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The two Janus faces in oxygen evolution electrocatalysis: Activity versus stability of layered double hydroxides
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Review Article The two Janus faces in oxygen evolution electrocatalysis: Activity versus stability of layered double hydroxides Stefan Barwe, Corina Andronescu, Justus Masa and Wolfgang Schuhmann∗ Electrochemical water splitting is among the most promising candidates for conversion of intermittently produced renewable energy into the portable energy carrier hydrogen. Hence, overcoming the sluggish kinetics of the oxygen evolution reaction (OER) is a focus of enormous research efforts in the field of electrocatalysis. Recently, layered double hydroxides (LDHs) have emerged as very promising low-cost catalysts for the OER in alkaline media. Exfoliation of bulk LDH into ultra-thin sheets, preparation of high surface area 3D structures and the combination with carbon materials are only a few examples used to enhance the activity toward the OER in the last years. The intrinsic stability of the LDHs and the stability of related electrode assemblies represent the second highly important aspect of catalyst evaluation. Although OER activity improvement of LDHs seems to have reached a plateau, their stability is much less optimized and more development in this direction is critically needed. Address Analytical Chemistry – Center for Electrochemical Sciences (CES), Ruhr-Universität Bochum, Universitätsstr. 150, D-44780 Bochum, Germany Corresponding author: Schuhmann, Wolfgang ([email protected])
Current Opinion in Electrochemistry 2017, 4:4–10 This review comes from a themed issue on Electrocatalysis 2017 Edited by Shi-Gang SUN For a complete overview see the Issue and the Editorial Available online 25 May 2017 http://dx.doi.org/10.1016/j.coelec.2017.05.006 2451-9103/© 2017 Elsevier B.V. All rights reserved.
Introduction Layered double hydroxides (LDHs) are presently the most active oxygen evolution reaction (OER) catalysts in alkaline media. LDHs are layered solid materials having brucite type structure. The general chemical formula is M2+ 1 − x M3+ x (OH)2 (An − )x / n •yH2 O, where M2+ Current Opinion in Electrochemistry 2017, 4:4–10
and M3+ are di- and trivalent metal cations, respectively. An − is an anion having the valence n and x represents the M2+ /(M2+ +M3+ ) ratio with usually reported values between 0.2 and 0.33. Partial exchange of bivalent cations with trivalent metal cations introduces a net positive charge in the hydroxide layers, which is balanced by anions present in the LDH galleries. In current literature, LDHs are often associated with transition metal oxyhydroxides with ambiguous delimitation. Additionally, materials with layered (oxy)hydroxide domains may be formed during the OER. This review exclusively focusses on deliberately prepared and wellcharacterized LDH materials. Mixed transition metal oxides and hydroxides which may contain layered hydroxide domains but whose structure is not elucidated, as well as materials with layered (oxy)hydroxide domains formed under oxygen evolution conditions, are therefore not covered in the review. LDHs are well known materials in heterogeneous catalysis [1], pharmaceutics, as polymer additives, as adsorbent materials, in electrochemistry and photochemistry [2]. However, the use of LDH-based materials as OER catalyst was only intensified recently. In 2009, Silva et al. used a Zn-based LDH for photochemical visible lightdriven oxygen evolution [3]. Initial observation of electrocatalysis of the OER by LDHs was reported by Hu et al. in 2011 [4]. Zhang et al. subsequently reported the use of LDHs as electrocatalysts for the OER in neutral electrolytes [5,6]. However, broad interest in the use of LDHs for OER intensified substantially after the work of Gong et al. in 2013, who used a NiFe LDH/CNT composite as OER catalyst in alkaline electrolyte. Note that in the following, LDHs are named according to M+2 M3+ LDH. By using a conductive material such as e.g. carbon nanotubes (CNT), the low conductivity, which is usually associated with metal hydroxides, was overcome, making NiFe LDH/CNT one of the most active catalyst for the OER in alkaline environment [7●● ]. Beside the high intrinsic activity, the material also showed very promising stability under OER conditions. The same group further explored the use of NiFe LDH/CNT as cathode material in a Zn–air battery [8]. Since these reports, the number of papers published in the last two years considerably increased. Understanding the OER mechanism www.sciencedirect.com
The two Janus faces in oxygen evolution electrocatalysis: Activity versus stability of layered double hydroxides Barwe et al.
