Spray-Coated Thin-Film Ni-Oxide Nanoflakes as Single Electrocatalysts for Oxygen Evolution and Hydrogen Generation from Water Splitting.

Electrochemical water splitting is a key process in many electrochemical energy conversion and storage phenomena. Simple synthesis methods to make highly porous and active nanostructured catalytic materials with large electroactive surface areas are very important to implement water-to-fuel conversion schemes. Herein, ultrafine, transparent thin-film nickel-oxide (NiO x ) nanoflakes are facilely synthesized following a simple spray-coating method from a solution-phase precursor. The NiO x nanoscale structures are grown on the FTO surface in the form of highly uniform smooth thin films. They are employed as promising bifunctional electrocatalysts for the overall water splitting process under alkaline conditions. During water oxidation catalysis, NiO x -SC/FTO initiates the oxygen evolution reaction (OER) at an overpotential η of just 250 mV while generating current decade at just 300 mV and demonstrates well-balanced kinetics toward OER. 10 mA cm-2 current density remains persistent for many hours of continuous electrolysis at just 1.53 VRHE illustrating high robustness of the system. The catalyst also showed substantial activity and durability toward the hydrogen evolution reaction (HER) under the same electrochemical conditions. Tafel slopes of just 57 and 89 mV dec-1 for OER and HER in 0.5 M aqueous KOH solution, respectively, showing high intrinsic kinetics for electrocatalysis. Having high electrochemical surface area and an optimum number of electrochemically active sites, these transparent NiO x thin films can be advantageously combined with photoelectrochemical devices for light-driven water-to-fuel conversion systems.

Introduction

Intensified research activities in the energy and conservation sector have mainly focused on the production of clean and sustainable carbon-free and carbon-neutral fuel sources having zero net emissions.[1] In this regard, electrocatalytic water splitting has been a center of interest for decades due to its plentiful precursors and green by-products. Typically, water electrolysis encompasses two half-reactions including oxygen evolution reaction (OER) and hydrogen evolution reaction (HER).[2] Between them, OER occurring at the anode is kinetically sluggish involving an uphill proton-coupled electron transfer reaction to release four electrons and making facile O–O bonds.[3] Therefore, communally, both OER and HER have to overcome a large energy barrier, which is reflected by an extensively required overpotential.[4] To overcome this energy barrier and to initiate water splitting, mainly water oxidation reaction (WOR) at lower energy input, competent electrocatalysts are required and employed.[2] They can be molecular systems or inorganic nanoscale materials coated on electrode surfaces.[4,5]

Highly efficient electrocatalysts such as oxides or ruthenium,[5] iridium,[6] and Pd[2] for OER and platinum[7] for HER have been considered to be best materials owing to their promising catalytic activities. However, their large-scale commercialization is significantly hampered due to their high cost and scarcity.[4] Therefore, to make the overall process economical, precious-metal-based catalytic systems must be replaced with non-noble-metal-based assemblages without compromising catalytic efficacy. In pursuit of this, materials like 3d transition-metal oxides,[4] hydroxides,[8] oxy-hydroxides,[9] double-layer hydroxides,[10] chalcogenides, sulfides,[11] and carbides[12] have been frequently reported as better OER electrocatalysts in alkaline conditions. Recently, Co3S4@MoS2 heterostructures prepared via a metal–organic framework strategy and hydrothermal method have been introduced as efficient bifunctional HER/OER electrocatalysts under acidic and alkaline conditions,[13a,13b] although less research has been conducted to develop very active bifunctional catalysts catalyzing both OER and HER under similar electrochemical conditions. Therefore, it is imperative to develop non-noble-metal electrode/electrocatalytic materials for overall water splitting using the same electrolyte.

A large number of homogeneous and nanoscale heterogeneous catalytic structures have been synthesized, which are later immobilized on various conductive supports to be employed as electrodes in an electrochemical cell.[4] However, these materials often undergo agglomeration and detachments from their support during catalysis.[13c] Consequently, some facile synthetic strategies are needed to chemically, electrically, and mechanically anchor the nanoscale electrocatalysts on the conductor surface.[14] Direct growth of the electroactive material on the conductor surface to obtain a thin-film catalyst can be a plausible solution as thin-film metals[2] and inorganic oxide type systems[4,20] are relatively more stable due to their compact structures and ease of electron transfer. Enormous synthetic stratagems have been disclosed to fabricate ultrafine and fine thin-film electrocatalysts such as electrodeposition,[15] anodization,[16] microemulsion,[17] colloidal,[4] solid-state,[17b] sol–gel method,[18] hydrothermal method,[19] drop-casting,[20] chemical vapor deposition,[2] atomic layer deposition,[21] laser ablation method,[22] and self-assembly method.[23] Most of the said methods to synthesize thin-film inorganic oxides are either very expensive or complicated and not feasible to use for large-scale implementation. Therefore, some self-sustainable, time-effective, and cost-effective methods are required to design facile catalytic thin films.

