Dynamic Hydrogen Bubble Templated NiCu Phosphide Electrodes for

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Dynamic Hydrogen Bubble Templated NiCu Phosphide Electrodes for pH-Insensitive Hydrogen Evolution Reactions Majid Asnavandi, Bryan Harry Rahmat Suryanto, Wanfeng Yang, Xin Bo, and Chuan Zhao ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02492 • Publication Date (Web): 30 Jan 2018 Downloaded from http://pubs.acs.org on January 31, 2018

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ACS Sustainable Chemistry & Engineering

Dynamic Hydrogen Bubble Templated NiCu Phosphide Electrodes for pH-Insensitive Hydrogen Evolution Reactions Majid Asnavandi+, Bryan H. R. Suryanto+, Wanfeng Yang, Xin Bo and Chuan Zhao* School of Chemistry, The University of New South Wales, High Street, Sydney, NSW 2052, Australia

Corresponding Author Chuan Zhao, E-mail: [email protected]

ABSTRACT

A highly efficient porous NiCu-phosphide (NiCu-P) electrode is reported for hydrogen evolution reaction (HER) in acidic, neutral and basic electrolytes. The NiCu-P electrode was prepared via hydrogen bubbles dynamic templated electrodeposition of NiCu alloy onto nickel mesh, followed by phosphidation. Due to the synergistic interaction in NiCu-P, the material exhibits excellent HER activity in wide pH range from 0.3 - 14. Current density of -10 mA cm-2 was achieved at low overpotentials of -226, -250, and -175 mV in 0.5 M H2SO4, 1 M phosphate buffer solution, and 1 M KOH, respectively. The fabricated NiCu-P electrode also shows an

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outstanding electrochemical stability, regardless of electrolyte pH. The NiCu-P has a slope value of -53 mV dec-1, indicating that HER at NiCu-P follows the Heyrovsky mechanism in 1 M KOH.

KEYWORDS: water splitting, catalyst, hydrogen evolution reaction mechanism, bimetallic, neutral media


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Production of hydrogen (H2) via electrochemical water-splitting driven by renewable energy is key to the success of the forthcoming transition from fossil fuel economy into the hydrogen economy.1 Hitherto, the cost of state-of-the-art HER electrocatalysts based on Pt has been considered as one of the major issues. Hence enormous current researches are focusing on the development of alternative catalysts based on earth-abundant elements. Several electrocatalysts based on Ni, Co, Fe and Mo have since been reported and showed excellent HER activity in either extremely basic or acidic electrolytes.2 However, it is noteworthy that the pH of earth’s largest body of water reserve lies within the neutral range. Therefore, the development of a HER catalyst that is active in a wider active pH range is imperative. By rational design, an increased electrode activity can be achieved by the synergistic interaction of bi-metallic system and active surface area improvements.3 Among the available bimetallic systems, preliminary studies have shown that Ni-Cu expedites synergistic interaction for HER in alkaline media.4 It is also worth to mention that, with a reference to a well-established volcano plot correlation between exchange current density and hydrogen bonding energy shows that Ni is located on the left branch of the plot (strong hydrogen binding), while Cu is on the right branch (weak hydrogen binding).5 Therefore, it is reasonable to deduce that the observed high HER activity results from the synergistic interaction between Ni and Cu which leads to an optimum hydrogen binding energy for catalyzing HER. NiCu electrocatalysts can be prepared by electrodeposition approach, which eliminates the need of using chemical binders (e.g. Nafion) for electrocatalyst immobilization on electrode substrates and also for improved electron transport and mechanical robustness.6 Further, electrodeposition via hydrogen bubble template also can be applied to achieve high surface area. In this technique, vigorous hydrogen bubbling occurs on the surface and acts as a dynamic template, resulting in a highly porous structure.7,8


