Imperial College London

Dr Ifan E. L. Stephens

Faculty of EngineeringDepartment of Materials

Reader in Electrochemistry
 
 
 
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Contact

 

+44 (0)20 7594 9523i.stephens Website

 
 
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Location

 

Molecular Sciences Research HubWhite City Campus

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Summary

 

Publications

Publication Type
Year
to

91 results found

Iriawan H, Zamany S, Zhang X, Comer B, Barrio J, Chen P, Medford A, Stephens I, Chorkendorff I, Shao-Horn Yet al., 2021, Methods for nitrogen activation by reduction and oxidation, Nature Reviews Methods Primers, ISSN: 2662-8449

The industrial Haber-Bosch process to produce ammonia (NH3) from dinitrogen (N2) is crucial for modern society. However, N2 activation is inherently challenging and the Haber-Bosch process has significant drawbacks, as it is highly energy intensive, not sustainable due to substantial CO2 emissions primarily from the generation of H2 and requires large-centralized facilities. New strategies of sustainable N2 activation, such as low-temperature thermochemical catalysis and (photo)electrocatalysis, have been pursued, but progress has been hindered by the lack of rigor and reproducibility in the collection and analysis of results. In this Primer, we provide a holistic step-by-step protocol, applicable to all nitrogen-transformation reactions, focused on verifying genuine N2 activation by accounting for all contamination sources. We compare state-of-the-art results from different catalytic reactions following the protocol’s framework, and discuss necessary reporting metrics and ways to interpret both experimental and density functional theory results. This Primer covers various common pitfalls in the field, best practices to improve reproducibility and cost-efficient methods to carry out rigorous experimentation. The future of nitrogen catalysis will require an increase in rigorous experimentation and standardization to prevent false positives from appearing in the literature, which can enable advancing towards practical technologies for the activation of N2.

Journal article

Westhead O, Jervis R, Stephens IEL, 2021, Is lithium the key for nitrogen electroreduction?, Science, Vol: 372, Pages: 1149-1150

Journal article

Bagger A, Wan H, Stephens IEL, Rossmeisl Jet al., 2021, Role of Catalyst in Controlling N-2 Reduction Selectivity: A Unified View of Nitrogenase and Solid Electrodes, ACS CATALYSIS, Vol: 11, Pages: 6596-6601, ISSN: 2155-5435

Journal article

Yadegari H, Koronfel MA, Wang K, Thornton DB, Stephens IEL, Molteni C, Haynes PD, Ryan MPet al., 2021, Operando measurement of layer breathing modes in lithiated graphite, ACS Energy Letters, Vol: 6, Pages: 1633-1638, ISSN: 2380-8195

Despite their ubiquitous usage and increasing societal dependence on Li-ion batteries, there remains a lack of detailed empirical evidence of Li intercalation/deintercalation into graphite even though this process dictates the performance, longevity, and safety of the system. Here, we report direct detection and dissociation of specific crystallographic phases in the lithiated graphite, which form through a stepwise staging process. Using operando measurements, LiC18, LiC12, and LiC6 phases are observed via distinct low-frequency Raman features, which are the result of displacement of the graphite lattice by induced local strain. Density functional theory calculations confirm the nature of the Raman-active vibrational modes, to the layer breathing modes (LBMs) of the lithiated graphite. The new findings indicate graphene-like characteristics in the lithiated graphite under the deep charged condition due to the imposed strain by the inserted Li. Moreover, our approach also provides a simple experimental tool to measure induced strain in the graphite structure under full intercalation conditions.

Journal article

Rao RR, Stephens IEL, Durrant JR, 2021, Understanding What Controls the Rate of Electrochemical Oxygen Evolution, JOULE, Vol: 5, Pages: 16-18, ISSN: 2542-4351

Journal article

Duarte R, Rao R, Durrant J, Stephens I, Ryan M, M Bonastre A, Sharman Jet al., 2020, Towards Active and Stable Bifunctional NiCo2O4 Catalysts for O2 Evolution and Reduction in Alkaline Media, ECS Meeting Abstracts, Vol: MA2020-02, Pages: 3860-3860