on LDH materials, especially on NiFe LDH, was anticipated using in operando techniques and theoretical calculations such as density functional theory (DFT). Koper et al. described Ni-based double hydroxide (DH) materials with different OER activity. They showed that the active center depends on the nature of the two metals. While Mn and Fe were the active center in NiMn and NiFe DH, respectively, Ni was found to be the active center in NiCr, NiCo, NiCu or NiZn DH [9]. For NiFe oxyhydroxides it was found that high OER potentials change the oxidation state of Ni from 2+ to 3.6+, while Fe remains in the 3+ state [10]. Fan et al. employed DFT calculations in order to get an insight into the OER mechanism occurring at NiV LDH that shows superior OER activity than NiFe LDH. They found that different reactions are the limiting step on the two catalysts [11● ]. Despite the overall effort done in the last years, the OER mechanism on LDH materials is still under debate [12●● ]. In consideration of the enormous interest in electrocatalysis aiming on developing non-precious LDH-based catalysts for water splitting, this review aims to give an overview of the activity and stability of LDHs as OER catalysts for alkaline water splitting.
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Activity of LDH-based materials supported on inert and flat electrodes Vast efforts on elucidating electrocatalysis of the OER by LDHs in the last years mainly focused on enhancing the intrinsic OER activity of the catalysts. In order to minimize support and structure effects, the intrinsic OER activity of newly developed materials was studied on inert and flat electrode surfaces. The approaches applied to enable LDH materials to facilitating the OER at low overpotentials can be split up into three major categories: (i) variation of the transition metals and their composition [13,14]; (ii) development of hybrids of LDHs and carbon materials [7●● ,15–26● ]; (iii) exfoliation of bulk LDHs into ultrathin nanosheets [27●● –29]. Following the publication in 2013 of a highly active NiFe LDH/CNT nanohybrid OER catalyst by Gong et al. [7●● ], many different LDHs have been synthesized by various groups, including CoCo LDH, NiCo LDH, FeCo LDH [27●● ], CoMn LDH [30], NiMn LDH [20] and NiV LDH [11● ], among others, and employed for the OER. Increasing charge transfer during electrocatalytic oxygen evolution by LDH catalysts was performed either by mixing them with highly conductive materials or by incorporating highly conductive elements into the LDH structure (Figure 1). For example, Tang et al. achieved enhanced OER performance of a LDH
Figure 1
Strategies used to increase the OER activity of LDH materials: (a) exfoliation of LDHs to nanosheets (reproduced from [27●● ]); (b) growing of the LDH on CNT (reproduced from [7●● ]); (c) intercalating rGO between the LDH layers (reproduced from [36]); (d) mixing LDH with quantum dots (reproduced from [31● ]); or (e) incorporation of high conductivity elements in the synthesis of LDH (V instead of Fe) (reproduced from [11● ]). www.sciencedirect.com
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catalyst by fabricating a LDH/carbon quantum dot composite [31● ]. In 2015, Long et al. and Ma et al. decreased the overpotential necessary to attain a current density of 10 mA cm−2 (the current density matching photoelectrochemical water splitting with a 12.3% efficiency [32]), by incorporating Co into NiFe LDH [33● ], and by stacking LDH and reduced graphene oxide (rGO) alternately in a superlattice-type structure [16], respectively. Of all the LDHs reported for the OER, NiFe LDH appears to be the most active making it exemplarily worth for studying the influence of the interlayer anion, crystallinity and sheet size on its oxygen evolution activity. Systematic studies of size and crystallinity of NiFe LDH revealed a decrease of OER activity with crystallinity and sheet size [34]. The amount of defect sites, a prerequisite for high OER activity, also decreases with increasing crystallinity. With increasing sheet size, the number of active sites which are supposed to be predominantly located at the sheet edges, decreases, resulting in decreased electrochemical activity. However, it should be pointed out that the study does not clearly distinguish between influences imposed by crystallinity or sheet size since both change together during the described preparation. The pristine interlayer anions play a crucial role for the initial OER activity of NiFe LDH. It was shown that the overpotential to provide a current density of 1 mA cm−2 can be correlated to the pKa values of the conjugate acid of the interlayer anion and is decreasing with increasing pKa values [35●● ]. On the other hand, the basal spacing defined by the interlayer anion cannot be correlated to the initial activity of NiFe LDH, at