Black TiO₂ Material for Highly Efficient Green Hydrogen Production Enabled by Advanced Surface Engineering

Abstract

Black TiO₂ (H-TiO₂), as a promising photoanode material, can be used for direct green hydrogen production without emissions to pollute the environment, but the reported surface engineering approaches for the preparation of black TiO₂ suffer from high temperatures, long processing time, or chemical residues, limiting its practical application in green hydrogen production. Here, we developed two advanced surface engineering technologies, overcoming the above limitations, to prepare a black TiO₂ photoanode that achieved the maximum photocurrent density reported to date. Moreover, we theoretically and experimentally revealed the formation mechanism of black TiO₂ and its enhanced photoelectrochemical (PEC) performance. These surface engineering technologies are not only suitable for the preparation of efficient photoanode materials for PEC hydrogen production but also play a beneficial and promoting role in the research and development of new materials for hydrogen fuel cells and hydrogen storage.

1. Introduction

Photoelectrochemical (PEC) technology for solar fuel generation has garnered international recognition as a promising solution for addressing both energy and environmental challenges [1, 2]. This technology has the capability to mimic photosynthesis by converting solar energy into hydrogen energy and also has the potential to convert CO₂ into hydrocarbon fuel, making it an ideal solution for a sunshine economy.

The technology of PEC hydrogen production was initiated in 1972 by Fujishima and Honda, which discovered that single crystal TiO₂ can generate sufficient photovoltage when exposed to light to facilitate the splitting of water into hydrogen and oxygen [3]. This breakthrough demonstrated the potential of utilizing solar energy to directly produce green hydrogen without CO₂ emissions, which is regarded as the most promising method for achieving zero-carbon hydrogen in the future. Subsequently, semiconductor photocatalysts have triggered significant interest within the academic community.

TiO₂ has emerged as a prominent semiconductor photocatalyst among various materials due to its stable chemical properties, robust oxidation-reduction capabilities, and cost-effectiveness. However, as a wide bandgap semiconductor, TiO₂ can only absorb ultraviolet light within the solar spectrum. Enhancing the responsiveness of TiO₂ to visible light has become a key focus of research in this field. Recent studies have explored surface engineering technologies via doping of various elements to improve visible light absorption of TiO₂, yet the resulting photocurrent remains below 1 mA cm⁻² [4]. The field of surface engineering for TiO₂ is currently grappling with the obstacle of achieving high solar-to-hydrogen conversion efficiency.

The global interest in the synthesis of black TiO₂ through high-pressure hydrogenation of TiO₂ has been significant. In 2011, Chen et al. presented an innovative approach to creating black TiO₂ in Science [5]. Figure 1 visually represents the unique core-shell structure of black TiO₂ and an accompanying energy diagram illustrating a decreased band gap. The researchers verified that black TiO₂ displays a wider band for light absorption, allowing it to capture visible, ultraviolet, and infrared light, exhibiting outstanding photocatalytic characteristics. Since its inception, black TiO₂ has attracted considerable interest in the scientific community due to its unique combination of high sunlight absorption and exceptionally high photocatalytic abilities. However, its applications are hampered by impractical hydrogenation conditions (20 bar, 200°C, and 5 days). Developing new hydrogenation technologies at mild conditions is highly demanded.

Figure 1.

(a) Core-shell structure and narrowing band gap models; (b) white pristine TiO₂ to black TiO₂; (c) and (d) high-resolution transmission electron microscope (HR-TEM) images of pristine TiO₂ and black TiO₂, respectively.

2. Recent advances in hydrogen technologies promoted by emerging hydrogenation methods

Hydrogenation technology is known as one of the most important material surface engineering technologies today, and here, we highlighted the latest research progress on how hydrogenation technologies can solve the core scientific problems of hydrogen-related technologies like PEC hydrogen production, hydrogen fuel cells and hydrogen storage.