For the large-scale applications, bifunctional thin-film OER/HER electrocatalysts with optimum redox chemistries, stabilities, and catalytic capabilities for oxidative and reductive proton-coupled electron transfer reactions are prerequisites to address the challenges associated with OER/HER processes.[14] Furthermore, along with the abovementioned characteristics, nanotexturing and reducing the size down to the nanometer scale for the generation of more active sites on the catalyst surface are also important factors governing catalytic efficiency of film structures.[24]

By considering all these points, here, we report a scalable strategy to develop a highly active, competent, and durable NiO nanoflake type thin film as a bifunctional electrocatalyst grown uniformly on the FTO conductor surface via simple spray-coating (NiO-SC/FTO). Among transition-metal oxides, NiO is copious and technologically significant semiconducting oxide and has been reported widely to be employed in catalysis, batteries, supercapacitors, and sensors.[25] Hence, the NiO type thin-film electrocatalyst is fabricated via a low-temperature spray pyrolysis method and is employed as an OER/HER catalyst under alkaline conditions (Scheme S1). NiO-SC/FTO achieved η (at 10 mA cm–2) of 300 and 350 mV and impressive Tafel plots of just 57 and 89 mV dec–1 for OER and HER in 0.5 M aqueous KOH solution. The improved electrocatalytic performance can be generally ascribed to the formation of adherent, transparent, nanoparticulate type catalytic structures that are durable and enhance the active sites and greatly facilitate the electronic transfer during the catalytic process.

Results and Discussion

Physicochemical Characterizations of NiO-SC/FTO

Ultrafine catalytic films are fabricated by a simple spray-coating method. The morphology and surface structure of the as-grown NiO-SC/FTO catalytic layer is determined via scanning electron microscopy. The SEM images presenting the surface features and cross-sectional structure of the catalyst are shown in Figure a–d.

Figure 1

Physical and physicochemical characterizations. Scanning electron microscopy images for NiO-SC/FTO: (a) overview surface image; (b) high-resolution surface image; (c) overview cross-sectional image; (d) high-resolution cross-sectional image.

The SEM images show surface features with nanoflake type structures, having homogeneous morphological attributes that are uniformly distributed on the conductor surface in the form of the thin film. A magnified view shows very uniform nanoflake type structures, which are smaller and have nanoscale features with porous hollow morphology (Figure b). The individual nanoflake can also be seen on the NiO-SC/FTO exterior in the high-resolution SEM image (Figure S1). Also, definite spaces and pores among nanoflakes can be observed, which enhance the surface areas of materials. A cross-sectional view of thin films clearly reveals the uniform thickness of the film in the range of about 700 nm. A naked eye view of the as-prepared NiO film appears to have good transparency and smoothness. Therefore, these catalytic films can advantageously be aligned with photocatalytic devices for solar water splitting as well.

The elemental composition of the as-grown catalytic film is investigated via energy-dispersive X-ray spectroscopy, which is commenced along with the SEM device. Figure S2 presents the EDS spectrum for the NiO-SC/FTO sample. The EDS measurements indicate the presence of nickel, oxygen as a major contributor, and some signals of tin and silicon that originated from the substrate as fluorine-doped tin oxide was used as a conductor for coating of the catalytic films. Some residual chlorine was also seen, which may have come from nickel-chloride salt. EDS results show that NiO thin films are highly pure. It is seen that the percentage of nickel is 16.52% and that of oxygen is 40%. Next, the active catalytic phase and purity of the as-grown NiO-SC/FTO catalytic film are investigated via X-ray diffraction pattern analysis. Figure illustrates the XRD spectrum for NiO-SC/FTO (red) and simulated pattern for the NiO type structure (black).

Figure 2

Physical and physicochemical characterizations. XRD (X-ray diffraction pattern) spectrum for NiO-SC/FTO.

The XRD pattern shows the presence of the NiO type phase where the relative intensity and position of 2θ diffraction peaks match well with those reported for the standard NiO (PDF #44-1159).[26] 2θ peaks represented at 37.255, 43.287, 62.880, 75.416, 79.3, and 79.501° are indexed to the (101), (012), (110), (113), (202), and (006) planes of NiO, respectively. All the diffraction peaks reflected that Ni crystallizes with the cubic crystal structure in the space group Fm3m. Mean crystallite size is found to be 84.3 nm. Three peaks at 29, 30, and 31° on the XRD spectrum may arise due to silicon and tin as FTO-coated glass substrates were used as the support for catalyst coating.[26a,26b] Furthermore, the appearance of narrow and sharp peaks suggests that the Ni-based thin-film catalyst obtained by a facile low-temperature spray-coating pyrolysis approach is highly pure and mainly comprises a NiO type catalytic structure.[26c] Further, Raman spectroscopy is commenced to know more about the catalyst structure. Figure S3 illustrates the Raman spectrum for the NiO-SC/FTO thin-film catalyst. Obvious peaks in the vicinity of 500 to 600 cm–1 originate from the Ni–O stretching mode as reported previously.[26b] Also, peaks at 740 and 1020 cm–1 are also indicative of the NiO active phase at the catalyst surface.[27a]

