Recent advances in solar cells and photo-electrochemical water splitting by scanning electrochemical microscopy

Abstract

Investigation on the mechanism and kinetics of charge transfer at semiconductor/electrolyte interface is significant for improving the photoelectric conversion efficiency and developing novel and high-efficiency photovoltaic devices. Scanning electrochemical microscopy (SECM), as a powerful analytical technique, has a potential advantage of high spatial and temporal resolution. It has been expanded into a broad range of research fields since the first inception of SECM in 1989 by Bard groups, which includes biological, enzymes, corrosion, energy conversion and storage (such as solar cells, hydrogen and battery). Herein, we review the basic principles and the development of SECM, and chiefly introduce the recent advances of SECM investigation in photoelectrochemical (PEC) cells including solar cells and PEC water splitting. These advances include rapid screening of photocatalysts/photoelectrodes, interfacial reaction kinetics and quantitation of reaction intermediates, which is significant for evaluating the performance, choosing catalysts and developing novel composite photoanodes and high efficiency devices. Finally, we briefly describe the development trends of SECM in energy research.

Key words： scanning electrochemical microscopy (SECM); solar cells; photoelectrochemical (PEC) water splitting; screening; kinetics; intermediates

Introduction

Developing of a long-term, sustainable energy economy is one of the most important technical issues facing humanity [¹]. Solar energy is abundant (90 PW), inexhaustible and fairly well distributed over the planet [²]. Solar energy can be converted into usable energy via photoelectrochemical (PEC) cells which can be divided into two categories: regenerative solar cells and photosynthetic cells (e.g., hydrogen) [³]. Regenerative solar cells (e.g., dye-sensitized solar cells (DSSCs), perovskite solar cells (PSCs) and organic solar cells) can convert solar energy to electricity. Photosynthetic cells such as hydrogen can be obtained through PEC water splitting approach, which has been widely investigated since the first demonstration in 1972 by Fujishima and Honda [⁴]. For these devices, the semiconductor/electrolyte interface (SEI) investigation plays a vital role in improving charge carrier separation, transport and recombination pathways. Up to date, some in situ techniques have been developed to probe the dynamics of SEI reactions on the surface of electrodes or solar cells devices, such as photoelectrochemical impedance spectroscopy (PEIS) [⁵–⁷], intensity modulated photocurrent or photo-voltage spectroscopy (IMPS/IMVS) [⁸], transient absorption spectroscopy (TAS) [⁹,¹⁰], scanning-probe measurement (SPM) [¹¹] and synchrotron-based X-ray techniques [¹²], and so on. Cen et al. reviewed the in situ techniques for PEC water splitting and summarized the advantages and limitations of these techniques [¹³]. Among them, SPM techniques, using a probe to scan the surface of samples, can provide the information of the interactions between the probe and the surface of samples, although they exhibit limited temporal resolution [¹⁴]. Thus the SPM techniques such as atomic force microscopy (AFM), scanning tunneling microscopy (STM) can be utilized to characterize different surface properties of the samples [¹⁵–¹⁸]. And the most popular SPM techniques for in situ PEC studies are scanning photocurrent microscopy (SPCM) [¹⁹] and scanning electrochemical microscopy (SECM) [²⁰]. Among them, SECM overcomes many typical problems, for example the effects of the resistive potential drop in solution (iR drop), and charging current. SECM is a “non-contact” scanning probe technique, which can provide chemical and/or topographic information about surfaces immersed in a solution. Therefore, SECM cannot only analyze the local chemical and PEC activity of sample surface, but also can be utilized to investigate the surface reaction intermediates and its kinetics as well as screen the active center. Hence, analytical measurements can be performed in the interfacial region. Besides, the researchers have also integrated SECM with alternating current (AC) impedance spectroscopy [²¹], atomic force microscopy (AFM) [²²] and scanning ion-conductance microscope (SICM) [²³] to investigate different properties.

In this review, we will describe the basic principle and the development of SECM, and chiefly introduce the recent advances in solar cells and PEC water splitting based on the different operation modes of SECM.

