A cobalt oxide–polypyrrole nanocomposite as an efficient and stable electrode material for electrocatalytic water oxidation

Abstract

Developing electrolyzers operating under neutral or near-neutral conditions with catalysts based only on earth-abundant metals is highly desirable with a view to reduce the cost of hydrogen production from the water splitting reaction and avoid the environmental issues related to corrosion usually encountered with alkaline electrolyzers. Herein, we report a highly active and stable anode material for the oxygen evolution reaction (OER) under mild-pH conditions based on cobalt oxide-nanoparticles embedded into a poly(pyrrole-alkylammonium) matrix (denoted as PPN⁺-CoO_x). Examples of hybrid materials combining metal oxide nanoparticles as OER catalysts within a polymer film are still rare. However, they are very promising to control the formation and the size of metal particles in view of enhancing the electrochemically active surface area and thus the electrocatalytic performances. Our strategy consists in electroprecipitating Co⁰ nanoparticles by the reduction of an anionic cobalt oxalate complex into a cationic PPN⁺ film, the latter being previously deposited onto an electrode surface by electropolymerization. The Co⁰ nanoparticles within the composite are then partially in situ oxidized under air exposure to CoO, and then finally fully oxidized to CoO_x by successive scans between 0 and 1.2 V vs. Ag/AgCl in a borate buffer at pH 9.2. This nanocomposite material is highly structured with around 30 nm-large CoO_x nanoparticles well dispersed into the polypyrrole film conferring a high OER electrocatalytic activity at a near neutral pH of 9.2 with exceptional values of mass activity and turnover frequency of 3.01 A mg⁻¹ and 0.46 s⁻¹ respectively, at an overpotential of 0.61 V and with a cobalt loading of 1.34 μg cm⁻². These performances place the PPN⁺-CoO_x electrode among the most active anodes described in the literature employing cobalt oxide under mild pH conditions. In addition, when the PPN⁺-CoO_x material is electrodeposited on carbon paper with a higher roughness than a simple carbon electrode, the physisorption of the film on the electrode is considerably enhanced resulting in a stable catalytic current for over more than 43 h. Post electrolysis characterization by SEM and EDX confirms the integrity of the PPN⁺-CoO_x material after many hours of electrocatalysis. This demonstrates the beneficial role of the polypyrrole matrix in the achievement of very stable and highly active anodes for water oxidation.

---

1. Introduction

Molecular hydrogen (H₂) is considered as the cleanest energy carrier of the future and one of the best alternatives to fossil fuels. Indeed, H₂ can be used in a fuel cell in the presence of oxygen to produce electricity with a relatively good conversion efficiency of around 50%, while releasing only water as the by-product.^1,2 However, most of the worldwide H₂ is currently industrially produced from fossil fuels via the steam reforming process, releasing CO₂, a greenhouse gas.³ Another process, namely water electrolysis, is very attractive since it can produce hydrogen from water without using fossil raw materials, and could compete with steam reforming provided that clean, renewable and low-cost electricity is employed.^4,5 Basically, an electrolyzer consists of an anode and a cathode, respectively, for water oxidation to oxygen (i.e. oxygen evolution reaction, denoted as OER) and water reduction to hydrogen (i.e. hydrogen evolution reaction, denoted as HER), associated with a polymeric solid-state electrolyte, which is usually a proton exchange membrane (PEM) like Nafion®, or with a liquid alkaline electrolyte (i.e. a 20–40 wt% aqueous KOH solution).^6,7 PEM electrolyzers usually employ metallic Pt and IrO₂ electrocatalysts at the cathode and anode, respectively, and operate under strongly acidic conditions between 50 and 80 °C with high current densities up to 2 A cm⁻².^7,8 However, the high cost of rare metal-based electrocatalysts and the perfluorinated Nafion®-based membrane makes PEM electrolysis an expensive technology and thus questionable for mass production of H₂. By contrast, alkaline electrolysis is a much cheaper technology since it requires only nickel, an earth-abundant metal as the electrocatalyst, deposited on stainless steel electrodes for the cathode and anode, both being separated by a simple diaphragm conductive for hydroxide ions.^7,9 Nevertheless, such alkaline electrolyzers exhibit lower current densities (from 0.2 to 0.4 A cm⁻² at 80 °C) compared to PEM electrolyzers and suffer from corrosion, which necessitates extensive maintenance and causes environmental and safety issues for operating personnel.¹⁰ In order to avoid the problems related to corrosion and reduce the global cost of hydrogen produced from the electrocatalytic water-splitting reaction, it is of utmost importance to develop electrolyzers that efficiently work under neutral or near-neutral conditions with a cheap membrane along with electrocatalysts based on earth-abundant metals.¹¹

The OER occurring at the anode is the bottleneck of the water splitting reaction and is more challenging than the HER at the cathode, more specifically at neutral or near-neutral pH, due to its sluggish kinetics and large anodic overpotential.^12–15 Several earth-abundant metal oxides such as Fe, Mn, Cu, Co and Ni and their alloys have been investigated as electrocatalysts for the OER under mild pH conditions.¹¹ Among them, cobalt oxide (CoO_x) has been intensively studied by many groups under alkaline conditions.^16–45 Originally identified as an electrocatalyst for water oxidation at pH 14 in 1950 by El Wakkad and Hickling,⁴⁶ CoO_x received renewed interest in 2008, owing to the work of Nocera and co-workers demonstrating that thin films of this earth-abundant metal oxide electrodeposited on an electrode surface in a proton-accepting phosphate (pH 7)^23,24 or borate (pH 9.2)²⁴ buffer are also very active OER electrocatalysts under such neutral or near-neutral conditions. A key factor for the performance of an electrocatalyst is its nanostructuration, the efficiency being significantly improved by increasing the active area/volume ratio, while reducing the cost of manufacture. However, the reduction of the particle size to increase the specific surface area of the catalyst leads to a reduction in their stability and can lead to aggregation. To overcome such limitations, we recently reported an original electrochemical approach to design nanocomposite electrode materials in which iridium or nickel oxide nanoparticles of small sizes (2–21 nm) are electrogenerated in a polypyrrole-alkylammonium matrix (denoted as PPN⁺) of controllable thickness, previously deposited on the electrode surface by electropolymerization of the pyrrole monomer (3-pyrrole-1-yl-propyl)-triethylammonium (denoted as PN⁺) (Scheme 1 and S1†).^47,48 These anodes displaying high OER activity prove to be much more effective and stable for electrocatalytic water oxidation than thin films of MO_x anodes (M = Ir or Ni) directly deposited on the electrode surface. Although examples of nanocomposites combining metal oxide particles and a polymer film for electrocatalytic water oxidation are quite rare,^47–50 these results highlight that such hybrid materials are very promising to control the formation and the size of metal particles and thus positively influence the catalytic behaviour and the stability of the anode.⁵¹

In this context, we have extended this interesting strategy of designing an efficient nanocomposite material to cobalt oxide and we report herein a very active and stable anode for the OER operating at pH 9.2 based on a cobalt oxide–poly(pyrrole-alkylammonium) nanocomposite (denoted as PPN⁺-CoO_x). The PPN⁺-CoO_x nanocomposite material electrodeposited onto vitreous carbon (denoted as C) or indium tin oxide (ITO) electrodes has been characterized by various electrochemical and microscopy techniques and the amount of cobalt was also quantified by inductively coupled plasma mass spectrometry (ICP-MS). The electrocatalytic properties of this nanocomposite material towards water oxidation have been evaluated in a borate buffer at pH 9.2 and compared with those of CoO_x-based anodes of the literature under similar catalytic conditions and with those of CoO_x films directly deposited on a naked electrode by the same electrochemical procedure. The objective was to highlight the beneficial role of the PPN⁺ matrix in generating non-agglomerated CoO_x particles with higher OER activity.

