Morphology Control of Carbon-Free Spinel NiCo2O4 Catalysts for

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Morphology Control of Carbon-free Spinel NiCo2O4 Catalysts for Enhanced Bifunctional Oxygen Reduction and Evolution in Alkaline Media Surya Vamsi Devaguptapu, Sooyeon Hwang, Stavros Karakalos, Shuai Zhao, Shiva Gupta, Dong Su, Hui Xu, and Gang Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16389 • Publication Date (Web): 06 Dec 2017 Downloaded from http://pubs.acs.org on December 6, 2017

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ACS Applied Materials & Interfaces

Morphology Control of Carbon-free Spinel NiCo2O4 Catalysts for Enhanced Bifunctional Oxygen Reduction and Evolution in Alkaline Media Surya V. Devaguptapu,† Sooyeon Hwang, ‡ Stavros Karakalos,§ Shuai Zhao,¶ Shiva Gupta,† Dong Su,‡ Hui Xu,¶, * and Gang Wu†,* †

Department of Chemical and Biological Engineering, University at Buffalo, The State University of New York, Buffalo, New York 14260, United States ‡

Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, United States ¶

§

Giner Inc., Newton, MA 02466, United States

Department of Chemical Engineering, University of South Carolina, Columbia, South Carolina 29208, United States Corresponding authors.

E-mail addresses: [email protected], (H Xu), [email protected] (G. Wu)

Abstract: Spinel NiCo2O4 is considered a promising precious metal-free catalyst that is also carbon-free for oxygen electrocatalysis. Current efforts mainly focus on optimal chemical doping and substituent to tune its electronic structures for enhanced activity. Here, we study its morphology control and elucidate the morphology-dependent catalyst performance for bifunctional oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Three types of NiCo2O4 catalysts with significantly distinct morphologies were prepared using temple-free, Pluronic-123 (P-123) soft, and SiO2 hard templates, respectively, via hydrothermal methods following by a calcination. While the hard-template yields sphere-like dense structures, softtemplate assists the formation of a unique nano-needle cluster assembly containing abundant

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meso- and macro pores. Furthermore, the effect of morphology of NiCo2O4 on their corresponding bifunctional catalytic performance was systematically investigated. The flowerlike nano-needle assembly NiCo2O4 catalyst via the soft template method exhibited the highest catalytic activity and stability for both ORR and OER. In particular, it exhibited an onset and half-wave potentials of 0.94 and 0.82 V vs. RHE, respectively, for the ORR in alkaline media. Although it is still inferior to Pt, the NiCo2O4 represents one of the best ORR catalyst compared to other reported carbon-free oxides. Meanwhile, remarkable OER activity and stability were achieved with an onset potential of 1.48 V and a current density of 15 mA/cm2 at 1.6 V, showing no activity loss after 20,000 potential cycles (0 to 1.9 V). The demonstrated stability is even superior to Ir for the OER. The morphology-controlled approach provides an effective solution to create a robust 3D architecture with increased surface areas and enhanced mass transfer. Importantly, the soft template can yield high degree of spinel crystallinity with ideal stoichiometric ratios between Ni and Co, thus promoting structural integrity with enhanced electrical conductivity and catalytic properties. Keywords: spinel NiCo2O4 oxide; oxygen reduction; oxygen evolution; electrocatalysis; energy conversion; alkaline media 1. Introduction Recently, a concept was proposed to integrate a fuel cell and an electrolyzer together for reversible energy conversion and storage devices, i.e. regenerative fuel cells. Similar to rechargeable batteries, a water electrolyzer can store renewable electricity via water splitting to produce H2 and O2 gases. In turn, a fuel cell can release energy later through generating electricity by consuming these H2 and O2.1 However, slow kinetics of the oxygen reduction reaction (ORR) in fuel cells and the oxygen evolution reaction (OER) in electrolyzers often cause a major 2 ACS Paragon Plus Environment

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energy loss.2 Platinum group metals (PGM) such as Pt and Ir can significantly catalyze these oxygen reactions, thus improving energy conversion efficiency. However, their prohibitive cost and scarcity limit their large-scale applications for these clean electrochemical energy technologies.3 Development of cost-effective and high-performance PGM-free catalysts therefore plays a pivotal role in effectively utilizing renewable energy.2 Relative to acidic electrolytes, alkaline fuel cells and electrolyzers have many advantages including high tolerance to impurities, favorable oxygen reaction kinetics, and less corrosive environment for catalysts.4 PGM-free catalysts would be more desirable in alkaline media and hold greater promise to completely replace PGM catalysts. Among studied PGM-free catalysts, carbon-based catalysts showed good activity for the ORR and some promise for the OER, which have been explored extensively as possible bifunctional catalysts.5-8 In principle, carbon is thermodynamically unstable (C(s) + 4OH− → CO2 (g)+ 2H2O(l) + 4e−, 0.207 V vs. SHE) and often suffers significantly from corrosion especially during the OER above 1.23 V.9-12 Therefore, in such an oxidative environment, most carbon-based catalysts exhibited significant performance degradation. Oppositely, transition metal oxides are intrinsically stable in oxidative OER environments.11, 13-15 Many metal oxide catalysts have been studied for the OER,16-25 but very few was for bifunctional ORR/OER, often due to their poor ORR activity.14,

