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<front>
<journal-meta>
<journal-id journal-id-type="publisher-id">Front. Energy Res.</journal-id>
<journal-title>Frontiers in Energy Research</journal-title>
<abbrev-journal-title abbrev-type="pubmed">Front. Energy Res.</abbrev-journal-title>
<issn pub-type="epub">2296-598X</issn>
<publisher>
<publisher-name>Frontiers Media S.A.</publisher-name>
</publisher>
</journal-meta>
<article-meta>
<article-id pub-id-type="publisher-id">612093</article-id>
<article-id pub-id-type="doi">10.3389/fenrg.2020.612093</article-id>
<article-categories>
<subj-group subj-group-type="heading">
<subject>Energy Research</subject>
<subj-group>
<subject>Original Research</subject>
</subj-group>
</subj-group>
</article-categories>
<title-group>
<article-title>NiFeRu Layered Double Hydroxide and Its Derivatives Supported on Graphite Foam as Binder-Free Cathode for Nonaqueous Li-O<sub>2</sub> Batteries</article-title>
<alt-title alt-title-type="left-running-head">Song et al.</alt-title>
<alt-title alt-title-type="right-running-head">Binder-Free NiFeRu-LDH@GF and Derivatives Cathode</alt-title>
</title-group>
<contrib-group>
<contrib contrib-type="author" corresp="yes">
<name>
<surname>Song</surname>
<given-names>Ming</given-names>
</name>
<xref ref-type="aff" rid="aff1">
<sup>1</sup>
</xref>
<xref ref-type="corresp" rid="c001">&#x2a;</xref>
<uri xlink:href="http://loop.frontiersin.org/people/1100694/overview"/>
</contrib>
<contrib contrib-type="author">
<name>
<surname>Tan</surname>
<given-names>Hua</given-names>
</name>
<xref ref-type="aff" rid="aff2">
<sup>2</sup>
</xref>
</contrib>
<contrib contrib-type="author">
<name>
<surname>Tian</surname>
<given-names>Lin</given-names>
</name>
<xref ref-type="aff" rid="aff1">
<sup>1</sup>
</xref>
</contrib>
<contrib contrib-type="author">
<name>
<surname>Li</surname>
<given-names>Jing</given-names>
</name>
<xref ref-type="aff" rid="aff1">
<sup>1</sup>
</xref>
</contrib>
<contrib contrib-type="author">
<name>
<surname>Xu</surname>
<given-names>Yan</given-names>
</name>
<xref ref-type="aff" rid="aff1">
<sup>1</sup>
</xref>
</contrib>
<contrib contrib-type="author">
<name>
<surname>Shi</surname>
<given-names>Liluo</given-names>
</name>
<xref ref-type="aff" rid="aff1">
<sup>1</sup>
</xref>
</contrib>
<contrib contrib-type="author">
<name>
<surname>Sun</surname>
<given-names>Limei</given-names>
</name>
<xref ref-type="aff" rid="aff1">
<sup>1</sup>
</xref>
</contrib>
<contrib contrib-type="author">
<name>
<surname>Zhuang</surname>
<given-names>Wenchang</given-names>
</name>
<xref ref-type="aff" rid="aff1">
<sup>1</sup>
</xref>
</contrib>
<contrib contrib-type="author">
<name>
<surname>Du</surname>
<given-names>Xihua</given-names>
</name>
<xref ref-type="aff" rid="aff1">
<sup>1</sup>
</xref>
</contrib>
</contrib-group>
<aff id="aff1">
<sup>1</sup>
<institution>College Materials and Chemical Engineering, Xuzhou University of Technology</institution>, <addr-line>Xuzhou</addr-line>, <country>China</country>
</aff>
<aff id="aff2">
<sup>2</sup>
<institution>School of Physical and Mathematical Sciences, Nanyang Technological University</institution>, <addr-line>Singapore</addr-line>, <country>Singapore</country>
</aff>
<author-notes>
<fn fn-type="edited-by">
<p>
<bold>Edited by:</bold> Xinghui Wang, Fuzhou University, China</p>
</fn>
<fn fn-type="edited-by">
<p>
<bold>Reviewed by:</bold> Chao Lai, Jiangsu Normal University, China</p>
<p>Yongqi Zhang, University of Electronic Science and Technology of China, China</p>
</fn>
<corresp id="c001">&#x2a;Correspondence: Ming Song, <email>mings@xzit.edu.cn</email>
</corresp>
<fn fn-type="other">
<p>This article was submitted to Electrochemical Energy Conversion and Storage, a section of the journal Frontiers in Energy Research</p>
</fn>
</author-notes>
<pub-date pub-type="epub">
<day>12</day>
<month>11</month>
<year>2020</year>
</pub-date>
<pub-date pub-type="collection">
<year>2020</year>
</pub-date>
<volume>8</volume>
<elocation-id>612093</elocation-id>
<history>
<date date-type="received">
<day>30</day>
<month>09</month>
<year>2020</year>
</date>
<date date-type="accepted">
<day>22</day>
