A highly efficient electrocatalyst based on double perovskite cobaltites with immense intrinsic catalytic activity for water oxidation

Håkon Andersen a, Kaiqi Xu a, Dmitry Malyshkin bc, Ragnar Strandbakke *a and Athanasios Chatzitakis *a
aCentre for Materials Science and Nanotechnology, Department of Chemistry, University of Oslo, FERMiO, Gaustadalléen 21, NO-0349 Oslo, Norway. E-mail: a.e.chatzitakis@smn.uio.no; ragnar.strandbakke@kjemi.uio.no
bDepartment of Physical and Inorganic Chemistry, Institute of Natural Sciences and Mathematics, Ural Federal University, Ekaterinburg, 620000, Russia
cInstitute of High Temperature Electrochemistry, 620137 Ekaterinburg, Russia

Received 9th November 2019 , Accepted 16th December 2019

First published on 16th December 2019


High temperature electrocatalysts based on double perovskite cobaltites that are typically employed in proton ceramic fuel cells and electrolyzers are exploited here for room temperature water oxidation. The double perovskites are assessed by the RctCdl product and we show that their intrinsic catalytic activities exceed that of IrO2.


Converting and storing electrical energy into chemical is crucial in order to sustain the ever-growing energy demands, but it should be in a form that can mitigate the reliance on fossil fuels. Hydrogen from water electrolysis is a green, renewable and highly efficient fuel that can replace coal and hydrocarbons.1 The key challenge in water electrolysis is the sluggish kinetics of the oxygen evolution reaction (OER), which requires large overpotentials to assist a complex, four-electron process.2–4 On top of that, the currently efficient electrocatalysts (ECs) are based on precious metals (IrO2, RuO2), prohibiting the large scale use of water electrolysis for hydrogen production. To further highlight this drawback, we have recently calculated that the amount of Ir and Ru needed to cover 1 TW of hydrogen production from the state-of-the-art PEM electrolyzer, equals 180 and 12 years of the current annual Ir and Ru productions, respectively.5 Therefore, much of the current research is dedicated to the development of earth-abundant ECs for both the hydrogen evolution reaction (HER) and the OER.6

Inspired by recent advances in our group in Proton Ceramic Fuel Cells and Electrolyzers (PEFCs and PCEs), which operate typically at 350 to 650 °C,7,8 we here exploit the high temperature ECs for room temperature alkaline water electrolysis. In this work, we investigate a selection of double perovskite-based oxides as ECs for the OER in alkaline media at room temperature.9,10 The materials used are, BaPrCo1.4Fe0.6O6 (BPCF), BaGdCo1.8Fe0.2O6 (BGCF) and BaPrCo2O6 (BPC), and their activities for the OER are compared against IrO2. These double perovskite cobaltites with a layered structure have special properties due to their oxygen non-stoichiometry and variable oxidation state of Co (+2, +3 and +4).11–14 Furthermore, alternating layers of basic and acidic character may be beneficial for water splitting. Several recent examples from the literature show the great promise that perovskites hold as electrocatalysts for efficient water electrolysis based on earth-abundant elements.10,15–18 Consequently, there is an increasing global interest in perovskites due to their structural and compositional flexibility and versatility in several other applications, such as photovoltaics (PV), ferroelectrics, superconductors and ionic conductors.19–21