least not in a range of 7.5–8.6 A˚ [35●● ]. However, the nature of the interlayer anion only has an impact in virtually carbonate free electrolytes. Using NiFe LDH as a model, Hunter et al. showed that interlayer anions are exchanged for carbonate anions when electrochemical measurements are conducted in potassium hydroxide solutions exposed to the atmosphere due to dissolution of CO2 , leading to an increase of LDHs OER activity [35●● ]. Thus, control of crystallinity and sheet size rather than the choice of the interlayer anion is of pivotal importance in designing highly active LDH catalysts. A more detailed discussion of the factors which influence the catalytic activity of NiFe LDH was recently summarized by Dionigi and Strasser [12●● ]. A survey of the benchmark OER activity of LDH materials over the past four years, 2013–2016, shows no striking increase of the intrinsic electrocatalytic activity despite intensive research efforts. Actually, the average overpotential needed to provide a current density of 10 mA cm−2 was lower in 2013 (0.28 V) than in 2016 (0.32 V). Thus, although the number of papers on LDH catalysts increased dramatically, there has been no net gain with regard to enhancement of the intrinsic OER activity of LDH materials over these years (Figure 2). Therefore, new strategies Current Opinion in Electrochemistry 2017, 4:4–10
mostly based on the use of porous high surface area materials as supports for the LDHs were developed to overcome the apparent limitation of the intrinsic OER activity of LDH catalysts. Tafel slopes, a further important figure of merit for the evaluation of an electrocatalyst’s activity, have to be carefully discussed when comparing different LDH-based OER catalysts. The Tafel slope for NiFe LDH was reported to be between 50 and 65 mV dec−1 if not exfoliated or supported on a carbon material [11● ,27●● ,33● ,43,44]. However, exfoliating the bulk LDH to ultra-thin sheets decreases the Tafel slope substantially [30,38], reaching values of around 40 mV dec−1 [27●● ]. A further decrease of the Tafel slope may be achieved by combining LDHs with functional carbon materials [7●● ,16,22,31● ].
Activity of LDH-based catalysts supported on three dimensional structures The efficiency of electrocatalysts can be enhanced significantly by using three dimensional support structures, or by supporting the catalyst on conductive and porous high surface area electrode surfaces. In particular, the use of nickel foam as a catalyst support has gained a lot of attention [11● ,43,44,45,46]. The highly porous and three dimensional structure of nickel foam make it possible to achieve high current densities at low overpotentials thereby enhancing the apparent electrocatalytic activity as has been observed for LDH-based catalysts (Figure 3). Besides nickel foam, high surface area carbon supports, e.g. electrochemically exfoliated graphene foil [47], three dimensional graphene networks [23] or carbon fiber paper [26● ] also appear to have a promotional effect on OER activity when used as support materials. Another strategy to increase the activity of LDHs is the formation of high surface area three dimensional catalyst structures in which the active LDH material is grown on flat electrode supports [48,49]. The difficulty to reliably determine the real surface area of 3D structure and the normalization of current by the geometric area of the electrode increase the apparent catalytic activity of catalysts supported on porous 3D structures with respect to the activity obtained employing flat inert electrodes. The combined effect of the influence of the catalyst support, catalyst structure and measurement conditions complicates quantitative comparisons of results reported by different groups.
Stability Besides activity, intrinsic catalyst stability as well as the fabrication of stable functional electrodes are of crucial importance for technical applications of OER electrocatalysts. Electrochemical long-term assessments are necessary to reveal on the one hand intrinsic catalyst changes during operation, and on the other hand to develop strategies to immobilize catalytically active materials on electrode surfaces in a stable manner to withstand the harsh corrosive conditions and physical stress upon gas bubble www.sciencedirect.com
The two Janus faces in oxygen evolution electrocatalysis: Activity versus stability of layered double hydroxides Barwe et al.
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Figure 2
Box plot of the average overpotential required to reach a current density of 10 mA cm−2 using LDH catalysts immobilized on flat and OER inert electrode surfaces for the years 2013, 2014, 2015 and 2016. Values taken from [7●● ] (2013, grey); [27●● ,28,30,31● ,37] (2014, black); [16,17,18,29,33● ,38,39] (2015, red); [11● ,14,19,20–22,24,40,41,42] (2016, purple).