<month>10</month>
<year>2020</year>
</date>
</history>
<permissions>
<copyright-statement>Copyright &#x00A9; 2020 Song, Tan, Tian, Li, Xu, Shi, Sun, Zhuang and Du.</copyright-statement>
<copyright-year>2020</copyright-year>
<copyright-holder>Song, Tan, Tian, Li, Xu, Shi, Sun, Zhuang and Du</copyright-holder>
<license xlink:href="http://creativecommons.org/licenses/by/4.0/">
<p>This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.</p>
</license>
</permissions>
<abstract>
<p>Li-O<sub>2</sub> batteries (LOBs) generally referred as Li-air batteries (LABs) potentially have a high specific capacity among all kinds of metal-ion batteries. However, it suffers severely from sluggish discharge/charge kinetics. A binder-free integrated cathode is fabricated which contains NiFeRu layered double hydroxide (NiFeRu LDH) nanosheets grown on a 3D conductive graphite foam (NiFeRu LDH@GF) in this work. This integrated 3D cathode exhibits a specific capacity of 3,085&#xa0;mAh&#xa0;g<sup>&#x2212;1</sup> and a cycle life of 46. The electrochemical performances of NiFeRu LDH derived metal oxides (DMO@GF) by calcining NiFeRu LDH@GF at 350&#xb0;C, 450&#xb0;C, and 550&#xb0;C are also been investigated. The 450&#xb0;C obtained DMO@GF (450-DMO@GF) exhibits the highest catalytic activity for oxygen reduction/evolution reaction. The unique floccule-like and cross-connected structure of the 450-DMO@GF and its low charge transfer resistance are responsible for the outstanding electrochemical performance.</p>
</abstract>
<kwd-group>
<kwd>Li-O<sub>2</sub> battery</kwd>
<kwd>layered double hydroxide</kwd>
<kwd>binder-free cathode</kwd>
<kwd>graphite foam</kwd>
<kwd>derived metal oxide</kwd>
</kwd-group>
<contract-num rid="cn001">BK20171169</contract-num>
<contract-num rid="cn002">19KJA430020</contract-num>
<contract-num rid="cn003">2020</contract-num>
<contract-num rid="cn003">2018</contract-num>
<contract-num rid="cn004">22075115</contract-num>
<contract-num rid="cn005">xcx2020135</contract-num>
<contract-sponsor id="cn001">Natural Science Foundation of Jiangsu Province<named-content content-type="fundref-id">10.13039/501100004608</named-content>
</contract-sponsor>
<contract-sponsor id="cn002">Natural Science Research of Jiangsu Higher Education Institutions of China<named-content content-type="fundref-id">10.13039/501100010023</named-content>
</contract-sponsor>
<contract-sponsor id="cn003">Qinglan Project of Jiangsu Province of China<named-content content-type="fundref-id">10.13039/501100013088</named-content>
</contract-sponsor>
<contract-sponsor id="cn004">National Natural Science Foundation of China<named-content content-type="fundref-id">10.13039/501100001809</named-content>
</contract-sponsor>
<contract-sponsor id="cn005">National College Students Innovation and Entrepreneurship Training Program<named-content content-type="fundref-id">10.13039/501100013254</named-content>
</contract-sponsor>
<counts>
<page-count count="0"/>
</counts>
</article-meta>
</front>
<body>
<sec id="s1">
<title>Introduction</title>
<p>The modern e-society has put increasing demand for new energy sources that can enable long-lasting power and fast charging and/or release of the energy. Among various available batteries, Li-O<sub>2</sub> batteries (LOBs), which are also called Li-air batteries (LABs), have the highest theoretical specific energy of 3,505&#xa0;Wh&#xa0;kg<sup>&#x2212;1</sup>. This specific energy is &#x223c;10&#xa0;times that of Li-ion batteries (<xref ref-type="bibr" rid="B2">Bruce et al., 2012</xref>) and higher than other Li metal batteries (<xref ref-type="bibr" rid="B31">Xue et al., 2019</xref>; <xref ref-type="bibr" rid="B27">Sun et al., 2020</xref>). Therefore, LOBs have attracted immense interest since 1996 as the &#x201c;holy grail&#x201d; of batteries (<xref ref-type="bibr" rid="B1">Abraham and Jiang, 1996</xref>). However, the development of LOBs are slow because of their low energy efficiency caused by decomposition of nonaqueous electrolyte and carbon-based cathode (<xref ref-type="bibr" rid="B8">Freunberger et al., 2011</xref>; <xref ref-type="bibr" rid="B20">Ottakam Thotiyl et al., 2013</xref>), short cycle life caused