Basic principles of scanning electrochemical microscopy (SECM)

Instrument constitutes

SECM is based on these changes of the faradaic current when the SECM tip is moved across the surface of a sample, which is useful in obtaining topographic and surface reaction kinetic information. Figure 1 shows the typical structure of SECM equipment which includes six components (bipotentiostat, piezoelectric controller or positioning system, probe, electrolytic cell, substrate and computer). Among them, the bipotentiostat is used to precisely measure and control the current and potential of the probe and substrate (or the test samples). A high resolution three-dimensional positioning system is controlled by piezoelectric controller, which allows for accurate movement of the probe and sample using x, y, z stages. Another very important component is the probe or SECM tip (e.g., ultra-microelectrode (UME)), which can offer high mass transport rates, low ohmic effects, low double layer charging [²⁴,²⁵]. UME is optimal for many applications from kinetics measurements to electrochemical imaging. Furthermore, the electrolyte cell provides the reaction site, where the substrate is placed.

Fig.1 Simple schematic of a SECM instrument (WE: working electrode, RE: reference electrode, CE: counter electrode)

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Working modes of scanning electrochemical microscopy (SECM)

SECM measurements can be performed using different working modes. After the first report on SECM in 1989 which introduced feedback and generation/collection (G/C) modes [²⁶,²⁷], a variety of new operation modes have been developed in order to expand its application, such as surface interrogation mode, ion transfer feedback mode, potentiometric detection mode, constant current mode, reverse imaging mode, redox competition mode and direct mode, and so on [²⁸]. Among them, the feedback mode, G/C and surface interrogation (SI) modes are widely used in the fields of PEC cells to study the imaging and interfacial kinetics.

Feedback mode

SECM in feedback mode shows the great advantages in detecting active sites of the surface. In general, the SECM experiment is performed under four-electrode system, in which SECM tip usually serves as one of the dual working electrodes, and the substrate acts as the other one. These electrodes are all immersed in the solution containing redox mediators. The basic principle of the feedback mode is shown in Fig. 2. When the positive potential is applied to the SECM tip, the mediators will take place an oxidation reaction as the following Eq. (1),

(1)\mathrm{R}-{\mathrm{ne}}^{-}\to \mathrm{O}\mathrm{.}

Here, R and O represent a reduced and oxidized species in the solution, respectively. On the contrary, a reduction reaction will occur when the negative potential is applied. When the tip is far from the substrate, the steady-state current (i_T,_∞) can be given from Eq. (2),

(2)\mathit{i}{}_{\mathrm{T},\infty }=\mathrm{4}nDFca\beta ,

where n is the number of the electrons of electrode, F is the Faraday constant (96485 A·s·mol^-1), D is the diffusion coefficient, c is the concentration of the reactant, a is the radius of the UME tip, and b is a geometric co-efficient [²⁸]. When the tip is close to a conductive substrate or electrochemical active species substrate, the oxidized species formed in reaction (1) diffuse to the substrate and can be reduced back to R (Fig. 2(a)). In this condition, the flux of R to the tip is increased, and i_T>i_T,_∞. This phenomenon is termed “positive feedback” (Fig. 2(c)). On the other hand, if the substrate surface is an insulator, the electron cannot recycle in the redox reaction (i_T<i_T,_∞, Fig. 2(b)), that is to say, “negative feedback” appears (Fig. 2(d)). If a steady-state is established, the normalized current becomes independent of the ratio of diffusion coefficients and depends only on the tip/substrate distance (d). Moreover, larger i_T is obtained when the d value is smaller. And the approach curve can be presented in form of i_T/i_T,_∞ vs. L (here L = d/a, and d is the distance between the tip and the substrate), which provides information on the kinetics of the process at the substrate.

Fig.2 Basic principle of the feedback mode

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Generation/collection (G/C) mode

The generation/collection mode includes the substrate generation/tip collection (SG/TC) and tip generation/substrate collection (TG/SC) mode. In these modes, both SECM tip and substrate are working electrodes, and one generates some species which can be collected by the other [²⁸]. In SG/TC mode, the electro-active species are generated at substrate and collected by SECM tip according to the following Eqs. (3) and (4),

(3)R-{\mathrm{ne}}^{-}\to \mathrm{O\; substrate\; reaction},

(4){\mathrm{O\; +\; ne}}^{-}\to \mathrm{R\; tip\; reaction}\mathrm{.}

However, the reaction occurs in TG/SC mode is counter to SG/TC mode, as exhibited in Eqs. (5) and (6),

(5){\mathrm{O\; +\; ne}}^{-}\to \mathrm{R\; substrate\; reaction},

(6)R-{\mathrm{ne}}^{-}\to \mathrm{O\; tip\; reaction}\mathrm{.}

Typically, SG/TC mode is used for the measurements of concentration profiles or chemical flux from a substrate. And the TG/SC mode is predominantly used to study reaction kinetics or perform modifications to the substrate.

