Electrochemically tuned cobalt hydroxide carbonate with abundant grain boundaries for highly efficient electro-oxidation of hydrazine

Abstract

Grain boundaries bear many active sites for surface reactions (e.g. electrocatalysis and energy storage), and increasing the grain boundaries has become an important strategy to tune the number of surface active sites and the reaction kinetics. Herein, we show that a simple in situ electrochemical tuning method can be used to turn Co(OH)_x(CO₃)_0.5(2−x) crystals into interconnected ultrafine nanoparticles with enriched grain boundaries. The grain boundaries offer numerous unparalleled active sites for highly efficient electro-oxidation of hydrazine. The electrochemically-tuned Co(OH)_x(CO₃)_0.5(2−x) shows an onset potential (−1.12 V vs. Ag/AgCl) 180 mV smaller than that of the pristine Co(OH)_x(CO₃)_0.5(2−x), and delivers a high current density of 62.4 mA cm⁻² at −0.90 V vs. Ag/AgCl, which is 27 times higher than that of the pristine Co(OH)_x(CO₃)_0.5(2−x). This work produces the most active non-metallic catalyst for hydrazine oxidation with catalytic activity comparable to the state-of-the-art catalysts. Moreover, our electrochemical tuning method is applicable to other materials (e.g. Co₃O₄, Co(OH)₂ and Ni(OH)₂).

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1. Introduction

Fuel cells, batteries and supercapacitors are the three major electrochemical systems for energy storage and conversion, and potentially the mainstream energy supply systems for portable electronics and vehicles in future green and sustainable economies.^1–8 Batteries, especially lithium-ion batteries, and supercapacitors have been successfully commercialized for decades, and are recently booming owing to their success in powering electric vehicles.^9,10 However, the commercial batteries and supercapacitors suffer from low volumetric energy, limiting their lifetime per charge to days and even hours. Fuel cells, which directly convert chemical energy from fuels (methanol, hydrazine, et al.) to mechanical, electrical and/or thermal energy, are an ideal primary energy conversion device for uninterruptible power supply.^1,11,12 Current fuels cells, however, rely on noble metal catalyst systems, whose high cost and scarcity greatly hampers the mass production and extensive applications of fuel cells.¹³ There is thus an urgent need for the development of low-cost, earth-abundant and highly-efficient catalyst materials for fuel cells.^14–18