Journal article

Jensen KD, Pedersen AF, Zamburlini E, Stephens IEL, Chorkendorff I, Escudero-Escribano Met al., 2020, X-ray Absorption Spectroscopy Investigation of Platinum-Gadolinium Thin Films with Different Stoichiometry for the Oxygen Reduction Reaction, CATALYSTS, Vol: 10

Journal article

Rao RR, Kolb MJ, Giordano L, Pedersen AF, Katayama Y, Hwang J, Mehta A, You H, Lunger JR, Zhou H, Halck NB, Vegge T, Chorkendorff I, Stephens IEL, Shao-Horn Yet al., 2020, Operando identification of site-dependent water oxidation activity on ruthenium dioxide single-crystal surfaces, NATURE CATALYSIS, Vol: 3, Pages: 516-525, ISSN: 2520-1158

Journal article

Francas L, Corby S, Selim S, Lee D, Mesa CA, Godin R, Pastor E, Stephens IEL, Choi K-S, Durrant JRet al., 2020, Spectroelectrochemical study of water oxidation on nickel and iron oxyhydroxide electrocatalysts (vol 10, 5208, 2019), NATURE COMMUNICATIONS, Vol: 11, ISSN: 2041-1723

Journal article

O'Mullane AP, Escudero-Escribano M, Stephens IEL, Krischer Ket al., 2019, The Role of Electrocatalysis in a Sustainable Future: From Renewable Energy Conversion and Storage to Emerging Reactions, CHEMPHYSCHEM, Vol: 20, Pages: 2900-2903, ISSN: 1439-4235

Journal article

Francàs L, Corby S, Selim S, Lee D, Mesa C, Godin R, Pastor E, Stephens I, Choi K-S, Durrant Jet al., 2019, Spectroelectrochemical study of water oxidation on nickel and iron oxyhydroxide electrocatalysts, Nature Communications, Vol: 10, ISSN: 2041-1723

Ni/Fe oxyhydroxides are the best performing Earth-abundant electrocatalysts for water oxidation. However, the origin of their remarkable performance is not well understood. Herein, we employ spectroelectrochemical techniques to analyse the kinetics of water oxidation on a series of Ni/Fe oxyhydroxide films: FeOOH, FeOOHNiOOH, and Ni(Fe)OOH (5% Fe). The concentrations and reaction rates of the oxidised states accumulated during catalysis are determined. Ni(Fe)OOH is found to exhibit the fastest reaction kinetics but accumulates fewer states, resulting in a similar performance to FeOOHNiOOH. The later catalytic onset in FeOOH is attributed to an anodic shift in the accumulation of oxidised states. Rate law analyses reveal that the rate limiting step for each catalyst involves the accumulation of four oxidised states, Ni-centred for Ni(Fe)OOH but Fe-centred for FeOOH and FeOOHNiOOH. We conclude by highlighting the importance of equilibria between these accumulated species and reactive intermediates in determining the activity of these materials.

Journal article

Andersen SZ, Colic V, Yang S, Schwalbe JA, Nielander AC, McEnaney JM, Enemark-Rasmussen K, Baker JG, Singh AR, Rohr BA, Statt MJ, Blair SJ, Mezzavilla S, Kibsgaard J, Vesborg PCK, Cargnello M, Bent SF, Jaramillo TF, Stephens IEL, Norskov JK, Chorkendorff Iet al., 2019, A rigorous electrochemical ammonia synthesis protocol with quantitative isotope measurements (vol 570, pg 504, 2019), NATURE, Vol: 574, Pages: E5-E5, ISSN: 0028-0836

Journal article

Mezzavilla S, Katayama Y, Rao R, Hwang J, Regoutz A, Shao-Horn Y, Chorkendorff I, Stephens IELet al., 2019, Activity-or Lack Thereof-of RuO2-Based Electrodes in the Electrocatalytic Reduction of CO2, JOURNAL OF PHYSICAL CHEMISTRY C, Vol: 123, Pages: 17765-17773, ISSN: 1932-7447

Journal article

Sebastian-Pascual P, Mezzavilla S, Stephens IEL, Escudero-Escribano Met al., 2019, Structure-sensitivity and Electrolyte Effects in CO2 Electroreduction: From Model Studies to Applications, CHEMCATCHEM, Vol: 11, Pages: 3624-3643, ISSN: 1867-3880