Figure 3
LSV curves in 1.0 M KOH showing differences registered in the catalyst activity deposited on different electrodes (a) Ni0.75 Fe0.25 -LDH and Ni0.75 V0.25 -LDH on GCE, and (b) Ni0.75 V0.25 -LDH on Ni foam. (reproduced from [11● ]).
formation and departure during the OER. The importance of sufficiently long-term stability tests becomes obvious considering the influence of the interlayer anion on the OER activity of LDHs. As mentioned before, all interlayer anions are exchanged over time for carbonate due to the dissolution of ambient CO2 in the electrolyte, leading to a change of the initial activity [35●● ]. These changes in the material can only be observed if the time scale of the stability test is long enough for the changes to occur. In contrast to electrocatalytic activity, which has well accepted figures of merit such as the overpotential at a given current density, normally 10 mA cm−2 , or the Tafel slope, no consensus exists for reporting the stability of LDHbased OER catalysts. Stability is commonly assessed either potentiodynamically [17], chronopotentiometrically [33● ,38,43,40] or chronoamperometrically [39,48]. Similar to the activity, the electrode material and catalyst fixation have a significant influence on stability. For chronopotentiometric measurements, applied current densities between 1 [18] and 200 [47] mA cm−2 can be found, dewww.sciencedirect.com
pending on the used electrolyte and electrode material. A general difficulty in evaluating stability data of LDH materials is the differentiation between the intrinsic stability of the material and the stability of a prepared catalyst film on an electrode surface. LDHs available as powders are often fixed onto an electrode surface with the aid of a binding matrix, typically Nafion [11● ,20,22,24,26● ,46], which possibly causes a decrease of active site availability and film conductivity. The latter is often compensated by the addition of carbon additives like carbon black [26● ,40] or by direct preparation of hybrids of LDH and functional carbon. However, changes in the potential or current response may be either due to a decrease in activity caused by a change in the materials properties, or due to loss of catalytically active material or conducting additive. The duration of a stability measurement is a very important measure of the potential for catalyst application. For LDH-based catalysts, the duration of the reported stability measurements range from a few hours [38,29,33● ] to days [19,11● ]. One group achieved Current Opinion in Electrochemistry 2017, 4:4–10
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a stability measurement over a span of 500 h at a current density of 20 mA cm−2 employing a nickel foam electrode with unusually high loading (20 mg cm-2 ) of NiFe LDH/Ni [50]. The lack of common standards for performing and reporting stability measurements hampers proper discussion not only of results obtained for different LDH-based catalysts but also for materials within the same LDH family. It is imperative that the importance of catalyst stability is given equal significance in catalyst evaluation as catalytic activity. Moreover, stability studies should not only be confined to discussion of performance degradation, but to in-depth understanding of the materials´ intrinsic stability through examination of structural and chemical changes, employing both in-situ and post-mortem characterization methods. To obtain reliable stability data over a reasonably long time scale, it is crucially important that the catalyst/electrode adhesion is significantly improved to allow enough time for kinetically slower changes to occur.
Concluding remarks LDHs and their hybrids are undebatable among the most active electrocatalysts for oxygen evolution in alkaline media. Enormous efforts presently aim at further enhancement of the activity of this class of materials. However, except for a few rare cases, no striking improvement in the inherent OER activity of LDHs has been achieved since 2013. Supporting LDHs on 3D structures, particularly nickel foam, results in a significant enhancement of their OER activity on a mass and geometric area basis, attributed to improved dispersion, faster mass transport, and apparently from a synergistic interaction of the catalysts with the support. A second approach to still further enhance the activity is the design and preparation of LDH catalysts with three dimensional structures by themselves. However, three dimensional catalyst structures have yet to proof their durability during long-term application under the corrosive and physical stress inducing conditions of oxygen evolution. The differences in electrode preparation as well as measurement procedures and conditions make it difficult to identify a specific combination of transition metal cations for the potentially most active LDH catalyst. Nevertheless, other parameters like the crystallinity and sheet size are crucial for tuning the initial activity of LDH materials. Alongside the activity enhancement efforts, the inherent stability of the catalysts and also importantly, the fabrication of stable functional electrodes using powder-based catalysts still have to be improved. In contrast to activity, the stability of LDH catalysts is much less investigated and there is lack of common standards and procedures for reporting stability, which hampers the possibility to reference published stability data in the context of a state of the art benchmark. For technical application of LDHs in OER, stability as a crucial requirement has to be more stressed comparable to the interest in the activity. That notwithCurrent Opinion in Electrochemistry 2017, 4:4–10
standing, LDHs represent clearly very promising class of materials with the potential to be further developed.
Acknowledgments The authors are grateful to the Deutsche Forschungsgemeinschaft (DFG) in the framework of the Cluster of Excellence Resolv (EXC1069) and the Bundesministerium für Bildung und Forschung (BMBF) in the frameworks of the project “Mangan” (FKZ 03EK3548).
