Solvent-free Solid Polymer Electrolytes Research Articles

Ultra-stable all-solid-state sodium metal batteries enabled by perfluoropolyether-based electrolytes.

Rechargeable batteries paired with sodium metal anodes are considered to be one of the most promising high-energy and low-cost energy-storage systems. However, the use of highly reactive sodium metal and the formation of sodium dendrites during battery operation have caused safety concerns, especially when highly flammable liquid electrolytes are used. Here we design and develop solvent-free solid polymer electrolytes (SPEs) based on a perfluoropolyether-terminated polyethylene oxide (PEO)-based block copolymer for safe and stable all-solid-state sodium metal batteries. Compared with traditional PEO SPEs, our results suggest that block copolymer design allows for the formation of self-assembled nanostructures leading to high storage modulus at elevated temperatures with the PEO domains providing transport channels even at high salt concentration (ethylene oxide/sodium = 8/2). Moreover, it is demonstrated that the incorporation of perfluoropolyether segments enhances the Na+ transference number of the electrolyte to 0.46 at 80 °C and enables a stable solid electrolyte interface. The new SPE exhibits highly stable symmetric cell-cycling performance at high current density (0.5 mA cm-2 and 1.0 mAh cm-2, up to 1,000 h). Finally, the assembled all-solid-state sodium metal batteries demonstrate outstanding capacity retention, long-term charge/discharge stability (Coulombic efficiency, 99.91%; >900 cycles with Na3V2(PO4)3 cathode) and good capability with high loading NaFePO4 cathode (>1 mAh cm-2).

Synthesis and Comparative Electrochemical Studies of Bottlebrush Solid Polymer Electrolytes (SPEs) for Li-Ion Transport

Solid polymer electrolytes in solvent-free lithium batteries may overcome some of the disadvantages of liquid electrolytes such as flammability and instability.1 Thereby, the polymer electrolyte acts as both ion transport medium and electrical separator between the electrodes. Compared to rigid separators (e.g. fiber glass) a higher shape flexibility is a further advantage, as these SPEs can compensate volume changes of the electrodes by elastic and plastic deformation.2 Poly(ethylene glycol) (PEG) possesses one of the highest ionic conductivities among solvent-free SPEs but suffers from a conductivity drop below its melting temperature about 50-60°C due to high crystallinity depending on the molecular weight.1 In addition to linear PEG polymers, ion-conducting bottlebrush graft copolymers can be obtained by attaching PEG side chains to a polymer backbone in order to reduce the crystallinity maintaining very high molecular weight.In this work we synthesized five new bottlebrush polymers using free radical polymerization as well as ring-opening metathesis polymerization (ROMP). These brush polymers contain different lengths of PEG side chains (1 kg mol-1 and 2 kg mol-1) and two different backbones, poly(methacrylate) and poly(norbornene). We present the influence of the polymer architecture on mechanical stability, ionic conductivity, Li-ion transport number and electrochemical stability of a series of SPEs obtained thereof by mixing with different amounts of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). We also compare the results to the respective linear PEG counterparts. The differences in ionic conductivity were analyzed and correlated with respect to thermal properties such as T g and ΔH melt in order to understand the fundamental factors which influence the properties of solvent-free PEG-containing bottlebrush SPEs.We also examined the implications of changing a linear polymer system to a brush architecture for potential applications in batteries and correlate the occurring processes in the cell with the distribution of relaxation times (DRT). Detailed comparative measurements under similar cell configurations for diverse O/Li ratios in a temperature range of 25 to 80 °C for different SPEs were carried out to elucidate structure-property relationships. The interesting findings are that by applying a brush architecture, we suppress the crystallinity of PEG and improve the mechanical strength without losing ionic conductivity. We obtained conductivities in the range of 10-3 to 10-4 S cm-1 for solvent-free SPEs. Furthermore, the best ionic conductivities for any system correlate strongly with their respective T g.3 Nevertheless, there are still several unresolved questions regarding these bottlebrushes compared to linear PEG in terms of interfacial as well as bulk processes in both blocking steel (ionic conductivity) and lithium (lithium plating/stripping, interfacial resistance) electrode setups. In addition to the polymer component, the lithium salt has a major effect on the properties of the electrolyte. LiTFSI is probably the most common Li-ion source in SPEs.4 Besides that, lithium borate salts have gained high interest due to their high thermal stability, cost-effectiveness, favorable solid electrolyte interface (SEI) formation and ionic conductivities in the same range as LiTFSI.5 For example, Lithium bis(oxalate)borate (LiBOB) and its asymmetric counterpart Lithium difluoro(oxalate) borate (LiDFOB) are stable in organic solvents and the electrochemical stability is higher than 4.5 V vs. Li/Li+.6 Different salts in an electrolyte can influence the Li-ion transport as well as the processes at the interfaces or the formation of the SEI. This again requires a detailed comparative analysis.For this, we prepared promising bottlebrush polymer electrolytes (1 kg mol-1 PEG sidechain) containing LiBOB and LiDFOB and subsequently analyzed them electrochemically by impedance spectroscopy. The measurement data was finally interpreted by the extended Distribution of Relaxation Times (eDRT). Both SPEs showed similarities in the lithium-ion conducting process in possessing one major, resistive-capacitive bulk conductivity mechanism. The resulting, temperature-dependent conductivities were evaluated and are in the range of 10-4 to 10-5 S cm-1.During cycling, the SPEs showed increased interface resistances over time, which are higher than respective bulk resistances. By applying eDRT, the time-dependent formation of an interphase layer in the SPEs is identified, separated from the slower charge transfer process and quantified. Thus, the electrolytes cannot be considered electrochemically stable against metallic Li, which is similar for liquid electrolytes.7 References (1) Scrosati, B.; Energy Environ. Sci. 2011, 4, 3287.(2) Janek, J.; Nat. Energy 2016, 1, 16141.(3) Rosenbach, D.; ACS Appl. Energy Mater. 2019, 2, 3373–3388.(4) Etacheri, V.; Energy and Environmental Science 2011, pp 3243–3262.(5) Xu, K. Chem. Rev. 2014, 114, 11503–11618.(6) Liu, Z.; Coord. Chem. Rev. 2015, 292, 56–73.(7) Hahn, M.; Electrochim. Acta 2020, 344, 136060. Figure 1