Among various fuel cell technologies, direct hydrazine fuel cells, which were pioneered in the 1960s,¹⁹ have drawn increasing attention due to the ease in transportation and storage of the liquid hydrazine fuel, the high theoretical cell voltage (+1.61 V), the high energy and power density, and the pollution-free emissions (H₂O and N₂).^{11,12,20–24} The key component for fuel cells is the catalysts.^{14,16–18,25–34} So far, a variety of earth-abundant catalyst materials have been developed for hydrazine fuel cells.^25–32 Among them, the first-row transition metal based nanomaterials (mainly Co, Ni and Cu) have attracted the most attention because of their relatively high earth abundance, low cost and decent catalytic activity.^27–32 However, only metal nanocrystals have so far been shown to exhibit high catalytic performances. Their low-cost, easily-processable oxides and hydroxides are seldom reported as high-performing electrocatalysts towards hydrazine oxidation. The activity of an electrocatalyst depends strongly on its structure, composition, surface area, etc., and thus structure engineering as an effective tool in tuning the functional properties of the existing materials has spurred wide interest. For instance, recent researches show that structure-engineered metal/metal oxide heterostructures exhibit tunable catalytic activity towards water electrolysis,^35–40 and present high activity towards hydrazine electro-oxidation.⁴¹ The enhanced performances of these metal/metal oxide heterostructures can be attributed, at least partially, to the increased interphase boundaries and/or grain boundaries.^35–38,41 As indicated by numerous recent studies, grain boundaries that often include abundant defects greatly contribute to the active sites for surface electrochemical reactions. Aaronson et al. found that grain boundaries on platinum contributed significantly to its overall electrochemical behavior towards the Fe²⁺/Fe³⁺ redox reaction.⁴² Feng et al. reported that the CO₂ electro-reduction activity of gold nanoparticles and the CO electro-reduction activity of copper nanoparticles were grain-boundary-dependent.^43,44 Maillard and co-workers found that the active sites on the carbon-supported Pt nanoparticles towards CO monolayer oxidation was partially derived from the grain boundaries.⁴⁵ Wang's group introduced a lithium electrochemical tuning method to controllably reduce zinc oxide into interconnected metal nanoparticles with enriched grain boundaries, resulting in a five-fold improvement towards CO₂ reduction at an overpotential of −948 mV.⁴⁶ Inspired by these studies, herein, we report that a simple in situ electrochemical tuning method can convert the Co(OH)_x(CO₃)_0.5(2−x) crystals into interconnected ultrafine nanoparticles with abundant grain boundaries, turning an otherwise inactive catalyst into one of the most effective non-metallic catalysts for hydrazine oxidation. To the best of knowledge, this is the first report on the application of Co(OH)_x(CO₃)_0.5(2−x) as a potential cost-effective yet highly efficient catalyst for hydrazine fuel cells. The abundant grain boundaries of the electrochemically tuned Co(OH)_x(CO₃)_0.5(2−x) (ECT-Co(OH)_x(CO₃)_0.5(2−x)) contribute to a high cathodic current density of 62.4 mA cm⁻² at a potential of −0.90 V vs. Ag/AgCl, representing a 27-fold improvement compared to the pristine Co(OH)_x(CO₃)_0.5(2−x). This opens a new avenue for endowing traditionally inactive materials with high activities towards electro-oxidation of hydrazine.

2. Experimental section

2.1. Material synthesis

Synthesis of cobalt hydroxide carbonate (Co(OH)_x(CO₃)_0.5(2−x)). Co(OH)_x(CO₃)_0.5(2−x) was prepared following previously reported hydrothermal methods with some modifications.^47–49 In a typical synthesis, 1.0 mmol Co(NO₃)₂·6H₂O and 4.0 mmol urea were dissolved in a mixture solution of 25 mL deionized (DI) water and 10 mL ethanol. A piece of nickel foam (∼12 cm²) was sonicated in 3 M HCl for 10 min to remove the possible surface oxide layer, and then washed with DI water. The nickel foam was then transferred into the reaction solution, and then hydrothermal reaction was carried out in a Teflon lined stainless steel autoclave (50 mL) at 100 °C for 10 h. After cooled down to room temperature, the product was collected, washed with DI water, and dried at 100 °C for 10 h. The Co(OH)_x(CO₃)_0.5(2−x) powder on the bottom of the autoclave was also collected, washed with DI water, and dried at 100 °C for 10 h. The mass loading of Co(OH)_x(CO₃)_0.5(2−x) on the nickel foam was about 1.0 mg cm⁻².

Synthesis of cobalt oxide (Co₃O₄). Co₃O₄ was prepared by annealing the as-prepared Co(OH)_x(CO₃)_0.5(2−x) at 350 °C in air for 2 h.

Synthesis of cobalt hydroxide (Co(OH)₂). Co(OH)₂ was fabricated by following a previously reported method.³⁵ Typically, 4.0 mmol Co(NO₃)₂·6H₂O and 8.0 mmol hexamethylene tetramine were dissolved in a mixture solution of 40 mL DI water and 20 mL ethanol. A piece of acid-treated nickel foam (∼12 cm²) was transferred into the reaction solution, and then hydrothermal reaction was carried out in a capped glass bottle (100 mL) at 90 °C for 10 h. After cooled down to room temperature, the product was collected, washed with DI water, and dried at 100 °C for 10 h. The mass loading of the Co(OH)₂ on the nickel foam was about 1.0 mg cm⁻².