by nonrecovery of the reaction surface/interface (<xref ref-type="bibr" rid="B16">Lu et al., 2016</xref>), and poor power performance led by low kinetics of electron/Li<sup>&#x2b;</sup>/O<sub>2</sub> transport during the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) (<xref ref-type="bibr" rid="B15">Liu et al., 2015</xref>). These issues are all related to the properties of cathode materials, and achieving cathode with high electrochemical performances is still a big challenge. During the discharging process, O<sub>2</sub> should be first reduced at the gas/liquid/solid (O<sub>2</sub>/Li<sup>&#x2b;</sup>/e<sup>&#x2212;</sup>) three-phase reaction interface of cathode. Then, the reduced O<sub>2</sub>
<sup>&#x2212;</sup> will combine with Li<sup>&#x2b;</sup> to form Li<sub>2</sub>O<sub>2</sub>. In the charging process, Li<sub>2</sub>O<sub>2</sub> should be decomposed to O<sub>2</sub> and Li<sup>&#x2b;</sup>, which guarantees reuse of the reaction interface of cathode during the following cycles. In addition to the Li anode and electrolyte, these two discharging/charging processes are dependent on the catalytic activity of cathode for ORR and OER. The slow kinetics of ORR and OER taking place in the cathode will undoubtedly give rise to poor battery performances. Thus, there is consensus that a stable cathode material with high ORR and OER activity is the key player of LOBs.</p>
<p>To realize a stable cathode and a superior Li-O<sub>2</sub> battery, numerous research efforts concerning catalyst design and catalytic mechanism have been devoted to the electro-catalytic materials field (<xref ref-type="bibr" rid="B4">Chang et al., 2017</xref>). Various cathode materials based on carbon materials (<xref ref-type="bibr" rid="B21">Qin et al., 2019</xref>), heteroatom doping carbon material (<xref ref-type="bibr" rid="B12">Jiang et al., 2019</xref>), noble metal (<xref ref-type="bibr" rid="B34">Zhang et al., 2019</xref>), and metal oxides (<xref ref-type="bibr" rid="B33">Yuan et al., 2019</xref>) have been used to the LOBs. The aim is to enhance the performances of LOBs by promoting the reaction kinetics associated with Li<sub>2</sub>O<sub>2</sub> generation/decomposition. In particular, the larger overpotential during the charging process than discharging process reveals more sluggish OER kinetics (<xref ref-type="bibr" rid="B19">Lyu et al., 2017</xref>). Layered double hydroxide (LDH) has become the protagonist in different areas including photochemistry (<xref ref-type="bibr" rid="B36">Zhao et al., 2014</xref>), adsorption (<xref ref-type="bibr" rid="B10">Gu et al., 2015</xref>), drug delivery (<xref ref-type="bibr" rid="B23">Ribeiro et al., 2014</xref>), and water electrolysis due to its advantages such as more exposed catalytic active sites than 0D or 1D materials, large surface-to-bulk ratios, and easily controllable layered structure adjustment (<xref ref-type="bibr" rid="B3">Cai et al., 2019</xref>). Especially, LDHs have attracted increased attention because of their OER catalytic activity since 2013 (<xref ref-type="bibr" rid="B9">Gong et al., 2013</xref>). This implies that LDHs are promising in accelerating the kinetics during the charging process for LOBs. However, there are only limited research so far on LDH cathode of LOBs. Recently, cathodes of nonaqueous LOBs based on CoNiFe LDH and conductive RuO<sub>2.1</sub> super-lattice composites have been fabricated by casting these composites with polyvinylidene fluoride (PVDF) binder on stainless steel mesh (<xref ref-type="bibr" rid="B17">Lu et al., 2020</xref>). It is worth mentioning that LDHs usually have poor conductivity and insufficient active edge sites. Therefore, binder-free cathodes with LDHs supported on conductive substrate will be more superior.</p>
