A layered Na_1−xNi_yFe_1−yO₂ double oxide oxygen evolution reaction electrocatalyst for highly efficient water-splitting

Abstract

Transition metal Ni- and Co-based oxides are potential candidates to replace expensive and scarce noble metal-based oxygen evolution reaction (OER) catalysts such as IrO₂ and RuO₂, which are required for efficient hydrogen production from solar water splitting and rechargeable energy storage technologies. So far, layered NiFe double hydroxide represents the best OER activity among all Ni- and Co-based oxides. Here, we report new layered Na_1−xNi_yFe_1−yO₂ double oxide OER catalysts exhibiting activity and stability surpassing those of noble metal OER catalysts including IrO₂ and RuO₂, and a layered NiFe double hydroxide OER catalyst. The superior catalytic properties can be ascribed to the layered structure as well as the enhanced covalency of Ni and Fe. Powered by a lead halide perovskite solar cell with a power conversion efficiency of 14.69%, a two-electrode solar water-splitting device combining a Na_0.08Ni_0.9Fe_0.1O₂ OER catalyst with a NiP hydrogen evolution reaction catalyst delivers a solar-to-hydrogen conversion efficiency of 11.22%. Our design and fabrication strategies offer insights for developing highly active electrocatalysts for water splitting and metal–air batteries.

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Broader context

Producing clean and low-cost hydrogen fuels via solar-driven water splitting requires efficient oxygen evolution reaction (OER) catalysts. Currently, the most efficient OER catalysts in acidic or alkaline solutions are IrO₂ and RuO₂, which suffer from the disadvantages of scarcity and high cost of noble metals. Here, we demonstrate that layered Na_1−xNi_yFe_1−yO₂ double oxide electrocatalysts exhibit OER activity and stability superior to the state-of-the-art noble metal-based oxide and FeNi layered double hydroxide catalysts. The strategy for enhancing the OER activity of transition metal oxides and approach for synthesizing layered provide insights for exploiting highly efficient noble metal-free electrocatalysts for water splitting and metal–air batteries.

Introduction

Solar-driven water splitting to produce clean and recyclable hydrogen fuels represents a desirable solution to the energy shortage and environmental pollution issues that the society is facing.^1–12 Water splitting involves both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). Achieving an efficient OER is significantly more challenging than achieving an efficient HER, because OER involves multiple electron transfer and is kinetically sluggish.^13–22 An efficient OER requires highly active OER catalysts, of which IrO₂ and RuO₂ in acidic or alkaline solutions are currently the most efficient ones. However, these OER catalysts suffer from the scarcity and high cost of noble metals, significantly limiting the production of clean and recyclable hydrogen fuels through solar-driven water splitting. Therefore, it is urgently required to develop highly active OER catalysts based on low-cost and earth-abundant elements; extensive efforts have been taken in this research direction.^23–38

Transition metals such as Ni- and Co-based oxides and hydroxides are promising candidates for the development of highly active, durable, and low-cost OER catalysts.^26–34 For example, lithium cobalt oxide (LiCoO₂), lithium nickel oxide (LiNiO₂), and their analogies have been intensively studied for OER applications.^36–42 LiCoO₂ and LiNiO₂ crystallize in the O3 phase (hexagonal R3m), in which the Li⁺ and Co³⁺/Ni³⁺ ions order on alternate {111} planes.^36–39 It was understood that the inherent OER activity of LiCoO₂ and LiNiO₂ is attributed to the presence of [Co₄O₄]ⁿ⁺ and [Ni₄O₄]ⁿ⁺ cubane structural units, which provide a lower oxidation potential to Co⁴⁺ and Ni⁴⁺ and lower inter-cubane hole mobility.^36–38 However, OER activities of LiCoO₂ and LiNiO₂ underperform that of IrO₂ and RuO₂.^40,41 Various efforts have been made to improve their activity and stability. In general, the OER activity depends predominantly on two factors: the available active sites that can contact electrolyte^1,33,34,40 and the high oxidation states of transition metal elements.^43,44 It has been reported that the catalytic activity of LiCoO₂ can be significantly improved by forcing Co to a higher oxidation state, which can be realized by reducing the Li content through de-lithiation or low temperature synthesis of Li_1−xCoO₂.⁴¹ However, de-lithiation of LiNiO₂ is not successful due to the formation of Li_1−xNi_1+xO₂, which cannot force Ni to higher charge states. Alternatively, Gupta et al. reported that Al incorporation in LiNiO₂ very could improve the OER activity. They showed that Al incorporation pushed Ni to higher charge states.⁴² However, though pushing Co and Ni to high oxidation states has shown improved performance, the OER activities of LiCoO₂ and LiNiO₂ still underperform that of IrO₂ and RuO₂, due to the lack of high enough density of active sites for the three-dimensional crystalline structure of the O3 phase.⁴²

