Laser-Induced Hydrogen Generation from Methanol with Graphene Aerogel as the Target.

The present work demonstrates a new concept of the efficient generation of hydrogen from methanol by the continuous wave laser diode irradiation of an immersed graphene aerogel (GA) scaffold as the target. It was observed that the process occurred very intensively when it was assisted by bright white light emission in the spot of a laser-irradiated GA scaffold. The yield of hydrogen emission increased exponentially with the applied laser power. The light emission was assisted by the intense production of H2, CH4, and CO gases. It was found that with increasing excitation laser power, the H2 generation increased at the expense of CO. It is shown that the volume of CO decreases because of the formation of C2 molecules and CO2 gases. The mechanism of the laser-driven dissociation of methanol was discussed in terms of the violent ejection of hot electrons from the GA surface as a result of the laser-induced light emission of the graphene target.

Introduction

The seeking of new green energy sources is a subject of intense investigation. In the face of climate change and increasing efficiency of renewable energy sources, the way to store it becomes a challenge. Methanol is one of the promising materials for chemical storage of energy because of high overall energy efficiency of its production.[1] As recent research shows, in combination with hydrogen, it can be used to increase efficiency of new types of dual-injection engines.[2−4] The H2 blending positively influences raising of thermal efficiency and mean effective pressure, as well as leads to extension of lean burn limit (air to fuel ratio) in methanol engines, which allows to reduce the fuel consumption.[5] The blending of H2 also caused significant reduction of emission of harmful byproducts such as hydrocarbons and carbon monoxide.[6]

The laser-induced (LI) synthesis of hydrogen from alcohols was the subject for a number of experiments. The processes of hydrogen emission proceeding by the pulse laser irradiation of the solutions of carbon allotropes dispersed in different solutions have been profoundly investigated in the recent years. The emission of hydrogen by the pulse laser irradiation of carbon powder in water was reported by Akimoto et al.[7] and by the pulse and continuous wave (CW) irradiation of graphene by Fasciani et al.[8] The photochemical reaction of laser-stimulated hydrogen generation from water with carbon and alcohol additives was discussed by Maeda et al.[9] who showed that ethanol improved the efficiency of the process more than methanol and isopropanol. Kawai and Sakata[10] reported hydrogen evolution from water using carbon and light. The application of intense pulsed lasers for the production of H2 from graphite and coal by using nanosecond pulsed Nd:YAG laser was reported by several groups.[11−16]

Graphene is a two dimensional (2D) network of sp2 hybridized carbon atoms, demonstrating a number of interesting physical and chemical properties, discovered by Geim and Novoselov.[17] Its unique photoelectric properties allow to use graphene hybrid materials for the light-stimulated production of hydrogen. A review of graphene-based materials as photocatalysts for light-driven hydrogen generation from water was presented by Xie et al.[18]

Hydrogen instead of methanol can effectively solve the harmful emissions from the use of methanol.[5,19] The present work discusses the efficient production of hydrogen form methanol (CH3OH) that is characterized by low carbon content, allowing to significantly reduce the emission of carbon dioxides.[20]

Studies of hydrogen generation from methanol were conducted through different reforming reactions, such as pyrolysis and catalytic techniques by using Pt, Pd, and Cu catalysts.[21] A noncatalytic reformation of methanol by using a microwave plasma reactor was described by Wang et al.[22]

The purpose of these studies was the application of a focused beam of low-power CW infrared laser diodes (LD) and graphene aerogel (GA) as a photocatalyst. The GA, being a three-dimensional multilayer bulk structure, preserves 2D graphene electronic features.[13] Such a structure is characterized by a large specific surface (several thousand square meters per gram), allowing for efficient irradiation with a laser beam. This study presents hydrogen production experiments performed by the laser reformation of methanol using graphene as a photocatalyst. A novel strategy for hydrogen generation by using a graphene scaffold immersed in methanol irradiated with focused beam of CW LD is proposed as a result. The efficient production of hydrogen was assisted by LI white light emission from the surface of the graphene scaffold linked with the simultaneous ejection of photoelectrons responsible for methanol reformation.

