Direct Toluene Electro-hydrogenation Using Anion Exchange Membrane Electrolyzer

Abstract

Direct toluene electro-hydrogenation is gaining considerable attention for storing and transporting large amounts of energy. When a proton exchange membrane (PEM) is used in this system, dragged water molecules migrate with the protons from the anode to the cathode, and water inhibits the toluene hydrogenation reaction at the cathode. An anion exchange membrane (AEM) is expected to facilitate the migration of water from the cathode to the anode, and less water suppresses the side reactions. In this study, the quantity of water reaching the cathode is determined. Further, direct toluene electro-hydrogenation without side reactions except hydrogen generation is successfully performed using the AEM. Moreover, optimization of the catalyst loading results in an improved current efficiency exceeding 1.5 mg cm⁻². This technology is valuable as a first step in direct toluene electro-hydrogenation using AEM.

1. Introduction

Energy carriers, such as organic hydrides, liquid hydrogen, and ammonia, have generated remarkable interest in storing and transporting large quantities of time- and regionally uneven renewable energy.¹ However, hydrogen must be cooled to −253 °C for liquefaction to obtain liquid hydrogen. Hence, the development of a new exclusive infrastructure is essential.² Further, using ammonia is challenging owing to its physical properties, such as toxicity, corrosiveness, and pungent smell.³ Although organic hydrides, especially toluene/methylcyclohexane (MCH) systems, are inferior to liquid hydrogen and ammonia in terms of efficiency, they offer several advantages, such as easy handling owing to their liquid state at ambient temperature and pressure and low toxicity. Further, toluene is included in gasoline; therefore, the existing petroleum infrastructure could be effectively used.^1,4 These energy carriers require further advancements to overcome their limitations. We have developed an electrolyzer for direct toluene electro-hydrogenation, which acts as an energy carrier for synthesizing toluene/MCH systems using renewable electricity and water. Hydrogenation of toluene and water electrolysis are performed simultaneously by PEM electrolysis in this system.⁵ Various studies have been performed, including optimizing the cathode flow field, loading the hydrogenation Pt catalyst in the porous transport layer for chemical hydrogenation, identifying cathode catalyst types, and determining cathode catalyst loading amounts.^5–8 However, there are two issues associated with PEM; the first is the mass transfer limitation at the cathode, which is attributed to an inhibition in the hydrophobic toluene supply due to the dragged water migrating from the anode to the cathode; therefore, the side reaction, i.e., hydrogen reduction occurs.⁹ The second is an increase in cost due to the usage of precious acid-resistant metal catalysts and constituent materials required under acidic conditions.¹⁰ To overcome these issues, AEM is increasingly used for water electrolysis and fuel cells.^11,12 Using precious metals in PEM ensures high-performance technology. Additionally, polymer electrolyte membrane water electrolyzer (PEMWE) can operate stably for over several thousand hours with demineralized water at 1.5–2 A cm⁻² and 1.7–2 V.¹³ However, AEMWE operates at only a few hundred mA cm⁻² and 2 V with dilute electrolyte or deionized water, resulting in poor performance.^13,14 The greatest challenge in the AEM is the poor conductivity and stability of the membrane and ionomer. Hydroxide ions are chemically and mechanically unstable and exhibit lower conductivity compared to that of protons. An improvement in ionic conductivity increases the water content, thus resulting in poor mechanical durability.¹³ Hence, remarkable studies have been conducted to improve the performance.^15,16 In this study, we investigated the direct toluene electro-hydrogenation with an AEM. The direct electro-hydrogenation of toluene using AEM proceeds via the following reactions (Eqs. 1–2):

  

\begin{equation} \text{Anode:}\ \text{4OH$^{-}$} \to \text{O$_{2}$} + \text{2H$_{2}$O} + \text{4e$^{-}$} \end{equation} (1)

  

\begin{equation} \text{Cathode:}\ \text{Toluene} + \text{6H$_{2}$O} + \text{6e$^{-}$} \to \text{MCH} + \text{6OH$^{-}$} \end{equation} (2)

No difference is there in the total chemical equation and electromotive force utilizing AEM compared to that utilizing PEM; only the direction of the ion gets reversed. Here, oxygen evolution proceeds at the anode (E° = 1.23 V vs. RHE) and toluene hydrogenation reactions at the cathode (E° = 0.15 V vs. RHE), respectively.⁹ Therefore, the theoretical decomposition voltage of the system is 1.08 V, which is lower than that of water electrolysis.⁹ Details of the cathode reaction with H⁺ type Nafion^® and OH⁻ type AS4 ionomers reactions are as follows.