Furthermore, the surface electronic state and exact chemical nature of NiO-SC/FTO are analyzed via X-ray photoelectron spectroscopy and are presented in Figure . The XPS survey spectrum illustrates the obvious signatures for Ni and O atoms in the catalytic film, and there is a minute peak for background C also observed (Figure a). The Ni 2p, O 1s, and C 1s deconvoluted core spectra of Ni, O, and C are shown in Figure b–d, respectively. Figure b represents two spin–orbit doublets and two shakeup satellites for the nickel element. The peak centered at 854.5 eV (2p3/2) corresponds to the Ni2+ phase, whereas the peak at 855 eV (2p3/2) with respective shakeup satellites can also be ascribed to the Ni2+ nickel state, which is characteristic of the NiO type structure.[27b] The intense peak at 871.9 eV (2p1/2) represents metallic nickel Ni0, which may be due to the reduction of some NiO during vacuum/argon exposure,[28a] while the peak at 873.5 eV (2p1/2) corresponds to the Ni2+ species.[28b]

Figure 3

Physical and physicochemical characterizations. XPS (X-ray photoelectron spectroscopy) spectrum for NiO-SC/FTO: (a) survey spectrum; (b) N 2p core-level spectrum; (c) O 1s core spectrum; (d) C 1s core spectrum.

Figure c presents obvious peaks at 530.3 and 531.3 eV, which are ascribed to the lattice oxygen from the metal–oxygen bond and adsorbed oxygen from surface oxygen species, respectively.[29] Some signatures of background carbon are also observed, and a minute peak at a binding energy of 282 to 284 eV is well ascribed to C 1s.[4] Thus, the XPS analysis confirms the NiO as an active catalytic structure constituting the thin-film catalyst.

It is worthy to mention that the spray-coating method can generate hollow catalytic films due to subsequent nucleation processes that occur directly on the conductor surface during deposition as shown by flake type morphology (Figure , SEM analysis). The porous texture of the catalytic film is further studied by sorption isotherm and pore size distribution analysis. During the spray-coating process, the material upon striking the heated FTO250–500 surface undergoes decomposition along with evaporation of water and gases, which ultimately results in the hollow porous structure of the catalyst. Figure presents that the sorption isotherm is found to be “S”-shaped. According to IUPAC classifications, this is ascribed to a type 3 isotherm with an H-3 hysteresis loop. Consequently, a high surface area of 189 m2 g–1 is found for NiO-SC/FTO along with highly order porosity averaging at 4.2 and 6 nm. Large surface area and high porosity are highly desirable for application in catalysis as they allow optimum adsorption/desorption of various intermediates on the catalyst surface during gas generation. Conclusively, all the analytical characterizations confirm the presence of the ultrafine nanoflake type, porous, pure NiO type catalytic phase on the FTO surface following a simple spray-coating method. All these features are advantageous for catalytic investigations of an electroactive material.

Figure 4

Physical and physicochemical characterizations. Adsorption–desorption isotherm for NiO-SC/FTO with the corresponding pore size distribution curve in the inset.

Preliminary Electrochemical Investigation of NiO-SC/FTO

Before testing the catalytic activity of the electrode toward OER and HER, the preliminary electrochemical characterization of the catalytic film is performed. In this quest, electrochemical stability of the catalyst surface,[30] electrochemically active surface area,[2,4] and electrochemical accessibility of active sites[31] are thoroughly evaluated following standard electrochemical methods (Figure a–d). Cyclic voltammetry measurements are used to characterize the behavior of Ni-oxide in the 0.5 M aqueous KOH electrolyte in the potential range of 0.9 to 1.48 VRHE (Figure a). The appearance of broad redox peaks in the vicinity of 1.15 to 1.45 VRHE is indicative of high valent nickel oxide Ni2+/3+ formation, which is desirable for promising catalytic activity.[20] Electrochemical activity, stability of oxide, and catalyst aging are investigated by measuring concurrent 200 CV cycles between 0.9 and 1.48 VRHE at a scan rate of 50 mV s–1 through their current density evolution. It is observed that the shape of the voltammogram changes slightly during long-term testing, not any significant alteration occurs in the catalysts, and current densities also remain the same during the 1st and 200th consecutive cycles. The fact that anodic and cathodic current densities do not change drastically and no extra peak appears ultimately reveals the high stability of the thin-film oxide materials.

Figure 5

Electrochemical characterizations. (a) Cyclic voltammograms representing the 1st, 20th, 40th, and up to 200th concurrent sweeps of NiO-SC/FTO during repetitive potential cycles under 0.9 to 1.48 VRHE at a scan rate of 50 mV s–1; (b) charge q* evolution; (c) q*/q*initial ratio during stability test measurements; (d) plot of scan rate versus current for calculation of double-layer capacitance in 0.5 M aqueous KOH electrolyte solution.