19, 23, 26-27

It should be noted that most of current oxide-based bifunctional

catalysts are supported on various carbon to improve both electrical conductivity and ORR activity,14, 22, 28-29 but still rising stability concern due to carbon corrosion. Thus, it is of utmost goals to develop carbon-free oxide catalysts, which are active and stable for both the ORR and the OER.14 Among promising oxides, spinel NiCo2O4 has attract significant attention for electrochemical applications due to its good activity and stability.18, 20, 28, 30-36 Various NiCo2O4 oxides were synthesized by different methods for Li-ion battery anode due to its high capacity



for Li+ ions.37 As for oxygen electrocatalysts, most of works focused on its OER activity by substituting with different metal elements or creating surface oxygen deficiency to achieve optimal electronic structures.22, 38-40 Few work is related to its ORR and bifunctional ORR/OER catalysis, probably due to its poor ORR activity similar to other oxides. Motived by the successes of other oxides with controlled morphology showing enhanced electrochemical performance,41-44 we hypothesized that tuning nanostructures and morphologies of NiCo2O4 oxides can achieve high-performance bifunctional ORR/OER applications without the assistance of carbon supports. Using templates is an effective way to control catalyst morphology.45 For example, SiO2 particles were used to prepare porous carbon through a hightemperature carbonization of carbon precursors, following by a leaching treatment with KOH to remove SiO2.45 Triblock copolymers are one class of soft-template surfactant that is capable of obtaining ordered mesopores through a self-assembly process. Especially, P-123, a poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) block copolymers has been introduced to synthesize a wide variety of nanomaterials,46 which is an effective template for controlling the nucleation and growth of crystals. In previous reports, it was employed to synthesize nanoparticles of MnO2 and NiCo2O4, but the performance of OER is insufficient.47-49 In this work, using various templates including template-free, SiO2 hard template, and P-123 soft template, three spinel NiCo2O4 catalysts with significantly different morphologies were prepared respectively. Among others, the P-123 templated NiCo2O4, with optimal synthetic conditions, exhibits high crystallinity and unique nano-needle cluster assembly, showing the highest ORR and OER activity and stability. The effect of morphology on catalyst performance was elucidated by using extensive characterization. The high-performance NiCo2O4 oxide with ideal morphology and microstructure holds promise to totally eliminate the addition of carbon


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supports and can eventually overcomes the stability issues resulting from carbon corrosion for bifunctional ORR/OER catalysis. 2. Experimental details 2.1 Catalyst synthesis Three NiCo2O4 catalysts were synthesized through one-step hydrothermal synthesis methods by using different templates. In the case of template-free, 1.74 g Co(NO3)2.6H2O and 0.87 g Ni(NO3)2.6H2O with a mole ratio of 2:1 were dissolved in water with addition of 1.68 g urea. The solution was then stirred for 30 minutes until a clear light pink solution appears. The solution was then transferred to a 100 ml Teflon lined stainless steel autoclave and heated to 120oC for 12 hours with a ramp rate of 5 oC/min. After the hydrothermal process, the obtained precipitate was dried overnight at 70oC, followed by calcination in air at 350oC for 2 hours at an optimal ramping rate of 1 oC/min. As for the hard template catalyst, 0.5 grams of SiO2 was added along with the metal nitrates. In addition to the identical conditions, the sample was treated with 6.0 M KOH at 70oC for 6 hours to remove SiO2 template. For the soft template method, a triblock polymer, an optimal amount of 3wt.% Pluronic-123(P-123) was added for the hydrothermal process. A synthesis scheme is shown in Figure S1. The possible chemical reactions during the synthesis are summarized follows: NH2CONH2(s)+H2O(l) NH3(g)+CO2 (g) (Initial dissolution of urea in water)

(1)

NH3(g)+H2O(l)NH4++OH- (Ammonium ion generation)

(2)

Ni2++2Co2++6OH-  NiCo2(OH)6(s) (Hydroxide formation)

(3)

NiCo2(OH)6(s)+1/2 O2(g) NiCo2O4(s)+ 3H2O(g) (Annealing in air)

(4)