<p>Herein, in this work, we design and realize an integrated binder-free cathode for LOBs by directly growing LDH on a graphite foam (LDH@GF). NiFe LDH doped by Ru (NiFeRu LDH) is selected since catalysts based on trimetal usually offer higher catalytic activity for OER in comparison with bimetal-centered LDH, and Ru has high adsorption capabilities toward oxygen-containing intermediates (<xref ref-type="bibr" rid="B24">Seh et al., 2017</xref>). In addition, their derived metal oxides (DMOs) supported on the GF (DMO@GF), by calcination under different temperatures, are also investigated as cathodes of LOBs since transitional metal oxides (e.g., NiO (<xref ref-type="bibr" rid="B28">Tong et al., 2015</xref>)) and spinel oxides (e.g., NiCo<sub>2</sub>O<sub>4</sub> (<xref ref-type="bibr" rid="B26">Sun et al., 2014</xref>)) have exhibited catalytic activity in LOBs.</p>
</sec>
<sec id="s2">
<title>Experiment Section</title>
<sec id="s2-1">
<title>Preparation of Cathode Materials</title>
<p>The 3D graphite foam (GF) substrate was prepared by using the chemical vapor deposition (CVD) method using the nickel foam as a template according to our previous work (<xref ref-type="bibr" rid="B5">Chao et al., 2014</xref>; <xref ref-type="bibr" rid="B25">Song et al., 2020</xref>). After etching the nickel template using a solution of 1&#xa0;M FeCl<sub>3</sub> and 0.5&#xa0;M&#xa0;HCl, a free-standing GF (&#x223c;0.5&#xa0;mg&#xa0;cm<sup>&#x2212;2</sup>) was obtained. Then, a piece of GF was immersed into a mixed solution of 98% HNO<sub>3</sub> and 70% H<sub>2</sub>SO<sub>4</sub> with a mole ratio of 1:3. After washing with deionized water, this hydrophilic-treated GF was immersed into ethylene glycol for 1&#xa0;h and then transferred into a teflon autoclave containing a mixed solution of Ni(NO3)2&#xb7;6H2O/Fe(NO3)2&#xb7;9H2O/RuCl3/NH<sub>4</sub>F/urea (mole ratio is 5:4:1:35:35). The hydrothermal reaction was carried out at 120&#xb0;C for 12&#xa0;h. Then, the NiFeRu LDH supported on GF (NiFeRu LDH@GF, &#x223c;1.3&#xa0;mg&#xa0;cm<sup>&#x2212;2</sup>) cathode was obtained after cooling naturally and washing with deionized water. The fabrication process for NiFeRu LDH@GF cathode is shown in <xref ref-type="fig" rid="F1">Figure 1</xref>.</p>
<fig id="F1" position="float">
<label>FIGURE 1</label>
<caption>
<p>Schematic illustration for the synthesis of NiFeRu-LDH@GF cathode.</p>
</caption>
<graphic xlink:href="fenrg-08-612093-g001.tif"/>
</fig>
<p>The DMOs supported on the GF (DMO@GF, &#x223c;1.0&#xa0;mg&#xa0;cm<sup>&#x2212;2</sup>) cathode was achieved through heating NiFeRu LDH@GF in a quartz tube furnace under Ar gas at 350&#xb0;C, 450&#xb0;C, and 550&#xb0;C for 4&#xa0;h.</p>
</sec>
<sec id="s2-2">
<title>Characterization</title>
<p>X-ray diffraction (XRD) was performed on an X-ray diffractometer (Bruker-AXS D8 Advance, CuK&#x3b1;, <italic>&#x3bb;</italic> &#x3d; 1.54186). The morphology and microstructure of the cathode were observed through an transmission electron microscope (JEOL 2100 F HRTEM) and scanning electron microscope (JEOL 7600F FESEM). Raman spectra were measured using a Raman system with a laser wavelength of 532&#xa0;nm (WITec-CRM200).</p>
</sec>
<sec id="s2-3">
<title>LOBs Assembly and Electrochemical Tests</title>
<p>The homemade Swagelok-type cells which were assembled under pure argon gas (&#x3c;0.5&#xa0;ppm H<sub>2</sub>O and O<sub>2</sub>) in a glovebox were used for the electrochemical measurements. A free-standing NiFeRu LDH@GF cathode or a DMO@GF cathode and a Li anode were separated by a Whatman glass fiber separator. The glass fiber separator was first impregnated with 1&#xa0;M bis(trifluoromethane)sulfonamide (LiTFSI). Tetraethylene glycol dimethyl ether (TEGDME) was used as the solution. The galvanostatic discharge/charge tests were conducted using a Neware battery tester. Electrochemical impedance spectroscopy (EIS) was carried out using CHI660E. The frequency range is from 1&#xa0;kHz to 1&#xa0;Hz, and an AC amplitude of 5&#xa0;mv was applied.</p>
</sec>
</sec>
<sec sec-type="results|discussion" id="s3">
<title>Results and Discussion</title>
<p>The crystal structure of NiFeRu-LDH was first researched by XRD. As disclosed in <xref ref-type="fig" rid="F2">Figure 2A</xref>, the diffraction peaks at 11.3&#xb0;, 22.6&#xb0;, and 33.9&#xb0; are the characteristic (009), (006), and (003) facets of the NiFeRu-LDH, respectively (<xref ref-type="bibr" rid="B6">Chen et al., 2018</xref>). The peaks appear at 26.6&#xb0;, 44.7&#xb0;, and 54.7&#xb0; (red triangle symbol) are assigned to the graphite substrate (JCPDS &#x23;89-8487).</p>