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Phosphidation has been used widely9–12 and shown to be an effective approach for improving the activity and electrochemical stability of transition metal-based HER electrocatalysts in acidic media. It is known that transition metal phosphides can act as a proton-acceptor to initiate HER and also weaken the bond strength to the hydrogen molecules which facilitate delivery of hydrogen from the catalyst surface.13,14 These materials also exhibit good electrical conductivity due to its metalloid character. Research work has shown that phosphorous doping even as low amount of 1% results in favorable phenomena such as refining the microstructure and improving top coating adhesion.15 Here we show a porous NiCu-phosphide (NiCu-P) electrode for efficient hydrogen evolution reaction in all pH 0.3 - 14. The highly porous NiCu has been electrodeposited on nickel mesh (NM) substrate by dynamic hydrogen bubble templating from an aqueous solution containing Ni2+ and Cu2+ ions (see details and Table S1 in supporting information). Subsequently, phosphidation has been performed through am annealing process at 500°C under Ar atmosphere (the schematic of NiCu-P synthesis is shown in Figure 1a). The NiCu-P loading on NM is 65 mg cm-2. The as-synthesized NiCu-P on NM substrate exhibits a highly porous structure, with micropores of ca. 2 – 50 µm in diameters as shown in Figure 1b,c. The highly porous structures were formed by dendritic microstructures (Figure 1c and S1). As confirmed by electrochemical active surface area (ECSA) obtained through double layer capacitance measurements16 in Figure S2, a significant increase of active surface area from 0.51 cm2 (non-porous NiCu) to 32.2.00 cm2 (porous NiCu) was recorded. SEM also demonstrates that highly porous structures are formed in the electrodeposition of NiCu bimetallic system. The formation of NiCu-P (atomic ratio of Ni:Cu:P, 7:3:1) was also confirmed by energy dispersive X-ray elemental mapping as shown in


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Figure S3. The appropriate ratio of Ni to Cu was achieved after optimization experiments (Figure S4) and is consistent with previous results.17

Figure 1. (a) schematic of preparation steps of porous NiCu-P catalysts, (b-e) SEM images of; (b) top view of NiCu-P on nickel mesh substrate, (c) cross-section of NiCu-P, (d) left image is Ni and the right one in Ni-P, (e) left image is Cu and the right one in Cu-P. Furthermore, SEM was also used to reveal the synergistic effect between Ni and Cu in the formation of the highly porous structure. By comparing morphological features from the SEM of electrodeposited Ni, Ni-P, Cu and Cu-P shown in Figure 1d and e, it is evident that the porosity of electrodeposited Ni is significantly lower than the electrodeposited Cu. In the absence of Cu, the number of pores per unit area and pore diameters are reduced significantly. The reductions in both of the structural qualities are ascribed to the difference in both of the standard reduction


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potential and the intrinsic hydrogen evolving catalytic activities of Ni and Cu.18 The lower porosity of Ni electrodeposit can be ascribed to the higher HER kinetics (finer H2 bubble evolutions) and the greater energy barrier for Ni2+ electroreductions which results in slower deposition kinetics.19 On the other hand, in the absence of Ni, Cu deposit exhibits highly porous structure (Figure 1e), since metallic copper is not active towards HER. In addition, as it can be seen in Figure 1d and 1e, the structure of individual electrodeposited Ni or Cu was not retained following phosphidation, and the porosity decreased significantly, while high ECSA was only obtained for NiCu following phosphidation procedure. The breakdown of Cu-P porous structure can be attributed to the formation of unstable copper phosphide as reported by previous studies.20

Figure 2. X-ray photoelectron spectroscopy elemental analysis, (a) survey scan of NiCu-P, (b) Ni 2p scan, (c) Cu 2p scan and (d) P 2p scan of NiCu-P.