Synthesis of nickel hydroxide (Ni(OH)₂). Typically, 1.5 mmol Ni(NO₃)₂·6H₂O and 6.0 mmol urea were dissolved in 35 mL DI water. A piece of acid-treated nickel foam (∼12 cm²) was transferred into the reaction solution, and then hydrothermal reaction was carried out in a Teflon lined stainless steel autoclave (50 mL) at 120 °C for 8 h. After cooled down to room temperature, the product was collected, washed with DI water, and dried at 100 °C for 10 h. The mass loading of the Co(OH)₂ on the nickel foam was about 1.1 mg cm⁻².

Electrochemical tuning. The electrochemically tuned Co(OH)_x(CO₃)_0.5(2−x) (ECT-Co(OH)_x(CO₃)_0.5(2−x)) was achieved as follows. Typically, eighty cycles of cyclic voltammograms was performed on the Co(OH)_x(CO₃)_0.5(2−x) electrode in the voltage range from −1.2 to −0.95 V vs. Ag/AgCl at a scan rate of 20 mV s⁻¹ in 1.0 M KOH with the presence of 0.5 M N₂H₄ in the electrolyte. The Co₃O₄, Co(OH)₂ and Ni(OH)₂ were also electrochemically tuned under similar conditions before further electrochemical characterization, and named ECT-Co₃O₄, ECT-Co(OH)₂ and ECT-Ni(OH)₂, respectively.

2.2. Property characterization

Morphologies of the samples were examined using scanning and transmission electron microscopy (SEM and TEM). The SEM images were collected using a field emission scanning electron microscope (Supra55, Carl Zeiss). The TEM analyses were conducted on a high-resolution transmission electron microscope (HRTEM, Tecnai G2 F30 S-TWIN) equipped with a scanning transmission electron microscope. The electron accelerating voltage for the transmission electron microscope was 200 kV. Structural analysis was performed with the collected Co(OH)_x(CO₃)_0.5(2−x) powder on a wide-angle X-ray diffractometer (WAXD, D8 Advance, Bruker) using Cu Kα radiation (wavelength = 1.5418 Å). The elemental composition and chemical status of the electrodes were analyzed by using X-ray photoelectron spectroscopy (XPS, EscaLab 250, Thermo Fisher Scientific) with a monochromatic Al X-ray (hν = 1486.6 eV) as the excitation source.

2.3. Electrochemical characterization.

Electrochemical measurements were carried out in a three-electrode system at room temperature. A Pt wire and an Ag/AgCl electrode were used as the counter and reference electrodes, respectively. Typically, 0.5 M N₂H₄ aqueous solution with 1.0 M KOH was used as the electrolyte. Linear sweep voltammetry was conducted at a scan rate of 5 mV s⁻¹ to evaluate the catalytic performance of the working electrode. Long-term stability test was carried out by cyclic voltammetry in the voltage range from −1.2 to −0.9 V vs. Ag/AgCl with repeated scans at a scan rate of 20 mV s⁻¹.

To estimate the electrochemically active surface area (ECSA), following procedures were carried out. The electrochemically tuned electrode was first undergone three cycles of linear sweep voltammograms at a scan rate of 5 mV s⁻¹. Subsequently, cyclic voltammograms were collected at varied scan rates of 5, 10, 20, 40, 60, and 80 mV s⁻¹. The voltage window of the cyclic voltammograms was between −1.0 and −0.9 V vs. Ag/AgCl. The cyclic voltammograms from different electrodes were shown in Fig. S1 (ESI†). The double layer capacitance (C_dl) was estimated by plotting the Δj (Δj = j_a − j_c) against the scan rate, and the slope was twice of the C_dl. j_a and j_c are the anodic and cathodic current densities, respectively, from the voltammograms at the potential of −0.95 V vs. Ag/AgCl.

