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Enhanced oxygen evolution at hydrous nickel oxide electrodes via electrochemical ageing in alkaline solution
Electrochemistry Communications 32 (2013) 39–42
Contents lists available at SciVerse ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Short communication
Enhanced oxygen evolution at hydrous nickel oxide electrodes via electrochemical ageing in alkaline solution I.J. Godwin ⁎, M.E.G. Lyons Trinity Electrochemical Energy Conversion and Electrocatalysis (TEECE) Group, School of Chemistry, University of Dublin, Trinity College, Dublin 2, Ireland
a r t i c l e
i n f o
Article history: Received 12 March 2013 Received in revised form 26 March 2013 Accepted 26 March 2013 Available online 6 April 2013
a b s t r a c t The effect of electrochemically ageing hydrous nickel oxide films via slow repetitive potential multi-cycling across the main nickel (II/III) redox peak was investigated in an aqueous base environment using cyclic voltammetry and steady state polarisation curves in the oxygen evolution reaction (OER) region. Similarities between hydrous nickel oxide films and electroprecipitated ‘battery type’ nickel oxide were shown due to their similar change in redox and oxygen evolving properties as a result of film ageing. This ageing method was found to significantly enhance the OER performance of the hydrous nickel oxide electrode with the OER overpotential decreasing by 60 ± 2 mV and experiencing a 10 fold increase in OER rate for a fixed overpotential over that of an un-aged electrode. The OER turnover frequency for an aged electrode was found to be 1.16 ± 0.07 s−1 in comparison to 0.05 ± 0.003 s−1 for a hydrous nickel oxide electrode not subjected to ageing. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Electrochemical water splitting via alkaline water electrolysis is currently a hot topic in research due to the need for a clean, reliable and sustainable method for large scale production of high purity hydrogen gas for use as a fuel [1–3]. However, one of the grand challenges fully utilising alkaline water electrolysis for hydrogen production is in the large anodic overpotential associated with the oxygen evolution reaction (OER). The OER is therefore said to be the ‘bottleneck’ or most energy intensive step in the overall water electrolysis process. Over the past 30 years considerable research effort and resources have been focused on the development and improvement of novel anode materials, with the aim of achieving useful rates of the OER at the lowest possible overpotential and cost in order to improve the economic viability of this technology. Dimensionally stable anode (DSA ®) electrodes, based on RuO2 and IrO2 replaced traditional graphite anodes and currently exhibit the lowest overpotential for the OER at practical current densities [4]. Despite their excellent OER performance, the relative high cost of these materials, in particular iridium, combined with their poor long term chemical stability in alkaline media renders their long term use as anode materials for water electrolysers impractical. Because of this problem we have attempted to overcome this problem by using oxides/hydroxides/
⁎ Corresponding author. Tel.: +353 18962032. E-mail address: [email protected] (I.J. Godwin). 1388-2481/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.elecom.2013.03.040
oxyhydroxides of first row transition metals which offer comparable OER performance but at significantly lower cost [5,6]. In previous work [7–12] we have shown that one can generate a catalytically active oxide on the surface of transition metals such as iron and nickel by application of a repetitive potential multicycling regime in aqueous base which causes disruption of the surface metal atoms from the bulk of the parent metal. These disrupted surface atoms, or adatoms, then coordinate to surrounding oxy and hydroxy groups in an octahedral arrangement forming an extremely disordered hydrated layer. These hydrated films have been shown by Lyons and co-workers to offer enhanced performance for the OER i.e. increased rates for a fixed potential over that of the uncycled metal [7–9]. In this work we have abstracted an ‘ageing’ method, discovered initially for electrodeposited nickel hydroxide to further enhance the OER performance of these hydrous nickel oxide films. This ageing method consists of repetitively cycling the potential across the main Ni(II/III)/ (III/II) redox peaks at a slow scan rate which we suggest allows rearrangement of the film to its thermodynamically most stable arrangement.
