Tailored ZnO Nanostructure Based Quasi-Solid-State Electrolyte and Mesoporous Carbon Electrocatalyst for Solar Energy Conversion

Materials

Preparation of tailored ZnO nanostrucrues

Preparation of tailored ZnO quasi-solid-state electrolyte

Preparation of mesoporous carbon electrocatalyst based counterelectrode

Preparation of tailored ZnO quasi-solid-state electrolyte based DSSCs

Characterization

The morphologies were acquired by field emission scanning electron microscopy (FE-SEM) (SU8010, Hitachi, Tokyo, Japan). The crystalline structure of tailored ZnO was obtained by X-ray diffraction (XRD) spectroscopy (Smartlab, Rigaku, Tokyo, Japan) with Cu–Kα radiation (λ = 1.5406 Å). UV–vis diffuse-reflectance spectra were characterized by spectrophotometry (Mega-900, Scinco, Seoul, South Korea). The specific surface area and pore volume of the mesoporous carbon electrocatalysts were measured using the N₂ adsorption-desorption isotherm via the Brunauer–Emmett–Teller (BET) methods. Before the adsorption-desorption measurement, the mesoporous carbon was degassed at 70 °C under a dynamic vacuum (10⁻² Torr) for 1 h. Intensity-modulated photocurrent/voltage spectroscopy (IMPS/IMVS) was used to investigate electron diffusion and recombination properties. Thickness of photoanode and counterelectrode was measured using an alpha-step IQ surface profile (KLA Tencor). The current density–voltage (J–V) measurements were performed using a potentiostat (Compactstat.h, Ivium Technologies, Eindhoven, The Netherlands). The solar simulator equipped with a 1000 W xenon lamp was used as light source (SunLite, Abet Technologies, Inc., Milford, CT, USA). Electrochemical impedance spectroscopy (EIS) characteristics were measured by a potentiostat under open-circuit voltage in a frequency range of 0.01 Hz–100 kHz. Incident photo-to-electron conversion efficiency (IPCE) spectra were obtained by IPCE system (IPCEPEIPCE100S, HS Technologies, Plano, TX, USA). The photovoltaic parameters were calculated using Eqs. 1 and 2:

$$\begin{eqnarray}&&FF={V}_{\max }\cdot {J}_{\max }/{V}_{oc}\cdot {J}_{sc}\end{eqnarray} \tag{ 1 }$$

$$\begin{eqnarray}&&\eta ={V}_{\max }\cdot {J}_{\max }/{P}_{in}\cdot 100={V}_{oc}\cdot {J}_{sc}\cdot FF/{P}_{in}\cdot \,100\end{eqnarray} \tag{ 2 }$$

where η is the photovoltaic conversion efficiency, J_sc is short-circuit current density, V_oc is the open-circuit voltage, P_in is incident light power, FF is fill factor, and J_max and V_max are current density and voltage, respectively, when maximum power is achieved.

DSSCs fabricated with different electrolyte of (a) polymer gel (b) ZnO nanoalmond quasi-solid-state, and (c) ZnO nanorod quasi-solid-state is shown in Scheme 1. Redox couple transport in (a) polymer gel electrolyte is explained by polymer segment motion and ion diffusion of PEG that shows a low ionic conductivity in DSSCs due to the high crystallinity of the PEG crystalline region. In comparison, (b) and (c) shows DSSCs with quasi-solid-state electrolyte by using either ZnO nanoalmond and ZnO nanorods as a nanofiller. While ZnO nanostructure interacts with PEG main chain in an amorphous region, a free volume in the crystalline region of a quasi-solid-state electrolyte is created thus reduce the crystallinity of polymer main chain of PEG. Such changes in the amorphous region allows an effective path for redox couple transfer and leads to an enhanced ionic conductivity. As a result, this improves the photoelectrochemical performance in DSSCs. ^30,31

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Figure 1 shows the FE-SEM images of the ZnO structures (ZnO nanoalmond and ZnO nanorod) that were obtained through hydrothermal reaction with different reaction time of 12 h and 15 h, respectively. Both ZnO nanostructures were grown into a radial shape where the core of the homogeneous structure reaching towards the outside in a circular direction. Still, ZnO nanoalmond (a,b) and ZnO nanorods (c,d) were significantly distinguishable to one another by their shape, length and width of the nanostructure. ZnO nanoalmond shows a shape of multi layered almonds in its lengthwise direction that is similar to an oval shape with a sharpen vertex. Also, a number of flower-like shape was found at each center of the multi layered almond structures, similar to that of a daffodil. Lengths of each ZnO nanoalmond were found between 0.3 μm and 1 μm while the widths were found between 0.1 μm and 0.8 μm. ZnO nanorods showed a shape of multiple blades arranged in a radial form, heading towards outer circle from the center. Lengths of each nanorods were found between 0.9 μm and 3 μm, while the widths were between 0.1 μm and 0.8 μm. The structural difference of ZnO nanoalmonds and nanorods has been realized by the different reaction time of ZnO solution during the hydrothermal growth condition inside the autoclave.