Furthermore, for the different cycles of this stability test charge values (q*) are measured to study the active sites’ loss and catalyst degradation over time. From Figure b, it is observed that the charge value during the stability test remains almost the same with no observable change, and this is attributed to the stable and consistent catalytic behavior of the catalyst. Further, Figure c demonstrates q*/q*initial as the function of the number of cycles. As the charges are considered to be directly proportional to the electroactive sites on the catalyst surface, therefore, the ratio of q*/q*initial can also demonstrate the activity of the catalyst during stability testing. The remaining active sites after 200 synchronized cycles are measured to be 96% of the initial charge. It seems that the coating of the thin-film catalytic layer on the conductor surface generates stable catalysts, which might be due to the formation of a more compact nanoscale catalytic layer on the substrate FTO by a low-temperature spray coating method.

To know about the electroactive sites on the thin-film NiO catalytic structure, electrochemically active surface area (ECSA) of the catalyst is further evaluated by double-layer capacitance measurements at the solid–liquid interphase through the CV approach. The cyclic voltammograms presented in Figure S4 were collected in the non-faradaic region under the potential range of 0.86 to 0.98 VRHE where all the current is supposed to be due to double-layer charging only. The corresponding plot of current density at a fixed potential of 0.92 VRHE versus the scan rate gives a straight line, the slope of which gives the value of Cdl (Figure d). The ECSA is calculated by dividing the Cdl by specific capacitance, which is 0.040 cm–2,[32] and a high surface area of 100.25 cm2 is observed for NiO-SC/FTO that is indicative of the larger surface area and more exposed active sites on the catalyst surface, which is highly desirable for efficient water splitting catalysis.

The electrochemical accessibility is also investigated by integrating the charge (QNi,O) under the reduction peak from the polarization curve investigated at 20 mV s–1 in 0.5 M aqueous KOH. It is assumed that each nickel atom corresponds to one chemisorbed oxygen atom. The area under the reduction peak considering the redox couple Ni2+/3+ directly measured from the polarization curve is calculated to be 0.000050 VA as shown in Figure S5. Hence, the charge can be given as 0.000050 VA/0.02 V s–1 = 0.00250 A s or C. Then,

The surface concentration of the nickel atom is calculated by dividing the number of atoms involved in the redox reaction, which is 1 for the Ni2+/3+ redox couple

Conclusively, all the above measurements provide us compelling remarks that facilely prepared NiO-SC/FTO demonstrates significant oxide stability, high ECSA, and an optimum number of electroactive sites on the electrode surface, which are crucial for boosting activity in electrocatalysis applications.

OER Activity Investigations

Considering the unique structural and surface properties and electrochemical redox capabilities of thin-film nanoflake type electrocatalysts, their performance for water splitting catalysis is investigated under the alkaline conditions. Anodic polarization measurements are performed to evaluate the electrocatalytic properties of nickel-oxide toward OER. Figure a presents a polarization curve for NiO-SC/FTO, and the curve is corrected for the ohmic drop. The minor preoxidative features appear at 1.15 VRHE followed by a Ni/NiO oxidation peak where the current density increases to around 1.36 VRHE and decreases drastically to 1.43 VRHE. This oxidation peak after undergoing a slight decline and presenting low current density is followed by a catalytic wave at an onset potential of 1.48 VRHE, which is lower than those of previously reported Ni-based systems (Table ). The peroxidation peak mainly arises from the Ni2+/3+ and Ni3+/4+ transitions as observed previously in this potential regime.[20] The OER onset overpotential is just 250 mV, and a current decade is obtained at 1.53 V (η = 300 mV), which is even lower than the benchmark overpotential of 350 mV required to produce 10 mA cm–2 for 10% efficient conversion of solar energy into fuels/chemicals.[2,4] A significantly high current density of >75 mA cm–2 is observed at a peak potential of 1.74 VRHE. The realization of such a high current density under this narrow potential range is highly required to effectively discourse the contests associated with slothful OER reactions (Table S1).

Figure 6

OER electrocatalysis. (a) Polarization measurement of NiO-SC/FTO recorded at 5 mV s–1; (b) consecutive 500 CV scans recorded at 20 mV s–1; (c) multicurrent step chronopotentiometric curves; (d) log of current density versus overpotential curve in 0.5 M aqueous KOH electrolyte solution.

Table 1

Comparison of OER Activity of NiO-Cat/FTO with Previously Reported Catalytic Systems

catalyst/system	η at onset (mV)	η (mV) at 10 mA cm–2	Tafel slope (mV dec–1)	current density at 2 V	ref
NiOx-SC/FTO	1.48	300	57	>70 mA cm–2a	this work
RuO2	1.48	510	39	7.2 mA cm–2	(5,6)
IrO2	1.58	540	45	5.0 mA cm–2	(6)
NiOx	1.75	720	59/120b	2.8 mA cm–2	(15,33)
NiPc350@FTO (nickel phthalocyanine coated on FTO following by heating at 350 °C)	1.55	619	147	12 mA cm–2c	(20)
NiCO@NC (nitrogen-doped carbon nanofibers)		539	98		(34)

A current density of >70 mA cm–2 is achieved at 1.74 VRHE.