2. Experimental 2.1. General setup All experiments were conducted in a conventional three electrode cell kept at 25 °C. Prior to each experiment the surface of the working electrode was polished with 1200 grit carbimet paper (Buehler), and
40
I.J. Godwin, M.E.G. Lyons / Electrochemistry Communications 32 (2013) 39–42
wiped until a “mirror bright” finish was achieved. A graphite rod was employed as the counter electrode and a mercury–mercuric oxide (Hg/HgO) reference electrode (CH Instruments, cat. no. CHI 152) was utilised as the reference standard, therefore all voltages are quoted against this reference electrode unless otherwise stated. 1 Aqueous 1 M NaOH served as both the growth medium and the supporting electrolyte for the redox switching and electrocatalytic studies. This solution was prepared from sodium hydroxide pellets (Sigma-Aldrich, minimum 99% purity) using Millipore water (resistivity > 15 MΩ cm−1). Before commencing each experiment, nitrogen gas was bubbled through the electrolyte solution for 20 min to ensure a consistent oxygen free environment. The electrochemical measurements were performed on a high performance CH instrument Model 760 D digital potentiostat and a Gamry model 3000 potentiostat.
2.2. Hydrous nickel oxide growth The polymeric hydrous nickel oxide films were prepared via multicycling a nickel wire electrode (as supplied by Alfa Aesar-Johnson Matthey, purity 99.9945% (metals basis)) with a geometric surface area of 0.3 cm2 between the potentials of −1.45 V and +0.65 V in 1 M NaOH at a scan rate of 150 mV/s. Films of different thicknesses were prepared by varying the number of growth cycles. The charge storage capacity or redox capacity (Q) was determined, following the growth of each film, by integration of the peaks in a voltammetric profile recorded at a slow sweep rate of 40 mV/s. The redox charge capacity is directly proportional to the layer thickness.
2.3. Electroprecipitated nickel hydroxide growth Electroprecipitated nickel hydroxide thin films were prepared following the method of Fantini and Gorenstein [13]. The electroprecipitation route consists of cycling the potential from − 0.9 V to + 1.2 V (potentials were measured with respect to a saturated standard calomel reference electrode) at a potential sweep rate of 50 mV/s in an acetate buffer electrolyte consisting of nickel sulphate (0.1 M), sodium acetate (0.1 M) and potassium hydroxide (0.001 M) with a pH value of 7.6. A gold disc electrode with a geometric area of 0.125 cm 2 (CH Instruments catalogue no. CHI 101) was used as the deposition substrate.
3. Results & discussion 3.1. Redox changes of hydrous nickel oxide and electroprecipitated nickel hydroxide due to ageing Although the exact mechanism by which ageing of the hydrous nickel oxide film occurs has yet to be elucidated, with further studies to be carried out for a more in depth and thorough analysis, we can however postulate a plausible pathway by which it might occur based on our preliminarily observable experimental data shown in Fig. 1. The formal potential of the Ni(II/III) redox couple can be seen in Fig. 1(a) and (c) to increase by ca. 50 mV for the hydrous nickel oxide film while decreasing by ca. 10 mV for the electroprecipitated nickel hydroxide electrode respectively due to electrochemical ageing. Of note and shown in Fig. 1(a) for the hydrous nickel oxide film, the anodic Ni(II/III) oxidation peak potential shifts markedly with ageing and exhibits a positive peak potential shift of ca. 45 mV. The higher potential required to oxidise the aged β-Ni(II) to the β-Ni(III) oxidation state could be indicative of a more stable phase being formed due to the electrochemical ageing over that of the un-aged film. This line of argument follows suit with that of the ageing experiments carried out by Lyons and co-workers [10] and others [15] on electroprecipitated nickel hydroxide where a similar anodic Ni(II/ III) positive peak shift of approx. 40 mV was observed due to ageing by slow potential multicycling in base with a more thermodynamically stable and crystalline phase being formed. For experimental consistency we have reproduced the ageing effect on the electroprecipitated nickel hydroxide to ensure direct comparison with our hydrous nickel oxide films. The process by which ageing occurs on electroprecipitated nickel is largely understood, with an excellent paper detailing the process given by Delahaye-Vidal and Figlarz [14], and is due to the initially formed hydrated and turbostratic α-Ni(OH)2 phase gradually changing into the more crystalline β-Ni(OH)2 phase due a dehydration of α-Ni(OH)2 when subjected to a slow potential multicycling regime in strong base. Following this rationale, and noting that our hydrous nickel oxide films are largely hydrated with coordinated water and hydroxide groups, the ageing process associated with our hydrous nickel oxide electrodes, therefore, could be thought of as a rearrangement of the film from an initial, kinetically more accessible phase generated from the initial fast potential multicycling regime, to a thermodynamically more favourable and stable arrangement due to the slow scan rate employed during the ageing process and hence would account for the positive anodic peak potential shift observed.
