First Author: Yushen Xiao
Corresponding Authors: Kai-Hang Ye and Songcan Wang
DOI: 10.1002/adfm.202528859
Research Highlights
Conventional photoelectrochemical (PEC) water splitting generally involves the kinetically sluggish oxygen evolution reaction (OER), which not only limits the hydrogen evolution reaction but also produces oxygen with relatively low economic value. In this study, a Nafion/TiO2-x photoanode was designed to couple the PEC glucose oxidation reaction (GOR) with hydrogen production. Charge transfer between the Nafion layer and TiO2-x effectively suppresses nonradiative recombination caused by vacancy-related surface traps. It also induces band bending, thereby enhancing the directional separation of photogenerated charge carriers. Compared with pristine TiO2, the photoanode delivers a fivefold increase in photocurrent density, excellent long-term stability, and an absorbed-photon-to-current efficiency of up to 100%. In addition, the glucose conversion reaches 97.8%, with an 80% Faradaic efficiency for the selective oxidation of glucose into the high-value product glucaric acid. The Faradaic efficiency for cathodic hydrogen production reaches 99%, enabling the simultaneous production of value-added organic chemicals and hydrogen. The Nafion/TiO2-x photoanode also exhibits an ultralow detection limit and a linear response at very low concentrations, demonstrating its potential for photoelectrochemical sensing.
Introduction
In February 2026, Advanced Functional Materials published online the latest photoelectrochemical research from Professor Shanqing Zhang's team at Guangdong University of Technology. The study reports a new photoanode in which oxygen defects and self-assembled Nafion are introduced onto a TiO2-x electrode, enabling highly selective PEC oxidation of glucose to glucaric acid while simultaneously coupling the reaction with cathodic hydrogen production. The first author is Yushen Xiao, and the co-corresponding authors are Associate Professor Kai-Hang Ye and Professor Songcan Wang of Northwestern Polytechnical University.
Background
Driven by rising global energy demand and carbon-neutrality goals, photoelectrochemical (PEC) hydrogen production has become an important direction in renewable-energy technology because of its high energy-conversion efficiency and relatively low system cost. However, its development is constrained by two major bottlenecks. First, the sluggish kinetics of the oxygen evolution reaction (OER) at the photoanode limit solar-to-hydrogen conversion efficiency. Second, the oxygen by-product has very low economic value, weakening the overall economics of the system. To address these challenges, researchers have proposed coupling cathodic hydrogen production with the synthesis of value-added chemicals at the photoanode. The glucose oxidation reaction (GOR) has attracted considerable attention because it can generate high-value bio-based chemicals such as gluconic acid and has a much lower reaction barrier than OER. Nevertheless, practical application remains challenging. The valence-band potentials of most photoanode materials satisfy the thermodynamic requirements of both GOR and OER, allowing OER to compete for photogenerated holes and significantly reduce target-product yield and energy utilization. Although reported platinum-modified catalysts can enhance GOR activity, the high cost of precious metals and the intrinsic OER activity of platinum may intensify the competing reaction, making it difficult to achieve both cost-effectiveness and high selectivity. To overcome these limitations, this study developed a Nafion-modified oxygen-vacancy TiO2 photoanode (Nf/TiO2-x). Through the combined effects of interfacial band-bending regulation, defect-state passivation, and active-site optimization, the photoanode enhances GOR activity while effectively suppressing the competing OER, enabling highly selective production of gluconic acid. This design provides a new strategy for developing low-cost, highly selective GOR photoanodes and offers a technically feasible route toward scalable systems that couple PEC hydrogen production with value-added chemical synthesis.
Equipment Used in the Study
Figure Analysis
Figure 1. Scheme 1 from the paper: (a) Schematic illustration of the light source and PEC cell configuration; (b) structure of the Nf/TiO2-x photoanode; and (c) organic oxidation process on the Nf/TiO2-x photoanode.