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X-ray photoelectron spectroscopy further confirms the presence of Ni, Cu and P in the electrodeposited and phosphidized samples, respectively as shown in Figure 2a. The Figure 2b, c and d showcase the high-resolution XP spectra for Ni 2p, Cu 2p and P 2p. As it is shown in Figure 2b, the Ni 2p scan displays Ni 2p3/2 core-level peaks at ~857.32 eV and Ni 2p1/2 core level peaks at ~873-875 eV in NiCu and NiCu-P samples which can be related to Ni2+ oxidation state in nickel oxides.21 The effect of phosphidation is observed by the formation of Ni 2p3/2 peak at lower binding energy values of 853.70 eV, confirming that Ni has formed a partial positive charge (δ+) as expected in Ni-P.22 On the other hand, the high-resolution Cu 2p XP spectra displays Cu 2p3/2 core-level peaks at 932-935 eV and Cu 2p1/2 core level peaks at ~952-955 eV. The Cu 2p3/2 at 934.52 eV in NiCu sample can be assigned to Cu2+ in copper oxides. In NiCu-P sample, the position Cu 2p3/2 was found in the lower binding energy of 932.89 eV due the formation of positively charged Cuδ+ in Cu-P.23 Furthermore, the presence of metal-phosphides were also validated the high-resolution P 2p XP spectra displayed in Figure 2d. The peak at 129.8 eV can be assigned to Pδ- from metal phosphides formation (i.e. Ni-P and Cu-P),24 while the peaks at 130.68 eV and 134.85 eV are related to the unreacted red-phosphorus and phosphate formation, respectively.22,23 Further physical characterization using X-ray Diffraction (XRD) spectroscopy was also performed to provide more structural insights. The XRD patterns in Figure 3 demonstrate that the electrodeposited layer prior to phosphidation is a crystalline NiCu with merged slightly shifted peaks of Ni and Cu25 as it is expected for dendritic materials. The phosphidation process leads to the formation of new phases as shown by the emergence of several diffraction peaks. Further investigation and comparison with Ni-P (Figure S5), reveals that these peaks are formed from X-ray diffraction of Ni2P26,27 (ICDD: 03-065-1989), Ni3P at 2Ɵ of 29.2° and 43.8°28 and


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Cu3P (JCPDS no. 71-2261).29 In addition, the remaining unassigned peaks were found to be related to Cu-P constituents as revealed from the XRD of Cu-P as shown in Figure S5.

Figure 3. X-ray diffraction pattern (XRD) generated by NiCu and NiCu-P. The HER electrocatalytic activity of NiCu-P was further assessed by linear sweep voltammetry (LSV) at three pH values, pH 0.3 (0.5 M H2SO4), pH 7 (1 M phosphate buffer, PB) and pH 14 (1 M KOH). The synergistic effect and the high performance of NiCu-P were also demonstrated by comparison with Ni-P, Cu-P and Pt/C modified nickel mesh electrodes. Shown by the LSV in Figure 4a, in 0.5 M H2SO4 NiCu-P electrode exhibits a HER onset potential (see Figure S6 for method) of -122 mV and a current density of j = -10 mA cm-2 can be achieved at overpotential (η) of -226 mV. In neutral media, the HER onset potential is detected at -41 mV and j = -10 mA cm-2 can be achieved at η of -250 mV (Figure 4b). Irrespective of the pH, the NiCu-P demonstrates higher HER activity than Cu-P or Ni-P suggesting the strong synergistic effect for HER. The generally lower current observed in neutral pH is expected due to the lack of reactant species and solute concentration.30 The observed behaviour indicates the significant roles of Cu for increasing the activity of Ni for HER, due to change in electronic structure.31 As shown in Figure 4c, in alkaline media, NiCu-P delivers HER at an onset potential of -134 mV, comparable


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to benchmark Pt/C catalysts. A current density of -10 mA cm-2 can be achieved at η of -175 mV. These values show that NiCu-P compares favorably to the recently reported HER catalysts for alkaline media (Table S2).