2.4. Hydrous nickel oxide & electroprecipitated nickel hydroxide ageing 3.2. Effect of ageing on the oxygen evolution reaction The hydrous nickel oxide film was electrochemically aged by potential multicycling at a slow scan rate of 10 mV/s between fixed limits of 0.1 V–0.6 V for 150 cycles in 1 M NaOH, while the electroprecipitated nickel oxide was aged in 5 M NaOH for 85 cycles using the same limits. The 5 M NaOH was found to produce more marked ageing than 1 M NaOH for the electroprecipitated nickel hydroxide. The cycle numbers noted above reflected the situation where there was no observable change in redox potential of the Ni(II/III) transition upon further cycling.
1 The equilibrium potential of the cell Pt/H2/OH−/HgO/Hg is 0.926 V at 298 K. Since the equilibrium oxygen electrode potential is 1.229 V vs. Reversible hydrogen electrode (RHE), it follows that the corresponding value is 0.303 V vs. Hg/HgO in the same solution. Hence EHg/HgO = ERHE − 0.926 V. It is a common practice in the literature on the OER to express potential in terms of the oxygen evolution overpotential η, when the reference electrode is a Hg/HgO electrode in the same solution as the working anode. Clearly, in this case the overpotential η is related to E meas measured on the Hg/HgO scale as follows: E = E meas − 0.303 V (at T = 298 K).
Herein, we use three key performance indicators to compare the nickel oxide films before and after ageing, these are: Measuring the OER onset potential, comparing the current density for a fixed overpotential and by comparing the turnover frequencies (TOF). The first benchmark for comparing the performance of OER anode materials is by comparing the potential at which the OER begins to be known as the OER onset potential or loosely as the overpotential. The overpotential of the aged nickel oxide film decreases by 60 ± 2 mV over that of the non-aged hydrous oxide film, representing a reduction of almost 20%. This drop in overpotential suggests that the newly formed phase due to ageing lowers the activation barrier for the OER. Secondly, if one picks a fixed arbitrary overpotential of, for example, η = 0.38 V, one sees a near ten-fold increase in current density and, hence, OER rate due to ageing. From this we can clearly say that a more catalytically active phase is formed due to hydrous oxide film ageing in base with similar increase in current densities seen as a result of ageing α-Ni(OH)2, shown in Fig. 1(d), formed by electroprecipitation which further bridges the links between our hydrous nickel oxide and the more studied and understood electroprecipitated nickel hydroxide. The Tafel slope was
I.J. Godwin, M.E.G. Lyons / Electrochemistry Communications 32 (2013) 39–42
41
Fig. 1. (a) Typical cyclic voltammogram (b) Tafel plot of a hydrous nickel hydroxide electrode before and after electrochemical ageing by slow potential multicycling in 1 M NaOH across the Ni(II/III)/Ni(III/II) redox peaks shown. (c) Cyclic voltammogram (d) Tafel plot of an electroprecipitated nickel hydroxide electrode on an Au substrate before and after aging using the same slow potential multicycling regime. Potentials quoted against Hg/HgO. Scan rate 40 mV/s for cyclic voltammograms and 1 mV/s for Tafel plots.
