Fundamental Understanding on Selenium Electrochemistry: From Electrolytic Cell to Advanced Energy Storage

Selenium (Se), as an important quasi‐metal element, has attracted much attention in the fields of thin‐film solar cells, electrocatalysts and energy storage applications, due to its unique physical and chemical properties. However, the electrochemical behavior of Se in different systems from electrolytic cell to battery are complex and not fully understood. In this article, we focus on the electrochemical processes of Se in aqueous solutions, molten salts and ionic liquid electrolytes, as well as the application of Se‐containing materials in energy storage. Initially, the electrochemical behaviors of Se‐containing species in different systems are comprehensively summarized to understand the complexity of the kinetic processes and guide the Se electrodeposition. Then, the relationship between the deposition conditions and resulting structure and morphology of electrodeposited Se is discussed, so as to regulate the morphology and composition of the products. Finally, the advanced energy storage applications of Se in thin‐film solar cells and secondary batteries are reviewed, and the electrochemical reaction processes of Se are systematically comprehended in monovalent and multivalent metal‐ion batteries. Based on understanding the fundamental electrochemistry mechanism, the future development directions of Se‐containing materials are considered in view of the in‐depth review of reaction kinetics and energy storage applications.


Introduction
Starting from the discovery of selenium (Se), the great value brought by the unique physical and chemical properties has been periodically explored in the past 200 years, [1][2][3][4] and now Se has become an indispensable part in scientific and technological significance (Figure 1).Among the five known Se isomers, three of them are crystalline structures, namely as α-monoclinic, β-monoclinic and gray trigonal.][10][11] These remarkable properties make Se highly promising for addressing the growing demands of efficient and sustainable energy storage technologies, thus possessing a relatively good prospect for future development and widespread application in energy storage fields.
[14][15][16] As a result of the many valence states, Se, during the electrochemical redox process, participate in disproportionation reactions, resulting in low current efficiency in electrochemical cells (Figure 2).Therefore, it is important to understand the electrochemical process of Se, with a view to minimize the occurrence of disproportionation reactions.Saji et al. [17] summarized the electrochemical redox mechanisms and discussed the Se deposition on different electrodes in aqueous media and ionic liquids 10 years ago.Nevertheless, some issues still need to be understood in depth: 1) the guideline for the electrodeposition mechanism on the morphology of electrodeposited Se-containing materials; 2) the relationship between the morphology and compositions of Se-containing materials and their electrochemical properties; and 3) complex reaction mechanisms requiring further clarity concerning Se-containing materials in secondary batteries.
In this review, we focus on the electrochemical reactions and deposition processes of Se in different electrochemical systems.The mechanisms of doping, nucleation, growth, and electrodeposition conditions relating to electrodeposition of Se-containing materials are summarized.Through a systematic understanding of the fundamental Se electrochemistry, it is anticipated that solutions for addressing insufficient Se electrodeposition can emerge.Further, research progress in the use of Se-containing materials in energy storage is presented, with an emphasis on the electrochemical behavior and energy storage performance in secondary batteries.Finally, the future development of Se-containing materials is concluded, to further the understanding of the electrochemical processes of high-performance Se-containing materials ranging from aspects of material preparation to that of applications.

Electrochemical Behaviors of Se
In the nonferrous metallurgical industry, the main raw material for the extraction of Se is the anode slime produced during electrolysis of other metals, such as copper.The anode slime of copper electrolysis accounts for about 90% of the raw material source, followed by the anode slime from nickel and lead electrolysis. [18]Se(IV) is the main valence state of Se in the anode slime, and the reduction of Se 4+ ions is the dominant electrochemical reaction in the intermediate redox potential range.Competing electrochemical behavior of Se 4+ will be discussed in aqueous solutions, molten salts and ionic liquid systems, in order to facilitate further understanding of Se electrodeposition.

Redox Processes
In addition to elemental Se, Se exists in the following oxidation states: À2, 0, +2, +4 and +6.Due to the narrow potential window of an aqueous solution (1.23 V at 298 K), it is easy to cause side effects from one of the many competing redox reactions.The standard electrode reduction potentials at 298 K between stable species of different valence states can be expressed as: [19] In acidic solutions: H 2 Se !À0:40 V Se !þ0:74 V H 2 SeO 3 !þ1:15 V SeO 2À H 2 Se, which also has a slight solubility in water, both can be expected to help to encourage this reaction.
Any resulting H 2 Se from reactions (4) and ( 5) can combine with the starting material H 2 SeO 3 by a non-redox reaction: [20] H The above discussion pertains to behavior of H 2 SeO 3 in an aqueous solution on a cathode surface undergoing electrochemical reduction reactions, which can generate Se and a soluble H 2 Se, which, in turn, can further react by oxidation to form Se on the cathode surface by the reverse of reaction (5).Mauter et al. [22] systematically evaluated the effects of Se concentrations, pH ranges and temperatures on the electrochemical behaviors of Se reduction on Au electrode, further confirming the difference of four-electron and six-electron reaction processes.Through linear sweep voltammetry (LSV) measurements, it was verified that besides two reduction peaks for HSeO 3 À /Se and HSeO 3 À /H 2 Se, additional reduction peaks for H 2 SeO 3 /Se and H 2 SeO 3 / H 2 Se were also observed in a more acidic solution (pH = 4.0).Higher concentrations of H + ions would boost the positive shift in reduction peak, and promote the reaction rate.Furthermore, an increase in Se(IV) concentration could shift the reduction potential in the positive direction, and enhance the reduction rate (Figure 3a).Additionally, the increase of temperature not only led to higher reaction rate, but also caused the deposited Se change from amorphous red Se to crystalline gray Se.Formation of crystalline Se on the electrode surface effectively converted the conductive Au electrode interface to a conductive Se electrode interface, thereby achieving six-electron reduction pathway at higher temperature (Figure 3b).Kinetic analysis indicated that the diffusion coefficient and standard rate constant via four-electrode pathway was 6.94 × 10 À5 cm 2 s À1 and 3.16 × 10 À7 cm s À1 , respectively.Maranowski et al. [23] confirmed that the transfer coefficient, diffusion coefficient, standard rate constant and exchange current density of fourelectron reduction pathway was determined as 0.3, 2.2 × 10 À5 cm 2 s À1 , 9.1 × 10 À7 cm s À1 and 1.1 × 10 À6 A cm À2 , respectively.While the standard rate constant of Se(IV)/Se(-II) was three orders of magnitude higher than that of Se(IV)/Se(0).Dilmi et al. [24] confirmed the two-step reduction process of SeO 2 → Se → H 2 Se in medium of sodium citrate solution, and that the diffusion of the electroactive species was the rate-determining step of Se deposition.Myung's group [25] further investigated the electrochemistry of Se at PH 1.5 on Au substrate by CV technique, which exhibited five reduction peaks (Figure 3c).A 1 to A 3 peaks were attributed to UPD of Se onto Au, A 4 peak was related to the bulk deposition of Se, and A 5 peak was ascribed to the formation H 2 Se.
The electrochemical behavior of Se had been extensively studied on varying conducting substrates, but the exact electrochemical mechanism of Se redox reactions in aqueous solutions remain unclear due to complexity.Kowalik [26] studied the electrodeposition of Se on different substrates by cyclic voltammogram (CV) measurements.The first two reduction peaks were related with the strong interaction between Au substrate and Se, implying underpotential deposition (UPD) of Se on Au. [22,27] The third peak at 0.1 V (vs.Ag/AgCl) was attributed to the  Energy Environ.Mater.2024, 7, e12664 surface limited process according to reaction (3).The next reduction peak at À0.4 V (vs Ag/AgCl) was related with the decrease of electrode quality, indicating the occurrence of reaction (5).The appearance of more than one anodic peak was related to the oxidation of Se, due to the effect of many different forms of the deposit produced by interaction with the substrate. [23]On an Ag surface, the peak at 0.05 V (vs Ag/AgCl) was associated with the surface phenomena and the formation of Ag 2 Se, and the small peak at À0.1 V (vs Ag/AgCl) was related to the bulk deposition of Se.When the potential was more negative than À0.55 V (vs Ag/AgCl), Se would further reduce to H 2 Se according to reaction (5).Similar phenomena were observed on the Cu electrode.Fan et al. [28] studied the relationship between the light conditions and the electrochemistry when the Se was deposited on indium tin oxide (ITO) substrate.From the linear scanning photoelectricity of ITO electrode in Figure 3d, two reduction peaks (peaks B 1 and B 2 ) corresponded to four-electron and six-electron reduction processes of Se, respectively.When the substrate was illuminated, the current densities of B 1 and B 2 peaks increased significantly.The nucleation and growth processes through chronoamperometry further revealed that Se nucleated at a relatively slow rate in the dark with 3D progressive nucleation model, whereas a very rapid nucleation rate when illuminating with 3D instantaneous nucleation model.
The effects of other metals on Se electrodeposition are also considered in aqueous solution.Li et al. [29] successfully prepared copper selenide (CuSe) nanosheets on Au-plated PET substrate by a simple electrodeposition method.The reduced Cu + would be combined by as-deposited Se to generate CuSe during the potential range from 0.1 to À0.2 V (vs SCE).Similarly, Jee et al. [30] clarified the mechanism of electrodeposition of cobalt selenide (CoSe) film on Pt-plated quartz electrode.Due to the existence of Se, it ultimately led to UPD process of Co. Further, CoSe could be electrodeposited by the reaction of Co 2+ and Se 2À at a more negative potential.Additionally, the co-deposition of nanosheetstructured NiCoSe 2 layer could be achieved at À0.8 V (vs SCE). [31]The calculated charge-transfer coefficient through CV measurements increased from 0.30 to 0.49 when increasing the temperature from 20 to 50 °C, and the corresponding diffusion coefficient increased from 2.58 × 10 À7 cm 2 s À1 to 3.35 × 10 À7 cm 2 s À1 .Some noteworthy findings have been observed in the current electrochemical studies.For example, SeO 2 as a selenium source is favorable for the production of gray Se.Elevated temperatures can significantly promote the reaction rate, but the electrochemical process of Se reduction will be more complicated.Furthermore, the electrochemical behaviors of Se co-deposited with heavy metal ions are particularly complicated.Therefore, it is necessary to further clarify the related electrochemical mechanisms in order to gain a more comprehensive understanding of these phenomena.