Partially Oxidized Cellulose grafted with Polyethylene Glycol mono-Methyl Ether (m-PEG) as Electrolyte Material for Lithium Polymer Battery

Herein, a novel cellulose derivative has been synthesized and investigated as a nature-derived solid polymer electrolyte for lithium batteries. Cellulose is oxidized in a two-step process to dicarboxylic acid cellulose to allow for grafting low molecular weight poly(ethylene glycol) monomethyl ether (550 g mol-1) via Fischer–Speier esterification at the thus obtained carboxyl groups. The chemical structure of the synthesized materials is confirmed by Fourier-transform infrared (FT-IR) and nuclear magnetic resonance (NMR) spectroscopy as well as X-ray diffraction. Incorporating lithium bis(trifluoromethane-sulfonyl)imide (LiTFSI) as conducting salt and N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr14TFSI) ionic liquid as plasticizer results in the realization of an amorphous and solvent-free solid polymer electrolyte. These electrolyte membranes are characterized by high thermal and electrochemical stability and ionic conductivities of about 1×10−5 S cm−1 at 20 °C and 2.5×10−4 S cm−1 at 80 °C, which enables very stable lithium stripping and plating for more than 800 h.

High voltage, solvent-free solid polymer electrolyte based on a star-comb PDLLA-PEG copolymer for lithium ion batteries.

In this work, a novel star-comb copolymer based on poly(d,l-lactide) (PDLLA) macromonomer and poly(ethylene glycol)methyl ether methacrylate (PEGMA) was prepared, and the electrochemical properties were studied, with the aim of using it as a solid polymer electrolyte in lithium ion batteries. The six-arm vinyl functionalized PDLLA macromonomer was synthesized by a ring-opening polymerization (ROP) of d,l-lactide and subsequently an acylation of the hydroxy end-groups. A series of free-standing solid polymer electrolyte membranes from different ratios of PDLLA, PEGMA and LiTFSI were prepared through solvent-free free radical polymerization under UV radiation. The chemical structure of the obtained polymers was confirmed by 1H NMR and FTIR. The as-prepared six-arm star-comb solid polymer electrolytes (PDLLA-SPEs) exhibit good thermal stability with Td5%s of ∼270 °C and low Tgs of −48 to −34 °C. The electrochemical characterization shows that the PDLLA-SPEs possess a wide electrochemical window up to 5.1 V with an optimal ionic conductivity of 9.7 × 10−5 S cm−1 at 60 °C at an EO/Li+ ratio of 16 : 1. Furthermore, the all-solid-state LiFePO4/Li cells display extraordinary cycling and rate performances at 60 °C by curing the PDLLA-SPEs directly on the cathode. These superior properties of the six-arm star-comb PDLLA-SPE make it a promising candidate solid electrolyte for lithium batteries.

