1. Introduction
Considering the low abundance of platinum, the research interest in common metals to replace it for electrochemical and catalytic reactions is renewed. In this context, nickel based materials (sharing the same group of the periodic table with Pt) play a crucial role in hydrogen production, as well as in several clean energy conversion and storage devices. Metallic nickel, as well as its oxides and nitrides, due to their great (electro) catalytic activity and low cost, are widely used to generate H
2 by reforming reactions [
1,
2], borohydride decomposition [
3], water electrolysis [
4,
5,
6] and splitting [
7]. Nickel and its compounds are also employed as electrocatalysts, co-catalysts or catalyst supports in fuel cells operating in various conditions and temperatures, including solid oxide [
8], proton exchange membrane [
9,
10,
11,
12], alkaline [
13,
14], and microbial fuel cells [
15]. Nickel materials also catalyze other environmentally relevant reactions including the electrochemical conversion of CO
2 [
16,
17]. Of particular interest is the use of nickel nanowires as the starting point for the synthesis of
[email protected] [email protected] one-dimensional (1D) electrocatalysts, using a simple galvanic displacement reaction, which was able to surpass the U.S. Department of Energy target of oxygen reduction mass activity for proton exchange membrane fuel cells (PEMFC) [
9,
18,
19], which provided one source of motivation for this work.
To further improve the physico-chemical and (electro) catalytic properties of nickel, the preparation of nickel nanofibers with a three-dimensional (3D) network structure is of particular interest. Indeed, not only their considerable surface to volume ratio could be exploited, but also their porous 3D architecture could be beneficial for electrochemical and catalytic reactions.
Ni-based nanowires have been synthesized by several chemical and physical routes, including electrodeposition on 1D materials (e.g., carbon nanotubes [
20]) or templates (e.g., polycarbonate membranes [
21]), electroless plating [
22], chemical vapor deposition [
23], microwave-assisted synthesis [
24], solvothermal [
25] and wet chemical processes [
26]. Ni and NiO nanowires have been also prepared by electrospinning [
27,
28,
29,
30,
31]. This technique presents several advantages compared to the other physico-chemical synthesis routes cited above: efficiency, reproducibility, high yield, simplicity, low-cost, and high versatility [
32]. The latter concerns not only the chemical compositions, but also the wide range of morphologies and architectures achievable. Furthermore, the preparation of electrospun materials is easily up-scalable through the availability of setups including multiple needle or needle-less electrospinning. In the recent papers exploring electrospinning of Ni-based materials, however, no systematic study has been made of the parameters allowing targeted compositions and morphologies to be achieved.
The rational design of Ni-based nanomaterials with controlled morphologies and compositions is crucial to fine-tune their properties in view of specific catalytic and electrocatalytic applications. In the present work, we describe a general route using electrospinning to obtain nickel-based 1D nanomaterials. Several synthesis parameters have been investigated, including the metal precursor, solvent and carrier polymer, as well as the thermal post-treatments performed (number of steps, gas atmosphere, and temperature). We demonstrate a range of strategies to tune the morphology (nanofibers, nanoribbons, sponge-like structures) and composition (Ni, NiO, Ni3N, Ni/C as well as their combinations), thus giving a means of obtaining the appropriate materials for the targeted application. Electrospinning is straightforward and up-scalable, allowing for large-scale production and enabling new and industrially relevant opportunities for the manufactured materials.
3. Materials and Methods
3.1. Materials
Nickel(II) nitrate hexahydrate (Sigma-Aldrich St. Louis, MO, USA, puriss, p.a. > 98.5%) and nickel(II) acetate tetrahydrate (Aldrich, St. Louis, MO, USA, purum, p.a. > 99.0%) were used as nickel precursors salts. Polyvinylalcohol (Aldrich, Milwakee WI, USA, 98%–99% hydrolized Mw 85,000–146,000), polyvinylbutyral (Butvar B98, Sigma, St. Louis, MO, USA) and polyvinylpyrrolidone (Aldrich, Steinheim, Germany, Mw ~ 1,300,000) were used as carrier polymers. MilliQ water (18 MΩ), N,N-dimethylformamide (Sigma-Aldrich Chromasolv®Plus for HPLC > 99.9%, Irvine, UK) and ethanol (Sigma-Aldrich, Steinheim, Germany, puriss.) were used as solvents. Chloroplatinic acid hexahydrate (ACS reagent, ≥37.50% Pt basis, Aldrich, St. Louis, MO, USA) was used for galvanic displacement.