Determination of Trace Se
In addition to the deposition and reduction processes of Se in an aqueous solution, the detection of trace Se in an aqueous solution is particularly important because metal dusts or Se compounds emitted during industrial production and applications can enter into the aqueous system and cause environmental pollution.Due to the high detection sensitivity and low cost, a stripping voltammetry method was developed for the determination of trace Se based on the formation of complex compounds and gaseous or soluble H 2 Se. [32,33]etal selenides: The hanging mercury (Hg) electrode was used as the working electrode for the determination of Se content, and the reactions are as follows: [34] Moreover, a higher-detection-sensitivity method was established by the generation of intermetallic compounds (Pb 2+ , Cu 2+ , Hg 2+ , Rh 3+ ) from Se ions and other metal ions: [35,36] Se and Mauter [22] .Copyright 2021, American Chemical Society.c) CV curves of 1 mM SeO 2 solution at 200 °C.Reproduced with permission from Seyedmahmoudbaraghani et al. [25] Copyright 2020, Frontiers Media S.A. d) Linear scanning photoelectricity of ITO electrode in electrolyte containing 8 mM H 2 SeO 3 and 100 mM NH 4 Cl.Reproduced with permission from Fan et al. [28] Copyright 2017, IOP Publishing.
When using copper amalgam as a working electrode, the following reactions were considered in the determination of Se content: [37] Cu Se complex compounds: The adsorptive voltametric behavior of Se 4+ using a Hg electrode was studied in buffer solution containing ophenylenediamine (PDA): [38] Se IV To facilitate the operation (without oxygen removal), a bismuth film coated glassy carbon (GC) electrode was used as the working electrode to determine the content of Se in a HAc-NaAc aqueous solution containing p-aminobenzene sulfonic acid (ABSA): [39] H

Se Electrochemistry in a Molten Salt
The electrochemical process of Se is complex in the molten salt system, and especially, as Se has many intermediate valence states in an alkaline molten salt system.The reduction of Se can be effectively simplified when increasing the acidity of the molten salt.When Se and other metals co-electrodeposit, Se will positively shift the reduction potential of other ions.The reduction of Se 4+ in an acidic melt is a four-electron process with the following reaction: [40] Se When Se 2 is formed, deposited Se will be dissolved: [40] Se However, the reduction process of Se 4+ in alkaline melts is very complicated.There are many oxidation states (Se 2þ , Se 2þ 2 , Se 2þ 4 , Se 2þ 8 , Se 2þ 12 and Se 2þ 16 ).The reactions between the oxidation states are as follows: [41] qSe xþ u , uSe γþ q þ ne À Unlike the aqueous systems, the "multiple peaks" in molten salt systems were due to the stable presence of multivalent Se ions in high-temperature molten salts.Rasmus et al. [41] proved the existence of Se 2þ 4 , Se 2þ 12 , and Se 2þ 16 in alkaline melts.Robinson et al. [40] confirmed that as pCl À increased from 1.9 to 6.2 in AlCl 3 -NaCl melts, the redox process of Na 2 Se became more complex, and the redox peaks moved in the positive direction.Simultaneously, the oxidation process of Se to Se 4+ was a two-step two-electron process in basic melts and a single four-electron process in acidic melts.The calculated charge-transfer coefficient through rotating-disk electrode experiments was almost steady at 0.55, independent of pCl À .The standard rate constant increased from 4.6 × 10 À5 cm s À1 to 17.8 × 10 À5 cm s À1 with melt acidity, and the corresponding diffusion coefficient exhibited a slight increase from 1.99 × 10 À6 cm 2 s À1 to 2.04 × 10 À6 cm 2 s À1 .Matsunaga et al. [42] revealed that the electrochemical reaction from Se to Se 4+ in alkaline AlCl 3 -NaCl melts consisted of several steps, with the average oxidation number of Se close to 2, 1 and 1/4, respectively.Using the Nernst equation, Se 4þ =Se 2þ , Se 2þ =Se 2þ 2 , Se 2þ 2 =Se 2þ 8 reactions were determined.Sakamura et al. [43] investigated the redox behaviors of Se on various electrodes in CaCl 2 melts containing Na 2 Se and CaO.Se could be deposited on the GC electrode in two steps: Se 2À to Se 2 2À and Se 2 2À to elemental Se gas (Figure 4a).Moreover, the anodic peak at about 1.85 V was attributed to the formation of liquid Ni-Se alloy (Figure 4b), which provided the insight into extracting Se from CaCl 2 molten salt.
Considering the ultra-low solubility of SeO 2 in a neutral NaCl-AlCl 3 molten salt, our group research has recently confirmed that SeO 2 could be used as a solid cathode for electro-deoxidation at 190 °C. [44]In CV curves in Figure 4c, the peak O-R 3 at À0.49 V (vs Pt) was assigned to the reduction of elemental Se (Se Through further square wave voltammogram (SWV) measurements in Figure 4d, the average electron transfer numbers of O-R 1 peak at À0.034 V (vs Pt) and O-R 2 peak at À0.109 V (vs Pt) are both equal to 2, indicating that a two-step two-electron electro-deoxidation process of SeO 2 was confirmed (Se(IV) → Se(II) → Se(0)).The clear electrodeoxidation mechanism of solid SeO 2 will help lay the foundation for the recovery of elemental Se.