2.1 PEC green hydrogen production

Wang et al. utilized high-temperature H₂ hydrogenation to prepare H-TiO₂ nanowires with significantly improved PEC performance for solar water splitting [6]. The H-TiO₂ nanorods treated at 350°C achieve the best photocurrent density of 2.4 mA cm⁻² at 1.23 V_RHE compared to untreated nanorods, indicating efficient charge separation and transportation (Figure 2a). The incident photon to current efficiency (IPCE) analyses confirm that the enhancement of photocurrent density of H-TiO₂ is attributed to the enhanced photoactivity in the ultraviolet region (100%) and a significant increase in the donor density (3×), achieved through the creation of a high density of oxygen vacancies that act as electron donors. However, the harsh conditions of the high-temperature H₂ approach (400°C, 1 h) limit its practical application in PEC water splitting.

Figure 2.

Photocurrent density versus potential (J-V) measurements of H-TiO₂ via (a) high-temperature H₂ and (b) Pd-catalyzed H hydrogenations.

Xu et al. explored a Pd-catalyzed H hydrogenation strategy for reducing TiO₂, leading to the generation of Ti³⁺ species in TiO₂ and improved photocatalytic activity (Figure 2b) [7]. The hydrogenation process creates intrinsic defects in the materials, narrowing the band gap and enhancing solar energy utilization. The unique crystalline core/disordered shell structure of H-TiO₂ formed during the process contributes to improved photocatalytic activity and long-term stability. However, the high cost of noble metal and chemical residues in this hydrogenation method hinder its practical application in PEC hydrogen production.

2.2 Hydrogen fuel cells

The hydrogen fuel cell is divided into two chambers, which pass into H₂ and O₂, respectively; in the left chamber, under the catalytic Pt, H₂ dissociates to protons and electrons (2H₂ → 4H⁺ + 4e⁻), protons reach the counter electrode of the right chamber through the solid electrolyte, and at the same time, the electrons pass through the peripheral circuit to the counter electrode, so as to cooperate with the reduction reaction of protons to O₂ to generate H₂O (O₂ + 4H⁺ + 4e⁻ → 2H₂O), which completes the transfer of chemical energy to electrical energy (Figure 3a). It is evident that conversion efficiency depends strongly on the proton conductivity of solid electrolytes.

Figure 3.

(a) Scheme of hydrogen fuel cells. (b) Temperature-dependent proton conductivity: HSrCoO_2.5 vs. state-of-the art. (c) Scheme of noble metal catalyzed H hydrogenation and resulting organized oxygen vacancy channels in HSrCoO_2.5.

Solid oxide ionic conductors are utilized to serve as solid electrolytes in hydrogen fuel cells. Meanwhile, traditional low ionic conductors based on metal oxides typically necessitate temperatures exceeding 500°C to facilitate ionic transport (Figure 3b). Very recently, Lu et al. reported a new solid oxide proton conductor, HSrCoO_2.5, prepared by noble metal catalyzed H hydrogenation, exhibiting remarkably high proton conductivity within the temperature range of 40–140°C [8]. The proton conductivity in the specified temperature range ranged from 0.028 to 0.33 S cm⁻¹ (Figure 3b), which is ascribed to the elevated proton concentration and the organized oxygen vacancy channels facilitated by the noble metal catalyzed H hydrogenation (Figure 3c). However, the prohibitive expense of noble metals and the presence of chemical residues in the hydrogenation process pose significant obstacles to its feasibility for use in practical hydrogen fuel cells.

2.3 Hydrogen storage

Solid-state hydrogen storage materials, facilitating the storage and controlled release of hydrogen, have been a focal point of extensive research for numerous decades, establishing ambitious goals for their advancement in recent years. Developing such materials that can store an appreciable amount of hydrogen reversibly without having to use high pressure and/or low temperature is a significant challenge.

