Phosphorus-Doped 3D RuCo Nanowire Arrays on Nickel Foam with Enhanced Electrocatalytic Activity for Overall Water Splitting.

It is especially significant to design and construct high-performance and stable three-dimensional (3D) bifunctional nanoarchitecture electrocatalysts toward overall water splitting. Herein, we have constructed 3D self-supported phosphorus-doped ruthenium-cobalt nanowires on nickel foams (RuCoP/NF) via a simple hydrothermal reaction followed by a low-temperature phosphating reaction. Doping P can not merely enhance the intrinsic activity of electrocatalysts for overall water splitting but at the same time increase electrochemical surface areas (ECSAs) to expose more accessible active sites. As a 3D bifunctional catalyst, RuCoP/NF demonstrates superior performance for HER (44 mV@10 mA cm-2) and OER (379 mV@50 mA cm-2) in 1.0 M KOH electrolyte solution. The overall water-splitting system was assembled using RuCoP/NF as both anode and cathode. Besides, it exhibits a voltage of 1.533 V at a current density of 10 mA cm-2 and long-term durability within 24 h. P-dopant changes the electron structure of Ru and Co, which is conducive to the formation of Ruδ- and Coδ+, resulting in the adjustment of binding H*/OH* and the improvement of the overall water-splitting reaction kinetics. This work provides a facile method to produce heteroatom-doped and high-performance catalysts for efficient overall water splitting.

Introduction

With increasing concerns about the rapid depletion of natural resources, environmental problems, and global warming, cheap and renewable resources have attracted more and more attention.[1−5] Because of high gravimetric energy density and environmentally friendly energy, H2 has been paid much attention as a candidate to replace fossil fuels.[6,7] Nowadays, overall water splitting is considered as a promising and highly effective technique to realize high-purity hydrogen generation with zero-carbon pollution.[8−11] The electrocatalytic overall water-splitting reaction is segmented into two-half reactions: the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) in the cathodic and anodic electrode, respectively.[12,13] However, to overcome the sluggish reaction kinetics, the electrochemical process always is accompanied by a high overpotential.[14−16] Accordingly, it is crucial to take a lot of efforts to reduce the use of a noble metal to develop bifunctional nanoarray electrocatalysts with a low price, high efficiency, and easy preparation for electrocatalytic overall water splitting under alkaline media.

Noble metals and their derivatives have been widely considered as the up-to-date catalysts to drive the HER and OER.[17−20] As a low-cost alternative to Pt, Ru has a 65 kcal mol–1 bond strength with hydrogen and the cost of Ru is approximately 1/3 that of Pt.[21,22] Besides, Ru tends to agglomerate during the reaction.[23] At present, there have only been a few reports about the preparation of Ru-based nanoarray electrocatalysts with outstanding overall water-splitting performance. A number of research studies have shown that adding a transition metal to the Ru-based nanoarrays can play a synergetic role of two different metal species to improve the catalytic performance of the catalyst.[24−26] On the other hand, heteroatom dopants can also effectively strengthen the catalytic activity. Heteroatom doping (e.g., N, S, and P) can rationally regulate the electronic structures between two metals to increase electrochemical surface areas (ECSAs) to expose more accessible active sites and finally increase the intrinsic activity for overall water splitting.[27−29] Li and co-workers have latterly reported phosphorus-doped Fe3O4 nanoflowers with outstanding HER performance, identifying that the P-doping modifies the electronic state of Fe3O4 and optimizes the Gibbs free energy of the samples.[30,31] Wang and co-workers prepared phosphorus-doped Co3O4 nanowire arrays with good overall water-splitting performance which modulates the electronic structure of the catalysts and benefits for the formation of O2 molecules from OOH*.[32] Considering these advantages, the design and preparation of phosphorus-doped bimetallic nanoarrays have extensive application prospect.

Herein, by choosing NF as the substrate, phosphorus-doped RuCo nanowires (RuCoP/NF) are successfully synthesized through the hydrothermal method followed by the phosphating reaction. The water-splitting system just requires a cell voltage of 1.533 V to operate at a current density of 10 mA cm–2 and demonstrates almost 100% Faradaic yield in alkaline solutions. The excellent electrochemical measurement confirmed that the phosphorus dopant can adjust the binding H*/OH* to dramatically enhance the electrocatalytic activity toward overall water splitting in alkaline media. High electrocatalytic activity makes RuCoP nanowire arrays a very promising catalyst for overall water-splitting application.