Se Electrochemistry in an Ionic Liquid
Due to the unique physical and chemical properties, such as non-or low-volatility, low melting points, broad electrochemical windows, and selective solubility, ionic liquids have promising applications in organic catalysis, separation and analysis, and purification electrochemistry. [45]In addition, there are fewer side reactions in ionic liquids than in aqueous solutions.It has been confirmed that different Se sources and electrolysis temperatures could affect the deposition kinetics of Se ions in ionic liquids.
The electrochemical reduction behaviors of different precursors are similar within the ionic liquids.The electrochemical reactions are as follows: [17] Se Energy Environ.Mater.2024, 7, e12664 Steichen et al. [46] reported the effect of Se 4+ precursor on the electrodeposition of Se on Mo electrode in 1-ethyl-3-methylimidazolium tetrafluoroborate/1-ethyl-3methylimidazolium chloride ([C 2 mim][BF 4 ]/ [C 2 mim]Cl) ionic liquid.It was found that SeO 2 and SeCl 4 possessed similar reduction behaviors: Se 4+ to Se and Se to Se 2À .The difference in open circuit potential (OCP) between SeO 2 and SeCl 4 indicated that the Se 4+ ions formed after dissolution had a different redox potential.And the reduction current of SeCl 4 system was three times that of the SeO 2 system.These results implied that Se was thermodynamically and kinetically easier to deposit in the SeCl 4 system.Alsayed et al. [47] studied the deposition process of crystalline Se from air-and water-stable 1-butyl-1methylpyrrolidinium trifluoromethylsulfonate ([Py 1,4 ]TFO) ionic liquids at lower temperatures.The two reduction processes involved a four-electron reaction (H 2 SeO 3 to Se) and a six-electron reaction (H 2 SeO 3 to H 2 Se).Bougouma et al. [48] investigated the electrochemical behavior of SeO 2 as a-Se source in choline chloride and urea (ChCl-U) on the polycrystalline Au electrode by voltammetry and chronoamperometry.With the increase of temperature, the electrochemical reduction process of Se became more complex (Figure 5a).At 110 °C, three reduction peaks around À0.075, À0.2 and À0.7 V (vs Ag) were attributed to the UPD of Se, the bulk deposition of Se, and the cathodic stripping of Se 2À , respectively.With the increase of scanning rate, the charge density of C 2 peak decreased, implying that the reduction process was controlled by diffusion.The charge density of C 1 peak did not change much with the increase of the scanning rate, indicating that it was a surface confinement process.Furthermore, the diffusion coefficient of Se(IV) species was calculated as 6.3 × 10 À7 cm 2 s À1 by chronoamperometry, confirming that Se electrodeposition was a 3D nucleation process with diffusion-controlled growth of overlapping nuclei.The above results confirmed that the electrochemical reduction of Se(IV) to Se(0) is a one-step four-electron process.
In addition, the co-deposition of Se with other metals will result in the formation of metal selenides.CV curves of SeO 2 and Cu 2 O at a polycrystalline Au electrode in ChCl-U electrolyte were exhibited in Figure 5b. [49]In the electrolyte containing only Cu 2 O, Cu deposition took place at À0.3 V (vs Ag).Moreover, the cathodic peak at 0.1 V (vs Ag) was assigned to the formation of Cu 2 Se in the electrolyte containing SeO 2 and Cu 2 O. Similarly, the Reproduced with permission from Sakamura et al. [43] Copyright 2021, IOP Publishing.c) CV curves of Se electrode system at 100 mV s À1 and SeO 2 electrode system at 10-100 mV s À1 .d) SWV curves of SeO 2 electrode system at 5-50 Hz and fitting curves under the O-R 1 and O-R 2 peaks.Reproduced with permission from Chang et al. [44] Copyright 2022, The Royal Society of Chemistry.
Energy Environ.Mater.2024, 7, e12664 electrodeposition processes of PbSe film in choline chloride/ethylene glycol [50] and CdSe film in tricaprylmethylammonium chloride/ formamide [51] were also confirmed.As discussed above, the complexity of the Se reduction process depends on not only the types of electrolytes, but also on the acid-base property of the given electrolyte, the source of Se, nature of the deposited substrate, and the temperature.The clarification on the electrochemical behaviors of Se would be beneficial to the comprehensive understanding of the reaction kinetics of Se in different conditions, and promote the development of electrodeposition of Se and Se alloys and compounds.

Electrodeposition Strategies of Se
As a simple and low-cost preparation method, different morphologies of the compounds containing O, S, Se, and Te could be synthesized by the electrodeposition method without a template, [52][53][54][55] such as nanorods, spherical flowers, nanosheets, nanocubes, and nanodendrites.Some functional and thermoelectric materials were also obtained by the electrodeposition method, such as metal oxides, [56] and metal tellurides. [57,58]As mentioned in the previous section, monolithic Se or Se-containing materials could be efficiently obtained through adjusting the electrodeposition conditions.In this section, we focus on the electrodeposition strategies of Se products under varying media and experimental conditions.

Se Electrodeposition in an Aqueous Solution
Depositing Se materials in an aqueous solution has the advantage of relatively simple operation, easy to control, and low operating temperature.However, the aqueous system is limited by low-efficiency production due to the solubility of the Se compounds and the formation of hydrogen selenide.Herein, the changes on the deposited Secontaining materials are summarized and discussed in the presence or absence of light, precipitation with other metal ions, as well as varying substrates, temperatures, potentials, and pH.
It has been found that the change in potential could greatly change the structures of deposited Se.Li et al. [29] studied sediments obtained at different deposition potentials (Figure 6a).Copyright 2017, IOP Publishing.b) Cathodic current of Se electrodeposition under short time interval illumination at À0.4 and À0.65 V (vs SCE).Reproduced with permission from Fan et al. [28] Copyright 2017, IOP Publishing.c, d) Optical photos and AFM images of the deposition products for nucleation process at different potentials and temperatures.Reproduced with permission from Guarneros-Aguilar et al. [64] Copyright 2019, IOP Publishing.

I (mA cm
Energy Environ.Mater.2024, 7, e12664 With the increase of deposition potential, the deposition products tended to be uniform.But H 2 Se bubbles would hinder the growth of the nanosheets.In addition, the effect of light on Se deposition was also investigated.When Se was electrodeposited in a selenite solution [59] and an acidic solution of Se dioxide, [60] the cathode polarization in the dark was very high.Fan et al. [28] showed that light absorption could enhance the reduction of Se 4+ /Se 0 and Se 4+ /Se 2À (Figure 6b), from the increase of current density under illumination.When the light was irradiated on the Se film, the semiconductor absorbed more photons than the band gap and produced photoelectrons and holes.The resulted photoinduced electrons and holes could be used as free charge carriers to improve the conductivity of the cathodic electrode.Different substrates also lead to changes in product morphology.A smooth and black amorphous Se coating was obtained on Pt and dull Ni, and pulverulent and orange-red amorphous Se was on Cr and bright Ni in the acidic system. [61]Meanwhile, reddish polycrystalline Se film could be deposited on Au substrate at a room temperature. [62]olaliendres et al. [63] studied the Se products deposited on the Au substrate by using atomic force microscopy (AFM).The deposited film showed some holes or blank positions, which may be caused by the conversion of pre-deposited Se to H 2 Se.Guarneros-Aguilar et al. [64] thoroughly investigated the deposition of Se thin films on fluorinedoped SnO 2 substrate.The reduction peaks at À925, À960 and À1120 mV at 75, 50 and 25 °C corresponded to a four-electron reduction process from Se 4+ to Se 0 .Meanwhile, the reduction peaks at À1060, À1150 and À1250 mV at 75, 50 and 25 °C were attributed to the six-electron reduction process from Se 4+ to Se 2À .The resulting Se 2À underwent a purely chemical reaction with Se 4+ to deposit Se 0 , which was a nucleation-growth process.Figure 6c,d showed the optical photographs and AFM images of the deposited products.Due to the multiple nucleation processes around the substrate, the grains growing from the nuclei (Se 4+ → Se 0 ) had a random height distribution in the film.Whereas, from the fast nucleation-growth process mechanism (Se 4+ → Se 2À , Se 2À + Se 4+ → Se 0 ), the grain size height distribution was uniformly similar.
The formation of alloy phases could make products with different morphologies.Souza et al. [65] mainly studied the co-deposition of Se-Bi alloy at different temperatures and with or without stirring.Surface and cross-section SEM images of the alloy deposited at different temperatures in Figure 7a indicated that large grains (1 μm) grew anisotropically on the substrate and formed columns.This irregular deposition was caused by the unstable diffusion from the aqueous solution.Upon further increase of temperature, the surface became flatter, aided by the close arrangement of sedimentary columns.Shen et al. [66] studied the electrodeposition of CdSe phase in an acidic aqueous solution at different potentials.AFM images of the deposited samples at the different potentials indicated that the surface roughness of CdSe films could be increased along with the larger deposition current density.A uniform and continuous distribution of grains with an average diameter of 80 nm was obtained at À0.70 V (vs SCE).Recently, the morphological evolution of Bi 2 Se 3 films with the orthorhombic phase was investigated at a potential of À0.45 V versus Ag/AgCl when applying different charge densities, as presented in Figure 7b. [67]With the increase of total charge densities, the elemental Se:Bi ratio delivered an increasing trend from 1.3 to 1.8.Meanwhile, the electrodeposited Bi 2 Se 3 films exhibited a layer of sub-micrometer-sized hemispherical seeds for a charge density of 0.3 C cm À2 .With further increasing the charge density, the platelet crystallites nucleated from the hemispherical seeds and branched.
As summarized above, the morphology and particle size of the electrodeposited Se can be regulated in an aqueous solution by changing the electrodeposition substrate, light intensity, potential and temperature.Moreover, the phase and composition of the deposited products can be effectively controlled by co-deposition with other metal ions.

