Prompt Electrodeposition of Ni Nanodots on Ni Foam to Construct a High-Performance Water-Splitting Electrode: Efficient, Scalable, and Recyclable
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Facile electrodeposition for fabricating active Ni nanodots (NiNDs) on Ni foam (NF) is shown.
Binder- and heteroatom-free recyclable NiO/NiNDs@NF electrodes are efficiently made.
NiO/NiNDs@NF bifunctional catalytic electrodes are used for water splitting.
KeywordsElectrodeposition Ni nanodots Bifunctional catalysts Water splitting Large-size
Growing concerns regarding the energy crisis and environmental pollution prompts the exploration of sustainable energy sources as substitutes for traditional fossil fuels [1, 2]. As an environmentally friendly energy carrier, molecular hydrogen (H2) plays a critical role in sustainable energy systems [3, 4]. Among the technologies for H2 production, the electrocatalytic H2 evolution reaction (HER) from water splitting is the most effective and economical route because of its high energy-conversion efficiency and environmentally benign process [5, 6, 7]. It has been confirmed that precious platinum (Pt)-based materials play a leading role in current H2-production technology, such as water-alkali electrolysis [8, 9]. However, the scarcity and high cost of Pt severely hamper its large-scale industrial applications. Therefore, it is crucial to explore inexpensive, alternative electrocatalysts that are made from earth-rich elements and have good activity and durability. Furthermore, water electrolysis should be carried out in either strongly acidic or alkaline electrolyte to minimize the overpotentials in the electrolyte . Hence, development of a bifunctional catalyst that is based on earth-rich elements and has high activity for both HER and oxygen evolution reaction (OER) in the same electrolyte is essential for simplifying the system and reducing manufacturing costs of H2.
In the past decades, a number of non-noble HER electrocatalysts based on transition metals and their compounds have been explored. Among the transition metals explored, Ni atoms possess an appropriate hydrogen surface adsorption energy, which makes them broadly recognized as excellent water dissociation centers [11, 12]. However, during the catalytic process on a Ni surface, the adsorption sites for H atoms may be occupied by OH− species. This causes a decrease in the active sites, which leads to a dramatic decline of catalytic activity. Ni foam (NF) is a low-cost and three-dimensional porous structure that is commonly used as the electrodes of alkaline electrolyzers for commercial applications . Nanostructuring and surface engineering research has aimed to improve the catalytic performance of NF through maximizing the number of catalytic active sites and promoting mass transport. Different types of Ni-based materials (including metallic Ni, Ni-based alloys, oxides, nitrides, phosphides, and sulfides) coated on NF have been intensively studied as HER and OER catalysts for water splitting [14, 15, 16, 17, 18, 19, 20, 21, 22, 23]. Despite the largely improved catalytic performance, the process for large-scale preparation and application of these catalysts is less impressive and satisfactory. Moreover, the introduction of heteroatoms (e.g., N, S, P, and Se) is not beneficial for the recovery and recycling of the electrode. With increasing demands for Ni worldwide, increasing Ni recycling and reducing waste are tangible goals for making substantial strides toward sustainability .
Recently, some nanoscale metal and metal oxide (M/MOx) heterostructures have been fabricated and exhibit high HER catalytic activity and stability; this is probably because of the synergistic effects of M and MOx, including Ni/NiO heterostructures [25, 26, 27, 28, 29, 30, 31, 32, 33]. However, these porous Ni/NiO composites are commonly powder and are fabricated through a complicated process, involving a sequence of hydrothermal, chemical reduction, and high-temperature processing [25, 26]. In addition, the preparation process of a traditional catalytic electrode that normally contains the addition of binder additives and carbon is also inconvenient. Hence, the search for economic, environmentally friendly, and feasible approaches to fabricate high-performance Ni-based catalysts for large-scale water splitting is still pursued and highly challenging.