2. Results and discussion

2.1. Electrosynthesis and electrochemical characterization of cobalt oxide and poly(pyrrole-alkylammonium)–cobalt oxide nanocomposite electrode materials

The four-step electrochemical procedure used to prepare the C or ITO/PPN⁺-CoO_x modified electrodes is depicted in Scheme 1. For analytical electrochemical studies, C electrodes (surface of 0.071 cm²) were used for film deposition, while larger surface ITO electrodes (surface of 1.0 cm²) were employed for microscopy studies (see below). The first step consists of the electropolymerization in acetonitrile of PN⁺ (4 mM) at the electrode surface at an applied potential of +0.95 V vs. Ag/AgNO₃ for an anodic charge of 4 mC.⁴⁸ The surface coverage in ammonium units Γ_N⁺ (mol cm⁻²) corresponding to the amount of polypyrrole deposited on the electrode was estimated to be 1.20 (±0.1) × 10⁻⁷ mol cm⁻² by the integration of the reversible redox process of polypyrrole (see the ESI†). In a second step, the PPN⁺ electrode is dipped for one hour in an aqueous borate buffer at pH 6 (0.1 M Na₂SO₄ and 0.1 M H₃BO₃) containing CoSO₄ (4 mM) and Na₂C₂O₄ (20 mM), to incorporate by ion exchange within the cationic polypyrrole film^{47,48,52–54} an anionic cobalt oxalate complex, most probably in the form of [Co(C₂O₄)₂]²⁻ generated in situ. The Co⁰ nanoparticles were then electrogenerated within the film by the reduction of the cobalt oxalate complex inserted into the polymer at an applied potential of −1.3 V vs. Ag/AgCl for a cathodic charge of 4 mC (Fig. S1†). Of note, in this article, the potentials are given versus Ag/AgCl for all electrochemical experiments performed in aqueous solution. During this electroreduction step, the C/PPN⁺ modified electrode is maintained in the cobalt oxalate solution (0.1 M borate buffer at pH 6). For comparison purposes, Co⁰ particles were also electrodeposited on naked C electrodes (without PPN⁺) from the same cobalt oxalate aqueous solution using also a cathodic charge of 4 mC. Typically for Co⁰ electrodeposition on C and in C/PPN⁺ electrodes, about 130–170 s are required to pass a cathodic charge of 4 mC (Fig. S1†). A cathodic potential of −1.3 V for Co⁰ electrogeneration on naked C and C/PPN⁺ electrodes was chosen from the cyclic voltammetry (CV) of the cobalt oxalate solution recorded in borate buffer at pH 6 (Fig. 1).

Scheme 1 General strategy for the electrosynthesis of poly(pyrrole-alkylammonium)/CoO_x nanocomposite film modified electrodes. (i) Formation of PPN⁺ by oxidative electropolymerization of monomer PN⁺ (4 mM) at E_app = +0.95 and +1.1 V vs. Ag/AgNO₃ in CH₃CN, 0.1 M [Bu₄N]ClO₄, respectively, for C and ITO electrodes; (ii) incorporation of [Co(C₂O₄)₂]²⁻ into PPN⁺ upon soaking for 1 h in a 0.1 M borate buffer solution (pH 6) of 4 mM CoSO₄, 20 mM Na₂C₂O₄ and 0.1 M Na₂SO₄; (iii) electroprecipitation of Co⁰ within PPN⁺ at E_app = −1.3 and −1.5 V vs. Ag/AgCl, respectively, for C and ITO electrodes, in the 0.1 M borate buffer solution (pH 6) of cobalt oxalate; (iv) air oxidation and electro-oxidation of Co⁰ to CoO_xvia 5 consecutive cycles between 0 and +1.2 V vs. Ag/AgCl in a 0.1 M borate buffer solution (pH 9.2).

Fig. 1 Cyclic voltammetry performed at C (black line) and C/PPN⁺ (Γ_N⁺ = 1.2 ± 0.1 × 10⁻⁷ mol cm⁻²) (red line) electrodes (3 mm of diameter) in a 0.1 M borate buffer solution (pH 6) of 4 mM CoSO₄, 20 mM Na₂C₂O₄ and 0.1 M Na₂SO₄; the C/PPN⁺ electrode was dipped in the aforementioned aqueous solution for 1 h before the cyclic voltammetry experiment.

In the CV shown in Fig. 1, a large irreversible process corresponding to the reduction of Co²⁺ (coordinated to oxalate) to metallic Co⁰ leading to its deposition at the electrode surface starts at −1.0 V. The Co²⁺ reduction process is concomitant with the electrocatalytic reduction of protons to H₂ catalyzed by the Co⁰ electrodeposited.⁴⁸ In the reverse scan, the irreversible oxidation peak observed at −0.4 V for C/PPN⁺-Co⁰ (−0.5 V for C/Co⁰) is ascribed to the reoxidation of the electrodeposited Co⁰ to Co²⁺, which is then dissolved in water.²⁶ Consequently, we chose to electrodeposit Co⁰ on C and C/PPN⁺ at −1.3 V, since at this potential the Co⁰ electrogeneration can be promoted while minimizing H₂ generation. The Co loading (Γ_Co) on the C/PPN⁺ and C electrodes was estimated to be 2.27 (±0.45) × 10⁻⁸ and 5.07 (±0.33) × 10⁻⁸ mol cm⁻², corresponding to deposition yields of 7.8 and 17.4%, respectively, calculated by ICP-MS measurements. Thus, for the same quantity of charge (4 mC), the amount of Co⁰ deposited on the C/PPN⁺ film is 2.23 times smaller than that deposited on the naked C electrode. The average Γ_Co/Γ_N⁺ ratio within the C/PPN⁺-Co⁰ electrodes has been determined to be 0.19, considering the average surface coverage values of Co and PPN⁺, which are respectively Γ_Co = 2.27 × 10⁻⁸ mol cm⁻² and Γ_N⁺ = 1.2 × 10⁻⁷ mol cm⁻².

The last step is the oxidation of Co⁰ to cobalt oxide. Firstly, a partial oxidation of Co⁰ to CoO occurs almost instantaneously under air exposure during the transfer of the C/PPN⁺-Co⁰ and C/Co⁰ electrodes into an aqueous borate buffer solution at pH 9.2. The spontaneous air oxidation of Co⁰ to CoO is well known and was previously reported by the groups of Rice⁵⁵ and Zhao.⁵⁶ Then, to oxidize CoO to higher oxidation states than II (i.e. to CoO_x), five consecutive cycles between 0 and +1.2 V were performed in a 0.1 M borate buffer solution (pH 9.2) (Fig. 2 and Scheme 1). The generation of cobalt oxide is evidenced by the appearance of its typical small intensity reversible Co^II/Co^III redox process^13,57,58 with anodic and cathodic peaks respectively located at +0.58 and +0.53 V for C/PPN⁺-CoO_x, and at +0.56 and +0.51 V for C/CoO_x (Fig. 2D). Increasing the number of anodic cycles does not lead to a significant increase of the Co^II/Co^III redox process. The similarity of the Co^II/Co^III redox processes for C/PPN⁺-CoO_x and C/CoO_x indicates that the same nature of cobalt oxide is generated on both electrodes (Fig. 2D). X-ray photoelectron spectroscopy (XPS) measurements performed on C/PPN⁺-CoO_x after the electro-oxidation of CoO particles indeed suggest a mixture of a mixed valence cobalt oxide (Co₃O₄) and a cobalt oxyhydroxide (CoOOH) (see below). Note that a similar Co^II/Co^III redox process in the same potential range was obtained by the group of Spiccia⁵⁹ for cobalt oxide electrodeposited in a borate buffer (pH 9.2) from solutions of negatively charged cobalt complexes.

Fig. 2 Electro-induced generation of CoO_x from CoO by repeated cyclic voltammetry (CV) scans (5 consecutive scans; scan rate of 50 mV s⁻¹) in a 0.1 M borate buffer solution (pH 9.2) at (A) the C/PPN⁺-CoO electrode (Γ_N⁺ = 1.2 ± 0.1 × 10⁻⁷ mol cm⁻² and Γ_Co = 2.27 ± 0.45 × 10⁻⁸ mol cm⁻²) and (B) the C/CoO electrode (Γ_Co = 5.07 ± 0.33 × 10⁻⁸ mol cm⁻²). (C) Overlay of the 5^th scans of the C/PPN⁺-CoO (red) and C/CoO (blue) electrodes (in black, the electroactivity of a naked C electrode of 3 mm diameter) and (D) zoom of part (C) showing the redox process of the cobalt oxide.

