Ni–NC and Fe–NC single-atom catalysts on Ti-doped hematite for selective photoelectrochemical glycerol oxidation

Abstract

Glycerol, a major byproduct of biodiesel production, can be selectively oxidized into high-value chemicals; however, efficient and selective conversion remains challenging due to complex C–O bond cleavage pathways. Photoelectrochemical glycerol oxidation with α-Fe₂O₃ photoanodes could offer a sustainable solution, but α-Fe₂O₃ suffers from inadequate charge separation and surface catalytic activity. Herein, we address these challenges by constructing hybrid photoanodes through integration of Ti-doped α-Fe₂O₃ (Ti–Fe₂O₃) with single metal atom catalysts in N-containing carbon (M–NC, M = Fe or Ni). The structural and functional synergy of Ti–Fe₂O₃/M–NC photoanodes was investigated using physicochemical characterization, photoelectrochemical measurements, and density functional theory (DFT) calculations. Formation of a Schottky junction at the Ti–Fe₂O₃/M–NC interface generates a built-in electric field that enhances charge-carrier separation and directional hole transfer to M–NC active sites. Consequently, Ti–Fe₂O₃/Ni–NC and Ti–Fe₂O₃/Fe–NC achieve photocurrent densities of 2.2 and 1.9 mA cm⁻² at +1.23 V_RHE, representing ∼3- and ∼2.4-fold improvements over bare Ti–Fe₂O₃ (0.8 mA cm⁻²). Moreover, M–NC incorporation enables tuning of the product selectivity: Ni–NC favors glyceric acid formation (∼80% selectivity), whereas Fe–NC promotes glycolic acid production (∼74% selectivity) at +0.8 V_RHE. With increasing applied potential (from +0.8 to +1.2 V_RHE), Ni–NC induces pronounced changes in product distribution to formic acid, while Fe–NC maintains high glycolic acid selectivity with enhanced yields. DFT calculations attribute these metal-dependent selectivity trends to intrinsic differences in oxidation activity between Fe–NC and Ni–NC. Selectivity changes of Ni–NC may also arise from oxidized Ni clusters formation, as confirmed by post-reaction microscopy. This work highlights the potential of Ti–Fe₂O₃/M–NC photoanodes for selective biomass valorization and solar-driven photoelectrochemical reactions.

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1 Introduction

As a major byproduct of biodiesel production, glycerol (C₃H₈O₃) can be selectively oxidized into value-added chemicals such as dihydroxyacetone (DHA), lactic acid (LA), glyceric acid (GLA), glycolic acid (GA), and formic acid (FA).^1,2 The glycerol oxidation reaction (GOR) involves complex cleavage of C–O bonds at different molecular positions, necessitating the development of highly selective catalysts with well-defined active sites to enhance reaction kinetics and product selectivity.³ Photoelectrochemical glycerol oxidation (PEC GOR) using n-type transition metal-based photoanodes such as hematite (α-Fe₂O₃),⁴ bismuth vanadate (BiVO₄),⁵ graphitic carbon nitride (g-C₃N₄),⁶ or different metal oxides represents a sustainable approach for glycerol valorization under mild conditions.^7,8 These photoanodes are attractive due to their abundance, affordability, and favorable interactions with carbon-containing species, enabling environmentally benign production of solar-driven chemicals.^9,10 Among various n-type transition metal-based electrodes, hematite has been widely studied as a photoanode material owing to its suitable bandgap (∼2.0 eV) for solar absorption, excellent chemical stability, and low cost.¹¹ However, α-Fe₂O₃ suffers from intrinsic limitations, including low charge-carrier mobility (∼10⁻² cm² V⁻¹ s⁻¹), poor electrical conductivity (10⁻¹⁴ Ω⁻¹ cm⁻¹), short minority carrier diffusion lengths (2–4 nm), and poor surface catalytic activity, which collectively hinder charge transport, separation, and interfacial reaction kinetics.^7,8 To mitigate these challenges, the integration of co-catalysts with α-Fe₂O₃ to construct a composite catalytic system has emerged as a promising approach.