Tafel slopes of 59 and 120 are taken with and without preconditioning for 12 h of pre-electrolysis at a current density of 1.0 mA cm–2.

A current density of 12 mA cm–2 is achieved at 1.82 VRHE.

Furthermore, catalytic films with varying numbers of spray-coating attempts of precursor solutions on FTO are synthesized to see their effect on OER activity. In this quest, the number of spray-coating steps is repeated from 5 to 20, and resultant catalytic films are directly tested for OER catalysis. It is seen that the catalyst film obtained by doing five spray attempts on FTO is relatively less active, whereas those obtained by doing 10 to 20 spray attempts show somewhat similar catalytic responses (Figure S6). Therefore, for optimum catalytic application, NiO ultrafine catalytic films are fabricated by making 8–10 spray-coating attempts of precursor solution on the FTO surface using a single nozzle spray bottle. However, the OER onset potential remains similar in all the coating samples.

Next, OER catalytic activity of NiO-SC/FTO is also investigated under various electrolyte/pH conditions such as in 0.5 M borate buffer solution (pH ≈ 9.2), 0.5 M carbonate solution (pH ≈ 8.2), and 0.5 M phosphate buffer solution (pH ≈ 6.9) using a similar electrochemical setup. The catalyst presents promising catalytic activity in all the electrolyte solutions while producing optimum current densities (Figures S7–S9). This feature confirms the substantial activity of the catalyst toward OER under various electrolytes and pH conditions from neutral to basic. However, owing to better catalytic performance under alkaline conditions, further analyses are carried out in 0.5 M aqueous KOH, mainly due to low ohmic constrains. High catalytic durability is of great importance for a good electrocatalyst. The durability of the catalyst is evaluated via consecutive multiple-scan cyclic voltammetry experiments.[33−35]

From Figure b, it is observed that the catalyst presents similar catalytic signatures during concurrent 1st and 500 CV cycles where the CV curve of 500th almost overlaps the initial one. The consecutive CVs measured under the same conditions suggest the high stability of the NiO-SC/FTO electrocatalyst for water oxidation reaction under employed conditions. The catalytic stability is further tested via multicurrent multistep chronopotentiometry experiments (without IR corrections) as presented in Figure c. The current density increases from 4 to 48 mA cm–2 (4 mA cm–2 per 180 s). The potential immediately levels off at 1.50 VRHE and remains constant for the next 180 s. The analogous outcomes are obtained for all other consecutive steps measured up to an applied current density of 48 mA cm–2. This feature confirms the consistency in the catalytic behavior for OER catalysis and demonstrates high conductivity and facile charge transfer process at the electrode surface.

Next, the Tafel plot analysis is conducted to study the kinetic behavior of the electrode/electrocatalyst. Tafel plots are calculated from the linear region of the polarization curve at or near the onset potential considering the following equation: η = b log j + c, where b is the Tafel slope.[4] It gives some valuable insights into the mechanism of water oxidation reactions. The kinetics of OER inversely depends on the value of the Tafel slope. A smaller value of the Tafel slope is of importance as the catalyst is desired to produce a high current response over a narrow potential range. Under alkaline conditions, the water oxidation reaction is considered as the following processes:

Here, A can be regarded as an active catalytic site. Moreover, according to the Tafel equation, the rate-determining step of OER can directly be predicted via the corresponding Tafel slope value.

The NiO-SC/FTO presents a smaller Tafel slope of just 57 mV dec–1 (Figure d). Here, the 57 mV dec–1 indicates the third step as the rate-determining step. Further, this value presents a proton-coupled electron transfer mechanism for water oxidation that involves one-electron transfer simultaneously associated with one proton transfer (Table S1).[36]

Electrochemical impedance spectroscopy is further carried out to get more insight into the intrinsic OER kinetics of NiO-SC/FTO at a fixed overpotential of 0.35 V. The corresponding Nyquist plots are presented in Figure a. The charge transfer resistance at the electrode–electrolyte interphase is calculated by fitting the simplified Randles circuit as presented in Figure a (inset). A lower Rct of just 2.3 Ω is observed that is indicative of a faster electron transfer process at the electrode surface, which might be due to the presence of ultrafine nanoparticles on the conductor surface. Moreover, the small diameter of the semicircle is representative of faster kinetics of the electrode for the OER process and signifies that the electrode is exclusively directing OER in the presence of any other residual electrochemical process that may or may not be taking place in the electrochemical cell.[4]

Figure 7

OER electrocatalysis. (a) Nyquist plot recorded at 1.58 VRHE (inset: simplified Randles circuit to calculate solution resistance and charge transfer resistance); (b) TOF at various applied potentials; (c) chronoamperometry curve at an applied potential of 1.58 VRHE for continuous 18 h; (d) cyclic voltammetry curves calculated before and after long-term controlled-potential electrolysis experiment to study catalyst aging over time in 0.5 M aqueous KOH electrolyte solution.