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Review Article The two Janus faces in oxygen evolution electrocatalysis: Activity versus stability of layered double hydroxides Stefan Barwe, Corina Andronescu, Justus Masa and Wolfgang Schuhmann∗ Electrochemical water splitting is among the most promising candidates for conversion of intermittently produced renewable energy into the portable energy carrier hydrogen. Hence, overcoming the sluggish kinetics of the oxygen evolution reaction (OER) is a focus of enormous research efforts in the field of electrocatalysis. Recently, layered double hydroxides (LDHs) have emerged as very promising low-cost catalysts for the OER in alkaline media. Exfoliation of bulk LDH into ultra-thin sheets, preparation of high surface area 3D structures and the combination with carbon materials are only a few examples used to enhance the activity toward the OER in the last years. The intrinsic stability of the LDHs and the stability of related electrode assemblies represent the second highly important aspect of catalyst evaluation. Although OER activity improvement of LDHs seems to have reached a plateau, their stability is much less optimized and more development in this direction is critically needed. Address Analytical Chemistry – Center for Electrochemical Sciences (CES), Ruhr-Universität Bochum, Universitätsstr. 150, D-44780 Bochum, Germany Corresponding author: Schuhmann, Wolfgang ([email protected])
Current Opinion in Electrochemistry 2017, 4:4–10 This review comes from a themed issue on Electrocatalysis 2017 Edited by Shi-Gang SUN For a complete overview see the Issue and the Editorial Available online 25 May 2017 http://dx.doi.org/10.1016/j.coelec.2017.05.006 2451-9103/© 2017 Elsevier B.V. All rights reserved.
Introduction Layered double hydroxides (LDHs) are presently the most active oxygen evolution reaction (OER) catalysts in alkaline media. LDHs are layered solid materials having brucite type structure. The general chemical formula is M2+ 1 − x M3+ x (OH)2 (An − )x / n •yH2 O, where M2+ Current Opinion in Electrochemistry 2017, 4:4–10
and M3+ are di- and trivalent metal cations, respectively. An − is an anion having the valence n and x represents the M2+ /(M2+ +M3+ ) ratio with usually reported values between 0.2 and 0.33. Partial exchange of bivalent cations with trivalent metal cations introduces a net positive charge in the hydroxide layers, which is balanced by anions present in the LDH galleries. In current literature, LDHs are often associated with transition metal oxyhydroxides with ambiguous delimitation. Additionally, materials with layered (oxy)hydroxide domains may be formed during the OER. This review exclusively focusses on deliberately prepared and wellcharacterized LDH materials. Mixed transition metal oxides and hydroxides which may contain layered hydroxide domains but whose structure is not elucidated, as well as materials with layered (oxy)hydroxide domains formed under oxygen evolution conditions, are therefore not covered in the review. LDHs are well known materials in heterogeneous catalysis [1], pharmaceutics, as polymer additives, as adsorbent materials, in electrochemistry and photochemistry [2]. However, the use of LDH-based materials as OER catalyst was only intensified recently. In 2009, Silva et al. used a Zn-based LDH for photochemical visible lightdriven oxygen evolution [3]. Initial observation of electrocatalysis of the OER by LDHs was reported by Hu et al. in 2011 [4]. Zhang et al. subsequently reported the use of LDHs as electrocatalysts for the OER in neutral electrolytes [5,6]. However, broad interest in the use of LDHs for OER intensified substantially after the work of Gong et al. in 2013, who used a NiFe LDH/CNT composite as OER catalyst in alkaline electrolyte. Note that in the following, LDHs are named according to M+2 M3+ LDH. By using a conductive material such as e.g. carbon nanotubes (CNT), the low conductivity, which is usually associated with metal hydroxides, was overcome, making NiFe LDH/CNT one of the most active catalyst for the OER in alkaline environment [7●● ]. Beside the high intrinsic activity, the material also showed very promising stability under OER conditions. The same group further explored the use of NiFe LDH/CNT as cathode material in a Zn–air battery [8]. Since these reports, the number of papers published in the last two years considerably increased. Understanding the OER mechanism www.sciencedirect.com
The two Janus faces in oxygen evolution electrocatalysis: Activity versus stability of layered double hydroxides Barwe et al.
