Showing papers by "Jens K. Nørskov published in 2022"

Acid anion electrolyte effects on platinum for oxygen and hydrogen electrocatalysis

Abstract: Abstract Platinum is an important material with applications in oxygen and hydrogen electrocatalysis. To better understand how its activity can be modulated through electrolyte effects in the double layer microenvironment, herein we investigate the effects of different acid anions on platinum for the oxygen reduction/evolution reaction (ORR/OER) and hydrogen evolution/oxidation reaction (HER/HOR) in pH 1 electrolytes. Experimentally, we see the ORR activity trend of HClO 4 > HNO 3 > H 2 SO 4 , and the OER activity trend of HClO 4 $$ > $$ > HNO 3 ∼ H 2 SO 4 . HER/HOR performance is similar across all three electrolytes. Notably, we demonstrate that ORR performance can be improved 4-fold in nitric acid compared to in sulfuric acid. Assessing the potential-dependent role of relative anion competitive adsorption with density functional theory, we calculate unfavorable adsorption on Pt(111) for all the anions at HER/HOR conditions while under ORR/OER conditions $${{{{{\rm{Cl}}}}}}{{{{{{\rm{O}}}}}}}_{4}^{-}$$ Cl O 4 − binds the weakest followed by $${{{{{\rm{N}}}}}}{{{{{{\rm{O}}}}}}}_{3}^{-}$$ N O 3 − and $${{{{{\rm{S}}}}}}{{{{{{\rm{O}}}}}}}_{4}^{2-}$$ S O 4 2 − . Our combined experimental-theoretical work highlights the importance of understanding the role of anions across a large potential range and reveals nitrate-like electrolyte microenvironments as interesting possible sulfonate alternatives to mitigate the catalyst poisoning effects of polymer membranes/ionomers in electrochemical systems. These findings help inform rational design approaches to further enhance catalyst activity via microenvironment engineering.

A spin promotion effect in catalytic ammonia synthesis

Abstract: Abstract The need for efficient ammonia synthesis is as urgent as ever. Over the past two decades, many attempts to find new catalysts for ammonia synthesis at mild conditions have been reported and, in particular, many new promoters of the catalytic rate have been introduced beyond the traditional K and Cs oxides. Herein, we provide an overview of recent experimental results for non-traditional promoters and develop a comprehensive model to explain how they work. The model has two components. First, we establish what is the most likely structure of the active sites in the presence of the different promoters. We then show that there are two effects dictating the catalytic activity. One is an electrostatic interaction between the adsorbed promoter and the N-N dissociation transition state. In addition, we identify a new promoter effect for magnetic catalysts giving rise to an anomalously large lowering of the activation energy opening the possibility of finding new ammonia synthesis catalysts.

Engineering metal–metal oxide surfaces for high-performance oxygen reduction on Ag–Mn electrocatalysts

Abstract: Diverse Ag–MnOx surface sites/structures in Ag–Mn electrocatalysts afford robust local electronic structures tuned for efficient oxygen reduction.

Opportunities and Challenges in Electrolytic Propylene Epoxidation.

Abstract: Propylene oxide (PO) is an important chemical. So far, its synthesis protocol relies on expensive oxidants. In contrast, direct epoxidation of propylene (DEP) using molecular oxygen is considered ideal for PO synthesis. Unfortunately, DEP has not met industrial demands due to the low propylene conversion and high side-product selectivity for known catalysts. Instead of a thermal process using molecular oxygen, electrolytic propylene oxidation can synthesize PO at room temperature, using the atomic oxygen generated from water-splitting. Herein, using density functional theory, surface Pourbaix analysis, scaling relation analysis, and microkinetic modeling, we show that (i) propylene epoxidation is facile on weak-binding catalysts if reactive atomic oxygen preexists; (ii) electrolytic epoxidation is facile to provide atomic oxygen for epoxidation, while hydroperoxyl formation does not overwhelm the epoxidation process at the potential of interest; (iii) propylene dehydrogenation is a competing step that forms side products. Finally, we discuss the opportunities and challenges of this green PO synthesis method.