Izumi et al. prepared crystalline samples of the material La_1−xSr_xH_3−x−2yO_y (0.1 ≤ x ≤ 0.6, y < 0.171) through ball-milling, followed by annealing under high-pressure H₂ hydrogenation [9]. They conducted an examination of the samples at ambient temperature, determining their ability to facilitate the conduction of hydride ions at a notable rate. Subsequently, they assessed the efficacy of the material in an all-solid-state cell composed of Ti| La_1−xSr_xH_3−x−2yO_y|LaH_3−δ, manipulating the quantities of strontium and oxygen within the composition. Upon identifying an optimal strontium content of at least 0.2, they observed the complete conversion of titanium to titanium hydride (TiH₂) at a rate of 100%, indicating minimal wastage of hydride ions. Based on the results, they focus on enhancing performance and developing electrode materials with the capability to reversibly absorb and release hydrogen. This advancement is crucial for enabling the recharging of batteries and facilitating the storage and controlled release of hydrogen, thereby meeting the demands of hydrogen-based energy applications. While the above findings have made important advances in hydrogen storage, the high-temperature and high-pressure hydrogen hydrogenation method (400°C, 0.5 MPa, 12 h) they employed will prevent the development of its practical applications.

3. Advanced hydrogenation technologies for PEC green hydrogen production

The above-hydrogenated surface engineering technologies have shown great potential in PEC green hydrogen production, fuel cells, and hydrogen storage applications; however, the efficacy of their applications is hindered by impractical hydrogenation conditions and ambiguous hydrogenation mechanisms. Here, two advanced methods, low-temperature H [10] and room-temperature H⁺ hydrogenations [11], are developed to address the challenging conditions associated with traditional high-temperature H₂ reductions and the constraints posed by chemical residues resulting from chemical reductions.

3.1 Low-temperature H hydrogenation

3.1.1 Experimental set-up

The concept of low-temperature hot wire H hydrogenation draws inspiration from previous research. Langmuir’s study in 1912 investigated the efficiency of a tungsten filament in facilitating the dissociation of H₂ molecules into H atoms [12]. Experimental results demonstrated that when a tungsten filament is heated to temperatures exceeding 1000°C in a hydrogen-rich environment, H₂ molecules can be catalytically dissociated upon contact with the hot tungsten filament. In contrast to the H⁺ hydrogenation approach, this method exclusively involves only hydrogen atoms rather than hydrogen ions.

Figure 4a depicts the experimental configuration utilized for the low-temperature H hydrogenation. Within a vacuum chamber, a parallel array of 10× tungsten wires, each measuring 600 mm in length, 0.53 mm in diameter, and 50 mm in space, was positioned, resulting in a total activated area of 600 × 450 mm². The samples were subjected to treatment at varying wire temperatures (1600, 1700, and 1800°C), with a separation of 75 mm between the wires and the samples.

Figure 4.

(a) Experimental set-up of low-temperature H hydrogenation and (b) plot of the flux of atomic hydrogen vs. wire temperature.

The regulation of the flux rate of atomic hydrogen is contingent upon the wire temperature and pressure, with the pressure typically preset at 1 Pa. Figure 4b shows the plot of the flux of atomic hydrogen vs. wire temperature. It is evident that the flux of atomic hydrogen increases exponentially with increased wire temperature. This means that the low-temperature H hydrogenation [10] can achieve precise H-doping by tuning the wire temperature, demonstrating the superior features over the uncontrollable traditional high-temperature or high-pressure H₂ hydrogenations.

3.1.2 Hydrogenation-PEC performance relation

In this section, we examine in detail the effects of wire temperature on the structural, optical, electrical, and PEC properties of black TiO₂.

3.1.2.1 Structure

Scanning electron microscopy (SEM) and HR-TEM were used to investigate the microstructure of H-TiO₂ nanorods treated at different wire temperatures. H-TiO₂ treated at T_wire = 1700°C closely resembles pristine TiO₂ in morphology. HR-TEM images show that pristine TiO₂ has a single crystalline structure (Figure 5a–c), while H-TiO₂ at T_wire = 1700°C has a core-shell structure with a 2 nm thick disordered shell (Figure 5d–f). Electron energy loss spectroscopy (EELS) analysis showed that the [O]/[Ti] ratio in pristine TiO₂ remains constant at about 2 throughout the nanorod, while in H-TiO₂ at T_wire = 1700°C, the ratio decreases gradually in the disordered region.

Figure 5.