A Long-Life 3-Stack 12 V Dry Polymer Battery Using LiNi1/3Mn1/3Co1/3O2 and Graphite

Introduction Safety is more important than high energy in stationary batteries. In addition, material choices based on the low-cost and easy production of larger batteries is another critical issue for developing large energy storage systems comparable to pumped hydro systems. A solvent-free solid polymer electrolyte (SPE) has the advantages of being safe, inexpensive and barrier-free for large battery production. However, polyether-based SPEs are believed to oxidize at >4 V. Carbon-based anodes are also thought to be incompatible with polyether-based SPEs. Therefore, such SPEs are now in categorized as traditional electrolytes. However, we demonstrated (i) the compatibility of 4 V-5 V cathodes, such as LiCoO2 1) and LiNi0.5Mn1.5O4 2), by modifying the cathode and polyether-based SPE interface and (ii) sufficient graphite reversibility with the SPE using suitably prepared electrodes. Recently, we drastically improved the cycle operation (>1,500 cycles) of LiNi1/3Mn1/3Co1/3O2 (NMC)3) and graphite using an optimized lithium salt and by introducing carboxymethylcellulose (CMC) at the interface. These materials are already mass produced; therefore, the active material cost will be equivalent to conventional lithium-ion batteries. There are no flammable vapors in the electrolyte (a pure polymer rather than a gel); therefore, an intrinsic safety improvement is expected for abused cells. In addition, a simple, multi-cell design using one external package is another advantage of all solid state batteries. Based on these advantages and renovations, we readopted such traditional SPE systems for stationary batteries. Here, we further improved the cycle performance by adding a new lithium salt and demonstrated the long term operation of a [Graphite | SPE | NMC] × 3 stack 12 V single-package multi-cell. Experimental The electrode materials were mixed with CMC, styrene-butadiene rubber (SBR) and conductive additives in a solvent without any SPEs before coating on the current collector. Subsequently, a polyether-based SPE (M=1.5 M, Daiso Co., Ltd.) was overcoated on the electrode using acetonitrile (AN). LiBF4 was used in the overcoat solution for NMC. In contrast, LiTFSI was used in the overcoat solution for graphite and the SPE sheet between the two electrodes. A lithium-ion cell composed of NMC, SPE, and graphite was fabricated using an Al-laminate-type cell. For the multi-cell, three cells (effective surface area 10 ×10 cm) were stacked in series in one package. All cells operated at 50°C. Results and Discussion We added 5 % LiBOB to the NMC overcoat solution. This LiBOB additive remarkably improved the reversibility, as shown in Fig. 1. The LiBOB additive was reported to suppress Al corrosion and/or Mn dissolution. While the detailed mechanism for these improvements is currently being studied, the high retention capacity obtained (70 % at 3,000 cycles) is practical for stationary use. Furthermore, over 1,000 operating cycles were successfully performed using a 10×10 cm three stack 12 V multi-cell, as shown in Fig. 2. The stacked multi-cell was vacuum sealed so no additional pressure was required during operations, which is significant progress for a large format battery design. References Y. Kobayashi et al., Journal of Power Sources, 146, 719 (2005).H. Miyashiro et al., Electrochem. Commun., 7, 1083 (2005).T. Kobayashi et al., ACS Applied Materials & Interfaces, 5, 12387 (2013). H. Xie et al., Electrochem. Solid State Lett., 11, C19 (2008). M. Xu et al., J. Electrochem. Soc., 160, A2005 (2013).

