Driving electrochemical reactions with a bias in an aqueous environment is an attractive approach for sustainable chemical synthesis, but understanding reaction dynamics remains a serious challenge in electrocatalysis. Computational techniques and concepts are necessary to elucidate the underpinnings of creating catalytic sites that are highly active, selective, and stable. Herein, we elucidate the intrusive role of hydroxide ions in the running of electrochemical reactions under alkaline conditions. Through an overhaul of the computational hydrogen electrode (CHE) model, we show that hydroxide ions can adsorb on many late transition metals, even on metals like Cu and Pt, where the OH* binding energy is energetically uphill relative to H2O (l). We provide a computational framework for modeling reaction energetics with OH-* relative to OH-(aq), using HER and CO2R as examples of how to model electroreduction reactions under alkaline conditions.

Cathodic corrosion of copper (Cu) has posed a significant challenge for over a century, impeding various technological progresses such as electrochemical conversion of CO2 (eCO2RR) into fuels and other value-added carbon products. In this study, employing a combined Density Functional Theory (DFT) and kinetic Monte Carlo (kMC) simulation approach, we delve into the atomistic level mechanism driving this phenomenon in Cu. Our hypothesis posits the pivotal role of alkaline hydrogen evolution reaction (HER) in facilitating cathodic corrosion in Cu. We rigorously develop a pH-dependent hydroxide (OH) adsorption mechanism and calculate the equilibrium OH coverage (𝜃OH) at varying pH levels, the thermodynamic stability of subsurface oxygen (Osub), as well as the Cu-vacancy mediated diffusion of subsurface oxygens (Osub). Through comprehensive analysis, we establish correlation among various microenvironments, including oxygen diffusion in subsurface layers, pH-dependent OH adsorption, and Cu dissolution into the electrolyte as (Cu- OH) complexes. Furthermore, our investigation explores the correlation between surface coordination environment of active sites and cathodic corrosion of Cu. Finally, by integrating DFT- derived thermodynamic data into a kMC model, we successfully predict the formation of experimentally observed corrosion pits on Cu-surfaces. This combined approach not only advances our fundamental understanding of Cu cathodic corrosion but also offers insights crucial for developing effective corrosion mitigation strategies.

The electrochemical reduction of nitrate (NO3R) to ammonia is a bold yet conceivable way of producing ammonia using renewable electricity. However, serious challenges remain in finding optimal electrocatalysts for the process. An atomistic understanding of the surface energetics behind the NO3R is needed in order to design an efficient catalyst. Herein, we combine energetics from density functional theory and microkinetic modeling to demonstrate how surface descriptors can help simplify the search for efficient NO3R electrocatalysts. We illustrate the strong correlations between transition-state energetics and O* binding energies for adsorbed nitrate and nitrite on transition metals. For intermediates from NO* and beyond, we compare the benefits of using either the N* or H* binding energies to predict reduction onset potentials. These insights enable us to develop a simple microkinetic model that elucidates the surface coverages of intermediates and the product selectivity of NO3R across a range of potentials and transition metals. We show that the model adequately corroborates with quasi-steady-state rates observed experimentally.

The computational hydrogen electrode (CHE) model has been a mainstay tool for understanding electrochemical reactions over electrocatalytic surfaces. While the electrode model has seen success in modeling catalysis under highly acidic solutions, over recent years, focus has shifted towards electrochemistry under neutral or alkaline solutions. In such cases, various correction schemes have been applied onto the computational hydrogen electrode in order to address its inability to describe pH dependent effects. Many of these schemes are often computationally cumbersome and not very transferrable across various material systems/ cell systems. In this presentation, we will breakdown the working assumptions of the computational hydrogen electrode and more recent grand canonical based models and present a new simple electrode model using density functional theory (DFT) and thermochemical data to describe reduction pathways as well as mechanisms that lead to corrosion on the electrode surface. Consistent with the original CHE model, the new electrode model is capable of referencing any charged surface intermediates to their aqueous ionic counterpart in solution in the standard concentration, such that they can be used to address problems of the original CHE model. The thermodynamic implications of our model can be incorporated into experiments and cell designs to understand the effects of ions in aqueous electrocatalytic reactions. Figure 1

