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      Operando characterization techniques for electrocatalysis

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          Abstract

          Significant progress toward the understanding of the electrochemical process has been achieved in the past decades, owning to the advancements in operando characterization techniques.

          Abstract

          The energy crisis and climate change are growing threats to human society. Fuel cells and electrolyzers using solar and wind as energy sources provide a promising carbon-free and sustainable energy future as an alternative to the present fossil fuels. Robust and efficient electrocatalysts constitute the cornerstone for practical applications of fuel cells and electrolyzers. However, their rational design is hindered by the lack of knowledge of the active site structures, the reaction pathways, and the degradation mechanisms. This calls for the operando characterization of the electrode–electrolyte–gas interface of electrocatalytic processes under working conditions. This review describes the understanding of various operando techniques for diverse nano-, molecular and single atom electrocatalysts. We highlight the progress achieved via operando techniques toward the elucidation of the structure–activity relationships of electrocatalysts.

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          Combining theory and experiment in electrocatalysis: Insights into materials design

          Electrocatalysis plays a central role in clean energy conversion, enabling a number of sustainable processes for future technologies. This review discusses design strategies for state-of-the-art heterogeneous electrocatalysts and associated materials for several different electrochemical transformations involving water, hydrogen, and oxygen, using theory as a means to rationalize catalyst performance. By examining the common principles that govern catalysis for different electrochemical reactions, we describe a systematic framework that clarifies trends in catalyzing these reactions, serving as a guide to new catalyst development while highlighting key gaps that need to be addressed. We conclude by extending this framework to emerging clean energy reactions such as hydrogen peroxide production, carbon dioxide reduction, and nitrogen reduction, where the development of improved catalysts could allow for the sustainable production of a broad range of fuels and chemicals.
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            Benchmarking heterogeneous electrocatalysts for the oxygen evolution reaction.

            Objective evaluation of the activity of electrocatalysts for water oxidation is of fundamental importance for the development of promising energy conversion technologies including integrated solar water-splitting devices, water electrolyzers, and Li-air batteries. However, current methods employed to evaluate oxygen-evolving catalysts are not standardized, making it difficult to compare the activity and stability of these materials. We report a protocol for evaluating the activity, stability, and Faradaic efficiency of electrodeposited oxygen-evolving electrocatalysts. In particular, we focus on methods for determining electrochemically active surface area and measuring electrocatalytic activity and stability under conditions relevant to an integrated solar water-splitting device. Our primary figure of merit is the overpotential required to achieve a current density of 10 mA cm(-2) per geometric area, approximately the current density expected for a 10% efficient solar-to-fuels conversion device. Utilizing the aforementioned surface area measurements, one can determine electrocatalyst turnover frequencies. The reported protocol was used to examine the oxygen-evolution activity of the following systems in acidic and alkaline solutions: CoO(x), CoPi, CoFeO(x), NiO(x), NiCeO(x), NiCoO(x), NiCuO(x), NiFeO(x), and NiLaO(x). The oxygen-evolving activity of an electrodeposited IrO(x) catalyst was also investigated for comparison. Two general observations are made from comparing the catalytic performance of the OER catalysts investigated: (1) in alkaline solution, every non-noble metal system achieved 10 mA cm(-2) current densities at similar operating overpotentials between 0.35 and 0.43 V, and (2) every system but IrO(x) was unstable under oxidative conditions in acidic solutions.
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              Benchmarking hydrogen evolving reaction and oxygen evolving reaction electrocatalysts for solar water splitting devices.

              Objective comparisons of electrocatalyst activity and stability using standard methods under identical conditions are necessary to evaluate the viability of existing electrocatalysts for integration into solar-fuel devices as well as to help inform the development of new catalytic systems. Herein, we use a standard protocol as a primary screen for evaluating the activity, short-term (2 h) stability, and electrochemically active surface area (ECSA) of 18 electrocatalysts for the hydrogen evolution reaction (HER) and 26 electrocatalysts for the oxygen evolution reaction (OER) under conditions relevant to an integrated solar water-splitting device in aqueous acidic or alkaline solution. Our primary figure of merit is the overpotential necessary to achieve a magnitude current density of 10 mA cm(-2) per geometric area, the approximate current density expected for a 10% efficient solar-to-fuels conversion device under 1 sun illumination. The specific activity per ECSA of each material is also reported. Among HER catalysts, several could operate at 10 mA cm(-2) with overpotentials <0.1 V in acidic and/or alkaline solutions. Among OER catalysts in acidic solution, no non-noble metal based materials showed promising activity and stability, whereas in alkaline solution many OER catalysts performed with similar activity achieving 10 mA cm(-2) current densities at overpotentials of ~0.33-0.5 V. Most OER catalysts showed comparable or better specific activity per ECSA when compared to Ir and Ru catalysts in alkaline solutions, while most HER catalysts showed much lower specific activity than Pt in both acidic and alkaline solutions. For select catalysts, additional secondary screening measurements were conducted including Faradaic efficiency and extended stability measurements.
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                Author and article information

                Contributors
                Journal
                EESNBY
                Energy & Environmental Science
                Energy Environ. Sci.
                Royal Society of Chemistry (RSC)
                1754-5692
                1754-5706
                November 12 2020
                2020
                : 13
                : 11
                : 3748-3779
                Affiliations
                [1 ]Key Laboratory for Green Chemical Technology of Ministry of Education
                [2 ]School of Chemical Engineering and Technology, Tianjin University
                [3 ]Collaborative Innovation Center of Chemical Science and Engineering (Tianjin)
                [4 ]Tianjin 300072
                [5 ]China
                Article
                10.1039/D0EE01706J
                82275848-8c46-4442-bb4f-936b6bb02550
                © 2020

                http://rsc.li/journals-terms-of-use

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