Among different co-catalysts, earth-abundant transition-metal-based catalysts have emerged as promising candidates for GOR, with Ni-based catalysts being particularly attractive due to their relatively low overpotentials.^12,13 In addition, single-atom catalysts have gained considerable attention due to their unique and tunable electronic structures, maximized atomic utilization, and uniform active centers.^14,15 Anchoring precious metal single atoms directly on metal oxide photoelectrodes can enhance surface reaction kinetics. For instance, Feng et al. designed Pt single site catalysts on the surface of WO₃, showing current densities of up to 2.85 mA cm⁻² and 60% faradaic efficiency toward dihydroxyacetone production.¹⁶ However, directly anchoring precious metal single sites onto the oxide surface of photoanodes present several challenges: (1) limited control over the coordination environment, which strongly influences band structure and reactivity,¹⁷ (2) leaching of precious metals in commonly used alkaline electrolytes,¹⁸ and (3) variable behavior of such sites, which may act as active catalysts or as spectators.¹⁹

In this context, transition-metal single atoms anchored on N-doped carbon (M–NC), such as Ni–NC and Fe–NC, have emerged as a promising alternative. Their synthesis typically requires high-temperature pyrolysis (800–1000 °C) to achieve sufficient electrical conductivity for electrochemical reactions. However, such conditions lead to undesirable side reactions during pyrolysis, such as carbothermal reduction of metal species at temperatures above 600 °C.^20,21 This limitation has been addressed through decoupled synthesis strategies employing inactive metal templates (e.g., Mg or Zn) that facilitate MN_x site formation without undergoing carbothermal reduction.^22,23 Subsequently, active metals such as Ni or Fe can be introduced at lower temperatures without affecting the bulk chemical or morphological properties of the NC support, enabling systematic structure–activity investigations of different single metal atoms on equivalent supports.^24–26 Owing to their excellent redox activity and tunable d-orbital electronic configurations, M–NC catalysts have demonstrated notable performance in various electrocatalytic reactions, including O₂, NO₃⁻ and CO₂ reduction.^25–27 Additionally, M–NC materials have been explored as bifunctional O₂ evolution and reduction systems;²⁸ however, the stability of their active sites under strongly oxidizing and reducing conditions remain a concern. For instance, Wan et al. reported the transformation of M–NC (M = Co, Ni or Fe) into amorphous (oxy)hydroxide clusters under O₂ evolution reaction conditions.²⁹ Furthermore, the NC support itself may undergo denitrogenation and carbon corrosion,^30–33 depending on its degree of graphitization. In contrast, GOR exhibits a significantly lower thermodynamic equilibrium potential than the O₂ evolution reaction (e.g. +0.091 V_RHE for glycerol oxidation to GLA versus +1.23 V_RHE for O₂ evolution),¹⁰ suggesting that M–NC catalysts could operate more stably in the less oxidative GOR environment. Nevertheless, the role of M–NC catalysts in PEC GOR systems remains largely unexplored, particularly with respect to their influence on photogenerated charge-carrier dynamics, reaction pathways, product selectivity, and operational stability.

Herein, we report the development of hybrid photoanodes composed of Ti-doped α-Fe₂O₃ (Ti–Fe₂O₃) integrated with single-atom M–NC catalysts (Fe–NC or Ni–NC) for enhanced PEC GOR. By combining physicochemical characterization, photoelectrochemical analysis, and density functional theory (DFT) calculations, we elucidate the role of M–NC catalysts in modulating surface reaction kinetics, improving photogenerated charge separation, and regulating product selectivity. The formation of a Schottky junction at the Ti–Fe₂O₃/M–NC interface facilitates charge-carrier separation and hole transfer to atomically dispersed active sites, resulting in markedly improved PEC performance. Moreover, Ti–Fe₂O₃/Ni–NC and Ti–Fe₂O₃/Fe–NC delivered photocurrent densities of 2.2 and 1.9 mA cm⁻² at +1.23 V_RHE, representing ∼3- and ∼2.4-fold enhancements over bare Ti–Fe₂O₃, respectively. M–NC incorporation also tuned product selectivity, with Ni–NC favoring glyceric acid (∼80%) and Fe–NC favoring glycolic acid (∼74%) at +0.8 V_RHE. At higher potentials, Ni–NC shifted selectivity toward formic acid, whereas Fe–NC retained high glycolic acid selectivity with improved yields. The distinct selectivity trends of Ni–NC and Fe–NC toward different glycerol oxidation products, highlighting the tunability of reaction pathways through the doped metal. This work demonstrates a promising strategy for integrating single-atom M–NC catalysts with hematite-based photoanodes, advancing the development of efficient and selective solar-driven biomass valorization systems.