OH Binding Energy as a Universal Descriptor of the Potential of Zero Charge on Transition Metal Surfaces

Abstract: The potential of zero charge (UPZC) is an important quantity of metal-water interfaces that are central in many electrochemical applications. In this work, we use ab initio molecular dynamics (AIMD) simulations to study a large number of (111), (100), (0001) and (211) and overlayers of transition metal-water interfaces in order to identify simple descriptors to predict their UPZC. We find a good correlation between water coverage and the work function reduction ∆φ which is defined by the difference of the work function in vacuum and in the presence of water. Furthermore, we determine the vacuum binding energies of H2O and *OH species as good descriptors for the prediction of water coverage and thereby

New challenges in oxygen reduction catalysis: a consortium retrospective to inform future research

Abstract: In this perspective, we highlight results of a research consortium devoted to advancing understanding of oxygen reduction reaction (ORR) catalysis as a means to inform fuel cell science. We demonstrate...

Limits to scaling relations between adsorption energies?

Abstract: Linear scaling relations have led to an understanding of trends in catalytic activity and selectivity of many reactions in heterogeneous and electro-catalysis. However, linear scaling between the chemisorption energies of any two small molecule adsorbates is not guaranteed. A prominent example is the lack of scaling between the chemisorption energies of carbon and oxygen on transition metal surfaces. In this work, we show that this lack of scaling originates from different re-normalized adsorbate valence energies of lower-lying oxygen vs higher-lying carbon. We develop a model for chemisorption of small molecule adsorbates within the d-band model by combining a modified form of the Newns-Anderson hybridization energy with an effective orthogonalization term. We develop a general descriptor to a priori determine if two adsorbates are likely to scale with each other.

Enhanced promotion of Ru-based ammonia catalysts by in situ dosing of Cs

Abstract: Ammonia synthesis via the high-temperature and -pressure Haber-Bosch (HB) process at large centralized facilities has a significant contribution to the global CO2 emission. Radically new catalysts should be discovered to...

Strategies for Modulating the Catalytic Activity and Selectivity of Manganese Antimonates for the Oxygen Reduction Reaction

Abstract: Strategies for improving the performance of nonprecious metal catalysts for the oxygen reduction reaction (ORR) can facilitate the cost-effective deployment of fuel cell devices. Electrocatalyst performance is typically improved via two approaches: increasing the number of active sites and increasing the intrinsic activity of the active site. Herein, we utilize these two methods of improving performance for MnSb2O6, which we have recently shown to be a promising ORR catalyst due to improvements in per-metal-site activity in the antimonate framework. First, electrode engineering is used to investigate the role of mass and conductive support loading in the observed ORR performance and selectivity. The apparent 2-electron selectivity is found to decrease with increases in mass and/or conductive support loading, indicating that rotating ring disk electrode studies do not necessarily measure the intrinsic selectivity of the catalyst. Second, theoretical calculations are used to identify Cr, Fe, and Ni as promising first-row transition metals for improving the intrinsic activity of MnSb2O6. Experimentally, the addition of Cr results in 3-fold and 2-fold increases in the mass and specific activities at 0.7 V vs the reversible hydrogen electrode, respectively. This enhancement is attributed to the modulation of the active site structure and Mn oxidation state with the addition of Cr. Through these studies, we gain insight into the intrinsic and extrinsic factors that govern ORR performance.

Analysing oxygen reduction electrocatalysis on transition metal doped niobium oxide(110).