also shown to be unchanged on ageing with values before and after ageing being ca. 60 mV/dec which suggests no change in OER mechanism. We can use the turnover frequency (TOF) concept often applied in molecular catalysis for the OER for our nickel oxide electrodes. TOF is defined as the number of molecules reacting for a given reaction per unit time. Evidently it is advantageous to have a catalyst with a high TOF for it to be practical for use in industrial catalysis. Following a method used by Yeo and Bell [15], one can calculate an electrochemical equivalent TOF using Eq. (1) TOF ¼ ðiNA Þ=ð4FNatoms Þ ¼ J=4Q
ð1Þ
where, i is current, Natoms is the no. of atoms or active sites, J is the current density, Q is the charge storage capacity of the main Ni(II/III) oxidation peak and F and NA have their usual meanings. Picking a fixed overpotential of η = 0.33 V in the OER region, one can calculate the TOF for the hydrous oxide film both before and after ageing. The TOF was found to be 0.05 ± 0.003 s −1 for the film before ageing while the TOF after ageing was shown to be 1.16 ± 0.07 s −1, representing a 23 fold improvement. This increase in TOF and the noted decrease in charge storage capacity of the main Ni(II/III) oxidation peak from 3.48 mC to 2.56 mC due to ageing suggests that although there is less charge storage material present in the film after ageing, the catalytically active material present after ageing catalyses the OER more effectively per unit charge than of the un-aged film. This is also in line with electroprecipitated Ni(OH)2 where an increase in TOF of 0.07 ± 0.004 s − 1 to 0.91 ± 0.05 s − 1 was observed from ageing from the α to β phase. The electroprecipitated β form has also been described at the ‘right type of oxide’ for the OER [3].
4. Conclusion In summary, we believe we have made significant progress in attempting to bridge the links between nickel hydroxide generated by electroprecipitation, used commonly in the battery industry or more recently as a possible material for non-enzymatic glucose detection [16], and our hydrous nickel oxide electrodes due to the similarities in changes to both the redox chemistry and to the OER performance observed when aged using the same technique. We have shown that electrochemical ageing by repetitive potential multicycling at a slow scan rate significantly increases the OER performance of hydrous nickel oxide electrodes using three key performance indicators. We believe that due to the promising results obtained in this preliminary study further work is warranted; in particular, elucidation of the mechanism by which ageing occurs on hydrous nickel oxide electrodes and SEM imaging of any associated surface rearrangements which may have occurred. Acknowledgements The authors are grateful for the financial support of Science Foundation Ireland (SFI) under grant number SFI/10/IN.1/I2969. The authors are also grateful to the reviewers for their useful and constructive comments. References [1] P. Häussinger, R. Lohmüller, A.M. Watson, Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH Verlag GmbH & Co., 2011. [2] J.P. Zheng, P.J. Cygan, T.R. Jow, Journal of the Electrochemical Society 142 (1995) 2699–2703. [3] P.W.T. Lu, S. Srinivasan, Journal of the Electrochemical Society 125 (1978) 1416–1422.
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I.J. Godwin, M.E.G. Lyons / Electrochemistry Communications 32 (2013) 39–42
[4] S. Floquet, M.E.G. Lyons, Physical Chemistry Chemical Physics 13 (2011) 5314–5335. [5] Y.H. Huang, S. Srinivasan, M.H. Miles, Journal of the Electrochemical Society 125 (1978) 1931–1934. [6] M. Hamdani, M.I.S. Pereira, J. Douch, A. Ait Addi, Y. Berghoute, M.H. Mendonça, Electrochimica Acta 49 (2004) 1555–1563. [7] M.E.G. Lyons, M.P. Brandon, Journal of Electroanalytical Chemistry 641 (2010) 119–130. [8] M.E.G. Lyons, R.L. Doyle, M.P. Brandon, Physical Chemistry Chemical Physics 13 (2011) 21530–21551. [9] M.E.G. Lyons, R.L. Doyle, I. Godwin, M. O'Brien, L. Russell, Journal of the Electrochemical Society 159 (2012) H932–H944.