2 Results and discussion

The preparation of Ti–Fe₂O₃/M–NC (M = Ni or Fe) photoanodes is schematically illustrated in Fig. 1a and described in detail in the Experimental Section (SI). Briefly, Ti–Fe₂O₃ photoanodes were obtained by annealing hydrothermally grown Ti-doped β-FeOOH films in air up to 800 °C. Ti doping was employed to enhance electrical conductivity, reduce recombination, and improve charge separation in hematite, thereby increasing the photocurrent.³⁴ 5% Ti doping level was selected based on our previous work.³⁵ Subsequently, the surface was modified with Ni–NC or Fe–NC single-atom catalysts prepared via a decoupled cation exchange protocol employing 2,4,6-triaminopyrimidine as an organic building block and MgCl₂·6H₂O as active site template and porogen.^25,36 The crystalline structures of the prepared samples were examined by X-ray diffraction (XRD). As shown in Fig. 1b, Ti–Fe₂O₃ exhibited characteristic diffraction peaks at 24.1, 33.2, 35.6, 49.5, and 54.3°, corresponding to the hematite (α) Fe₂O₃ phase (JCPDS No. 33-0664). Additional reflections at 26.6, 37.9, 51.8, 61.9, and 65.9° arose from the fluorine-doped tin oxide (FTO, JCPDS No. 41-1445) coated on the glass substrate. No additional diffraction peaks were observed after Ni–NC or Fe–NC loading, indicating the absence of detectable crystalline Ni- or Fe-containing phases. This is consistent with atomically dispersed metal sites embedded within a predominantly amorphous (partially graphitized) NC matrix or with loadings below the XRD detection limit. The morphology of the photoanodes was further investigated by field-emission scanning electron microscopy (FE-SEM). As shown in Fig. 1c–e and S1, Ti–Fe₂O₃ formed a highly porous nanostructure uniformly covering the FTO substrate, consisting of micrometer-scale agglomerates composed of ∼100 nm nanoparticles. After deposition of M–NC catalysts, the porous Ti–Fe₂O₃ framework remained largely exposed, while additional thin nanosheet-like features indicated with orange arrows attributed to the Ni–NC or Fe–NC species were observed (Fig. S1), showing that M–NC is uniformly loaded on the Ti–Fe₂O₃ porous layer and confirming the successful incorporation of the catalysts on the photoanode absorption layer.

Fig. 1 (a) Schematic illustration of the synthesis procedure for Ti–Fe₂O₃/M–NC (M = Ni or Fe) photoanodes. (b) XRD patterns of Ti–Fe₂O₃, Ti–Fe₂O₃/Ni–NC, and Ti–Fe₂O₃/Fe–NC. (c–e) FE-SEM, acquired using the in-column secondary electron (InLens) detector, of Ti–Fe₂O₃, Ti–Fe₂O₃/Ni–NC, and Ti–Fe₂O₃/Fe–NC. (f) High-resolution C 1s XPS spectra of Ti–Fe₂O₃, Ti–Fe₂O₃/Ni–NC, and Ti–Fe₂O₃/Fe–NC. (g) High-resolution N 1s XPS spectra of Ti–Fe₂O₃/Ni–NC and Ti–Fe₂O₃/Fe–NC. (h–k) HAADF-STEM micrographs and corresponding STEM-EDS elemental mappings of Ni–NC. Scale bars are identical for figure (h–k).

X-ray photoelectron spectroscopy (XPS) was employed to investigate the surface chemical composition and electronic states of the Ti–Fe₂O₃/M–NC hybrid photoanodes. The survey spectra (Fig. S2a) confirmed the presence of Ti, Fe, O, N, and C in the Ti–Fe₂O₃/M–NC samples. Owing to the pronounced multiplet splitting characteristic of Fe 2p core levels,³⁷ the Fe 2p spectra (Fig. S2b) were not deconvoluted; importantly, their overall spectral features remain essentially unchanged after M–NC loading, indicating that the electronic environment of Fe in Ti–Fe₂O₃ is not significantly perturbed by the incorporation of M–NC catalysts. High-resolution C 1s spectra of pristine Ti–Fe₂O₃ (Fig. 1g) were deconvoluted into four components corresponding to C–C, C–O, C=N–C, and O–C=O species. Upon incorporation of Ni–NC in Ti–Fe₂O₃/Ni–NC, broadened features were observed due to overlapping contributions from C–N and C–O bonds originating from the N-doped carbon framework of the M–NC catalyst. The corresponding N 1s spectra of Ti–Fe₂O₃/Ni–NC and Ti–Fe₂O₃/Fe–NC (Fig. 1h) exhibited four characteristic nitrogen species, assignable to pyridinic N, metal–N (M–N, M = Ni or Fe), pyrrolic N, and graphitic N, consistent with the formation of atomically dispersed M–N_x sites.²⁴ The presence of Ti in Ti–Fe₂O₃ was further confirmed by the Ti 2p spectra shown in Fig. S2c. There was no peak shift in both XPS (Fig. S2) and Raman spectra (Fig. S3) before and after the addition of Ni-CN, indicating that there is only physical contact between Ti–Fe₂O₃ and M–NC sheet. To directly verify the incorporation of Ni species, scanning transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy (STEM-EDS) was performed. Elemental mapping (Fig. 1h–k) confirmed the coexistence of Ni, N, and C in the Ni–NC catalyst, supporting the successful deposition of Ni–NC onto the Ti–Fe₂O₃ photoanode. Collectively, the XPS and STEM-EDS analyses confirmed the expected elemental composition and validated the successful construction of Ti–Fe₂O₃/M–NC hybrid photoanodes. Detailed atomic-scale characterization of the Fe–NC and Ni–NC catalyst powders prior to photoanode integration has been reported previously using high-angle annular dark-field STEM, energy-dispersive X-ray spectroscopy, X-ray absorption spectroscopy, time-of-flight secondary ion mass spectrometry (ToF-SIMS), and ⁵⁷Fe Mössbauer spectroscopy.^24,25,36 STEM-EDS and elemental mapping of Fe–NC are presented in Fig. S4.