  

\begin{equation} \text{H$^{+}$ type:}\ \text{H$^{+}$} + \text{e$^{-}$} \to \text{H$_{\text{ad}}$} \end{equation} (3)

  

\begin{equation} \text{OH$^{-}$ type:}\ \text{H$_{2}$O} + \text{e$^{-}$} \to \text{H$_{\text{ad}}$} + \text{OH$^{-}$} \end{equation} (4)

  

\begin{equation} \text{Both types:}\ \text{Toluene} + \text{6H$_{\text{ad}}$} \to \text{MCH} \end{equation} (5)

Here, the onset potential of electro-hydrogenation is above 0 V vs. RHE, so under potential deposited hydrogen can contribute hydrogenation, but over potential deposit hydrogen also contributes to hydrogenation in practical reaction conditions. AEM offers several advantages that overcome the challenges of PEM. First, the direction of the ion is reversed; therefore, dragged water moves from the cathode to the anode;¹⁶ thus solving the toluene supply inhibition. On the other hand, the cathode reaction needs water as a reactant, which must be diffused from the anode side. Second, it is cost-effective because it uses inexpensive catalysts such as Ni, Co, and Fe. Further, we evaluated the applicability of direct toluene electro-hydrogenation with AEM by changing the ionomer and catalyst loading in the cathode catalyst layer. The AS4 ionomer of OH⁻ and Nafion^® ionomer of H⁺ conductive types were compared to investigate toluene hydrogenation in both H⁺ and OH⁻ environments. Moreover, catalyst loading was explored to optimize the loading of the cathode catalyst layer.

2. Experimental

2.1 Preparing cathode catalyst

The cathode was prepared by supporting 0.02 mg cm⁻² of platinum on carbon paper (39BB, SGL Carbon Ltd.), acting as the substrate and toluene channel. The catalyst ink was prepared using deionized water, 1-propanol, ionomer, and PtRu/C (TEC61E54K, TKK) in a solid ratio of approximately 8 wt%. The ionomer/carbon weight ratio (I/C) was 0.8 and applied using a bar coater.

2.2 Electrolyzer configuration

Figure 1 shows a schematic of the electrolyzer, and Table 1 lists the experimental conditions used. The experimental device, a direct toluene electro-hydrogenation electrolyzer, was constructed using a standard PEM water electrolysis cell, exhibiting a total active area of 11.55 cm² and equipped with a reference electrode.¹⁷ Three types of the electrolyzers are compared in Table 1. First, an AEM with OH⁻ conductive ionomer was fabricated and the experiment was performed at 50 °C. A201 (Tokuyama Co.) was used as the membrane for AEM. DSE^® (Dimensionally Stable Electrode, De Nora Permelec Ltd.) was used as the anode electrode for oxygen evolution, and a pressure of 0.5 MPa was applied. Carbon paper coated with PtRu/C and a 5 % AS4 ionomer (Tokuyama Co.) was used as the cathode. The loadings were varied as 0.5, 1.0, 1.5, and 2.0 mg cm⁻². 1 M (= mol dm⁻³) KOH and 10 % toluene (volume ratio to MCH) were circulated at 10 cm³ min⁻¹ to the anode and the cathode, respectively. Second, AEM with H⁺ ionomer (see Table 1) was used to investigate the effect of the ionomer on the cathode catalyst layer. The loading was 0.5 mg cm⁻², and only the ionomer was changed to 5 % Nafion^® (DuPont Inc.). Third, PEM with H⁺ ionomer (see Table 1) was utilized for comparison with conventional methods. The experiment was performed at 60 °C using Nafion^®117 (DuPont Inc.) as a membrane, and 1 M H₂SO₄ was circulated at 10 cm³ min⁻¹ to the anode.

Figure 1.

The schematic illustration of the electrolyzer.

Table 1. Specifications of the electrolyzer.