Surface interrogation (SI) mode

The surface interrogation mode of scanning electrochemical microscopy (SI-SECM) is a new in situ electrochemical technique based on SECM operating in a transient feedback mode [²⁹], which is developed by Bard and coworkers [²⁸] and can be used to analyze, detect and quantify the adsorbed surface species on the electrode. The SI-SECM mode consists of a specific application of the feedback mode under transient conditions that allows for the quantification of finite amounts of reacting species on the surface of substrate. Different from the feedback mode, the reactive species (A) are absorbed at the substrate electrode in SI-SECM mode [²⁸,³⁰]. Figure 3 illustrates the schematic of the mechanism of the surface interrogation mode. First, the substrate is pulsed or scanned to a potential where oxidation occurs and an adsorbed species A is formed. At this time, the tip is kept at open circuit (OC), and the solution only contains the initial mediators in oxidized states (O) (Fig. 3(a)). Figure 3(b) shows the substrate is taken to OC and the tip is pulsed and reduces O to R. Then O can be regenerated via the diffused R reacting with adsorbed species A which is consumed and transferred to final product P. Ideally, regeneration of O should only be obtained through reaction with A. Figure 3(c) exhibits the final state of the system, which shows that all A is depleted and O reacts at the tip at the rate governed by the hindered diffusion into the tip-substrate gap. In summary, the SI mode acts as a transient feedback mode, in which a SECM tip is used to generate a titrant R which can react with A to regenerate O, producing positive feedback. Finally, a negative feedback can be detected as A is depleted from the surface.

Fig.3 Schematic diagram of mechanism for the surface interrogation (SI) mode [³¹,³²] (OC is denoted as open circuit)

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Based on these different operation modes, many excellent works have been carried out in various fields such as solar cells, water splitting and battery as listed in Table 1.

Tab.1 Application of SECM based on the above modes

applications	measurement	mode	Refs.
dye-sensitized solar cells	dye regeneration kinetics	feedback mode	[33–35]
dye-sensitized solar cells	imaging	feedback and SG/TC mode	[36]
perovskite solar cells	kinetics	feedback mode	[37]
oxygen reduction reaction	detect hydrogen peroxide	TG/SC mode	[38]
hydrogen evolution reaction	kinetics	SG/TC mode	[39]
formic acid oxidation	electrocatalytic activity	TG/SC mode	[40]
oxygen reduction reaction	mechanism	SG/TC mode	[41]
Li-ion batteries	Li-ion intercalation	SG/TC mode	[42,43]
supercapacitor	charge transfer kinetics	feedback mode	[44]
corrosion	localized corrosion	TG/SC mode	[45]
photoelectrochemical	kinetics	SI-SECM mode	[46]
photocatalysis	screening	TG/SC mode	[31,47]
electroreduction of CO2	CO2 reduction	SG/TC mode	[32]
hydrogen evolution reaction	kinetics	feedback mode	[48]
decomposition of formic acid	quantification of adsorbed hydrogen	SI-SECM mode	[49]

Application of scanning electrochemical microscopy (SECM)

Since the first introduction in the 1990s, SECM has been widely used to characterize hydrogen oxidation reaction (HER), oxygen evolution reaction (OER), and CO₂ reduction via electrochemical or PEC technologies [⁵⁰–⁵²]. As the hot topic, application of SECM in solar cells and PEC water splitting has attracted more attention for researchers described as follows.

Nanostructured solar cells

With the development of PEC technology, more and more new technology is applied to solve relevant scientific problem. Charge transfer process in solar cells is a field worthy of extensive study topic, which includes the regeneration process of the photosensitizers, photoelectric catalyst, and the transfer behavior of photo-generated electron and hole pairs at the interface of photoelectrode/electrolyte. These processes are the necessary steps for generating photocurrent, and paly an essential role in improving the photoelectric conversion efficiency of device. Due to the reaction occurring in the PEC interface is a fast charge transfer process, some conventional research methods cannot be able to monitor real time information under working condition. Attributed to the high time and spatial resolution, and sensitivity, SECM is very suitable to detect the trace change of electrochemical active material or chemical group located at the micro-region, which has been proven to be an effective method and plays unique role in studying the interfacial charge transfer kinetics of solar cells.