The PEC performance of the Ti–Fe₂O₃ photoanode was first evaluated in a stirred electrolyte containing 0.5 M glycerol and 1 M NaOH under 1 sun illumination (AM 1.5G, 100 mW cm⁻²), using back-side illumination through the glass substrate. The current density–potential (j–V) curves recorded under chopped illumination are shown in Fig. 2a, while the optimization of M–NC catalyst loading and glycerol concentration are presented in Fig. S5 and S6. Pristine Ti–Fe₂O₃ delivered a photocurrent density of 0.8 mA cm⁻² at +1.23 V_RHE. Upon modification with M–NC catalysts, the PEC activity was markedly enhanced. Ti–Fe₂O₃/Ni–NC exhibited a photocurrent density of 2.2 mA cm⁻² at +1.23 V_RHE, corresponding to an approximately threefold increase relative to bare Ti–Fe₂O₃, while Ti–Fe₂O₃/Fe–NC achieved a photocurrent density of 1.9 mA cm⁻² under the same conditions. To exclude the influence of the N-containing carbon matrix alone, a control photoanode comprising Ti–Fe₂O₃ coated with NC (without Fe or Ni metal sites) was evaluated. This composite exhibited a substantially lower photocurrent density of 1.0 mA cm⁻² at +1.23 V_RHE (Fig. S7a), confirming that the enhanced activity originated from the presence of M–NC (M = Ni or Fe) rather than the NC support itself. Collectively, these results demonstrate that the incorporation of M–NC catalysts can effectively promote glycerol oxidation on Ti–Fe₂O₃ photoanodes. Incident photon-to-current efficiency (IPCE) measurements further corroborated these findings. As shown in Fig. 2b, pristine Ti–Fe₂O₃ exhibited an IPCE of approximately 22% at 350 nm, whereas Ti–Fe₂O₃/Ni–NC and Ti–Fe₂O₃/Fe–NC achieved significantly higher IPCE values of approximately 58% and 55%, respectively, at the same wavelength. The enhanced IPCE across the UV-visible region confirmed that M–NC modification improved photon-to-current conversion efficiency, in good agreement with the trends observed in the j–V characteristics.

Fig. 2 (a) (Photo) current–potential (j–V) curves of Ti–Fe₂O₃, Ti–Fe₂O₃/Fe–NC, and Ti–Fe₂O₃/Ni–NC photoanodes recorded under 1 sun illumination (Xe lamp, AM 1.5G filter, 100 mW cm⁻²) with chopped light at a scan rate of 10 mV s⁻¹ in 0.5 M glycerol and 1 M NaOH (pH = 13.7). (b) IPCE spectra measured at +1.23 V_RHE under monochromatic illumination (Xe lamp with monochromator) and corresponding integrated photocurrents for Ti–Fe₂O₃, Ti–Fe₂O₃/Ni–NC, and Ti–Fe₂O₃/Fe–NC. (c) Photoanodic production rates of glycerol oxidation products on different photoanodes at applied potentials of +0.8, +1.0, and +1.2 V_RHE. (d) Product selectivity toward glyceric acid (GLA), glycolic acid (GA), and formic acid (FA) under the corresponding conditions.