3. Results and discussion

Co(OH)_x(CO₃)_0.5(2−x) was first grown on nickel foam substrates by hydrothermal reaction in the presence of 1.0 mmol Co(NO₃)₂ and 4.0 mmol urea in a mixture solution of water and ethanol. As shown in Fig. 1A and B, the Co(OH)_x(CO₃)_0.5(2−x) film was uniformly grown on the surface of the nickel foam. On the surface of the Co(OH)_x(CO₃)_0.5(2−x) film, thinly distributed nanosheets and nanowires were observed (Fig. 1B). TEM characterization of the nanosheets showed that the Co(OH)_x(CO₃)_0.5(2−x) nanosheets were single crystals with continuous lattice fringes (Fig. 1C and D). The interplanar spacing of 0.442 nm is close to the (001) plane of Co(OH)(CO₃)_0.5 (JCPDS 48-0083), confirming the formation of the Co(OH)_x(CO₃)_0.5(2−x). Note that the holes in the Co(OH)_x(CO₃)_0.5(2−x) nanosheet in Fig. 1D were derived from the high-energy electron beam during the HRTEM characterization. The elevated local temperature by the high-energy electron beam decomposed part of the Co(OH)_x(CO₃)_0.5(2−x) nanosheet.

Fig. 1 SEM (A, B, E and F), TEM (C and G) and HRTEM (D and H) images of Co(OH)_x(CO₃)_0.5(2−x) before (A–D) and after (E–H) electrochemical tuning.

ECT-Co(OH)_x(CO₃)_0.5(2−x) was prepared by performing cyclic voltammetry cycles on the pristine Co(OH)_x(CO₃)_0.5(2−x) electrode until the catalytic performance was stabilized. The morphology of the resulting ECT-Co(OH)_x(CO₃)_0.5(2−x) was studied by SEM and TEM. The SEM images (Fig. 1E and F) of the ECT-Co(OH)_x(CO₃)_0.5(2−x) showed similar feature to the pristine Co(OH)_x(CO₃)_0.5(2−x). While the TEM image (Fig. 1G) of the ECT-Co(OH)_x(CO₃)_0.5(2−x) nanosheets confirmed its well-maintained morphology, the HRTEM image taken from the same nanosheet showed that the ECT-Co(OH)_x(CO₃)_0.5(2−x) nanosheets were composed of numerous ultrafine interconnected nanocrystals of sizes around 2–3 nm (Fig. 1H and Fig. S2, ESI†). This proves the high effectiveness of our electrochemical tuning method in transforming relatively large nanocrystals of Co(OH)_x(CO₃)_0.5(2−x) into ultrafine nanocrystals with abundant grain boundaries. The spacing of 0.201 nm between adjacent planes (Fig. S2, ESI†) is indexed to the (050) plane of Co(OH)(CO₃)_0.5 (JCPDS 48-0083). XRD analysis was further used to probe the phase composition of the Co(OH)_x(CO₃)_0.5(2−x) and ECT-Co(OH)_x(CO₃)_0.5(2−x). The XRD patterns (Fig. 2A) of the Co(OH)_x(CO₃)_0.5(2−x) and ECT-Co(OH)_x(CO₃)_0.5(2−x) resemble each other and both match well with the XRD patterns of the Co(OH)_x(CO₃)_0.5(2−x) reported in the literature.^47–50 Most of the XRD peaks of ECT-Co(OH)_x(CO₃)_0.5(2−x) are weaker, likely due to the ultra-small sizes of the nanocrystals in the ECT-Co(OH)_x(CO₃)_0.5(2−x) and their decreased crystallinity caused by the abundant grain boundaries. On the other hand, diffraction signals at 2θ ≈ 14.4° and 24.0° in the XRD pattern of the ECT-Co(OH)_x(CO₃)_0.5(2−x), which correspond to the (200) and (301) crystal planes of hexagonal cobalt hydroxide carbonate, respectively,⁴⁹ are stronger, indicating that the percentage of those crystal planes increased after electrochemical tuning.

