Article Cite This: Acc. Chem. Res. 2018, 51, 1590−1598
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Superwetting Electrodes for Gas-Involving Electrocatalysis Wenwen Xu,† Zhiyi Lu,*,§ Xiaoming Sun,*,†,‡ Lei Jiang,∥ and Xue Duan† †
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State Key Laboratory of Chemical Resource Engineering, Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, P. R. China ‡ College of Energy, Beijing University of Chemical Technology, Beijing 100029, P. R. China § Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, United States ∥ CAS Key Laboratory of Bio-inspired Materials and Interfacial Sciences, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Future Technology College, University of Chinese Academy of Sciences, Beijing 100190, P. R. China CONSPECTUS: Gas-involving electrochemical reactions, including gas evolution reactions and gas consumption reactions, are essential components of the energy conversion processes and gathering elevating attention from researchers. Besides the development of highly active catalysts, gas management during gas-involving electrochemical reactions is equally critical for industrial applications to achieve high reaction rates (hundreds of milliamperes per square centimeter) under practical operation voltages. Biomimetic surfaces, which generally show regular micro/nanostructures, offer new insights to address this issue because of their special wetting capabilities. Although a series of nanoarray-based structured electrodes have been constructed and demonstrated with excellent performances for gas-involving electrochemical reactions, understanding of bubble wetting behavior remains elusive. In this Account, our recent works including understanding the way to achieve the superwetting properties of solid electrode surfaces, and our advanced design and fabrication of superwetting electrodes for different types of electrochemical gas-involving electrochemical reactions are summarized. To begin, we first put forward several criteria of superwetting surfaces, including superaerophobic surfaces and superaerophilic surfaces. Then, we discuss how the nanoarray-based surface engineering technology can achieve the superwetting properties, in which high roughness of the nanoarray architecture is discovered to be a critical factor for constructing superaerophobic and superaerophilic surfaces. Finally, the feasibility of superwetting electrodes for enhancing the performances of gas-involving electrochemical reactions is also analyzed. Based on theoretical guidance, a series of superaerophobic and superaerophilic electrodes with various methods, such as hydrothermal reactions, electrodeposition technology and high-temperature vapor phase growth, have been built for practice. By comparing with the traditional planar electrodes fabricated by drop-casting method, the superaerophobic electrodes afford a low adhesion force to gas products and accelerate gas bubbles evolution, resulting in fast current increase and stable current for gas evolution reactions. This phenomenon is confirmed by operating different gas evolution reactions (hydrogen evolution, oxygen evolution and hydrazine oxidation) using superaerophobic electrodes with different catalysts (e.g., MoS2, Pt and Cu). On the other side, the superaerophilic electrodes can improve the catalytic performance of gas consumption reaction (e.g., oxygen reduction reaction) by facilitating gas diffusion and electron transport. Following theoretical analyses and experimental demonstrations, we assemble several energy conversion systems (e.g., electrochemical water splitting and direct hydrazine fuel cells) based on superwetting electrodes and test their performances. By virtue of the structural advantages of electrodes, these energy conversion systems show much higher energy efficiencies than their counterparts. In the last section, we put forward several future fields which are worthy for further exploration as rational extensions of the superwetting electrodes.
1. INTRODUCTION
gaseous H2 oxidation in hydrogen fuel cells). Demand of energy devices with higher efficiency and faster rate of gas production has drawn much attention on the development of novel electrode materials. To increase efficiency of these processes, electrodes with minimized overpotentials, maximized reaction rates (i.e., high current densities), and optimized costeffectiveness are highly needed and appealing. It has been
Gas-involving electrochemical reactions, including gas-evolution reactions (GERs) and gas-consumption reactions (GCRs), are essential components of the energy conversion processes. For instance, water splitting, which is believed to be a promising approach to store solar energy into chemical energy, contains two typical GERs that convert water into H2 and O2 gases. In metal-air batteries and fuel cells, GCRs often take place on cathodes to reduce oxygen into water and combine with anodic fuel oxidation reactions to deliver energy (e.g., © 2018 American Chemical Society
Received: February 11, 2018 Published: June 8, 2018 1590
DOI: 10.1021/acs.accounts.8b00070 Acc. Chem. Res. 2018, 51, 1590−1598
Article
Accounts of Chemical Research