on LDH materials, especially on NiFe LDH, was anticipated using in operando techniques and theoretical calculations such as density functional theory (DFT). Koper et al. described Ni-based double hydroxide (DH) materials with different OER activity. They showed that the active center depends on the nature of the two metals. While Mn and Fe were the active center in NiMn and NiFe DH, respectively, Ni was found to be the active center in NiCr, NiCo, NiCu or NiZn DH [9]. For NiFe oxyhydroxides it was found that high OER potentials change the oxidation state of Ni from 2+ to 3.6+, while Fe remains in the 3+ state [10]. Fan et al. employed DFT calculations in order to get an insight into the OER mechanism occurring at NiV LDH that shows superior OER activity than NiFe LDH. They found that different reactions are the limiting step on the two catalysts [11● ]. Despite the overall effort done in the last years, the OER mechanism on LDH materials is still under debate [12●● ]. In consideration of the enormous interest in electrocatalysis aiming on developing non-precious LDH-based catalysts for water splitting, this review aims to give an overview of the activity and stability of LDHs as OER catalysts for alkaline water splitting.
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Activity of LDH-based materials supported on inert and flat electrodes Vast efforts on elucidating electrocatalysis of the OER by LDHs in the last years mainly focused on enhancing the intrinsic OER activity of the catalysts. In order to minimize support and structure effects, the intrinsic OER activity of newly developed materials was studied on inert and flat electrode surfaces. The approaches applied to enable LDH materials to facilitating the OER at low overpotentials can be split up into three major categories: (i) variation of the transition metals and their composition [13,14]; (ii) development of hybrids of LDHs and carbon materials [7●● ,15–26● ]; (iii) exfoliation of bulk LDHs into ultrathin nanosheets [27●● –29]. Following the publication in 2013 of a highly active NiFe LDH/CNT nanohybrid OER catalyst by Gong et al. [7●● ], many different LDHs have been synthesized by various groups, including CoCo LDH, NiCo LDH, FeCo LDH [27●● ], CoMn LDH [30], NiMn LDH [20] and NiV LDH [11● ], among others, and employed for the OER. Increasing charge transfer during electrocatalytic oxygen evolution by LDH catalysts was performed either by mixing them with highly conductive materials or by incorporating highly conductive elements into the LDH structure (Figure 1). For example, Tang et al. achieved enhanced OER performance of a LDH
Figure 1
Strategies used to increase the OER activity of LDH materials: (a) exfoliation of LDHs to nanosheets (reproduced from [27●● ]); (b) growing of the LDH on CNT (reproduced from [7●● ]); (c) intercalating rGO between the LDH layers (reproduced from [36]); (d) mixing LDH with quantum dots (reproduced from [31● ]); or (e) incorporation of high conductivity elements in the synthesis of LDH (V instead of Fe) (reproduced from [11● ]). www.sciencedirect.com
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catalyst by fabricating a LDH/carbon quantum dot composite [31● ]. In 2015, Long et al. and Ma et al. decreased the overpotential necessary to attain a current density of 10 mA cm−2 (the current density matching photoelectrochemical water splitting with a 12.3% efficiency [32]), by incorporating Co into NiFe LDH [33● ], and by stacking LDH and reduced graphene oxide (rGO) alternately in a superlattice-type structure [16], respectively. Of all the LDHs reported for the OER, NiFe LDH appears to be the most active making it exemplarily worth for studying the influence of the interlayer anion, crystallinity and sheet size on its oxygen evolution activity. Systematic studies of size and crystallinity of NiFe LDH revealed a decrease of OER activity with crystallinity and sheet size [34]. The amount of defect sites, a prerequisite for high OER activity, also decreases with increasing crystallinity. With increasing sheet size, the number of active sites which are supposed to be predominantly located at the sheet edges, decreases, resulting in decreased electrochemical activity. However, it should be pointed out that the study does not clearly distinguish between influences imposed by crystallinity or sheet size since both change together during the described preparation. The pristine interlayer anions play a crucial role for the initial OER activity of NiFe LDH. It was shown that the overpotential to provide a current density of 1 mA cm−2 can be correlated to the pKa values of the conjugate acid of the interlayer anion and is decreasing with increasing pKa values [35●● ]. On the other hand, the basal spacing defined by the interlayer anion cannot be correlated to the initial activity of NiFe LDH, at