Lithium bis(fluorosulfonyl)imide-based liquid electrolytes are promising for realizing high coulombic efficiency and long cycle life in next-generation Li-metal batteries. However, the role of anions in the formation of the solid–electrolyte interphase remains unclear. Here we combine electrochemical analyses and X-ray photoelectron spectroscopy measurements, both with and without sample washing, together with computational simulations, to propose the reaction pathways of electrolyte decomposition and correlate the interphase component solubility with the efficacy of passivation. We discover that not all the products derived from interphase-forming reactions are incorporated into the resulting passivation layer, with a notable portion present in the liquid electrolyte. We also find that the high-performance electrolytes can afford a sufficiently passivating interphase with minimized electrolyte decomposition, by incorporating more anion-decomposition products. Overall, this work presents a systematic approach of coupling electrochemical and surface analyses to paint a comprehensive picture of solid–electrolyte interphase formation, while identifying the key attributes of high-performance electrolytes to guide future designs. Li-metal batteries often utilize liquid electrolytes that yield a solid–electrolyte interphase on electrodes; however, the role of anions in interphase formation remains unclear. Now it has been shown that anion-decomposition products provide varying contributions to interphase formation and that high-performance electrolytes balance effective interfacial passivation with minimized degradation.

The electrochemical CO2 reduction reaction (eCO2RR) is one of the promising pathways which primarily leverages electrocatalysts such as copper (Cu) to facilitate the conversion of CO2 into higher value carbon products; C1 (e.g., CO, methane, formic acid, and methanol etc.) and C2 (e.g., ethylene, ethane, and ethanol etc.) using renewable electricity. Cu is the only monometallic electrocatalyst able to electrochemically transform CO2 into higher hydrocarbons with appreciable activity and selectivity. However, Cu and oxide-derived Cu surfaces (OD-Cu) fall short in stability or durability during eCO2RR. One significant challenge is the cathodic corrosion of Cu. Cathodic corrosion leads to degradation of Cu surfaces, reducing their catalytic activity, selectivity, and long-term operability. In this study, using a combination of density functional theory (DFT) and kinetic Monte Carlo (KMC) simulations, we delve into the intricate mechanisms driving this phenomenon. Our hypothesis posits the pivotal role of alkaline hydrogen evolution reaction (HER) in facilitating cathodic corrosion. Through comprehensive analysis, we establish correlations between various microenvironments, including subsurface oxygen (Osub) diffusion, pH-dependent OH adsorption on the Cu surface, and Cu dissolution as a (Cu-OH) complex into the electrolyte solution. We rigorously calculated the pH-dependent OH adsorption on the surface, as well as the stability and diffusion behavior of Osub. Furthermore, our investigation explores the significance of under-coordinated sites in Cu corrosion. By integrating DFT-derived thermodynamic data into our KMC model, we successfully predict the formation of experimentally observed corrosion pits on Cu surfaces. This combined approach not only advances our fundamental understanding of Cu cathodic corrosion but also offers insights crucial for developing effective corrosion mitigation strategies. The data obtained from theoretical calculations were also corroborated with available experimental observations. Finally, the details of this study will be presented during the conference. Figure 1