Se Electrodeposition in a Molten Salt
The electrochemical synthesis in high-temperature molten salt is a promising method to prepare thin metal or composite materials.There is no need for subsequent treatment to change the crystalline structure of the product or subsequent impurity removal to control the composition of the finished alloy.Thereinto, the electrodeposition of Se in molten salt has the advantages of a wide liquid temperature range and large electrochemical window.It is worth noting that it can, not only obtain the metal coating without internal stress because of the high liquid temperature of the molten salt system, but also realize the high diffusion based plating process by properly adjusting the electrolysis conditions.
The electrodeposition of Se in molten salts usually suffers from the disadvantage of the evaporation of the precursor and the deposited product Se from the elevated temperatures.Currently, the research focus on molten-salt electrodeposition is mainly confined to the preparation of metallic selenides.Sanchez et al. [68] obtained ZnSe thin films with 90% crystallinity in molten NaCl-CaCl 2 mixture at 550 °C.The Se/Zn mole ratio was about 1.1 and the average grain size was 1 μm.The deposited film was transparent with a yellowish hue, but the thickness and density of the films were not affected by the change in the potentials.But the crystal form of ZnSe was affected by the value of the applied potential.The hexagonal ZnSe (Se/Zn = 1.1) at À0.5 (vs Ag/ AgCl) and cubic ZnSe (Se/Zn = 0.7) at À1.0 V (vs Ag/AgCl) was obtained.The increase of zinc content in sediments under more negative potential was conducive to the reduction of elemental zinc.
In addition to the advantages of high conductivity and high crystallinity of products obtained in molten-salt electrolysis, it is necessary to explore the new route of electrodeposited Se, specifically in molten salts with lower eutectic temperatures, so as to achieve a low-energyconsumption, a higher-efficiency and a greener metallurgical process.Very recently, our research group proposed the idea of preparing Se by one-step direct electro-deoxidation of solid SeO 2 in a neutral NaCl-AlCl 3 molten salt. [44]The morphological evolution processes of SeO 2 before and after electro-deoxidation was demonstrated along with the change of potentials, as shown in Figure 8a,b.The more negative potential could accelerate the reaction kinetics of the electro-deoxidation, which is conducive to obtaining better crystallinity of Se.This method avoids the problems of low solubility of Se sources and low recovery efficiency in the traditional recovery process.

Se Electrodeposition in an Ionic Liquid
Electrodeposition in ionic liquids combines the advantages of both high-temperature molten salt and aqueous solution electrolytic systems, by offering a wide electrochemical window at ambient temperatures, and low-corrosion rates associated with the high-temperature molten salt.At the same time, most metals and alloys can be electrodeposited in ionic liquid with low side reactions.The effects of varying potential, temperature, substrate, Se source precursor and electrolysis Energy Environ.Mater.2024, 7, e12664 8 of 21 duration on Se deposition in presence of co-depositing ions are discussed.
The composition and morphology of Se-containing materials electrodeposited in ionic liquids are influenced by the types of Se source.Steichen et al. [46] studied the morphology change of precipitates from different precursors at higher temperatures (Figure 9a).When using SeO 2 precursor, the product consisted of randomly oriented Se nanorods and a small number of agglomerated particles.The deposition with SeCl 4 as precursor consisted of Se hemispheres with diameters ranging from 1 to 15 μm.It was also confirmed that the sediments from SeO 2 and SeCl 4 were composed of single-crystalline trigonal Se (t-Se) and amorphous Se (a-Se), respectively.Abedin et al. [69] revealed that t-Se films could be electrodeposited on Pt in 1-butyl-1-methylpyrrolidinium bis (trifluoromethylsulfonyl) amide (BMPTFSA) containing SeCl 4 at 150 °C.t-Se, a semiconductor consisting of a spiral polymer chain, has high photoconductivity (8 × 10 4 S cm À1 ), low melting point (217 °C), and nonlinear optical properties due to its anisotropic crystal structure. [70]Alsayed et al. [47] found that the deposited Se was a mixed crystal phase of texture rhombohedron and hexahedron when using H 2 SeO 3 as the Se source.The pure Se deposit consisted of a smooth, regular sphere <1 μm in diameter.
Changing the deposition substrate and controlling the formation conditions can also form products with varying compositions and structures.Feng et al. [71] proposed a template-free one-step electrochemical synthesis method of CdSe nanowires with hexagonal structure in dimethylformamide (DMF) containing CdCl 2 and elemental Se.Chou et al. [72] discovered that a dense t-Se layer could be deposited on the ITO electrode in [C 2 mim][BF 4 ]/[C 2 mim]Cl.It is worth noting that since Se is a semiconductor, Se deposited at lower temperatures can lead to decreasing rate in the subsequent electrodeposition.Daniel et al. [73] reported on the electrodeposition of Se in 1-propyl-1methylpiperidine bis (trifluoromethylsulfonyl) imide.The timedependent in-situ ultraviolet-visible (UV-vis) spectra of Se deposition revealed that the growth of Se nanoparticles was quite rapid in the first 20 min, but the growth speed slowed down as the film became more insulating.SEM images in Figure 9b implies that with the increase of deposition time, Se particles continued to coalesce to form a more uniform film.Reproduced with permission from Souza et al. [65] Copyright 2017, IOP Publishing.b) SEM images of Bi 2 Se 3 films grown on Si (111) electrodes at a potential of À0.45 V versus Ag/AgCl for different charge densities.Reproduced with permission from Luo et al. [67] Copyright 2022, American Chemical Society.
Energy Environ.Mater.2024, 7, e12664 Increasing the potential and the temperature can effectively improve the crystallinity and density of the deposited Se film.Chou et al. [72] investigated the electrodeposition of Se film on ITO coated glass electrode in an ionic liquid containing excess chloride ions.It was found that the deposition of crystalline Se required a more negative deposition potential.As shown in Figure 9c, with the increase of the deposition potential, the size of the deposited particles increased, and the film became denser.Shimul et al. [74] studied the electrodeposition of Se in an amide-type hydrophobic ionic liquid at 25 and 50 °C.Small particles and needles were observed at 25 °C, while denser leaf or flower-like particles were obtained at 50 °C (Figure 9d).The difference in crystal phase and morphology at different temperatures might be related to the crystal growth rate or the conductivity of Se.

Obstacles of Electrodeposited Se
Although the development in electrodeposition of Se has made great progress, it still faces the following two obstacles.One is the slow deposition rate of Se, which can be improved by considering and optimizing different aspects, including temperature, Se precursor source, illumination intensity, substrate choice and the applied potential (Figure 10).Another obstacle is that the morphology of the electrodeposited product is difficult to be predetermined unlike the template method.And it can only control the particle size and structure of the products to form single-component one-and twodimensional materials.In order to meet the current rapidly increasing needs, it requires further research in the future to obtain multicomponent non-homogeneous multi-dimensional Se-containing materials.