TEM, HR-TEM images, and fast Fourier transform (FFT) patterns of (a–c) pristine TiO₂ and (d–f) H-TiO₂ nanorods treated at T_wire = 1700°C.

3.1.2.2 Optical properties

The optical absorption spectra of pristine TiO₂ and H-TiO₂ were analyzed to study the improved optical absorption properties of H-TiO₂. Results show a red shift in the band edge of H-TiO₂ with increasing wire temperatures and increased absorption in the visible spectrum (Figure 6a). Tauc plots indicate a band edge of 2.9 eV for H-TiO₂ compared to 3.0 eV for pristine TiO₂ (Figure 6b).

Figure 6.

(a) Optical absorption and (b) Tauc plots for pristine TiO₂ and H-TiO₂.

3.1.2.3 PEC properties

J-V measurements were conducted to investigate the improved PEC properties of H-TiO₂ nanorods (Figure 7a). The comparison of J-V curves of pristine TiO₂ and H-TiO₂ samples showed an increase in photocurrent density after treatment at T_wire = 1600°C, peaking at T_wire = 1700°C, and decreasing at T_wire = 1800°C. The effect of wire temperature on the conductivity of H-TiO₂ was studied using Mott-Schottky measurements (Figure 7b). Higher wire temperatures led to a decrease in the slope of H-TiO₂, indicating an increase in donor density and conductivity. The donor densities of H-TiO₂ were found to be 3× higher than those of pristine TiO₂, showing a trend of increased conductivity in H-TiO₂ samples with higher temperatures.

Figure 7.

(a) J-V curves of pristine TiO₂ and H-TiO₂ nanorods in dark and under solar illumination. (b) Mott-Schottky plots of pristine TiO₂ and H-TiO₂ nanorods. (c) O1s and (d) Ti2p XPS spectra of pristine and H-TiO₂ samples.

3.1.2.4 Ti³⁺ identification

X-ray photoelectron spectra (XPS) analysis of H-TiO₂ samples at different wire temperatures showed that the photocurrent density was highest at T_wire = 1700°C. The O1s spectra revealed peaks at 530 and 531.5 eV, corresponding to TiO₂ and Ti-OH species (Figure 7c). The intensity of the Ti∙OH peak increased with increasing wire temperature up to 1700°C but decreased at 1800°C. The reduced Ti-OH peak in H-TiO₂ at 1800°C suggests less Ti³⁺ presence. XPS Ti2p spectra for both samples show similar characteristics, with binding energies of 458.5 eV for Ti2p_3/2 and 464.3 eV for Ti2p_1/2, typical of TiO₂ (Figure 7d). A Ti-H peak at 456.5 eV, rather than metallic Ti at 453.8 eV, is observed in the sample treated at T_wire = 1800oC. The formation of Ti‒H bonds in H-TiO₂ at 1800°C reduces Ti∙OH bonds and decreases Ti³⁺ concentration, confirming the insight into improved photocurrent density of H-TiO₂ at T_wire = 1700°C.

The results suggest that the low-temperature H hydrogenation process effectively facilitates the formation of a disordered shell on the H-TiO₂ nanorods, resulting in a decrease in band gap (3.0 → 2.9 eV), an increase in donor density (3×), and Ti³⁺ concentration, leading to the highest photocurrent density of 2.5 mA cm⁻² at 1.23 V_RHE of H-TiO₂ at T_wire = 1700°C reported to date.

3.1.3 Formation mechanism

The low-temperature H hydrogenation process is delineated in the subsequent three stages: (a) hydroxylation of the TiO₂(110) surface through hydrogen adsorption; (b) subsurface diffusion of hydrogen; (c) subsurface hydrogenation.

Density functional theory (DFT) simulations suggest that H treatment facilitates the occupation of O_3c surface sites by H, subsurface diffusion, and subsurface hydrogenation through the O_3c → O_sub low-energy-barrier pathway (0.87 eV), leading to the thermodynamically favorable formation of a disordered shell. Conversely, H₂ treatment typically results in only submonolayer degrees of hydroxylation, with the occupation of O_3c sites being thermodynamically unfavorable. Consequently, subsurface diffusion from O_2c to O_sub is kinetically hindered during H₂ treatment; only H₂O desorption and the formation of oxygen vacancies are deemed thermodynamically feasible, necessitating high pressure and/or high temperatures for the process to occur (Table 1).