Fig. 2 XRD patterns (A) and Co 2p XPS spectra (B) of the Co(OH)_x(CO₃)_0.5(2−x) (a) and ECT-Co(OH)_x(CO₃)_0.5(2−x) (b).

XPS analysis was conducted to survey the surface chemical states and elemental composition of the samples. All XPS spectra were calibrated with the C 1s peak (284.6 eV). The Co(OH)_x(CO₃)_0.5(2−x) and ECT-Co(OH)_x(CO₃)_0.5(2−x) presented very similar XPS survey spectra (Fig. S3, ESI†) with signals from cobalt, oxygen and carbon elements.³⁵Fig. 2B displays the Co 2p core-level XPS spectra of the Co(OH)_x(CO₃)_0.5(2−x) and ECT-Co(OH)_x(CO₃)_0.5(2−x). The similar Co 2p XPS spectra suggest similar surface chemical states of Co in the Co(OH)_x(CO₃)_0.5(2−x) and ECT-Co(OH)_x(CO₃)_0.5(2−x). The Co 2p_3/2 peak at 781.0 eV and the Co 2p_1/2 peak at 797.0 eV were assigned to the Co²⁺ species.^51,52 Note that the Co 2p peaks at 781.0 and 797.0 eV were intensified after electrochemical tuning, indicative of the increased surface Co²⁺ sites. This could be to the consequence of the abundant grain boundaries in the ECT-Co(OH)_x(CO₃)_0.5(2−x).

The electrocatalytic performances towards hydrazine oxidation over the ECT-Co(OH)_x(CO₃)_0.5(2−x) and pristine Co(OH)_x(CO₃)_0.5(2−x) electrodes were evaluated in a typical three-electrode system with a mixture solution of 0.5 M N₂H₄ and 1.0 M KOH as the electrolyte. Fig. 3 displays the polarization curves of the ECT-Co(OH)_x(CO₃)_0.5(2−x) and pristine Co(OH)_x(CO₃)_0.5(2−x) electrodes. The pristine Co(OH)_x(CO₃)_0.5(2−x) was found to be inactive towards electro-oxidation of hydrazine at potentials below −0.90 V vs. Ag/AgCl. As shown in Fig. 3A, the current density of the pristine Co(OH)_x(CO₃)_0.5(2−x) electrode only reached ∼2.2 mA cm⁻² at a relatively high potential of −0.90 V vs. Ag/AgCl. Excitingly, the ECT-Co(OH)_x(CO₃)_0.5(2−x) electrode reached a markedly high current density of ∼62.4 mA cm⁻² at the same potential, which represented a 27-fold improvement after electrochemical tuning. On the other hand, the onset potential, defined as the potential where the current density reaches 1.0 mA cm⁻², was found to be −1.12 vs. Ag/AgCl for the ECT-Co(OH)_x(CO₃)_0.5(2−x) electrode. This is a dramatic decrease of 180 mV as compared to that of the pristine Co(OH)_x(CO₃)_0.5(2−x) electrode, confirming the high activity of the ECT-Co(OH)_x(CO₃)_0.5(2−x) electrode owing to the numerous active sites primarily derived from the enriched grain boundaries. The onset potential of the ECT-Co(OH)_x(CO₃)_0.5(2−x) electrode is among the lowest reported in the literature (Table 1). More importantly, our preparation method is simple and does not involve complex processes and harsh conditions used in most of the other synthesis methods. In addition, our method uses inexpensive source materials, and is easily scalable.

Fig. 3 (A) Polarization curves of the ECT-Co(OH)_x(CO₃)_0.5(2−x) and pristine Co(OH)_x(CO₃)_0.5(2−x) electrodes. (B) Charging current density difference Δj (j_a − j_c) plotted against scan rate.