Key Points: The coupled photoelectrochemical reaction system uses Nf/TiO2-x as the photoanode and a Pt plate as the cathode. Under AM 1.5G simulated sunlight, it simultaneously drives the anodic glucose oxidation reaction (GOR) and the cathodic hydrogen evolution reaction (HER), as illustrated in Scheme 1a. The key structural feature of the Nf/TiO2-x photoanode is an ultrathin Nafion (Nf) modification layer uniformly self-assembled on the surface of vertically grown TiO2 nanorods, as shown in Scheme 1b. At the Nafion/TiO2 heterointerface, the strong electron-withdrawing character of Nafion drives defect electrons associated with oxygen vacancies from the TiO2 surface toward the Nafion layer, inducing upward band bending at the TiO2 surface. This band bending creates a built-in electric field at the interface, providing the driving force for directional migration of photogenerated holes toward the electrode surface and fundamentally improving charge-carrier separation. Through the combined effects of the interfacial built-in electric field, passivation of oxygen-vacancy-related surface defect states by the Nafion coating, and precise regulation of surface-reaction active sites, the Nf/TiO2-x photoanode enables efficient separation and utilization of photogenerated charge carriers, substantially accelerates GOR kinetics, and effectively suppresses the competing OER. The synergistic mechanism is illustrated in Scheme 1c.
Figure 2. Figure 1 from the paper: (a) Schematic synthesis procedure for Nf/TiO2-x; (b) high-resolution scanning electron microscopy (HR-SEM) image of Nf/TiO2-x; (c) cross-sectional HR-SEM image of Nf/TiO2-x; (d) transmission electron microscopy (TEM) image of Nf/TiO2-x; corresponding elemental maps of (e) Ti, (f) O, (g) S, and (h) F; (i) high-resolution transmission electron microscopy (HR-TEM) image of Nf/TiO2-x; (j) Fourier-transform infrared (FT-IR) spectra of Nf/TiO2-x; (k) O 1s XPS spectra of TiO2, TiO2-x, and Nf/TiO2-x; (l) scanning transmission electron microscopy (STEM) image; and (m) O K-edge and O pre-peak electron energy-loss spectra (EELS) of Nf/TiO2-x from the bulk to the surface.
Key Points: Vertically aligned rutile TiO2 nanorod arrays were first synthesized on an FTO substrate using a hydrothermal method followed by calcination. The nanorods had an axial length of approximately 800 nm and an average diameter of approximately 120 nm. Oxygen vacancies were then introduced through electrochemical reduction, followed by the self-assembly of an ultrathin Nafion coating on the TiO2-x nanorod surface without altering the original TiO2 morphology. Multiple characterization techniques verified the material structure and composition. SEM and TEM images showed well-dispersed nanorods, while HR-TEM revealed 0.259 nm lattice fringes corresponding to the TiO2-x (110) plane and a several-nanometer-thick disordered Nafion surface layer. XRD confirmed that the rutile TiO2 crystal structure was retained after modification. EDS elemental mapping showed uniform distributions of O, Ti, S, and F, confirming the homogeneous loading of Nafion. Characteristic Nafion absorption bands were detected by both FT-IR and Raman spectroscopy, while a characteristic SO3- peak appeared in the O 1s XPS spectrum, further confirming the successful introduction of Nafion. EPR spectroscopy verified the presence of oxygen vacancies, and EELS showed a gradient distribution in which the oxygen-vacancy concentration was enriched at the surface and gradually decreased toward the bulk. This structure promotes the migration of defect electrons from the bulk to the surface, providing the basis for band bending.