Dye-sensitized solar cells (DSSCs)

SECM acting as an elegant tool was used to directly determine the mobility of charge carriers and diffusion constant within dye-sensitized nanostructured semiconductors in 2006 [⁵³]. Hence the PEC parameters including conductivity under illumination or heterogeneous kinetics could be obtained by means of SECM. In the field of DSSCs, SECM has been used to investigate the dye regeneration and charge transfer kinetics in the interface of dye/electrode. For example, Wittstock group developed a series of study on dye regeneration kinetics based on the feedback mode of SECM, and distinguished the luminous region using G/C mode, including D149 sensitized ZnO and N719 sensitized TiO₂ solar cells [⁵⁴–⁵⁶]. Similarly, Martin group investigated the charge transfer kinetics constants of N719 and N749 sensitized TiO₂ solar cells using SECM [³⁵]. Shen group from Huazhong University of Science and Technology has also developed much valuable research since the first collaboration with Wittstock group [⁵⁷], in which the charge transfer kinetics between I^- and Eosin Y⁺ dye adsorbed on ZnO was first investigated and demonstrated the viability of SECM for understanding DSSCs under working conditions. Later, they extended SECM to investigate the dye-regeneration process based on feedback mode [³⁰,³⁴,⁵⁸], and dye regeneration process with different mediator concentrations and light intensities. First, they investigated the reaction kinetics of n-type dye sensitized solar cells and studied the influence of redox shuttles [Co(bpy)₃]³⁺/[Co(bpy)₃]²⁺, I₃^-/I^- on the dye regeneration kinetics of C106TBA and LD14. This work interpreted the essential rule for the effects of PEC reaction kinetics on the n-type photovoltaic device properties [⁴⁸]. Furthermore, they also investigated the reaction kinetics at the photo-cathode/electrolyte interface of p-type devices. They studied the influence of redox shuttles T₂/T^-, I₃^-/I^- on the dye regeneration kinetics of P1 [⁵⁹]. Both these works investigated the back transfer ability of the photo-generated electrons at photo-anode/electrolyte interface with different redox electrolyte, which was very helpful for assessing the interfacial recombination behavior of the photogenerated charges. Taking n-type TiO₂ and p-type CuCrO₂ for an example, Fig. 4 showed the basic principle of SECM and the reaction mechanism of different types DSSCs under short-circuit condition. The photoanodes (for example TiO₂/C 106TBA and TiO₂/LD 14) were placed in the presence of the oxidized species of redox couple and the photocathode (CuCrO₂/P1) with reduced species of redox couple. As shown in Fig. 4(a), under illumination, dye generated hole/electron pairs after absorbed the incident light. Then the holes (D⁺/TiO₂) reacted with redox species Co²⁺ regenerated dye (D/TiO₂) accompanied by the production of Co³⁺ which could be detected by the UME. Finally, Co²⁺ species was regenerated by given potentials relative to Co²⁺/Co³⁺. Therefore, the SECM feedback approach curves can be obtained and fitted, which could relate the tip current to the surface’s heterogeneous electron or hole transfer kinetics and eventually obtained the normalized apparent charge-transfer constant (k). To determine the apparent standard hetero-generous rate constant k⁰, the effective rate constant (k_eff) values were plotted in Fig. 4(c) as a function of the corresponding over-potential (h, where h = E_substrate−E_{redox,solution}^1/2). For p-type (CuCrO₂/P1) DSSCs, the mechanism was similar as illustrated in Fig. 4(b) and the apparent standard hetero-generous rate constant k⁰ also could be given from Fig. 4(c). It was observed that k⁰ for fluorine-doped tin oxide (FTO)/TiO₂ was slower in electrolyte I₃^-/I^- than that of Co³⁺/Co²⁺, and the hole transfer kinetics constant k⁰ for FTO/CuCrO₂ in the case of T₂/T^- was slower than that of I₃^-/I^-. Further comparison showed the recombination rate constant in p-type DSSCs was much higher than n-type devices, and the interfacial charge recombination kinetics constant were much smaller than the dye-regeneration kinetic constants. This ensures effective charge collection in the DSSC devices. These works may offer some new complementing aspects to establish methods for DSSCs.