2.2 Materials characterization



The morphology of NiCo2O4 catalysts were characterized using scanning electron microscopy (SEM) on a Hitachi SU 70 microscope at a working voltage of 5 kV. The microstructures and crystalline of various NiCo2O4 catalysts were further compared using high-resolution transition electron microscopy (HR-TEM) on a JEOL JEM-2010 or a JEOL 2100F microscope at a working voltage of 200 kV. The crystalline phases present in each sample were identified using x-ray diffraction (XRD) on a Rigaku Ultima IV diffractometer with Cu K-α X-rays. BET surface area was measured using N2 adsorption/desorption at 77 K on a Micromeritics TriStar II. Samples were degassed at 130 °C for 5 h under vacuum prior to nitrogen physisorption measurements. All Raman spectra were obtained using a Renishaw Raman system at 514 nm excitation. X-ray photoelectron spectroscopy (XPS) was performed using a Kratos AXIS Ultra DLD XPS system equipped with a hemispherical energy analyzer and a monochromatic Al Kα source operated at 15 keV and 150 W; pass energy was fixed at 40 eV for the high-resolution scans. 2.3 Electrochemical measurements All electrochemical measurements were performed using a CHI Electrochemical Station (Model 760b) equipped with high-speed rotators from Pine Research Instruments. A glassy carbon rotating ring disk electrode (GC-RRDE) was used as the working electrode. Each catalyst powder (10 mg) was mixed with isopropanol (1.0 mL) and a Nafion® solution (15 μL) to produce an ink that was deposited onto the GC-RRDE. Electrochemical measurements were performed in a three-electrode electrochemical cell using a graphite electrode as the counter electrode and Hg/HgO as the reference electrode. The ORR and OER activities were measured in 1.0 M NaOH electrolyte saturated with O2 at 900 and 1600 rpm, respectively. To accurately measure the Faradaic current density without contributions from capacitive current, stair case


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voltammetry was used to obtain the steady-state polarization curves for all ORR and OER tests. Each potential was held constantly for 30 seconds with an increment of 30 mV for the ORR and for 40 seconds with an increment of 50 mV for the OER. Electrochemical impedance spectroscopy (EIS) was recorded for each NiCo2O4 catalyst in both ORR and OER modes at 0.6 and 1.6 V, respectively. The frequency range is from 100 K to 0.01 Hz. The amplitude is 10 mV. Harsh accelerated stress tests (AST) by cycling potential from 0 to 1.9 V vs. RHE in O2 saturated 0.1 M NaOH was carried out to study the stability of NiCo2O4 catalysts during the bifunctional ORR/OER. 3. Results and discussion 3.1 Morphology controls of NiCo2O4 Figure 1 shows the SEM images of three NiCo2O4 catalysts with distinct morphologies synthesized from template free, P-123 soft, and SiO2 hard template methods, respectively. In the case of template-free catalyst, a typical needle-like structure appears (Figure 1a), which is in good agreements with reported NiCo2O4 prepared for anode materials in Li-ion batteries and electrodes for supercapacitors.50-51 But such morphology was not studied for OER/ORR catalysts yet. When P-123 soft template was used, the needle-like structures assembly together and form highly porous nanoflower clusters (Figure 1b). The possible mechanism is associated with the formation of the micelles consisting of hydrophilic (PEO) and hydrophobic (PPO) chains in aqueous solution.52 Thus, metal ions including Ni and Co adsorb onto their external surface. As a result, a continuous hybrid framework of the nanoparticles could be distributed around the P-123 polymer due to the possible hydrogen bonds in water, which can further convert to such needlelike assembly. During the hydrothermal process, urea is decomposed and form hydroxides with Ni2+ and Co2+. The formation of NiCo2(OH)6 was also confirmed by using both XRD (Figure



S2a) and XPS (Figure S2b) indicating dominant hydroxide in the sample after hydrothermal process. The subsequent calcination further converts the hydroxide into spinel NiCo2O4 with more opening spaces between each nanoflower cluster as shown in Figure. S2c. Hard templates yield spherical and dense structures containing less pores (Figure 1c). The obvious sphere-like morphology with a diameter of 1.0 µm is due to SiO2 particles used for hard template during catalyst synthesis. Notably, significant aggregates are observed throughout the catalyst.

Figure 1. Three morphologies of NiCo2O4 catalysts synthesized from (a) template-free, (b) P123 soft template, and (c) SiO2 hard template, respectively. The SEM images are presented from low to high magnifications.


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HR-TEM images for NiCo2O4 samples synthesized from various methods were studied to further illustrate their difference in microstructure, morphology, and crystalline lattice. At first, the detailed comparison between template-free and P-123 templated NiCo2O4 were shown in Figure 2 and S3. The nano-needles observed in the template-free NiCo2O4 catalyst consist of many small particles with a size around 10-15 nm. Each particle is a single crystal as evidenced by TEM images at high-magnification. Thus, the nano-needle assembly is typical polycrystals of NiCo2O4, consisting of many single crystals with dominant grain boundary and dislocation defects. Similar single crystal particles present in soft-templated NiCo2O4 samples as well, but with slightly larger size around 15-20 nm. Also, its nano-needle assembly shows loose structures with larger pore (~5 nm) between particles relative to those in template-free NiCo2O4. The mesopores are typically formed during the conversion of hydroxides into spinel oxides via a calcination process. The gaseous species produced at high temperature is a main factor in the development of mesopores in oxide catalysts.53 Therefore, the large mesopore morphology observed with the P-123 templated NiCo2O4 is likely due to the formation of larger amount of gaseous species associated with the decomposition of P-123 during the calcination. Furthermore, these high magnification images clearly show lattice fringes having d-spaces of 0.47 nm and 0.29 nm for the template free catalyst corresponding to (220) and (111) planes. Likewise, d-spaces of 0.25 nm and 0.31 nm for the soft template assisted catalyst may correspond to (311) and (220). This suggests that soft template alters the crystalline orientation during the formation of single crystals of NiCo2O4. In addition, the selected-area diffraction (SAED) patterns for both catalysts shows well-defined diffraction rings, revealing its typical polycrystalline characteristics. The diffraction rings can be readily indexed to various planes of the NiCo2O4, which is consistent with the XRD patterns. Figure S4 present HR-TEM images of



SiO2 templated NiCo2O4. Unlike single crystal assembly in nano-needle morphology observed with P123-tempalted and template free samples, significant agglomeration of single crystals are observed. Also, 2D- sheet like morphology is dominant containing amorphous structures. This comparison indicated that templates play an important role in controlling NiCo2O4 crystalline and morphology, which affects resulting catalytic activity and stability.