Figure 3. Figure 2 from the paper: (a) Linear sweep voltammetry curves of TiO2, TiO2-x, and Nf/TiO2-x measured under AM 1.5G illumination at a scan rate of 25 mV s-1 in 0.1 M Na2SO4 solution at pH 11, using 0.1 M Na2SO3 as a hole scavenger; (b) APCE and IPCE spectra collected over an incident-wavelength range of 370–450 nm at 0.6 V vs. RHE in 0.1 M Na2SO4 electrolyte containing 0.1 M Na2SO3; (c) LSV curves of TiO2-x and Nf/TiO2-x measured under AM 1.5G illumination at a scan rate of 25 mV s-1 in 0.1 M Na2SO4 electrolyte with and without 0.5 M glucose; (d) surface charge-injection efficiencies (ηinj) of TiO2-x and Nf/TiO2-x; and (e) H2 production, gluconic-acid production, and corresponding Faradaic efficiencies of the Nf/TiO2-x photoanode at 0.6 V vs. RHE in 0.1 M Na2SO4 containing 0.5 M glucose under AM 1.5G illumination at 100 mW cm-2.
Key Points: A series of photoelectrochemical measurements revealed how surface-defect engineering and the self-assembled Nafion film regulate glucose oxidation on TiO2 photoanodes. Under AM 1.5G illumination in a glucose solution at pH 11 and at 1.2 V vs. RHE, the photocurrent densities of pristine TiO2, oxygen-vacancy-modified TiO2-x, and Nafion-modified Nf/TiO2-x were 0.54, 1.56, and 1.95 mA cm-2, respectively. Oxygen vacancies markedly improved performance by enhancing charge-separation efficiency, and Nafion further strengthened this effect. IPCE and APCE measurements showed that Nf/TiO2-x achieved a maximum IPCE of 83.44%, exceeding the 74.61% obtained for TiO2-x, while its APCE reached 97.41% at 0.6 V vs. RHE, confirming highly efficient utilization of absorbed photons. Selectivity measurements showed that the glucose-oxidation current of Nf/TiO2-x exceeded its water-oxidation current by 1.00 mA cm-2. Its glucose catalytic efficiency reached 99.21%, representing a 45.08% improvement over TiO2-x, while water-oxidation activity was substantially reduced. This behavior arises from the selective regulation provided by the hydrophobic Nafion interface, which restricts water molecules from accessing active sites while preferentially adsorbing glucose. In stability tests, Nf/TiO2-x retained 95% of its initial current after 40,000 s of continuous operation, demonstrating excellent long-term stability. Product analysis showed a cathodic hydrogen-evolution Faradaic efficiency above 98%, a glucose conversion of 97.8%, and a glucaric-acid yield of 81.3%. The reaction kinetics followed a multistep cascade pathway of glucose → gluconic acid → glucuronic acid → glucaric acid. The decline in yield during the later stage was attributed to overoxidation of glucaric acid, consistent with previous reports.
Figure 4. Figure 3 from the paper: (a) EIS spectra under AM 1.5G illumination in 0.1 M Na2SO4 solution with and without 0.5 M glucose, together with optical absorption spectra of TiO2-x and Nf/TiO2-x at 0.6 V vs. RHE; (b) representative IMPS responses of TiO2 and Nf/TiO2-x photoanodes measured from 0 to 0.4 V vs. RHE in 0.1 M Na2SO4 solution; (c) representative IMPS responses of TiO2 and Nf/TiO2-x photoanodes measured over the same potential range in 0.1 M Na2SO4 containing 0.5 M glucose; in situ ATR-FTIR spectra of (d) TiO2 and (e) Nf/TiO2-x collected from the open-circuit potential (OCP) to 0.6 V vs. RHE in 45 mV increments; (f) time-resolved in situ ATR-FTIR spectra of Nf/TiO2-x collected from 10 to 180 min; and (g) schematic illustration of photoelectrochemical glucose oxidation to gluconic acid accompanied by cathodic hydrogen production.