Fig.4 Basic arrangement for probing the heterogeneous reaction at the (a) n-type dye-sensitized semiconductor (TiO₂) and (b) p-type dye-sensitized semiconductor (CuCrO₂) interface in the feedback mode of SECM under short-circuit conditions. The mediator couple is Co³⁺/Co²⁺ and T₂/T⁻, respectively (Ref: reference electrode, Aux: auxiliary electrode, WE-1: working electrode 1, WE-1: working electrode 2). Plot of ln(k_eff) vs. h for (a) FTO/TiO₂ electrodes in acetonitrile corresponding to the reduction with I^- and Co²⁺ and for (b) FTO/CuCrO₂ electrodes in acetonitrile corresponding to the oxidation with T₂ and I₃⁻[⁵⁹]. Copyright © 2014, John Wiley and Sons

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Quantum dots sensitized solar cells

Except for DSSCs, inorganic semiconductor quantum dots (QDs) have attracted more interest due to their unique optoelectronic properties derived from the quantum confinement effect. Many narrow band gap QDs (such as CdS, PbS/CdS, CdSe/CdTe and CuInS₂) are suitable for sensitized solar cells (QDSSCs) because they can transfer electrons to large band gap semiconductors in order to further increase the charge separation and transfer efficiencies. Compared to DSSCs, the efficiency of QDSSCs is still very low. Up to now, the main factor restricting the improvement efficiency of QDSSCs is the hole-transfer process namely the regeneration process of QDs sensitizers which depends on the selection of redox electrolytes. Therefore, the investigation of the QDs regeneration process will be significant for improving the efficiency of such devices. Some novel technologies (such as nanosecond laser transient absorption spectroscopy (TAS)) have been used to measure the rate of quantum dots regeneration. In contrast to TAS, SECM is suitable to monitor the fast electron/hole transfer process in devices in working devices. So Shen group employed SECM to scrutinize the influence of redox couples (T₂/T^-, [Co(bpy)₃]³⁺/[Co(bpy)₃]²⁺, I₃^-/I^-) on the quantum dots regeneration rate (CdSe and CdS) [³³]. The measurement is also based on monitoring the feedback current, which is related to the small change in the concertation of the redox shuttle under the active area of UME probe caused by QDs regeneration. The results revealed that the kinetics of QDs regeneration depends on the nature of the QDs and the redox shuttles presented in QDSSCs. This work offers some new complementing aspects to establish the methods for QDSSCs characterization.

Perovskite solar cells

As the new star in the field of photovoltaic, perovskite solar cells, traced back to 2009 [⁶⁰], has possessed a higher efficiency up to ~23%. However, one of the main restricting factors is the relative poor stability for the environment and optical which are directly related to the interface. To further increase the efficiency of PSCs, it is crucial to select of interface materials, increase the interfacial charge separation and transportation efficiency except for developing new materials and optimizing of preparation technology. Based on the advantage of SECM, Hsu et al. prepared a variety of PbI₂/MAPbI₃ perovskites and determined how excess PbI₂ affects its PEC properties via a rapid screening technique using a modified SECM [⁶¹]. Subsequently, Wang group investigated the regeneration kinetics at perovskite/oxide interface with SECM [⁶²]. However, the application of SECM in PSCs is only a few and still needs to be explored might due to the instability of PSCs in moist environment or solution.

Photoelectrochemical water splitting

To efficiently and sustainably split water via PEC technology, several key criteria must be met simultaneously. For instance, (1) the light absorption, the semiconductor should have an adequate band gap (1.8−2.2 eV) so as to absorb a significant portion of the solar spectrum especially the visible light [⁶³,⁶⁴], (2) the appropriate band edge potentials which must straddle the hydrogen and oxygen redox potentials, (3) efficient charge transfer and separation process, (4) long-term stability in the electrolyte solution and low cost and so on [⁶⁵]. However, no single semiconductor satisfies all of the above requirements for practical hydrogen production. Therefore, various strategies have been developed in order to improve the PEC activity, such as doping [⁶⁶–⁶⁸], constructing hetero-junction [⁶⁹–⁷¹], coupling with co-catalysts [⁷²,⁷³], surface modification [⁷⁴–⁷⁷] as well as integrating with solar cells to forming tandem cells [⁷⁸–⁸⁰]. Among the factors affecting the PEC performance, the light absorption depends on the band gap of semiconductor, which is its intrinsic property. The application of SECM in PEC water splitting mainly includes the SECM imaging, kinetics investigation and quantitation of reaction intermediates.

