A 3D Porous Cobalt-Iron-Phosphorous Bifunctional Electrocatalyst for

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A 3D Porous Cobalt-Iron-Phosphorous Bifunctional Electrocatalyst for the Oxygen and Hydrogen Evolution Reactions HyoWon Kim, SeKwon Oh, EunAe Cho, and Hyuk-Sang Kwon ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00118 • Publication Date (Web): 17 Apr 2018 Downloaded from http://pubs.acs.org on April 18, 2018

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ACS Sustainable Chemistry & Engineering

A 3D Porous Cobalt-Iron-Phosphorous Bifunctional Electrocatalyst for the Oxygen and Hydrogen Evolution Reactions

HyoWon Kim, SeKwon Oh, EunAe Cho* and HyukSang Kwon*

Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST), Applied Engineering Building (W1-1, #2408), 291 Daehak-ro, Yuseonggu, Daejeon, Republic of Korea

Corresponding Author

EunAe Cho*: [email protected] HyukSang Kwon*: [email protected]

KEYWORDS: Co-Fe-P; Electrodeposition; OER; HER; Electrolysis. 1 ACS Paragon Plus Environment

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ABSTRACT

A 3D porous Co-Fe-P foam fabricated using electrodeposition is presented as a highperformance and durable catalyst for both oxygen and hydrogen evolution reactions. To establish optimal Fe/Co ratio of the catalyst, Co-Fe-P films were electrodeposited with Fe/Co ratio of 0.2, 0.4, 1.1 and 3.3. Among the prepared samples, the Co-Fe-P film with the Fe/Co ratio of 1.1 (CoFe-P-1.1) exhibited the highest activity for the oxygen evolution reaction, which could be attributed to the transfer of the valence electron from Co to Fe and P. To improve performance of the Co-Fe-P-1.1, a 3D porous foam structure was adopted using the electrodeposition. The CoFe-P foam had 94 times larger electrochemical active surface area (ECSA) than the Co-Fe-P film with similar Fe/Co ratios of 1.1, resulting in an distinguished activity for the oxygen evolution reaction (294 mV at 10 mA/cm2) and hydrogen evolution reaction (73 mV at 10 mA/cm2) in an alkaline solution. Since the electrodeposited Co-Fe-P foam itself can be directly used as an electrode, it is free from binders and microstructure of the electrode can be engineered by controlling the electrodeposition condition, leading to the enlarged ECSA and improved performance. Thus, the Co-Fe-P foam presented in this study offers a facile and controllable synthesis of catalyst and electrode through an easy electrodeposition process.

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INTRODUCTION Hydrogen has been considered as a promising alternative fuel owing to its clean and sustainable features. Currently, most of the hydrogen is manufactured by reforming fossil fuels with emission of carbon dioxide. Water electrolysis is the most environmentally friendly way of hydrogen production and can address the intermittency of renewable electricity.1-3 Hydrogen production cost and efficiency of the water electrolysis is determined by the overpotential of the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) at the anode and cathode, respectively. To minimize the overpotentials, Pt-based catalysts 4-5 and Ir/C 6 or RuO2 7 catalysts have been used for HER and OER, respectively. Nonetheless, the high cost and insufficiency of these noble metal catalysts have raised questions about economic issues.8-9 Therefore, there has been a strong demand to develop low-cost non-precious catalyst materials that are highly active towards OER and HER. Of the catalysts studied, transition metal phosphides (TMPs) have demonstrated high OER and/or HER activity with long-term durability and regarded as promising low-cost catalysts.10-21 Cobalt phosphides (CoP, Co2P),10-11 nickel phosphides (NiP, Ni2P),12-14 iron phosphides (FeP),1516

molybdenum phosphides (MoP),17 and copper phosphides (Cu3P)18 have been investigated in

various nanostructures such as nanoparticles,10,12,15-16 nanosheets,11 and nanowires.18 More recently, Co-Fe-P bimetallic phosphides demonstrated enhanced activity for the HER