In this work, we engineer the surface of commercial NF by quickly coating a cluster of Ni nanodots (NiNDs) (~ 2 nm) via electrochemical deposition. After drying in air, NiO/NiND composites can be obtained in a one-step procedure for constructing the binder-free and heteroatom-free NiO/NiNDs@NF electrodes. For comparison with commercial NF, the catalytic activity and durability of the NiO/NiNDs@NF electrodes toward HER and OER were greatly enhanced. The rough surface and porous structure of this composite with NiNDs can simultaneously expose more active sites with enhanced electrical conductivity. This NiO/NiND electrode consequently displays high activity and durability for electrocatalytic water splitting. The bifunctional catalytic electrode can enable highly efficient alkaline water electrolysis with 10 mA cm−2 at a cell voltage of only 1.70 V. More practically, a large-sized (S ~ 70 cm2) NiO/NiNDs@NF electrode fabricated using this method has also been demonstrated. This large-sized electrocatalytic electrode can enable alkaline water electrolysis with 13 mA cm−2 at 4.68 V (including electrical resistive loss in the electrolyte and electrode surfaces)  with superior durability. Importantly, these NiO/NiNDs can be easily removed using diluted HNO3 aqueous solution for the recovery and recycling of Ni foam and hydrated Ni(NO3)2. This NiO/NiNDs@NF electrode was prepared via one-step electrodeposition, which shortens the preparation process of the traditional electrode that normally contains added carbon and binder additives. With the low-cost, facile, and prompt fabrication strategy and the easy recycling property, this could be promising for water electrolysis devices used for large-scale H2 production.
2.1 Fabrication of Electrocatalytic Electrodes
Commercial Ni foam (thickness: 0.5 mm) was first cleaned with acetone and then soaked in 0.5 M HNO3 for 10 min to remove the NiO from the surface. It was then washed with water and dried at room temperature. Ni nanoparticles were electrodeposited on the Ni foam in a N2-saturated acetonitrile solution containing 0.1 M Ni(NO3)2·6H2O (Residual water should first be eliminated through electrolysis.) ITO glass was used to electrodeposit and collect sample for N2 adsorption–desorption isothermal measurements. The electrochemical deposition process was conducted in a three-electrode system at a potential of − 1.46 V (vs. RHE) using an electrochemical workstation (GAMRY-111000) with Ag/Ag+ as a reference electrode, Pt wire as a counter electrode, and Ni foam as the working electrode. The mass loading of active materials can be adjusted by controlling the deposition time. The NiNDs@NF electrode was then washed with acetonitrile and dried in air at room temperature for 10 min. The NiO/NiNDs@NF electrode was prepared via oxidization of the pre-electrodeposited NiNDs in the air after drying. The prepared electrode can be directly used to collect the polarization curves or stored under vacuum for future use. The 120-s deposited electrode (loading of NiO/NiNDs was determined from the difference of the weight of NF before and after electrodeposition and was found to be ~ 1 mg cm−2) was used for the material characterizations and electrochemical tests. The large-sized NiO/NiNDs@NF electrode was fabricated using the same procedure with a large-sized carbon plate as the counter electrode. To prepare the Pt/C@NF electrodes, 1 mg of 20 wt% Pt/C or RuO2 (99.9%) was mixed with 90 μL of water, 50 μL of ethanol, and 10 μL of 5 wt% Nafion solution. The mixture was sonicated for 1 h to form a homogeneous ink. Then, 25 μL of the suspension was drop dried onto NF (0.5 cm2 loading of 1 mg cm−2 for the active mass).
2.2 Materials Characterizations
The morphologies and structures of the NiO/NiND composites were investigated using scanning electron microscopy (SEM, LEO 1530) with an energy-dispersive X-ray (EDX) attachment (Zeiss) and using high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2100). X-ray photoelectron spectroscopy (XPS, Thermo Scientific, Escalab 250Xi) was employed to analyze the composition of the NiO/NiND composites. N2 adsorption/desorption isotherms were obtained using a Quantachrome Autosorb-1 system at 77 K.