[10] A. Cakara, M.E.G. Lyons, P. O' Brien, I. Godwin, R.L. Doyle, International Journal of Electrochemical Science 7 (2012) 11768. [11] R.L. Doyle, M.E.G. Lyons, International Journal of Electrochemical Science 7 (2012) 9488–9501. [12] L. Russell, M.E.G. Lyons, M. O'Brien, R.L. Doyle, I. Godwin, M.P. Brandon, International Journal of Electrochemical Science 7 (2012) 2710–2763. [13] M. Fantini, A. Gorenstein, Solar Energy Materials 16 (1987) 487–500. [14] A. Delahaye-Vidal, M. Figlarz, Journal of Applied Electrochemistry 17 (1987) 589–599. [15] B.S. Yeo, A.T. Bell, Journal of Physical Chemistry C 116 (2012) 8394–8400. [16] K.E. Toghill, R.G. Compton, International Journal of Electrochemical Science 5 (2010) 1246–1301.
Contents lists available at SciVerse ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Short communication
Enhanced oxygen evolution at hydrous nickel oxide electrodes via electrochemical ageing in alkaline solution I.J. Godwin ⁎, M.E.G. Lyons Trinity Electrochemical Energy Conversion and Electrocatalysis (TEECE) Group, School of Chemistry, University of Dublin, Trinity College, Dublin 2, Ireland
a r t i c l e
i n f o
Article history: Received 12 March 2013 Received in revised form 26 March 2013 Accepted 26 March 2013 Available online 6 April 2013
a b s t r a c t The effect of electrochemically ageing hydrous nickel oxide films via slow repetitive potential multi-cycling across the main nickel (II/III) redox peak was investigated in an aqueous base environment using cyclic voltammetry and steady state polarisation curves in the oxygen evolution reaction (OER) region. Similarities between hydrous nickel oxide films and electroprecipitated ‘battery type’ nickel oxide were shown due to their similar change in redox and oxygen evolving properties as a result of film ageing. This ageing method was found to significantly enhance the OER performance of the hydrous nickel oxide electrode with the OER overpotential decreasing by 60 ± 2 mV and experiencing a 10 fold increase in OER rate for a fixed overpotential over that of an un-aged electrode. The OER turnover frequency for an aged electrode was found to be 1.16 ± 0.07 s−1 in comparison to 0.05 ± 0.003 s−1 for a hydrous nickel oxide electrode not subjected to ageing. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Electrochemical water splitting via alkaline water electrolysis is currently a hot topic in research due to the need for a clean, reliable and sustainable method for large scale production of high purity hydrogen gas for use as a fuel [1–3]. However, one of the grand challenges fully utilising alkaline water electrolysis for hydrogen production is in the large anodic overpotential associated with the oxygen evolution reaction (OER). The OER is therefore said to be the ‘bottleneck’ or most energy intensive step in the overall water electrolysis process. Over the past 30 years considerable research effort and resources have been focused on the development and improvement of novel anode materials, with the aim of achieving useful rates of the OER at the lowest possible overpotential and cost in order to improve the economic viability of this technology. Dimensionally stable anode (DSA ®) electrodes, based on RuO2 and IrO2 replaced traditional graphite anodes and currently exhibit the lowest overpotential for the OER at practical current densities [4]. Despite their excellent OER performance, the relative high cost of these materials, in particular iridium, combined with their poor long term chemical stability in alkaline media renders their long term use as anode materials for water electrolysers impractical. Because of this problem we have attempted to overcome this problem by using oxides/hydroxides/
⁎ Corresponding author. Tel.: +353 18962032. E-mail address: [email protected] (I.J. Godwin). 1388-2481/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.elecom.2013.03.040
oxyhydroxides of first row transition metals which offer comparable OER performance but at significantly lower cost [5,6]. In previous work [7–12] we have shown that one can generate a catalytically active oxide on the surface of transition metals such as iron and nickel by application of a repetitive potential multicycling regime in aqueous base which causes disruption of the surface metal atoms from the bulk of the parent metal. These disrupted surface atoms, or adatoms, then coordinate to surrounding oxy and hydroxy groups in an octahedral arrangement forming an extremely disordered hydrated layer. These hydrated films have been shown by Lyons and co-workers to offer enhanced performance for the OER i.e. increased rates for a fixed potential over that of the uncycled metal [7–9]. In this work we have abstracted an ‘ageing’ method, discovered initially for electrodeposited nickel hydroxide to further enhance the OER performance of these hydrous nickel oxide films. This ageing method consists of repetitively cycling the potential across the main Ni(II/III)/ (III/II) redox peaks at a slow scan rate which we suggest allows rearrangement of the film to its thermodynamically most stable arrangement.