Journal article

Andersen SZ, Colic V, Yang S, Schwalbe JA, Nielander AC, McEnaney JM, Enemark-Rasmussen K, Baker JG, Singh AR, Rohr BA, Statt MJ, Blair SJ, Mezzavilla S, Kibsgaard J, Vesborg PCK, Cargnello M, Bent SF, Jaramillo TF, Stephens IEL, Norskov JK, Chorkendorff Iet al., 2019, A rigorous electrochemical ammonia synthesis protocol with quantitative isotope measurements, Nature, Vol: 570, Pages: 504-508, ISSN: 0028-0836

The electrochemical synthesis of ammonia from nitrogen under mild conditions using renewable electricity is an attractive alternative to the energy-intensive Haber–Bosch process, which dominates industrial ammonia production. However, there are considerable scientific and technical challenges facing the electrochemical alternative, and most experimental studies reported so far have achieved only low selectivities and conversions. The amount of ammonia produced is usually so small that it cannot be firmly attributed to electrochemical nitrogen fixation rather than contamination from ammonia that is either present in air, human breath or ion-conducting membranes, or generated from labile nitrogen-containing compounds (for example, nitrates, amines, nitrites and nitrogen oxides) that are typically present in the nitrogen gas stream, in the atmosphere or even in the catalyst itself. Although these sources of experimental artefacts are beginning to be recognized and managed concerted efforts to develop effective electrochemical nitrogen reduction processes would benefit from benchmarking protocols for the reaction and from a standardized set of control experiments designed to identify and then eliminate or quantify the sources of contamination. Here we propose a rigorous procedure using 15N2 that enables us to reliably detect and quantify the electrochemical reduction of nitrogen to ammonia. We demonstrate experimentally the importance of various sources of contamination, and show how to remove labile nitrogen-containing compounds from the nitrogen gas as well as how to perform quantitative isotope measurements with cycling of 15N2 gas to reduce both contamination and the cost of isotope measurements. Following this protocol, we find that no ammonia is produced when using the most promising pure-metal catalysts for this reaction in aqueous media, and we successfully confirm and quantify ammonia synthesis using lithium electrodeposition in tetrahydrofuran13. The use

Journal article

Nitopi S, Bertheussen E, Scott SB, Liu X, Engstfeld AK, Horch S, Seger B, Stephens IEL, Chan K, Hahn C, Norskov JK, Jaramillo TF, Chorkendorff Iet al., 2019, Progress and Perspectives of Electrochemical CO2 Reduction on Copper in Aqueous Electrolyte, CHEMICAL REVIEWS, Vol: 119, Pages: 7610-7672, ISSN: 0009-2665

Journal article

Chan AK, Tatara R, Feng S, Karayaylali P, Lopez J, Stephens IEL, Shao-Horn Yet al., 2019, Concentrated Electrolytes for Enhanced Stability of Al-Alloy Negative Electrodes in Li-Ion Batteries, JOURNAL OF THE ELECTROCHEMICAL SOCIETY, Vol: 166, Pages: A1867-A1874, ISSN: 0013-4651

Journal article

Mezzavilla S, Horch S, Stephens IEL, Seger B, Chorkendorff Iet al., 2019, Structure Sensitivity in the Electrocatalytic Reduction of CO2 with Gold Catalysts., Angew Chem Int Ed Engl

An understanding of the influence of structural surface features on electrocatalytic reactions is vital for the development of efficient nanostructured catalysts. Gold is the most active and selective known electrocatalyst for the reduction of CO2 to CO in aqueous electrolytes. Numerous strategies have been proposed to improve its intrinsic activity. Nonetheless, the atomistic knowledge of the nature of the active sites remains elusive. We systematically investigated the structure sensitivity of Au single crystals for electrocatalytic CO2 reduction. Reaction kinetics for the formation of CO are strongly dependent on the surface structure. Under-coordinated sites, such as those present in Au(110) and at the steps of Au(211), show at least 20-fold higher activity than more coordinated configurations (for example, Au(100)). By selectively poisoning under-coordinated sites with Pb, we have confirmed that these are the active sites for CO2 reduction.