Electrical and mechanical properties of poly(ethylene oxide)/intercalated clay polymer electrolyte

Solvent-free solid polymer electrolytes (SPEs) based on two different poly(ethylene oxide), PEO Mw 600,000 and 4,000,000 and intercalated clays are reported. The inorganic additives used were lithiated bentonite and the nanocomposite PEO@bentonite with the same polymer used as matrix. SPE films, obtained in the scale of grams by mixing the components in a Brabender-type batch mixer and molding at 130 °C, were characterized by X-ray diffraction analysis, UV–vis spectroscopy, and thermal analysis. During the preparation of the films, the unmodified clay got intercalated in situ. Comparative analysis of ionic conductivity and mechanical properties of the films show that the conductivity increases with the inclusion of fillers, especially for the polymer with low molecular weight. This effect is more pronounced when using PEO@bentonite as additive. Under selected work conditions, avoiding the presence of crystalline lithium complexes, observed effects are mainly centered on the polymer. An explanation, considering the higher affinity between the modified clay and PEO matrix which leads to differences in the micro homogeneity degree between both types of polymer electrolytes is proposed.

High reversible capacities of graphite and SiO/graphite with solvent-free solid polymer electrolyte for lithium-ion batteries

The combination of graphite or silicon monoxide (SiO)/graphite = 1/1 mixture with a solvent-free solid polymer electrolyte (SPE) was fabricated using a new preparation process, involving precoating the electrode with vapor-grown carbon fiber (VGCF) and binders (polyvinyl difluoride: PVdF or polyimide: PI), followed by the overcoating of the SPE. The reversible capacity of [graphite | SPE | Li] and [SiO/graphite | SPE | Li] cells were >360 and >1000 mAh g −1 with 78% and 77% for the 1st Coulombic efficiency, respectively. The reversible capacities were 75% at the 250th cycle for [graphite | SPE | Li] and 72% at the 100th cycle for [SiO/graphite | SPE | Li]. The electrode used was compatible with that of the conventional liquid electrolyte system, and the SPE film could be formed on the electrode by the continuous overcoating process, which will lead to a low-cost electrodes and low-cost battery production. The solid-state lithium-ion polymer battery (SSLiPB) developed in this study, which consisted of [LiFePO 4 | SPE | graphite], showed the reversible capacity of 128 mAh g −1 (based on the LiFePO 4 capacity) with favorable cycle performance.

Preparation of hybrid organic–inorganic materials based on a di-ureasil matrix doped with lithium bis(trifluoromethanesulfonyl)imide

In this presentation we describe the preparation of solvent-free solid polymer electrolytes (SPEs) by the sol–gel route with the incorporation of controlled quantities of lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) into the host matrix. The host framework of these xerogels, designated as di-ureasils and represented by d-U(900), contains oxyethylene oligomers with about 15 repeat units bonded at each end to a siliceous backbone through urea bridging links. Electrolytes were characterized by ionic conductivity measurements, cyclic voltammetry at a gold microelectrode and thermal analysis. The results obtained reveal that these hybrid materials are completely amorphous and exhibit appropriate electrochemical characteristics for a variety of applications.

Synthesis of microphase‐separated solvent‐free solid polymer electrolyte nanocomposite films using amphiphilic urethane acrylate precursors

AbstractA new solvent‐free solid polymer electrolyte (SPE) films could be fabricated through bulk copolymerization process of amphiphilic urethane acrylate nonionomer (UAN). Amphiphilic UAN chain having polypropylene oxide‐based hydrophobic segment and polyethylene oxide‐based hydrophilic segment can not only dissolve lithium salt by complex formation with lithium cations but also be copolymerized with various monomers to form microphase‐separated polymeric matrix. Unlike conventional SPE systems showing higher conductivity with polar polymers and polar solvents, our SPE films prepared by copolymerization of UAN and hydrophobic monomers exhibited relatively higher conductivity. Dissolving lithium salts in UAN/hydrophobic monomer mixtures caused hydrophilic/hydrophobic microphase separation, which was more favorable for ionic conduction of lithium ions, resulting in the higher ionic conductivity than the SPE films fabricated using UAN/hydrophobic monomer mixture. This microphase‐separated structure of SPE films could be also confirmed by transmission electron microscope (TEM) images. Ionic conductivity of our SPE films could be also improved by dispersing clay minerals within SPE films. Three types of clay having different surface properties were used to fabricate clay/SPE nanocomposite films. Ionic conductivity of nanocomposite films depended on dispersibliity of clay nanoparticles with a SPE film, which was confirmed by measuring X‐ray diffraction and TEM. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007

Ion transport and battery discharge characteristics of polymer electrolyte based on PEO complexed with NaFeF4 salt