Recently, the FeNi layered double hydroxide (LDH) OER catalyst has shown high activity.^43–45 It was shown that the high OER activity is due to high available active sites to the electrolyte.⁴³ Chemical exfoliation of FeNi-LDH combined with electric conducting graphene sheets resulted in low overpotential and high turnover frequency, due to the synergetic effect between the double hydroxide and graphene, which enhanced the electron transfer.^44,45 In addition, ultrathin NiFe-LDH nanoplates incorporated on mildly oxidized multiwalled carbon nanotubes exhibited higher OER electrocatalytic activity and better stability than a commercial precious metal IrO₂ catalyst.⁴⁵ Increasing the oxidation states of Ni is a viable approach to further improve the OER activity of FeNi-LDH. Unfortunately, increasing the oxidation states of layered FeNi-LDH is very hard due to the formation of meta-phases.^43–45 Thus there are few reports on study of high oxidation states of layered double hydroxide materials.

Herein, we report a unique two-step approach to combine the benefits of high chemical states and high active sites. We first synthesize NaNi_yFe_1−yO₂ and then reduce the Na content by chemical extraction of Na, forming Na_1−xNi_yFe_1−yO₂, which is a layered double oxide consisting of [MO₆] (M = Ni, Fe) octahedral layers with some residual Na atoms lying between the octahedral layers. Like NiFe-LDH, the layered Na_1−xNi_yFe_1−yO₂ double oxide structure provides more active sites than the O3 phase. The use and extraction of Na enforce Ni and Fe to high chemical states, which are confirmed by X-ray photoemission spectroscopy (XPS) measurements. As a result, the new Na_1−xNi_yFe_1−yO₂ electrocatalysts exhibit remarkable OER activity and excellent stability. For instance, Na_0.08Ni_0.9Fe_0.1O₂ reaches a 10 mA cm⁻² anodic current density at a potential of 1.49 V vs. RHE as well as a low onset potential of ∼1.35 V vs. RHE and a Tafel slope of 44 mV dec⁻¹ in alkaline solution. A water oxidation current density of ∼10 mA cm⁻² can be maintained for over 70 h. The performance is superior to the state-of-the-art noble metal-based oxide catalysts, RuO₂ and IrO₂ as well as the NiF-LDH catalyst. Moreover, powered by a 14.69% efficiency lead halide perovskite solar cell, an unassisted two-electrode solar water-splitting device using a Na_0.08Ni_0.9Fe_0.1O₂ OER catalyst and a NiP HER catalyst is able to deliver a 11.22% solar-to-hydrogen conversion efficiency under a simulated AM 1.5G illumination. We also find that the same strategy can be applied to synthesize efficient Na_1−xCo_1−yFe_yO₂ OER catalysts. Therefore, the synthesis strategy used here provides insights for exploiting highly efficient electrocatalysts for water splitting and metal–air batteries.