It can also be noted that in the first scan from 0 to +1.2 V of the C/PPN⁺-CoO electrode (Fig. 2A), two broad anodic processes are observed at +0.67 and +0.95 V in the cyclic voltammogram, which correspond respectively to the oxidation and the overoxidation of polypyrrole. The mechanism of the overoxidation of polypyrrole is complex but has been well investigated.^60–62 During its overoxidation, a nucleophilic attack at α- or β-carbon of a pyrrole unit occurs, resulting in the introduction of oxygen atoms on the polymer backbone and a loss of its conductivity. After the overoxidation, the polymeric chain remains globally intact, the ammonium substituents are still present in the film and the CoO_x particles stay embedded in the overoxidized polymer. Consequently, after the first scan, only the Co^II/Co^III redox process is observed in the CV below +0.7 V and the redox behavior of the polypyrrole has disappeared, meaning that the polymer conductivity is fully destroyed owing to its complete overoxidation. The electronic conductivity of the PPN⁺-CoO_x film, which is effective for potentials above +0.3 V (Fig. 2D), is thus only ensured by the cobalt oxide nanoparticles, presumably via percolation, while the overoxidized PPN⁺ acts as a nonconductive matrix maintaining the nanoparticles at the surface of the electrode, as we previously observed for C/PPN⁺-NiO_x electrodes.⁴⁸

The generation of the CoO_x particles on C and C/PPN⁺ electrodes is quasi-concomitant with the catalytic water oxidation observed in their CVs (Fig. 2) by a growing catalytic current above +0.80 V. It is important to note that the catalytic current density (j) of the C/PPN⁺-CoO_x electrode at +1.2 V is higher than that of the C/CoO_x electrode, with 5.77 vs. 4.76 mA cm⁻², respectively (Fig. 2A and B), evidencing the better catalytic performance of C/PPN⁺-CoO_x in spite of the lower loading of cobalt within the polymer film. This result could be ascribed to the greater nanostructuration of the CoO_x embedded into the polymeric PPN⁺ matrix. The difference of nanostructuration between C/PPN⁺-CoO_x and C/CoO_x, which is related to their electrochemically active surface area, can be qualitatively estimated from their CVs. The integration of the overall current of the CVs for both electrodes between +0.3 and +0.7 V (Fig. 2D) indicates that the charge involved in the redox processes leading to the insulator/conductor conversion⁶³ of C/PPN⁺-CoO_x as well as the Co^II/Co^III conversion of redox active sites exposed to the electrolyte⁵⁷ is 1.81 times greater than that of C/CoO_x. Given that there is 2.23 times less cobalt in C/PPN⁺-CoO_x compared to C/CoO_x (see above), the electrochemically active surface area of the nanocomposite electrode is then estimated to be greater than that of the simple cobalt oxide electrode.

In order to confirm the higher nanostructuration of the composite material, the capacitance of C/PPN⁺-CoO_x and C/CoO_x electrodes was estimated by electrochemical impedance spectroscopy (EIS). Indeed, the capacitance of an electrode is directly correlated with its electrochemically active surface area and thus with its structuration.^64–66 Bode phase and Nyquist plots (Fig. 3 and S5†) were recorded with both electrodes in borate buffer solution at pH 9.2 for different potentials (0.2, 0.6, 0.8, 0.9 and 1.0 V). The impedance spectra obtained were fitted with the equivalent circuit conventionally used for the catalytic oxidation of water.^13,67–69 The equivalent circuit is shown in the inset of Fig. 3C and is composed of the resistance of the electrolyte (R_s) between the working and reference electrodes, the double layer capacitance of the working electrode (C_dl), the electron transport resistance (R_tr, also called polarization resistance) of the film (i.e. PPN⁺-CoO_x or CoO_x), and the capacitance (C_sub) and resistance (R_sub) related to the charge transfer between the cobalt oxide particle and its surface intermediates involved in the water oxidation process. For the curve fitting, we chose to replace the capacities with constant phase elements (CPEs) in order to take into account the inhomogeneity of the system, which is commonly associated with nanostructured electrodes with a high roughness factor.^69–72 The values of the equivalent circuit elements obtained by the experimental data fitting are reported in Table 1 and are averages calculated from measurements performed on three different electrodes for each film.

Fig. 3 Bode (module (A) and phase (B)) and Nyquist plots (C) recorded at the C/PPN⁺-CoO_x (Γ_N⁺ = 1.2 ± 0.1 × 10⁻⁷ mol cm⁻² and Γ_Co = 2.27 ± 0.45 × 10⁻⁸ mol cm⁻²) electrode (3 mm of diameter) in a 0.1 M borate buffer solution (pH 9.2) at different potentials of 0.2 (a), 0.6 (b), 0.8 (c), 0.9 (d) and 1.0 V (e) vs. Ag/AgCl; inset in (C) shows the equivalent circuit model used to fit the EIS data.

Table 1 Values of equivalent circuit elements (R_tr, C_dl, and n₁, and R_sub, C_sub, and n₂) obtained for the fitting of the experimental data obtained for various potentials with C/PPN⁺-CoO_x and C/CoO_x in a 0.1 M borate buffer at pH 9.2

E (V)	R tr (Ω cm^−2)	C dl (mF cm^−2)	n 1	R sub (Ω cm^−2)	C sub (mF cm^−2)	n 2
a Potentials are given versus Ag/AgCl.
C/PPN ^+ -CoO x
0.6	12 ± 1	0.67 ± 0.06	0.936 ± 0.004	>10 (ref. 17)	—	—
0.8	10 ± 1	1.1 ± 0.1	0.930 ± 0.006	>10 (ref. 14)	—	—
0.9	7.2 ± 0.5	1.2 ± 0.1	0.89 ± 0.01	145 ± 9	1.32 ± 0.09	0.905 ± 0.006
1.0	5.6 ± 0.7	0.8 ± 0.2	0.875 ± 0.006	30 ± 1	1.4 ± 0.1	0.91 ± 0.01
C/CoO x
0.6	18 ± 1	0.15 ± 0.03	0.956 ± 0.003	>10 (ref. 17)	—	—
0.8	16 ± 1	0.30 ± 0.05	0.958 ± 0.003	>10 (ref. 15)	—	—
0.9	14 ± 1	0.38 ± 0.06	0.952 ± 0.003	350 ± 40	0.34 ± 0.06	0.922 ± 0.002
1.0	8.2 ± 0.8	0.30 ± 0.03	0.929 ± 0.007	40 ± 0.5	0.39 ± 0.07	0.901 ± 0.095

---

Three potential domains with different electrochemical behaviours for C/PPN⁺-CoO_x and C/CoO_x were observed by EIS, the latter being correlated with their CVs: the regions around +0.2 V, between +0.6 and +0.8 V and above +0.9 V. At a potential of +0.2 V, the impedance spectra of C/PPN⁺-CoO_x and C/CoO_x could not be correctly fitted because the impedance measured at this potential is very high and different from those of the other spectra obtained with potentials between +0.6 and +1.0 V (Fig. 3C and S5C†). Therefore only the values of the equivalent circuit elements obtained between +0.6 and +1.0 V are reported in Table 1. This corroborates the fact that no redox process of cobalt is observable in the CVs of C/PPN⁺-CoO_x and C/CoO_x in the region of +0.2 V (Fig. 2D) and both electrodes are mainly insulating in this potential region. Hence, EIS measurements confirm that the conductivity within the PPN⁺-CoO_x film is only ensured by the cobalt oxide particles through the various oxidation states of cobalt (i.e. Co^II/Co^III and Co^III/Co^IV).

In the region between +0.6 and +0.8 V, the EIS spectra allow recovery of only the C_dl and R_tr parameters which are here related to the Co^II/Co^III redox system. Since the water oxidation catalysis is weak or non-operative between +0.6 and +0.8 V, R_sub and C_sub cannot be determined. Otherwise, EIS measurements reveal that, irrespective of the potential, the R_tr resistance is weaker for the C/PPN⁺-CoO_x electrode than for the C/CoO_x electrode, and conversely, the C_dl value of C/PPN⁺-CoO_x is higher than those of C/CoO_x. At ⁺0.6 V in the Co(II)/Co(III) redox process (see Fig. 2D), the C_dl value of C/PPN⁺-CoO_x (670 μF cm⁻²) is 4.5 times greater than that of C/CoO_x (150 μF cm⁻²). Therefore, EIS measurements confirm that C/PPN⁺-CoO_x exhibits greater nanostructuration than C/CoO_x. At +0.8 V, C_dl values drastically increase to 1100 and 300 μF cm⁻², respectively, for C/PPN⁺-CoO_x and C/CoO_x, most probably due to further oxidation of cobalt active sites leading to the onset of the OER (Fig. 2D).¹³

For potentials above +0.9 V, all the parameters of the equivalent circuit were recovered from the spectra. Indeed, the R_subC_sub loop in the equivalent circuit correctly models the interfacial charge transfer with the surface intermediates produced during the OER,¹³ with the latter operating efficiently above +0.9 V. C_dl values are higher at +0.9 V (1200 μF cm⁻² for C/PPN⁺-CoO_x and 380 μF cm⁻² for C/CoO_x) than those at +0.6 and +0.8 V for both films for the reason mentioned above. It is worth noting that, when the potential goes from 0.9 to 1.0 V, the decrease in C_dl (800 μF cm⁻² for C/PPN⁺-CoO_x and 300 μF cm⁻² for C/CoO_x at 1.0 V) was ascribed by Bisquert et al.⁷³ to the strong release of O₂ bubbles obtained at the higher potentials that diminish the electrode surface area exposed to the substrate and thus reduce its capacitive behavior. The values of n₁ and n₂ are the parameters associated with the transition from the CPE to a pure capacity.⁷⁰ When n is close to 1, it means that we are approaching a pure capacity. Here, the n values are all greater than 0.87 indicating a relatively good homogeneity⁷⁰ of both film/solution interfaces, PPN⁺-CoO_x and CoO_x. This may reflect the fact that the catalytic sites are certainly distributed relatively homogeneously. In addition, the n value associated with the double-layer capacitance is slightly lower for the PPN⁺-CoO_x film, which shows that in the presence of the polymer, the roughness at the film/solution interface is slightly greater. This increase in the film surface roughness was also observed by scanning electron microscopy (SEM) (see below and Fig. S7†). Therefore it can be envisaged that, even if the catalytic sites are distributed in a homogeneous manner for the two interfaces, the roughness of the polymer lowers very slightly the value of n₁. The change in this parameter would then be an illustration of the increase in the surface area of the interface provided by the roughness of the polymer, this greater surface area inducing higher catalytic activity.