Figure 2. TEM and HR-TEM images along with SAED patterns of (a-d) template free and (e-h) soft template assisted NiCo2O4 catalysts.

Such unique single crystal assembly in the form of nano-needle clusters of NiCo2O4 derived from soft templated was further studied using STEM couple with elemental mapping. Figure 3 indicates that Co and Ni are uniformly distributed throughout the NiCo2O4 polycrystals. The STEM images further verify dominant Co over Ni in NiCo2O4 catalysts. Elemental mapping on


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catalysts shows that the green dots corresponding to Co are nearly twice the red dots corresponding to Ni.

Figure 3. STEM images and elemental mapping of the soft-templated NiCo2O4 showing uniform distribution of Co and Ni.



3.2 Catalyst structure XRD patterns of three NiCo2O4 with different morphologies were analyzed to identify their crystalline phases (Figure 4a). The peaks for template-free and P-123 templated NiCo2O4 are well indexed based on JCPDS card No. 73-1702 corresponding to spinel NiCo2O4, which verify the exclusive spinel structure in the catalysts.54 The well-defined peaks suggest highly crystalline nature of the NiCo2O4 catalysts, which was in good agreement with the HR-TEM images. Relative to template free and P-123 soft templated samples, the degree of crystallinity is lower for the SiO2 hard templated NiCo2O4 with an absence of (111) and (222) peaks. In addition, the presence of minor peaks at 43.0o and 63.5o corresponds to SiO2, indicating that it has not been completely removed from the final catalyst. The trace of SiO2 impurity in NiCo2O4 catalyst may hamper catalyst performance. Raman analysis was further performed to understand the composition and structure of these NiCo2O4 samples (Figure 4b). The peaks are fitted using a Gaussian mechanism at 497.2, 605.8 and 651.3cm-1 corresponding to the Eg, F2g and A1g modes of NiCo2O4. They only show Co-O and Ni-O vibrations and no any signal corresponding to OH group. This also indicates that cobalt and nickel hydroxide salts are completely converted to spinel during a calcination at 350oC during the synthesis. However, the shift in the peak at 605.8 cm-1 with higher intensity for softtemplated NiCo2O4 is slightly different for the template-free one (594.5 cm-1). This can be attributed to the growth of a cluster of nano-needles, which show higher vibration frequency than an evenly spread polycrystal needle structure.55 There are also significant peak shifts between the soft and the hard templated catalysts with peaks at 669.1, 477 and 530 cm-1, likely due to the interactions between NiCo2O4 phase and trace of SiO2. In addition, the Raman analysis shown in


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Figure S5 clearly show the absence of D and G peaks associated with carbon structures at 1360 and 1560 cm-1, further confirming that these NiCo2O4 samples are carbon-free. P123-NiCo2O4

(533)

(440)

(422) (511)

(400)

(222)

(220) (311)

(b) (111)

(a)

P123-NiCo2O4

Intensity (a.u)

Intensity (a.u)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Template free-NiCo2O4

Template free NiCo2O4

SiO2-NiCo2O4

* SiO2

*

*

SiO2-NiCo2O4 20

30

40

50

60

70

80

200

300

400

500

600

700

800

Raman Shift (cm-1)

2(Degree)

Figure 4. (a) XRD patterns and (b) Raman spectra for these three NiCo2O4 catalysts with different morphologies.

XPS was used to analyze metal and oxygen species in surface layers of catalysts (Figure 5). As shown in Figure 5a, Co 2p spectrum is well fitted with two spin-orbit doublets;56 the fitting peaks at (779.3 eV, 793.65 eV) and (780.8 eV, 797.35 eV) are assigned to valence states corresponding to Co3+ and Co2+. Similarly, the fitting peaks at 787.9 and 803.2 eV are assigned as shakeup satellites (Sat.) at the high binding energy side of Co 2p3/2 and Co 2p1/2, respectively. Likewise, Ni 2p spectrum (Figure 5b) can be de-convoluted into two spin-orbit doublets and two shake-up satellites, indicating the existence of Ni2+ and Ni3+. The fitting peaks of O 1s spectra (Figure 5c) at 529.3, 530.7, and 531.8 eV typically correspond to a metal-oxygen bond, oxygen in cobalt hydroxyl group, and oxygen in nickel hydroxyl group. The SiO2 templated sample