Table 1 Comparison of onset potentials for various materials in electro-oxidation of hydrazine

Sample	Electrolyte	v (mV s^−1)	Con. of N2H4 (M)	Onset potential^a (V)	Ref.
a All the potentials are relative to Ag/AgCl.
Co/carbon fiber cloth	1.0 M KOH	10	0.02	−1.10	27
Cu nanowires on Cu foil	3.0 M NaOH	25	1.0	−0.82	29
Ni60Co40 alloy	1.0 M KOH	20	0.1	−1.17	31
Ni nanoflowers on Ni foam	3.0 M KOH	1	0.5	−1.05	53
Nanostructured Cu	9.0 M KOH	20	2.1	−0.96	54
Cu film on Cu Foil	3.0 M NaOH	5	1.0	−0.76	55
NiS2 nanosheets on Ti mesh	1.0 M KOH	5	0.5	−0.983	56
FeP nanosheets on Ni foam	1.0 M KOH	5	0.5	−1.023	57
Ni2P on Ni foam	1.0 M KOH	5	0.5	−1.12	58
ECT-Co(OH)x(CO3)0.5(2−x)	1.0 M KOH	5	0.5	−1.12	This work

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It is suspected that the improved catalytic performance is correlated to the increased grain boundaries in the ECT-modified electrode. While it is impossible to quantitatively measure the grain boundaries, the ECSA may be measured and to some extent reflect the quantity of the grain boundaries, since ECSA is proportional to the number of surface active sites. ECSA was estimated from the C_dl, as C_dl was linearly proportional to the effective active surface area.^59,60 The profile of the charging current density difference Δj (j_a − j_c) versus scan rate is shown in Fig. 3B. The C_dl values of Co(OH)_x(CO₃)_0.5(2−x) before and after electrochemical tuning were found to be 0.0007 and 0.0113 F, respectively. This indicates that the ECSA of the ECT-Co(OH)_x(CO₃)_0.5(2−x) electrode is 16.1 times as high as that of the pristine Co(OH)_x(CO₃)_0.5(2−x) electrode. It is believed that the large ECSA of the ECT-Co(OH)_x(CO₃)_0.5(2−x) leads to its dramatically improved catalytic performance, while the large ECSA is undoubtedly derived from the abundant grain boundaries. Further, normalized current density was calculated by dividing the current density with the relative ECSA (The relative ECSA of the pristine Co(OH)_x(CO₃)_0.5(2−x) was set as 1). The normalized current density of the ECT-Co(OH)_x(CO₃)_0.5(2−x) electrode (3.9 mA cm⁻²) was about 1.8 times that of the pristine Co(OH)_x(CO₃)_0.5(2−x) electrode (2.2 mA cm⁻²) at the potential of −0.9 V vs. Ag/AgCl. This suggests that the active grain boundary sites are superior to the regular active sites on the surface of the pristine Co(OH)_x(CO₃)_0.5(2−x). This can be explained as follows. Adsorption of OH⁻ and N₂H₄ onto the surface of the catalyst is reported to be the rate-determining processes (N₂H_4 ads + OH⁻_ads → N₂H_3 ads + H₂O + e⁻),^61,62 and the adsorption of OH⁻ onto the surface Co sites herein is the first step towards hydrazine oxidation.⁶¹ The rich grain boundaries exposed more accessible Co sites as evidenced by the XPS, where many defective Co sites may exist, thus enhancing the OH⁻ adsorption and contributing to the greatly decreased overpotential necessary for the electro-oxidation of hydrazine.^61,63

The effect of mass loadings of Co(OH)_x(CO₃)_0.5(2−x) on the catalytic performance was evaluated. As shown in Fig. 4A, a mass loading of about 1.0 mg cm⁻² appears to be optimal: lower loading led to poorer performance while higher loading did not further improve the catalytic performance. We also investigated the effect of the concentration of the hydrazine on the performance (Fig. 4B). As was expected, no redox reaction was detected without N₂H₄. At a low N₂H₄ concentration of 0.05 M, the reaction was highly diffusion-controlled, evidenced by the limited current increase with potential. This is consistent with the literature.^26–28 At high N₂H₄ concentrations of ≥0.1 M, the current density increased linearly with potential when the potential was over −1.0 V vs. Ag/AgCl, and the catalytic performance was slightly but continuously improved with increasing the concentration of N₂H₄ from 0.2 to 1.0 M.