19

and for

the OER. 20-22 Zhang et al.21 presented excellent OER performance (260 mV @ 10 mA/cm2 in 1 M KOH) of bimetal organic framework based Co-Fe phosphides. Kibsgaard et al.25 reported Co0.5Fe0.5P had superior HER activity based on density functional theory computation for the hydrogen adsorption free energies (∆GH). Most of the previously-reported Co-Fe-P catalysts were produced into powder through multi step methods with toxic chemicals and heat treatment 3 ACS Paragon Plus Environment


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for more than several hours at high temperatures.19-22 These synthetic methods increase the catalyst fabrication cost and cause environmental issues. Furthermore, to fabricate electrodes, catalyst powders are mixed with a solvent and a polymer binder and then coated onto a substrate, followed by a drying process. The polymer binder in the electrode can block the catalyst powders from the electrolyte and those overall electrode fabrication processes make it difficult to control pore structure of the electrode, leading to loss in active surface area. Electrodeposition can be a powerful alternative method to prepare the catalyst. The catalyst aselectrodeposited on a substrate can be used as an electrode, whose composition and pore structure can be engineered by controlling the electrodeposition condition such as current density and solution composition. Moreover, the prepared electrode is binder-free, contributing to enhancement of active surface area, electrical conductivity and electrode performance. In a word, as a well-established scalable technique, electrodeposition can provide the tailored shape or composition for industrial requirements. In this study, Co-Fe-P catalysts were synthesized using the facile electrodeposition. Based on the OER activity results obtained from Co-Fe-P films with various Fe/Co ratios, the optimal Fe/Co ratio was determined. To increase the active surface area and performance of the Co-Fe-P catalyst, a 3D porous foam structure was adopted. The Co-Fe-P foam, fabricated with the optimal Fe/Co ratio using electrodeposition, exhibited an excellent bifunctional OER and HER activity and stability in an alkaline solution.

RESULTS AND DISCUSSION To optimize the compositional ratio of Fe to Co in the Co-Fe-P, electrodeposition was conducted onto a Cu substrate in 0.1 M CoCl2, 0.8 M NaH2PO2 • H2O, 0.6 M NH2CH2COOH 4 ACS Paragon Plus Environment

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and 0.93 M NH4Cl solutions with addition of different concentration (0, 0.01, 0.03, 0.1 and 0.5 M) of FeCl2. Chemical composition of the electrodeposits was summarized in Table S2 and Fig. S2. Without addition of FeCl2 to the electrodeposition bath, Co-10 at% P layer was obtained. With increasing the FeCl2 concentration from 0.01 to 0.5 M, the Fe content increased from 15 to 56 at% and the Co content decreased from 65 to 17 at% (the Fe/Co ratio increased from 0.2 to 3.3) while the P content was in the range from 20 to 27 at%. Fig. S1 presents that all of the electrodeposited layers had a smooth and flat surface. XRD patterns (Fig. S3) exhibited only the peaks from the Cu substrate, indicating that the electrodeposited layers had an amorphous structure. All these results demonstrate that amorphous Co-P and Co-Fe-P films were obtained with various Fe/Co ratios of 0.2, 0.4, 1.1 and 3.3. In this study, Co-Fe-P films were denoted as Co-Fe-P-0.2, 0.4, 1.1 and 3.3 according to the Fe/Co ratio. To evaluate OER activity of the prepared catalysts, linear sweep voltammetry (LSV) was performed in a potential range from 1.2 to 1.8 V at a scan rate of 10 mV/s in an Ar-saturated 1 M KOH solution (Fig. 1a). At a current density of 10 mA/cm2, the overpotential of the Co-P (382 mV) was higher than that of the Co-Fe-P catalysts. With increasing the Fe content in the Co-Fe-P catalysts (increasing the Fe/Co ratio from 0.2 to 1.1), the overpotential at 10 mA/cm2 (η @ 10 mA/cm2) was reduced from 374 to 330 mV (Fig. 1b). However, with a Fe/Co of 3.3, the overpotential increased to 351 mV. These results show that the Co-Fe-P with a Fe/Co of 1.1 (CoFe-P-1.1) had the highest OER activity among the prepared samples, which is in a good accordance with a previous report that showed Co-Fe-P had the best performance with a Fe/Co close to 1.25