	[H]	H2
Hydroxylation	Almost no barrier for adsorption of atomic hydrogen → low temperature is sufficient H2 desorption is suppressed due to high-energy barrier	High barrier for dissociative adsorption molecular hydrogen → high temperature and flux are needed. High surface coverages are not thermodynamically favorable
Subsurface diffusion	Lower barrier for subsurface diffusion pathway is O3c → Osub subsurface diffusion is faster than H2 and H2O desorption at low temperatures	High barrier for subsurface diffusion pathway is O2c → O3c → Osub subsurface diffusion is very slow
Subsurface H-doping	Incorporation of H in TiO2 lattice is thermodynamically favorable up to high concentrations	Incorporation of H in TiO2 lattice is not thermodynamically favorable. At high temperatures, incorporation becomes even less favorable
Formation of disordered shell	Formation of disordered shell is thermodynamically favorable through O3c → Osub → H-doping in TiO2 lattice with relatively low barriers. Formation pathway: low temperatures are enough	O2c → O3c → Osub → H-doping in TiO2 is not thermodynamically favorable and kinetically suppressed. Different formation pathway: only H2O desorption and formation of oxygen vacancies are thermodynamically favorable → high temperatures are needed

Table 1.

Different formation pathways of disordered shell: H vs. H₂.

In conclusion, we have experimentally and theoretically confirmed that our low-temperature H hydrogenation method is indeed superior to the traditional H₂ hydrogenation method: the highest photocurrent density and highly efficient hydrogenation are achieved.

3.2 Room-temperature H⁺ hydrogenation

3.2.1 Experimental set-up

Recent interest in H⁺ hydrogenation using kinetic hydrogen ion species has increased because it does not require thermal activation. Traditional methods of H⁺ hydrogenation are limited by the need for high temperatures or high power output from plasma equipment to generate high-energy hydrogen ions, leading to inefficient and uncontrollable processes as well as limited photocurrent density of <1 mA cm⁻² at 1.23 V_RHE [13].

The magnetic field from an electromagnetic coil affects hydrogen ions and free electrons differently. Electrons change direction, but ions are not affected due to their greater mass (1000×). The magnetic field causes electrons to orbit in a circular path, increasing the chance of ionization and creating more hydrogen ions.

Inspired by the physical picture above, unlike traditional plasma hydrogenation methods, we used a circular electromagnetic coil in a radio frequency (RF) plasma physical vapor deposition (PVD) system to create a magnetic field (Figure 8). This field increases hydrogen ion concentration and decreases RF power and self-bias voltage, allowing for controlled low-energy hydrogen ion production for room-temperature H⁺ hydrogenation [11].

Figure 8.

Experimental set-up of room-temperature H⁺ hydrogenation. The role of a circular electromagnetic coil in the H⁺ hydrogenation process.

A programmable recipe controlled the RF power, magnetic field strength, hydrogen flow rate, processing pressure, and treatment duration for the H⁺ hydrogenation process. This included using RF power of 20 W, a magnetic field strength of 30 mT, a hydrogen flow rate of 50 sccm, and a processing pressure of 1.5 Pa to achieve a low self-bias voltage of −250V at the substrate electrode.

3.2.2 Hydrogenation-PEC performance relation

H-TiO₂ nanorods were synthesized at different time intervals (2.5, 5, 10, 20, and 40 mins) to investigate their structural, optical, electrical, and PEC properties.