Turnover frequency (TOF) measurements associated with generating an oxygen molecule per second is also employed to further evaluate the intrinsic kinetics of the electrocatalytic system under employed conditions.[20] In this regard, after electrochemically quantifying the surface concentration of electroactive sites, TOF is calculated using the formula TOF = I × NA/A × n × F × r (where I is the current in ampere at 0.35 V, here, the choice of 0.35 V is based on the assumption that 10% efficient solar water splitting devices should operate for OER generating 10 mA cm–2 at a maximum overpotential input of 0.35 V,[37]NA is Avogadro’s number, A is the geometrical area of the electrode, n is the moles of electron consumption for 1 mol of oxygen generation, F is Faradaic constant, and r represents the surface concentrations of electroactive sites constituting the catalytic layer). The resulting TOF at 1.58 VRHE (η = 0.35 V) is calculated to be 1.2 s (details are mentioned in the Supporting Information). Furthermore, TOFs calculated at various applied potentials are presented in Figure b and Table S2.

To further estimate the intrinsic activity of the catalyst and effects of charge transfer resistance during the OER catalysis exchange, current density is calculated from the Nyquist plot using the formula J0 = RT/nAFθ (where R is the universal gas constant of 8.314 J (kg m2 s–2)/(K mol), T is a temperature of 298 K, n is the number of electrons, which is 4, F is the Faraday constant of 96485 C (A s)/mol, θ is charge transfer resistance Ω (kg m2 s–3 A–2), and A is the geometrical area of the working electrode)[31] (details are mentioned in the Supporting Information). The calculated value of 2.79 mA cm–2 shows the high intrinsic activity of NiO-SC/FTO for water oxidation catalysis and faster charge transfer across the electrode–electrolyte interphase. Thus, the Tafel slope analysis, TOF, impedance measurements, and intrinsic activity analysis reveal that the as-synthesized electrode reveals the facile kinetics of water oxidation, which is a very useful feature in governing the catalytic activity of the system.

Long-term stability is of immense importance for scaling up of any electrocatalytic system for bulk production of hydrogen-based fuels. After having remarkable catalytic activity and well-balanced kinetics of NiO-SC/FTO electrodes toward water oxidation catalysis, the long-term stability of the electrode is further evaluated. The long-term durability is tested via controlled potential electrolysis (CPE) and controlled-current electrolysis experiments conducted in 0.5 M aqueous KOH solution. Figure c presents the time versus current density curve for NiO-SC/FTO at an applied voltage of 1.58 VRHE for continuous 18 h where the change in the value of current density is observed as a function of time. The catalyst is shown to produce a very stable current density with little degradation over a long time. This might be due to the accumulation of air bubbles on the catalyst surface that might have blocked some active sites as a rich stream of oxygen bubbles can be seen coming out of the electrode surface during catalysis.[20] The stability of the catalyst is also confirmed via CV measured just after the CPE test (Figure d). The magnified view of the CV shows that onset potential after the durability test remains the same; however, a broad preoxidative curve is seen prior to the catalytic wave, and this may indicate the formation of more and more active NiO type structures on the catalyst surface during continuous electrolysis. The durability of the catalyst is further tested via extended period controlled-current electrolysis experiments in 0.5 M aqueous KOH solution. The catalyst is held at a constant current density of 10 mA cm–2, and the change in the value of potential is studied as a function of time. Figure S10 presents the time versus operating potential curve for the NiO-SC/FTO type catalyst. It is seen that a constant value of nearly 1.53 VRHE produces the current decade and remains persistent throughout the course of analysis, thus demonstrating that the system is highly stable and can be employed for industrial applications as well. Lastly, XRD results after the OER process reveal that the catalyst still retains all of its characteristic peaks of the pristine NiO sample that indicate high stability of the catalytic system (Figure S11).[34]

The OER activity of NiO-SC/FTO is also compared with other noble- and non-noble-metal-based catalytic systems reported earlier. This comparison can be considered only as an approximate guide only to relatively study catalytic activity. Direct comparison is problematic because of the difference in film thickness, ECSA, electrochemical accessibilities, and overall electrochemical system. Table presents that catalytic activity of facilely prepared NiO-SC/FTO is comparable and better than those of other precious- and transition-metal-based systems mentioned above.

HER Activity Analysis

After having remarkable OER performance, the NiO-SC/FTO electrode is also tested for the HER activity under alkaline conditions using a similar three-electrode configuration as described above. The thin-film electrocatalyst is compared with the Pt electrode for HER activity (Figure S12). Interestingly, the NiO-SC/FTO type catalytic system shows very promising HER activity with an onset overpotential of 0.24 V and a current density of 10 mA cm–2 at 0.350 V, which are comparable with formerly reported catalytic assemblage (Figure a, Table S1). To know about the kinetics of HER, the Tafel slope is calculated from the linear polarization curve, and a low Tafel slope of 89 mV dec–1 indicates the well-balanced kinetics on the electrode surface and is lower than the previously reported system[38] (Figure b, Table S3). The Tafel slope value indicates that the catalyst is following the Volmer–Heyrovsky mechanism for hydrogen generation. Furthermore, a lower charge transfer resistance of just 4.5 Ω at 0.35 V is also indicative of faster charge transfer kinetics at the electrode/electrocatalyst–electrolyte interphase for HER catalysis (Figure c).