		AEM with OH− conductive ionomer	AEM with H+ conductive ionomer	PEM with H+ conductive ionomer
Electrode size/cm2		11.55	11.55	11.55
Cathode	Ionomer	AS4	Nafion®	Nafion®
	Catalyst	PtRu/C	PtRu/C	PtRu/C
	Loading/mg cm−2	0.5, 1.0, 1.5, 2.0	0.5	0.5
	Carbon Paper	39BB	39BB	39BB
	Catholyte	10 % toluene	10 % toluene	10 % toluene
Membrane		A201	A201	Nafion®117
Anode	Electrode	DSE®	DSE®	DSE®
	Anolyte	1 M KOH	1 M KOH	1 M H2SO4
Operating temperature/°C		50	50	60

2.3 Evaluation of cell performance

The cell voltage, cell potential, current efficiency, and quantity of dragged water were determined to evaluate the cell performance based on the type of ionomer in the cathode catalyst layer and the supported quantity of the cathode catalyst. To explore the applicability of direct toluene electro-hydrogenation with an AEM, the reaction products obtained using an AEM with an OH⁻ conductive ionomer were confirmed by gas chromatography (GC-2014 AFsc, Shimadzu Co.). In this case, toluene (100 %) was circulated at the cathode. Moreover, the electrochemical measurements were performed as follows: the cell voltages and potentials were recorded after 3 min of constant-voltage measurements, AC impedance measurements (0.1–10⁵ Hz) were determined 3 minutes after the constant voltage measurement, and the internal resistance was obtained by reading high-frequency intercept. In this study, the cell voltage is described as the value obtained after subtracting the product of internal resistance with current density. The ratio of toluene hydrogenation to hydrogen evolution, or the current efficiency, was determined using Faraday’s law from the volume ratio of the toluene/MCH solution and H₂ gas after 3 minutes of constant-current electrolysis. To measure the quantity of dragged water, the amount of water collected in the cathode reservoir after constant-current electrolysis was determined. After evaluating the AEM with the OH⁻ conductive ionomer and the H⁺ ionomer, the cathode catalyst layer surface was observed by scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX).

3. Results and Discussion

Figure 2 reveals the internal resistances, R, as a function of current for AS4 (square) and Nafion^® (rhombus) ionomers with A201 membrane under 1 M KOH circulation and Nafion^® ionomer using Nafion^®117 with 1 M H₂SO₄ anolyte (triangle). The internal resistances with the A201 membrane are approximately 0.5 Ω cm² for both the ionomers, which are higher than that with Nafion^®117, although A201 is 28 µm, thinner than 183 µm of Nafion^®117.^18,19 The internal resistance of the high-frequency intercept would correspond to the membrane properties without any effect on the current distribution in the membrane or catalyst layer. Perhaps the decrease in ionic conductivity due to water scarcity may be a factor. Figure 3 shows the cell voltages as a function of current without any effect of the internal resistance. The cell voltage with Nafion^® (rhombus) is lower than that with AS4 (square) using the A201 membrane. At 0.18 A cm⁻², the cell voltages are 1.7 and 1.8 V, approximately with Nafion^® and AS4, respectively. The cell voltage with PEM (triangle) is lower than that with AEM (square). Comparing the results with those of previously published reports reveals that the reduction in cell voltage for direct toluene electro-hydrogenation using AEM is challenging, as the iR-free performance of PEMWE and AEMWE is 1.0 and 0.50 A cm⁻², respectively, at 1.8 V.^13,20,21

Figure 2.

Internal resistance as a function of current for AS4 (square) and Nafion^® (rhombus) ionomer with A201 membrane and 1 M KOH anolyte, and Nafion^® ionomer and membrane (triangle) with 1 M H₂SO₄ anolyte.

Figure 3.

Cell voltage as a function of current for AS4 (square) and Nafion^® (rhombus) ionomer with A201 membrane and 1 M KOH anolyte, and Nafion^® ionomer and membrane (triangle) with 1 M H₂SO₄ anolyte.

Figure 4 shows the cathode potentials, which decrease in the order: PEM (triangle) > AEM with Nafion^® ionomer (rhombus) > AEM with AS4 ionomer (square). The cathode potential with Nafion^® ionomer is higher than that with AS4 ionomer using A201 membrane, suggesting that the overvoltage for hydrogen generation or toluene hydrogenation is smaller when Nafion^® is used. Excellent cell voltage and cathode potential values are achieved when PEM is used as the membrane. This is because protons react to adsorbed hydrogen under acidic conditions, while adsorbed hydrogen must form from water under alkaline conditions. Therefore, the cathode potential is higher under alkaline conditions than that under acidic. In general, it is known that cathode overvoltage is larger in alkaline systems than in acid systems in water electrolysis.²²

Figure 4.

Cathode potential as a function of current for AS4 (square) and Nafion^® (rhombus) ionomer with A201 membrane and 1 M KOH anolyte, and Nafion^® ionomer and membrane (triangle) with 1 M H₂SO₄ anolyte.