Ion-conducting solvent-free solid polymer electrolytes based on polyethylene oxide (PEO) complexed with sodium ferric tetrafluoride (NaFeF4) were prepared using a solution casting technique. The complexation of the films was investigated through X-ray diffraction and Fourier transform infrared spectroscopic studies. Measurements of DC conductivity in the temperature range 300–370 K and the transference numbers were carried out to investigate the nature of charge transport in the polymer electrolyte system. Transference number data shows that the charge transport in this system is found to be predominantly due to ions. Using these polymer electrolytes, electrochemical cells were fabricated with the configuration of Na/(PEO+NaFeF4)/(I2+C+electrolyte). Various cell parameters, such as open circuit voltage, short circuit current, power density, and energy density of the device were evaluated and reported.

Investigation on solvent-free solid polymer electrolytes for advanced lithium batteries and their performance

Seven solvent-free solid polymer electrolytes based on cross-linked polyethers and doped with LiN(CF 3SO 2) 2, were investigated for usage in large-scale rechargeable lithium batteries. They are classified into two categories: one contains five poly(ethyleneoxide-co-propyleneoxide) (poly(EO-co-PO)) network polymers with different degrees of cross-linking and precursor molecular weight and the other includes two polyethers with borate structure having a Lewis acid functionality as an anion trap. The ionic conductivity, direct current characteristics, thermal properties and transference numbers were measured. To evaluate the battery performance for the polymer electrolytes, test cells of the typical Li/solvent-free solid polymer electrolyte/LiCoO 2, were fabricated. High discharge performance was obtained for the solvent-free electrolytes made of the less cross-linked poly(EO-co-PO) and for the PEO including borate structure. A 10 Wh class lithium battery was entirely fabricated from solid components, and 500 charge–discharge cycles with 100% capacity retention was demonstrated. The present limitations and problems for further improvement of polymer batteries are also discussed.

Practical applications of modified poly(ethylene oxide) networks

Modified poly(ethylene oxide) (PEO) networks have been studied as phase transfer catalysts, flocculates and solvent-free polymer electrolytes. The activity of the hydrogels has been investigated with respect to the structure and crosslinking density of the networks, their degree of quaternization and amphiphilic properties (hydrophilicity coefficients). It has been found that the microenvironment of the active sites (EO segments and ammonium ions) affects the catalytic activity and sorption ability of the modified networks. Hydrophobic organic compounds such as sodium picrate and bromophenolblue are bound predominantly to the lipophilic quaternary ammonium ions. A stable level of electrical conductivity of 5.0×10 −5 S cm −1 was achieved without using of additives. A probable mechanism of ion transport within the networks has been proposed. Potential applications of PEO-based materials as solvent-free solid polymer electrolytes are also discussed.

Preparation and Characterization of a New Solvent-Free Polymer Electrolyte Based on Spiroketal Structure

Solvent-free solid polymer electrolytes based on spiropolymers were prepared and their properties were confirmed by conductance, differential scanning calorimetry, and X-ray diffraction measurements. The spiropolymer was synthesized from the bicyclic diketone and pentaerythritol. The spiro-polyketal (SP) dissolves lithium perchlorate and the conductivity of the complex is at 30°C and at 60°C. © 2001 The Electrochemical Society. All rights reserved.

Correlating the NMR self-diffusion and relaxation measurements with ionic conductivity in polymer electrolytes composed of cross-linked poly(ethylene oxide-propylene oxide) doped with LiN(SO2CF3)2