The global nitrogen cycle has been severely skewed since the widespread adoption of the Haber-Bosch process to produce ammonia (NH3). Currently, removal of reactive nitrogen (inorganic forms besides N2) from the environment lags behind its production and emission to the environment. Nitrate is one of the most prevalent waterborne nitrogen pollutants and, in its excess, threatens the health of ecosystems. Enabling a sustainable food-energy-water nexus requires feeding a growing population while minimizing environmental impacts. Therefore, selective electrochemical nitrate reduction (NO3RR) to NH3 can couple water purification and NH3 production, helping offset the energy- and carbon-intensive Haber Bosch process. NH3 recovery from emissions could contribute over 20 million tons N per year by 2050 (~ 10% of projected N demand). Noble and transition metal catalysts including single metals (e.g., Pt, Rh, Ru, Ir, Pd, Cu, Ag, Au)[1,2] and alloys (e.g., CuNi)[3] have been studied for NO3RR. Notably, under acidic conditions, polycrystalline titanium has been demonstrated to display robust and efficient NO3RR.[4] Ti is corrosion-resistant, a poor hydrogen evolution catalyst, and a readily available, abundant metal. However, under acidic and reducing conditions, Ti forms a water-stable hydride (TiHx, 0<x≤2).[5] It remains unclear how the degree of surface hydride formation – which alters the physical and electronic properties of the electrode surface – impacts NO3RR performance. Thus, rationally implementing Ti-catalyzed NO3RR requires improved understanding of how near-surface Ti-hydride forms and influences NO3RR activity and selectivity. In this work, we show that near-surface Ti-hydride formation is a function of the duration and magnitude of applied NO3RR potential. A combination of ex situ grazing-incidence X-ray diffraction (GIXRD – probes long-range, crystalline order) and X-ray absorption spectroscopy (XAS – probes short-range, atom-specific probe into local coordination environments) enabled quantitative near-surface characterization of Ti-hydride electrodes. Through electrochemical testing and density functional theory calculations, we investigated the role near-surface Ti-hydride may play in electrochemical nitrate conversion. Our results preliminary suggest that near-surface hydride content plays a relatively minor role in steering NO3RR performance compared to applied potential and electrolyte effects. In addition, we are performing ongoing in situ GIXRD and XAS measurements to interrogate the transient nature and interplay of near-surface hydride formation and NO3RR. Put in context, our results help prioritize how Ti-catalyzed NO3RR processes can be optimized for electrified ammonia production and wastewater remediation. [1] G. E. Dima, A. C. A. de Vooys, M. T. M. Koper, Journal of Electroanalytical Chemistry 2003, 554–555, 15–23. [2] G. E. Dima, G. L. Beltramo, M. T. M. Koper, Electrochimica Acta 2005, 50, 4318–4326. [3] Y. Wang, A. Xu, Z. Wang, L. Huang, J. Li, F. Li, J. Wicks, M. Luo, D.-H. Nam, C.-S. Tan, Y. Ding, J. Wu, Y. Lum, C.-T. Dinh, D. Sinton, G. Zheng, E. H. Sargent, J. Am. Chem. Soc. 2020, 142, 5702–5708. [4] J. M. McEnaney, S. J. Blair, A. C. Nielander, J. A. Schwalbe, D. M. Koshy, M. Cargnello, T. F. Jaramillo, ACS Sustainable Chem. Eng. 2020, 8, 2672–2681. [5] Y. Liu, Z. H. Ren, J. Liu, R. F. Schaller, E. Asselin, J. Electrochem. Soc. 2019, 166, C3096–C3105.