Screening of photocatalysts/photoelectrodes

As we all know, the solar to hydrogen efficiency (STH) of the photoelectrodes is still too low to practical application. Attributed to the varieties of photoelectrode materials, it is still a challenge to rapidly screen photocatalysts in order to economize the evaluation time and cost. Up to dates, many in situ technologies have been introduced into PEC field as described in Introduction section. Screening of catalysts for PEC application by SECM has drawn wide attention of many researchers.

Doping has been widely used to extend the light absorption and improve the conductivity of photo-electrodes. However, the essential relation between doping element or content and PEC activity is not investigated in detail. Previously, SECM was used to search for more efficient doped Fe₂O₃ photocatalysts which doped with different elements Sn, Ti, Be, Al [⁸¹]. For the screening of single dopants, the array pattern was prepared as shown in Figs. 5(a) and 5(b). Figure 5(c) shows a typical SECM image obtained from a photocatalyst spot array under visible light (l≥420 nm). The dark brown color (Red dotted box in Fig. 5(c)) implied a largest photocurrent which was obtained from 4% Sn-Fe₂O₃. Similarly, Fig. 5(d) shows the SECM image obtained from photocatalyst spot arrays consisting of Fe, Sn, and Be under visible light illumination. The highest photocurrent (130 nA) was observed at the composition of the 6% Be−4% Sn-Fe spot. The photocurrent then decreased gradually with further increase of the amount of Be. Lately, Bard group extended SECM to investigate PEC performance by screening various dopants for BiVO₄ [⁸²]. The results demonstrated the photocurrent of W or Mo doped BiVO₄ was more than 10 times higher than undoped BiVO₄.

Fig.5 Dispensed pattern of photocatalyst spot array with different mol % of Sn in Fe₂O₃ (a) and Be in 4% Sn-Fe₂O₃ (b). SECM image of (c) Sn doping Fe₂O₃ and (d) Be doping Sn-Fe₂O₃ measured with spot arrays at 0.2 V vs. Ag/AgCl in 0.2 mol NaOH under visible light irradiation (l≥420 nm) [⁸¹]. Copyright © 2009, American Chemical Society

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Leonard et al. reported a rapid screening technique based on the SG/TC mode of a modified SECM to study how metal doping affects its PEC properties [⁸³]. As shown in Fig. 6(a), the colors represented the photocurrent for sulfite oxidation at 0.2 V vs. Ag/AgCl in which browns represented higher currents and greens represented lower currents. From the SECM screening, we could see that a maximum photocurrent occurred at a 9% atomic ratio of Zn/W, which was consistent with the LSV results under chopped light (Fig. 6(b)). The results demonstrated SECM can act as a tool to choose the photoelectrodes with better PEC performance.

Fig.6 (a) SECM images for the typical photocurrent response of Zn/WO₃ composites under full UV irradiation and with a 420 nm long-pass filter. (b) PEC response of electrodes with chopped light under full UV irradiation at 20 mV/s [⁸³]. Copyright © 2013, American Chemical Society

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Except for doping, coupling semiconductor (SC) with electro-catalyst is also a common method to improve photo-catalytic (PC) and PEC performance. However, we cannot randomly couple SC with an excellent electro-catalyst as co-catalyst to expect a higher PC or PEC performance. Ye et al. developed some co-catalysts (IrO_x, Pt, Co₃O₄) dropped onto the W doped BiVO₄ for PEC water oxidation, which was quickly identified the effect of co-catalysts on W doped BiVO₄ by the optical fiber-SECM technique [⁸⁴]. Fig. 7 shows the principle of SECM and the corresponding image results of the electrocatalyst Ir/Co oxides Co₃O₄ and Pt. The optical fiber-modified SECM technique was used to quickly identify the effect of various electrocatalysts for PEC water oxidation. The experimental results demonstrated Pt and Co₃O₄ were effective on BiVW-O films, while IrO_x was not active, even though IrO_x films showed the highest electrocatalytic activity for water oxidation as electrocatalyst. This work inspired us that the SECM method can be used to optimize electro-catalysts rapidly, e.g., by investigating different compositions with mixed electro-catalysts and by changing their thickness by dispensing blank solutions or with changing concentration of precursor solutions. Subsequently, Bard group also recently reported a new combinatorial methodology using a modified SECM technique for rapid preparation and screening of semiconductor photo-catalysts [⁸⁵], which is analogous to the SECM for screening of other electro-catalysts.