Fig. 4 (A) Polarization curves of the ECT-Co(OH)_x(CO₃)_0.5(2−x) with different mass loading. (B) Polarization curves of the ECT-Co(OH)_x(CO₃)_0.5(2−x) with varied concentrations of N₂H₄. (C) Cycling stability of the ECT-Co(OH)_x(CO₃)_0.5(2−x) electrode: current density at the potential of −0.90 V vs. Ag/AgCl against cycle number. (D) Polarization curves of the ECT-Co(OH)_x(CO₃)_0.5(2−x) electrode before and after the stability test.

The catalyst stability of the ECT-Co(OH)_x(CO₃)_0.5(2−x) electrode was evaluated by cyclic voltammetry measurements. As shown in Fig. 4C, the current density at the potential of −0.90 V vs. Ag/AgCl maintained 89% of its initial value after 350 cycles. Polarization curves collected before and after the stability test showed that the catalytic performance only decreased slightly after 350 cycles (Fig. 4D). For instance, the degradation in current density at −0.90 V vs. Ag/AgCl from the polarization curves before and after the stability test was calculated to be 5.4%. These results show that the ECT-Co(OH)_x(CO₃)_0.5(2−x) catalyst has good stability. The SEM image of the ECT-Co(OH)_x(CO₃)_0.5(2−x) electrode after 350 cycles was collected and is shown in Fig. S4 (ESI†).

To find out whether the electrochemical tuning method can be applied to other materials, the Co₃O₄, Co(OH)₂ and Ni(OH)₂ electrodes were subjected to the same electrochemical tuning process. As shown in Fig. S5–S7 (ESI†) and Fig. 5, compared with their corresponding pristine electrodes, improved catalytic performances were observed for all electrochemically tuned electrodes. The extent of improvement, however, is different for different materials. The Co₃O₄ showed the least amount of improvement while cobalt hydroxide carbonate presented the largest increase and the highest catalytic performance after the electrochemical tuning. The current density of the ECT-Co(OH)_x(CO₃)_0.5(2−x) electrode doubled that of the second best one (the ECT-Co(OH)₂ electrode). This suggest that the intrinsic structure of the cobalt hydroxide carbonate plays a critical role in its restructuring during the electrochemical tuning process.

Fig. 5 Current density at the potential of −0.90 V vs. Ag/AgCl plotted against catalyst before and after electrochemical tuning.

4. Conclusions

In summary, a simple in situ electrochemical tuning method has been shown to be able to convert an inactive cobalt hydroxide carbonate into a highly efficient catalyst for electro-oxidation of hydrazine. Structurally, the electrochemical tuning turns single crystals into interconnected ultrafine crystalline particles of a few nanometers, creating abundance grain boundaries. As a catalyst for electro-oxidation of hydrazine, the electrochemically tuned Co(OH)_x(CO₃)_0.5(2−x) showed a performance comparable to the state-of-the-art catalysts, delivering a low onset potential (−1.12 V vs. Ag/AgCl), a high current density (62.4 mA cm⁻² at −0.9 V vs. Ag/AgCl) and good long-term stability. The active grain boundary sites are found to be superior to the regular surface-active sites towards catalytic oxidation of hydrazine due to the unique structure of the gain boundary regions. This study provides new insights in developing low-cost, advanced electrocatalysts for hydrazine fuel cells.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

P. Z. acknowledges the support from the National Science Foundation (DMR1308577). X. Y. thanks the funds provided by the University of Missouri-Kansas City, School of Graduate Studies.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7qm00511c

‡ These authors contributed equally.

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