Results and discussion

The powder X-ray diffraction (PXRD) patterns show that the as-synthesized NaNi_yFe_1−yO₂via solid-reactions is highly crystalline (Fig. 1a). The NaNi_yFe_1−yO₂ samples exhibit cubical particle morphology with sizes ranging from 300 to 700 nm. As an example, a SEM image of the as-prepared NaNi_0.9Fe_0.1O₂ powder is shown in Fig. S1a (ESI†). The XRD patterns are in good agreement with the O3-type phase (Fig. 1b) by Rietveld fitting of the XRD patterns of NaNi_0.9Fe_0.1O₂ and NaCo_0.8Fe_0.2O₂ samples (Fig. S1b, ESI†). The crystal structure of NaNi_1−yFe_yO₂ is shown in Fig. 1b. NaMO₂ can have two structures: O3 type and P2 type, in which sodium ions are accommodated at octahedral and prismatic sites, respectively.⁴⁶ The O3-type NaMO₂ has a hexagonal lattice with a space group of R3m and consists of a cubic close-packed (ccp) oxygen array with Na cations occupying one layer, and Ni/Fe a second adjacent metal layer (Fig. 1b). Metal atoms bind with oxygen atoms forming edge-sharing [MO₆] octahedra. The adjacent [NaO₆] and [MO₆] octahedra share O atoms. Because Na–O bonding is much weaker than M–O bonding, and Na atoms are mobile in NaMO₂, the [NaO₆] octahedra are not shown in Fig. 1b. It is worth noting that the [MO₆] octahedra layers resemble the [M(OH)₆] octahedral layers in NiFe-LDH, CoFe-LDH (Fig. S1c, ESI†).^43–45 The O3-type NaMO₂ and LDH are structurally similar, but chemically different. The LDH has transition metals bonded with OH with an O/H ratio of 1.22 (Fig. S1c, ESI†). On the other hand, the transitional metals in NaMO₂ are bonded with O. Therefore, the transition metals are expected to exhibit higher valence states, leading to enhanced OER activity.^29,43–45

Fig. 1 Structure characterization. (a) XRD patterns of NaNi_yFe_1−yO₂ samples (b) crystal structure of O3-type NaNi_yFe_1−yO₂.

The electrochemical activities of the NaNi_yFe_1−yO₂ were evaluated using a 1.0 M KOH electrolyte. The results of LiNiO₂ and NaNiO₂ are also shown for comparison. LiNiO₂ exhibits an onset potential of 1.57 V vs. RHE (Fig. 2a). The potential required to generate a current density of 10 mA cm⁻² is about 1.63 V, and the corresponding Tafel slope is 60 mV dec⁻¹ (Fig. S2, ESI†). The activity of LiNiO₂ is comparable with the results reported in the literature.^36–38 The NaNiO₂ catalyst shows an onset potential of 1.46 V vs. RHE, and reaches 10 mA cm⁻² at a potential of 1.56 V vs. RHE. The corresponding Tafel slope is 60 mV dec⁻¹. Apparently, the NaNiO₂ OER catalyst performs significantly better than the LiNiO₂ catalyst. More strikingly, upon Fe incorporation, all NaNi_yFe_1−yO₂ catalysts exhibit remarkably improved OER activities (Fig. 2a) compared to the NaNiO₂ catalysts. With the increase of y value, the onset potentials and potentials at 10 mA cm⁻² first gradually decrease to a minimum, and then increase. The catalyst shows the highest activity at y = 0.9: the NaNi_0.9Fe_0.1O₂ catalyst exhibits an onset potential of 1.40 V vs. RHE. The potential for generating a current density of 10 mA cm⁻² is about 1.52 V vs. RHE, and the corresponding Tafel slope is 44 mV dec⁻¹ (Fig. S2, ESI†). NaNi_0.8Fe_0.2O₂ and NaNi_0.75Fe_0.25O₂ exhibit the same onset potential of 1.43 V vs. RHE, but their potentials at 10 mA cm⁻² are 1.55 V and 1.56 V vs. RHE, respectively, and their corresponding Tafel slopes are 52 and 60 mV dec⁻¹, respectively. The effect of the y value on activity has a similar trend to that in Ni_yFe_1−y-LDH and/or perovskite OER catalysts (BaSrCo_yFe_1−yO₃),⁴³ indicating that the content of Fe would affect the e-filling level of Ni, which closes to unity at y = 0.9. To further comprehend the catalytic activity of NaNi_yFe_1−yO₂, the relative electrochemically active surface areas (ECSA) of the samples are identified by their double-layer capacitance (C_dl) (Fig. S3, ESI†), which is determined from cyclic voltammetry measurements. Then the current densities are normalized by the ECSA.⁴⁶ As shown in Table S1 (ESI†), NaNi_0.9Fe_0.1O₂ shows the highest normalized current density, which stands for the number of active sites per relative surface area, among all NaNi_yFe_1−yO₂ samples and it is more than 2.8 times higher than that of RuO₂. To the best of our knowledge, the OER performance of NaNi_0.9Fe_0.1O₂ is the highest among all previously reported OER catalysts coated on glassy carbon substrates (Table S2, ESI†).