Results and Discussion

Structural Characterization of Catalysts

As shown in Figure , the 3D RuCoP/NF nanowire arrays were synthesized through a two-step method. First, RuCoOH nanowires were grown in situ on a cleaned 3D microporous NF substrate by a simple hydrothermal process. Second, RuCoP/NF nanowires can be obtained by a low-temperature phosphatizing reaction with phosphorous salt. As shown in Figure S1, the color changes of NF, RuCoOH/NF, and RuCoP/NF are clearly distinguished from the photographs.

Figure 1

Schematic diagram of the fabrication of RuCoP/NF nanowires.

SEM is usually used to illustrate the micromorphologies of the as-synthesized samples. The SEM images of NF reveal that it has a 3D open-pore and cross-linked grid structure with a smooth surface (Figure S2). As exhibited in Figure a, the SEM images of the RuCoOH/NF precursor indicate that the RuCoOH nanowires are composed of radial nanowires with a length of ∼1 μm and a diameter of ∼200 nm. Besides, nanowires are uniformly distributed on the surface of NF. After the low-temperature phosphatizing reaction, the morphology of nanowires is maintained and the diameter has barely changed, as shown in Figure b. It is worth mentioning that the nanowire arrays can increase the contact area with the electrolyte to enhance the charge transfer.[33]Figure S3 shows the SEM morphologies of Co(OH)F/NF and CoP/NF. Compared with CoP/NF, the addition of a small amount of Ru does not change its basic morphology. As shown in Figure S4, when the molar ratio of Ru/Co is changed from 0.03 to 0.05, Ru0.03CoP/NF and RuCoP/NF (Ru0.05CoP/NF) nanowires are successfully synthesized. While the molar ratio of Ru/Co is 0.1, there is no nanowire arrays. More importantly, due to the addition of Ru easily to aggregation, it is essential for the preparation of RuCoP/NF nanowires to regulate and control the amount of Ru. The active species aggregation decreases the contact area and active sites of catalysts. Among the as-prepared catalysts, RuCoP/NF (namely, Ru0.05CoP/NF) nanowire arrays exhibit the best surface structure and the largest specific surface area, which is beneficial for the transport of electrolyte ions.[34]

Figure 2

SEM images of (a) RuCoOH/NF and (b) RuCoP/NF and TEM images of (c) RuCoOH/NF and (d) RuCoP/NF (the insets are the HRTEM images of the corresponding samples). Elemental mapping images of Ru, Co, P, and O for (e) RuCoOH/NF and (f) RuCoP/NF.

The abundant porous channels of the wire-like nanoarrays can accelerate the diffusion of produced gas to avoid blocking the active sites. Also, a high surface-to-volume ratio is helpful to improve the electrocatalytic overall water-splitting performance. To further investigate “a single wire” micromorphology of RuCoOH/NF and RuCoP/NF, transmission electron microscopy (TEM) measurement is a good choice to carry out. Figure d shows the microstructure of RuCoP/NF nanowires, which has a diameter of about 200 nm with highly rough surface. Also, the HRTEM image reveals an interplanar distance of 0.220 nm, which is consistent with the (111) plane of Co2P. Moreover, the lattice fringe spacing of RuCoP/NF with 0.220 nm is slightly smaller than that of RuCoOH/NF with 0.252 nm (Figure c), which is attributed to the phosphating process. In addition, Figure e,f displays the EDX spectroscopy elemental mapping of RuCoOH/NF and RuCoP/NF, respectively, indicating the uniform distribution of Ru, Co, and P elements. All results suggest the successful formation of the RuCoP nanowire array on nickel foam. In addition, the incorporation of phosphorus provides more active sites and the nanowire structure improves the surface area for catalyzing reactant and accelerates the diffusion of the electrolyte.