Figures 5 and 6 show the gas chromatograms of circulating toluene during constant-cell-voltage electrolysis. Here we checked the post-electrolysis sample for the presence of substances other than MCH. The cathode was circulated with 100 % toluene for 180 min at 2 V and the circulating reactant was sampled every 60 min for the measurements. Figure 5 shows the sample after 180 min of electrolysis (solid line) and the standard sample with 99 % toluene and 1 % MCH, (dotted line), revealing that the standard sample and sample after electrolysis overlap. The peaks at approximately 5.1 and 6 min correspond to MCH and toluene, respectively. Other peaks are not observed up to a retention time of 15 min except a slight peak at t = 11 min approximately, which is also present in the standard sample. Therefore, no products other than MCH could be identified in the AEM-type toluene direct hydrogenation. Bubble formation was observed during electrolysis, and it must be hydrogen since other gas evolution reactions are difficult in the reducing cathode condition. Figure 6 shows the retention time in the 5.0–5.4 min range of Fig. 5 for (a) standard sample with 99 % toluene and 1 % MCH, and at various electrolysis times, i.e., (b) 0, (c) 60, (d) 120, and (e) 180 min. The intensities of MCH peaks increase with increasing electrolysis time, confirming the direct electro-hydrogenation of toluene using AEM electrolysis.

Figure 5.

Gas chromatogram of the circulated toluene of the electrolyzer for AS4 ionomer with A201 membrane and 1 M KOH anolyte at 180 min in 2 V constant voltage electrolysis. Solid and dotted lines represent after electrolysis and standard samples with 99 % toluene and 1 % MCH, respectively.

Figure 6.

Gas chromatogram of circulated toluene during electrolysis in 5.0–5.4 min retention time for (a) standard sample with 99 % toluene and 1 % MCH, and various electrolysis times at (b) 0, (c) 60, (d) 120, and (e) 180 min in 2 V constant voltage electrolysis using AS4 ionomer with A201 membrane and 1 M KOH anolyte.

The quantities of dragged water as a function of current for AS4 ionomer (square) with A201 membrane under 1 M KOH circulation and Nafion^® membrane (triangle) under 1 M H₂SO₄ circulation are given in Fig. 7. The AEM type exhibits a remarkable reduction in the quantity of dragged water compared to that in the PEM type. In the PEM type, water is proportional to the current density; however, in AEM, it shows no dependence on the current density. It is constant at all current densities, which could be attributed to the ions moving through the membrane in opposite directions. In the AEM, OH⁻ migrates from the cathode to the anode owing to electrical flow, and water migrates along with it; hence, no water accumulates at the cathode. The small amount of water observed could be attributed to back-diffusion. We assume that the amount of the water always appears to be constant due to back-diffusion to compensate for the decrease in water with OH⁻. In contrast, protons migrate from the anode to the cathode using PEM owing to the electrical flow with water, resulting in a larger quantity of water migration. Therefore, a current density dependence is observed for the amount of dragged water.

Figure 7.

The flux of dragged water as a function of current for AS4 (square) ionomer with A201 membrane and 1 M KOH anolyte, and Nafion^® ionomer and membrane (triangle) with 1 M H₂SO₄ anolyte.

Figure 8 exhibits higher current efficiency with AS4 ionomer (square), which conducts hydroxide ions, than that with Nafion^® ionomer (rhombus) with A201 membrane. Compared to that of PEM (triangle), both AEMs result in lower current efficiency. PEM exhibits approximately 90 % current efficiency at 0.5 A cm⁻², whereas AEM shows approximately 20 % at 0.1 A cm⁻². The lower cathode potential could lead to a lower current efficiency of the AEM despite the lower dragged water content. The reaction mechanism of hydrogen chemisorption is more complicated under alkaline conditions than under acidic ones since it must be adsorbed from water, not from protons. Therefore, we think that the alkaline conditions lead to a lower cathode potential, and the current efficiency decreases because hydrogen generation, E° = 0 V vs. RHE, becomes dominant. In contrast, the toluene electro-hydrogenation occurs at E° = 0.15 V vs. RHE. A comparison of the cathodic potentials at 0.2 A cm⁻² in Fig. 4 reveals that the values are approximately −0.1 and −0.25 V for PEM and AEM with AS4 ionomer, respectively. Theoretically, when PEM is used, hydrogen generates below 0 V, but toluene hydrogenation was predominately dominant because toluene adsorption inhibits hydrogen evolution by the combination of adsorbed hydrogens or reaction of adsorbed hydrogen and proton. Therefore, the behavior and reactivity of toluene are also important to understand this phenomenon.