Figure 8

HER electrocatalysis. (a) Polarization measurement of NiO-SC/FTO recorded at 5 mV s–1; (b) log of current density versus overpotential curve and corresponding Tafel slope value; (c) Nyquist plot recorded at −0.35 VRHE (inset: simplified Randles circuit to calculate solution resistance and charge transfer resistance); (d) chronopotentiometry curve at an applied current density of 10 mA cm–2 for continuous 10 h in 0.5 M aqueous KOH electrolyte solution.

The stability of the catalyst for long-term HER is determined via chronopotentiometry experiments applying a current density of 10 mA cm–2 for continuous 10 h of electrolysis. The catalyst is shown to produce a constant current decade, just an operating overpotential of 0.33 V, and demonstrates high durability for HER as illustrated in Figure d. Furthermore, LSV measured after long-term chronopotentiometry experiments also exactly overlaps the initial one, thus suggesting remarkable stability of the catalyst under the employed cathodic conditions (Figure S13).

Furthermore, XRD analysis indicates that original signals for NiO disappear and new signals appeared, which are indexed to elemental nickel (Figure S14). Conclusively, these electrochemical investigations confirm the remarkable performance of the easily obtainable NiO nanoflakes for water splitting catalysis both under anodic and cathodic conditions. This study also provides a very promising way of producing a highly applied metal-oxide-based anode and electrocatalytic coatings for overall electrochemical water splitting catalysis.

Electrocatalytic performance of NiO-SC/FTO for overall water splitting catalysis in a two-electrode system at various current densities such as 10 and 20 mA cm–2 is also investigated as presented in Figure S15a,b. The catalyst is shown to produce current densities of 10 and 20 mA cm–2 for overall water splitting catalysis at overpotentials of 370 and 480 mV, respectively, in 0.5 M KOH solution. Furthermore, the catalytic performance in the two-electrode system is also evaluated at various concentrations of KOH such as 0.1 M aqueous KOH and 0.25 M aqueous KOH. To make the system simple, the activity was not evaluated using much higher concentration of electrolytes such as 1 to 2 M KOH. The catalysis presented promising activity even in low concentration of electrolytes as illustrated in Figure S16a,b. All these results presented promising bifunctional activity of NiO-SC/FTO for water splitting catalysis, which is very interesting from an economical perspective.

Summary and Outlook

Nanoscale thin-film metal-oxide/-hydroxide catalytic electrodes or electrocatalysts are highly demanded for an efficient water splitting scheme. Designing and subsequent fabrication of bifunctional OER/HER electrocatalysts can offer vast advantages such as reduction in material poisoning, cross contaminations, and great technological interest. However, facile preparation methods that are time-effective and cost-effective are needed to introduce for industrial application of the overall system. Nanoscale catalytic films comprising NiO-based active structures are promising electrocatalysts; however, simple preparations, surface anchoring, and catalytic activation on the conductor surface via easily accessible strategies for long-term catalysis operation remain a challenging hurdle. In this work, we have presented the highly efficient Ni-based nanoflake patterned electrodes prepared following a simple low-temperature spray-coating approach as bifunctional electrocatalysts for overall water splitting. Interestingly, the ultrafine NiO catalytic film is highly uniform, having nanoflake morphological attributes covering the entire surface of FTO. The as-grown transparent catalytic materials are successfully employed as efficient electrocatalysts for OER and HER catalysis under alkaline conditions. The catalyst is proven to be highly efficient, electroactive, durable, and presents facile kinetics of both OER and HER in the same electrolyte solution. Owing to their low onset potential and high stability toward OER and HER, we believe that such a promising water splitting activity of nanoscale materials under harsh oxidative conditions permits its recycling potential for OER/HER. We anticipate that a firm, highly pure, homogeneous catalytic film comprising a crystalline, porous nano-interphase favors the redox fluctuations of Ni2+/3+ species in a cyclic manner that imparts remarkable stability to the system for better catalytic performance. Unique features of these transparent NiO nanoscale catalytic electrodes prepared by a low-temperature spray-coating method provide great potential for further fabrication of thin-film catalytic materials for applications in electrochemical and photoelectrochemical water splitting. From these outcomes, we foresee the potential behavior of new single- and mixed-metal-based alike materials for promising water splitting activity under employed conditions. Furthermore, insertion of heteroatoms and defects in highly uniform catalytic films obtained via a simple spray-coating approach can further pave the way for their potential applications for more efficient water splitting catalysis and other redox electrochemical reactions.