<fig id="F2" position="float">
<label>FIGURE 2</label>
<caption>
<p>Characterization of the as-fabricated NiFeRu-LDH@GF cathode. Typical <bold>(A)</bold> XRD, <bold>(B)</bold> Raman, <bold>(C, D)</bold> SEM, <bold>(E)</bold> TEM spectra, and corresponding elemental mapping images of <bold>(F)</bold> Ni, <bold>(G)</bold> Fe, and <bold>(H)</bold> Ru.</p>
</caption>
<graphic xlink:href="fenrg-08-612093-g002.tif"/>
</fig>
<p>The Raman technique was carried out to further identify composition of the as-prepared cathode. The G band (1,575&#xa0;cm<sup>&#x2212;1</sup>) in the Raman spectrum (<xref ref-type="fig" rid="F2">Figure 2B</xref>) is identified for the graphite (<xref ref-type="bibr" rid="B29">Tuinstra and Koenig, 1970</xref>). D band (1,348&#xa0;cm<sup>&#x2212;1</sup>) of graphite is not observed, which indicates that the GF is highly ordered (<xref ref-type="bibr" rid="B7">Ferrari et al., 2004</xref>). The broad band located at around 547&#xa0;cm<sup>&#x2212;1</sup> and the faint broad feature at around 458&#xa0;cm<sup>&#x2212;1</sup> are in accordance with the observation of NiFe-LDH (<xref ref-type="bibr" rid="B18">Lu et al., 2014</xref>). The doping of Ru is responsible for the small shift of Raman peaks for NiFeRu-LDH in comparison with those of NiFe-LDH.</p>
<p>Field emission scanning electron microscopy (FESEM) was utilized to reveal the morphology of NiFeRu-LDH. NiFeRu-LDH grows uniformly on the surface of GF (<xref ref-type="fig" rid="F2">Figure 2C</xref>). Numerous vertically aligned nanosheets with size of 300&#x2013;600&#xa0;nm and thickness of 20&#x2013;40&#xa0;nm are observed (<xref ref-type="fig" rid="F2">Figure 2D</xref>). This interconnected structure of nanosheet may be helpful in improving the conductivity and providing more active edge sites of NiFeRu-LDH. The transmission electron microscopy (TEM) photograph further uncovers the nanosheet morphology of NiFeRu-LDH (<xref ref-type="fig" rid="F2">Figure 2E</xref>), and the corresponding energy-dispersive X-ray (EDX) elemental mapping discloses the existence of Ru in the nanosheets (<xref ref-type="fig" rid="F2">Figure 2F&#x2013;H</xref>). This 3D-structured porous binder-free cathode with a large amount of nanosheets can provide a large reaction interface and abundant highly efficient catalytic active sites for discharging products accommodation.</p>
<p>The electrochemical activity of NiFeRu-LDH@GF is revealed by using the galvanostatic discharge/charge method. Home-made Swagelok-type cells are used to characterize the electrochemical performance. The Li metal is used as anode, and the free-standing NiFeRu-LDH@GF without additional current collectors is the cathode. The cathode and anode are separated by a glass fiber containing electrolyte (details are in the Experimental Section). The first galvanostatic discharging and charging curves of LOBs at various current densities are shown in <xref ref-type="fig" rid="F3">Figure 3A</xref>. The specific discharge capacities of LOBs at 200&#xa0;mA&#xa0;g<sup>&#x2212;1</sup>, 300&#xa0;mA&#xa0;g<sup>&#x2212;1</sup>, and 400&#xa0;mA&#xa0;g<sup>&#x2212;1</sup> are 3,085&#xa0;mAh g<sup>&#x2212;1</sup>, 2,155&#xa0;mAh&#xa0;g<sup>&#x2212;1</sup>, and 1,085&#xa0;mAh&#xa0;g<sup>&#x2212;1</sup>, respectively. This result indicates that the specific capacity is sensitive to the current density, which is because of the limited conductivity of discharging products and O<sub>2</sub> transport limitation (<xref ref-type="bibr" rid="B30">Viswanathan et al., 2011</xref>; <xref ref-type="bibr" rid="B38">Zhu et al., 2013</xref>). The mid-capacity potential (defined as the potential at half of the specific capacity) for the discharging process at 200&#xa0;mA&#xa0;g<sup>&#x2212;1</sup> is 2.49&#xa0;V, which is close to 2.46&#xa0;V at 300&#xa0;mA&#xa0;g<sup>&#x2212;1</sup> and 2.43&#xa0;V at 400&#xa0;mA&#xa0;g<sup>&#x2212;1</sup>. This means the catalytic activity of NiFeRu-LDH@GF for ORR is not the controlling step for the discharging process. However, the mid-capacity potential for the charging process increases from 4.20&#xa0;V at 200&#xa0;mA&#xa0;g<sup>&#x2212;1</sup> to 4.32&#xa0;V at 400&#xa0;mA&#xa0;g<sup>&#x2212;1</sup>. The obviously increased overpotential implies that the catalytic activity of NiFeRu-LDH@GF is not sufficient to OER.</p>