The Co-Fe-P-1.1 exhibited excellent OER activity (η @ 10 mA/cm2: 330 mV),

similar to that of the Ir/C (η @ 10 mA/cm2: 335 mV). To evaluate the HER activity, LSV was carried out in a potential range from 0 to - 0.3 V at a scan rate of 2 mV/s in an Ar-saturated 1 M 5 ACS Paragon Plus Environment


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KOH solution (Fig. 1c). In terms of the overpotential at 10 mA/cm2, the Co-P and Co-Fe-P exhibited a similar overpotential (Fig. 1d) of about 96 mV, irrespective of the Fe/Co ratio. Considering that η @ 10 mA/cm2 for the commercial Pt/C was 58 mV, the Co-P and Co-Fe-P demonstrated slightly higher overpotential. However, with increasing overpotential, current density increased more rapidly for the Co-P and Co-Fe-P than for Pt/C, implying faster HER kinetics on the Co-P and Co-Fe-P. Based on those results, it was concluded that the Co-Fe-P-1.1 could be the optimal composition for a bifunctional catalyst for the OER and HER. It should be noted in Fig. 1a and 1c that for the OER and HER, the Tafel slope of the Co-Fe-P-1.1 is clearly lower than that of the Ir/C and Pt/C, respectively, as discussed below. To explore the origin of the excellent catalytic activity of the Co-Fe-P-1.1, XPS analysis was performed and compared with Co-P. The catalyst activity of the Co-P and Co-Fe-P could be related to the valence and coordination of the electrons.21 Fig. 2a and b demonstrate the XPS spectra of Co 2p, Fe 2p and P 2p for the Co-P and Co-Fe-P-1.1. For the Co-P (Fig. 2a), the Co peaks (2p3/2 and 2p1/2) were detected at 779.5 and 795.0 eV. The Co3+ and Co2+ peaks appeared at 779.3 and 780.4 eV, respectively. The P 2p3/2, P 2p1/2 and POx peaks were identified at 128.4, 129.5 and 133.0 eV, respectively. Compared with the reference peaks of the Co 2p3/2 and P 2p3/2,19 the Co 2p3/2 peak in the Co-P was positively shifted by 1.4 eV, and the P 2p3/2 peak was negatively shifted by 1.8 eV. Positive shifts of the binding energies correspond to a decreased electron occupation resulting in a strong electron-accepting site. In contrast, negative shifts of the binding energies mean an increased electron occupation resulting in a strong electron-donating site. Thus, those peak shifts indicate that the Co atoms are in a cationic state and the P atoms in an anionic state by electron transfer from the Co to the P 19 enabling the Co-P to have a catalytic activity.26-27 6 ACS Paragon Plus Environment

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For the Co-Fe-P-1.1 in Fig. 2b, the Co peaks (2p3/2 and 2p1/2) were detected at 780.3 and 796.0 eV. Co3+, and the Co2+ peaks appeared at 780.2 and 783.2 eV, respectively. The P 2p3/2, P 2p1/2 and POx peaks were identified at 128.5, 129.5 and 132.7 eV, respectively. Additionally, the Fe peaks (2p3/2 and 2p1/2) were observed at 710.6 and 723.4 eV and the Fe3+and Fe2+ peaks at 710.3 and 713.7 eV. Compared with the reference peaks of the Co 2p3/2, Fe 2p3/2 and P 2p3/2,19, 21 the Co 2p3/2 peak in the Co-Fe-P-1.1 was positively shifted by 2.2 eV, and the Fe 2p3/2 and P 2p3/2 peaks were negatively shifted by 0.6 and 1.7 eV, respectively. Compared with the XPS spectra for the Co-P, the Co-Fe-P-1.1 had a larger binding energy for the Co 2p (more positively shift) with a similar binding energy for the P 2p (a similar negative shift) due to the addition of the negatively shifted Fe. Thus, the Co can become a stronger electron-accepting site. The high valence of Co ions improve the O- adsorption ability of catalyst.21 As a result, the incorporation of Fe in Co-P can promote the binding energy shift of Co, which leads to an excellent catalytic performance.