2.2. Characterization of cobalt oxide and poly(pyrrole-alkylammonium) cobalt oxide nanocomposites by AFM, TEM, SEM and XPS

For atomic force microscopy (AFM), transmission electron microscopy (TEM) and scanning electron microscopy (SEM) studies, the films of PPN⁺-CoO_x or CoO_x were deposited on ITO electrodes (surface of 1.0 cm²). ITO/PPN⁺-CoO_x and ITO/CoO_x electrodes were prepared with a charge of 56 mC for both the PPN⁺ and Co⁰ deposition to obtain a charge density of 56 mC cm⁻², similar to that used for the preparation of the C/PPN⁺-CoO_x and C/CoO_x electrodes (4 mC for a C electrode with a surface of 0.071 cm²) (see ESI† Section 3 for more details).

The surface topography of ITO/PPN⁺-CoO_x and ITO/CoO_x electrodes was studied by AFM (Fig. 4). The 3D and 2D AFM images for ITO/CoO_x display a homogeneous nodular topography composed of aggregated CoO_x particles with a root-mean square (r.m.s.) roughness of 10.2 nm (Fig. 4A and B), while the images of ITO/PPN⁺-CoO_x exhibit an ill-defined cauliflower-like topography with a similar r.m.s. roughness of 11.6 nm (Fig. 4D and E). Such a cauliflower topology is usually observed for electrodeposited polypyrrole films.^47,48,74 From the height profile analysis of the AFM image of ITO/CoO_x (Fig. 4C), the size of CoO_x nodules was estimated to be 58 ± 31 nm. For ITO/PPN⁺-CoO_x, nanoparticles of CoO_x are buried within the PPN⁺ film preventing the determination of the particle size by AFM. Secondary electron SEM images also confirmed the nodular and cauliflower morphologies observed for ITO/CoO_x and ITO/PPN⁺-CoO_x, respectively (Fig. S7†).

Fig. 4 3D and 2D AFM images in the peak force mode of (A and B) ITO coated with CoO_x (56 mC used for the deposition of Co⁰); (D and E) ITO coated with the PPN⁺-CoO_x nanocomposite (56 mC used for the deposition of PPN⁺ and for the subsequent deposition of Co⁰); (C and F) section analysis of the AFM images of ITO/CoO_x and C/PPN⁺-CoO_x shown respectively in (B) and (E).

To further characterize the size of CoO_x particles electrogenerated inside the PPN⁺ film or directly electrodeposited on the electrode surface, the PPN⁺-CoO_x and CoO_x materials were observed by TEM (Fig. 5). PPN⁺-CoO_x and CoO_x fragments of films were scraped off from the ITO plates and deposited on carbon-coated copper grids. TEM images reveal large agglomerates (>100 nm) of CoO_x nanoparticles for ITO/CoO_x (Fig. 5A), while for ITO/PPN⁺-CoO_x the cobalt nanoparticles are not aggregated and are well-dispersed in the polymer film with an average particle size of 31 ± 20 nm (Fig. 5B). These images clearly show that the PPN⁺ polymer prevents the nanoparticle agglomeration, resulting in a larger electrochemically active surface area of PPN⁺-CoO_x compared to that obtained from a direct electrodeposition of CoO_x on the electrode surface.

Fig. 5 TEM images of (A) CoO_x (56 mC used for Co⁰ deposition on ITO, scale 100 nm) and (B) PPN⁺-CoO_x nanocomposite (56 mC used for PPN⁺ deposition and subsequent Co⁰ deposition on ITO, scale 100 nm).

The nature of the electrogenerated cobalt oxide particles within the PPN⁺ film, after 5 cycles between 0 and +1.2 V in 0.1 M borate buffer (pH 9.2), was also studied by X-ray photoelectron spectroscopy (XPS) measurements (Fig. 6 and S6†). For XPS analysis, PPN⁺-CoO_x was electrodeposited on carbon pellets (6 mm of diameter, denoted as C_pel) which can be easily removed from the electrode support by unscrewing and placed in the vacuum chamber of the spectrometer (see ESI† Section 3 for their preparation and Section 10 for XPS measurement). High resolution XPS spectroscopy of the Co 2p_3/2 region reveals a broad signal, which could be deconvoluted into a series of peaks corresponding to various transitions (shake-up, plasmon loss, satellites, etc.) due to the complex nature of this transition metal (Fig. 6). The deconvolution of the peak at ca. 781 eV indicates that the cobalt deposit for the C_pel/PPN⁺-CoO_x anode could be a mixture of a mixed valence cobalt oxide (Co₃O₄) and a cobalt oxyhydroxide (CoOOH).⁷⁵ Foelske et al.⁷⁶ demonstrated by coupling electrochemical experiments with XPS measurements that poising an electrode of metallic cobalt at a potential superior to +1.18 V vs. Ag/AgCl in a borate buffer at pH 9.3 results in the formation of a Co₃O₄ layer on its surface. In parallel, the group of Spiccia⁵⁹ demonstrated by X-ray absorption fine structure measurements (EXAFS) that cobalt oxide, electrodeposited in a borate buffer (pH 9.2) by oxidizing a cobalt complex, closely resembles the heterogeneous CoOOH phase. Given the similarities between our study and that of Spiccia⁵⁹ in the electrochemical behavior of CoO_x in borate buffer at the same pH 9.2 (see discussion above), as well as with the study of Foelske⁷⁶ in the electrogeneration of CoO_x from Co⁰ in borate buffer, we can assume that our cobalt oxide within PPN⁺ could indeed contain a mixture of both Co₃O₄ and CoOOH species.

Fig. 6 XPS spectra (Co 2p_3/2 region) of the electrode surfaces of C_pel/PPN⁺-CoO_x after 5 cycles between 0 and +1.2 V vs. Ag/AgCl in a 0.1 M borate buffer (pH 9.2).

2.3. Electrocatalytic performance of cobalt oxide and poly(pyrrole-alkylammonium) cobalt oxide nanocomposite electrodes

The electrocatalytic activity of C/PPN⁺-CoO_x and C/CoO_x electrodes has been investigated by cyclic voltammetry under mild basic conditions in a borate buffer solution at pH 9.2 (Fig. 2). Table 2 summarizes the catalytic OER performance of these modified electrodes and that of C/PPN⁺-NiO_x recently reported by our group⁴⁸ in a 0.1 M borate buffer solution at pH 9.2 together with those of cobalt oxide-based electrodes reported in the literature in a similar medium. In all reported cases, cobalt oxide was electrogenerated on the surface of fluorine doped tin oxide electrodes (FTO). Developing electrolyzers operating at nearly neutral pH, such as pH 9.2, is of utmost importance in view of avoiding the corrosion issue under strongly acidic or alkaline conditions.¹¹ Herein, the C/PPN⁺-CoO_x anode exhibits a j value of 4.04 mA cm⁻² at a η of +0.61 V and a η value of +0.44 V at 1 mA cm⁻² with a cobalt mass loading of 1.34 μg cm⁻². This performance places our nanocomposite anode among the best reported cobalt oxide-based anodes for water oxidation operating in a 0.1 M borate buffer solution at pH 9.2.