This effect can be potentially applied for simultaneous generation of hydrogen from methanol by laser irradiation for injection into new types of methanol engines.

Results and Discussion

In the course of the experiment, it was observed that the sample of the GA scaffold immersed in methanol emitted intense bright white light upon irradiation with the focused beam of infrared laser. The photo of the GA scaffold emitting light in methanol is shown in Figure . A movie presenting the dynamics of the process is attached in the Supporting Information. The intensity of hydrogen generation starts to occur for the focused laser beam at only 2 W power. The diameter of the excitation spot of the focused laser beam was 175 μm,[23] so the density of excitation power was very high 1.4 × 104 W/cm2.

Figure 1

Photos of emitting spot of laser-irradiated GA in methanol.

The generation of gases due to the LI dissociation of methanol for lower excitation power was observed in the form of bubbles growing at the irradiation spot that was floating up to the surface of the solution. The mean diameter of bubbles was determined to be 1–2 mm. Figure (left), which shows an interesting observation made during the experiment, and there were several simultaneous electrical discharges occurring inside the bubble. Their appearance may have resulted from the high-temperature pyrolysis of methanol due to the laser heating of graphene in the spot of laser irradiation or due to the LI dissociation of methanol. The pyrolysis of methanol occurs in the temperature range of 1073–1223 K[24] that is much higher than the temperature measured in the spot of irradiated graphene between 300 and 600 K;[23] hence, LI dissociation should be responsible for the observed bubble emission of gases. For higher excitation power, a funnel chimney transporting the gas products directly to the surface of liquid was formed (see Figure right). The time evolution of single gas bubble formation is shown in Figure S2 (Supporting Information).

The spectrum of LI white emission of GA in methanol excited with a 975 nm CW LD is illustrated in Figure . The measurement system scheme of LI emission is shown in Figure S3.

Figure 2

Emission spectra for the GA scaffold immersed in methanol irradiated with a focused beam of 975 nm LD for different excitation power values.

The spectrum was measured as a function of excitation power in the power range of 2–3.8 W. It consists of a broad band centered at 680 nm. Its origin is associated with the photoinduced sp2 → sp3 phase transition because of the multiphoton induced ionization of graphene in vacuum.[23,25] It is important to notice that the sp3 phase is transient and switching off the LD causes the return to the initial configuration. The LI white emission is the threshold phenomenon characterized by an exponential increase with the excitation power of a focused laser beam. In the present experiment, the observed spectrum of GA white emission in methanol is almost identical to that observed in vacuum however, it is characterized by a much higher excitation threshold. It was observed that the volume of emitted gases is closely related to the LI white emission of the GA scaffold. The LI light emission process was assisted by the efficient generation of gases from the illuminated spot with a diameter of 175 μm. The process of gas generation (hydrogen, methane, and carbon monoxide) is a threshold phenomenon. It was found that the intense generation of gases starts at a higher excitation laser power of around 2.8 W, which is comparable with the threshold of LI white emission of 1.8 W. The main products of the LI dissociation of methanol under the irradiation of the target with the power of 3.8 W apart from H2 were CO and CH4.

The measured gas products obtained in the experiments using argon as a carrier gas (5 mL/min) are shown in Figure and listed in Table . The calculated efficiency of H2 production is 0.3817 mL/min after LD irradiation for excitation power 3.8 W.

Figure 3

Gas products of LI dissociation of methanol by using the GA scaffold as the target (in Ar flow 5 mL/min).