Formation of a porous structure is a way to promote the catalytic activity by exposing more electrochemically active sites to electrolyte.28 In this study, the Co-Fe-P-1.1 catalyst was fabricated into a highly porous 3D foam structure using the facile electrodeposition. As a substrate, a Cu foam with a dendritic nanostructure (Fig. S6) was prepared using the electrodeposition process at a high cathodic overpotential in which the porous foam can be effectively formed with help of the vigorous generation of hydrogen bubbles.29 Onto the Cu foam, the Co-Fe-P foam was electrodeposited in the same solution as the Co-Fe-P-1.1. (in a 0.1 M CoCl2, 0.8 M NaH2PO2• H2O, 0.6 M NH2CH2COOH, 0.93 M NH4Cl, and 0.1 M FeCl2 solution). 7 ACS Paragon Plus Environment


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Fig. 3a presents the surface morphology of the prepared Co-Fe-P foam revealing a highly porous 3D structure. The chemical composition of the Co-Fe-P foam was similar to that of the Co-Fe-P-1.1 (Table S3). To obtain the Fe/Co ratio close to 1.1, applied current density was adjusted to -0.085 A/cm2. (Co-Fe-P-1.1 film was fabricated at -0.05 A/cm2.) These results show that by controlling applied current density, composition of the electrodeposits can be affected depending on the substrate structure. EDS mapping images in Fig. S7 reveal that the Co, Fe and P were uniformly distributed in the Co-Fe-P foam. The TEM images (Fig. 3b) reveal that the CoFe-P layer was deposited onto the Cu foam with a thickness of about 15 nm. The crystallinity of the Co-Fe-P foam catalysts was examined by the XRD pattern (Fig. S8). As was the case in the Co-Fe-P-1.1, only the peaks from the Cu substrates (foam and RDE) were observed, demonstrating that the amorphous Co-Fe-P foam was electrodeposited on Cu substrates. Fig. 4a presents the OER activity of the Co-Fe-P-1.1 (denoted as ‘Co-Fe-P film’) and the CoFe-P foam compared with the Ir/C and the Cu foam substrate in a 1 M KOH solution. The prepared the Co-Fe-P foam exhibited an excellent OER activity. The overpotential at 10 mA/cm2 (η @ 10 mA/cm2) was 294 mV, which was considerably lower than that of the Co-Fe-P film (330 mV) and the Ir/C (335 mV). Compared with the previously-reported materials, (Table S4), the Co-Fe-P foam exhibited relatively high OER activity. In addition to the intrinsic OER activity of the Co-Fe-P composition, the large area of the foam structure could lead to the excellent OER activity. From the data in Fig. 4a, the Tafel slopes of the Co-Fe-P film, Co-Fe-P foam, and Ir/C were plotted to be 40, 40 and 107 mV/dec, respectively, reflecting that the Co-FeP film and the Co-Fe-P foam had the same rate determining step of the OER with even a lower Tafel slope than that of the Ir/C (Fig. 4b). Therefore, at a higher operating current density in a water electrolysis system, the Co-Fe-P foam could be more advantageous than Ir/C. From the 8 ACS Paragon Plus Environment