shows addition water peak at 535.9 eV corresponding to physically and chemically bonded water. In addition, the SiO2-templated sample shows a Si peak at 102.3 eV (Figure S6), indicating the existence of trace of SiO2. XPS spectra also indicate that the as-obtained NiCo2O4catalyst products doesn’t contain other impurity including carbon. These results along with Raman (Figure 4b and Figure S5) verify that the NiCo2O4 catalysts are carbon-free, which is crucial for bifunctional catalysts eliminating carbon supports.57 The O concentration are around 38.4, 38.0 and 27.3 wt% for the template free, P-123, and SiO2 template assisted samples, respectively. The ratios of atomic concentration between Co and Ni should be ideally 2, but are 1.4, 1.9 and 2.7 for the template free, soft, and hard template assisted catalysts, respectively. Although NiCo2O4 crystalline structure have been identified in the bulk, the slight difference in stoichiometry in surface layers may be a key factor to affect catalytic activity due to possible surface oxygen-deficiency and metal site defects. (a)

(b) Co 2p

Ni 2p SiO2-NiCo2O4

Co3O4 779.3 eV

NiO 854.1 eV

(c)

Metal oxides 529.3 eV

O 1s SiO2-NiCo2O4

SiO2-NiCo2O4

H2O 535.9 eV

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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CoO 780.8 eV


Ni2O3 855.6 eV


C-O-C, Co(OH)2


530.7 eV

P123-NiCo2O4

810

805

800

795

790

785

Ni(OH)2 856.8 eV

P123-NiCo2O4

Co(OH)2 782.6 eV

P123-NiCo2O4 O-C=O, Ni(OH)2 531.8 eV

780

890

880

870

860

850

Binding Energy (eV)

Binding Energy (eV)

538

536

534

532

530

528

526

524

Binding Energy (eV)

Figure 5. The high-resolution (a) Co 2p, (b) Ni 2p, and (c) O 1s spectra of three NiCo2O4 catalysts with different morphologies.


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Figure 6a illustrates N2 adsorption-desorption isotherm of three NiCo2O4 catalysts. While the template-free catalyst only has a surface area of 16.9 m2/g, the use of soft template is able to increase up to 55.3 m2/g. The increased surface area should ideally provide abundant active sites for oxygen reactions. Figure 6b further indicates that all oxide catalysts contain large amount of meso and macropores, but are in absence of micropore. The soft template method is also favorable for generating larger pore volume (0.23 cm3/g) vs. the template free catalyst (0.078 cm3/g). On the other hand, the SiO2 templated catalyst shows the lowest BET surface area of 8.9 m2/g with an average pore width of 27.7 nm and pore volume of 0.054 cm3/g. A possible explanation is the highly dense structure of the catalyst and insufficient removal of SiO2. Both soft template and template-free catalysts present mesopore feature with narrow pore size distribution mainly centering at 15 and 17 nm, respectively. The highest surface areas and largest pore volume of the soft-templated NiCo2O4 may correspond to improved electrochemical properties with facile accessibility of electrolyte within the pores. Micropores

Mesopores

Macropores

0.010

0.008

0.006

(b) 180

P 123-NiCo2O4

Template free -NiCo2O4

SiO2-NiCo2O4

0.004

0.002

Quantity Adsorbed (cm3/g)

(a) Incremental Volume (cm3/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


P123-NiCo2O4

160


140

SiO2-NiCo2O4

Macropores

120 100 80 60 40

Mesopores Micropores

20 0

0.000

-20 1

10

100

0.0

0.2

Pore Size (nm)

0.4

0.6

0.8

1.0

Relative Pressure (P/P0)

Figure 6. (a) Pore size distributions and (b) N2 adsorption–desorption isotherms of three NiCo2O4 catalysts with different morphologies.



Electrochemically accessible/active surface areas (EASA) are measured for three oxides by using cyclic voltammetry (CV) recorded in 1.0 M NaOH in the absence of O2 (Figure S7). The non-Faradaic capacitances associated with the EASA on the NiCo2O4 catalysts are reflected by current densities of the CVs between 0.3 and 0.8 V. The gravimetric double layer capacitance C (F g−1) at a given scan rate (v) and mass of catalyst deposited on the electrode (m) can be related to the capacitance current.58-59 The EASA follows the same trend as the BET surface area, which the soft templated catalyst has the highest EASA (64.7 m2/g) followed by template-free (14 m2/g) and hard template assisted catalysts (7.2 m2/g). Thus, the controlled nano-needle flowerlike cluster assembly in the soft-templated NiCo2O4 catalyst would ideally provide the optimal morphology for oxygen reactions. We also speculate that the assembly could be more robust architecture with enhanced stability than a simple needle like or a spherical structure. 3.3 Catalyst activity and stability Following understanding of difference in morphology and structure between three NiCo2O4 catalysts, their corresponding catalytic performance are studied. Figure 7 displays the activity of three NiCo2O4 catalysts for the ORR and OER. The best activity is observed with the softtemplated NiCo2O4 catalyst, achieving onset (Eonset) and half-wave potential (E1/2) of 0.94 V and 0.82 V, respectively (Figure 7a). The catalyst shows only an 80 mV difference in E1/2 when compared to Pt/C at a loading of 60 g/cm2. It should be noted that the ORR activity is exceptional for carbon-free oxide catalysts, and represents as one of best oxide ORR catalysts (Table S1).60-61 On the contrary, the SiO2-tempaled NiCo2O4 exhibits the lowest activity only showing an E1/2 at 0.66 V, probably due to its dense and less porous morphology along with the existence of SiO2 impurity.