Anthropogenic perturbations to the global nitrogen cycle due to industrial Haber-Bosch fertilizer production threaten large-scale food production, energy inputs for chemical manufacturing, and protection of water quality. Enabling a sustainable food-energy-water nexus requires feeding a growing population while minimizing environmental impacts. In this talk, I introduce Electrodialysis and Nitrate Reduction (EDNR), a novel electrochemical process that couples water purification with electrified ammonia production from nitrogen-polluted wastewaters. The EDNR reactor consists of three chambers and operates in two stages (Figure 1), with the influent entering the middle chamber and products recovered from the left and right chambers. In Stage 1, influent nitrate () and ammonium () are separated via electrodialysis (ED) and ammonia is recovered in the right chamber. In Stage 2, ammonia is synthesized from the electrochemical nitrate reduction (NR) in the left chamber. EDNR enables rational design of electrochemical environments in each chamber (e.g., electrolyte pH; cationic and anionic constituents; species concentrations) through tunable operating parameters such as applied potential, electrolyte flow rate, and duration of the ED and NR stages. This modular, tunable design facilitates robust water remediation and ammonia production from wastewaters of transient composition. We have demonstrated proof-of-concept EDNR reactors using titanium foil (left chamber, NR electrode), Ti/IrO2-Ta2O5 mesh (left and middle chambers, ED electrode), and platinum foil (right chamber, ED electrode). With recirculating batches of simulated wastewater (100 ppm + 500 ppm ), 75% influent NH4 + was recovered into the right chamber and 25% influent was converted to in the left chamber after three EDNR cycles. As the rate-limiting stage, NR on titanium merited further fundamental investigation. In particular, the reasons for titanium’s electrocatalytic NR performance remain largely unclear to date, especially regarding the role of the role of titanium hydride (TiHx, 0<x<2), which forms during NR. Rationally implementing Ti-catalyzed NR requires improved understanding of how near-surface Ti-hydride forms and influences NR activity and selectivity. Through systematic synchrotron x-ray characterization of Ti-hydride electrodes, electrochemical testing, and density functional theory calculations, we found that near-surface hydride content plays a relatively minor role in steering NR performance compared to applied potential and electrolyte effects. Put in context, our results help prioritize how EDNR operation can be optimized for ammonia production. As a validated platform with ongoing work to improve performance, EDNR shows great potential in realizing sustainable and distributed water remediation and ammonia production.

The electrochemical conversion of carbon di-/monoxide into commodity chemicals paves a way towards a sustainable society but it also presents one of the great challenges in catalysis. Herein, we present the trends in selectivity towards specific dicarbon oxygenate/hydrocarbon products from carbon monoxide reduction on transition metal catalysts, with special focus on copper. We unveil the distinctive role of electrolyte pH in tuning the dicarbon oxygenate/hydrocarbon selectivity. The understanding is based on density functional theory calculated energetics and microkinetic modeling. We identify the critical reaction steps determining selectivity and relate their transition state energies to two simple descriptors, the carbon and hydroxide binding strengths. The atomistic insight gained enables us to rationalize a number of experimental observations and provides avenues towards the design of selective electrocatalysts for liquid fuel production from carbon di-/monoxide.

Understanding the hydrogen evolution reaction (HER) behaviors over 2D transition metal dichalcogenides (2D-TMDs) is critical for the development of non-precious HER electrocatalysts with better activity. In this work, by combining density functional theory calculations with microkinetic modelling, we thoroughly investigated the HER mechanism on 2D-TMDs. We find there is an important dependence of simulated cell size on the calculated hydrogen adsorption energy and the activation barrier for MoS2. Distinct from previous “H migration” mechanisms proposed for the Heyrovsky reaction − the rate-determining step for MoS2, we propose the Mo site only serves as the stabilized transition state rather than H adsorption. In comparison to transition metal electrocatalysts, we find that the activation barrier of the Heyrovsky reaction on 2D-TMDs scales with the hydrogen adsorption energy exactly as for transition metals except that all activation energies are displaced upwards by ca. 0.4 eV. This higher Heyrovsky activation barrier is responsible for the substantially lower activity of 2D-TMDs. We further show that this higher activation barrier stems from the more positively charged adsorbed hydrogen on the chalcogenides interacting repulsively with the incoming proton. Based on these insights, we discuss potential strategies for the design of non-precious HER catalysts with activity comparable to Pt.

The electrochemical nitrate reduction reaction (NO3RR) on titanium introduces significant surface reconstruction and forms titanium hydride (TiHx, 0 < x ≤ 2). With ex situ grazing-incidence X-ray diffraction (GIXRD) and X-ray absorption spectroscopy (XAS), we demonstrated near-surface TiH2 enrichment with increasing NO3RR applied potential and duration. This quantitative relationship facilitated electrochemical treatment of Ti to form TiH2/Ti electrodes for use in NO3RR, thereby decoupling hydride formation from NO3RR performance. A wide range of NO3RR activity and selectivity on TiH2/Ti electrodes between -0.4 and -1.0 VRHE was observed and analyzed with density functional theory (DFT) calculations on TiH2(111). This work underscores the importance of relating NO3RR performance with near-surface electrode structure to advance catalyst design and operation.