3.2.2.1 Structure

SEM was used to study morphological changes in TiO₂ before and after H⁺ treatment. No significant changes were observed in SEM images of pristine TiO₂ and H-TiO₂ treated at 5 and 40 mins. X-ray diffraction (XRD) patterns showed only a rutile phase with (101) and (002) peaks, indicating no phase change in either sample. HR-TEM images show that H-TiO₂ samples have a composite structure with a crystalline core and a disordered shell. The core remains single crystalline, with the shell thickness increasing from 2 to 5 nm as treatment time increases from 5 to 40 mins (Figure 9). The line-scan EELS data of [O]/[Ti] ratio changes in pristine TiO₂ and H-TiO₂ samples. Oxygen vacancies form in the shell of H-TiO₂ over time but not in the core of H-TiO₂ treated for 5 mins.

Figure 9.

TEM, HR-TEM, and FFT images of H-TiO₂ at (a–c) 5 and (d–f) 40 mins, respectively.

3.2.2.2 Optical properties

We recorded optical absorption spectra to compare the optical properties of pristine TiO₂ and H-TiO₂. The absorption band edges of H-TiO₂ shift toward red and absorb more in the visible to infrared range with longer treatment time. The band gap decreases from 3.0 eV in pristine TiO₂ to 2.91 eV in H-TiO₂ after 5 mins of treatment.

3.2.2.3 PEC properties

Figure 10a shows that the photocurrent density of H-TiO₂ increases up to 5 mins of treatment time, then decreases. At 5 mins, a photocurrent density of 2.55 mA cm⁻² at 1.23 V_RHE is achieved, the highest reported for H-TiO₂ studies. Increasing treatment time increases donor density and decreases depletion region width. The negative shift in flat band potential of H-TiO₂ at 2.5 and 5 mins is due to a higher donor density, shifting the Fermi level toward the conduction band. The largest band bending of 1.1 V is observed for H-TiO₂ at 5 mins, determined from M-S plots (Figure 10b).

Figure 10.

(a) J-V curves of pristine TiO₂ and H-TiO₂ at different treatment times in dark and under solar illumination. (b) M-S plots of pristine TiO₂ and H-TiO₂ at different treatment times. (c) O1s and (d) Ti2p XPS spectra of pristine TiO₂ and H-TiO₂ at 5 and 40 mins, respectively.

3.2.2.4 Ti³⁺ identification

XPS analysis of the disordered shell showed the presence of Ti‒O and Ti‒OH in H-TiO₂. Ti³⁺ was detected in H-TiO₂ treated at 5 mins, with a strong reduction to Ti²⁺ observed in H-TiO₂ treated at 40 mins (Figure 10c and d).

Our experiment results confirmed that the highest photocurrent density of 2.55 mA cm⁻² at 1.23 V_RHE of H-TiO₂ at 5 mins is due to a smaller band gap (2.91 eV), increased band bending (1.1 eV) and bulk defect suppression.

3.2.3 Formation mechanism

We studied how the penetration ratio of vertically impinging H⁺ changes with initial kinetic energy E_kin,0. At the lowest E_kin,0 of 0.1 eV, all 15 H⁺ were reflected, consistent with the above DFT study of low-temperature H hydrogenation predicting a minimal energy barrier of 0.87 eV for subsurface diffusion. At E_kin,0 = 1 eV, 1 out of 15 H⁺ bonded to a surface O ion. At E_kin,0 = 10 eV, 8 out of 15 H⁺ penetrated the TiO₂ surface. At the maximum velocity of 80 eV, 10 out of 15 H⁺ penetrated the TiO₂ surface, resulting in a 67% penetration ratio. H⁺ with kinetic energies of at least 1 eV can penetrate the TiO₂ surface without needing thermal activation. The penetration ratio for H⁺ with kinetic energies of at least 10 eV can reach approximately 0.5.

It is intriguing to compare how energetic H⁺ particles interact with the TiO₂ surface versus thermal H species like H₂ and H in hydrogenation processes. Impinging thermal H₂ or H cannot directly penetrate; instead, hydrogen uptake in the subsurface region involves chemisorption and subsurface diffusion. Atomic H is more efficient than molecular H₂ for subsurface diffusion due to its ability to adsorb at higher energy surface sites, resulting in a lower energy barrier. However, efficient hydrogenation only occurs at high substrate temperatures. In our previous studies, we found that the lowest energy barrier for the movement of a chemisorbed H atom into the subsurface area is 0.87 eV and occurs near an O_3c site. In the H⁺ case, as the initial kinetic energy increases, additional penetration pathways with higher energy barriers surrounding O_3c → O_sub are available. This phenomenon results in a rapid increase in the penetration probability with increasing initial kinetic energy, ultimately contributing to the high efficiency of room-temperature H⁺ hydrogenation.