Table 1

Volume Ratio of Gas Components Following LI Dissociation of Methanol Determined for Various Values of Laser Excitation Power

	gas products
laser power PO [W]	hydrogen H2 [%]	methane CH4 [%]	carbon monoxide CO [%]
2.50	43.96	19.78	36.26
2.75	48.48	20.35	31.17
3.00	53.45	20.88	25.66
3.15	57.63	20.22	22.14
3.30	57.79	20.04	22.17
3.50	56.75	19.67	23.58
3.65	56.82	20.1	23.08
3.80	57.30	19.81	22.89

The power dependencies of the LI light intensity of the GA scaffold in methanol and the volumes of evolved gases (H2, CH4, and CO) are plotted in Figure a. It is interesting to note that the increases in light intensity and gas volumes with excitation are closely related. The light emission intensity starts to increase at a much lower excitation power threshold 2 W, whereas all gases start to grow at 2.8 W. The maxima of gas evolutions were observed at an excitation power of 3.8 W, and for higher powers, both the light and gas emissions drop. It is known that hot electrons assist the white emission of graphene.[26] One can suppose that a higher intensity threshold is related to the requirement of a higher number of hot electrons to reach the ionization state of methanol. A higher number of hot electrons appear at a higher excitation intensity.[23]

Figure 4

Influence of excitation laser power on light emission and the volumes of generated gases (top). Comparison of LI light emission intensity and emitted gas fractions (H2, CO, and CH4) of the GA scaffold on excitation LD power in log/log scale (bottom).

Figure b shows the power dependence of light intensity I and gas volumes in a log/log scale using the power law expression I ∝ P, where N is the order parameter that may be related, according to the Keldysh theory,[27] to the number of absorbed photons, assuming multiphoton absorption or the electron impact factor.[27,28] The ionization energy of methanol 10.8 eV is much higher compared to graphene 4.2 eV. We suppose that this may be a reason why the intensity threshold for gas emission is 2 and 5 times higher compared to graphene. It means that for multiphoton ionization of methanol, the order of N-photon absorption is at least 2–3 photons more than that for multiphoton ionization of graphene. With the increasing order of multiphoton absorption, the excitation threshold shifts to higher values. This effect was observed by us for ionization of graphene, which was pumped with different excitation-energy CW lasers.[25] The theoretical model of increase of the multiphoton ionization threshold with excitation laser power was proposed by Krajewska et al.[29]

A comparative plot of the intensities of LI white light emission and emitted the gas fractions is shown in Figure .

It is important to note that the order parameter N for graphene foam in vacuum changed from 3.02 to 6.11, depending on the excitation power density.[23] The dominant fraction of the LI dissociation of methanol for an excitation power of 3.80 W is hydrogen—57.30%. An inspection of Table shows that all gas products of dissociation change with increasing excitation power up to 3.8 W.

The formation of gaseous stable products (H2, CO, and CH4) results from the direct dissociation of methanol molecules by ejected electrons from irradiated graphene. Because the ionization energy of graphene is 4.2 eV[23] and the ionization energy of methanol is much higher ∼10.5 eV,[30] at least three electrons must be involved in the ionization process. It is directly related to the higher-excitation intensity threshold required for emission of gases.

Table also shows that the volume of hydrogen increases with excitation power, whereas methane does not change significantly and carbon oxide strongly decreases. A hydrogen-to-carbon monoxide ratio changes from 1.26 for 2.5 W to 2.89 for excitation powers 3.8 W. This means that hydrogen increased 2.3 times relative to carbon monoxide. The power dependence of H2 and CO volumes is shown in Figure . One can see a strong increase in the H2 volume when the excitation power changed from 3 to 3.5 W, and an opposite behavior was observed for CO gas when the enhancement of hydrogen production occurred at the cost of consumption of CO.

Figure 5

Influence of excitation laser power on H2 and CO gas products because of photoreformation of CH3OH solution with graphene foam as the photocatalyst.

A similar behavior was observed for hydrogen generation by the laser irradiation of carbon powder in water by Akimoto et al.[7] They attributed this behavior to the reaction of carbon monoxide with water: CO + H2O → H2 + CO2. In our system, we can propose the following reaction with graphene as a photocatalyst: 2CO + CH3OH → H2 + H2O + C2 + CO2.