XRD measurement was conducted to investigate the crystal structure of the prepared catalysts. As illustrated in Figure a, the XRD pattern of RuCoOH/NF shows the main characteristic peaks of Co(OH)F and metallic Ni. The three strong diffraction peaks at 44.5, 51.8, and 76.4° correspond to (111), (200), and (220) planes of NF (JCPDS# 00-004-0850), respectively. The main characteristic peak at 35.6° of RuCoOH/NF is matched well with the (111) plane of crystalline Co(OH)F (JCPDS# 00-050-0827).[40] Meanwhile, in the XRD pattern of RuCoP/NF, well-matched diffraction peaks located at 40.8° can be mainly observed, corresponding to the (111) crystal plane. Notably, there are no peaks in regard to Ru compounds, showing that Ru is successfully introduced into catalysts.[35] This result is consistent with TEM results. As shown in Figure S5, compared with CoP/NF, the main characteristic peaks (40.8°) exhibited a slight shift, indicating that Ru was successfully introduced into nanowire arrays.

Figure 3

(a) XRD patterns of the samples. (b) XPS survey spectra of RuCoOH/NF and RuCoP/NF. High-resolution XPS spectra of (c) Ru 3d peak, (d) Ru 3p peak, (e) Co 2p peak in RuCoOH/NF and RuCoP/NF, and (f) P 2p peak in RuCoP/NF.

XPS measurement was studied to explore the chemical composition and valence of the prepared samples. In order to avoid the impacts of the matrix, the XPS samples were obtained by scraping the nanoarray growing on NF. As shown in Figure b, RuCoP/NF has an obvious P 2p peak, confirming that P is successfully incorporated. Figure c shows the XPS spectra of Ru 3d and C 1s in RuCoOH/NF and RuCoP/NF. The C 1s spectrum of RuCoP/NF can be divided into four peaks that can be assigned to OH–C=O (290.1 eV), C=O (288.7 eV), C–O/sp2–C (286.2 eV), and C–C/C=C (284.9 eV).[36] The additional peaks of RuCoP/NF at 283.7 and 280.7 eV were attributed to Ru 3d3/2 and 3d5/2, respectively. On account of the overlapping of the Ru 3d peak with the C 1s peak, the Ru 3p spectrum was mainly used to explore the electronic property. Figure d shows the peaks of the Ru 3p spectrum located at 483.4 and 462.0 eV which were attributed to Ru 3p1/2 and Ru 3p3/2, respectively. Compared with RuCoOH/NF, the Ru 3d and Ru 3p peaks of RuCoP/NF shift noticeably to lower binding energy regions, indicating the increase in electron density of Ru species. As listed in Figure e, the peaks at 778.4 and 793.7 eV can be assigned to Co 2p3/2 and Co 2p1/2 spectra of Coδ+ in RuCoP/NF, respectively, while the peaks at 781.7 and 798.2 eV can be presumably assigned to the cobalt oxide, originating from the superficial oxidation of RuCoP/NF. What is more, it can be observed that the Co 2p BEs in RuCoP/NF exhibit positive shifts than that in RuCoOH/NF. The BEs at 129.2 and 129.9 eV are ascribed to P 2p3/2 and P 2p1/2 from the metal-P bonding in RuCoP/NF, respectively.[37] Also, the peak of oxidized P is located at 134.0 eV (Figure f). All the XPS results indicate that P is successfully introduced into RuCoOH/NF, leading to the obvious enhancement electron transfer from Co to Ru. It showed that RuCoP/NF has a strong electron interaction that existed in Ru, Co, and P, which implied the formation of the Ru–Co–P ternary structure.

Electrochemical Characterization for HER

The electrocatalytic performances of the obtained samples for HER and OER were tested in a 1.0 M KOH electrolyte at a scan rate of 5 mV s–1 using a standard three-electrode system without iR correction. Furthermore, the HER and OER property of the commercial Pt/C and RuO2 was also measured to compare.