Journal article

Mezzavilla S, Horch S, Stephens IEL, Seger B, Chorkendorff Iet al., 2019, Structure Sensitivity in the Electrocatalytic Reduction of CO2with Gold Catalysts, Angewandte Chemie, Vol: 131, Pages: 3814-3818, ISSN: 0044-8249

Journal article

Winiwarter A, Silvioli L, Scott SB, Enemark-Rasmussen K, Saric M, Trimarco DB, Vesborg PCK, Moses PG, Stephens IEL, Seger B, Rossmeisl J, Chorkendorff Iet al., 2019, Towards an atomistic understanding of electrocatalytic partial hydrocarbon oxidation: propene on palladium, ENERGY & ENVIRONMENTAL SCIENCE, Vol: 12, Pages: 1055-1067, ISSN: 1754-5692

Journal article

Wei C, Rao RR, Peng J, Huang B, Stephens IEL, Risch M, Xu ZJ, Shao-Horn Yet al., 2019, Recommended Practices and Benchmark Activity for Hydrogen and Oxygen Electrocatalysis in Water Splitting and Fuel Cells, Advanced Materials, ISSN: 0935-9648

© 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Electrochemical energy storage by making H 2 an energy carrier from water splitting relies on four elementary reactions, i.e., the hydrogen evolution reaction (HER), hydrogen oxidation reaction (HOR), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR). Herein, the central objective is to recommend systematic protocols for activity measurements of these four reactions and benchmark activities for comparison, which is critical to facilitate the research and development of catalysts with high activity and stability. Details for the electrochemical cell setup, measurements, and data analysis used to quantify the kinetics of the HER, HOR, OER, and ORR in acidic and basic solutions are provided, and examples of state-of-the-art specific and mass activity of catalysts to date are given. First, the experimental setup is discussed to provide common guidelines for these reactions, including the cell design, reference electrode selection, counter electrode concerns, and working electrode preparation. Second, experimental protocols, including data collection and processing such as ohmic- and background-correction and catalyst surface area estimation, and practice for testing and comparing different classes of catalysts are recommended. Lastly, the specific and mass activity activities of some state-of-the-art catalysts are benchmarked to facilitate the comparison of catalyst activity for these four reactions across different laboratories.

Journal article

Engstfeld AK, Maagaard T, Horch S, Chorkendorff I, Stephens IELet al., 2018, Polycrystalline and Single-Crystal Cu Electrodes: Influence of Experimental Conditions on the Electrochemical Properties in Alkaline Media, CHEMISTRY-A EUROPEAN JOURNAL, Vol: 24, Pages: 17743-17755, ISSN: 0947-6539

Journal article

Roy C, Sebok B, Scott SB, Fiordaliso EM, Sørensen JE, Bodin A, Trimarco DB, Damsgaard CD, Vesborg PCK, Hansen O, Stephens IEL, Kibsgaard J, Chorkendorff Iet al., 2018, Impact of nanoparticle size and lattice oxygen on water oxidation on NiFeO<inf>x</inf>H<inf>y</inf>, Nature Catalysis, Vol: 1, Pages: 820-829, ISSN: 2520-1158

NiFeOxHy are the most active catalysts for oxygen evolution in a base. For this reason, they are used widely in alkaline electrolysers. Several open questions remain as to the reason for their exceptionally high catalytic activity. Here we use a model system of mass-selected NiFe nanoparticles and isotope labelling experiments to show that oxygen evolution in 1 M KOH does not proceed via lattice exchange. We complement our activity measurements with electrochemistry–mass spectrometry, taken under operando conditions, and transmission electron microscopy and low-energy ion-scattering spectroscopy, taken ex situ. Together with the trends in particle size, the isotope results indicate that oxygen evolution is limited to the near-surface region. Using the surface area of the particles, we determined that the turnover frequency was 6.2 ± 1.6 s−1 at an overpotential of 0.3 V, which is, to the best of our knowledge, the highest reported for oxygen evolution in alkaline solution.