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In the case of OER (Figure 7b), both soft-templated and template-free NiCo2O4 catalysts present similar Eonset at 1.49 V as defined when generates a current density of 1.0 mA/cm2. This value is even 30 mV negative than that of Ir catalyst (1.52 V), indicating superior OER activity relative to precious metal catalyst reference. Furthermore, soft-templated NiCo2O4 also exhibited the best catalytic activity, as evidenced by the lowest overpotential (c.a. 350 mV) when generating a current density of 10 mA/cm2, relative to the template-free (370 mV) and SiO2templated catalysts (400 mV). The significant enhancement of the activity resulting from the use of the P-123 soft template is attributed to the formation of nanoneedle self-assembly and flowerlike clusters with a large amount of mesopores, which can increase reaction surface areas and efficient mass transfer. The underlying mechanism of the formation of the nano-needle assembly cluster is likely due to the P-123 micelles consisting of hydrophilic (PEO) and hydrophobic (PPO) chains, which acts as a template for the adsorption of metal ions and subsequent hydrolysis process to form hydroxides. The optimal synthesis to yield the unique nano-needle assembly cluster was found dependent on serval key factors including the calcination temperature and the content of P-123 template used for synthesis (Figure S8). Notable, a calcining temperature over 375°C would not yield the pure stoichiometric NiCo2O4, since NiO starts to segregate from the NiCo2O4 lattice.62 In addition to the favorable morphology, higher degree of crystalline for NiCo2O4 for the nano-needle cluster assembly compared to others may enhance the catalytic activity for the ORR and OER. Because distortion of spinel structure may reduce the electrical conductivity, which partially explain the inferior performance from template-free and SiO2 templated samples. Based on our XPS and XRF elemental analysis, the soft-templated sample shows that the chemical stoichiometry of Co to Ni ratio is closer to 2, a theoretical value. This could be another possible explanation that soft-templated sample yielded



the highest catalytic activity. During the ORR and OER on spinel oxide catalysts, Xu et. al revealed the governing role of octahedral-coordinated metal sites, and further demonstrated that eg occupancy of the active cation on the octahedral sites is the activity descriptor for spinels. 63 Here, Ni is located at octahedral sites and likely is active sites for the NiCo2O4 catalysts. In addition to morphology change, various templates may yield slightly difference in eg filling of Ni due to the effects on stoichiometry and crystalline structures. (a) 0.0

(b) 50

(c) 40

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-288 mV dec-1 SiO2-NiCo2O4

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-179 mV dec-1 P123-NiCo2O4

0.75


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.9

-71 mV dec-1 Template Free-NiCo2O4

SiO2-NiCo2O4 0.8

-59 mV dec-1

OER

1.65 Equation Weight Residual Sum of Squares Pearson's r Adj. R-Square

1.60 B

y = a + b*x No Weighting 1.70521E-4

65.46 mV dec-1 SiO2-NiCo2O4

-0.99048 0.97474 Intercept Slope

Value Standard Error 0.47975 0.02783 -0.0593 0.00476

109.88 mV dec-1 P123-NiCo2O4

1.55

0.45

-183 mV dec-1 Template Free-NiCo2O4

0.40 -3.5

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-3.5

-3.0

-2.5

-2.0

Log (jk)

Figure 7. Steady state polarization plots for the ORR (a) and the OER (b) of various NiCo2O4 catalysts recorded in O2 saturated 1.0 M NaOH at 900 and 1600rpm, respectively. (c) Peroxide yield and electron transfer numbers during the ORR. (d-e) Tafel plots in the high and low overpotential regions during the ORR, respectively. (f) Tafel plots at low overpotential during the OER.


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To study the selectivity of NiCo2O4 catalysts during the O2 reduction toward 4e- electron, the formation of peroxide HO2− was monitored by using ring currents measured from RRDE tests (Figure 7c). For the template-free and soft-templated catalysts, the yield of HO2− was below 5 % in a wide potential range from 0.4 to 0.9 V vs. RHE, giving corresponding electron transfer number (n) between 3.8 and 3.9. Conversely, the SiO2 templated catalyst yielded a high amount of peroxide (20 % at all potentials) giving clear indications that a two-electron transfer might be dominant and the catalyst might not be active for the ORR. Tafel slopes for the ORR and OER were further determined to elucidate the overall reaction mechanisms. For the ORR, we found a clear trend in Tafel slopes (Figure 7d and 7e), showing a transition from low to high overpotentials. This suggests a change in the rate-determining step (RDS) with the overpotential, which is consistent with previous reports.11, 64 Both template-free and P-123-templated NiCo2O4 catalysts shows Tafel slopes of -53 mV/dec in the low overpotential region, suggesting the addition of the soft-template doesn’t alter reaction pathways. This slope is close to that of commercial Pt/C catalysts (−62 mV/dec). In principle, a Tafel slope of -59 mV/dec is due to the RDS associated with the migration of adsorbed oxygen intermediates on a catalyst, resulting from coverage-dependent activation barrier for the ORR. As for the OER, Tafel slopes of Ir catalysts is close to 40 mV/dec. In theory, this is due to that second electron transfer associated with surface coverage of intermediate is the slowest step. However, NiCo2O4 catalysts demonstrated Tafel slopes closer to 118 mV/dec, suggesting that the RDS is the first electron transfer with a first-order kinetic in OH− concentration. We also evaluated the dependency of OER on the concentration of OH− ions for NiCo2O4 in different electrolytes 1.0 M vs. 0.1 M NaOH. A clear trend of increasing OER activity with an increase in alkaline strength of electrolyte was observed, as shown from Figure S9a. As expected, no such significant trend was