2.3 Electrochemical Measurements
Cyclic voltammetry (CV) for HER and OER catalytic activity measurements was performed using a standard three-electrode system controlled by a GAMRY 11100 electrochemistry workstation. The current densities were calculated based on the projected geometric area of an electrode. All of the electrolytes were saturated by N2 bubbles for 30 min before the experiments. Different catalyst electrodes were used as the working electrode, a graphite plate was used as the counter electrode, and Ag/AgCl was used as the reference electrode. The reference was calibrated against and converted to the reversible hydrogen electrode (RHE; E (RHE) = E (Ag/AgCl) + 1.024 V; pH = 14). Water electrolysis measurements were carried out in a standard two-electrode system using the same deposited catalyst electrodes as the cathode and anode. Linear sweep voltammetry was carried out at 2 mV s−1 for the polarization curves. Chronopotentiometry was measured under a constant current density of 13 mA cm−2. All of the polarization curves in the three-electrode system were iR-corrected. Before measurements were made, a resistance test was conducted and iR compensation was applied using the GAMRY software. R is the equivalent series resistance, which was determined from electrochemical impedance spectroscopy (EIS). All of the data for the two-electrode electrolyzer were recorded without iR compensation. The faradaic efficiency was calculated by comparing the amount of gas determined from theoretical calculations and that determined from experimental measurements. H2 and O2 were collected using a water-drainage method, and the amounts of each were calculated using the moles of H2 and O2 generated from the overall water splitting with the ideal gas law. The theoretical amounts of H2 and O2 were calculated using I–t curve and by applying Faraday’s law . The content of active materials was obtained by comparing the weight of the electrode before and after electrodeposition. EIS measurements were carried out by applying an AC voltage with 5 mV amplitude in a frequency range from 0.01 to 100 kHz at an overpotential of 0 V in 1.0 M KOH.
3 Results and Discussion
3.1 Fabrication of NiO/NiNDs Composites on NF
3.2 Materials Characterization
3.3 Electrochemical Properties of NiO/NiNDs@NF
3.4 Catalytic Activity of NiO/NiNDs@NF
The OER characteristics of these different electrodes are shown in Fig. 3b. The NiO/NiNDs@NF electrode also exhibits excellent catalytic activity toward OER. In the three-electrode configuration, the NiO/NiNDs@NF electrode has a much higher current density than the bare NF and an onset potential as low as 1.55 V versus RHE. It requires an overpotential (ηOER) of 360 mV to reach a (projected geometric area) current density of 50 mA cm−2, which is 21 mV more than that of the RuO2@NF electrode. Meanwhile, this overpotential is comparable to the behavior of other modified NF (Table S1) and reported state-of-the-art Ni- and Co-based bifunctional catalysts, such as Ni–P/Cu foam (410 mV) , Ni3Se2/Cu foam (343 mV) , Ni5P4/Ni foil (363 mV) , and CoSe/Ti mesh (341 mV) . The anodic peak at 1.41 V corresponds to the oxidation of NiO. Moreover, a minor anodic peak is observed at 1.35 V in the magnified LSV curve, which corresponds to the oxidation of NiNDs (Fig. S12). The Tafel slope for the NiO/NiNDs@NF electrode is calculated to be 90 mV dec−1, which is close to the corresponding value of the RuO2@NF (84 mV dec−1) electrode and smaller than the corresponding value of bare NF (98 mV dec−1) (Fig. 3d). Moreover, to further investigate the effects of NiNDs, we used LSV and the heat-treated NiO/NiNDs@NF as the electrode to test its electrocatalytic properties for HER and OER. Obviously, the heating treatment (at 200 °C in air) has large effects on HER and OER performance, which gradually declined with an increase in the heating time (Fig. S13). These results further indicate that the NiNDs play an important role in the excellent electrocatalytic activity toward HER and OER.
3.5 Catalytic Stability of NiO/NiNDs@NF
Long-term electrocatalytic stability is another important criterion for water-splitting electrocatalysts because a longer lifetime of a device reduces the cost of the resulting H2. To assess the durability of the NiO/NiNDs@NF electrodes, an applied current density was set at 13 mA cm−2. A constant potential of 1.74 V can be well maintained for at least 20 h without any decay (Fig. 4b), and this indicates the high stability of the NiO/NiNDs@NF electrode. For comparison, the applied potential of RuO2@NF‖Pt/C@NF (1.61 V) is nearly 120 mV less than that of the NiO/NiNDs@NF electrode, and the NF (2.0 V) electrodes show a poor electrocatalytic activity and stability. The potential of the NF electrode increased greatly during the first few hours, suggesting a dramatic decline in the catalytic activity. Meanwhile, the faradaic efficiency during the overall water splitting is almost 100% for both HER and OER, and the molar ratio of H2 to O2 remains at 2:1 (Fig. S14). After the durability assessments, the NiO/NiNDs@NF electrodes were also tested using SEM and TEM, and the results indicate no topographic changes. This highlights the superior structural robustness of the NiO/NiND composites during the electrocatalytic HER and OER processes (Fig. S15). However, the NiNDs content decreased obviously after the long-term OER durability test (Fig. S16). The survival of some of the NiNDs is probably because of the existence of carbon-containing groups on the surface of NiNDs, which may retard the oxidation of NiNDs. It is worth mentioning that the excellent catalytic stability of NiO/NiND composites should be attributed to the NiND survival in the electrochemical tests (Fig. S17).