PEC GOR was performed at applied potentials of +0.8, +1.0, and +1.2 V_RHE for 2 h to analyze the liquid-phase oxidation products (Fig. 2c). The SEM micrographs in Fig. S8 show the macro-scale morphologies of Ti–Fe₂O₃/Ni–NC and Ti–Fe₂O₃/Fe–NC remain intact after reaction. The effective reaction potential comprises the externally applied potential and the photovoltage generated by the hematite photoanode under illumination. Although the apparent photovoltage inferred from the shift in open-circuit potential was ∼0.15 V, larger apparent potential shifts (>0.5 V) were typically observed at operating current densities, reflecting enhanced charge separation and interfacial kinetics under illumination and higher applied potentials.^35,38 The major liquid-phase products, identified by high-performance liquid chromatography (HPLC), were glyceric acid (GLA), glycolic acid (GA), and formic acid (FA). Although the corresponding species formed under the PEC alkaline conditions were glycerate, glycolate, and formate, respectively, they are herein referred to by their acid forms for consistency with the literature and their commercial relevance after acidification. The production rates and selectivities of the oxidation products on different photoanodes at various applied potentials are summarized in Fig. 2c and d. For all photoanodes, total product formation rates increased with increasing applied potential. At +0.8 V_RHE, pristine Ti–Fe₂O₃ predominantly produced GLA (∼58% selectivity) and GA (∼42% selectivity). Upon Ni–NC modification, the total production rate increased markedly, with product distribution strongly shifted toward GLA, reaching a selectivity of ∼80%, highlighting the pronounced catalytic effect of Ni–NC. In contrast, Fe–NC loading also substantially enhanced the production rate but favored GA formation, achieving a selectivity of ∼74% at the same potential. As the applied potential was increased from +0.8 to +1.2 V_RHE, both the production rate and product selectivity of pristine Ti–Fe₂O₃ remained largely unchanged. In contrast, Ti–Fe₂O₃/Ni–NC exhibited a pronounced evolution in product distribution, with the emergence of FA at higher potentials. The selectivity toward FA increased from 0% at +0.8 V_RHE to 49% at +1.2 V_RHE, indicating that either the applied potential plays a critical role in steering product pathways in the presence of Ni–NC or that the oxidative conditions drive the dissolution and aggregation of Ni atoms into clusters, which have been shown to promote the C–C scission in glycerol oxidation towards formate.^39–41 To exclude contributions from the NC matrix, a Ti–Fe₂O₃/NC control photoanode was evaluated under identical conditions. It displayed similar production rates and selectivity to pristine Ti–Fe₂O₃ (Fig. S7b), confirming that the observed selectivity modulation originated from the doped metal sites. In comparison, Ti–Fe₂O₃/Fe–NC maintained high selectivity toward GA with increasing potential, accompanied by enhanced production rates, while no FA formation was detected across the investigated potential range. Finally, calculated faradaic efficiencies (FE) for liquid GLA, GA, and FA at different applied potentials are shown in Fig. S9. At +0.8 V_RHE, the summation of these FEs reached 100%, confirming all the photocurrent observed is linked to glycerol oxidation to liquid products at this potential. However, at +1.0 and +1.2 V_RHE, some deviation from 100% was observed (e.g. 22% and 44% for Ti–Fe₂O₃/Ni–NC, respectively) suggesting competing gas evolution with increasing applied potentials. LSV measurements without glycerol (Fig. S5a) consistently indicated increasing photocurrents with applied potential from +1.2 V_RHE for Ti–Fe₂O₃ and from +1.0 V_RHE for Ti–Fe₂O₃/Ni–NC. Based on the response of similar hematite systems, these photocurrents (and the deviation from 100% summation of GLA, GA, and FA FEs) at the highest applied potentials were assigned to mainly O₂ evolution.³⁵ A small contribution from complete glycerol oxidation to CO₂ gas at the highest applied potentials cannot be ruled out.