Key Points: The photoelectrochemical mechanism of the Nf/TiO2-x photoanode was systematically investigated using photoelectrochemical measurements and in situ ATR-FTIR spectroscopy. Electrochemical impedance spectroscopy (EIS) and fitting with a dual resistance-capacitance equivalent circuit showed that Nf/TiO2-x had higher charge-injection efficiency for GOR but lower charge-injection efficiency for OER. In the glucose-free system, introducing Nafion increased the charge-transfer resistance at both the TiO2-x/Nafion and Nafion/electrolyte interfaces, suppressing the participation of photogenerated holes in OER. In the glucose-containing system, however, the Nafion coating reduced the series resistance and both interfacial charge-transfer resistances, accelerated bulk charge transport, and enhanced the transfer of photogenerated holes into the aqueous glucose solution. Intensity-modulated photocurrent spectroscopy (IMPS) further confirmed that, in the absence of glucose, the hydrophobic Nafion layer reduced the charge-transfer rate of Nf/TiO2-x, thereby weakening the competing OER. In the glucose-containing system, neither photoanode displayed a semicircle in the low-frequency IMPS region, indicating the absence of charge recombination at the electrode-electrolyte interface. In situ ATR-FTIR spectra showed that characteristic O-C-O absorption bands associated with GOR products appeared at lower potentials on Nf/TiO2-x than on pristine TiO2-x, demonstrating substantially higher GOR activity. Meanwhile, the absorption band assigned to the OER intermediate *OOH showed no significant enhancement, confirming that Nafion effectively suppresses the competing OER. Time-resolved in situ infrared spectroscopy also detected the time-dependent formation of a characteristic COO- band associated with reaction intermediates, supporting the formation of glucaric acid as the final product.
Figure 5. Figure 4 from the paper: (a) PL spectra of TiO2-x and Nf/TiO2-x; (b) TRPL spectra of TiO2-x and Nf/TiO2-x; (c) Ti 2p XPS spectra of TiO2-x and Nf/TiO2-x; calculated charge-density differences of (d) TiO2, (e) TiO2-x, and (f) Nf/TiO2-x, where blue indicates charge depletion and yellow indicates charge accumulation; (g) band-bending diagram of Nf/TiO2-x; and in situ XPS spectra of Nf/TiO2-x: (h) O 1s and (i) S 2p.
Key Points: To clarify how surface defects and Nafion self-assembly enhance the glucose oxidation performance of TiO2 photoanodes, the mechanism was systematically examined from the perspectives of charge separation and surface catalysis. Photoluminescence (PL) and time-resolved photoluminescence (TRPL) measurements showed a substantial increase in PL emission intensity and an extension of the carrier lifetime to 4.60 ns after Nafion treatment of TiO2-x. This improvement arises because the sulfonic-acid groups in Nafion passivate oxygen-vacancy-induced surface trap states and suppress nonradiative electron-hole recombination. Their electron-accepting character also facilitates photogenerated charge separation. In the XPS spectra, the Ti 2p peaks of Nf/TiO2-x shifted by 0.6–0.8 eV toward higher binding energy. Combined with density functional theory (DFT) simulations of differential charge density, these results confirm that bonding between sulfonic-acid groups and the TiO2 surface induces electron transfer from TiO2 to Nafion and causes band bending in TiO2. UPS, CV, and band-structure analyses further indicate that this band bending drives the directional separation of photogenerated holes from the TiO2 valence band toward Nafion, improving charge separation in the photoanode. The decrease in O 1s binding energy and increase in S 2p binding energy observed by in situ XPS before and after illumination directly verify this charge-transfer direction. Together, these mechanisms reduce photogenerated carrier recombination, provide more active holes for GOR, and ultimately improve GOR performance.
Figure 6. Figure 5 from the paper: Water contact angles of (a) TiO2, (b) TiO2-x, and (c) Nf/TiO2-x; (d) adsorption energies of H2O and glucose on TiO2, TiO2-x, and Nf/TiO2-x; and adsorption models of (e) water molecules and (f) glucose molecules on TiO2, TiO2-x, and Nf/TiO2-x.