Figure a displays the linear sweep voltammogram (LSV) curves. The overpotential at 10 mA cm–2 is a significant evaluation parameter for HER. Among these samples, bare NF, P-doped NF, and RuCoOH/NF present an overpotential of 211, 172, and 175 mV at 10 mA cm–2, respectively (Figure b). Interestingly, RuCoP/NF has an overpotential (49 mV) at a current density of 10 mA cm–2. It is near to the overpotential of 20 wt % Pt/C (27 mV). As shown in Figure S6, the effect of the content of Ru on the catalytic activity of these catalysts was also researched. CoP/NF has an overpotential (143 mV) at a current density of 10 mA cm–2, clearly identifying that RuCoP/NF (Ru0.05CoP/NF) has better catalytic performance and the best electrocatalytic performance was mainly attributed to the doping of P. As shown in Figure c, the Tafel slopes of the obtained catalysts were calculated. The consequence revealed that the Tafel slope of RuCoP/NF is the lowest except for Pt/C (45.0 mV dec–1), indicating that the Volmer step is the rate-determining step for HER.[38] The consequence demonstrates that P-dopant accelerates the water dissociation to improve the HER kinetics.[39]

Figure 4

HER catalytic performances of RuCoOH/NF and RuCoP/NF in 1.0 M KOH. (a) HER polarization curves on the various samples at a scan rate of 5 mV s–1. (b) Corresponding overpotentials at a current density of 10 mA cm–2. (c) Corresponding Tafel slopes. (d) Plots of the current density vs the scan rate for the catalysts. (e) Nyquist curves of the obtained catalysts (the inset is the equivalent circuit). (f) Current density between 10 and 190 mA cm–2 with an interval of 20 mA cm–2 every 500 s.

To better understand the superior HER activity of RuCoP/NF, the double-layer capacitance (Cdl) was used to calculate the ECSA. The value of Cdl was estimated via cyclic voltammograms (CVs) in the potential range of −0.8 to −0.9 V with the scan rate from 15 to 40 mV s–1 (Figure S7). As listed in Figure d and Table S1, the values of Cdl and ECSA for RuCoP/NF are 65.1 mF cm–2 and 1627.5 cm2, respectively, higher than NF (2.5 mF cm–2 and 62.5 cm2), P-doped NF (9.2 mF cm–2 and 237.5 cm2), and RuCoOH/NF (3.7 mF cm–2 and 92.5 cm2). This consequence shows that a p-dopant can efficiently improve the value of the ECSA.[40]Figure e reveals the charge-transfer impedance of NF (20.9 Ω), P-doped NF (14.3 Ω), RuCoOH/NF (4.1 Ω), and RuCoP/NF (2.5 Ω). As listed in Table S2, all parameters of EIS spectra were acquired by fitting the Nyquist plots. RuCoP/NF exhibits the smallest radius of the circle on behalf of the minimum charge-transfer resistance (Rct).[41] It indicates that P-doping can increase the intrinsic HER kinetics for electrocatalysts.

Figure f demonstrates a multicurrent step chronopotentiometric curve for RuCoP/NF. When the current density is from 10 to 190 and then to 10 mA cm–2, the potential in each segment has barely changed. The consequence shows that the RuCoP/NF has superior mass transport property and mechanical robustness.[42] According to the abovementioned experimental results, the introduction of P modulates the electron distribution of Ru and Co, enhances intrinsic conductivity to optimize the electrochemical adsorption step, and accelerates the water dissociation. After the incorporation of P, Ru species tending to Ruδ− serve as the boosted active sites under catalytic conditions for HER, which lead to the charge redistribution at the atomic interface of the catalyst, meanwhile improving the H adsorption behavior and further promoting the HER kinetics.[43]

Electrochemical Characterization for OER

The OER electrocatalytic performances of the catalysts in an alkaline electrolyte were also investigated under the same measurement condition as HER. LSV curves and the corresponding overpotentials at 50 mA cm–2 for bare NF, P-doped NF, RuCoOH/NF, RuCoP/NF, and RuO2/NF are shown in Figure a,b. Obviously, the OER activity of RuCoP/NF is significantly improved after the introduction of P. Compared with pure NF and P-doped NF, RuCoP/NF has formed uniform nanowire arrays. The unique nanowire structure provides large surface area and abundant active sites to improve electrochemical performance. As listed in Figure b, RuCoP/NF merely needs an overpotential of 385 mV at 50 mA cm–2, superior to RuCoOH/NF (433 mV). More importantly, this value is close to that of commercial RuO2 (379 mV). Meanwhile, the OER performance of different ratios of doped Ru in RuCoP/NF was also measured, as shown in Figure S8. CoP/NF has an overpotential (403 mV) at a current density of 50 mA cm–2. The overpotential of RuCoP/NF is superior to other catalysts. As listed in Figure c, RuCoP/NF shows the smallest Tafel slope at 89.7 mV dec–1 compared to other control catalysts except RuO2 (70.7 mV dec–1), suggesting faster dynamics for OER. The smallest Tafel slope in RuCoP/NF is attributed to abundant nanostructures and P-doping effects, which favor the reaction dynamics of OER.