Table 2 OER performances of C/PPN⁺-CoO_x and C/CoO_x anodes prepared in this work compared to those of C/PPN⁺-NiO_x and FTO/CoO_x anodes previously reported with a 0.1 M borate buffer solution at pH 9.2

Catalyst/electrode	j (mA cm^−2)@η (V)	η (V)@j (mA cm^−2)	Metal mass loading (μg cm^−2)	Mass activity (A mg^−1)@η (V)	TOFmin (s^−1)@η (V)	Author et al. (ref.)
a Values were extrapolated from the cyclic voltammograms reported by the authors. b Data non calculated by the authors but estimated herein from the current density (j) at defined overpotentials (η) and the mass loading of Co given in the article. c The current density was measured from cyclic voltammetry corrected for the ohmic drop. nd: not determined. FTO: fluorine doped tin oxide electrode.
C/PPN^+-CoOx	4.04@0.61	0.44@1	1.34	3.01@0.61	0.46@0.61	This work
C/PPN^+-CoOx	2.17@0.51		1.34	1.62@0.51	0.25@0.51	This work
C/PPN^+-CoOx	0.53@0.40		1.34	0.40@0.40	0.06@0.40	This work
C/CoOx	3.40@0.61	0.46@1	2.99	1.14@0.61	0.17@0.61	This work
C/CoOx	1.32@0.51		2.99	0.44@0.51	0.07@0.51	This work
C/CoOx	0.22@0.40		2.99	0.07@0.40	0.01@0.40	This work
C/PPN^+-NiOx	2.17@0.61	0.51@1	1.9	1.12@0.61	0.17@0.61	Fortage^48
FTO/CoOx	2.89^a@0.51	0.37^a@1	nd	nd	nd	Nocera^24
FTO/CoOx	0.41@0.40	0.53@1	46.0	0.009^b@0.40	0.001^b@0.40	Nocera^28
FTO/CoOx	1.90^a@0.61	0.51^a@1	1.11	1.71^b@0.61	0.26^b@0.61	Spiccia^59
FTO/CoOx	0.39^a@0.61	0.75^a@1	0.003	130^b@0.61	20@0.61	Sun^79
FTO/CoOx	2.60^c@0.61	nd	nd	nd	nd	Du^77
FTO/CoOx	1.40^c@0.61	0.51^a@1	3.53	0.40^b@0.61	0.06^b@0.61	Du^78

---

For comparison, the C/CoO_x anode presents a j value of 3.40 mA cm⁻² at a η of +0.61 V and a η value of +0.46 V at 1 mA cm⁻² with a cobalt mass loading of 2.99 μg cm⁻². The C/PPN⁺-CoO_x nanocomposite electrode presents a higher j and a lower η values compared to those of C/CoO_x with ∼2.2 times less cobalt loading, demonstrating the beneficial effect of using a poly(pyrrole-alkylammonium) matrix to enhance the nanostructuration of the electrode material and thus its electrocatalytic performance. Of note, in the three compartment cell, the resistance R between the anode (i.e. the working electrode) and the Ag/AgCl reference, which causes the ohmic drop (related to the iR value) in the cell, could be different between C/PPN⁺-CoO_x and C/CoO_x, making the comparison of their electrocatalytic performance unfair. In view of suppressing the ohmic drop and making a fair comparison, the resistance R in the cell has been determined to be 1 and 4.6 Ω respectively with C/PPN⁺-CoO_x and C/CoO_x. Since these R values are low, the CVs and the catalytic current density of both electrodes are weakly affected by the iR correction (see in the ESI the CVs with and without iR correction; Fig. S2†), which means that the previous comparison of the catalytic activities of both anodes was correct. According to TEM images and EIS measurements, C/PPN⁺-CoO_x contains small CoO_x particles (31 nm) with a capacitance of 670 μF cm⁻², while C/CoO_x exhibits big agglomerates of CoO_x particles (>100 nm) associated with a lower capacitance of 150 μF cm⁻² (see above). Hence the higher catalytic activity of the C/PPN⁺-CoO_x nanocomposite compared to the direct deposition C/CoO_x can be directly correlated with a greater nanostructuration of the nanocomposite that increases the electrochemically active surface area of the anode and ensures a great accessibility of the active catalytic sites towards substrates such as water or OH⁻ ions. The higher values of mass activity and turnover frequency (TOF) of C/PPN⁺-CoO_x (3.01 A mg⁻¹ and 0.46 s⁻¹) compared to those of C/CoO_x (1.14 A mg⁻¹ and 0.17 s⁻¹) at pH 9.2 at an overpotential of 0.61 V confirm the higher OER catalytic activity of the nanocomposite PPN⁺-CoO_x material (Table 2). In order to facilitate the comparison with the electrocatalytic performance of previously reported anodes based on cobalt oxide operating at pH 9.2 with a 0.1 M borate buffer (Table 2), the CVs of C/PPN⁺-CoO_x have been recorded at different scan rates between 5 and 100 mV s⁻¹ (Fig. S3†), since various scan rates have been used to measure the performance of the reported anodes. The CVs of C/PPN⁺-CoO_x are weakly impacted by the scan rate value, indicating that the diffusion of the borate buffer in the polypyrrole film is efficient. In other words, the electrocatalytic activity of C/PPN⁺-CoO_x measured with a scan rate of 50 mV s⁻¹ is reliable to make a fair comparison with those of the reported anodes. It appears that the mass activity and TOF values of the C/PPN⁺-CoO_x electrode are among the highest in the literature (Table 2). Otherwise, although the CoO_x films of the literature have been realized with the same range of mass loadings (1–4 μg cm⁻²) as that of the PPN⁺-CoO_x nanocomposite, the comparison of the performances of these anodes is not easy and could be erroneous due to the fact all the oxide deposits reported in Table 2 were made on an FTO electrode (i.e. fluorine tin oxide) as a conductive support, whose conductivity is lower than that of a carbon electrode, thus possibly decreasing the overall performance of the anodes. Otherwise, it is worth noting that the comparison of the electrocatalytic activities of anodes, for which the ohmic drop correction is applied, such as those reported by Du^77,78 in Table 2, is relevant. One can see that the catalytic currents of C/PPN⁺-CoO_x and C/CoO_x (respectively 4.04 and 3.40 mA cm⁻² at η = 0.61 V) are higher than those of the FTO/CoO_x anodes (2.60 and 1.40 mA cm⁻² at η = 0.61 V),^77,78 in spite of the iR correction and the higher cobalt loading on FTO/CoO_x in ref. 78 (3.53 μg cm⁻²vs. 1.34 and 2.99 μg cm⁻² for C/PPN⁺-CoO_x and C/CoO_x respectively). It is also interesting to note that the OER performance of C/PPN⁺-CoO_x is greater than that of the analogous C/PPN⁺-NiO_x previously reported by our group,⁴⁸ which displays lower values of j (2.17 mA cm⁻²), mass activity (1.12 A mg⁻¹) and TOF (0.17 s⁻¹) for an overpotential of 0.61 V. If we consider a similar nanostructuration for C/PPN⁺-CoO_x and C/PPN⁺-NiO_x anodes (particle sizes of CoO_x and NiO_x (ref. 48) within PPN⁺ of 31 and 21 nm, respectively) and a lower metal loading of cobalt compared to that of nickel (Γ_Co ∼ 23 nmol cm⁻²vs. Γ_Ni ∼ 33 nmol cm⁻²),⁴⁸ CoO_x particles display a higher intrinsic catalytic activity than NiO_x for water oxidation in borate buffer at pH 9.2.

The stability of C/PPN⁺-CoO_x and C/CoO_x electrodes at pH 9.2 in a 0.1 M borate buffer was evaluated by chronoamperometry measurements over a period of about 2 h at an applied potential of +1.2 V vs. Ag/AgCl (Fig. 7). The current density of the C/CoO_x electrode decreases from 6 to 5.30 mA cm⁻² after 5 min of electrolysis and then gradually decreases to reach a pseudo-plateau at 4.90 mA cm⁻² after 2.5 h of electrolysis. Meanwhile the current density of C/PPN⁺-CoO_x increases during the first hour from 7.16 to 7.80 mA cm⁻² and then gradually decreases to reach a pseudo-plateau at 7.00 mA cm⁻² after 2.5 h of electrolysis. Over this time range, the catalytic current of C/PPN⁺-CoO_x is relatively stable and 1.43 times higher than that of C/CoO_x, even though there is 2.2 times less cobalt in the former. Once again, this demonstrates the superiority of the nanocomposite electrode over a simple Co-based electrode.

Fig. 7 Chronoamperograms recorded at the C/PPN⁺-CoO_x (Γ_N⁺ = 1.2 ± 0.1 × 10⁻⁷ mol cm⁻² and Γ_Co = 2.27 ± 0.45 × 10⁻⁸ mol cm⁻²) and C/CoO_x (Γ_Co = 5.07 ± 0.33 × 10⁻⁸ mol cm⁻²) electrodes (3 mm of diameter) in a 0.1 M borate buffer solution (pH 9.2), at a constant potential of +1.2 V vs. Ag/AgCl under stirring.