To investigate the evolution of active sites in M–NC catalysts at the minimal loadings employed in the photoanodes, highly surface-sensitive ToF-SIMS was performed on fresh and post-reaction Ti–Fe₂O₃/M–NC photoanodes, focusing on the detection of NiN_xC_y fragments. Fe–NC was not examined by ToF-SIMS, as previous studies have reported no observable FeN_xC_y fragments in this catalyst, likely due to the lower ionization efficiency of Fe-containing species.²⁵ Measurements were conducted in negative ion mode to enhance the ionization yield of NiN_xC_y fragments generated by bond-cleavage events induced by the negative primary ion beam.⁴² A bare NC sample was also analyzed as a reference to aid fragment assignment and distinguish Ni-derived species from other surface contaminants. Quantitative comparison was enabled through algebraic deconvolution of overlapping ⁵⁸Ni and ⁶⁰Ni isotopic fragments following previously reported methods.^25,42 The detected NiN_xC_y fragments (Fig. 3a–c) were consistent with atomically dispersed Ni species within N-containing carbon, in agreement with prior characterization of these materials.²⁵ Importantly, no m/z signals corresponding to dinuclear Ni species (Ni₂N_xC_y⁻) were detected, suggesting the absence of detectable Ni–Ni aggregation. Interestingly, normalized NiN_xC_y fragment intensities were lower for fresh photoanodes and increased after PEC GOR testing (Fig. 3c), which may have resulted from surface cleaning during operation or changes in surface chemistry, such as adsorption of electrolyte-derived species that enhance fragment ionization. Calibration and reference peak intensities of the NC-related peaks, including C⁻, C₂⁻, C₄⁻, and C₆⁻ in Fig. S10a, as well as Fe₂O₃⁻ in Fig. 3a and b, remained comparable before and after testing, although the slightly lower NC peak intensities in the post-reaction sample indicate minor NC degradation during oxidation. In addition, the increased KHCO₃⁻ signal (Fig. S10b) indicated the presence of residual electrolyte species on the post-reaction samples. The relevant fragment mapping counts showed that the Ni-NC was well dispersed on the surface of Ti–Fe₂O₃ (Fig. S11). While ToF-SIMS revealed enhanced NiN_xC_y signals after PEC operation, it has been recently established that M–NC can undergo oxidation and structural reconstruction under anodic potentials.²⁹ According to the Ni Pourbaix diagram, Ni readily oxidizes to higher-valence species (e.g., NiOOH/Ni(OH)₂ or Ni₂O₃/NiO₂) at pH ≈ 14 and ≈ +1 V_SHE (10⁻⁶ mol L⁻¹),⁴³ and such transformations may involve partial aggregation into small (oxy)hydroxide clusters featuring M–O–M or M–O–M′ (M/M′ = Fe or Ni) linkages.^29,43 To further assess the dispersion and structural integrity of Ni sites, high-resolution transmission electron microscopy (HRTEM) and STEM-EDS were employed. For fresh Ti–Fe₂O₃/Ni–NC photoanodes, STEM-EDS mapping (Fig. 3d) revealed uniformly distributed Ni and N signals alongside Fe from the Ti–Fe₂O₃ substrate, while aberration-corrected HAADF-STEM (Fig. 3e) confirmed atomic-level dispersion of Ni sites. Following a 28 h stability test at +1.23 V_RHE under 1 sun illumination (Fig. S12), Ti–Fe₂O₃/Ni–NC retained approximately 82% of its initial photocurrent, indicating operational durability. Post-reaction STEM-EDS mapping (Fig. 3f) and atomic-resolution imaging (Fig. 3g) showed a reduced Ni signal associated with the NC relative to fresh samples as well as the presence of Ni-rich particles, suggesting the partial oxidation and aggregation of Ni single-atom sites during PEC GOR, which may have contributed both to the observed photocurrent decay during prolonged operation and to the shift in product distribution towards formate at high anodic potentials.^39,40

Fig. 3 (a–c) Negative-polarity ToF-SIMS analysis of fresh and post-tested Ti–Fe₂O₃/Ni–NC photoanodes after PEC operation for 2 h at +1.23 V_RHE in 0.5 M glycerol and 1 M NaOH under 1 sun illumination. Fragment ion counts, normalized to total ion counts, for mass spectral regions assigned to ⁵⁸NiNC₃⁻, ⁶⁰NiNC₃⁻/⁵⁸NiN₂C₂⁻, and identified major ⁵⁸NiN_xC_y⁻ fragments. The bars represent the mean values, and the error bars the deviation of two distinct measurements at locations where Ni–NC was visibly deposited. (d) STEM-EDS elemental mapping of fresh Ti–Fe₂O₃/Ni–NC. Scale bars are identical for all panels in d. (e) HAADF-STEM micrograph of fresh Ni–NC. (f) STEM-EDS elemental mapping of Ti–Fe₂O₃/Ni–NC after PEC operation at +1.23 V_RHE for 28 h under 1 sun illumination in 0.5 M glycerol and 1 M NaOH. Scale bars are identical for all panels in f. (g) HAADF-STEM micrograph of used Ni–NC. (h) Schematic illustration of the Ti–Fe₂O₃/M–NC (M = Ni or Fe) photoanode and the atomic structure of M–NC. (i and j) Schematic energy band diagrams of the Ti–Fe₂O₃/M–NC photoanode before and after contact.

The atomic structure of the Ti–Fe₂O₃/M–NC (M = Ni or Fe) photoanode is illustrated in Fig. 3h, while the corresponding energy band alignment of the individual components prior to contact is schematically shown in Fig. 3i. Ti–Fe₂O₃ exhibits a work function of approximately −4.1 eV,³⁵ corresponding to a relatively shallow Fermi level compared with that of M–NC (−5.31 eV for Ni–NC and −5.29 eV for Fe–NC).^44,45 Upon contact between Ti–Fe₂O₃ and M–NC, Fermi-level equilibration is expected to occur, leading to upward band bending in the Ti–Fe₂O₃ near the interface with M–NC (Fig. 3j). This interfacial energy-level alignment is characteristic of a Schottky junction, generating a built-in electric field that promotes charge-carrier separation and facilitates hole transfer from Ti–Fe₂O₃ toward the M–NC layer. Such a charge-transfer configuration is consistent with the enhanced photocurrent densities observed for Ti–Fe₂O₃/M–NC photoanodes (Fig. 2a).⁴⁶ Under illumination, photogenerated holes in Ti–Fe₂O₃ preferentially migrate toward the M–NC component, where they can participate in interfacial glycerol oxidation reactions at the M–NC/electrolyte interface.⁴⁶ However, care must be taken to avoid high potentials where photogenerated holes could also contribute to partial self-oxidation of the carbon framework.⁴⁷