Figure 5

OER catalytic performances of RuCoOH//NF and RuCoP/NF in 1.0 M KOH. (a) OER polarization curves on the electrocatalysts at a scan rate of 5 mV s–1. (b) Corresponding overpotentials at 50 mA cm–2. (c) Corresponding Tafel slopes. (d) Plots of the current density vs the scan rate for the catalysts. (e) Nyquist curves for the obtained catalysts (the inset is the corresponding equivalent circuit). (f) Current density between 10 and 190 mA cm–2 with an interval of 20 mA cm–2 every 500 s.

Moreover, by contrasting slope of the current density versus the scan rate (Figure S9), the RuCoP/NF electrode owns the largest Cdl value of 85.6 mF cm–2 (Figure d), meaning it possesses the maximal ECSA (Table S3). This is beneficial for exposing more accessible active sites for the improved catalytic activity.[44] The EIS spectra were measured to further understand the electrochemical kinetics. All electrochemical parameters are also acquired by fitting the Nyquist plots of the samples (Table S4). As shown in Figure e, EIS curves display that the semicircle radius of RuCoP/NF (0.03 Ω) is smaller than that of RuCoOH/NF (0.9 Ω), further demonstrating that RuCoP/NF possesses higher electron transfer rate and quicker electrocatalytic kinetics.

A multistep chronopotentiometric curve of RuCoP/NF is presented in Figure f in 1.0 M KOH. When the current density is from 10 to 190 and then to 10 mA cm–2, the potential has barely changed. The result shows that the RuCoP/NF also is provided with excellent mass transport property and mechanical robustness for electrocatalysis. Hence, P-doping leads to a decrease in the electron density around Co, as well as the increase in Co3+ numbers, which can generate enriched empty d orbitals to promote the adsorption of the OH group. Such strengthened OH adsorption is conducive to overcome reaction kinetics to improve the catalytic activity for OER.[32]

Electrochemical Characterization for Overall Water Splitting

Based on excellent catalytic performance for HER and OER, by employing RuCoP/NF as both anode and cathode, the overall water splitting was tested in the two-electrode system. As listed in Figure a, abundant H2 and O2 bubbles can be produced on the surface of the cathode and anode, indicating the excellent catalytic property. The overall water splitting for RuCoP/NF merely needs 1.533 V to reach a current density of 10 mA cm–2. Meanwhile, the overall water splitting performance of different ratios of doped Ru in RuCoP/NF was tested, as shown in Figure S10. CoP/NF needs 1.607 V to reach a current density of 10 mA cm–2. The results showed that RuCoP (Ru0.05CoP) has the best water-splitting performance, which can be contributed to the tight mechanical cohesion between the substrate NF and RuCoP/NF and the introduction of metals. As presented in Figure b, the RuCoP/NF||RuCoP/NF system shows outstanding durability with a slight amplitude of potential variation for 24 h at 10 mA cm–2. Besides, to make sure that all the current is served for water decomposition, the theoretical and experimental H2/O2 evolved from the RuCoP/NF electrode at a constant current density of 50 mA cm–2 were tested. The experimental yield of H2 and O2 was collected and calculated by means of drainage gas collection shown in Figure c. It exhibits 99.6% Faradaic efficiency for 1 h during overall water splitting. In addition, the volume ratio of hydrogen to oxygen is close to 2:1, and it is consistent with the theoretical values of H2 and O2.[45] These consequences clearly demonstrate that RuCoP/NF is exactly an excellent overall water-splitting catalyst.