Unfortunately, the current density of C/PPN⁺-CoO_x decreases suddenly and drastically after 2.5 h of electrolysis reaching 2.36 mA cm⁻² after 6 h of electrolysis, whereas the current density of C/CoO_x decreases slowly until 4.3 mA cm⁻² over the same period of time. As we previously observed for the C/PPN⁺-NiO_x anode,⁴⁸ the decrease of catalytic current with C/PPN⁺-CoO_x is not due to the decomposition of the material or to the gradual release of cobalt oxide particles but to the partial film detachment from the carbon electrode induced by the oxygen bubbles released during the electrolysis (see in Picture S1 in the ESI,† the view of a C/PPN⁺-CoO_x electrode before and after the electrolysis for 6 h, and the view of the PPN⁺-CoO_x film floating in the borate electrolyte after its partial detachment from the C electrode induced by the electrolysis). Note that the relatively good stability of C/CoO_x (i.e. without PPN⁺) could be ascribed to the good physisorption of the cobalt oxide on the carbon electrode and also to the self-healing phenomenon occurring during electrocatalytic water oxidation with cobalt oxide in a borate buffer.^25,80,81

To avoid the film detachment from the electrode surface under prolonged electrolysis at +1.2 V, the physisorption of the polypyrrole film on the electrode has been increased by the electrodeposition of the PPN⁺-CoO_x material on a carbon paper electrode (denoted as C_pap) with a high roughness and a larger surface of 2.4 cm², following the same procedure used for the preparation of the PPN⁺-CoO_x nanocomposite on a small glassy carbon electrode (see Section 3 in the ESI for its preparation and Fig. S4A†). The deposition of the PPN⁺-CoO_x film on the C_pap electrode was evidenced by SEM images (Fig. 8A and B) and EDX spectroscopy (Fig. 8C) by comparison with a pristine C_pap electrode (Fig. S8†). The EDX spectrum of C_pap exhibits only the characteristic peaks of C and O atoms respectively at 0.28 and 0.50 keV (Fig. S8C†), while the EDX spectrum of C/PPN⁺-CoO_x displays broader peaks below 0.6 keV ascribed to the C, N and O atoms of the overoxidized polypyrrole film, in addition to the C/O peaks of C_pap (see above), along with the typical peaks of cobalt at 0.78 (Lα) and 6.92 keV (Kα) (Fig. 8C). As observed above on the CV of C/PPN⁺-CoO_x recorded in a borate buffer at pH 9.2 (Fig. 2), C_pap/PPN⁺-CoO_x displays the redox signature of Co^II/Co^III between 0.3 and 0.8 V vs. Ag/AgCl with a strong catalytic current attributed to water oxidation to O₂, reaching 3.6 mA cm⁻² at 1.2 V vs. Ag/AgCl (Fig. 9 and S4†). Otherwise, on this rough support, no PPN⁺-CoO_x film detachment is observed and the catalytic current of C_pap/PPN⁺-CoO_x is stable over 43 h of electrolysis at a constant potential of +1.2 V vs. Ag/AgCl in a borate buffer at pH 9.2 (Fig. 10). In addition, the stability of the catalytic current is a good indication of the stability of the CoO_x nanoparticles which remain embedded in the PPN⁺ film during electrocatalysis. This is confirmed by the post-electrolysis characterizations of the PPN⁺-CoO_x material deposited on C_pap.

Fig. 8 SEM images of the C_pap/PPN⁺-CoO_x electrode at (A) 200× magnification and (B) 1000× magnification and (C) corresponding EDX spectrum.

Fig. 9 Cyclic voltammograms recorded at C_pap/PPN⁺-CoO_x (red, Γ_N⁺ = 1.53 ± 0.03 × 10⁻⁷ mol cm⁻² and Γ_Co = 1.04 ± 0.16 × 10⁻⁸ mol cm⁻²), C_pap/CoO_x (blue, Γ_Co = 3.70 ± 0.27 × 10⁻⁸ mol cm⁻²) and C_pap (black) electrodes (2.4 cm²) in a 0.1 M borate buffer solution (pH 9.2); scan rate of 50 mV s⁻¹.

Fig. 10 Chronoamperograms recorded at C_pap/PPN⁺-CoO_x (red, Γ_N⁺ = 1.53 ± 0.03 × 10⁻⁷ mol cm⁻² and Γ_Co = 1.04 ± 0.16 × 10⁻⁸ mol cm⁻²), C_pap/CoO_x (blue, Γ_Co = 3.70 ± 0.27 × 10⁻⁸ mol cm⁻²) and C_pap (black) electrodes (2.4 cm²) in a 0.1 M borate buffer solution (pH 9.2) at a constant potential of +1.2 V vs. Ag/AgCl maintained for ca. 43 h under stirring.

Indeed, the stability of the film on C_pap allows us to perform post-electrolysis characterization by SEM and EDX on this electrode material. Post-electrolysis characterization on ITO electrodes was not possible due to the detachment of the PPN⁺-CoO_x film under electrolysis at +1.2 V. The SEM images and EDX spectra of C_pap/PPN⁺-CoO_x before (Fig. 8) and after 43 h of electrolysis (Fig. 11 and 12) are very similar. The EDX spectra exhibit the same signals with similar intensity for the C/N/O peaks below 0.6 keV and for cobalt at 0.78 and 6.92 keV (Fig. 12). This result strongly indicates that the overoxidized PPN⁺ film and the embedded CoO_x nanoparticles remain unchanged even after being maintained at +1.2 V vs. Ag/AgCl in a borate buffer at pH 9.2 over such a long period of time. The PPN⁺ film thus imparts a great stability to the nanocomposite material by preventing the corrosion and the degradation of CoO_x nanoparticles, in addition to the phenomenon of self-healing that is known for cobalt oxide in a borate buffer.^25,80,81 We also verified the formation of oxygen by gas chromatography. A high faradaic yield of 97% towards O₂ evolution was measured after 2 h of electrolysis (see ESI† Section 12), confirming that C_pap/PPN⁺-CoO_x is an efficient and selective anode for water oxidation to O₂.

Fig. 11 SEM image of the C_pap/PPN⁺-CoO_x electrode: (A) 200× magnification, (B) 2000× magnification and (C) 500× magnification, along with zones 1 and 2 for which EDX analysis (see below Fig. 12) has been performed on the C_pap/PPN⁺-CoO_x electrode after electrolysis for 43 h at 1.2 V vs. Ag/AgCl in a 0.1 M borate buffer (pH 9.2).

Fig. 12 EDX spectra of the SEM images respectively in Fig. 11 of C_pap/PPN⁺-CoO_x before (red) and after electrolysis for 43 h at +1.2 V vs. Ag/AgCl in a 0.1 M borate buffer (pH 9.2) performed in zone 1 (black) and in zone 2 (green).

To compare the performance of the PPN⁺-CoO_x nanocomposite with that of a simple CoO_x film, CoO_x was also electrodeposited on the rough C_pap electrode (without PPN⁺). The CV of C_pap/CoO_x exhibits a similar Co^II/Co^III signature between 0.3 and 0.8 V vs. Ag/AgCl, but despite a cobalt loading 3.5 times higher (3.70 ± 0.27 × 10⁻⁸ mol cm⁻²) than that on C_pap/PPN⁺ (1.04 ± 0.16 × 10⁻⁸ mol cm⁻²), the corresponding catalytic current density of 2.37 mA cm⁻² at 1.2 V remains lower than that of C_pap/PPN⁺-CoO_x (3.61 mA cm⁻²) (Fig. 9 and S4†), further demonstrating the benefits of using PPN⁺ films. It is interesting to note that, after a long electrolysis for 43 h at 1.2 V vs. Ag/AgCl in a borate buffer at pH 9.2, the catalytic current of C_pap/CoO_x is also very stable (Fig. 10) as previously observed on a small carbon electrode with C/CoO_x, most probably due to the self-healing phenomenon.²⁵

To obtain some mechanistic information, Tafel analysis was performed through stepwise chronoamperometry at pH 9.2 on C/PPN⁺-CoO_x and C/CoO_x electrodes (Fig. 13, see Section 7 in the ESI for more details†). A Tafel slope value of 56 mV dec⁻¹ was determined for C/CoO_x corresponding to a fast equilibrated electron transfer. The catalytic process is thus not controlled by an electron transfer but by a chemical step following this electron transfer.^13,26,82 Similar values were reported with cobalt oxide catalysts in borate buffer at pH 9.2 by several groups such as those of Nocera,²⁸ Du^77,78 and Spiccia.⁵⁹ For C/PPN⁺-CoO_x, a Tafel slope of 65 mV dec⁻¹, close to that of C/CoO_x, was estimated within a region of corrected overpotentials between +0.41 and +0.47 V (i.e. within a potential region between +0.9 and +1.02 V vs. Ag/AgCl). Above +0.47 V, the Tafel plot of C/PPN⁺-CoO_x deviates from linearity most probably due to the limitations by mass transport of the buffer within the polypyrrole film, which could not be easily obviated by the rotation of the electrode.⁸³ However, the similar Tafel slopes of C/PPN⁺-CoO_x and C/CoO_x confirm that the O₂ evolution occurring with these two electrodes involves the same catalytic mechanism.