Fig. 2 OER polarization curves of NaNi_yFe_1−yO₂, NaNiO₂, LiNiO₂, and RuO₂ in 1.0 M KOH solution with various y values. (a) LSV curves of Pt, RuO₂, and NaNi_yFe_1−yO₂ with y = 0.75, 0.80, 0.90 and 1.00 respectively. (b) Durability test for the NaNi_0.9Fe_0.1O₂ and RuO₂ samples at 10 mA cm⁻² in 1.0 M KOH solution.

The OER performance of NaNi_yFe_1−yO₂ was also compared with that of Pt and RuO₂. For Pt and RuO₂ catalysts, the potential required to generate a current density of 10 mA cm⁻² is about 1.78 V and 1.55 V vs. RHE, respectively, and the corresponding Tafel slopes are 246 and 73 mV dec⁻¹, respectively. These values are considerably higher than that of NaNi_yFe_1−yO₂ with y = 0.80 and 0.90, confirming the superior electrochemical OER performance of NaNi_0.9Fe_0.1O₂ as compared to the-state-of-the-art noble metal catalyst. The stability of the NaNi_0.9Fe_0.1O₂ catalyst was evaluated by chronocoulometry measurements (Fig. 2b). Under a galvanostatic current density of 10 mA cm⁻², a constant operating potential of 1.52 V vs. RHE is maintained for over 70 h, whereas the RuO₂ catalyst exhibits a continuous current loss during the test. These results suggest that NaNi_0.9Fe_0.1O₂ is an efficient and stable electrocatalyst for OER. Moreover, the bulk and surface compositions of the sample were determined using inductively coupled plasma mass spectroscopy (ICP-MS) and energy-dispersive X-ray spectroscopy (EDS), respectively. The ICP-MS shows a slight decrease (0.2%) in the Na content after the OER test (Table S3, ESI†). The EDS measurements (Fig. S4, ESI†) of the sample before the stability test show an even Na distribution between the surface and the bulk, while the measurements of the sample after the stability test reveal a slight Na deficiency at the surface (Table S4, ESI†). The alkaline metal extraction during the OER test may be due to their strong ionic nature, which is common in alkaline metal-containing materials.^36–42

The activity improvements observed in the NaNi_yFe_1−yO₂ catalysts can be understood by their crystal structure and the chemical states of Ni and Fe. For example, the Na–O and Ni–O bond lengths in NaNi_0.9Fe_0.1O₂ are 2.408(3) Å and 1.971(2) Å, respectively, which are longer than the Li–O and Ni–O bond lengths in LiNiO₂ (2.0946(3) Å) and (1.9220(2) Å).^47–49 As a result, the unit cell volume of NaNi_0.9Fe_0.1O₂ is 1.26 times larger than that of LiNiO₂. The volume expansion of [MO₆] units is caused by the lower electronegativity of Na and Fe compared to their counterparts, Li and Ni. The unit cell volume of NaNi_yFe_1−yO₂ decreases with the increase of the y value due to the smaller ionic radius and higher electronegativity of Ni compared to Fe. Thus, the corresponding peaks in XRD patterns shift to higher angles with the increase of y (Fig. S5, ESI†). It was proposed that a lower electronegativity of introduced metals can push up the valence state of Ni and increase the Ni–oxygen covalency.^35,43 Therefore, the lower electronegativity of Na over Li and Fe over Ni leads to enhanced Ni–oxygen covalency, which is beneficial for charge transfer and hence favors OER catalytic activity.⁴⁶ This is consistent with our results that NaNi_yFe_1−yO₂ catalysts perform much better than the LiNiO₂ catalysts.

