation. Potential approaches that facilitate the electronic structure modulation of transitional metals to optimize their surface oxophilicity, such as alloying, are suggested.
Recent advances in the use of nonstoichiometric mixed metal oxides belonging to the perovskite family as cost-effective catalysts for various oxygen-related heterogeneous thermochemical and ...electrochemical reactions have led to the need for the development of robust design criteria to tune their catalytic performance. The current paradigm for describing the electrocatalytic activity of these oxides relies on the averaged oxidation state of the transition metal in the structure, which often fails to systematically describe activity. In addition, the current design strategies mainly focus on activity and often overlook oxide stability. Therefore, the development of robust criteria that rely on measurable oxide properties and can shed light on the electrochemical activity and stability of these oxides still remains a challenge. Herein, we demonstrate an approach for correlating experimentally measurable oxide properties (i.e., oxide surface reducibility) with the oxide activity and stability. This is demonstrated through the use of electrochemical oxygen reduction reaction (ORR) as a probe reaction. We show that the oxide surface reducibility describes the transition metal–lattice oxygen bond strength and captures effects from both the oxide composition and crystal symmetry on the binding energetics of the oxygenated intermediates and consequently the ORR activity and stability. Comprehensive design strategies for efficient ORR on nonstoichiometric mixed metal oxides are devised. Such strategies have the potential to be extended to other oxygen-related catalytic reactions on these nonstoichiometric mixed metal oxides.
Identifying structure−performance relationships is critical for the discovery and optimization of heterogeneous catalysts. Recent theoretical contributions have led to the development of d-band ...theory, which relates the calculated electronic structure of a metal to its chemical and catalytic activity. While there are many contributions where quantum-chemical calculations have been utilized to validate the d-band theory, experimental examples relating the electronic structures of commercially relevant nonmodel catalysts to their performance are lacking. We show that even small changes in the near-Fermi-level electronic structures of nonmodel supported catalysts, induced by the formation of surface alloys, can be measured and related to the chemical and catalytic performance of these materials. We demonstrate that critical shifts in the d-band center in alloys are related to the formation of new electronic states in response to alloying rather than to charge redistribution among constitutive alloy elements, i.e., the number of d holes and d electrons localized on the constitutive alloy elements is constant. On the basis of the presented results, we provide a simple, physically transparent framework for predicting shifts in the d-band center in response to alloying and relating these shifts to the chemical characteristics of the alloys.
Selective electrochemical reduction of CO2 using renewable energy sources to create platform molecules for synthesis of fuels and chemicals has become a contemporary research area of interest because ...of its potential for recycling and minimizing the adverse environmental impacts of CO2. Solid oxide electrolysis cells (SOECs) are solid-state electrochemical devices with significant potential in this area because of their ability to efficiently and selectively convert CO2 to CO or, when coupled with water electrolysis, to produce syngas (CO and H2). Both CO and syngas are precursors for the synthesis of fuels and chemicals using existing technologies. While promising, SOECs are limited by the instability of the state-of-the-art cathode electrocatalyst, Ni/yttria-stabilized zirconia (YSZ) cermet, due to its limited redox properties and deactivation by carbon deposits. Nonstoichiometric mixed ionic and electronic conducting oxides are promising alternatives because of their redox stability and resistance to deactivation by carbon. Herein, we summarize the literature in this area and derive trends that relate changes in composition and oxygen defects in these oxides to activity, selectivity, and stability for the electrochemical reduction of CO2 to CO in SOECs using both experimental and theoretical studies. We also evaluate the factors that present challenges in a direct comparison of the performance of SOEC cathode electrocatalysts for CO2 reduction reported in the literature and suggest possible solutions and standardized protocols for benchmarking the performance of SOECs. We conclude by summarizing and providing an overview of challenges in the field along with potential solutions and opportunities for electrochemical reduction of CO2 by nonstoichiometric mixed metal oxides in SOECs.
Recent efforts to design selective catalysts for multi‐step reactions, such as hydrodeoxygenation (HDO), have emphasized the preparation of active sites at the interface between two materials having ...different properties. However, achieving precise control over interfacial properties, and thus reaction selectivity, has remained a challenge. Here, we encapsulated Pd nanoparticles (NPs) with TiO2 films of regulated porosity to gain a new level of control over catalyst performance, resulting in essentially 100 % HDO selectivity for two biomass‐derived alcohols. This catalyst also showed exceptional reaction specificity in HDO of furfural and m‐cresol. In addition to improving HDO activity by maximizing the interfacial contact between the metal and metal oxide sites, encapsulation by the nanoporous oxide film provided a significant selectivity boost by restricting the accessible conformations of aromatics on the surface.
Nanoscale morphology control over active sites consisting of Pd and TiO2 specifies binding orientation of reactant molecules to provide unprecedented reaction specificity toward hydrodeoxygenation during catalytic conversion of biomass‐derived aromatic alcohols/aldehydes and phenolic compounds.
Electrochemical high-temperature oxygen reduction and evolution play an important role in energy conversion and generation using solid oxide electrochemical cells. First-series Ruddlesden–Popper ...(R-P) oxides (A2BO4) have emerged as promising electrocatalysts for these reactions due to their suitable mixed ionic and electronic conductivities. However, a detailed understanding of the factors that govern their performance is still elusive, making their optimization challenging. In the present work, a systematic theoretical study is used to investigate the underlying factors that control the process of surface oxygen exchange, which governs oxygen reduction and evolution on these oxides. The effects of A- and B-site composition and surface termination of these oxides on their activities are elucidated. Among the different compositions, Co-based, B-site-terminated R-P oxides are predicted to exhibit the highest activity due to providing the best compromise between the energetics associated with oxygen dissociation and surface oxygen vacancy formation. A “volcano”-type relation between the calculated rates for surface oxygen exchange and O2 binding energy on a surface oxygen vacancy is found, suggesting the O2 binding energy might be used as an activity descriptor to identify R-P oxides with optimized performance. These findings shed light on the factors that govern the reported experimental behaviors of these oxides and lay the groundwork for the development of predictive models to design optimal mixed ionic and electronic conducting oxides for high-temperature oxygen reduction and evolution.
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