To rationalize the experimentally observed differences in glycerol oxidation product selectivity between Ti–Fe₂O₃/Ni–NC and Ti–Fe₂O₃/Fe–NC photoanodes, DFT calculations were performed to evaluate the energetics of glycerol electro-oxidation intermediates and reaction pathways. The modeled catalyst surface consisted of a metal–N₄–pyridine-doped graphene structure, representing Ni–NC or Fe–NC, as shown in Fig. 3h. Glycerol oxidation was modeled as described elsewhere,⁴⁸ in which two reaction types [dehydrogenation and hydroxyl (OH) incorporation] were considered for each elementary oxidation step (described in the Computational Section of the SI). All possible intermediates evaluated at each step are summarized in Tables S3 and S4, and the reaction pathways constructed from the most stable intermediates at U_{vs. RHE} = 0 V are presented in Fig. 4a and b. Owing to model limitations, including the exclusion of the Ti–Fe₂O₃ photoactive component and solvation effects, the predicted final products did not exactly reproduce experimental observations; nevertheless, the calculations provided valuable mechanistic insights into metal-dependent oxidation activity and pathway energetics.

Fig. 4 Free-energy diagrams for possible electro-oxidation pathways of (a and b) glycerol and (c and d) formic acid (FA) on Fe–NC and Ni–NC catalysts evaluated at U_{vs. RHE} = 0 V: (a) glycerol on Fe–NC, (b) glycerol on Ni–NC, (c) FA on Fe–NC, and (d) FA on Ni–NC.

The DFT results indicated that Fe–NC should exhibit intrinsically higher glycerol oxidation activity than Ni–NC. For Ni–NC, the potential-determining step was the initial glycerol dehydrogenation step, in which C₃H₈O₃ was oxidized to CH₂OH–CHOH–CH₂O*, requiring a minimum potential of +1.24 V_RHE to become thermodynamically favorable [unlike Ni(111)], which shows favorable glycerol electro oxidation even at 0 V_RHE.^43,49 Above this +1.24 V_RHE threshold, all subsequent reaction steps were downhill in free energy, enabling multiple reaction pathways (Table S4), which was consistent with the experimentally observed formation of multiple oxidation products (GLA, GA, and FA), even though specific products were not explicitly predicted by the model. In contrast, Fe–NC required a significantly lower activation potential of only +0.22 V_RHE for the initial glycerol dehydrogenation step, nearly 1 V lower than that of Ni–NC, highlighting its superior intrinsic oxidation activity. For Fe–NC, the potential-determining step occurred at the fourth oxidation step, where CH₂OH–CO–*CHOH is oxidized to CH₂OH–CO–CHO, with a calculated ΔG ranging from 0.43 to 0.76 eV. Once the potential exceeded +0.33 V_RHE, all six oxidation steps became thermodynamically downhill, facilitating rapid glycerol oxidation.

Experimentally, FA was observed during glycerol photoanodic oxidation on Ti–Fe₂O₃/Ni–NC but not on Ti–Fe₂O₃/Fe–NC (Fig. 2c and d). To elucidate this discrepancy, the electro-oxidation pathways of FA were further examined on both Ni–NC and Fe–NC catalysts. Two reaction mechanisms (dehydrogenation and OH incorporation) were again considered, with all possible intermediates listed in Tables S5 and S6. The resulting pathways constructed from the most stable intermediates at 0 V_RHE are shown in Fig. 4c and d. On Fe–NC, both oxidation steps for FA were thermodynamically favorable (downhill), indicating that FA was readily oxidized to CO₂ and therefore did not accumulate to detectable levels. In contrast, on Ni–NC, the first oxidation step of FA to HCOO* was endergonic, with ΔG = 0.75 eV, requiring a potential of at least 0.75 V_RHE to proceed, allowing FA to persist under reaction conditions, consistent with its experimental detection on Ti–Fe₂O₃/Ni–NC. These results highlighted the higher intrinsic oxidation activity of Fe–NC relative to Ni–NC and provided a mechanistic explanation for the observed metal-dependent product selectivity.