The above explanations are supported by the XPS results. Fig. 3 shows the XPS spectra of Ni 2p and Fe 2p obtained from LiNiO₂, NaNiO₂, and NaNi_0.9Fe_0.1O₂ samples. The peaks at 856.1 and 873.3 eV in Fig. 3a are assigned to Ni 2p_3/2 and Ni 2p_1/2, respectively,⁵⁰ and they can be fitted with the two chemical states of Ni³⁺ and Ni²⁺, consistent with the reported literature.⁴² The Ni²⁺ chemical states may be caused by non-stoichiometric Li_1−xNi_1+xO₂ impurities in the alternate (111) planes of the cubic close-packed oxygen sublattices of LiNiO₂.⁴² The satellite peaks in the range of 860 to 890 eV for Ni 2p can be ascribed to Ni³⁺ in NiOOH, which is produced by reacting with the absorbed water from the ambient air.⁵⁰ For NaNiO₂, the peak positions of Ni 2p_3/2 and Ni 2p_1/2 shift to 857.1 and 874.1 eV, respectively. Such a position shift indicates an increase in the amount of Ni³⁺, further confirming that the presence of Na forces Ni to its higher oxidation state. Upon Fe incorporation, the peak positions shift to even higher binding energies. The peak positions of Ni 2p_3/2 and Ni 2p_1/2 in the NaNi_0.9Fe_0.1O₂ shift to 857.6 and 876.1 eV, indicating the mixed chemical state of both Ni³⁺ and Ni⁴⁺.⁵⁰ Furthermore, the peaks at 711.7 eV and 725.0 eV (Fig. 3b), respectively, are the characteristic signatures of Fe³⁺ and Fe⁴⁺.⁵¹ Higher chemical oxidation states would favor the absorption of active species and charge transfer in the rate-determining steps of OER,^35,43 which explains the superior activity of NaNi_0.9Fe_0.1O₂ as compared to NaNiO₂ and LiNiO₂. The ascending chemical oxidation state order of Ni in LiNiO₂, NaNiO₂ and NaNi_0.9Fe_0.1O₂ samples is in good accordance with the ascending OER performance order of these samples. It is already well known that the Fe⁴⁺ chemical state is also highly favorable for OER catalysts,⁵² even though only few studies on OER catalysts with the Fe⁴⁺ chemical state have been reported due to complicated synthesis conditions (i.e. CaCu₃Fe₄O₁₂).³⁵ Therefore, the electrocatalytic activity improvements of NaNi_yFe_1−yO₂ are ascribed to the combination of high chemical states of Ni and Fe.

Fig. 3 XPS spectra obtained from LiNiO₂, NaNiO₂, NaNi_0.9Fe_0.1O₂ and Na_0.08Ni_0.9Fe_0.1O₂ samples. (a) Ni 2p, (b) Fe 2p states.

As an analogy of NaNi_yFe_1−yO₂, NaCo_yFe_1−yO₂ samples were also studied. NaCo_yFe_1−yO₂ samples also crystallize in the O3-type crystal structure (Fig. S6, ESI†). Among all synthesized catalysts in this family of alloys, NaCo_0.8Fe_0.2O₂ exhibits the best performance (Fig. S6b, ESI†). Even though the activity is not as good as that of NaNi_0.9Fe_0.1O₂, it still outperforms noble metal electrocatalysts including RuO₂ (Table S2, ESI†).