Journal article

Escudero-Escribano M, Pedersen AF, Ulrikkeholm ET, Jensen KD, Hansen MH, Rossmeisl J, Stephens IEL, Chorkendorff Iet al., 2018, Active-Phase Formation and Stability of Gd/Pt(111) Electrocatalysts for Oxygen Reduction: An In Situ Grazing Incidence X-Ray Diffraction Study, CHEMISTRY-A EUROPEAN JOURNAL, Vol: 24, Pages: 12280-12290, ISSN: 0947-6539

Journal article

Rao RR, Kolb MJ, Hwang J, Pedersen AF, Mehta A, You H, Stoerzinger KA, Feng Z, Zhou H, Bluhm H, Giordano L, Stephens IEL, Shao-Horn Yet al., 2018, Surface Orientation Dependent Water Dissociation on Rutile Ruthenium Dioxide, JOURNAL OF PHYSICAL CHEMISTRY C, Vol: 122, Pages: 17802-17811, ISSN: 1932-7447

Journal article

Roy C, Rao RR, Stoerzinger KA, Hwang J, Rossmeisl J, Chorkendorff I, Shao-Horn Y, Stephens IELet al., 2018, Trends in activity and dissolution on RuO2 under oxygen evolution conditions: particles versus well-defined extended surfaces, ACS Energy Letters, Vol: 3, Pages: 2045-2051, ISSN: 2380-8195

Rutile RuO2 catalysts are the most active pure metal oxides for oxygen evolution; however, they are also unstable toward dissolution. Herein, we study the catalytic activity and stability of oriented thin films of RuO2 with (111), (101), and (001) orientations, in comparison to a (110) single crystal and commercial nanoparticles. These surfaces were all tested in aqueous solutions of 0.05 M H2SO4. The initial catalyst activity ranked as follows: (001) > (101) > (111) ≈ (110). We complemented our activity data with inductively coupled plasma mass spectroscopy, to measure Ru dissolution products occurring in parallel to oxygen evolution. In contrast to earlier reports, we find that, under our experimental conditions, there is no correlation between the activity and stability.

Journal article

Colic V, Yang S, Revay Z, Stephens IEL, Chorkendorff Iet al., 2018, Carbon catalysts for electrochemical hydrogen peroxide production in acidic media, ELECTROCHIMICA ACTA, Vol: 272, Pages: 192-202, ISSN: 0013-4686

Journal article

Yang S, Verdaguer-Casadevall A, Arnarson L, Silvio L, Colic V, Frydendal R, Rossmeisl J, Chorkendorff I, Stephens IELet al., 2018, Toward the decentralized electrochemical production of H2O2: A focus on the catalysis, ACS Catalysis, Vol: 8, Pages: 4064-4081, ISSN: 2155-5435

H2O2 is a valuable, environmentally friendly oxidizing agent with a wide range of uses from the provision of clean water to the synthesis of valuable chemicals. The on-site electrolytic production of H2O2 would bring the chemical to applications beyond its present reach. The successful commercialization of electrochemical H2O2 production requires cathode catalysts with high activity, selectivity, and stability. In this Perspective, we highlight our current understanding of the factors that control the cathode performance. We review the influence of catalyst material, electrolyte, and the structure of the interface at the mesoscopic scale. We provide original theoretical data on the role of the geometry of the active site and its influence on activity and selectivity. We have also conducted a series of original experiments on (i) the effect of pH on H2O2 production on glassy carbon, pure metals, and metal–mercury alloys, and (ii) the influence of cell geometry and mass transport in liquid half-cells in comparison to membrane electrode assemblies.

Journal article

Arnarson L, Schmidt PS, Pandey M, Bagger A, Thygesen KS, Stephens IEL, Rossmeisl Jet al., 2018, Fundamental limitation of electrocatalytic methane conversion to methanol, PHYSICAL CHEMISTRY CHEMICAL PHYSICS, Vol: 20, Pages: 11152-11159, ISSN: 1463-9076

Journal article

Jensen KD, Tymoczko J, Rossmeisl J, Bandarenka AS, Chorkendorff I, Escudero-Escribano M, Stephens IELet al., 2018, Frontispiz: Elucidation of the Oxygen Reduction Volcano in Alkaline Media using a Copper-Platinum(111) Alloy, Angewandte Chemie, Vol: 130, ISSN: 0044-8249

Journal article

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