The double perovskite ECs were synthesized by the glycerol nitride method as described in our previous work, but more detailed information and characterization can be found in the ESI.7 All the electrochemical experiments were carried out in 1 M NaOH solution in a typical three-electrode set up. A rotating disc electrode (RDE) with glassy carbon (GC) as the tip (RDE710 Rotating Electrode from Gamry Instruments) was used as the working electrode, a standard calomel electrode (SCE) was used as the reference and a graphite rod as the counter electrodes, respectively. The standard potential of the SCE was measured and calibrated after each experiment. The RDE was coated by the perovskite and IrO2 (Sigma-Aldrich, CAS 12030-49-8) powders according to the procedure suggested by S. Jung et al.22 Briefly, the catalyst ink was prepared by adding 160 mg of powder in 8 mL water, 1 mL isopropanol and 40 μL of Nafion 5 wt% solution. The powder inks were sonicated for a few hours until a homogeneous suspension was obtained. The inks were drop-cast on the GC tip (0.196 cm2) by applying 40 μL of ink and then dried for 10 min at 60 °C. This procedure resulted in a loading of approx. 1 mg of the electrocatalyst on the GC. For long-term stability experiments, the same amount of powder (∼1 mg) was loaded by drop casting on Ni foam of a nominal surface area of 0.25 cm2; therefore the mass loading was 4 mg cm−2. In this case, Hg/HgO (1 M NaOH) was used as the reference electrode as SCE is not appropriate. All the electrochemical measurements were performed with a Gamry Reference 3000 potentiostat/galvanostat/ZRA. All overpotentials are given against the normal hydrogen electrode (NHE) taking into account that water electrolysis takes place thermodynamically at 1.23 V vs. NHE.

While a comprehensive material characterization can be found in our previous report as well as in the ESI,7 herein we focus on the performance of these high temperature ECs for water electrolysis under alkaline conditions at room temperature. In high temperature operation, these double perovskite ECs are used on the positive electrode (positrode) as the fuel cell cathode or anode for steam electrolysis. Herein, they are investigated in anodic operation for the OER. Fig. 1 shows the linear sweep voltammograms (LSV) of all the double perovskites, as well as that of the state-of-the-art IrO2 for comparison. The onset for IrO2 is at approx. 262 mV, while for BGCF it is at 373 mV and for BPCF and BPC it is at 254 and 296 mV, respectively. It is obvious that BPCF has the lowest onset overpotential among all ECs and even for IrO2. The picture changes a lot when we look at the overpotential for 10 mA cm−2. In this case, IrO2 needs 100 mV more from the onset overpotential, while BGCF, BPCF and BPC need 96, 176 and 184 mV, respectively.


image file: c9cc08765f-f1.tif
Fig. 1 LSV curves of the double perovskites and IrO2. The potential scanning rate was 10 mV s−1 in 1 M NaOH. The dashed lines are the IR corrected curves and are given for comparison purposes, as argued by S. Anantharaj et al.6

Therefore and in order to get more insights into the activity of these ECs, we need to look into their kinetics. The Tafel slopes were calculated by the LSV curves in Fig. 1 and are given in Table 1 and schematically presented in Fig. 2. It is evident and expected that IrO2 has the lowest Tafel slope, but BGCF has also a remarkably low Tafel slope of 60.1 mV dec−1. BPC and BPCF have much higher Tafel slopes, which explains the fact that although BPC and BPCF have low onset for the OER, their overpotentials for 10 mA cm−2 are much higher than for BGCF. The difference in performance between BPC and BPCF may be explained by the difference between iron and cobalt with respect to the electronic states that are involved in the oxidation reaction. The higher catalytic activity for BGCF as compared to the two Pr-based ECs, we attribute this to the larger non-stoichiometry on the oxygen sublattice for the former,7 providing oxygen vacancies as suitable active sites for water splitting.

Table 1 Electrochemical parameters of the different double perovskite cobaltites and IrO2
Catalyst Onset@0.3 mA cm−2 (mV vs. NHE)24,25 η@10 mA cm−2 (mV vs. NHE)a Tafel slope (mV dec−1) 1000 rpm R ct C dlτ (s)
a Not IR corrected.
BPC 296 480 96.5 1.3 × 10−3
BPCF 254 430 92 5.4 × 10−4
BGCF 373 477 60.1 1.4 × 10 −4
IrO2 262 361 50.1 5.8 × 10−3



image file: c9cc08765f-f2.tif
Fig. 2 Tafel slopes of the ECs as calculated from the LSV curves of Fig. 1. The non-IR corrected curves were used.