Progresses of Application of Se in Energy Storage
With the continuous growth of the world population and the increase in material demand, the requirement for various kinds of energy storage and energy conversion has increased significantly in recent years.Copyright 2011, Elsevier.b) SEM images of Se deposition at different times.Reproduced with permission from Redman et al. [73] Copyright 2014, American Chemical Society.c) SEM images of Se films at different deposition potentials.Reproduced with permission from Chou et al. [72] Copyright 2010, IOP Publishing.d) SEM images of sediments obtained at 25 and 50 °C.Reproduced with permission from Saha et al. [74] Copyright 2016, IOP Publishing.
Figure 10.Factors affecting the Se deposition rate.

Solar Cells
It is well known that Se can improve the photoelectric conversion efficiency of solar cells.It is of significant importance that the high reactivity of Se to metals allows the synthesis of chalcogenide semiconductors, [75] such as Cu(In,Ga)Se 2 (CIGS) films, which are widely used in photovoltaic applications.CuSe thin-film solar cells, in which thin film was deposited on a substrate with a thickness of only a few millimeters, had the advantages of good stability, high-energy exchange efficiency, and low manufacturing cost. [76]Under the same conditions, the power generation efficiency of CuSe thin-film cells was 5-10% higher than that of commercial silicon solar cells.When glass substrate was substituted for aluminum foil or polymer films, the lightweight and flexible solar cells could be curled during production, facilitating continuous production and thus further reducing production cost.However, the photoelectric conversion effect of such thin-film cells varied greatly from small variations of elemental ratios and from its complex multilayer structure.James et al. [77] developed a CIGS thin film grown by physical evaporation on soda-lime glass substrate by an improved three-step method.The resulted CIGS device could achieve 23.3% photoelectric efficiency at 14.7 Suns optical concentration.
Similarly, the electrochemical method is also widely applied in the preparation of solar cell materials.Torane et al. [78] explored the possibility of electrodeposition of Se and Bi under different molar ratios of Se and Bi.Moreover, it was found that adding copper, [79] nickel, [80] antimony [81] and polyaniline [82] in the electrolyte could enhance the photoelectric efficiency of the products.Esmaeili-Zare et al. [83] studied the electrodeposited CIGS absorption layer, which was prepared by a three-stage electrodeposition process.With the increase of deposition time, the size of the nanostructures increased, and the agglomerated CIGS flower-like nanostructures were formed.Ye et al. [84] reported that the effect of the crystal quality of CIGS thin-film on properties, which was prepared by one-step electrodeposition in the mixture of 1butyl-3-methylimidazolium tetrafluoroborate (BMImBF 4 ) and ethanol.The Hall coefficient, carrier concentration, Hall mobility, and conductivity of the annealed CIGS films were found to be 2.852 × 10 2 cm 3 C À1 , 2.188 × 10 20 cm À3 , 1.206 × 10 2 cm 2 V s À1 , and 4.226 × 10 3 S cm À1 , respectively.The measurements indicated that the film was a p-type semiconductor, with a band gap of 1.41 eV.
It is well known that numerous defects on the surface of CIGS film can lead to dramatic carrier recombination at hetero-interface, which hinders further efficiency improvement of CIGS solar cells.Yuan et al. [85] employed the thioacetamide induced surface sulfurization process to passivate interface defects, as shown in Figure 11a.Sulfurized thin-film samples had good crystallization, with a higher surface roughness, yielding a more efficient carrier-separation (Figure 11b,c).A power conversion efficiency of 15.25% was achieved, with an  and c) two-dimensional topography/surface potential mapping of the CIGS thin films before/after sulfurization process.Reproduced with permission from Yuan et al. [85] Copyright 2020, American Chemical Society.d) Admittance spectroscopy and density of defect states of CIGS solar cells treated by KSCN, GaCl 3 , and GaCl 3 & KSCN.Reproduced with permission from Gao et al. [86] Copyright 2021, American Chemical Society.
Energy Environ.Mater.2024, 7, e12664 improved OCP of 0.65 V and a fill-factor of 72.21%.Gao et al. [86] spun GaCl 3 solution onto CIGS surface for increasing the Ga content on the CIGS surface.As shown in Figure 11d, after CIGS was treated by potassium thiocyanate (KSCN), GaCl 3 and GaCl 3 & KSCN solutions, the activation energy changed to 57, 79, and 59 meV, respectively.Through GaCl 3 & KSCN post deposition treatment, the conversion efficiency of the CIGS solar cell reached 13.5%, and the OCP was improved to 610 mV.
Electrochemical deposition method is one of the non-vacuum deposition technologies for preparing GIGS thin films, which is capable of obtaining uniform-thickness, large-area, multi-component CIGS films.
Electrodeposition can be performed at or near ambient temperatures, so there is negligible residual thermal stress in the deposited films, which is beneficial to enhance the bonding between substrate and film.And it is also possible to precisely control the thickness of the film by controlling process parameters such as current, voltage, pH, temperature and concentration.However, outstanding issues such as the selection of the deposition potential, the instability of In/Ga-ions in neutral and alkaline solutions, and the co-deposition of oxides and other hazardous binary phases will be addressed in the current study.

Secondary Batteries
Se has high potential in the field of power batteries due to their high bulk energy density. [87,88]However, similar to sulfur-containing batteries, [89] the shuttle effect and the inevitable volume changes during charge and discharge process will lead to rapid capacity decay, poor cycling performance, and low coulombic efficiency. [90]Meanwhile, the pure Se bulk material has low activity and a weak reaction interface, making the capacity far removed from the theoretical value.[93][94] In this section, we mainly discuss the reaction kinetics and electrochemical performance of Se in monovalent and multivalent metal-ion batteries.