<fig id="F3" position="float">
<label>FIGURE 3</label>
<caption>
<p>Galvanostatic discharge/charge profiles of NiFeRu-LDH@GF electrodes <bold>(A)</bold> at different current densities and <bold>(B)</bold> different cycles.</p>
</caption>
<graphic xlink:href="fenrg-08-612093-g003.tif"/>
</fig>
<p>To reveal the cycling performance, the NiFeRu-LDH@GF cathode is tested at a current density of 200&#xa0;mA&#xa0;g<sup>&#x2212;1</sup>, and a fixed specific capacity of 500&#xa0;mAh&#xa0;g<sup>&#x2212;1</sup> is a commonly used parameter in LOBs (<xref ref-type="bibr" rid="B37">Zhou et al., 2017</xref>; <xref ref-type="bibr" rid="B35">Zhang et al., 2018</xref>). The overpotential gradually increases for the charging process, and the terminal voltage is higher than 4.4&#xa0;V after 46 cycles (<xref ref-type="fig" rid="F3">Figure 3B</xref>). Meanwhile, the terminal voltage for the discharging process is about 2.15&#xa0;V. The performance up to 46 cycles is similar to that of CoNiFe-LDH cathode reported recently (<xref ref-type="bibr" rid="B17">Lu et al., 2020</xref>). These results reveal LDHs may not have superior electrochemical properties for LOBs.</p>
<p>To improve the electrochemical performances of LOBs, we further investigate the electrochemical activity of NiFeRu-LDH DMOs supported on the GF (DMO@GF) obtained by calcining NiFeRu-LDH@GF at 350&#xb0;C, 450&#xb0;C, and 550&#xb0;C, respectively. XRD and Raman spectra reveal the component of DMO@GF in <xref ref-type="fig" rid="F4">Figure 4</xref>. The peaks of graphite substrate still appear at 26.6&#xb0;, 44.7&#xb0;, and 54.7&#xb0; (red triangle symbol, JCPDS &#x23;89-8487), and the peaks of NiFeRu-LDH disappear. The diffraction peaks at 35.7&#xb0; and 43.4&#xb0; are the characteristic (311) and (400) facets of the NiFe<sub>2</sub>O<sub>4</sub>, respectively, and the (111) and (200) facets of NiO at the peak position of 37.2&#xb0; and 43.3&#xb0; are also observed in <xref ref-type="fig" rid="F4">Figure 4A</xref> (<xref ref-type="bibr" rid="B32">Xue et al., 2020</xref>). These results prove that NiFeRu-LDH has changed to metal oxides including NiFe<sub>2</sub>O<sub>4</sub> and NiO. In addition, the diffraction peak intensity of DMOs increases. In addition, the full width at half maximum (FWHM) decreases when the calcination temperature increases. This means more metal oxides are formed, and their crystal size is larger at higher calcination temperature. The diffraction peaks of Ru oxides are not detected because of the low Ru atom ratio in NiFeRu-LDH.</p>
<fig id="F4" position="float">
<label>FIGURE 4</label>
<caption>
<p>Characterization of the NiFeRu-LDH&#x2013;derived metal oxides supported on graphite foam. Typical <bold>(A)</bold> XRD and <bold>(B)</bold> Raman spectra.</p>
</caption>
<graphic xlink:href="fenrg-08-612093-g004.tif"/>
</fig>
<p>Raman spectra of DMO@GF further identify the DMOs (<xref ref-type="fig" rid="F4">Figure 4B</xref>). The G band (1,575&#xa0;cm<sup>&#x2212;1</sup>) of graphite in Raman spectrum still exists, which is consistent with the result of XRD (<xref ref-type="fig" rid="F4">Figure 4A</xref>). The Raman peaks at 188&#xa0;cm<sup>&#x2212;1</sup>, 305&#xa0;cm<sup>&#x2212;1</sup>, 468&#xa0;cm<sup>&#x2212;1</sup>, and 549&#xa0;cm<sup>&#x2212;1</sup> are related to the symmetric and antisymmetric bending of oxygen atom in iron&#x2013;oxygen bond at FeO<sub>6</sub> octahedral voids (<xref ref-type="bibr" rid="B13">Kumar et al., 2020</xref>). The band at 673&#xa0;cm<sup>&#x2212;1</sup> is responsible for the symmetric stretching vibrations of oxygen atoms against Ni ion in NiO<sub>4</sub> tetrahedra. The NiO-related Raman bands at 508&#xa0;cm<sup>&#x2212;1</sup> and 1,014&#xa0;cm<sup>&#x2212;1</sup> are also observed in <xref ref-type="fig" rid="F4">Figure 4B</xref> (<xref ref-type="bibr" rid="B22">Qiu et al., 2019</xref>). Furthermore, the characteristic peaks of RuO<sub>2</sub> at 528&#xa0;cm<sup>&#x2212;1</sup>, 646&#xa0;cm<sup>&#x2212;1</sup>, and 716&#xa0;cm<sup>&#x2212;1</sup> are included in the broad bands between 450&#xa0;cm<sup>&#x2212;1</sup> and 750&#xa0;cm<sup>&#x2212;1</sup> (<xref ref-type="bibr" rid="B11">Huang and Pollak, 1982</xref>). These results predict a conclusion that NiFeRu-LDH has transformed to metal oxides on GF.</p>