It should be mentioned that we checked whether these double perovskites can be used for the hydrogen evolution reaction (HER) too, as there are such bifunctional ECs reported in the literature.17,23 The results are given is Fig. S2 (ESI) and it can be seen that an overpotential as high as 535 mV for −10 mA cm−2 is required for BGCF, which shows the best HER activity among the studied double perovskites. Although the HER activity will no longer be considered here, it is of interest to change the stoichiometry of these perovskites and assess the HER activity in future studies.

The stability and steady state conditions of the LSV curves were assessed by galvanostatic experiments at 10 mA cm−2 on Ni foam (Fig. 3). From Fig. 3a it can be seen that the galvanostatic performance of IrO2 agrees well with the expected overpotential values from the LSV curves, but not for the double perovskites. In fact, the overpotentials for the three perovskites were significantly reduced. Especially for BGCF, the overpotential for galvanostatic operation at 10 mA cm−2 is even lower than that for IrO2, which is surprising as the loading of BGCF on Ni foam (4 mg cm−2) is even lower than that on the RDE (5 mg cm−2). It is worth noticing that the stability of all the ECs is very good and no apparent degradation is observed after 24 h of operation at 10 mA cm−2. The faradaic efficiency (FE) for the OER is also very high and reaches approx. 95% at 10 mA cm−2, as shown in Fig. S3 (ESI).


image file: c9cc08765f-f3.tif
Fig. 3 (a) Galvanostatic experiments at a current density of 10 mA cm−2 in 1 M NaOH for 24 h and Hg/HgO (1 M NaOH) as the reference electrode. (b) Stability testing over 24 h in 1 M NaOH at a current density of 100 mA cm−2. The nominal area of the Ni foam was 0.25 cm2 and the loading of the electrocatalyst was 4 mg cm−2. Compensation of the electrolyte solution was needed during the long term experiments.

Before arguing the reasons behind this outcome, we looked into the stability of BGCF at 10 mA cm−2 for 48 h (Fig. S4, ESI) and at much higher current densities, at 100 mA cm−2 for 24 h (Fig. 3b). It can be seen that BGCF shows very good stability at both chosen current densities.

We speculate that this behavior is related to the change of the surface area between the GC and Ni foam. We also considered to use BGCF as both the anode and cathode electrocatalyst for overall water splitting. The results are given in Fig. S5 (ESI). Although the expected operating potential (2.25 V) from the HER and OER LSVs is heavily reduced due to the higher exposed surface area with the use of Ni foam as the substrate, the cell voltage increased slightly during 48 h of operation. We have not investigated whether the increase is due to water evaporation or cathode degradation, as the HER reaction is not the scope of this work.

To further investigate this behavior we looked into the intrinsic catalytic activity (ICA) of the electrocatalysts.26,27 For this reason, we performed electrochemical impedance spectroscopy (EIS) at the operational potential at 10 mA cm−2 of each electrocatalyst with the main aim to find the double layer capacitance and the charge transfer resistance for the OER; Fig. 4.


image file: c9cc08765f-f4.tif
Fig. 4 Nyquist plots recorded in the frequency range from 100 kHz to 1 Hz with an imposed sinusoidal voltage of 10 mV. The EIS measurements were performed at overpotentials for 10 mA cm−2 for each electrocatalyst, as given in Table 1, with the RDE at 1000 rpm in 1 M NaOH. The raw data were fitted to the equivalent circuit given in the ESI (Fig. S6), where the capacitor was replaced by a CPE element to account for the non-ideal capacitive behavior. The effective capacitance was then determined by image file: c9cc08765f-t1.tif.28 All fittings were performed with Zview. The inset shows higher magnification in the high frequency domain.