In summary, our study has experimentally and theoretically validated the superiority of our room-temperature H⁺ hydrogenation over the thermal hydrogen hydrogenation approaches, as evidenced by the attainment of the highest photocurrent density and exceptional efficiency in hydrogenation processes.

4. Comparison of low-temperature H and room-temperature H⁺ methods with state-of-the-art hydrogenations

Through a comparative analysis of the synthesis of black TiO₂ using the traditional hydrogenation techniques and the subsequent evaluation of photocurrent density, our two advanced hydrogenation methods, low-temperature H and room-temperature H⁺, exhibit the highest reported photocurrent density (Table 2), while simultaneously effectively addressing the challenges associated with traditional H₂ hydrogenation, such as high temperature, high pressure, and prolonged treatment, as well as chemical residues from chemical reductions.

Hydrogenation method	Temperature	RF power	Treatment time, pressure	Photocurrent density of H-TiO2 at 1.23 VRHE	Features and references
High-pressure H2	200°C	N/A	5 days, 20 bar	N/A	X High pressure; prolonged treatment [5]
High-temperature H2	400°C	N/A	1 h, 1 bar	2.4 mA cm^−2	X High temperature; prolonged treatment [6]
Low-temperature H	265°C	N/A	20 mins, 1 Pa	2.5 mA cm^−2	✓ Low temperature; short treatment [10]
Noble metal-catalyzed H	250°C	N/A	10 mins, 1 bar	1.5 mA cm^−2	X High cost: noble metals; chemical residues [7]
High-temperature H^+	425°C	200 W	1 h	0.9 mA cm^−2	X High temperature and power; prolonged treatment [13]
Room temperature H^+	25°C	20 W	5 mins, 1.5 Pa	2.55 mA cm^−2	✓ Room temperature; short treatment [11]

Table 2.

A comparison of the low-temperature H hydrogenation and room-temperature H⁺ hydrogenation with state-of-the-art hydrogenations for PEC applications.

Table 3 shows that the hydrogenation conditions of the hydrogenation technologies recently applied to hydrogen fuel cells and hydrogen storage are not suitable for practical applications, and it is evident that the two advanced hydrogenation technologies can compensate for their shortcomings.

Hydrogenation method	Temperature	Treatment time, pressure	Applications	Features and references
Noble metal-catalyzed H	250°C	10 mins	Hydrogen fuel cells	X High cost: noble metals; chemical residues [8]
High-pressure H2	400°C	12 h, 0.5 MPa	Hydrogen storage	X High pressure; prolonged treatment [9]

Table 3.

Reported hydrogenations for hydrogen fuel cells and storage applications.

5. Conclusions

In summary, we highlighted the advancements in surface engineering-hydrogenation technology for use in PEC hydrogen production, hydrogen fuel cells, and hydrogen storage. We presented the operational principles and experimental set-up of our invented hydrogenation technologies, and subsequently investigated the correlation between hydrogenation parameters and the structural, optical, electrical, and PEC properties of materials through combined experimental and theoretical studies. Finally, we conducted a comparative analysis of the benefits and drawbacks of the two hydrogenation technologies in relation to reported hydrogenation technologies. Our findings suggest that these two advanced technologies hold significant potential for widespread applications in various hydrogen-related technologies, such as PEC hydrogen production, hydrogen fuel cells, and hydrogen storage.

Acknowledgments

X. Wang acknowledges the support of the research fund by the Institute of Carbon Neutrality, ShanghaiTech University. H. Shen and B. Wang thank the support of the research fund by Yiwu Research Institute, Fudan University.

Conflict of interest

The authors declare no conflict of interest.