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EIS results measured at a potential of 1.52 VRHE in a 1 M KOH solution (Fig. S4), the charge transfer resistance (Rct) of the Co-Fe-P foam and the Co-Fe-P film was estimated as 97 and 130 Ω, respectively. These results imply that electron transfer kinetics of the Co-Fe-P foam is better than the Co-Fe-P film, contributing to the higher OER catalytic activity. To evaluate the durability of the Co-Fe-P foam, the chronovoltammetric responses (V–t) was measured at 10 mA/cm2 in an Ar-saturated 1 M KOH solution, as presented in Fig. 4c. The potential at 10 mA/cm2 gradually increased from 294 to 320 mV during first 14,000 s and then remained almost constant up to 50,000 s, demonstrating superior durability of the Co-Fe-P foam as an OER catalyst. The HER activity of the Co-P, Co-Fe-P film and Co-Fe-P foam were estimated and compared with the Pt/C and Cu foam substrate in a potential range from 0 to - 0.3 V (Fig. 4d). The Co-Fe-P foam and Co-Fe-P film have HER overpotential of 73 and 96 mV (η @ 10 mA/cm2), respectively. Although the commercial Pt/C exhibits the lower HER overpotential (58 mV), the HER activity of Co-Fe-P foam can be regarded as one of the highest among the previouslyreported non-precious HER catalysts (Table S5). Moreover, the Tafel slopes for the Co-Fe-P film, Co-Fe-P foam and Pt/C were estimated to be 43, 44 and 70 mV/dec, respectively, showing that the Co-Fe-P film and Co-Fe-P foam have the same rate determining step of the HER with a significantly lower Tafel slope than that of the Pt/C (Fig. 4e). Regarding HER, Tafel, Heyrovsky and Volmer mechanisms have been proposed with corresponding Tafel slope of 30, 40 and 120 mV/dec. Thus, the Co-Fe-P film and Co-Fe-P foam Tafel slope implies that HER is mainly induced by the Heyrovsky mechanism. 31-32 Therefore, at a higher operating current density in a water electrolysis system, the Co-Fe-P foam could be more advantageous than that of the Pt/C. From the EIS results obtained in a 1 M KOH solution at a potential of -0.07 VRHE (Fig. S5), the 9 ACS Paragon Plus Environment


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Rct of the Co-Fe-P film and Co-Fe-P foam was determined to be 49 and 30 Ω, respectively, implying the faster electron transfer kinetics on the Co-Fe-P foam than on the Co-Fe-P film and the higher HER activity of the Co-Fe-P foam than the Co-Fe-P film. The HER durability of the Co-Fe-P foam was evaluated from chronovoltammetric responses (V–t) at 10 mA/cm2 in an Arsaturated 1 M KOH solution. As shown in Fig. 4f, the potential slightly decreased from -73 to 80 mV during first 700 s and then remained almost constant up to 50,000 s, indicating that the Co-Fe-P foam has an remarkable durability as a HER catalyst. All those result demonstrate that the Co-Fe-P foam has a superior bifunctional activity and durability towards OER and HER in alkaline solution. The Co-Fe-P foam also demonstrated high HER activity in acid solution (0.5 M H2SO4). Fig. S9 compares the HER activity of Co-P, Co-Fe-P film, Co-Fe-P foam and Pt/C. The Co-Fe-P foam exhibited higher HER activity (η @ 10 mA/cm2 = 70 mV) than the Co-P film (104 mV) and Co-Fe-P film (87 mV). Although the Co-Fe-P foam has lower HER activity than the commercial Pt/C (η @ 10 mA/cm2 = 24 mV), it exhibited superior activity as a non-precious catalyst. For comparison, HER activity of previously-reported non-precious catalysts in acidic media is summarized in Table S6. The porous structure of the Co-Fe-P foam related to large electrochemical active surface area induces the promoted OER and HER activities. To investigate the electrochemically active surface area (ECSA), cyclic voltammetry (CV) was conducted at various scan rates from 0.01 to 0.1 V/s (Fig. S10a and S10b). From the non-faradic region in the cyclic voltammograms, the double layer capacitance (CDL) of the Co-Fe-P film and Co-Fe-P foam was calculated shown in Fig. S10c and S10d. Then, the ECSA was obtained with the following equation: ECSA = CDL/CS, (CS was the atomically smooth plane capacitance and was assumed as 40 µF cm2.) In 10 ACS Paragon Plus Environment

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terms of the calculated ECSA, Co-Fe-P foam (798.2 cm2) had about 94 times larger active surface than the Co-Fe-P film (8.5 cm2). Since the Co-Fe-P foam and the Co-Fe-P film had a loading mass of 1.12 and 0.51 mg/cm2, the ECSA per unit mass was 71.27 and 1.67 m2/g, respectively. Consequently, the highly porous 3D Co-Fe-P foam with the large electrochemical active surface area has a superior catalytic activity towards the OER and HER.