Figure 4. Linear swap voltammetry of NiCu-P in various pH and electrolytes (a) acidic (0.5 M H2SO4), (b) neutral (1 M phosphate buffer), (c) basic (1 M KOH), all voltammetries were collected with scan rate of 5 mV.s-1; (d) Tafel slope comparison in pH = 14.0. The kinetics of HER catalysed by NiCu-P was also studied by Tafel slopes (Figure 4d and S7). The determination is made possible due to the dependency of HER Tafel slope to possible rate determining steps, Volmer, Heyrovsky, and Tafel reactions. Molecular hydrogen can be obtained by combination of Volmer and either Heyrovsky or Tafel reactions. Tafel slopes of around -30 and -40 mV dec-1 suggest that the HER proceeds through the Tafel and Heyrovsky mechanism, respectively.15 If the Volmer reaction were the rate determining step, a Tafel slope of ca. -120 mV dec-1 would be obtained. For instance, a benchmark electrocatalyst such as Pt/C is known to follow Volmer-Heyrovsky reaction pathway in alkaline electrolyte and has a typical Tafel slope


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of ~-40 mV dec-1, which is consistent with our own results. Correspondingly, NiCu-P also shows a slope value of -53 mV dec-1, indicating that HER at NiCu-P follow a similar Heyrovsky mechanism to Pt/C. The synergistic interaction between Ni, Cu and P is also confirmed by the significantly higher Tafel slopes from both Cu-P (-138 mV dec-1) and Ni-P (-110 mV dec-1), suggesting that the HER at Cu-P and Ni-P follows Volmer mechanism. Table 1 lists Tafel slopes of the electrodes in different electrolytes which reveal that the Volmer-Heyrovsky reaction pathway is dominant for HER. In acidic media, HER on the surface of NiCuP also follows Volmer-Heyrovsky reaction pathway (-37 mV dec-1), while the Volmer mechanism is prominent in neutral solutions (-96 mV dec-1). From the overall HER catalytic activity assessments, the as synthesized NiCu-P displays a high HER activity in acidic media as reflected from the lowest values of both Tafel slope and onset potential for HER. The high catalytic activity could be attributed to the low pH of the electrolyte that favours HER due to pH-dependent nature of Hbinding energy, which is optimum at acidic environment.32 Although NiCu-P displays a slightly lower Tafel slope and HER onset potential in acidic solutions compared to those in alkaline ones, Table 1 shows that the j0 and TOF values of NiCu-P in pH = 14 are significantly higher than those in pH = 0.3. The HER faradic efficiency has been calculated for NiCu-P electrode in the three solutions by gas chromatography and it was found ca. 100% in all different solutions. From our characterization results, we propose that the high HER activity of the electrodeposited NiCu-P is generated from the synergies between the chemistry, chemical compositions and morphological features of the electrodeposited NiCu-P. First, phosphidation has been established as a method to improve the HER activity of most 3d-transition metal elements. One of the main reason is the higher electronegativity of P atom,23 strongly linked to the alteration of the electronic property of the neighboring metal atoms. The negatively polarized


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P atoms in metal phosphides are known to effectively reduce the Gibbs free energy of proton adsorption, influencing a shift of the rate determining step in the HER mechanism from the Volmer reaction to one of desorption steps such as Heyrovsky and Tafel, thus facilitating the HER process.33 Furthermore, dendritic-structured NiCu alloys show hydrophobic properties25 which can impede the HER catalytic activity. Nevertheless, Figure S8 reveals that phosphidation decreases the contact angle from 93° for the electrodeposited NiCu to 30° for NiCu-P and accordingly improve the catalyst wetting and reduces the activation overpotential. Second, the presence of zero-valent metallic elements is essential in providing fast-electron shuttle between the electrode substrate and the catalytically active sites in NiCu-P. Furthermore, Ni-Cu has been recognized as a diffusion couple.34 The application of sufficient heat results in the formation of NiCu alloy through a solid-state atomic diffusion process. This allows the formation of unique catalytic active sites composed of Ni, Cu and P, which more effectively catalyze HER. Although this would be an attractive object for a density functional theory (DFT) study, it is beyond the scope of this manuscript. Third, the high-surface profile of the electrodeposited NiCu-P effectively exposes high number of these catalytically active sites which results in the observation of high HER performance. Table 1. Tafel slopes and other electrochemical parameters of the prepared electrodes in different electrolytes for HER

Media pH = 0.3

pH = 7 pH = 14

Catalyst NiCu-P Ni-P Cu-P NiCu-P Ni-P Cu-P NiCu-P

Tafel Slope (mV dec-1) -37 -88 -93 -96 -218 -115 -53

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(mV) -226 -260 -323 -250 -639 -388 -175