2. Experimental 2.1. General setup All experiments were conducted in a conventional three electrode cell kept at 25 °C. Prior to each experiment the surface of the working electrode was polished with 1200 grit carbimet paper (Buehler), and
40
I.J. Godwin, M.E.G. Lyons / Electrochemistry Communications 32 (2013) 39–42
wiped until a “mirror bright” finish was achieved. A graphite rod was employed as the counter electrode and a mercury–mercuric oxide (Hg/HgO) reference electrode (CH Instruments, cat. no. CHI 152) was utilised as the reference standard, therefore all voltages are quoted against this reference electrode unless otherwise stated. 1 Aqueous 1 M NaOH served as both the growth medium and the supporting electrolyte for the redox switching and electrocatalytic studies. This solution was prepared from sodium hydroxide pellets (Sigma-Aldrich, minimum 99% purity) using Millipore water (resistivity > 15 MΩ cm−1). Before commencing each experiment, nitrogen gas was bubbled through the electrolyte solution for 20 min to ensure a consistent oxygen free environment. The electrochemical measurements were performed on a high performance CH instrument Model 760 D digital potentiostat and a Gamry model 3000 potentiostat.
2.2. Hydrous nickel oxide growth The polymeric hydrous nickel oxide films were prepared via multicycling a nickel wire electrode (as supplied by Alfa Aesar-Johnson Matthey, purity 99.9945% (metals basis)) with a geometric surface area of 0.3 cm2 between the potentials of −1.45 V and +0.65 V in 1 M NaOH at a scan rate of 150 mV/s. Films of different thicknesses were prepared by varying the number of growth cycles. The charge storage capacity or redox capacity (Q) was determined, following the growth of each film, by integration of the peaks in a voltammetric profile recorded at a slow sweep rate of 40 mV/s. The redox charge capacity is directly proportional to the layer thickness.
2.3. Electroprecipitated nickel hydroxide growth Electroprecipitated nickel hydroxide thin films were prepared following the method of Fantini and Gorenstein [13]. The electroprecipitation route consists of cycling the potential from − 0.9 V to + 1.2 V (potentials were measured with respect to a saturated standard calomel reference electrode) at a potential sweep rate of 50 mV/s in an acetate buffer electrolyte consisting of nickel sulphate (0.1 M), sodium acetate (0.1 M) and potassium hydroxide (0.001 M) with a pH value of 7.6. A gold disc electrode with a geometric area of 0.125 cm 2 (CH Instruments catalogue no. CHI 101) was used as the deposition substrate.