Figure 8.

Current efficiency as a function of current for AS4 (square) and Nafion^® (rhombus) ionomer with A201 membrane and 1 M KOH anolyte, and Nafion^® ionomer and membrane (triangle) with 1 M H₂SO₄ anolyte.

Figure 9 shows the SEM-EDX images of the catalyst layer on the carbon paper after electrolysis. F was observed by polytetrafluoroethylene (PTFE) treatment of carbon paper and F, S by Nafion^® dispersion. Potassium was observed only using Nafion^® in Fig. 9a — the proton-exchange group of the ionomer exchanges with potassium ions derived from the crossover of the anolyte. Therefore, the reactant forming the adsorbed hydrogen changes from protons to water. Thus, the overpotential of adsorbed hydrogen formation increases, and hydrogen gas generation is enhanced compared to the toluene hydrogenation, which leads to a sharper slope of Nafion^® than that of AS4 in Fig. 8.

Figure 9.

The surface of the cathode catalyst after testing ionomer with (a) Nafion^® and (b) AS4 with A201 membrane and 1 M KOH anolyte. Names of elements observed below the figure; red, orange, light orange, yellow, green, and blue represent C, F, S, K, Ru, and Pt, respectively.

Figure 10 shows the cell voltages for various catalyst loadings at 0.5 (square), 1.0 (triangle), 1.5 (circle), and 2.0 (rhombus) mg cm⁻². No correlation is observed between the cell voltages and the quantities of catalyst loading. Figure 11 shows the current efficiencies for various catalyst loadings, which increase with increasing loading, indicating that the hydrogenation reaction of toluene advances with increasing loadings. A large difference is observed between 1.0 and 1.5 mg cm⁻², suggesting that a 1.5 mg cm⁻² or higher loading should be used. The current efficiency increased despite no change in cell voltages with increasing loading. Another paper has reported an increase in current efficiency with increasing loadings, although the cell voltage remains the same.⁸ When the loading amount is high, the relative percentage of water present is low due to the thick catalyst layer. On the other hand, when the loading amount is low, the proportion of water present relative to the catalyst layer increases. This water promoted hydrogen generation.

Figure 10.

Cell voltage as a function of current for 0.5 (square), 1.0 (triangle), 1.5 (circle), and 2.0 (rhombus) mg cm⁻² for AS4 ionomer with A201 membrane and 1 M KOH anolyte.

Figure 11.

Current efficiency as a function of current for 0.5 (square), 1.0 (triangle), 1.5 (circle), and 2.0 (rhombus) mg cm⁻² for AS4 ionomer with A201 membrane and 1 M KOH anolyte.

4. Conclusions

The gas chromatogram of circulated toluene during electrolysis showed that toluene could be converted to MCH, suggesting that AEM direct toluene electro-hydrogenation is feasible. Further, no by-products were obtained except hydrogen evolution, and the quantity of dragged water in AEM was reduced compared to that in the PEM. Despite the reduction in water content, the current efficiency decreased; however, the highest current efficiency was achieved using the hydroxide type AS4 as the ionomer and with the cathode catalyst layer loading of over 1.5 mg cm⁻². Therefore, this study is useful since remarkable reports were not dated. Future experiments with higher cathode potentials are needed.

Acknowledgments

This study was based on the results obtained from the Development of Fundamental Technology for Advancement of Water Electrolysis Hydrogen Production in the Advancement of Hydrogen Technologies and Utilization Project (JPNP14021), commissioned by the New Energy and Industrial Technology Development Organization (NEDO). Tokuyama Corporation supplied the anion exchange membranes and ionomers, and De Nora Permelec Ltd. provided DSE^®. We are grateful to all the people concerned.

CRediT Authorship Contribution Statement

Rio Shinohara: Writing – original draft (Lead)

Kensaku Nagasawa: Supervision (Supporting)

Yoshiyuki Kuroda: Supervision (Supporting)

Kaoru Ikegami: Validation (Equal), Writing – review & editing (Supporting)

Shigenori Mitsushima: Supervision (Equal)

Conflict of Interest

The authors declare no conflict of interest in the manuscript.

Funding

New Energy and Industrial Technology Development Organization: P14021

Footnotes

A part of this paper has been presented in the 90th ECSJ Annual Meeting in 2023 (Presentation #2W07).

R. Shinohara: ECSJ Student Member

K. Nagasawa, Y. Kuroda, K. Ikegami, and S. Mitsushima: ECSJ Active Members

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