CONCLUSIONS In summary, highly active and durable bifunctional Co-Fe-P catalysts for the OER and HER were simply prepared by electrodeposition. Among the prepared Co-Fe-P films (Fe/Co = 0.2, 0.4, 1.1, 3.3), the Co-Fe-P with a Fe/Co of 1.1 exhibited the highest OER activity (η @ 10 mA/cm2: 330 mV) due to the electron transfer from the Co to the Fe and P. By fabricating the Co-Fe-P with a Fe/Co of 1.1 in a porous foam structure, the ECSA was increased from 1.67 (CoFe-P film) to 71.27 m2/g (Co-Fe-P foam). As a result, the OER activity of the Co-Fe-P foam (η @ 10 mA/cm2: 294 mV) had one of the best activities even better than that of the Ir/C (η @ 10 mA/cm2: 335 mV) in a 1 M KOH solution. For the HER, although lower than that of the Pt/C (η @ 10 mA/cm2: 54 mV), the prepared Co-Fe-P foam exhibited the highest activity (η @ 10 mA/cm2: 73 mV) among the previously reported non-precious catalysts. The superior performance can be attributed to the characteristic of the electrodeposition method that can make a highly-porous 3D structure with a designed chemical composition. The OER and HER overpotential at 10 mA/cm2 gradually increased during 50,000 s, demonstrating excellent durability. In conclusion, the Co-Fe-P foam prepared in this study exhibited a superior OER/HER activity and durability in an alkaline solution. Since the electrodeposition is a well-



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established technique, the electrodeposited 3D porous Co-Fe-P foam can be directly applied to commercial water electrolyzers with high activity and stability.

EXPERIMENTAL SECTION Preparation of the Co-Fe-P film catalysts: Co-Fe-P films were formed by electrodeposition using a three-electrode cell in an electrolyte composed of 0.1 M CoCl2, 0.8 M NaH2PO2 • H2O, 0.6 M NH2CH2COOH, 0.93 M NH4Cl and 0.01~0.5 M FeCl2 (Table S1). A copper rotating disk electrode (RDE) was used as the electrodeposition substrate. As a counter electrode, a Pt mesh was employed. All the potential was measured with respect to the saturated calomel electrode (SCE). The Co-Fe-P film was deposited galvanostatically at a cathodic current density of – 0.05 A/cm2 for 300 s in the prepared electrolytic solution while rotating the RDE at 3600 rpm. All the electrodeposition processes were conducted at room temperature.

Preparation of the Co-Fe-P foam catalysts: Cu foam substrates were formed by electrodeposition using a two-electrode cell in an electrolyte composed of 0.2 M CuSO4, 0.7 M H2SO4, 1.2 M (NH4)2SO4 and 0.4 mM BTA.29 A copper rotating disk electrode (RDE) was used as the electrodeposition substrate. A pure Cu plate was used as the counter electrode. Cu foam was deposited galvanostatically at a cathodic current density of – 3 A/cm2 for 4 s in the prepared 12 ACS Paragon Plus Environment

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electrolyte without rotating. The prepared Cu foam was used as the electrodeposition substrate for the Co-Fe-P foam using a three-electrode cell in an electrolyte composed of 0.1 M CoCl2, 0.8 M NaH2PO2 • H2O, 0.6 M NH2CH2COOH, 0.93 M NH4Cl and 0.1 M FeCl2. A Pt mesh counter electrode was used and the potential was measured with respect to the saturated calomel electrode (SCE). The Co-Fe-P foam was deposited galvanostatically at a cathodic current density of – 0.085 A/cm2 for 300 s in the prepared electrolytic solution while rotating the RDE at 3600 rpm. All the electrodeposition processes were conducted at room temperature.

Preparation of the Pt/C and Ir/C catalysts: The commercial Pt/C and Ir/C catalysts (20 wt % on Vulcan XC-72) were supplied from Premetek. First, 5 mg of Pt/C (or Ir/C) were suspended in a mixture of isopropyl alcohol (400 µL) and Nafion (30 µL) to form a catalyst ink. Then, 5 µL of the catalyst ink were loaded onto a freshly polished glassy carbon rotating disk electrode (GC RDE) and dried at room temperature.