Exchange current density, j0 (mA cm-2) 0.066 0.035 0.012 0.288 0.112 0.063 0.158

TOF, s-1 HER mechanism (at = -400 mV) Heyrovsky 7.1 × 10-7 Volmer 2.9 × 10-8 Volmer 1.9 × 10-7 Volmer 1.2 × 10-7 Volmer 3 × 10-9 Volmer 8.1 × 10-9 Heyrovsky 2.6 × 10-6


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Ni-P Cu-P

-110 -138

-255 -392

0.026 0.166

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Volmer Volmer

3.2 × 10-7 6 × 10-9

The phosphidation also contributes favorable to the general electrochemical and physical stabilities of NiCu alloy. The phosphidation increases the material durability against acid dissolution or oxidation by alkaline media. The increased chemical stability is imperative for the preservation of catalytically active centers. As demonstrated by the accelerated degradation tests (ADT) shown in Figure S9, the HER activities of NiCu-P following 1000 electrochemical cycles in acid, neutral or alkaline remain the same, indicating excellent electrochemical stability. On the other hand, long-term chronopotentiometry experiments revealed that without phosphidation, NiCu-alloys destabilize quite rapidly, and quite notably, particularly in an acidic electrolyte (Figure S10). Figure S11 also displays appropriate stability of chemical composition and morphology of the NiCu-P electrode. Facile H2 bubbles evolution was also physically evidenced in all reported electrolytes. The fast generation of H2 bubbles from NiCu-P can be attributed to the shape of the pores. We propose that the use of H2 templated electrodeposition contributes significantly to the observed high HER activity through two mechanisms: i) increasing the surface area and ii) improving the gas dissipation ability because of the pore shapes. According to the principle of H2 templated electrodeposition, the porous nature of hollow conical channels is caused by the evolution of small bubble of H2, which grows by releasing from the surface (Figure 1a). Figure S12 and S13 compare morphology and HER activity of NiCu-P fabricated by normal and hydrogen templating electrodeposition, respectively. Based on Figure S12, the HER current density of the porous NiCu-P normalized by ECSA is two times higher than non-porous one, suggesting a suitable structure and morphology of the catalyst. Therefore, it can be said that the cone shape of the


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pores (Figure 1c) facilitates the generation of H2 gas during the electrode operation and makes it more efficient. Furthermore, the 3D NM is also contributing to increasing the electrode surface area. In summary, NiCu-P/NM electrode showed high HER activity in the wide range of pH. NiCuP demonstrated more catalytic activity than Ni-P and Cu-P because of the synergistic effects of Cu on Ni alloys. Hydrogen dynamic template electrodeposition was carried out to increase the catalysts surface area and gas dissipation ability. Phosphidation is also found as a useful method to increase the NiCu catalytic activity and electrochemical stability. Furthermore, conical pores facilitate the dissipation of generated hydrogen gases and phosphidation improves the wettability properties of the nanostructured catalysts. The 3D-NiCu-P expedites HER a current density of 10 mA cm-2 at pH values of 0.3, 7, and 14 at overpotentials of -226 mV, -250 mV, and -175 mV, respectively. Based on the measured Tafel slope of the fabricated electrodes, the HER follows the Heyrovsky mechanism in acidic and alkaline media, while the Volmer mechanism is dominant in neutral solutions. Supporting Information Supporting Information (SI) is available from the authors. Experimental and some detailed figures are in the SI. AUTHOR INFORMATION +

Dr M. Asnavandi and Dr B. Suryanto had equivalent contribution in this work. Associate

Professor Chuan Zhao is the paper corresponding author. E-mail: [email protected] REFERENCES (1)

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Synopsis Hierarchy porous NiCu-P electrode was made for HER catalyzing. HER mechanism studies reveal a synergistic effect of Ni and Cu.


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Dynamic Hydrogen Bubble Templated NiCu Phosphide Electrodes for

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