least not in a range of 7.5–8.6 A˚ [35●● ]. However, the nature of the interlayer anion only has an impact in virtually carbonate free electrolytes. Using NiFe LDH as a model, Hunter et al. showed that interlayer anions are exchanged for carbonate anions when electrochemical measurements are conducted in potassium hydroxide solutions exposed to the atmosphere due to dissolution of CO2 , leading to an increase of LDHs OER activity [35●● ]. Thus, control of crystallinity and sheet size rather than the choice of the interlayer anion is of pivotal importance in designing highly active LDH catalysts. A more detailed discussion of the factors which influence the catalytic activity of NiFe LDH was recently summarized by Dionigi and Strasser [12●● ]. A survey of the benchmark OER activity of LDH materials over the past four years, 2013–2016, shows no striking increase of the intrinsic electrocatalytic activity despite intensive research efforts. Actually, the average overpotential needed to provide a current density of 10 mA cm−2 was lower in 2013 (0.28 V) than in 2016 (0.32 V). Thus, although the number of papers on LDH catalysts increased dramatically, there has been no net gain with regard to enhancement of the intrinsic OER activity of LDH materials over these years (Figure 2). Therefore, new strategies Current Opinion in Electrochemistry 2017, 4:4–10
mostly based on the use of porous high surface area materials as supports for the LDHs were developed to overcome the apparent limitation of the intrinsic OER activity of LDH catalysts. Tafel slopes, a further important figure of merit for the evaluation of an electrocatalyst’s activity, have to be carefully discussed when comparing different LDH-based OER catalysts. The Tafel slope for NiFe LDH was reported to be between 50 and 65 mV dec−1 if not exfoliated or supported on a carbon material [11● ,27●● ,33● ,43,44]. However, exfoliating the bulk LDH to ultra-thin sheets decreases the Tafel slope substantially [30,38], reaching values of around 40 mV dec−1 [27●● ]. A further decrease of the Tafel slope may be achieved by combining LDHs with functional carbon materials [7●● ,16,22,31● ].
Activity of LDH-based catalysts supported on three dimensional structures The efficiency of electrocatalysts can be enhanced significantly by using three dimensional support structures, or by supporting the catalyst on conductive and porous high surface area electrode surfaces. In particular, the use of nickel foam as a catalyst support has gained a lot of attention [11● ,43,44,45,46]. The highly porous and three dimensional structure of nickel foam make it possible to achieve high current densities at low overpotentials thereby enhancing the apparent electrocatalytic activity as has been observed for LDH-based catalysts (Figure 3). Besides nickel foam, high surface area carbon supports, e.g. electrochemically exfoliated graphene foil [47], three dimensional graphene networks [23] or carbon fiber paper [26● ] also appear to have a promotional effect on OER activity when used as support materials. Another strategy to increase the activity of LDHs is the formation of high surface area three dimensional catalyst structures in which the active LDH material is grown on flat electrode supports [48,49]. The difficulty to reliably determine the real surface area of 3D structure and the normalization of current by the geometric area of the electrode increase the apparent catalytic activity of catalysts supported on porous 3D structures with respect to the activity obtained employing flat inert electrodes. The combined effect of the influence of the catalyst support, catalyst structure and measurement conditions complicates quantitative comparisons of results reported by different groups.
Stability Besides activity, intrinsic catalyst stability as well as the fabrication of stable functional electrodes are of crucial importance for technical applications of OER electrocatalysts. Electrochemical long-term assessments are necessary to reveal on the one hand intrinsic catalyst changes during operation, and on the other hand to develop strategies to immobilize catalytically active materials on electrode surfaces in a stable manner to withstand the harsh corrosive conditions and physical stress upon gas bubble www.sciencedirect.com
The two Janus faces in oxygen evolution electrocatalysis: Activity versus stability of layered double hydroxides Barwe et al.
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Figure 2
Box plot of the average overpotential required to reach a current density of 10 mA cm−2 using LDH catalysts immobilized on flat and OER inert electrode surfaces for the years 2013, 2014, 2015 and 2016. Values taken from [7●● ] (2013, grey); [27●● ,28,30,31● ,37] (2014, black); [16,17,18,29,33● ,38,39] (2015, red); [11● ,14,19,20–22,24,40,41,42] (2016, purple).