observed for ORR activity in different concentration of OH- ions (Figure S9b). Furthermore, we carried out EIS measurements to study the dependence of charge transfer resistances Rct during the ORR and OER on catalyst morphologies. As shown in Figure 8, Nyquist plots were recorded for three NiCo2O4 catalysts at 0.6 V and 1.6 V for the ORR and OER, respectively. The spectra with well-defined semi-arc are apparent. Using a relevant equivalent circuit,65 the fitting results indicated that P-123 templated NiCo2O4 have the lowest Rct in both ORR and OER, which is in good agreement with polarization plots shown in Figure 7. It should be noted that Rct for these NiCo2O4 catalysts is much larger than that of conventional carbon catalysts due to insufficient electrical conductivity. These oxides are not conductive enough to allow electrons going through the oxide. The electrochemical reactions only occur at the interfaces of oxides and electrolytes, where the reactants and electron can reach the catalysts. Thus, the boundary between metal oxide and the electrolyte for these oxide catalysts are even more crucial for activity improvement, which is likely governed by morphology control as we investigated in this work.66

(a)

(b) 250

5000 Template Free - NiCo2O4

Template Free - NiCo2O4

@0.6 V in 0.1 M NaOH

@1.6 V in 0.1 M NaOH

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P123 - NiCo2O4 SiO2 - NiCo2O4

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-Z'' ()

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3000

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Figure 8. EIS to determine charge transfer resistance of ORR (a) and OER (b) on NiCo2O4 catalyst prepared from template-free, P-123, and SiO2 templates.


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Stability of PGM-free catalyst is more challenging relative to high initial activity. Here, we conducted long-term accelerated stress testing (AST) to evaluate stability of three NiCo2O4 catalysts within a wide potential window covering the ORR and OER (0 to 1.9 V in O2 saturated 0.1 M NaOH). As shown in Figure 9, the best performing soft-templated NiCo2O4 is also highly stable up to 20,000 cycles in the high potential region of OER even with an 20% increase in current density generated at 1.6 V. Under identical AST conditions, template-free and SiO2templated NiCo2O4 catalysts loss 27% and 57 % OER activity, respectively. The Pt-Ir mixture, as a precious metal-based bifuncational reference catalyst, also exhibited a loss of 40% in current density at 1.6 V after 10, 000 cycles (Figure S10). Therefore, the soft-templated NiCo2O4 catalyst has achieved superior stability to precious metals for the OER under the harsh AST. During the ORR region, the P-123 templated catalyst shows a decrease in the potential of about 80 mV after 20,000 cycles at a current density of 1.0 mA/cm2. Meanwhile, the precious metal based Pt-Ir bifunctional catalyst also loses 90 mV in E1/2 of ORR after 10,000 cycles. Much larger ORR degradation was observed with template-free NiCo2O4 catalyst with a loss of 220 mV in E1/2. Due to the poor ORR activity, the decline observed with the SiO2-templated NiCo2O4 is not significant.



(b)10

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Figure 9. Stability tests of the ORR (a-c) and the OER (d-f) for the soft template, template-free, and hard template NiCo2O4 catalysts. Testing conditions are in O2 saturated 0.1 M NaOH at 25oC and 900rpm with continuous potential cycling: 0-1.9 V vs. RHE. All catalyst loading is 1.0 mg/cm2.

To understand the possible reasons of activity loss during the ORR, morphology changes were studied by using SEM images after the AST (Figure S11). Unlike template-free catalyst showing complete loss of needle-like porous morphology and becoming dense, the selfassembled needle cluster in soft-templated NiCo2O4 is nearly retained, while some of needle assembly are collapsed. Furthermore, HR-TEM images of the sample after potential cycling were analyzed as shown in Figure S12. Although overall shape of the sample is somewhat changed, crystallinity of each NiCo2O4 particles is well retained after 20, 000 potential cycling from 0-1.9 V in 0.1 NaOH electrolyte. To correlate the activity decline with the changes of surface species