3. Results & discussion 3.1. Redox changes of hydrous nickel oxide and electroprecipitated nickel hydroxide due to ageing Although the exact mechanism by which ageing of the hydrous nickel oxide film occurs has yet to be elucidated, with further studies to be carried out for a more in depth and thorough analysis, we can however postulate a plausible pathway by which it might occur based on our preliminarily observable experimental data shown in Fig. 1. The formal potential of the Ni(II/III) redox couple can be seen in Fig. 1(a) and (c) to increase by ca. 50 mV for the hydrous nickel oxide film while decreasing by ca. 10 mV for the electroprecipitated nickel hydroxide electrode respectively due to electrochemical ageing. Of note and shown in Fig. 1(a) for the hydrous nickel oxide film, the anodic Ni(II/III) oxidation peak potential shifts markedly with ageing and exhibits a positive peak potential shift of ca. 45 mV. The higher potential required to oxidise the aged β-Ni(II) to the β-Ni(III) oxidation state could be indicative of a more stable phase being formed due to the electrochemical ageing over that of the un-aged film. This line of argument follows suit with that of the ageing experiments carried out by Lyons and co-workers [10] and others [15] on electroprecipitated nickel hydroxide where a similar anodic Ni(II/ III) positive peak shift of approx. 40 mV was observed due to ageing by slow potential multicycling in base with a more thermodynamically stable and crystalline phase being formed. For experimental consistency we have reproduced the ageing effect on the electroprecipitated nickel hydroxide to ensure direct comparison with our hydrous nickel oxide films. The process by which ageing occurs on electroprecipitated nickel is largely understood, with an excellent paper detailing the process given by Delahaye-Vidal and Figlarz [14], and is due to the initially formed hydrated and turbostratic α-Ni(OH)2 phase gradually changing into the more crystalline β-Ni(OH)2 phase due a dehydration of α-Ni(OH)2 when subjected to a slow potential multicycling regime in strong base. Following this rationale, and noting that our hydrous nickel oxide films are largely hydrated with coordinated water and hydroxide groups, the ageing process associated with our hydrous nickel oxide electrodes, therefore, could be thought of as a rearrangement of the film from an initial, kinetically more accessible phase generated from the initial fast potential multicycling regime, to a thermodynamically more favourable and stable arrangement due to the slow scan rate employed during the ageing process and hence would account for the positive anodic peak potential shift observed.
2.4. Hydrous nickel oxide & electroprecipitated nickel hydroxide ageing 3.2. Effect of ageing on the oxygen evolution reaction The hydrous nickel oxide film was electrochemically aged by potential multicycling at a slow scan rate of 10 mV/s between fixed limits of 0.1 V–0.6 V for 150 cycles in 1 M NaOH, while the electroprecipitated nickel oxide was aged in 5 M NaOH for 85 cycles using the same limits. The 5 M NaOH was found to produce more marked ageing than 1 M NaOH for the electroprecipitated nickel hydroxide. The cycle numbers noted above reflected the situation where there was no observable change in redox potential of the Ni(II/III) transition upon further cycling.
1 The equilibrium potential of the cell Pt/H2/OH−/HgO/Hg is 0.926 V at 298 K. Since the equilibrium oxygen electrode potential is 1.229 V vs. Reversible hydrogen electrode (RHE), it follows that the corresponding value is 0.303 V vs. Hg/HgO in the same solution. Hence EHg/HgO = ERHE − 0.926 V. It is a common practice in the literature on the OER to express potential in terms of the oxygen evolution overpotential η, when the reference electrode is a Hg/HgO electrode in the same solution as the working anode. Clearly, in this case the overpotential η is related to E meas measured on the Hg/HgO scale as follows: E = E meas − 0.303 V (at T = 298 K).