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Figure 2a shows (a) the XRD patterns of ZnO nanalmonds and ZnO nanorod. Sharp peaks at 2 thata values of 31.84°, 34.52°, 36.33°, 47.63°, 56.71°, 62.96°, 68.13°, and 69.18° have been identically found in both ZnO nanostructures with lattice constants a = b = 0. Each peak corresponds to (100), (002), (101), (102), (110), (103), (200), (112), and (201) planes of wurtzite hexagonal crystalline phase (JCPDS file no. 36-1451). ³² This also indicates that the preparation of tailored ZnO nanostrucrues (ZnO nanoalmonds and ZnO nanorod) using the hydrothermal process has been successful. Figure 2b shows the scattering effect of each electrolytes that was characterized by UV–vis diffuse reflectance spectra. At short wavelength region between 300 nm and 550 nm, the diffuse reflectance value of all three electrodes was found at a very similar range. The sequence of diffuse reflectance showed ZnO nanorod > ZnO nanoalmond > polymer gel, while the maximum difference of each electrolytes were below 1.5%. However, at the wavelength between 550 nm and 740 nm, the difference in diffuse reflectance between ZnO nanorod quasi-solid-state electrolyte and two other electrolytes has gradually increased. At 740 nm, the diffusion reflectance of ZnO nanorods (13%) showed the approximately twice the amount of that of ZnO nanoalmond (7%) and polymer gel (6%). This comparably higher diffusion reflectance with a steep increasement in a particular wavelength indicates that the use of ZnO nanorods in quasi-solid-state electrolyte has enhanced scattering effect, which is advantageous for leading a higher light-harvesting property of DSSCs.

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Figure 3 shows the N₂ adsorption–desorption isotherm of mesoporous carbon electrocatalyst derived from PVDC-g-POEM double comb copolymer. Results of BET surface area (472.15 m² g⁻¹) and specific pore volume (0.74 cm³ g⁻¹) indicates the successful formation of highly porous carbon material with well-defined mesoporous structure was carried through a self-assembled double comb polymer template carbon source. ²⁶ Also, the mesoporous carbon electrocatalyst derived from PVDC-g-POEM double comb copolymer also exhibited a Type IV N₂ isotherm where the hysteresis loop approached relatively higher pressures, indicating multilayer adsorption followed by capillary condensation.

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Figure 4 shows the results of electron transport properties of three different types of electrolyte through diffusion coefficient (D_n), electron lifetime (τ_r), electron diffusion length (L_n), and electron collection efficiency (η_cc). As a function of J_sc , η_cc was characterized using IMPS/IMVS analysis to evaluate electron transport time (τ_t ) and electron lifetime (τ_r ), which the τ_t being converted to diffusion coefficient (D_n) as below:

$$\begin{eqnarray}&&{D}_{n}={d}^{2}/2.35{\tau }_{t}\end{eqnarray} \tag{ 3 }$$

while d represents the film thickness on FTO photoanode. Equations 4 and 5 was used to obtain electron diffusion length (L_n), and electron collection efficiency (η_cc) as below:

$$\begin{eqnarray}&&{L}_{n}={\left({D}_{n}{\tau }_{r}\right)}^{0.5}\end{eqnarray} \tag{ 4 }$$

$$\begin{eqnarray}&&{\eta }_{cc}=1-{\tau }_{t}/{\tau }_{r}\end{eqnarray} \tag{ 5 }$$

with τ_r and τ_t values being calculated by IMVS and IMPS analysis, respectively. ^33,34 Compared to polymer gel electrolyte, higher values of D_n and τ_r of DSSCs was achieved for both ZnO nanostructure (nanoalmond and nanorod) quasi-solid-state electrolyte. This further indicats that the electron transfer of photoelectrochemical devices has been enhanced by using ZnO as nanofiller in electrolyte. Moreover, both quasi-solid-state electrolyte with ZnO nanostructure (ZnO nanoalmond and ZnO nanorod) showed a higher L_n and η_cc values compared to polymer gel electrolyte at all ranges of Jsc. This indicates that the facile formation of amorphous region of polymer part in ZnO nanstructure quasi-solid-state electrolyte has provided an effective path for redox couple transfer, which has successfully led the enhanced the charge transport while reducing the recombination of the photoelectrochemical devices. Also, micrometer-sized inorganic nanofiller in ZnO nanostructure quasi-solid-state electrolyte could potentially lead to enhanced scattering effect on the interface of electrolyte/photoanode. It should be noted that similar observations have previously been reported regarding the influence of the micrometer-sized inorganic nanofiller on the electron transfer properties. ¹⁴