3.2. Synthesis of Ni-Based Nanofibers
A common synthesis procedure was used for all the materials presented in this paper. The electrospinning solutions were prepared by first dissolving the carrier polymer in the solvents of choice, and, in a second stage, the nickel salt precursor was added; once a clear solution was obtained, it was transferred to a plastic syringe and fed to a linearly sliding electrospinning needle using Teflon tubes. The flow rate was regulated with a syringe pump. A Spraybase® electrospinning system (Dublin, Ireland) equipped with a rotating drum and a 0–30 kV power supply was used in all of the experiments.
Details of the optimized electrospinning solution/condition used are gathered in
Table 1, while details of the thermal treatments are collected in
Table 2.
3.3. Characterization of Ni-Based Nanofibers
The morphology of the electrospun materials before and after the different thermal treatments was characterized by SEM using a FEI Quanta FEG (Field Emission Gun) 200 (Hillsboro, OR, USA) equipped with EDS, and TEM using a JEOL 1200 EXII (Tokyo, Japan).
XRD patterns of the Ni supports were recorded at room temperature in Bragg–Brentano configuration using a PANAlytical X’pert diffractometer (Almelo, Netherlands), equipped with a hybrid monochromator, operating with CuKα radiation (λ = 1.541 Å), and using a step size of 0.1° 2θ in the 2θ domain from 20° to 80°.
Thermogravimetric analysis to determine the carbon content on the Ni/C composite fibers was performed in air up to 1000 °C (10 °C/min) using a Netzsch TG 439 thermobalance (Selb, Germany).
The dynamic viscosity of the electrospinning solutions was measured with a shear rheometer (Paar Physica UDS 200, Graz, Austria) at RT.
3.4. Pt Deposition on to Nickel Nanofibres
Platinum was deposited on the nickel nanofibres from aqueous chloroplatinic acid hexahydrate solution using an approach described elsewhere [
46]. At the end of the exchange process, the nanofibres were collected by filtration and carefully washed with ethanol and water. A complete exchange leads to 30% platinum loading.
3.5. Characterisation of [email protected] Nanofibres
The morphology of the
[email protected] 1D materials has been characterized by TEM using a JEOL 2200FS (Tokyo, Japan) (Source: FEG) microscope operating at 200 kV equipped with a Charge Coupled Device camera Gatan USC (16 MP) (Pleasanton, CA, USA). For TEM analyses, samples were suspended in ethanol and sonicated before deposition on to carbon-coated copper grids.
Electrochemical characterisation was carried out in a conventional three-electrode cell consisting of a glassy carbon rotating disk electrode (RDE) (working electrode, geometric area of 0.196 cm2), a reversible hydrogen electrode (reference electrode, RHE) and a platinum wire (counter electrode). A Pine bipotentiostat model AFCBP1 (Grove City, PA, USA) was used. All of the potential values are referred to the RHE. Inks were prepared dispersing 5 mg of catalysed support in 300 μL of isopropanol, 20 μL of water, and 15 μL of 5 wt % Nafion. These inks were then deposited onto the RDE surface with a micropipette to give a final Pt loading of ~100 μg·cm−2. Cyclic voltammetry was carried out at 100 mV/s in N2 saturated 0.1 M aqueous HClO4 and the electrochemical surface area of platinum was calculated by integrating the peak of hydrogen desorption from the Pt sites. ORR was conducted on an RDE in an oxygen saturated 0.1 M HClO4 aqueous electrolyte at increasing rotating speeds (400, 900, 1600, 2500 RPM) chosen in order to achieve equally spaced saturation currents (Koutecky-Levich).
4. Conclusions
We investigated the effect of electrospinning and thermal treatment parameters on morphology and composition of 1D nickel, nickel oxide and nickel nitride. This systematic screening has given insights, enabling improved understanding of the formation process of hybrid and inorganic spun nanomaterials, leading to an informed choice of synthesis procedure by the possibility of tuning their structure and the related properties. The obtained nanofibers/ribbons/sponges of nickel, nickel oxide, nickel/carbon, and nickel nitride are promising candidates for application in catalysis as well as energy conversion and storage. As an example, the use of nickel electrospun materials as electrocatalyst supports for fuel cell electrodes after their surface modification by Pt galvanic displacement has been proofed in this work.
Considering the flexibility and good scalability of electrospinning, as well as the versatile applicability of Ni-based compounds, this work opens the way for the rational and large-scale production of metallic materials towards binary and multi-metallic systems with a widened application range.