Monovalent Metal-Se Batteries
Li, Na and K belong to the Group IA of alkali metal elements, which share similar physical and chemical properties and have been widely used as metal-ion carriers for secondary batteries.Owing to the multielectron transfer behavior, Se delivers high theoretical capacity and desirable energy density in Li, Na and K secondary batteries.
[97] The conversion between Se allotypes (r-Se, c-Se, Se 8 , Se n ) is irreversible, [98][99][100] but the reduction of Se to polyselenide (Li 2 Se n ) is reversible. [101,102]The possible reaction mechanisms involved in the discharge/charge process of positive electrode for Li-Se batteries were reported: [103][104][105][106][107][108] Mechanism i: The soluble polyselenides can diffuse in the organic liquid electrolyte from the +ve to the Àve electrode and vice-versa, leading to the socalled "shuttle effect", reminiscent of Li-S batteries, thus decrease the capacity and cycling stability.The currently researches mainly focus on the modification of the Se-containing materials with special physical and chemical properties to impede or stop altogether any polyselenide generation during the discharge process, and/or the development of high-surface-area Se-containing materials to absorb the generated polyselenides.
Great efforts have been attempted to improve the cycling performance and energy density of Li-Se batteries.Zhou et al. [109] prepared a-Se nanowire and crystalline Se (c-Se) nanowire by ball milling method.It was found that the first discharge capacity of a-Se nanowire positive electrode reached 750 mAh g À1 , much higher than that of c-Se (at 500 mAh g À1 ).In addition, the difference in activity and specific surface area between a-Se and c-Se allowed a-Se to have an additional redox process to Li 2 Se n during discharge.Zhou et al. [110] designed ZnO-Se nanocomposites by pulsed laser deposition (PLD), which delivered a reversible specific capacity of 505 mAh g À1 .Zhang et al. [111] constructed a new kind of nanocomposite material with graphenecoated Se/polyaniline (PANI) core-shell (G@Se/PANI) nanowires, which retained a superior cycling stability (567.1 mAh g À1 after the 200th cycle at 0.2C) and high-rate capability (510.9 mAh g À1 at 2C).
The addition of carbon materials can provide a large specific surface area and encapsulate a large amount of Se, which will suppress the shuttle effect of the conventional pure Se electrode, thus obtaining better cycling performance.Zhang et al. [112] prepared a series of composite materials of lignin porous carbon (LPC) and elemental Se (Se/LPC) by combining carbonization and activation of industrial alkali lignin and subsequent Se loading.At 0.5C, the reversible capacity of Se/LPC composite electrode was initially 596.4 mAh g À1 , with an average attenuation of 0.08% per cycle for >300 cycles.Park et al. [103] prepared bimodal porous carbon nanofibers containing Se (BP-CNF/Se) from metal-organic framework material by the combination of electrospinning and chemical activation.The unique microstructure of BP-CNF, enabled Se in micropores to contact the liquid electrolyte effectively, resulting in long cycle life and high-rate capability of BP-CNF/ Se (Figure 12a).At 0.5C, the discharge capacities at the second and 300th cycle were 742 and 588 mAh g À1 , respectively, with a capacity retention rate of 79.2%.Additionally, BP-CNF/Se delivered a reversible capacity of 568 mAh g À1 at a high current density of 10C rate (Figure 12b).They elucidated that the BP-CNF/Se underwent the reduction process from r-Se to c-Se to Li 2 Se during discharging, and r-Se to c-Se was an irreversible process in this process.Lv et al. [104] prepared a new type of nitrogen-doped carbon scaffolds (NCSs) impregnated with Se (Se-NCSs) composite positive electrode by a solution method.Unlike the one-dimensional materials, the layered and porous NCSs could effectively alleviate the volume change of the electrode during the charge-discharge process, thus improving the cycling stability and rate performance (Figure 12c).At 1C, the capacity retention rate of Se-NCSs after 500 cycles was 66%, and the average capacity attenuation per cycle was 0.068% (Figure 12d).Moreover, they confirmed the reduction process of elemental Se to polyselenides and then to Li 2 Se during the discharge process.Balakumar et al. [105] prepared nitrogen-containing hollow mesoporous carbon spheres as the host material of Se positive electrode (NHCS/Se-52) by SiO 2 hard Energy Environ.Mater.2024, 7, e12664 template method (Figure 12e).The resulting battery showed excellent electrochemical performance.At 2C, the capacity remained at 75% of the initial capacity after 10 000 cycles, with an average capacity loss of 0.0025% per cycle (Figure 12f).They confirmed the reduction of cyclic Se 8 to chain Se n and then to Li 2 Se during the discharge process.Han et al. [106] designed a kind of reduced graphene oxide layered structure containing 3D mesoporous carbon nanoparticles (MCN) composite (MCN-RGO).When the content of Se reached 62 wt%, the capacity attenuation was only 0.008%/per cycle after 1300 cycles at 1C current density.The excellent performance of Se/MCN-RGO was mainly attributed to the layered structure, which made the selenide "escape" from MCN mesoporous and was then adsorbed by RGO again, to achieve the two-stage capture effect of selenide, further effectively inhibiting the shuttle effect of the polyselenides.Similarly, the elemental Se within Se/MCN-RGO was reduced to polyselenides and then to Li 2 Se during the discharge process.
Compared with the common nonpolar carbon matrix, the strong interaction between the surface polarity of transition metal composites and polar selenides may be a better way to solve the problem of dissolution and diffusion of selenides.Yang et al. [108] prepared a Se@CoSe 2 composite with porous carbon (Se@CoSe 2 -PC).They proved that the existence of CoSe 2 could significantly accelerate the reduction reaction of polyselenides to Li 2 Se.Due to the low solubility of Li 2 Se in the electrolyte, the dissolution of Se compounds was limited, thus greatly improving the cycling performance of the Se electrode.
[115] The electrochemical behaviors of Se in the Na-Se batteries are similar to that of Li-Se batteries, and the final reduction product at the positive electrode from discharge is Na 2 Se. [116]The difference is that low-valence selenide (as NaSe 2 ) may appear during the reduction process. [117,118]These non-chain selenides are insoluble in the electrolyte and do not cause shuttle effect.Two Figure 12. a) Preparation process of BP-CNF/Se, and b) cycling and rate performance of BP-CNF/Se.Reproduced with permission from Park et al. [103] Copyright 2018, The Royal Society of Chemistry.c) Preparation process of Se-NCSs, and d) cycling performance of Se-NCSs.Reproduced with permission from Lv et al. [104] Copyright 2017, American Chemical Society.e) TEM images of NHCS/Se-52@2C, and f) cycling performance of NHCS/Se-52@2C.Reproduced with permission from Balakumar et al. [105] Copyright 2017, American Chemical Society.
Energy Environ.Mater.2024, 7, e12664 different discharge/charge processes of positive electrode for Na-Se batteries were proposed: [119][120][121] Mechanism i: under low current density: under high current density: The radius of Na + ion (0.99 Å) is larger than that of Li + ion (0.59 Å), resulting in slow electrochemical reaction kinetics. [122,123]herefore, it is necessary to build more active sites with high sodiumion diffusion kinetics.126][127] The addition of carbon materials allows Na-Se batteries to achieve better cycle performance.Zhao et al. [128] adopted poplar catkins as raw materials to realize N/O doped defective porous poplar catkins carbon (NOPCC) matrix for amorphous Se intercalation (Se@NOPCC; Figure 13a).Density functional theory (DFT) calculations revealed the interaction between NOPCC and the Se discharge product, as shown in Figure 13b.In the Na-Se batteries, Se was reduced to NaSe 2 and then to Na 2 Se without forming soluble polyselenides.Wang et al. [129] prepared a binder-free, self-interweave CNF/Se positive electrode through vacuum filtration, KOH activation and Se permeation processes (Figure 13c).The uniform distribution of Se in the CNF structure was capable of high Se content (72.1 wt%) and high loading (4.4 mg cm À2 ).As shown in Figure 13d, the formation energy of Na 2 Se (À2.12 eV) and Na 4 Se 2 (À2.26 eV) were much lower than Na 2 Se 6 (À1.32 eV) or Na 4 Se 12 (À1.41eV), and binding energy of Na 2 Se to the ethylene carbonate (EC) or dimethyl carbonate (DMC) (À1.52 eV for EC and À1.61 eV for DMC) was higher than that of Na 2 Se 6 (À1.66 eV for EC and À1.78 eV for DMC), indicating that higher-order Na 2 Se tend to coalesce into a solid network rather than dissolve in the electrolyte.In this battery, the charge/discharge process was the conversion between Se n and Na 2 Se.It is worth noting that, at low current densities, long chain polyselenides (Na 2 Se n ) from the discharge reactions combined with EC/DMC and dissolved into the electrolyte, increasing the "shuttle effect" and thus reducing cycling stability.
K-Se batteries: Se utilization in K-ion batteries is also a concern because it has higher theoretical discharge voltage than Na-Se batteries and lower costs.[132][133][134][135] Moreover, it may lead to the incomplete reduction of Se to Se 2À , [136,137] in which there is still a part of unreduced K 2 Se 2 during the discharge process.The discharge/charge reactions of positive electrode for K-Se batteries are as follows: [138][139][140][141] 2Se For K-Se batteries, the increase in ionic radius of K ions compared to Li and Na would greatly affect the charge-transfer and ionic diffusion  [128] Copyright 2021, American Chemical Society.c) Schematic illustration of the synthesis process of the CNF/Se electrode.d) Calculated binding energies of Na 2 Se 6 , Na 4 Se 12 , Na 2 Se and Na 4 Se 2 , and Na 2 Se 6 and Na 2 Se with EC and DMC.Reproduced with permission from Wang et al. [129] Copyright 2018, Wiley-VCH.
Energy Environ.Mater.2024, 7, e12664 processes during the electrochemical reaction.Currently, the researches mainly focus on the development of Se carriers with high ionic/electronic conductivity.
The introduction of carbon materials into Se is also one of the most important strategies for improving the electrochemical performance of K-Se batteries.Liu et al. [139] found that due to the dissolution of polyether intermediates in electrolyte, pure Se suffered from rapid capacity fading.For addressing the dissolution issue, Lin et al. [140] mixed Se, red phosphorus and expanded graphite (EG) to synthesize Se-2P/C (Se/ P = 1:2 mole rate) by P-milling method Figure 14a.Due to its minimum particle size, Se-2P/C@30 h positive electrode presented a shortened diffusion distance for potassium ions, which not only improved the conductivity of the active substances, but also avoided the generation of polyene ethers during discharge and charging.Figure 14b revealed the reaction mechanism during potassiation/depotassiation process in Se-2P/C@30 h electrode.In the discharge process, the reactions involved the intercalation of K + ions in carbon materials at 0.03 V (vs K + /K), generation of K-P compounds at 0.44 V (vs K + /K), and formation of K-Se compounds at more positive potential.SEM images in Figure 14c showed that the Se-2P/C@30 h electrode had good volume and structure reversibility.Zhou et al. [141] designed N and O dual-doped porous carbon nanocage anchored with carbon nanotube as the host to confine Se (Se@NO-nanocage/CNT) by solution method, crystal growth method and melt diffusion method (Figure 14d).The special structure of the carbon body shortened the diffusion distance of the K + ions, promoted the penetration of the electrolyte and improved the electrical conductivity, thus enhancing the redox kinetics.