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Photovoltaic properties of mesoporous carbon based DSSCs with different electrolyte are shown in Fig. 5a through (a) J-V curves. Respective data of each photovoltaic parameters of short-circuit current density (J_sc), open-circuit voltage (V_oc), FF, and overall energy conversion efficiency (η) are listed in Table I. J_sc, V_oc, FF, and η value of mesoporous carbon based DSSCs with polymer gel electrolyte has shown 0.72 V, 10.0 mA cm⁻², 0.64, and 4.7% that is comparably lower than those values collected from ZnO nanstructure quasi-solid-state electrolyte based to other types of DSSCs. ZnO nanorods quasi-solid-state electrolyte has achieved the highest achieved values of 0.77 V, 11.4 mA cm⁻², 0.75 and 6.4%, which followed by ZnO nanoalmond based DSSC with values of 0.75 V, 10.9 mA cm⁻², 0.73, and 5.4%. The highest efficiency (6.4%) of DSSC achieved in this research by using ZnO nanorod quasi-solid-state electrolyte and mesoporous carbon electrocatalyst is also identified as the one of the highest values among other type of Pt-free counter electrode based DSSCs to date. ^35–39 Moreover, the J_sc of DSSC with ZnO quasi-solid-state electrolyte reports each 9% (ZnO nanoalmond) and 14% (ZnO nanorod) higher compared to that of DSSCs with polymer gel electrolyte. These results indicates that the introduction of ZnO to quasi-solid-state electrolyte has improved the light-harvesting effect of DSSCs by the enhancement in both ion diffusion of the redox couple and the scattering effect. Interestingly, the Voc value also increased up to 0.77 V using the ZnO nanorod quasi-solid-state electrolyte, which resulted from the decreased charge recombination, electron back reaction loss in mesoporous carbon based DSSCs. Figure 5b shows the equivalent circuit diagram of the mesoporous carbon based DSSCs with ZnO nanostructure quasi-solid-state electrolyte, while the corresponding electrochemical parameters being listed in Table II. Equivalent circuit model includes the series resistance (R_s ) that is the ohmic resistance of the FTO substrate, interfacial charge transfer resistance between counter electrode/electrolyte (R₁), interfacial charge transfer resistance between photoanode/electrolyte (R₂), and Warburg diffusion resistance (W_s ). While matching the equivalent circuit with the Nyquist impedance data at Fig. 5c, a significant decrease in the total resistance of mesoporous carbon based DSSCs with ZnO nanstructure quasi-solid-state electrolyte was found, compared to that of polymer gel. This can be due to the increase of electron transport in mesoporous carbon based DSSCs with ZnO nanostructure quasi-solid-state electrolyte that may lead to the decrease of resistance at the photoanode/electrolyte interface and enhanced FF values. Incident photon-to-electron conversion efficiency (IPCE) spectra in Fig. 5d demonstrates the light-harvesting efficiencies and the internal quantum of mesoporous carbon based DSSCs with ZnO nanostructure quasi-solid-state electrolyte at an incident wavelength. Maximum IPCE value of 70% and 68% was respectively found at the DSSCs with ZnO nanorod and ZnO nanoalmond quasi-solid-state electrolytes, which is comparably higher to that of DSSCs based on the polymer gel electrolyte (63%). Also, the IPCE spectra were integrated in order to calculate the photocurrent density gained and were compared with the actual photocurrents obtained by J-V measurements. As shown in Table I, the photocurrent density values from the J-V curves were well consistent with those determined from IPCE curves for for every types of mesoporous carbon based DSSCs. Improved IPCE value of the mesoporous carbon based DSSCs with the ZnO nanostructure quasi-solid-state electrolyte was due to the improved light harvesting efficiency that is consistent to the previous diffuse reflectance.

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For improved photoelectrochemical performance of mesoporous carbon based DSSCs, ZnO nanomaterials were prepared as a nanofiller in a PEG-based quasi-solid-state electrolyte to enhance ion diffusion, scattering effect, effective path for redox couple transfer, and sufficient penetration of quasi-solid-state electrolyte. Two types of ZnO nanomaterials of ZnO nanoalmond and ZnO nanorods were prepared by hydrothermal method with respective reaction hours. Mesoporous carbon electrocatalyst on the FTO counterelectrode substrate was prepared through the carbonization of PVDC-g-POEM double comb copolymers. The mesoporous carbon based DSSCs based on ZnO nanorods quasi-solid-state electrolyte (6.4%) showed a higher cell performance than that with ZnO nanoalmond quasi-solid-state electrolyte (5.4%), and polymer gel that was prepared as a reference of quasi solid-state electrolyte (4.7%). By adding ZnO nanofiller to a quasi-solid-state electrolyte, significant improvement has been achieved in the light-harvesting, charge transport, reduces recombination, and decreases the resistance at the electrode/electrolyte interface of DSSCs. We believe that a promising method of fabricating a mesoporous carbon based photoelectrochemical application with enhanced performance has been successfully presented by introducing shape controlled ZnO nanostructures and mesoporous carbon electrocatalyst.

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (NRF-2019R1C1C1010283). This paper was written as part of Konkuk University's research support program for its faculty on sabbatical leave in 2021.