Characterization: Surface images of the Co-Fe-P film and foam catalyst were observed with a scanning electron microscope (SEM). The composition of the catalysts surface was evaluated by energy-dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). The microstructure of the Co-Fe-P foam catalyst was investigated with transmission electron microscopy (TEM). For the TEM analysis, the Co-Fe-P foam catalysts were raked out from the RDE with a sharp blade and then dispersed in ethanol by sonication for 30 min. The crystallinity and structure of the catalyst was analyzed by X-ray diffraction (XRD).



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Electrochemical Measurements: The electrochemical measurements were carried out in a three electrode cell. A Pt mesh and a standard calomel electrode (SCE) were used as the counter and reference electrode, respectively. The iR drop was compensated for using 4.8 Ω. All the potentials measured with respect to SCE were converted to the reversible hydrogen electrode (RHE). (ERHE = 1.05 V + ESCE).

The OER and HER activities were investigated by linear sweep voltammetry (LSV) at a scan rate of 10 mV/s (OER) and 2 mV/s (HER) in a 1 M KOH aqueous solution. Electrochemical impedance spectroscopy (EIS) was measured by frequency sweep from 105 Hz to 10-2 Hz using a superimposed sinusoidal signal of 10 mV rms at 1.52 VRHE and - 0.07 VRHE. To obtain the electrochemical capacitance (CDL) and electrochemically active surface area (ECSA), cyclic voltammograms in a non-Faradaic region were collected in the potential window at different scan rates (v) from 0.01 to 0.1 V/s. The ECSA was obtained based on the following equation: ECSA = CDL/CS, (CS was the atomically smooth plane capacitance and was assumed as 40 µF cm2. 30 For the EIS and CV measurements, the rotation rate of the RDE was 3600 rpm.


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FIGURE



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Fig. 1. (a) OER polarization curves, (b) overpotential for OER at a current density of 10 mA/cm2, (c) HER polarization curves and (d) overpotential for HER at a current density of 10 mA/cm2 for Co-P, Co-Fe-P-0.2, Co-Fe-P-0.4, Co-Fe-P-1.1 and Co-Fe-P-3.3 in 1 M KOH solution.


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Fig. 2. XPS spectra of Co 2p, Fe 2p and P 2p for (a) Co-P and (b) Co-Fe-P-1.1.



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Fig. 3. (a) SEM and (b) TEM images of the Co-Fe-P foam prepared by electrodeposition at a current density of - 0.085 A/cm2 for 300 s from 0.1 M CoCl2, 0.8 M NaH2PO2 • H2O, 0.6 M NH2CH2COOH, 0.93 M NH4Cl and 0.1 M FeCl2 solution.


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Fig. 4. (a) OER polarization curves and (b) Tafel plots for the Co-P film, Co-Fe-P film, Co-Fe-P foam and Ir/C. (c) Potential-time responses (V–t) of the Co-Fe-P foam at an OER current density of 10 mA/cm2. (d) HER polarization curves and (e) Tafel plots for the Co-P film, Co-Fe-P film, Co-Fe-P foam and Pt/C. (f) Potential-time responses (V–t) of the Co-Fe-P foam at a HER current density of -10 mA/cm2. (All experiments were conducted in 1 M KOH solution.)

ASSOCIATED CONTENT The Supporting Information is available free of charge. Preparation process, additional SEM images, EDS, XRD patterns, nyquist plots and cyclic voltammograms for ECSA (PDF)

AUTHOR INFORMATION



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Corresponding Author EunAe Cho*: [email protected] HyukSang Kwon*: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was sponsored by the National Research Foundation of Korea Grant, funded by the Korean Government (MSIT) (NRF-2017M1A2A2072597).

REFERENCES


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For Table of Contents Use Only


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Synopsis: A 3D porous Co-Fe-P foam fabricated using electrodeposition is presented as a highly active catalyst for oxygen and hydrogen evolution reactions.


A 3D Porous Cobalt-Iron-Phosphorous Bifunctional Electrocatalyst for

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