As argued by Papaderakis et al.,29 and Chatzitakis et al.,27 the RctCdl product of the EIS data (units of ΩF – effectively the units can be rearranged to (s) as the RctCdl product reflects the time constant (τ) of the studied reaction) is an indicator of the ICA, with Rct accounting for the overall activity in an inverse manner and C correcting for surface area. This means that the Rct is normalized by the double layer capacitance, which is analogous to the active surface area of the material. The lower the RctCdl product, the higher the ICA of a given material as any effects due to surface area are decoupled.

Surprisingly, BGCF has the highest ICA, i.e. lowest τ, followed by the rest of the double perovskites, while IrO2 showed the lowest ICA, i.e. highest τ. This suggests that given a similar microstructure, the double perovskites will outperform IrO2, and explains that when the perovskites were loaded on a substrate of higher surface area (Ni foam ≫ GC), it resulted in a better electrocatalytic activity for the OER. In particular, BGCF, which even outperforms IrO2 (see Fig. 3a), has a particle diameter in the range of 5–10 μm (Fig. S9, ESI), rendering significant electroactive area. The superior performance of BGCF over IrO2 is also evident from its much smaller semicircle in the Nyquist plot of Fig. 4.

Finally and in order to check the validity of the proposed method, we estimated the electrocatalytic surface area (ECSA) by the double layer capacitance from cyclic voltammetry at different scan rates.30 The capacitance as extracted by CV scanning at different scanning rates (Fig. S7, ESI) is compared with the capacitance extracted by EIS at two different potentials. The first one was obtained in the region where the CV scanning was conducted, while the second one at the overpotential for 10 mA cm−2 of each electrocatalyst. The results are presented in Table S1 (ESI). There is a deviation between the results, but the trends are more or less the same. IrO2 is also known for its pseudocapacitive properties and indeed, it also shows the highest capacitance among all samples. It is also very important to notice the big change in the capacitance of IrO2 at 10 mA cm−2 further relating to its pseudocapacitive nature. We can also correlate the τ factor with the capacitive behaviour of a material. Inversely to the high electrocatalytic activity for the OER and a low τ value, we can now suggest that a high τ value, i.e. low electrocatalytic activity, means a higher capacitive, or polarising behaviour of a material. Indeed, IrO2 has the highest τ, and therefore the highest capacitive behaviour. This is also obvious in the Nyquist plot (Fig. S8, ESI), where the line at the low frequency domain is almost vertical and parallel to the imaginary axis of the plot.31 In contrast, BPCF shows the lowest τ value and its EIS response deviates the most from the ideally capacitive behaviour.

In conclusion, we have shown that a state-of-the-art electrocatalyst for high temperature steam electrolysis based on non-precious elements has intrinsically a higher activity for the OER at room temperatures than IrO2. A manipulation of the grain size and surface area of the double perovskite is needed in order to fully unravel the actual activity of the material. Moreover, the composition of the double perovskites is easily tuned, a fact that can lead to derivatives with even higher electrocatalytic activity. This work highlights the versatility of this class of double perovskites and expands the material horizons for OER electrocatalysts based on non-precious metals. Finally, we hope that our interdisciplinary approach will inspire relevant research in order to unearth other promising materials for cost-effective and efficient water electrolysis.

R. S. acknowledges funding from the Research Council of Norway (272797 “GoPHy MiCO”) through the M-ERA.NET Joint Call 2016, the European Union, ERA.Net RUS project #197 – PROTON, and Dmitry S. Tsvetkov at Ural Federal University for valuable scientific input on materials development. A. C. acknowledges the PH2ON project (project number 288320) sponsored by the Research Council of Norway and MoZEES, a Norwegian Centre for Environment-friendly Energy Research (FME), co-sponsored by the Research Council of Norway (project number 257653) and 40 partners from research, industry and the public sector.

Conflicts of interest

There are no conflicts to declare.

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c9cc08765f

This journal is © The Royal Society of Chemistry 2020