rise to incomplete reaction and low efficiency, as shown in Figure 1B. In contrast to tremendous progresses in screening active catalysts, very limited research has been devoted to construct an effective electrode architecture. Actually, the architecture construction is of equal importance as an ideal electrode should either minimize or maximize the interaction of the gaseous species on the surface for promoting the reactions, depending on the identity of the gas-involving reactions. For GERs, the gas bubble adhesion on the electrode architecture should be reduced, resulting in promoted bubble releasing behavior. While, for GCRs, the surface should provide a facilitated gas diffusion pathway to continually supply sufficient gas reactant to catalytic active sites.4 Biomimetic surface provides new insights to engineer functional surfaces with special wetting capabilities.5 It is reported that the adhesion behavior of gas bubbles underwater could be well controlled (e.g., “pinning state”6 or “bursting state”7) by tailoring compositions and micro/nanoarchitectures of surfaces.8−12 Recently, the bioinspired materials show new possibilities of improving the electrochemical performance by tailoring the surface bubble-superwetting properties. The nanoarray-based superaerophobic electrodes afford a discontinuous three-phase (solid−liquid−gas) contact line (TPCL), which minimizes the gas bubble adhesion and accelerates gas evolution.13−15 On the other hand, the nanoarray-based superaerophilic electrodes accelerate the gas diffusion processfor the gas consumption reaction.12 In both cases, the nanoarray electrodes show the potential to approach the “ideal” electrodes for gas-involving electrocatalysis by providing three “highways”: structured electrodes for electron-ways, continual electrolytes for ion-ways, and tailored aerophilicity for gasways.16 In this Account aiming at providing a systematic understanding of superwetting electrodes for gas-involving electrochemical reactions, we first introduce a brief definition of superaerophobicity and superaerophilicity under aqueous media, and then propose optimized architectures (nanoarrays) for achieving superaerophobicity and superaerophilicity. In the following sections, we summarized several nanoarray-based superaerophobic and superaerophilic electrodes which have demonstrated noticeably enhancement of performance for GERs and GCRs. Afterward, some energy conversion systems containing superwetting electrodes are demonstrated with
widely recognized that the performances of the electrodes depend on both catalyst screening1 and structural optimization. Generally, GERs include hydrogen evolution reaction (HER), oxygen evolution reaction (OER), hydrazine oxidation reaction (HzOR with N2 as product), and chlorine evolution reaction (ClER). On the other side, GCRs include oxygen reduction reaction (ORR), hydrogen oxidation reaction (HOR), and carbon dioxide reduction reaction (CO2RR). The major restrictive factor to achieve high performances of the aforementioned reactions is sluggish intrinsic kinetics, and thus, great effort has made to identify active electrocatalysts with appropriate electronic structures and drive the electrocatalysis at relatively low overpotentials.2,3 However, several severe problems still exist in industrial productions such as industrial water splitting and fuel cells even if those highly active catalysts are employed. In water splitting industry, a great number of gas bubbles will generate on the electrode surface as large current densities are usually required. In this case, if the electrode shows strong adhesion to gas bubbles, a large number of gas bubbles will gather around the surface and block the diffusion of electrolyte, resulting in huge reaction resistance, as shown in Figure 1A. While in fuel cells or metal-air batteries, if channels
Figure 1. (A) Large number of big H2 bubbles are generated from the Pt foil electrodes and gather around surface during HER; (B) threephase contact lines (red lines) in fuel cell or metal-air positive electrode. Electrolyte fully floods the channels, and only the dissolved oxygen can participate in the reaction.
are flooded with the electrolyte, the diffusion kinetics of reactant gas (i.e., oxygen) will become extremely slow, giving
Figure 2. Schematic illustration of how the surface roughness affecting the bubble contact angle at given intrinsic aerophilicity: (A) flat aerophilic surface, (B) rough aerophilic surface, (C) flat aerophobic surface, and (D) rough aerophobic surface. 1591
DOI: 10.1021/acs.accounts.8b00070 Acc. Chem. Res. 2018, 51, 1590−1598
Article
Accounts of Chemical Research
Figure 3. Demonstration of superaerophobic electrode concept using MoS2 electrode. (A) Stress analysis of one single bubble on the electrode surface. (B) Continuous and discontinuous TPCL at flat (left) and nanostructured film (right); (C−E): (left) SEM images of flat, microstructured, and nanostructured MoS2 films. (right) Digital images of hydrogen bubbles on the three MoS2 films. (F) (top) Polarization curves of MoS2 and Pt/C catalysts; (bottom) stability testing of the three MoS2 electrodes. Reproduced with permission from ref 13. Copyright 2014 WILEY-VCH.
superb energy conversion efficiency. Finally, we discuss about future development in superwetting electrodes and their applications in electrocatalysis.
2. CONCEPT OF SUPERAEROPHOBICITY AND SUPERAEROPHILICITY The terms “superaerophobicity” and “superaerophilicity” are analogous to the terms like “superhydrophobicity” and “superhydrophilicity”, which are frequently used to describe the interaction between solid surface and water. Herein, since the investigated objects are moved from “liquid droplets” to “gas bubbles”, the “hydro-” term is replaced by “aero-” and the investigation environments are changed from air to water. Generally, a superhydrophobic surface shows a high liquid contact angle (LCA, >150°) and a tiny adhesion force, while a superhydrophilic surface is a surface with low LCA (e.g., 150°) underwater, which also shows a low adhesion force, while the superaerophilic surface means low BCA (usually