Fig.7 (a) Operation principle of SECM and the corresponding image results of the electrocatalyst (b) Ir/Co oxide array, (c) photocurrent at Co₃O₄ spot, and (d) photocurrent at Pt spot [⁸⁴]. Copyright © 2011, American Chemical Society

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Overall, SECM technique is extremely useful in mapping of photoelectrode surface, which can provide valuable spatially resolved information on the process kinetics occurring at the SEI of a PEC system. However, the resolution achieved by this technique is limited by the dimension of the UME. New developments in electrode design will be able to improve the special resolution of this technique.

Investigation on the kinetics of charge transfer at semiconductor (SC)/electrolyte interface (SEI) with scanning electrochemical microscopy (SECM)

Fast charge transfer kinetics at the photoelectrode/electrolyte interface is critical for highly efficient PEC water splitting. SECM is a powerful approach for measuring the kinetics of heterogeneous electron transfer, which can be determined with high lateral resolution while scanning a tip parallel to the surface. Lu group described the electron transfer kinetics at interfaces using SECM in details [⁸⁶]. Bard group performed surface-selective and time-dependent redox titrations to directly measure the surface OER kinetics of Ni^IV and Fe^IV in NiOOH, FeOOH, and Ni_1−xFe_xOOH (0<x<0.27), which were popular co-catalyst used in PEC water system [⁸⁷]. The results concluded that the remarkable OER activity of Ni_1−xFe_xOOH arising from the fast OER kinetics on Fe sites in NiOOH with a rate constant of 1.7 s^-1. Currently, we studied the interfacial charge transfer kinetics in PEC system by SECM based on its feedback mode as show in Fig. 8(a) [⁸⁸]. For investigating the catalytic reaction kinetics (reaction 1 in Fig. 8(b)) of BiVO₄ at the electrode/electrolyte interface, an extra Pt wire was ultrasonically welded on the substrate electrode FTO with ohmic contact (i.e., FTO/BiVO₄ was named as photoanode) as indicated as connection style 1 in Fig. 8(a). Thus, the PEC cell was operated in short-circuit condition. SECM revealed the surface reaction kinetics constant of BiVO₄ was 9.30´10⁷ mol^-1·cm³·s^-1. And the ratio of rate constant for hole to electron for Mo:BiVO₄ is ~30 times larger than that of BiVO₄. The results reveal that the Mo element can significantly improve the transfer property of the photo-generated holes and suppress the transfer of photo-generated electrons at the photo-anode/electrolyte interface when doped into BiVO₄. This study will provide new ideas for designing and choosing the materials with high performance, and allows us to make a comprehensive analysis of interfacial charge transfer kinetics in PEC system. Subsequently, SECM has also been used for recording the photo-induced charge transfer reactions at BiVO₄ at chemically polarized liquid/liquid (L/L) interfaces [⁸⁹].

Fig.8 (a) Basic principles for investigating of interfacial reaction kinetics in PEC water splitting under the feedback mode of SECM (Ref: reference electrode, Aux: auxiliary electrode, WE-1: working electrode 1, WE-1: working electrode 2). The semiconductor photocatalyst is BiVO₄ and the redox probe is [Fe(CN)₆]₃⁻/[Fe(CN)₆]₄⁻ (named Fe³⁺/Fe²⁺). (b) Energy scheme of BiVO₄ system on electrochemical and vacuum scale at pH 7.0. Reaction 1 is the catalytic reaction, and reaction 2 is back reaction at interface [⁸⁸]. Copyright © 2016, American Chemical Society

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Quantitation of reaction intermediates