3.6 Proposed Mechanism of Electrocatalytic Property of NiO/Ni composites
On the basis of these results, we propose three explanations for the superior electrocatalytic performance of NiO/Ni composites: First, the presence of metallic Ni nanoparticles increases the conductivity of the catalyst; this is beneficial for electron transport through NiO and improves the catalytic stability. Second, the rough surface and porous nanostructure enhance electron transfer by increasing the reaction area and preventing bubbles from growing; in turn, this increases the rate of electrolysis. Third, the synergistic effect of surface NiO and Ni nanoparticles can further improve the catalytic activity of NiO/Ni composites. On the one hand, Ni supplies the active catalytic sites and highly improves the conductivity of the catalyst. On the other hand, the OH− that is generated by H2O splitting can preferentially attach to a NiO site at the interface because of strong electrostatic affinity to the locally positively charged Ni2+ species and the larger number of unfilled d orbitals in Ni2+ than in Ni metal .
3.7 Fabrication of Large-sized NiO/NiNDs@NF Electrode
3.8 Recycling NF and Ni(NO3)2
These NiO/NiND composites can quickly dissolve in acid solution to form the corresponding salt of Ni2+, leading to the regeneration of NF. As seen in Fig. S18a, the color of the NiO/NiNDs@NF electrode changes gradually from black to silver-white when it is immersed in 0.5 M HNO3 solution. The SEM images further indicate that the spherical nodules on the surface of NF can be completely dissolved in 0.5 M HNO3 solution for 20 min (Fig. S18b). In addition, the corrosion behavior of pure NF in 0.5 M HNO3 solution has been investigated. No vigorous reaction was observed, and there was a merely 4% decrease in the weight of NF after 20 min of the immersion treatment (Fig. S18c). This dissolution process of NF in NiO/NiNDs@NF should be slow because of the surface coating of NiO/NiNDs. Meanwhile, the dissolved product of NF and the concentrated treatment solution can also be used as a source of Ni(NO3)2. Thus, the regenerated Ni foam can be recycled and used in the fabrication of the NiO/NiNDs@NF electrodes. In total, the whole recovery process of Ni foam and Ni(NO3)2 is simple and environmentally friendly without any emission of toxic gases or wasted energy. All of the above results demonstrate the convenient fabrication and recyclability of the electrodes for practical applications in water electrolysis.
To construct high-performance, low-cost, and environmentally friendly Ni-based catalytic electrodes for water-splitting, binder-free, heteroatom-free, and recyclable NiO/NiNDs@NF bifunctional catalytic electrodes were fabricated using a one-step quick electrodeposition method. Typically, active Ni nanodot clusters were electrodeposited on Ni foam in acetonitrile solution. After drying in the air, the NiO/NiND composites were obtained, leading to binder-free and heteroatom-free NiO/NiNDs@NF catalytic electrodes, which have superior performance during HER and OER processes. A large-sized (S ~ 70 cm2) catalytic electrode with high durability was also fabricated using this method. Importantly, the recovery process of the raw materials of these electrodes is convenient and environmentally friendly for their recycling use. This method provides a simple and fast technology for preparing recyclable Ni-based bifunctional electrocatalytic materials for large-scale real-world water-splitting electrolyzers.
H.T. Yu thanks the China and Germany Postdoctoral Exchange Program for this research in Helmholtz-Zentrum Berlin für Materialien und Energie, the Postdoctoral Science Foundation of China (2017M610324) and NSFC (21704040). The authors also thank the Joint Lab for Structural Research at the Integrative Research Institute for the Sciences (IRIS Adlershof).
Supplementary material 1 (AVI 2214 kb)
Supplementary material 2 (AVI 1047 kb)
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