Given that the active sites of NaNi_yFe_1−yO₂ materials are the layered [MO₆] octahedra, partial extraction of Na cations could expose more [MO₆] octahedra active sites to the electrolyte, which is beneficial for activity improvement. In sodium-ion batteries, de-intercalation of Na gradually converts NaMO₂ from the O3-type to the P3-type, because gliding of [MO₆] layers results in the formation of more symmetric structures. The electrochemical performance (Fig. 4) of Na de-intercalated NaNi_0.9Fe_0.1O₂ samples was evaluated. ICP-MS measurements suggest that the samples with 12, 24, 48 and 72 h Na-extraction have compositions of Na_0.41Ni_0.9Fe_0.1O₂, Na_0.24Ni_0.9Fe_0.1O₂, Na_0.08Ni_0.9Fe_0.1O₂ and Na_0.01Ni_0.9Fe_0.1O₂, respectively. With the increase of Na-extraction duration, the performance improves and reaches the highest activity at 48 h (Na_0.08Ni_0.9Fe_0.1O₂), but further increase of Na-extraction time leads to decreased activity. For Na_0.41Ni_0.9Fe_0.1O₂ and Na_0.24Ni_0.9Fe_0.1O₂ samples, the potentials at 10 mA cm⁻² decrease to 1.50 V and 1.49 V vs. RHE, respectively (Fig. 4a), which are lower than that of NaNi_0.9Fe_0.1O₂ (1.52 V vs. RHE). The corresponding Tafel slopes are 60 and 43 mV dec⁻¹, respectively (Fig. 4b). Na_0.08Ni_0.9Fe_0.1O₂ shows the best performance with an onset potential of 1.40 V vs. RHE and a Tafel slope of 40 mV dec⁻¹, outperforming the best noble metal catalysts, LiMO₂ analogies, and most of other reported catalysts on glassy carbon substrates (Table S2, ESI†). The SEM images reveal that the morphology of Na_0.08Ni_0.9Fe_0.1O₂ changes to layered flakes from the cubes before Na extraction (Fig. S7, ESI†). The XRD pattern obtained from the Na_0.08Ni_0.9Fe_0.1O₂ sample shows only two peaks at 12.0° and 24.1° (Fig. 4c), resembling the XRD pattern of (001)-oriented NiFe-LDH,^44,45 further confirming the layered structure. XRD simulations indicate that the structure of Na_0.08Ni_0.9Fe_0.1O₂ can be considered as the structure of NaNi_0.9Fe_0.1O₂ in the absence of Na atoms (Fig. S8b, ESI†). After Na extraction, the unit cell has an expansion of 0.2 nm along the [001] direction, possibly due to the weaker van der Waals interaction between two adjacent [MO₆] octahedral layers. To further verify the structure of Na de-intercalated NaNi_0.9Fe_0.1O₂, Raman spectroscopy was conducted (Fig. S9, ESI†). Two featured peaks centered around 450 and 650 cm⁻¹ are ascribed to Ni–O and Fe–O bonds.^53,54 The broadness of the peaks may be caused by the structural asymmetry. The similarity between NaNi_0.9Fe_0.1O₂ and Na_0.08Ni_0.9Fe_0.1O₂ reveals that the [MO₆] layers are maintained after 48 h Na-extraction. The improved activity after Na de-intercalation is attributed to the exposure of more active sites to the electrolyte. However, excessive extraction of Na will not allow a stable double oxide structure to persist, due to the relatively high chemical oxidation state of Ni and Fe. The stability of Na_0.08Ni_0.9Fe_0.1O₂ was evaluated by chronocoulometry measurements (Fig. 4d). Under a galvanostatic current density of 10 mA cm⁻², the Na_0.08Ni_0.9Fe_0.1O₂ catalyst exhibits a nearly constant operating potential of 1.50 V vs. RHE for more than 40 h, demonstrating an excellent stability of the layered oxide material. Further evaluation through ICP-MS (Table S5, ESI†), EDS (Table S5, ESI†), XRD (Fig. S10, ESI†), Raman spectra (Fig. S9, ESI†), and SEM (Fig. S11, ESI†) confirm that the structure, chemical state, and composition of a Na_0.08Ni_0.9Fe_0.1O₂ sample after the stability test exhibit negligible change, demonstrating that the Na_0.08Ni_0.9Fe_0.1O₂ sample is very stable under operation. Na_0.08Ni_0.9Fe_0.1O₂ may experience further Na extraction during OER testing. While the ICP-MS measurements show only 0.08% decrease in the Na content, EDS analysis did not exhibit Na content variation due to the sensitivity limit. Therefore, Na_0.08Ni_0.9Fe_0.1O₂ demonstrates a higher electrocatalytic activity and stability than NaNi_0.9Fe_0.1O₂. In addition, the EDS and ICP-MS did not detect K in our catalysts after activity tests in the KOH electrolyte. Therefore, the electrolyte should not affect the stability of NaNi_0.9Fe_0.1O₂.