Figure 6

(a) Polarization curves of RuCoP/NF in 1.0 M KOH for overall water splitting at a scan rate of 5 mV s–1. (b) Chronopotentiometry test of RuCoP/NF at a current density of 10 mA cm–2 in a two-electrode electrolyzer for 24 h. (c) Faradaic efficiency of H2/O2 generation over the RuCoP/NF electrode at 50 mA cm–2 for 1 h and (d) proposed reaction mechanism for electrocatalytic water splitting on RuCoP/NF nanowire arrays.

To further get insights into the stability of the overall water-splitting process, the XRD and XPS characterizations of the tested sample were conducted (Figures S11 and S12). As seen from Figure S11, the peaks of Co2P in the XRD pattern become weak. The XPS of RuCoP/NF (Figure S12) after the overall water-splitting process is also tested, and all peaks of the Co spectrum are consistent with oxidized and satellite peaks. At the same time, the separate existence of the P–O bond and the significant decrease in P intensity are in close relation to oxidation. These identify the superior stability of RuCoP/NF for electrocatalytic overall water splitting.

To sum up, the improved electrocatalytic performance and stability for the as-synthesized RuCoP/NF nanowire arrays are ascribed to several ways: (1) the unique nanowire structure provides large surface area and abundant active sites; (2) the tight mechanical cohesion between the substrate NF and the RuCoP/NF not merely provides an effective means for fast charge transport and bubbles escape and this maintains long-term stability of the electrochemical process; (3) after being P-doped, the electron structure of Ru and Co changes, which is conducive to the formation of Ruδ− and Coδ+, resulting in the adjustment of binding H*/OH* and the improvement of the overall water-splitting reaction kinetics. Based on the aforementioned discussion, a possible reaction mechanism of RuCoP/NF nanowire arrays for electrocatalytic water splitting was determined (Figure d).

Conclusions

In conclusion, 3D RuCoP/NF nanowire arrays have been successfully designed and prepared on nickel foam through the simple hydrothermal reaction and subsequent phosphating reaction, which adjust the amount of Ru introduced to control the morphology of nanoarrays. Besides, P-doping changes the electron distribution of metals to improve the overall water-splitting reaction kinetics. The P-doping in RuCoOH/NF regulates the electronic structure to improve the inherent catalytic activity. It displays superior bifunctional electrocatalytic property, with an overpotential of 49 mV at a current density of 10 mA cm–2 for HER and 385 mV at a current density of 50 mA cm–2 for OER, respectively. Also, the overall water-splitting system exhibits a voltage of 1.533 V at a current density of 10 mA cm–2 and the excellent long-term stability in 1.0 M KOH solutions. More importantly, the superior stability in overall water splitting is demonstrated within 24 h. There is no doubt that our work offers novel understanding for highly effective bifunctional nanoarray materials by phosphorus-doping and the introduction of trace amount of noble metal.

Experimental Section

Synthesis of a Hydroxide Precursor

Nickel foams (NF, 2 cm × 3 cm) were cleaned using a 1 M HCl solution, ethanol, and deionized water for 20 min by ultrasonication to remove residual surface oxide, respectively. To synthesize a hydroxide precursor, Co(NO3)2·6H2O (0.586 g), NH4F (0.185 g), urea (0.721 g), and RuCl3·xH2O (0.027 g) were dispersed in 30 mL of deionized water. After continuously stirring for 30 min, the mixed solution was transferred into a 50 mL Teflon-lined stainless-steel autoclave (Anhui Kemi Machinery Technology Co., Ltd.), and cleaned NF was immersed into the autoclave and kept at 120 °C for 6 h. After cooling down to room temperature, the hydroxide precursor was thoroughly rinsed with water (three times) and ethanol (one time) and then dried at 60 °C for 6 h.

Synthesis of RuCoP NWs/NF

The as-synthesized hydroxide precursor and 300 mg of sodium hypophosphite were put on both sides of the procelain boat inside a tube furnace. After being flushed with N2 gas flow of 700 sccm to about 30 min, the tube furnace was heated to 300 °C for 2 h at a ramping rate of 2 °C min–1 and under N2 gas flow of 40 sccm. After phosphorization, RuCoP NWs/NF (RuCoP/NF) changed from burgundy to dark black color. What is more, to make clear the effect of different Ru contents, CoP NWs/NF (CoP/NF) was prepared without RuCl3·xH2O and P-doped NF was prepared through directly phosphating NF.