<p>Galvanostatic discharge/charge method was further utilized to investigate the electrochemical performances of DMO@GF cathode by calcining NiFeRu-LDH@GF at 350&#xb0;C, 450&#xb0;C and 550&#xb0;C (denoted as 350-DMO@GF, 450-DMO@GF and 550-DMO@GF). The specific discharge capacities of 350-DMO@GF, 450-DMO@GF and 550-DMO@GF electrodes are 2,718&#xa0;mAh&#xa0;g<sup>&#x2212;1</sup>, 3,700&#xa0;mAh&#xa0;g<sup>&#x2212;1</sup>, and 2,555&#xa0;mAh&#xa0;g<sup>&#x2212;1</sup>, respectively (<xref ref-type="fig" rid="F5">Figure 5A</xref>). Obviously, only the 450-DMO@GF exhibits higher specific discharge capacity than that of NiFeRu-LDH@GF (<xref ref-type="fig" rid="F3">Figure 3A</xref>). Meanwhile, the mid-capacity potential for the discharging process of 450-DMO@GF is 2.55&#xa0;V which is close to 2.56&#xa0;V of 350-DMO@GF and 2.50&#xa0;V of 550-DMO@GF. This means the discharging over-potential of DMO@GF, especially 450-DMO@GF, is smaller than that of NiFeRu-LDH@GF (2.49&#xa0;V). More importantly, the mid-capacity potential of DMO@GF for the charging process (4.02&#xa0;V for 350-DMO@GF, 3.97&#xa0;V for 450-DMO@GF and 4.01&#xa0;V for 550-DMO@GF) is much lower than that of NiFeRu-LDH@GF (4.20&#xa0;V). The cycling properties of 450-DMO@GF cathode are further researched since it has highest specific discharging capacity and smallest charging over-potential (<xref ref-type="fig" rid="F5">Figure 5B</xref>). The cycle life of 58 is realized for 450-DMO@GF cathode at a current density of 200&#xa0;mA&#xa0;g<sup>&#x2212;1</sup> and a fixed specific capacity of 500&#xa0;mAh&#xa0;g<sup>&#x2212;1</sup>. The terminal voltage for charging and discharging process is 4.08&#xa0;V and 2.25&#xa0;V. These cycling performances of 450-DMO@GF are better than those of NiFeRu-LDH@GF (<xref ref-type="fig" rid="F3">Figure 3B</xref>). These results imply that the NiFeRu-LDH DMOs have higher catalytic activity for ORR and OER process in comparison with NiFeRu-LDH. It has been reported that metal oxides such as NiCo<sub>2</sub>O<sub>4</sub>, NiO and RuO<sub>2</sub> have better electrochemical performances than those of layered-double-hydroxides (<xref ref-type="bibr" rid="B4">Chang et al., 2017</xref>), which is consistent with our results.</p>
<fig id="F5" position="float">
<label>FIGURE 5</label>
<caption>
<p>
<bold>(A)</bold> Galvanostatic discharge/charge profiles of DMO@GF electrodes and <bold>(B)</bold> the cycling performance of 450-DMO@GF cathode. The current density is 200&#xa0;mA&#xa0;g<sup>&#x2212;1</sup>.</p>
</caption>
<graphic xlink:href="fenrg-08-612093-g005.tif"/>
</fig>
<p>To reveal the differences of electrochemical performance for NiFeRu-LDH DMOs, the morphology and structure of DMO@GF are analyzed by SEM. At a low temperature of 350&#xb0;C, the vertically aligned nanosheets of NiFeRu-LDH partially changed to floccule-like products because of the phase transformation from NiFeRu-LDH to DMOs (<xref ref-type="fig" rid="F6">Figure 6A</xref>). Then, all the nanosheets of NiFeRu-LDH change to floccule-like DMOs that are cross-connected after calcining 450&#xb0;C (<xref ref-type="fig" rid="F6">Figure 6B</xref>). The floccule-like DMOs further grow to thick nanosheets since the crystal grains grow up accompanying the temperature rising, and no floccule-like products are observed at 550&#xb0;C (<xref ref-type="fig" rid="F6">Figure 6C</xref>). Obviously, the high specific surface combined with the cross-connected structure of the floccule-like DMOs is beneficial to improve the electrocatalytic activity and meanwhile to lower the overpotential. Therefore, the 450-DMO@GF electrode has the best charge&#x2013;discharge performance among three DMO@GF electrodes.</p>
<fig id="F6" position="float">
<label>FIGURE 6</label>
<caption>
<p>SEM spectra of the <bold>(A)</bold> 350-DMO@GF, <bold>(B)</bold> 450-DMO@GF, and <bold>(C)</bold> 550-DMO@GF.</p>