Materials and Methods

Chemical Reagents

All the chemicals and reagents were of analytical grade, obtained from Aldrich, and used as received without any purification step, unless otherwise mentioned. All the solutions were made using ultrapure water (Milli-Q 18.2 MΩ cm, 2–4 ppb total organic content). Fluorine-doped tin oxide (FTO) glass slides obtained from Aldrich were used after the cleaning process as described previously.[20] All the glassware and electrochemical cells were well cleaned by boiling in a 1:1 solution of nitric acid and sulfuric acid followed by boiling in water. They were rinsed many times with ultrapure water and acetone and ultimately dried for 1 h at 80 °C by keeping inside an oven before use.[4]

Analytical Instruments

Surface structure and morphology of the as-grown thin-film electrocatalyst were investigated via a scanning electron microscopy technique using a Nova-Nano SEM (NOVA FEI SEM-450 equipped with an EDX detector). Surface composition was evaluated via an energy-dispersive X-ray spectroscopy technique on a NOVA FEI SEM-450 equipped with an EDX detector. The active catalytic phase of the as-grown catalytic structure was determined via X-ray diffraction pattern analysis using a Rigaku-Dmax 3C diffractometer (Rigaku Corp., Tokyo, Japan) with Cu Kα (λmax = 1.54056) radiation. The chemical nature, oxidation state, and bulk compositional analysis of the thin-film electrode/electrocatalyst were determined via an X-ray photoelectron spectroscopy technique on a VersaProbe III XPS (PHI 5000, ULVAC-PHI) X resource: 100u 25w 15KV instrument. For XPS analysis, peaks were deconvoluted using Origin software.

Electrochemical Depictions

All the electrochemical investigations were performed in a standard three-electrode configuration glass cell covered with a Teflon cap on a computer-controlled potentiostat (PG-Stat10). The NiO-SC/FTO type electrode prepared via a low-temperature spray-coating method was employed as a working electrode often called as the indicator electrode, a platinum wire as a counter electrode also known as an auxiliary electrode, and a saturated silver-silver chloride (sat. Ag/AgCl) and saturated calomel electrode (SCE) as reference electrodes. The platinum wire was well cleaned by immersing in a 20% solution of nitric acid and rinsed many times with ultrapure water before placing it into the electrochemical cell. During the HER study, the Pt counter electrode was used after inserting in a ceramic frit that ultimately helps in preventing the Pt electrode from dissolution during HER under alkaline conditions. All the potentials cited in this research were referenced to reversible hydrogen electrode potential using the Nernst equation: ERHE = EREF + E0REF + 0.059(pH). Here, E0REF for Ag/AgCl is 0.197 V, and that for SCE is 0.2416 V. For the ohmic drop correction, uncompensated solution resistance (Rs) was derived from impedance measurements. All the electrochemical data were presented with 60% IR correction, unless otherwise mentioned, using the equation Eactual – IR = Ecorrected. Here, Eactual is potential versus Ag/AgCl and SCE. Considering the best catalytic performance of transition-metal-based oxides under alkaline conditions, all the electrochemical data was collected in 0.5 M aqueous KOH electrolyte solution. To study kinetics details, the Tafel slope was calculated directly from linear polarization curves at or near the onset potential region. Electrochemical impedance spectroscopy was used to study solution resistance and charge transfer resistance (Rct) at the electrode–electrocatalyst interphase. Nyquist plots were collected under a frequency range of 100000 to 0.1 Hz. Rs and Rct were estimated from the Nyquist plot by fitting a simplified Randles circuit using Autolab-Nova software. Widely used kinetics parameters such as electrochemically active surface area, electrochemical accessibility, turnover frequency, and intrinsic activity were employed to study catalyst performance. The durability of the catalyst was studied using controlled-potential electrolysis (CPE), controlled-current electrolysis, and multiple scan cyclic voltammetry techniques.

Fabrication of Electrode–Electrocatalyst via Low-Temperature Spray-Coating Method

The NiO thin-film electrocatalyst constituting NiO nanoparticulates grown on the conductor (FTO) surface was developed via a simple low-temperature spray-coating method. In this quest, to a 5 mL clear solution of 0.1 M NiCl2·6H2O, the same volume of an organic solvent such as ethanol was added and mixed thoroughly. Ethanol was aimed to decrease the viscosity of the mixture and to maintain the temperature of the heated substrate (it releases heat on combustion, which may overcome heat loss from the preheated conductor surface that occurs due to water evaporation). The precleaned conductive support was kept on the hot plate and heated until the temperature was reached and maintained at about 250 °C. The mixture was sprayed 8–10 times by maintaining a time interval of 4–5 min between each consecutive spray attempt on the preheated FTO substrate using the single nozzle spray bottle. A minute spray mist constituting metal ions, water, and organic solvent struck the heated FTO substrate, its water and the organic solvent evaporated, and metal ions were deposited as nanoflake type metal oxide structures that grew on the conductor surface in the form of the uniform thin film (Scheme S1). As-obtained thin-film anodes were directly employed as bifunctional electrocatalysts for water splitting catalysis.