Multivalent Metal-Se Batteries
The development of Se in multivalent metal-ion rechargeable batteries is expected to improve energy storage density.49]  Reproduced with permission from Lin et al. [140] Copyright 2019, Elsevier.c) SEM images of Se-2P/C@30 h composite electrodes: 1) fresh electrode; 2) potassiation state (discharge to 0.01 V); and 3) Depotassiation state (charge to 3 V).d) Preparation diagram and SEM images of Se@NO-nanocage/CNT.Reproduced with permission from Zhou et al. [141] Copyright 2020, Wiley-VCH.

Heating
Energy Environ.Mater.2024, 7, e12664 Currently, there are limited positive electrode materials suitable for reversible intercalation/deintercalation of Mg ions. [150]Se-containing positive electrode materials, based on the conversion mechanism, have high specific capacity and good cycling performance. [151]In Mg-Se batteries, the discharge process of Se is the reduction of elemental Se to polyselenides and then to Mg-Se, which is similar to the behaviors of S. [152] The discharge/charge processes of positive electrode for Mg-Se batteries are as follows: [153,154] nSe þ Mg 2þ þ 2e À , MgSe n (38)   MgSe Similar to Se in Li, Na and K batteries, the shuttle effect of polyselenides in Mg-Se batteries would cause severe capacity decay.Therefore, some effective attempts have conducted on Mg-Se batteries to restrain the shuttle of polyselenides.Zhang et al. [153] designed Se-C materials in B-centered anion-based magnesium electrolytes (BCM).The positive electrode showed high reversibility and stable cycling performance.Yuan et al. [154] reported that polyacrylonitrile with Se content (45.37 wt%) positive electrode (Se/PAN) exhibited high capacity and cycling stability in (PhMgCl) 2 -AlCl 3 electrolyte.The high cycling performance of this Mg-Se battery was attributed to the formation of a highly stable and ion-permeable solid electrolyte interphase (SEI) layer on the Se/PAN electrode, in which the chain-structured Se n molecules were embedded in the polymer backbone through the chemical bond between Se/PAN.Moreover, Se was directly reduced to selenide without forming a polyselenide, thus avoiding the shuttle effect.
In addition to making composite electrodes with conductive materials, the other new modification methods are expected to decrease the voltage hysteresis and improve the specific capacity.Selenium-sulfur solid solution (SeS x ) positive electrode materials for high-energy rechargeable Mg batteries have been confirmed to compensate for the disadvantages of each of S and Se, achieving higher electrochemical reactivity and greater accessible capacity. [155]With the contribution of S, the specific capacity of the SeS 2 /CMK-3 positive electrode was apparently increased compared with that of Se/CMK-3 composite electrode. [156]l-Se batteries: In recent years, the application of Al-Se batteries has also been explored.[157,158] For Al-Se batteries, two different charge/ discharge electrochemical processes of Se were proposed, involving the conversion of Al complex ions: [159][160][161] Mechanism i: Mechanism ii: Recently, Li et al. [159] prepared TiO 2 @Se-RGO as a positive electrode material (Figure 15a).TiO 2 precursor was mixed with Se powder and heated at 500 °C to form TiO 2 @Se porous nanospheres.The obtained TiO 2 @Se and RGO were self-assembled to TiO 2 @Se-RGO composite by electrostatic effect (Figure 15b).The first discharge specific capacity of TiO 2 @Se-RGO reached up to 1127.3 mAh g À1 at 200 mA g À1 .In this case, Se was oxidized to Se 6+ after complete charging, and Se 6+ was gradually reduced to Se after Se 4+ and Se 2+ during discharging.The redox reaction of positive electrode was the conversion process between Se and Se n Cl m .Nevertheless, Liu et al. [160] confirmed that the redox reaction was the conversion process between Se and Al 2 Se 3 .From the in-situ TEM image in Figure 15c, the diameter of Se nanowire was increased from 130 to 151 nm during charging.After prolonged reaction, the selected area electron diffraction (SAED) patterns of the Se ) TEM images of TiO 2 , TiO 2 @Se and TiO 2 @Se-RGO.Reproduced with permission from Li et al. [159] Copyright 2020, Elsevier.c) Evolution of Se nanowire positive electrode in the in-situ Al-Se nanobattery after operating for different time (scale bar: 200 nm), and d) SAED patterns recorded at different positions of a partially reacted Se nanowire (scale bar: 2 1/nm).Reproduced with permission from Liu et al. [160] Copyright 2019, Elsevier.
Energy Environ.Mater.2024, 7, e12664 nanowire totally transformed from sharp points into amorphous haloes (Figure 15d).As discussed above, the different reaction mechanisms imply that a deeper understanding is eagerly needed to clarify the exact cause of the shuttle effect, in order to provide a basis for further modification of the Se-containing materials and improvement of cycling performance.To address this issue of conversion-type Se electrode for Al-Se batteries, many attempts have been explored in Al batteries.
Through the early research of Se-containing materials in rechargeable aluminum batteries, our group found that Se 2 Cl 2 produced during the charging process was soluble within the electrolyte, [161] which would lead to a serious shuttle effect, resulting in poor cycle life and rapid capacity decay.The adsorption of the carbon material to the soluble intermediate product can improve the cycling performance of the Al-Se battery.Huang et al. [162] proposed a composite of Se nanowires and mesoporous carbon (CMK-3) nanorods (Se/CMK-3) as a positive electrode material for the Al-Se battery.The reversible capacity of the Se/CMK-3 positive electrode was 178 mAh g À1 , with a discharge voltage >1.5 V. Further, Li et al. [163] prepared hollow carbon-coated Se nanowires (Se@CT) for the positive electrode.After 200 cycles, the capacity was kept at 162.9 mAh g À1 , with the capacity retention rate of 83.5%.Recently, our group designed a CMK-3 porous carbon or graphene-coated Se for Al-Se batteries to greatly improve its discharge capacity and cycling stability. [164,165]s discussed above, the charging/discharging process of Se in monovalent and multivalent metal-ion batteries often involves uncertain and inconsistent reaction products.This ambiguity poses challenges in understanding the real electrochemical reactions involved.Taking Al-Se batteries as the example, the formation of different products such as Se n Cl m and Al 2 Se 3 through various reaction mechanisms would lead to the difference in the number of transferred electrons, and the accompanying inconsistent theoretical capacities, as well as the varying modification strategies.To conclude, further investigation on the reaction mechanism is needed to implement in the future, in order to more accurately understanding of the reaction pathways and shuttle effects in Se-containing systems.By gaining a deeper understanding of these processes, researchers can develop more precise and effective strategies to maximize the utilization of Se in future energy storage applications.

Conclusions and Perspectives
In this review, the electrochemical behaviors of Se were systematically summarized in different systems ranging from aqueous solutions to molten salts to ionic liquids.The deposition strategies of different Secontaining materials were discussed based on the electrochemical behaviors of Se, and the evolution processes of the morphology and component of the products under different deposition conditions were also underlined.Moreover, the extensive applications of Se-containing materials in the energy systems were presented, including in solar cells and in secondary batteries.Although great progresses have been made in the electrodeposition and energy storage of Se, great challenges exist in electrolytic cells and energy storage fields regarding complex and unclear reaction processes, uncontrollable morphology and multidimensional structure design, as well as advanced and stable energy storage applications.
There remains to be in-depth studies in the electrochemical processes and energy storage applications of Se-containing materials.