Fig. 13 Tafel plots recorded at the C/PPN⁺-CoO_x (Γ_N⁺ = 1.2 ± 0.1 × 10⁻⁷ mol cm⁻² and Γ_Co = 2.27 ± 0.45 × 10⁻⁸ mol cm⁻²) (A) and C/CoO_x (Γ_Co = 5.07 ± 0.33 × 10⁻⁸ mol cm⁻²) (B) rotating disc electrodes (3 mm of diameter) in a 0.1 M borate buffer solution (pH 9.2) under rotation (1200 rpm).

3. Conclusion

We prepared a very efficient OER anode material based on a cobalt oxide-poly(pyrrole-alkylammonium) nanocomposite deposited on an electrode surface by an easy all-electrochemical procedure. TEM images and EIS measurements evidenced the high nanostructuration of the PPN⁺-CoO_x nanocomposite owing to the small CoO_x nanoparticles of ca. 30 nm well dispersed and not aggregated in the polypyrrole film. The nature of the cobalt oxide has also been identified by XPS as potentially being a mixture of Co₃O₄/CoOOH. By comparison with CoO_x directly deposited on an electrode surface (CoO_x particles > 100 nm), we demonstrated that the high nanostructuration of the composite material is the origin of the higher OER electrocatalytic performances. The nanocomposite material exhibits an exceptional mass activity of 3.01 A mg⁻¹ and TOF values of 0.46 s⁻¹ along with a faradaic yield of 97% for water oxidation at mildly basic pH 9.2 at an overpotential of 0.61 V, and a Tafel slope of 65 mV dec⁻¹. The cobalt oxide–polypyrrole nanocomposite electrode is among the most efficient cobalt oxide-based anodes previously reported operating in a 0.1 M borate buffer solution at pH 9.2, but it also outperforms cobalt oxide electrogenerated on a naked electrode surface by the same electrochemical procedure. In addition, when the PPN⁺-CoO_x material is electrodeposited on a rough carbon paper, the physisorption of the nanocomposite film is considerably enhanced and consequently its catalytic activity is very stable beyond 43 h of electrolysis. Post-electrolysis characterization by SEM and EDX also confirms the integrity of the PPN⁺-CoO_x material after many hours of electrocatalysis. This work demonstrates the beneficial role of a positively charged polypyrrole matrix in the preparation of small particles of cobalt oxide and in the achievement of a highly stable and active anode for water oxidation. The elaboration of such a polypyrrole-metal oxide nanocomposite material could be easily extended to other abundant first-row transition metals such as manganese and iron, and also could be useful to prepare mixed metal oxides such as NiFeO_x or CoFeO_x, which are the most efficient OER catalysts under alkaline or mildly basic conditions.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

ANID (ex Becas-Chile-Conicyt) “Programa de formación de capital humano avanzado” for the PhD fellowship of D. V. M. (No. 72150091) and C. N. A. (No. 72170399), ANID/FONDECYT Postdoctoral project of D. V. M. (Grant No. 3200467), “Université Grenobles Alpes” for the PhD fellowship of B. D., The CMTC platform of “Institut Poolythechinique de Grenoble” for SEM-EDX measurements, the NanoBio-ICMG Platform (UAR 2607, Grenoble) for granting access to the Electron Microscopy facility, the ECOS-CONICYT exchange program (C13E01), Ministères de l'Europe et des Affaires Etrangères (MEAE) et de l'Enseignement Supérieur, de la Recherche et de l’Innovation (MESRI) (PHC Brancusi program (43546QA)) and the COST CM1202 program (PERSPECT H₂O) are acknowledged. This work has been partially supported by LabEx Arcane and CBH-EUR-GS (ANR-17-EURE-0003).