This reaction leads to reduction/decrease in the amount of carbon monoxide and the production of carbon dioxide, hydrogen, and diatomic carbon (dicarbon) molecules C2 with a general formula C=C. The presence of carbon dots is connected with the yellow color of methanol solution after irradiation (see: Supporting Information), which can be indirect evidence of transient C2 formation. The formation of C2 dicarbon molecules has recently been reported to be conducted by the femtosecond LI chemical reaction from CO2 molecules in water by Nishi et al.[31] The absorption, emission, and excitation spectra of the yellow methanol solution are shown in the Supporting Information in Figure S5. The observed absorption spectra are due to the different nanoscale forms of carbon as dicarbon, graphitic, and graphene dots.[32] The solution exhibits behavior characteristic for the graphene dots—shifting of emission bands with excitation.[33]

Conclusions

The novel method of hydrogen generation from methanol by irradiation with the CW LD of an immersed GA scaffold as a target has been presented. The graphene served as a photocatalyst. It was found that the generated volume of gases was assisted by the intense emission of white light from the irradiation spot at the GA surface. The light emission increased exponentially with incident laser power above the characteristic excitation power threshold. Increase of the laser power gas emission initiated at the excitation spot occurred but at a much larger excitation power. An exponential increase in the observed gas volumes with excitation power was well correlated with the LI light intensity of graphene. It is confirmed by multiphoton absorption as a source of ejected electrons responsible for the dissociation of methanol.

The main products are hydrogen, H2, carbon monoxide, CO, and methane, CH4. It was observed that the volume of hydrogen increased with excitation power, whereas the volume of carbon monoxide decreased. The amount of CH4 was almost constant in the experiment and independent of the applied laser power. The mechanism of such behavior was discussed in terms of CO transformation into CO2, water, and C2 dicarbon molecules. The CO2 gas product was not measured in the experiment, most probably because of its dissolution in methanol. Similar results were reported by Akimoto et al.[7] for laser-irradiated carbon powders in water.

The LI dissociation of methanol with graphene as a photocatalyst opens new possibilities in the application of hydrogen-enriched methanol as a fuel in automotive engines.

The mechanism of the LI dissociation of methanol was discussed in terms of the LI ejection of hot electrons assisting LI white light emission. The power threshold for gas production was higher than that corresponding to LI white light emission from graphene. It means that the photoreformation of methanol needs more energetic hot electrons.

The efficiency of laser-driven hydrogen production in the experiment was determined to be 0.38 mL/min, which is 22.8 mL after 1 h of laser irradiation from one irradiated spot. Such large efficiency suggests that the technology presented in this work may be successfully applied for the production of hydrogen on an industrial scale even by the simple multiplication of irradiated spots.

Experimental Section

The GA scaffolds were synthesized using the method described earlier by us.[23] The photo of GA samples is shown in Figure .

Figure 6

Image of GA scaffolds and the scaffold immersed in the cuvette.

The scheme of the experimental setup is pictured in Figure . The sample of GA 2 × 2 × 2 mm3 was submerged in methanol of 99.9% purity kept in a quartz vessel.

Figure 7

Scheme of the experimental setup for generation of hydrogen by laser irradiation of GA in methanol.

The measurements were controlled by using a direct digital synthesis generator. The GA sample was subjected to irradiation with the focused beam of a 980 nm CW IR LD. The excitation power of the laser was tuned from 1 to 3.8 W. Methanol temperature during the irradiation time (10 min) measured during the experiment did not exceed 30 °C for the whole volume of methanol, that is, 3 mL. No visible changes on the surface of GA were observed after irradiation of IR laser. The specific surface area was determined by N2 adsorption at −196 °C using a Sorptomatic 1990 Fisons Instruments apparatus and calculated using the Brunauer–Emmett–Teller (BET) method. The pore size distribution was measured using the Barrett, Joyner, and Halenda (BJH) method. The size of pores of the aerogel sample, calculated from the BET and BJH measurements, are in the range of 8–29 nm, and the specific surface area was determined to be 40 m2/g (see Supporting Information). The emission spectra were measured using a focused CW LD 4 W 975 nm (CNI Laser) as the excitation source and AVS-USB2000 spectrometer (Avantes) as a detector in a 250–900 nm spectral range. It was found that the GA scaffold was not affected by laser irradiation.