Fig. 4 Electrochemical and structural characterization of NaNi_0.9Fe_0.1O₂ samples. (a) LSV curves of NaNi_0.9Fe_0.1O₂ samples for various Na extraction times. (b) Corresponding Tafel plots of (a). (c) XRD patterns of NaNi_0.9Fe_0.1O₂ samples for various Na extraction times. (d) Durability test for the Na_0.08Ni_0.9Fe_0.1O₂ sample (48 h of Na extraction) at 10 mA cm⁻² in 1.0 M KOH solution.

The Na_0.08Ni_0.9Fe_0.1O₂ sample exhibits higher chemical states of Ni and Fe compared to NaNi_0.9Fe_0.1O₂ (Fig. 2). The corresponding XPS peaks of Ni 2p_3/2, Ni 2p_1/2, Fe 2p_3/2 and Fe 2p_1/2 obtained from the sample Na_0.08Ni_0.9Fe_0.1O₂ are 858.7 and 880.5 eV, 712.6 eV and 725.7 eV, respectively. Therefore, the increased OER activity of Na_0.08Ni_0.9Fe_0.1O₂ is due to the higher covalency of Ni. Electrocatalysts often show broad redox waves, and the valence state always shuttles between oxidation states and resting states.^55,56 The slight variation in oxidation states will strongly affect the absorbance of oxidative oxygen species O₂²⁻/O⁻. In particular, in the rate-determining steps of OER, the higher chemical states could promote the absorption capacity, thus resulting in higher activities.^35,43 Electrochemical impedance spectroscopy (EIS) was carried out to evaluate the charge transfer during oxygen evolution (Fig. S12, ESI†). The semicircular arc diameter is related to charge transfer resistance (R_ct), and smaller diameter means smaller R_ct. and therefore, improved charge transfer in the rate-determining steps of OER.⁴⁰ The semicircular arc diameter of Na_0.08Ni_0.9Fe_0.1O₂ in the EIS plot is much smaller than that of RuO₂, indicating that Na_0.08Ni_0.9Fe_0.1O₂ has very high electron generation and fast charge transfer for OER. These results further confirm that Na_0.08Ni_0.9Fe_0.1O₂ is an excellent OER catalyst with high activity and superior stability.

Given the excellent activity and stability, Na_0.08Ni_0.9Fe_0.1O₂ and NaNi_0.9Fe_0.1O₂ OER catalysts coated on Ni foam were integrated with NiP nanoparticle HER catalysts coated on Ni foam to form a two-electrode water-splitting device. NiP was chosen as an alternative to a noble metal HER catalyst. The HER catalytic activities of NiP were also evaluated in 1.0 M KOH solution (Fig. S13a and b, ESI†). NiP has the lowest onset potential of around −100 mV vs. RHE. The cathodic current density reaches up to −10 mA cm⁻² at a potential of −150 mV vs. RHE. Furthermore, the stability test manifests that NiP can still maintain stable potentials at −10 mA cm⁻² for over 6 h (Fig. S13b, ESI†). Thus, it has been demonstrated that the NiP is a superior HER electrocatalyst exhibiting high activity and stability. The Na_0.08Ni_0.9Fe_0.1O₂ and NaNi_0.9Fe_0.1O₂ catalysts coated on nickel foam show even higher performance. They reach 10 mA cm⁻² current density at a potential of 1.39 and 1.41 V (vs. RHE) respectively (Fig. S13, ESI†). A water electrolysis polarization curve obtained using the combination of RuO₂(+) with Pt(−) coating on Ni foam exhibits an onset potential difference of 1.40 V, and achieves 10 mA cm⁻² current densities at a potential difference of 1.62 V (Fig. 5a). The IrO₂(+) and Pt(−) electrode combination shows inferior activities, with an onset potential difference of 1.50 V and a potential difference of 1.74 V at 10 mA cm⁻². The NaNi_0.9Fe_0.1O₂ and NiP electrode combination shows a slightly higher performance than the RuO₂(+) and Pt(−) electrode combination – with the same onset potential but a slightly lower potential of 1.61 V at 10 mA cm⁻². Strikingly, the Na_0.08Ni_0.9Fe_0.1O₂ and NiP electrode combination shows much higher activities – an onset potential difference of 1.4 V and a potential difference of 1.54 V at 10 mA cm⁻². The long-term stability test is further conducted by full water splitting at 1.60 V applied potential difference. The RuO₂(+) and Pt(–) electrode combination exhibits a gradual decay from 9.5 to 7.9 mA cm⁻² in 5 h. In contrast, the current density of the Na_0.08Ni_0.9Fe_0.1O₂ and NiP electrode combination is stable above 15.5 mA cm⁻² for over 12 h of continuous testing (Fig. 5b). Gas chromatography measurements confirm a high faradaic efficiency of H₂ and O₂ at the predicted ratio of 2 : 1 (Fig. S14, ESI†).

