Electrochemical CO₂ Reduction to Two-Electron Products

Climate change remains a major global challenge and with CO2 emissions still increasing, a rapid transition towards renewables is required. Power-2-X and in this context electrochemical CO2 reduction offers a promising solution to several issues related to this. First of all, it provides a means of storing excess energy from intermittent renewable energy sources in chemical bonds. At the same time, it also provides a viable synthesis route for the production of CO2 neutral fuels we can use in our existing energy infrastructure. The technology still needs significant improvements both in terms of the activity and efficient use of the electrons supplied, but also in terms of improving selectivity towards desired products such as ethanol. This thesis studies the formation of two-electron products in CO2 reduction, to understand what guides the activity and selectivity on the different metals. We map out the selectivity for different metals and identify palladium as a clear outlier. In addition, Pd is able to produce both two-electron products with high selectivity and even switch between them across a quite narrow potential range, which makes it interesting to us as a fundamental study of what guides CO2R product selectivity. We find that the entire Pd electrode undergoes structural changes in the presence of the electrochemical environment, forming a highly intercalated palladium hydride (PdH) structure. This changes the properties of the material and thus its ability to reduce CO2. It has been proposed that the selectivity towards formate is driven by the *OCO binding motif, as opposed to *CO2 driving CO production. This is not seen on PdH, instead formate is formed via a (surface) hydrogenation step. Furthermore, the electrochemical environment introduces strong differences in the driving forces between the two products, with the formate pathway initially being favored followed by a switch due to strong stabilization with potential of the CO pathway. Next, we move on to take a more general look at the structural and environmental factors that affect the CO2R activity. We construct a general model used to explore the possible CO2 binding motifs. We distinguish the structural effects of chemical bonding through surface hybridization from the environmental effects of changing CO2 binding through interactions between the electric field and the surface dipole. While the *OCO motif is initially destabilized by the field upon activation, ultimately both motifs benefit from the electric potential. The relative *OCO/*CO2 stability becomes a competition between the hybridization/chemical bonding, which appears to favor *OCO on our model Cu(211) surface, and dipole-field interaction/-electrostatic effects, which favors *CO2. Thus, given the right surface *OCO may become more stable even at slightly negative potentials. This was however not found to be the case for the post-transition metals known to produce formate.Finally, as the other works of this thesis shows, the electric field is of paramount importance in CO2 reduction, and with this in mind, we probe methods to intrinsically improve this. Specifically, we study whether curvature-induced field enhancements are large enough to drive the improved activity we observe. We find however, that the electric field effect is convoluted with structure changes in the most active systems. Ultimately, we find that the field enhancement associated with even high-curvature surfaces are negligible. Instead, we attribute the improved activity to increases in site density of step sites. The experimental data also verifies the theoretical hypothesis, that a region limited by *COOH formation exists at low overpotential, with a different potential response to that of CO2 adsorption

Iron(II) and Iron(III) Spin Crossover: Toward an Optimal Density Functional

Spin crossover (SCO) plays a major role in biochemistry, catalysis, materials, and emerging technologies such as molecular electronics and sensors, and thus accurate prediction and design of SCO systems is of high priority. However, the main tool for this purpose, density functional theory (DFT), is very sensitive to applied methodology. The most abundant SCO systems are Fe­(II) and Fe­(III) systems. Even with average good agreement, a functional may be significantly more accurate for Fe­(II) or Fe­(III) systems, preventing balanced study of SCO candidates of both types. The present work investigates DFT’s performance for well-known Fe­(II) and Fe­(III) SCO complexes, using various design types and customized versions of GGA, hybrid, meta-GGA, meta-hybrid, double-hybrid, and long-range-corrected hybrid functionals. We explore the limits of DFT performance and identify proficient Fe­(II)–Fe­(III)-balanced functionals. We identify and quantify remarkable differences in the DFT description of Fe­(II) and Fe­(III) systems. Most functionals become more accurate once Hartree–Fock exchange is adjusted to 10–17%, regardless of the type of functionals involved. However, this typically introduces a clear Fe­(II)–Fe­(III) bias. The most accurate functionals measured by mean absolute errors <10 kJ/mol are CAMB3LYP-17, B3LYP*, and B97-15 with 15–17% Hartree–Fock exchange, closely followed by CAMB3LYP and CAMB3LYP-15, OPBE, rPBE-10, and B3P86-15. While GGA functionals display a small Fe­(II)–Fe­(III) bias, they are generally inaccurate, except the O exchange functional. Hybrid functionals (including B2PLYP double hybrids and meta hybrids) tend to favor HS too much in Fe­(II) vs Fe­(III), which is important in many studies where the oxidation state of iron can vary, e.g. rational SCO design and studies of catalytic processes involving iron. The only functional with a combined bias <5 kJ/mol and a decent MAE (15 kJ/mol) is our customized PBE0-12 functional. Alternatively one has to sacrifice Fe­(II)–Fe­(III) balance to use the best functionals for each group separately. We also investigated the precision (measured as the standard deviation of errors) and show that the target accuracy for iron SCO is 10 kJ/mol for accuracy and 5 kJ/mol for precision, and DFT is probably not going to break this limit in the near future. Importantly, all four types of functional behavior (accurate/precise, accurate/imprecise, inaccurate/precise, inaccurate/imprecise) are observed. More generally, our work illustrates the importance not only of overall accuracy but also of <i>balanced</i> accuracy for systems likely to occur in context