Bimetallic catalysts with optimal CO and C affinity for electrochemical reduction of CO 2 (eCO 2 R) to high-value multi-carbon chemicals are identified by screening for transition and p-block metals with complementary strong and weak CO binding energy.

Challenges in improving catalysts for electrochemical CO2 reduction require a clear understanding of the reaction mechanisms that lead to products of higher value. In this work, we use density functional theory (DFT) to determine the most competitive coupling mechanisms leading to C-3 products on Cu(100) and Cu(511). We exhaustively consider surface coupling pathways between CO* and different C-2 intermediates. On Cu(100), CO* coupling with acetaldehyde was identified as a notable step for C-3 product formation. On Cu(511), local field stabilizations enable an additional coupling step between HCCH* and CO*. This suggests that there is more than one possible pathway toward forming C-3 intermediates. Our simulations show that much like C-2 formation, C-3 formation prefers stepped surfaces with (100)-like sites and that local field stabilization can play a pivotal role in certain coupling steps.

Electrochemical reduction of carbon-dioxide/carbon-monoxide (CO(2)R) to fuels and chemicals presents an attractive approach for sustainable chemical synthesis, but also poses a serious challenge in catalysis. Understanding the key aspects that guide CO(2)R towards value-added multicarbon (C2+) products is imperative in designing an efficient catalyst. Herein, we identify the critical steps toward C2 products on copper through a combination of energetics from density functional theory and micro-kinetic modeling. We elucidate the importance of atomic carbon in directing C2+ selectivity and how it introduces surface structural sensitivity on copper catalysts. This insight enables us to propose two simple thermodynamic descriptors that effectively describe C2+ selectivity on metal catalysts beyond copper and hence it identifies an intelligible protocol to screen for materials that selectively catalyze CO(2) to C2+ products.

Electrochemical reduction of carbon dioxide (CO2) over transition metals follows a complex reaction network. Even for products with a single carbon atom (C1 products), two bifurcated pathways exist: initially between carboxyl (COOH*) and formate (HCOO*) intermediates and the COOH* intermediate is further bifurcated by pathways involving either formyl (CHO*) or COH*. In this study, we combine evidence from the experimental literature with a theoretical analysis of energetics to rationalize that not all steps in the reduction of CO2 are electrochemical. This insight enables us to create a selectivity map for two-electron products (carbon monoxide (CO) and formate) on elemental metal surfaces using only the CO and OH binding energies as descriptors. In the further reduction of CO*, we find that CHO* is formed through a chemical step only whereas COH* follows from an electrochemical step. Notably on Cu(100), the COH pathway becomes dominant at an applied potential lower than −0.5V vs. RHE. For the elemental metals selective towards CO formation, the variation of the CO binding energy is sufficient to further subdivide the map into domains that predominantly form H2, CO, and ultimately more reduced products. We find Cu to be the only elemental metal capable of reducing CO2 to products beyond 2e− via the proposed COH pathway and we identify atomic carbon as the key component leading to the production of methane. Our analysis also rationalizes experimentally observed differences in products between thermal and electrochemical reduction of CO2 on Cu.

Despite the apparent simplicity of the hydrogen evolution reaction (HER) and the decades of research into it, controversy remains in the literature regarding the identity of the active site and the competition between the Heyrovsky and Tafel steps. In this work, we use charge-extrapolated ab initio simulations with explicit water in conjunction with mean-field microkinetic modeling to explore the mechanism for HER on both close-packed (111) and stepped (211) transition metals. First, we show that atop H*, beyond a monolayer of hollow H*, is unlikely to play a role in the HER mechanism, given its very positive adsorption energies. The energetics suggests the Volmer–Heyrovsky mechanism to predominate on fcc transition metals under typical operating conditions. We evaluate our theoretical results vs several experimental observations. We show that the Volmer–Heyrovsky mechanism predicts an activity volcano with its peak at a H* binding ΔGH* ≈ 0 eV, consistent with experiment. In contrast, the Volmer–Tafel volcano shows a broad rate plateau between ΔGH* ≈ 0 eV and ΔGH* ≈ – 0.4 eV. We find our theoretical Tafel slopes to be consistent with experimental ones on a range of transition metals. We show that, in line with experimental observations, the introduction of a CO(g) atmosphere shifts the strong binding metals toward the weak binding leg. Our study suggests that the simple thermodynamic approach to HER activity still holds, even when a detailed kinetic picture is considered.