References

1. A. A. Gewirth and M. S. Thorum, Inorg. Chem., 2010, 49, 3557.
2. O. Z. Sharaf and M. F. Orhan, Renew. Sustain. Energy Rev., 2014, 32, 810.
3. N. Armaroli and V. Balzani, ChemSusChem, 2011, 4, 21.
4. T. R. Cook, D. K. Dogutan, S. Y. Reece, Y. Surendranath, T. S. Teets and D. G. Nocera, Chem. Rev., 2010, 110, 6474.
5. F. Ausfelder and A. Bazzanella, in Hydrogen Science and Engineering: Materials, Processes, Systems and Technology, ed. D. Stolten and B. Emonts, Wiley-VCH, 2016, vol. 1, p. 19.
6. K. Ayers, N. Danilovic, R. Ouimet, M. Carmo, B. Pivovar and M. Bornstein, Annu Rev Chem Biomol, 2019, 10, 219.
7. K. E. Ayers, E. B. Anderson, C. B. Capuano, B. D. Carter, L. T. Dalton, G. Hanlon, J. Manco and M. Niedzwiecki, ECS Trans., 2010, 33, 3.
8. J. Mergel, D. L. Fritz and M. Carmo, in Hydrogen Science and Engineering : Materials, Processes, Systems and Technology, ed. D. Stolten and B. Emonts, Wiley-VCH, 2016, vol. 1, p. 331.
9. D. Zhang and K. Zeng, in Hydrogen Science and Engineering : Materials, Processes, Systems and Technology, ed. D. Stolten and B. Emonts, Wiley-VCH, 2016, vol. 1, p. 283.
10. G. Gahleitner, Int. J. Hydrogen Energy, 2013, 38, 2039.
11. P. Li, R. Zhao, H. Chen, H. Wang, P. Wei, H. Huang, Q. Liu, T. Li, X. Shi, Y. Zhang, M. Liu and X. Sun, Small, 2019, 15, 1805103.
12. J. O. M. Bockris, J. Chem. Phys., 1956, 24, 817.
13. R. L. Doyle, I. J. Godwin, M. P. Brandon and M. E. G. Lyons, PhysChemChemPhys, 2013, 15, 13737.
14. M. D. Kärkäs, O. Verho, E. V. Johnston and B. Åkermark, Chem. Rev., 2014, 114, 11863.
15. J. D. Blakemore, R. H. Crabtree and G. W. Brudvig, Chem. Rev., 2015, 115, 12974.
16. P. Benson, G. W. D. Briggs and W. F. K. Wynne-Jones, Electrochim. Acta, 1964, 9, 281.
17. V. Y. Shafirovich and V. V. Strelets, Nouv. J. Chim., 1978, 2, 199.
18. S. M. Jasem and A. C. C. Tseung, J. Electrochem. Soc., 1979, 126, 1353.
19. L. D. Burke, M. E. Lyons and O. J. Murphy, J. Electroanal. Chem., 1982, 132, 247.
20. Y. W. D. Chen and R. N. Noufi, J. Electrochem. Soc., 1984, 131, 731.
21. S. P. Jiang and A. C. C. Tseung, J. Electrochem. Soc., 1991, 138, 1216.
22. E. B. Castro, C. A. Gervasi and J. R. Vilche, J. Appl. Electrochem., 1998, 28, 835.
23. M. W. Kanan and D. G. Nocera, Science, 2008, 321, 1072.
24. Y. Surendranath, M. Dinca and D. G. Nocera, J. Am. Chem. Soc., 2009, 131, 2615.
25. D. A. Lutterman, Y. Surendranath and D. G. Nocera, J. Am. Chem. Soc., 2009, 131, 3838.
26. Y. Surendranath, M. W. Kanan and D. G. Nocera, J. Am. Chem. Soc., 2010, 132, 16501.
27. M. W. Kanan, J. Yano, Y. Surendranath, M. Dinca, V. K. Yachandra and D. G. Nocera, J. Am. Chem. Soc., 2010, 132, 13692.
28. A. J. Esswein, Y. Surendranath, S. Y. Reece and D. G. Nocera, Energy Environ. Sci., 2011, 4, 499.
29. J. B. Gerken, E. C. Landis, R. J. Hamers and S. S. Stahl, ChemSusChem, 2011, 3, 1176.
30. J. B. Gerken, J. G. McAlpin, J. Y. C. Chen, M. L. Rigsby, W. H. Casey, R. D. Britt and S. S. Stahl, J. Am. Chem. Soc., 2011, 133, 14431.
31. Y. Liang, Y. Li, H. Wang, J. Zhou, J. Wang, T. Regier and H. Dai, Nat. Mater., 2011, 10, 780.
32. Y. Surendranath, D. A. Lutterman, Y. Liu and D. G. Nocera, J. Am. Chem. Soc., 2012, 134, 6326.
33. S. Cobo, J. Heidkamp, P.-A. Jacques, J. Fize, V. Fourmond, L. Guetaz, B. Jousselme, V. Ivanova, H. Dau, S. Palacin, M. Fontecave and V. Artero, Nat. Mater., 2012, 11, 802.
34. C. L. Farrow, D. K. Bediako, Y. Surendranath, D. G. Nocera and S. J. L. Billinge, J. Am. Chem. Soc., 2013, 135, 6403.
35. T. Y. Ma, S. Dai, M. Jaroniec and S. Z. Qiao, J. Am. Chem. Soc., 2014, 136, 13925.
36. Y. Liu and D. G. Nocera, J. Phys. Chem. C, 2014, 118, 17060.
37. N. Jiang, B. You, M. Sheng and Y. Sun, Angew. Chem., Int. Ed., 2015, 54, 6251.
38. H. Kim, J. Park, I. Park, K. Jin, S. E. Jerng, S. H. Kim, K. T. Nam and K. Kang, Nat. Commun., 2015, 6, 8253.
39. Y. Zhao, S. Chen, B. Sun, D. Su, X. Huang, H. Liu, Y. Yan, K. Sun and G. Wang, Sci. Rep., 2015, 5, 7629.
40. G. Gardner, J. Al-Sharab, N. Danilovic, Y. B. Go, K. Ayers, M. Greenblatt and G. Charles Dismukes, Energy Environ. Sci., 2016, 9, 184.
41. H. Chen, Y. Gao and L. Sun, ChemCatChem, 2016, 8, 2757.
42. L. Han, S. Dong and E. Wang, Adv. Mater., 2016, 28, 9266.
43. J. Wang, W. Cui, Q. Liu, Z. Xing, A. M. Asiri and X. Sun, Adv. Mater., 2016, 28, 215.
44. Y. Chen, J. Hu, H. Diao, W. Luo and Y.-F. Song, Chem. Eur. J., 2017, 23, 4010.
45. I. Roger, M. A. Shipman and M. D. Symes, Nat. Rev. Chem., 2017, 1, 0003.
46. S. E. S. El Wakkad and A. Hickling, Trans. Faraday Soc., 1950, 46, 820.
47. Y. Lattach, J. F. Rivera, T. Bamine, A. Deronzier and J.-C. Moutet, ACS Appl. Mater. Interfaces, 2014, 6, 12852.
48. D. V. Morales, C. N. Astudillo, Y. Lattach, B. F. Urbano, E. Pereira, B. L. Rivas, J. Arnaud, J.-L. Putaux, S. Sirach, S. Cobo, J.-C. Moutet, M.-N. Collomb and J. Fortage, Catal. Sci. Technol., 2018, 8, 4030.
49. X. Cao, W. Yan, C. Jin, J. Tian, K. Ke and R. Yang, Electrochim. Acta, 2015, 180, 788.
50. L. Liu, Y. Hou, J. Wang, J. Chen, H.-K. Liu, Y. Wu and J. Wang, Adv. Mater. Interfaces, 2016, 3, 1600030.
51. Z. B. Shifrina, V. G. Matveeva and L. M. Bronstein, Chem. Rev., 2020, 120, 1350.
52. A. Zouaoui, O. Stephan, M. Carrier and J. C. Moutet, J. Electroanal. Chem., 1999, 474, 113.
53. A. Zouaoui, O. Stephan, A. Ourari and J. C. Moutet, Electrochim. Acta, 2000, 46, 49.
54. Y. Lattach, A. Deronzier and J.-C. Moutet, ACS Appl. Mater. Interfaces, 2015, 7, 15866.
55. D. W. Rice, P. B. P. Phipps and R. Tremoureux, J. Electrochem. Soc., 1979, 126, 1459.
56. B. H. R. Suryanto, X. Lu, H. M. Chan and C. Zhao, RSC Adv., 2013, 3, 20936.
57. C. Costentin, T. R. Porter and J.-M. Savéant, J. Am. Chem. Soc., 2016, 138, 5615.
58. C. N. Brodsky, D. K. Bediako, C. Shi, T. P. Keane, C. Costentin, S. J. L. Billinge and D. G. Nocera, ACS Appl. Energy Mater., 2019, 2, 3.
59. S. A. Bonke, M. Wiechen, R. K. Hocking, X.-Y. Fang, D. W. Lupton, D. R. MacFarlane and L. Spiccia, ChemSusChem, 2015, 8, 1394.
60. A. S. Liu and M. A. S. Oliveira, Mater. Res., 2007, 10, 205.
61. S. P. Ozkorucuklu, Y. Sahin and G. Alsancak, Sensors, 2008, 8, 8463.
62. T. W. Lewis, A Study of the Overoxidation of the Conducting Polymer Polypyrrole, Doctor of Philosophy thesis, Department of Chemistry, University of Wollongong, 1998, http://ro.uow.edu.au/theses/1107.
63. C. Costentin, T. R. Porter and J.-M. Savéant, ACS Appl. Mater. Interfaces, 2019, 11, 28769.
64. S. Trasatti and O. A. Petrii, Pure Appl. Chem., 1991, 63, 711.
65. M. W. Louie and A. T. Bell, J. Am. Chem. Soc., 2013, 135, 12329.
66. C. C. L. McCrory, S. Jung, J. C. Peters and T. F. Jaramillo, J. Am. Chem. Soc., 2013, 135, 16977.
67. A. Singh, M. Fekete, T. Gengenbach, A. N. Simonov, R. K. Hocking, S. L. Y. Chang, M. Rothmann, S. Powar, D. C. Fu, Z. Hu, Q. Wu, Y. B. Cheng, U. Bach and L. Spiccia, ChemSusChem, 2015, 8, 4266.
68. B. Klahr, S. Gimenez, F. Fabregat-Santiago, T. Hamann and J. Bisquert, J. Am. Chem. Soc., 2012, 134, 4294.
69. S. Anantharaj and S. Noda, ChemElectroChem, 2020, 7, 2297.
70. R. L. Doyle and M. E. G. Lyons, PhysChemChemPhys, 2013, 15, 5224.
71. M. E. Orazem, N. Pébère and B. Tribollet, J. Electrochem. Soc., 2006, 153, B129.
72. T. Pajkossy, Solid State Ionics, 2005, 176, 1997.
73. A. J. Terezo, J. Bisquert, E. C. Pereira and G. Garcia-Belmonte, J. Electroanal. Chem., 2001, 508, 59.
74. I. M. F. De Oliveira, J. C. Moutet and S. Hamarthibault, J. Mater. Chem., 1992, 2, 167.
75. M. C. Biesinger, B. P. Payne, A. P. Grosvenor, L. W. M. Lau, A. R. Gerson and R. S. C. Smart, Appl. Surf. Sci., 2011, 257, 2717.
76. A. Foelske and H.-H. Strehblow, Surf. Interface Anal., 2000, 29, 548.
77. A. Han, H. Wu, Z. Sun, H. Jia and P. Du, PhysChemChemPhys, 2013, 15, 12534.
78. H. Chen, Z. Sun, X. Liu, A. Han and P. Du, J. Phys. Chem. C, 2015, 119, 8998.
79. Q. Daniel, R. B. Ambre, B. Zhang, B. Philippe, H. Chen, F. Li, K. Fan, S. Ahmadi, H. Rensmo and L. Sun, ACS Catal., 2017, 7, 1143.
80. C. Costentin and D. G. Nocera, Proc. Natl. Acad. Sci., 2017, 114, 13380.
81. D. K. Bediako, A. M. Ullman and D. G. Nocera, in Solar Energy for Fuels, ed. H. Tüysüz and C. K. Chan, Springer International Publishing, Cham, 2016,  DOI:10.1007/128_2015_649, p. 173.
82. N.-T. Suen, S.-F. Hung, Q. Quan, N. Zhang, Y.-J. Xu and H. M. Chen, Chem. Soc. Rev., 2017, 46, 337.
83. S. Anantharaj, S. R. Ede, K. Karthick, S. Sam Sankar, K. Sangeetha, P. E. Karthik and S. Kundu, Energy Environ. Sci., 2018, 11, 744.

---

Footnotes

† Electronic supplementary information (ESI) available: See DOI: 10.1039/d1se00363a

‡ D. V. M., C. N. A. and V. A. contributed equally to this work.

---