Fig. 5 (a) Overall water-splitting characteristics of different catalyst electrodes in a two-electrode configuration. (b) Durability test for the Na_0.08Ni_0.9Fe_0.1O₂ sample in a two-electrode configuration at 1.60 V vs. RHE in 1.0 M KOH solution, compared with noble metal eletrodes. (c) J–V curves of the perovskite tandem cell under simulated AM 1.5G 100 mW cm⁻² illumination. (d) Current density–time curve of solar-driven water splitting under chopped simulated AM 1.5G 100 mW cm⁻² illumination. Inset image in (d) is the schematic of the solar-driven water splitting device using the Na_0.08Ni_0.9Fe_0.1O₂ electrocatalyst and perovskite solar cells.

The Na_0.08Ni_0.9Fe_0.1O₂ and NiP electrode combination was also tested in an unassisted solar-driven water splitting device. A series-connected lead halide perovskite solar cell, which is composed of two perovskite sub-cells with an open circuit potential (V_oc) of 1.10 V each, was used to power the system. The series-connected perovskite solar cell exhibits a V_oc of 2.16 V, a short circuit current density (J_sc) of 9.96 mA cm⁻², and a power conversion efficiency of 14.69% when illuminated under 100 mW cm⁻² AM 1.5G solar irradiation (Fig. 5c). The Na_0.08Ni_0.9Fe_0.1O₂ and NiP two-electrode system delivered a current density of 9.12 mA cm⁻² and was stable for up to 2 h (Fig. 5d). Further lifetime testing was limited by the stability of the lead halide perovskite cell. A corresponding solar-to-hydrogen conversion efficiency of ∼11.22% was obtained from the polarization curve. The current density response under chopped simulated sunlight illumination during the stability test indicates good reversibility of the electrodes and relatively rapid response to changes in photocurrent.

Conclusions

New noble metal-free layered Na_1−xNi_yFe_1−yO₂ oxide electrocatalysts have been successfully developed for efficient water splitting. Owing to Na extraction, enhanced charge states of Ni and Fe, and a layered microstructure, Na_1−xNi_yFe_1−yO₂ catalysts exhibit excellent OER activity. Among these catalysts, Na_0.08Ni_0.9Fe_0.1O₂ exhibits the highest OER activity, higher than LiMO₂, noble metal electocatalysts (e.g. RuO₂, IrO₂ and Pt) and most of reported OER catalysts including the NiFe or CoFe layered double hydroxides. When integrated with a 14.69% efficient perovskite solar cell, the Na_0.08Ni_0.9Fe_0.1O₂ and NiP two-electrode system delivered a solar-to-hydrogen efficiency of 11.22% under an AM 1.5G simulated illumination. Due to the scalable synthetic method, wide material availability, and high catalytic activity and stability, the Na_1−xNi_yFe_1−yO₂ OER catalysts are attractive candidates for substituting noble metal-based catalysts toward highly efficient and low-cost electrolysis or solar-driven water splitting.

Acknowledgements

This paper presents results from an NSF project (award number CBET-1433401) competitively-selected under the solicitation “NSF 14–15: NSF/DOE Partnership on Advanced Frontiers in Renewable Hydrogen Fuel Production via Solar Water Splitting Technologies”, which was co-sponsored by the National Science Foundation, Division of Chemical, Bioengineering, Environmental, and Transport Systems (CBET), and the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office. The work was also partially supported by the National Science Foundation under contract no. CHE-1230246 and DMR-1534686. This research used the resources of the National Energy Research Scientific Computing Center, which is supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

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Footnote

† Electronic supplementary information (ESI) available: Detailed experimental methods, characterization and supporting figures. See DOI: 10.1039/c6ee03088b

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