Sulfur (S) vacancies in MoS2 have been found to act as a new active center, which shows an unprecedented intrinsic HER activity under elastic strain. However, such S-vacancies are unstable and the activities are very sensitive to the vacancy concentration. A strategy to stabilize these abundant active sites is thus highly desirable. Herein, we rationally design a catalyst system to stabilize S-vacancies in the basal plane of 2H-MoS2 supported on defective vertical graphene network (VGN). The energetically favorable line-shaped S-vacancies in MoS2 show a consistently high HER activity that is insensitive to S-vacancy concentration. Moreover, the defective graphene support effectively stabilizes these S-vacancies. The optimized catalyst exhibits a superior HER activity with overpotential of 128 mV at 10 mA cm(-2) and Tafel slope of 50 mV dec(-1). Most importantly, the catalyst shows greatly increased stability over 500 h; benchmarking the most stable nonprecious HER catalyst in acidic media to date. (C) 2019 Elsevier Inc. All rights reserved.

Despite the incredible success in reducing the overpotential of nonprecious catalysts for acidic hydrogen evolution reaction (HER) in the past few years, the stability of most platinum-free electrocatalysts is still poor. Here, we report an ultrastable electrocatalyst for acidic HER based on two-dimensional (2D) molybdenum disulfide (MoS2) doped with trace amount of palladium (<5 mu g cm(-2)), which creates sulfur vacancies (S-vacancies). The optimized catalyst shows stable operation over 1000 h at 10 mA cm(-2) with overpotential of 106 mV. The MoS2 catalyst is stabilized on a defective vertical graphene support, where the strong interaction at the 2D-2D interface increases the adhesion between the catalyst and the support. Palladium (Pd) doping generates rich sulfur vacancies in MoS2 that have a twofold role: (1) increasing hydrogen adsorption energy, which enhances activity; and (2) further increasing the adhesion between graphene support and defective MoS2, and thus enhancing stability. Complementary theoretical studies reveal the reaction pathways for substitutional doping, where the Mo-vacancy sites are prior to be doped by Pd. Our work thus offers a strategy for making stable, efficient, and earth-abundant HER catalysts with strong potential to replace platinum for PEM electrolysis.

The present work was undertaken to elucidate the facet-dependent activity of Ag for the electrochemical reduction of CO2 to CO. To this end, CO2 reduction was investigated over Ag thin films with (111), (100), and (110) orientations prepared via epitaxial growth on single-crystal Si wafers with the same crystallographic orientations. This preparation technique yielded larger area electrodes than can be achieved using single-crystals, which enabled the electrocatalytic activity of the corresponding Ag surfaces to be quantified in the Tafel regime. The Ag(110) thin films exhibited higher CO evolution activity compared to the Ag(111) and Ag(100) thin films, consistent with previous single-crystal studies. Density functional theory calculations suggest that CO2 reduction to CO is strongly facet-dependent, and that steps are more active than highly coordinated terraces. This is the result of both a higher binding energy of the key intermediate COOH as well as an enhanced double-layer electric field stabilization over undercoordinated surface atoms located at step edge defects. As a consequence, step edge defects likely dominate the CO2 reduction activity observed over the Ag(111) and Ag(100) thin films. The higher activity observed over the Ag(110) thin film is then related to the larger density of undercoordinated sites compared to the Ag(111) and Ag(100) thin films. Our conclusion that undercoordinated sites dominate the CO2 reduction activity observed over close-packed surfaces highlights the need to consider the contribution of such defects in studies of single-crystal electrodes.