Key Points: In addition to the charge-separation effect, the enhanced GOR performance and reduced water-oxidation activity of the Nf/TiO2-x photoanode can also be explained by surface catalysis. Contact-angle measurements showed that pristine TiO2 was hydrophilic, with a water contact angle of only 18.9°. After oxygen vacancies were introduced by electrochemical reduction, the contact angle increased to 100.5°, indicating enhanced hydrophobicity. Nafion modification further increased the contact angle to 164°, and this strong hydrophobicity effectively minimized interference from water. Adsorption-energy calculations and first-principles simulations showed that, during modification from TiO2 to TiO2-x and then to Nf/TiO2-x, the adsorption energy of water increased monotonically from -1.192 to -0.901 eV, indicating weaker adsorption, whereas the adsorption energy of glucose decreased monotonically from -1.36 to -2.911 eV, indicating stronger adsorption. On stoichiometric TiO2, neither water nor glucose exhibited specific adsorption. After oxygen vacancies were introduced, glucose preferentially bound to undercoordinated Ti3+ sites near the vacancies, while water occupied more distant Ti4+ sites. Following Nafion modification, the perfluorosulfonic-acid chains preferentially anchored at vacancy sites through Ti-O-S bonds, producing a molecular gradient that favored glucose adsorption near Nafion-covered vacancy clusters while excluding water from nonfunctionalized regions. The coating simultaneously passivated defect states, ultimately improving overall GOR performance by promoting selective glucose adsorption.
Conclusion
In summary, this study developed a low-cost, high-performance Nf/TiO2-x photoanode for the photoelectrochemical glucose oxidation reaction. At 0.8 V vs. RHE in a glucose-containing Na2SO4 solution, the photoanode achieved a GOR photocurrent density of 1.96 mA cm-2. It delivered an APCE of 100% and excellent stability, retaining 95% of its current after 40,000 s of continuous operation. The glucose conversion reached 97.8%, with an 80% Faradaic efficiency for selectively oxidizing glucose into the high-value product gluconic acid. Meanwhile, the Faradaic efficiency for cathodic hydrogen production reached 99%. Coupling these processes enables the simultaneous production of value-added organic chemicals and hydrogen. In situ infrared spectroscopy and theoretical calculations confirmed that the Nafion layer enhances the selectivity of the glucose oxidation reaction. Charge transfer between Nafion and TiO2-x effectively suppresses nonradiative recombination associated with vacancy-related surface traps while inducing band bending that promotes the directional separation of photogenerated charge carriers. This self-assembled Nafion-modified TiO2-x photoanode employs a straightforward surface-modification strategy that simultaneously improves charge separation in TiO2 and glucose-oxidation selectivity, providing an innovative and reproducible solution for the co-production of value-added chemicals and clean energy.
About the Author
Shanqing Zhang is a professor at Guangdong University of Technology. His primary research interests include the development of nanosensors and energy-storage devices based on various metal-ion battery chemistries.
In 2009, he received an Australian Research Council Future Fellowship. In 2016, he was promoted to full professor at Griffith University, Australia. In 2020, he was elected a Fellow of the Royal Australian Chemical Institute (FRACI) and a Fellow of the Royal Society of Chemistry (FRSC). From 2023 to 2025, he was named a Clarivate Highly Cited Researcher and ranked among the world's top 2% of scientists. He has published more than 380 papers in leading international journals, including Proceedings of the National Academy of Sciences, Chemical Reviews, Nature Communications, Journal of the American Chemical Society, Angewandte Chemie International Edition, Energy & Environmental Science, Advanced Materials, Analytical Chemistry, and Environmental Science & Technology. His publications have received more than 29,000 citations, with an H-index of 95.
Research Group Website: https://gdutist.cn
Reference
Yushen Xiao, Junchen Wang, Tongxin Tang, Kai-Hang Ye, et al. Synergistic Defect Engineering and Self-Assembled Nafion on Photoanodes Enabling Selective Photoelectrochemical Glucose Oxidation Coupled H2 Production.
Advanced Functional Materials 2026, e28859.
https://advanced.onlinelibrary.wiley.com/doi/10.1002/adfm.202528859
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