A solvent-free solid-polymer electrolyte based on a cross-linked poly(ethylene oxide-propylene oxide) random copolymer doped with LiN(SO2CF3)2, was studied using multinuclear NMR and ionic conductivity. The NMR spin-lattice relaxation times, T1, of the bulk polymer (1H), the lithium ion (7Li) and the anion (19F) were analyzed using single exponential analysis above the glass transition temperatures. Since the temperature dependent H1 and Li7 NMR T1 values had minima, the reorientational correlation times were obtained for the segmental motion of the CH2CH2O/CH2CH(CH3)O moiety of the bulk polymer and the hopping motion of the lithium ions correlated with the segmental motion. The spin–spin relaxation of the anion signals appeared single exponential with respect to time, whereas that of the polymer and the lithium echo signals were at least bi-exponential. Since both the spin-lattice and spin–spin relaxation of the anion indicated a single component, the self-diffusion coefficients, D, were measured using F19 pulsed-gradient spin-echo (PGSE) NMR measurements. Although the PGSE attenuation data appeared single exponential at each value of the separation between the gradient pulses, Δ, the measured D values had a Δ-dependence. Phenomenologically, the anion diffuses quicker in a shorter range and the activation energy of the shorter-time diffusion is smaller than that of the longer-time diffusion. The apparent self-diffusion coefficients became smaller for longer Δ and approached a constant when Δ was longer than 0.05 s. The mean square displacements of the anion were inconsistent with standard diffusion models including “anomalous diffusion” as found for a neutral particle diffusing in a fractal network [i.e., 〈r2(Δ)〉∝Δκ with κ<1(κ≡2/dw where dw is the random walk fractal dimension)]. The apparent diffusion coefficients of the lithium ions at Δ=0.02 s are almost independent of temperature and smaller than the corresponding diffusion coefficients of the anion. Since the activation energy of the anion determined for Δ longer than 0.05 s correlates well with those obtained from the ionic conductivity, the ion conduction in the solid-polymer medium is driven mainly by fast transfer of the anions.

Electrochemical study of Li4Ti5O12 as negative electrode for Li-ion polymer rechargeable batteries

Abstract Li4Ti5O12 a zero-strain insertion material was prepared by conventional method and by high energy ball milling (HEBM) of precursor to form nanocrystalline phases. The electrochemical performance of solid-state negative electrode was carried out using a solvent-free solid polymer electrolyte at 60°C and 80°C. A Li4Ti5O12 vs. lithium cell discharged at C/12 delivered 155 mA h g−1 for the conventional method and 157 mA h g−1 for the method using high ball energy milling, corresponding respectively to 97% and 96% first-cycle coulombic efficiency. The chemical diffusion coefficient of Li4Ti5O12 spinel-type compound is about 2×10−8 cm2 s−1. This is one order of magnitude higher than that of carbonaceous negative electrodes. Li4Ti5O12, which is a zero-strain insertion material, offers advantage for the SPE cell including safety, long life, and reliability.

Electrochromic windows using a solid polymer electrolyte

We have constructed an all-solid-state transmissive electrochromic device with a novel configuration: indium-tin-oxide (ITO)/tungsten compound film (TCF)/a solid polymer electrolyte (SPE)/Prussian blue film (PBF)/ITO. As the electrolyte, a solvent-free solid polymer electrolyte based on poly(ethylene oxide) is used. The fundamental performances of the cell as electrochromic windows with dynamic control of solar radiant energy are evaluated. The large coloration effeciency (η) of 0.14 cm 2 mC −1, almost equal to the sum of η's of TCF and PBF, is obtained. The response speed increases with increasing conductivity (σ) of the electrolyte in the range of σ below 10 −4 S cm −1 and the open circuit memory is maintained over two weeks. It is noteworthy that the transmittance of solar radiation can be arbitrary controlled between 14% and 56%. These results demonstrate that this cell is a promising candidate for the electrochromic windows.

Charge Transfer Between Polypyrrole Coated n-Si Electrodes And Solid Polymer Electrolytes

Abstract We have developed photoelectrochemical cells based on rectifying junctions between n-Si and thin film polymer solid electrolytes. We describe various surface modifications applied to the n-Si electrodes which overcome a high activation energy barrier for efficient charge transfer to redox ions in the polymer solid electrolytes. Solvent free solid polymer electrolytes represent an important new class of ionically conducting materials (1). Their compatibility with thin film technology makes them potential candidates for a wide variety of technological applications. We have developed a new type of photoelectrochemical cell (PEC) based on rectifying junctions between semiconductor electrodes and thin film polymer solid electrolytes. This holds out the hope of being able t o manufacture all thin film solid state PEC's without the semiconductor surface corrosion problems associated with liquid electrolyte PEC's. In addition, it allows easy construction of multispectral thin film cells based on more than one semiconductor material i n optical and electrical series (2). Our present study has focussed on surface modification techniques for improving the rates of charge transfer between semiconductor electrodes and redox ion couples i n polymer electrolytes (3). Specifically, we have shown that surface modifications of n-Si electrodes with thin films of polypyrrole can dramatically reduce the large activation energy barrier against efficient charge transfer between bare semiconductor electrodes and solid polymer electrolytes.