</caption>
<graphic xlink:href="fenrg-08-612093-g006.tif"/>
</fig>
<fig id="F7" position="float">
<label>FIGURE 7</label>
<caption>
<p>Nyquist plots of DMO@GF and LDH@GF.</p>
</caption>
<graphic xlink:href="fenrg-08-612093-g007.tif"/>
</fig>
<p>EIS is further introduced to research the mechanism of differences in kinetic characteristic among DMO@GF and LDH@GF (<xref ref-type="fig" rid="F7">Figure 7</xref>). The impedance spectra of LOBs are interpreted and proposed by Laoire. The equivalent circuit is Rs (C (R<sub>p</sub>W), where Rs is the electronic resistance from the cathode, the current collector, and electrolyte resistance, C is the capacitive contribution of the cathode and anode, R<sub>p</sub> is the charge transfer resistance of the two electrodes, and W is the linear Warburg element that is attributed to the diffusion of the O<sub>2</sub> and Li<sup>&#x2b;</sup> species to the electrode (<xref ref-type="bibr" rid="B14">Laoire et al., 2011</xref>). Obviously, the charge transfer resistance R<sub>p</sub> of three DMO@GF electrodes is smaller than that of LDH@GF, which implies that DMO@GF electrodes have higher catalytic activity in LOBs. In addition, the charge transfer resistance of the 450-DMO@GF electrode is smallest among the three DMO@GF electrodes, which is responsible for its outstanding electrochemical performance. These results are consistent with the discharge/charge characteristic of DMO@GF and LDH@GF (<xref ref-type="fig" rid="F3">Figures 3</xref>, <xref ref-type="fig" rid="F5">5</xref>).</p>
</sec>
<sec sec-type="conclusion" id="s4">
<title>Conclusion</title>
<p>In summary, we present an integrated binder-free cathode for LOBs based on Ru-doped NiFe layered double hydroxide grown on a GF (NiFeRu LDH@GF) through hydrothermal reaction. The vertically aligned NiFeRu LDH nanosheets combined with the ultralight and conductive GF substrate provide a specific capacity of 3,085&#xa0;mAh&#xa0;g<sup>&#x2212;1</sup> at 200&#xa0;mA&#xa0;g<sup>&#x2212;1</sup> and a cycle life of 46. Furthermore, we also investigate the NiFeRu LDH DMOs supported on a GF (DMO@GF) by calcining NiFeRu LDH@GF at 350&#xb0;C, 450&#xb0;C, and 550&#xb0;C. The 450&#xb0;C-obtained DMO@GF (450-DMO@GF) shows the best electrochemical performances among the three DMO@GFs. The specific capacity of 450-DMO@GF is 3,700&#xa0;mAh&#xa0;g<sup>&#x2212;1</sup>, and its cycle life is 58. SEM reveals that the floccule-like 450-DMO@GF has a high specific surface combined with a cross-connected structure. EIS further reveals that the charge transfer resistance of the 450-DMO@GF electrode is smallest. Our results provide a new kind of integrated binder-free cathode to achieving LOBs with high performance.</p>
</sec>
</body>
<back>
<sec id="s5">
<title>Data Availability Statement</title>
<p>The original contributions presented in the study are included in the article/supplementary materials, further inquiries can be directed to the corresponding author/s.</p>
</sec>
<sec id="s6">
<title>Author Contributions</title>
<p>All authors extensively discussed the results, reviewed the manuscript, and approved the final version of the manuscript to be published.</p>
</sec>
<sec id="s7">
<title>Funding</title>
<p>This work was financially supported by the National Natural Science Foundation of China (No. 22075115), the Natural Science Foundation of Jiangsu Province (BK20171,169), the Natural Science Foundation of the Jiangsu Higher Education Institutions of China (19KJA430020, 18KJA430015), the Jiangsu Qing Lan Project (2020, 2018) and the innovation and entrepreneurship training program for college students (xcx2020135).</p>
</sec>
<sec sec-type="COI-statement" id="s8">
<title>Conflict of Interest</title>
<p>The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.</p>
</sec>
<sec sec-type="disclaimer" id="s9">
<title>Publisher&#x2019;s Note</title>
<p>All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations or those of the publisher, the editors, and the reviewers. Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher.</p>
</sec>
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