Considering the complex process of water oxidation, quantitative of the reaction intermediates (such as H₂O₂, OH⋅) is very important which is useful for understanding the mechanism. Previously, Ahn and Bard investigated the surface cobalt sites and the intermediates of the electrocatalyst CoPi by surface interrogation scanning electrochemical microscopy (SI-SECM), in which CoPi was a universal co-catalyst for PEC water splitting [⁹⁰]. Currently, SI-SECM was widely used to detect and quantify the intermediates of photoelectorde during PEC water splitting. For instance, Zigah et al. quantified adsorbed hydroxyl radical OH(ads) generated photoelectrochemically at the surface of a nanostructured TiO₂ substrate electrode [⁹¹]. In this study, the geometry of the simulation space was depicted in Fig. 9(a) and it was adjusted for the experimental conditions, where a tip radius a = 50 m and RG = 3, and the substrate was modeled as an active subdomain of radius b = 150 m and depth d′ = 10 m. The interelectrode distance d was adjusted to 27 m. Fig. 9(b) described the mode of operation of SI-SECM for the generation and the detection of OH(ads) on the TiO₂ substrate. Finally, the typical ⋅OH(ads) saturation coverage of 338 mC·cm^-2 was found in nanostructured samples. The decay kinetics of ⋅OH(ads) by dimerization to produce H₂O₂ were studied through the time dependence of the SI-SECM signal and the surface dimerization rate constant was found to be ~ k_OH = 2.2´10³ mol^-1·m²·s^-1. Subsequently, this group investigated the reaction kinetics and surface coverage of water oxidation intermediates (H₂O₂) at the W/Mo-BiVO₄ photoanode using SI-SECM [⁹²]. Besides, Cho et al. studied the PEC water oxidation of doping BiVO₄ and detected the oxygen evolution with using optical fiber incorporating a ring electrode via SECM [⁹³].

Fig.9 (a) Description of the general SI-SECM simulation space and conditions. All geometries are in axial 2D and described by z and r were shown. Boundary types: i, insulation; ii, bulk concentration (semi-infinite); iii, flux at the tip; iv, concentration of hydroxyl radical OH at the substrate; v, insulation. (b) Description of the surface interrogation technique for the reduction of ⋅OH(ads) on TiO₂. ① No ⋅OH(ads) on TiO₂; ②⋅OH(ads) are generated on TiO₂ through surface irradiation. ③ Light is turned off, interrogation of ⋅OH(ads) takes place by reduced species generated at the tip [⁹¹]. Copyright © 2012, Royal Society of Chemistry

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As a promising narrow semiconductor, the PEC performance of Fe₂O₃ was mainly limited by slow water oxidation and severe recombination in the surface states. Therefore, in order to better understand the pathway of PEC reaction and further improve the PEC performance, it is essential to quantitative analyze of the reaction intermediates of the surface states. Krumov et al. used SI-SECM mode to quantitatively in situ probe the reactivity and coverage of reactive surface states in Fe₂O₃ during PEC water oxidation [⁹⁴]. Besides, they also detected the evolution of H₂O₂ (1.0 mmol·m^-2) during the decay process based on the SG/TC mode of SECM, which provided strong evidence for the existence of the ⋅OH(ads) intermediate on Fe₂O₃ and clearly showed a mechanism for the generation of unwanted products in PEC cells. This study demonstrated how SI-SECM enabled direct observation of multiple adsorbates and reaction pathways on operating photoelectrodes. Bard group also quantified the density of photoactive sites (Fe⁴⁺ species) on the surface of Fe₂O₃ based on the SI-SECM [⁹⁵]. In addition, the PEC water oxidation reaction dynamic was elucidated by time-dependent redox titration experiment which was similar to Krumov et al. [⁹⁴].

Conclusions

In summary, we elaborated and summarized the basic principle and application of SECM in PEC fields in recent years. Particularly, discussion is focused on the screening, kinetics and quantitation of reaction intermediates in solar cells and PEC water splitting based on the different modes of SECM. Attributed to its better sensitivity, spatial resolution and more accurate control of the position of the probe, the application of SECM is continuously expanded to various areas up to now. However, it still has some disadvantages such as its spatial resolution is not high enough. So SECM has more and more commonly combined with other instruments, such as AFM and SICM, surface plasma resonance, and so on. And thus it will continue to broaden the scope of the study, and will become a more important electrochemical technique in the future.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 51602120), Natural Science Foundation of Education Department of Henan Province (No. 17A430023), and the opening project of Henan Provincial Key Laboratory of Nanocomposites and Applications (No. hkd20170106).

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