In-Situ Liquid Phase Transmission Electron Microscopy and Electron Diffraction Provides Mechanistic Insight into Electrochemical CO2 Reduction on Palladium/Palladium Hydride Catalysts

Electrochemical conversion of CO2 (CO2R) offers a sustainable route for producing fuels and chemicals. Pd-based catalysts are effective for selective conversion of CO2 into formate at low overpotentials and CO/H2 at high overpotentials. Furthermore, Pd catalysts undergo morphology and phase structure transformations under reaction conditions that are not well understood. Herein, in-situ liquid phase transmission electron microscopy (LP-TEM) and select area diffraction (SAD) measurements under CO2R conditions is applied to track the morphology and Pd/PdHx phase interconversion as a function of electrode potential, respectively. Correlating in-situ characterization with electrochemical CO2R activity/selectivity measurements, density functional theory and micro-kinetic analyses, the change in Pd/PdHx catalyst selectivity from formate at low overpotentials towards CO/H2 at higher overpotentials is found to result from electrode potential-dependent thermodynamic changes in the reaction energetics and not morphological or phase structure changes, providing insight that can guide advanced understanding and design of improved performance catalysts

Impact of palladium/palladium hydride conversion on electrochemical CO2 reduction via in-situ transmission electron microscopy and diffraction

Abstract Electrochemical conversion of CO2 offers a sustainable route for producing fuels and chemicals. Pd-based catalysts are effective for converting CO2 into formate at low overpotentials and CO/H2 at high overpotentials, while undergoing poorly understood morphology and phase structure transformations under reaction conditions that impact performance. Herein, in-situ liquid-phase transmission electron microscopy and select area diffraction measurements are applied to track the morphology and Pd/PdHx phase interconversion under reaction conditions as a function of electrode potential. These studies identify the degradation mechanisms, including poisoning and physical structure changes, occurring in PdHx/Pd electrodes. Constant potential density functional theory calculations are used to probe the reaction mechanisms occurring on the PdHx structures observed under reaction conditions. Microkinetic modeling reveals that the intercalation of *H into Pd is essential for formate production. However, the change in electrochemical CO2 conversion selectivity away from formate and towards CO/H2 at increasing overpotentials is due to electrode potential dependent changes in the reaction energetics and not a consequence of morphology or phase structure changes

Impact of palladium/palladium hydride conversion on electrochemical CO₂ reduction via in-situ transmission electron microscopy and diffraction

Electrochemical conversion of CO2 offers a sustainable route for producing fuels and chemicals. Pd-based catalysts are effective for converting CO2 into formate at low overpotentials and CO/H2 at high overpotentials, while undergoing poorly understood morphology and phase structure transformations under reaction conditions that impact performance. Herein, in-situ liquid-phase transmission electron microscopy and select area diffraction measurements are applied to track the morphology and Pd/PdHx phase interconversion under reaction conditions as a function of electrode potential. These studies identify the degradation mechanisms, including poisoning and physical structure changes, occurring in PdHx/Pd electrodes. Constant potential density functional theory calculations are used to probe the reaction mechanisms occurring on the PdHx structures observed under reaction conditions. Microkinetic modeling reveals that the intercalation of *H into Pd is essential for formate production. However, the change in electrochemical CO2 conversion selectivity away from formate and towards CO/H2 at increasing overpotentials is due to electrode potential dependent changes in the reaction energetics and not a consequence of morphology or phase structure changes.</p

Improving Neurosurgery Education Using Social Media Case-Based Discussions: A Pilot Study

Background: The increasing shift toward a more generalized medical undergraduate curriculum has led to limited exposure to subspecialties, including neurosurgery. The lack of standardized teaching may result in insufficient coverage of core learning outcomes. Social media (SoMe) in medical education are becoming an increasingly accepted and popular way for students to meet learning objectives outside formal medical school teaching. We delivered a series of case-based discussions (CbDs) over SoMe to attempt to meet core learning needs in neurosurgery and determine whether SoMe-based CbDs were an acceptable method of education. Methods: Twitter was used as a medium to host 9 CbDs pertaining to common neurosurgical conditions in practice. A sequence of informative and interactive tweets were formulated before live CbDs and tweeted in progressive order. Demographic data and participant feedback were collected. Results: A total of 277 participants were recorded across 9 CbDs, with 654,584 impressions generated. Feedback responses were received from 135 participants (48.7%). Participants indicated an increase of 77% in their level of knowledge after participating. Of participants, 57% (n = 77) had previous CbD experience as part of traditional medical education, with 62% (n = 84) receiving a form of medical education previously through SoMe. All participants believed that the CbDs objectives were met and would attend future sessions. Of participants, 99% (n = 134) indicated that their expectations were met. Conclusions: SoMe has been shown to be a favorable and feasible medium to host live, text-based interactive CbDs. SoMe is a useful tool for teaching undergraduate neurosurgery and is easily translatable to all domains of medicine and surgery