Abstract: A good oxygen reduction reaction (ORR) catalyst should be stable and active under electrochemical reaction conditions. Niobium pentaoxide (Nb2O5) is known to be stable under ORR conditions. However it has a large band gap, which makes conductivity a challenge during the reaction. In this work, we aim to understand if surface modification of the 110 facet of niobium pentaoxide with transition metal doping has any effect on its ORR activity and conductivity. While the problem of conductivity in the case of transition metal oxides (TMOs) can be partially rectified by transition metal doping, it has negligible influence on the ORR activity of the doped systems.

Increasing Ammonia Formation Rates of Li-Mediated Ammonia Synthesis with High Surface Area Copper Electrodes

Abstract: The Haber-Bosch process, which industrially produces ammonia, is one of the most important inventions of the 20th century. It is argued that without the ability to mass produce ammonia and therefore fertilizer, we would not be able to feed half of the current population. However, the Haber-Bosch process is harmful to the environment as it runs at high pressures and temperatures and is therefore very energy intensive. Furthermore, it utilizes H2 from steam reforming which causes large CO2 emissions. To mitigate the environmental strain of the Haber-Bosch process, electrochemical ammonia synthesis from renewable electricity sources would be an alternative, however the large activation barrier for dinitrogen splitting and the competition of hydrogen evolution reaction (HER) makes the electrochemical nitrogen reduction very difficult. (1) As of now, only the Li-mediated ammonia synthesis has been successfully proven by several labs to consistently produce ammonia at rates and faradaic efficiencies that could be industrially relevant. The exact mechanism is yet to be elucidated but it widely accepted that the first step is electrochemical plating of metallic Li from Li+ ions. The freshly plated Li metal is very reactive and is then believed to react with N2 in the electrolyte which produces an intermediate Li-N species. Lastly, ammonia is being formed by protonation of this Li-N species (2). Recent breakthroughs in the field managed to push the faradaic efficiencies to 78 %, however that was achieved at low current densities of -4 mA/cm2 (3). To make the process industrially relevant the current densities and therefore ammonia formation rates need to be increased significantly. The Department of Energy has stated in their REFUEL program a goal of 300 mA/cm2 at faradaic efficiencies of 90 % (4). In our latest publication, we have reached current densities of -100 mA/cm2 with high surface area Cu electrodes, however at relatively low faradaic efficiencies of 13 %. To synthesize the high surface area Cu electrodes we used the hydrogen bubble templating procedure that deposits metals at high overpotentials where the competing HER makes a templating structure and forms porous metal foams. We have further improved upon the deposition method by changing the substrate, which not only increased the physical stability but also the electrochemical performance. By varying the deposition conditions and optimizing the electrolyte for ammonia synthesis, we achieved a current density of -1 A/cm2 and high faradaic efficiencies of 75 %. The increase in faradaic efficiency is speculated to be due to changes in the solid electrolyte interface (SEI) layer, which we probe with X-ray photoelectron spectroscopy with the help of our in-house build transfer system that limits contact to air and moisture. The results are supported by theoretical models that calculate the Li+ conductivity of different constituents of the SEI layer. J. Kibsgaard, J. K. Nørskov, I. Chorkendorff, The Difficulty of Proving Electrochemical Ammonia Synthesis. ACS Energy Lett. 4, 2986–2988 (2019). N. Lazouski, Z. J. Schiffer, K. Williams, K. Manthiram, Understanding Continuous Lithium-Mediated Electrochemical Nitrogen Reduction. Joule. 3, 1127–1139 (2019). K. Li, S. Z. Andersen, M. J. Statt, M. Saccoccio, V. J. Bukas, K. Krempl, R. Sažinas, J. B. Pedersen, V. Shadravan, Y. Zhou, D. Chakraborty, J. Kibsgaard, P. C. K. Vesborg, J. K. Nørskov, I. Chorkendorff, Enhancement of lithium-mediated ammonia synthesis by addition of oxygen. Science (6575). 374, 1593–1597 (2021). G. Soloveichik, in 2019 AIChE Annual Meeting: Topical Conference - Ammonia Energy (AIChE, 2019).