Herein, we use three key performance indicators to compare the nickel oxide films before and after ageing, these are: Measuring the OER onset potential, comparing the current density for a fixed overpotential and by comparing the turnover frequencies (TOF). The first benchmark for comparing the performance of OER anode materials is by comparing the potential at which the OER begins to be known as the OER onset potential or loosely as the overpotential. The overpotential of the aged nickel oxide film decreases by 60 ± 2 mV over that of the non-aged hydrous oxide film, representing a reduction of almost 20%. This drop in overpotential suggests that the newly formed phase due to ageing lowers the activation barrier for the OER. Secondly, if one picks a fixed arbitrary overpotential of, for example, η = 0.38 V, one sees a near ten-fold increase in current density and, hence, OER rate due to ageing. From this we can clearly say that a more catalytically active phase is formed due to hydrous oxide film ageing in base with similar increase in current densities seen as a result of ageing α-Ni(OH)2, shown in Fig. 1(d), formed by electroprecipitation which further bridges the links between our hydrous nickel oxide and the more studied and understood electroprecipitated nickel hydroxide. The Tafel slope was
I.J. Godwin, M.E.G. Lyons / Electrochemistry Communications 32 (2013) 39–42
41
Fig. 1. (a) Typical cyclic voltammogram (b) Tafel plot of a hydrous nickel hydroxide electrode before and after electrochemical ageing by slow potential multicycling in 1 M NaOH across the Ni(II/III)/Ni(III/II) redox peaks shown. (c) Cyclic voltammogram (d) Tafel plot of an electroprecipitated nickel hydroxide electrode on an Au substrate before and after aging using the same slow potential multicycling regime. Potentials quoted against Hg/HgO. Scan rate 40 mV/s for cyclic voltammograms and 1 mV/s for Tafel plots.
also shown to be unchanged on ageing with values before and after ageing being ca. 60 mV/dec which suggests no change in OER mechanism. We can use the turnover frequency (TOF) concept often applied in molecular catalysis for the OER for our nickel oxide electrodes. TOF is defined as the number of molecules reacting for a given reaction per unit time. Evidently it is advantageous to have a catalyst with a high TOF for it to be practical for use in industrial catalysis. Following a method used by Yeo and Bell [15], one can calculate an electrochemical equivalent TOF using Eq. (1) TOF ¼ ðiNA Þ=ð4FNatoms Þ ¼ J=4Q
ð1Þ
where, i is current, Natoms is the no. of atoms or active sites, J is the current density, Q is the charge storage capacity of the main Ni(II/III) oxidation peak and F and NA have their usual meanings. Picking a fixed overpotential of η = 0.33 V in the OER region, one can calculate the TOF for the hydrous oxide film both before and after ageing. The TOF was found to be 0.05 ± 0.003 s −1 for the film before ageing while the TOF after ageing was shown to be 1.16 ± 0.07 s −1, representing a 23 fold improvement. This increase in TOF and the noted decrease in charge storage capacity of the main Ni(II/III) oxidation peak from 3.48 mC to 2.56 mC due to ageing suggests that although there is less charge storage material present in the film after ageing, the catalytically active material present after ageing catalyses the OER more effectively per unit charge than of the un-aged film. This is also in line with electroprecipitated Ni(OH)2 where an increase in TOF of 0.07 ± 0.004 s − 1 to 0.91 ± 0.05 s − 1 was observed from ageing from the α to β phase. The electroprecipitated β form has also been described at the ‘right type of oxide’ for the OER [3].
4. Conclusion In summary, we believe we have made significant progress in attempting to bridge the links between nickel hydroxide generated by electroprecipitation, used commonly in the battery industry or more recently as a possible material for non-enzymatic glucose detection [16], and our hydrous nickel oxide electrodes due to the similarities in changes to both the redox chemistry and to the OER performance observed when aged using the same technique. We have shown that electrochemical ageing by repetitive potential multicycling at a slow scan rate significantly increases the OER performance of hydrous nickel oxide electrodes using three key performance indicators. We believe that due to the promising results obtained in this preliminary study further work is warranted; in particular, elucidation of the mechanism by which ageing occurs on hydrous nickel oxide electrodes and SEM imaging of any associated surface rearrangements which may have occurred. Acknowledgements The authors are grateful for the financial support of Science Foundation Ireland (SFI) under grant number SFI/10/IN.1/I2969. The authors are also grateful to the reviewers for their useful and constructive comments. References [1] P. Häussinger, R. Lohmüller, A.M. Watson, Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH Verlag GmbH & Co., 2011. [2] J.P. Zheng, P.J. Cygan, T.R. Jow, Journal of the Electrochemical Society 142 (1995) 2699–2703. [3] P.W.T. Lu, S. Srinivasan, Journal of the Electrochemical Society 125 (1978) 1416–1422.
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