Figure 3
LSV curves in 1.0 M KOH showing differences registered in the catalyst activity deposited on different electrodes (a) Ni0.75 Fe0.25 -LDH and Ni0.75 V0.25 -LDH on GCE, and (b) Ni0.75 V0.25 -LDH on Ni foam. (reproduced from [11● ]).
formation and departure during the OER. The importance of sufficiently long-term stability tests becomes obvious considering the influence of the interlayer anion on the OER activity of LDHs. As mentioned before, all interlayer anions are exchanged over time for carbonate due to the dissolution of ambient CO2 in the electrolyte, leading to a change of the initial activity [35●● ]. These changes in the material can only be observed if the time scale of the stability test is long enough for the changes to occur. In contrast to electrocatalytic activity, which has well accepted figures of merit such as the overpotential at a given current density, normally 10 mA cm−2 , or the Tafel slope, no consensus exists for reporting the stability of LDHbased OER catalysts. Stability is commonly assessed either potentiodynamically [17], chronopotentiometrically [33● ,38,43,40] or chronoamperometrically [39,48]. Similar to the activity, the electrode material and catalyst fixation have a significant influence on stability. For chronopotentiometric measurements, applied current densities between 1 [18] and 200 [47] mA cm−2 can be found, dewww.sciencedirect.com
pending on the used electrolyte and electrode material. A general difficulty in evaluating stability data of LDH materials is the differentiation between the intrinsic stability of the material and the stability of a prepared catalyst film on an electrode surface. LDHs available as powders are often fixed onto an electrode surface with the aid of a binding matrix, typically Nafion [11● ,20,22,24,26● ,46], which possibly causes a decrease of active site availability and film conductivity. The latter is often compensated by the addition of carbon additives like carbon black [26● ,40] or by direct preparation of hybrids of LDH and functional carbon. However, changes in the potential or current response may be either due to a decrease in activity caused by a change in the materials properties, or due to loss of catalytically active material or conducting additive. The duration of a stability measurement is a very important measure of the potential for catalyst application. For LDH-based catalysts, the duration of the reported stability measurements range from a few hours [38,29,33● ] to days [19,11● ]. One group achieved Current Opinion in Electrochemistry 2017, 4:4–10
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a stability measurement over a span of 500 h at a current density of 20 mA cm−2 employing a nickel foam electrode with unusually high loading (20 mg cm-2 ) of NiFe LDH/Ni [50]. The lack of common standards for performing and reporting stability measurements hampers proper discussion not only of results obtained for different LDH-based catalysts but also for materials within the same LDH family. It is imperative that the importance of catalyst stability is given equal significance in catalyst evaluation as catalytic activity. Moreover, stability studies should not only be confined to discussion of performance degradation, but to in-depth understanding of the materials´ intrinsic stability through examination of structural and chemical changes, employing both in-situ and post-mortem characterization methods. To obtain reliable stability data over a reasonably long time scale, it is crucially important that the catalyst/electrode adhesion is significantly improved to allow enough time for kinetically slower changes to occur.
Concluding remarks LDHs and their hybrids are undebatable among the most active electrocatalysts for oxygen evolution in alkaline media. Enormous efforts presently aim at further enhancement of the activity of this class of materials. However, except for a few rare cases, no striking improvement in the inherent OER activity of LDHs has been achieved since 2013. Supporting LDHs on 3D structures, particularly nickel foam, results in a significant enhancement of their OER activity on a mass and geometric area basis, attributed to improved dispersion, faster mass transport, and apparently from a synergistic interaction of the catalysts with the support. A second approach to still further enhance the activity is the design and preparation of LDH catalysts with three dimensional structures by themselves. However, three dimensional catalyst structures have yet to proof their durability during long-term application under the corrosive and physical stress inducing conditions of oxygen evolution. The differences in electrode preparation as well as measurement procedures and conditions make it difficult to identify a specific combination of transition metal cations for the potentially most active LDH catalyst. Nevertheless, other parameters like the crystallinity and sheet size are crucial for tuning the initial activity of LDH materials. Alongside the activity enhancement efforts, the inherent stability of the catalysts and also importantly, the fabrication of stable functional electrodes using powder-based catalysts still have to be improved. In contrast to activity, the stability of LDH catalysts is much less investigated and there is lack of common standards and procedures for reporting stability, which hampers the possibility to reference published stability data in the context of a state of the art benchmark. For technical application of LDHs in OER, stability as a crucial requirement has to be more stressed comparable to the interest in the activity. That notwithCurrent Opinion in Electrochemistry 2017, 4:4–10
standing, LDHs represent clearly very promising class of materials with the potential to be further developed.
Acknowledgments The authors are grateful to the Deutsche Forschungsgemeinschaft (DFG) in the framework of the Cluster of Excellence Resolv (EXC1069) and the Bundesministerium für Bildung und Forschung (BMBF) in the frameworks of the project “Mangan” (FKZ 03EK3548).
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