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of the NiCo2O4, the sample derived from P-123 template was analyzed after AST by using XPS as shown in Figure 10. After 20, 000 potential cycles, Co species at the catalyst surface remains nearly the same, showing similar ratios between Co(OH)2, CoO, and Co3O4 to those of the asprepared sample. Notable, the content of CoO is slightly increased after cycling, which could promote the OER. On the other hand, Ni species is not stable during the AST. Ni(OH)2 becomes dominant over Ni2O3 and NiO. Thus, the formation of additional CoO and Ni(OH)2 is likely responsible for the activity loss of ORR. Meanwhile, we hypothesize that the activity declined especially during the ORR may be associated with concentration of oxygen vacancy on oxide catalysts. Because previous DFT simulation results suggest that the increasing oxygen vacancy concentration reduces the adsorption energy of water molecules and their dissociation energy barrier on the surface of the catalyst.67 Thus, the possible oxygen vacancies in oxide catalysts, which can be simultaneously stable under both reductive and oxidative environments for the ORR and OER, respectively, seems very challenging in such wide potential ranges. Therefore, new strategies are needed to stabilize transition metals and the surface defects in catalysts, which will be future research focus.



(a)

(b) Ni 2p

Co 2p

XPS Peak Intensity /a.u.


Co3O4 779.3 eV CoO 780.8 eV

Co(OH)2 782.6 eV

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890

775

Ni(OH)2

NiO 854.1 eV

856.8 eV

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880

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850

Binding Energy /eV

Binding Energy /eV

Figure 10. Co and Ni species on P-123 templated NiCo2O4 (a, b) after and (c, d) before stability ASTs (0-1.9 V in O2 saturated 0.1 M NaOH).

4. Conclusions Development of high-performance ORR/OER bifunctional catalyst is very crucial for advanced electrochemical energy technologies including reversible alkaline fuel cells and metal-air batteries. In this work, to eliminate the use of carbon as supports due to its stability issue, we identified an effective solution to realize morphology control for well-defined spinel NiCo2O4 catalysts, which can significantly enhance its activity and stability. In particular, the morphology dependent performance was systematically studied by using three types of NiCo2O4 catalysts with distinct morphologies via template-free, P-123 soft, and SiO2 hard template methods, respectively. Among others, the P-123-template can yield an NiCo2O4 catalyst with unique nano needle cluster assembly, which exhibited significantly enhanced ORR/OER activity and stability.


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The porous nano-needle clusters consist of many single crystals of NiCo2O4 featured with high specific surface area and a large amount of meso- and macro-pores. This effective morphology control approach can produce favorable structures for oxide catalysts with better dispersion and accessibility of active sites as well as the structural robustness. Importantly, optimal P-123 template method can produce NiCo2O4 single crystals with high degree of spinel crystallinity and present a theoretical stoichiometry ratio (i.e., 1: 2) of Ni to Co. These structural integrity properties can further enhance catalytic activity and stability along with enhanced electrical conductivity. Thus, morphology control of the spinel oxide was clearly identified as an effective solution to significantly enhance catalyst performance for both ORR and OER. Unlike previously reported methods, no additional carbon support is necessary for the catalyst to achieve good performance especially for the ORR, which avoids serious carbon corrosion and the associated performance loss. Notable, development of stable oxide catalysts in a wide potential window covering the ORR and the OER is very challenging. The observed performance loss of ORR in the best performing catalysts is due to the instability of Ni and Co in catalysts and the formation of additional CoO and Ni(OH)2. Future work will be focusing on exploration of new strategies to stabilize these transition metals by introducing co-catalysts or developing advanced oxide systems. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Additional characterization and result discussion AUTHOR INFORMATION 25 ACS Paragon Plus Environment


Corresponding Author * Email addresses for G.W.: [email protected]; H.X.: [email protected] Notes The authors declare no competing financial interest.

Acknowledgements The authors acknowledge financial support for the start-up funding from the University at Buffalo (SUNY) along with National Science Foundation (CBET-1604392) and U.S. Department of Energy, Fuel Cell Technologies Office (FCTO) Incubator Program (DEEE000696). TEM and STEM analysis were performed at the Center for Functional Nanomaterials, a U.S. DOE Office of Science Facility, at Brookhaven National Laboratory under Contract No. DE-SC0012704. References 1. Carrette, L.; Friedrich, K. A.; Stimming, U., Fuel Cells – Fundamentals and Applications. Fuel Cells 2001, 1, 5-39. 2. Gasteiger, H. A.; Kocha, S. S.; Sompalli, B.; Wagner, F. T., Activity Benchmarks and Requirements for Pt, Pt-Alloy, and Non-Pt Oxygen Reduction Catalysts for Pemfcs. Appl. Catal. B: Environ. 2005, 56, 9-35. 3. Zhang, H.; Hwang, S.; Wang, M.; Feng, Z.; Karakalos, S.; Luo, L.; Qiao, Z.; Xie, X.; Wang, C.; Su, D., Single Atomic Iron Catalysts for Oxygen Reduction in Acidic Media: Particle Size Control and Thermal Activation. J. Am. Chem. Soc. 2017, 139, 14143–14149. 4. McLean, G. F.; Niet, T.; Prince-Richard, S.; Djilali, N., An Assessment of Alkaline Fuel Cell Technology. Int. J. Hydrogen Energy 2002, 27, 507-526. 26 ACS Paragon Plus Environment

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