In-depth understanding of electrochemical processes
The complexity of the Se reduction process depends not only on the type of electrolyte, but also on the acid-base property, multivalent nature of Se, Se precursor source, deposition substrate and temperature.It has been reported that when using SeCl 4 as the Se source and Au as deposition substrate, elevating electrodeposition temperature is effective in enhancing the kinetics and thermodynamics of the deposition reactions.In the multistep reduction process of Se, the complex variations of valence states of Se with increasing alkalinity of the electrolyte, make the conventional CV detection difficult to accurately calculate the number of transferred electrons for the reactions and identify the kinetic process.To overcome these challenges, it is necessary to confirm the reduction mechanism of Se-containing materials through in-situ or exsitu physical characterization techniques coupled with electrochemical measurements.Additionally, the reaction mechanism of Se in the energy storage applications currently lacks consistent understanding, which should be further clarified through advanced multiscale characterizations combining with various in-situ techniques, like spectroscopy, microscopy, and diffraction methods.

Multiscale architectures
In the preparation process of Se-containing materials by electrochemical methods, three major obstacles are faced: 1) The formation mechanisms of Se-containing materials are less discussed in the current electrochemical preparation methods, including the effects of metal doping, nucleation and growth processes, the rules and causes of the influence of deposition conditions, as well as the role of complexing agents.2) The morphology of the Se-containing material is difficult to design and control in advance as in the template method, making the finally prepared material not meet the needs of the application.3) Due to the lack of kinetic studies in the complex ternary and above systems, the component regulation of Se-containing compounds in multisystems are difficult to control accurately.To address the above issues, the further research should focus on elucidating the electrodeposition reaction mechanism and the relationship between the composition, morphology and energy storage properties of the Se-containing materials, enabling better control and design of material properties.The effects of various process factors on kinetics should be determined to provide rational predictions for the morphology and composition, and to achieve a controllable structure and morphology design.Moreover, a variety of in-situ electrochemical methods and real-time physical characterizations should be coupled to elucidate the reaction characteristics, nucleation and growth processes, which can help regulate the composition and properties of Se-containing materials.

Advanced energy storage application
The high theoretical capacity and energy density of Se make it favorable to apply in the electrochemical energy storage fields.However, it eventually suffers from three serious impediments: 1) The weak conductivity of bulk Se can apparently hamper the capability of electronic transport and ionic diffusion, leading to increased polarization hysteresis and capacity decline.2) The soluble Se-containing intermediate compounds produced in the redox reaction can cause the Energy Environ.Mater.2024, 7, e12664 serious shuttle effect and structural collapse, resulting in the decreased utilization rate and poor cycling stability of the active Se material.3) The complex multistep redox reactions probably lead to the irreversible phase transformation and the failure of the electrode.Accordingly, the rational design approaches of structure and morphology of the active Se materials is crucial to enhance electronic transport and ionic diffusion, prevent the dissolution and shuttle effect of the soluble compounds into the electrolyte, thereby improving the utilization of active Se materials and the electrochemical performance.Designing active Se materials with appropriate structures, such as compositing, coating, hollow construction and integrated electrode, can effectively enhance the conductivity of Se, reduce polarization hysteresis and capacity decline, preserve the structural integrity of the electrode, further promote the electrochemical behavior.Moreover, employing advanced characterization techniques, including in-situ monitoring, can provide insights into the structural and chemical changes and transformation pathways of active Se materials occurring during cycling and aid in the development of stable energy storage systems.
In this review article, the basic electrochemistry properties of Se in the preparation and energy application are discussed.The authors have aimed to provide further insights to achieve a complete production process of high-performance Se-containing materials in the future, encompassing material preparation and applications.

Figure 1 .
Figure 1.Historical evolution of Se application.

Figure 2 .
Figure 2. The complicated redox processes between different valence states of Se.

Figure 3 .
Figure 3. a) LSV scans under various Se(IV) concentrations in the reduction region.b) Schematic of four-electron and six-electron pathways with their products.Reproduced with permission from Zou and Mauter[22] .Copyright 2021, American Chemical Society.c) CV curves of 1 mM SeO 2 solution at 200 °C.Reproduced with permission from Seyedmahmoudbaraghani et al.[25]Copyright 2020, Frontiers Media S.A. d) Linear scanning photoelectricity of ITO electrode in electrolyte containing 8 mM H 2 SeO 3 and 100 mM NH 4 Cl.Reproduced with permission from Fan et al.[28]Copyright 2017, IOP Publishing.

Figure 4 .
Figure 4. a, b) CV curves GC and Ni electrodes in CaCl 2 -Na 2 Se and CaCl 2 -CaO-Na 2 Se melts at 820 °C.Reproduced with permission from Sakamura et al.[43]Copyright 2021, IOP Publishing.c) CV curves of Se electrode system at 100 mV s À1 and SeO 2 electrode system at 10-100 mV s À1 .d) SWV curves of SeO 2 electrode system at 5-50 Hz and fitting curves under the O-R 1 and O-R 2 peaks.Reproduced with permission from Chang et al.[44]Copyright 2022, The Royal Society of Chemistry.

Figure 6 .
Figure 6.a) Scanning electron microscopy (SEM) images of sediments obtained at different deposition potentials.Reproduced with permission from Li et al.[29]Copyright 2017, IOP Publishing.b) Cathodic current of Se electrodeposition under short time interval illumination at À0.4 and À0.65 V (vs SCE).Reproduced with permission from Fan et al.[28]Copyright 2017, IOP Publishing.c, d) Optical photos and AFM images of the deposition products for nucleation process at different potentials and temperatures.Reproduced with permission from Guarneros-Aguilar et al.[64]Copyright 2019, IOP Publishing.

Figure 7 .
Figure7.a) Surface and cross-section SEM images of the Se-Bi alloy deposited at different temperatures and with or without stirring.Reproduced with permission from Souza et al.[65]Copyright 2017, IOP Publishing.b) SEM images of Bi 2 Se 3 films grown on Si(111) electrodes at a potential of À0.45 V versus Ag/AgCl for different charge densities.Reproduced with permission from Luo et al.[67]Copyright 2022, American Chemical Society.

Figure 8 . 21 ©Figure 9 .
Figure 8. a) SEM images of SeO 2 before and after electro-deoxidation at different potentials.b) Schematic diagram of the electro-deoxidation process of SeO 2 .Reproduced with permission from Chang et al.[44]Copyright 2022, The Royal Society of Chemistry.

Figure 11 .
Figure 11.a) Schematic diagram for the surface sulfurization process, b) surface/cross-sectional SEM images,and c) two-dimensional topography/surface potential mapping of the CIGS thin films before/after sulfurization process.Reproduced with permission from Yuan et al.[85]Copyright 2020, American Chemical Society.d) Admittance spectroscopy and density of defect states of CIGS solar cells treated by KSCN, GaCl 3 , and GaCl 3 & KSCN.Reproduced with permission from Gao et al.[86]Copyright 2021, American Chemical Society.

Figure 13 .
Figure 13.a) Schematic diagram of the synthesis route of Se@NOPCC.b) Simulated charge distribution between NaSe2 and defective carbon (up) or regular carbon (down) and simulated charge distribution between Na 2 Se and defective carbon (up) or regular carbon (down).Reproduced with permission from Zhao et al.[128]Copyright 2021, American Chemical Society.c) Schematic illustration of the synthesis process of the CNF/Se electrode.d) Calculated binding energies of Na 2 Se 6 , Na 4 Se 12 , Na 2 Se and Na 4 Se 2 , and Na 2 Se 6 and Na 2 Se with EC and DMC.Reproduced with permission from Wang et al.[129]Copyright 2018, Wiley-VCH.

Figure 15 .
Figure 15.a) Schematic diagram of the preparation process of TiO2 @Se-RGO.b) TEM images of TiO 2 , TiO 2 @Se and TiO 2 @Se-RGO.Reproduced with permission from Li et al.[159]Copyright 2020, Elsevier.c) Evolution of Se nanowire positive electrode in the in-situ Al-Se nanobattery after operating for different time (scale bar: 200 nm), and d) SAED patterns recorded at different positions of a partially reacted Se nanowire (scale bar: 2 1/nm).Reproduced with permission from Liu et al.[160]Copyright 2019, Elsevier.