Based on the above results and discussion, a proposed mechanism for solar-driven glycerol oxidation over Ti–Fe₂O₃/M–NC (M = Fe or Ni) hybrid photoanodes is illustrated in Fig. 5. Under light irradiation, photogenerated holes (h⁺) in Ti–Fe₂O₃ migrate toward the M–NC cocatalyst driven by the built-in electric field at the Ti–Fe₂O₃/M–NC Schottky junction. These holes subsequently reach the M–NC/electrolyte interface, where glycerol oxidation occurs. Fe–NC exhibits intrinsically higher glycerol oxidation activity than Ni–NC. For Ni–NC, the potential-determining step is the initial glycerol dehydrogenation step, in which C₃H₈O₃ is oxidized to CH₂OH–CHOH–CH₂O*. Once this step is overcome, all subsequent reaction steps are thermodynamically downhill, enabling multiple reaction pathways and leading to the formation of various oxidation products, including GLA, GA, and FA. In contrast, for Fe–NC, the potential-determining step occurs at the fourth oxidation step, where CH₂OH–CO–*CHOH is oxidized to CH₂OH–CO–CHO. All six oxidation steps become thermodynamically downhill at significantly lower potentials than Ni–NC, facilitating faster glycerol oxidation and favoring selective GLA formation. Meanwhile, photogenerated electrons (e⁻) are extracted from the photoanode and driven through the external circuit by the applied potential, with charge balance maintained by the potentiostat, resulting in electron delivery to the Pt counter electrode where the hydrogen evolution reaction occurs.

Fig. 5 Schematic illustration of the photoinduced charge transfer and reaction mechanism for PEC glycerol oxidation over Ti–Fe₂O₃/M–NC (M = Fe or Ni) hybrid photoanodes.

3 Conclusions

Ti–Fe₂O₃/M–NC (M = Fe or Ni) hybrid photoanodes were successfully developed to address the key challenges of low activity, inefficient charge separation, and limited selectivity control in photoelectrochemical glycerol oxidation (PEC GOR). The integration of single-atom M–NC cocatalysts with Ti–Fe₂O₃ provides two principal advantages. First, the formation of a Schottky junction at the Ti–Fe₂O₃/M–NC interface establishes a built-in electric field that promotes efficient charge-carrier separation and directional hole transfer, resulting in a substantial enhancement of photocurrent density, reaching up to 2.2 mA cm⁻² for Ti–Fe₂O₃/Ni–NC at +1.23 V_RHE. Second, the use of different M–NC catalysts enables modulation of product selectivity, with Ni–NC favoring glyceric acid formation (∼80% selectivity) and Fe–NC preferentially promoting glycolic acid production (∼74% selectivity) at +0.8 V_RHE, while maintaining stable or tunable performance across a broad potential window (+0.8 to +1.2 V_RHE). Density functional theory calculations reveal that the distinct selectivity trends observed for Ni–NC and Fe–NC originate from their intrinsic differences in oxidation activity, providing a mechanistic foundation for the experimentally observed product distributions and supporting the proposed selective PEC GOR pathways. Post-reaction analyses using ToF-SIMS and HAADF-STEM show that the Ni single-atom metal sites largely retain their dispersion after extended operation at +1.23 V_RHE. However, partial Ni oxidation to nanoclusters is evident, which can also explain the selectivity change of Ti–Fe₂O₃/Ni–NC to formic acid at these potentials. Increasing photocurrents with applied potential in the absence of glycerol and faradaic efficiencies indicate competing gas evolution at the higher applied potentials. Overall, this work advances the rational design of a selective photoanodes for biomass valorization by exploiting the structural and electronic synergy between hematite photoanodes and single-atom M–NC cocatalysts and extends the application of single-atom catalysts to solar-driven PEC energy conversion.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data that support the findings of this study are openly available in the following Figshare data repository at https://doi.org/10.6084/m9.figshare.32714346.

Supplementary information (SI): experimental and computational sections and figures and tables. See DOI: https://doi.org/10.1039/d6ta02778d.

Acknowledgements

The authors thank the Department of Chemical Engineering at Imperial College London for funding a PhD scholarship for M.Y. S.E. acknowledges financial support from the UK Engineering and Physical Sciences Research Council (EPSRC) through grant EP/S030727/1. J.B. acknowledges financial support from Imperial College London via the Imperial College Research Fellowship (ICRF) scheme. A. P. thanks the EPSRC Doctoral Prize Fellowship (EP/W524323/1). Z.M., H.H.K., and J.R. acknowledge financial support from the Independent Research Fund Denmark grant no. 1127-00372B and the Danish National Research Foundation, Center for High Entropy Alloy Catalysis (CHEAC) DNRF149. SJH acknowledges funding from EPSRC grant EP/Y024303/1. TEM/STEM was supported by the Royce Institute for Advanced Materials, funded through EPSRC grants EP/R00661X/1, EP/S019367/1, EP/P025021/1 and EP/P025498/1.

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