Application of Boron-doped Diamond Electrodes: Focusing on the Electrochemical Reduction of Carbon Dioxide

Abstract

Boron-doped diamond (BDD) electrodes are next generation electrode materials and their electrochemical applications have been actively developed in recent years. They are expected to be useful electrode materials for improving the environment and for bio-medical applications. Here, examples of practical applications as electrochemical sensors, the development of in vivo real time measurements, and electrochemical organic synthesis using BDD electrodes are briefly introduced. In the second part, our recent work on the production of useful chemicals by means of the electrochemical reduction of CO₂ using BDD electrodes is described. The work has attracted particular attention for its potential contribution to carbon neutrality and carbon recycling.

1. General Introduction

Boron-doped diamond (BDD) electrodes have unique electrochemical properties and can be considered as next-generation electrode materials. Based on their unique properties, they have recently been applied to several different electrochemical applications. The number of publications involving BDD electrochemistry research has been rapidly increasing year by year and several review articles and books on diamond-based electrodes have now been published.^1–10

For the last 20 years, our research group have reported not only fundamental studies on BDD, but also on several novel electrochemical applications such as electrochemical sensors (including biomedical),^11–18 electrochemiluminescence,^19–22 and electrochemical organic synthesis.^23–25 These applications are due to the unique properties of BDD electrodes, and they could not be achieved using conventional electrodes. In this article, we have focused on the synthesis of value-added chemicals from the electrochemical reduction of CO₂, which has attracted a lot of attention in recent years.

2. Development of Boron-doped Diamond Electrodes

Before the main topic of the electrochemical CO₂ reduction application, some other important electrochemical application examples using BDD electrodes are briefly described.

2.1 Practical application of electrochemical sensors

In 2008, we succeeded in achieving the highly sensitive detection of free chlorine as an example of utilizing the superior electrochemical properties of BDD electrodes, such as their wide potential window and low background current.²⁶ Previous work had shown that the oxidation peak of the hypochlorite ion (ClO⁻) observed at around 1.3 V (vs. Ag/AgCl) was difficult to detect it because it overlapped with oxygen evolution when using traditional electrodes. The successful detection of ClO⁻ is a good example of the superiority of BDD electrodes. However, to develop a measurement device for practical use, we encountered problems due to the pH fluctuation of the solution. Also, the detailed redox mechanism had not been completely clarified at that time. Later, in 2016, the electrochemical redox mechanism, including the reduction reaction of free chlorine, was carefully studied.²⁷ From these studies we could conclude that the total free chlorine concentration obtained by the sum of current values of both oxidation and reduction does not depend on the pH (Figs. 1a and 1b). Furthermore, the pH of the solution can be measured by calculating the abundance ratio. Based on these results, we designed a prototype system using BDD electrodes that can monitor the free chlorine concentration in real time. Following this work, HORIBA Advanced Techno, Co., Ltd. developed this approach into a commercial free chlorine sensor (Fig. 1c). This example shows the future potential of practical BDD electrochemical sensors.

Figure 1.

(a) Cyclic voltammograms for 100 ppm NaClO in 0.1 M NaClO₄ with different pH values at scan rates of 20 mV/s using BDD electrode. (b) The pH dependence of peak current at −0.4 V (vs. Ag/AgCl) (blue), at +1.6 V (vs. Ag/AgCl) (red), and the sum of both current values (green). (c) Commercial product for free chlorine sensor produced by HORIBA Advanced Techno, Co., Ltd.

2.2 In vivo real time monitoring using BDD microelectrodes

It is important to miniaturize the BDD electrodes for application in the study of electrochemical sensing in living beings. Here, we introduce the application of BDD microelectrodes to biomedical measurements. Firstly, in 2007, we succeeded in measuring dopamine in the mouse brain in vivo. By controlling the surface termination of the BDD microelectrode, we succeeded in achieving high-sensitivity, real-time, and selective dopamine detection.¹² Then, in 2012, we succeeded in measuring glutathione in tissues and showed that the effect of this cancer treatment could be measured in real time.¹³ Furthermore, in 2017, we developed a microsensing system for the in vivo real time detection of local drug kinetics and explored its physiological relevance (Fig. 2).¹⁶ The system consists of two different sensors with both a micro-sensor composed of BDD microelectrodes with tip diameter ∼40 µm and a glass microelectrode. By using this system, we have first tested bumetanide, a diuretic that is ototoxic, but applicable to epilepsy treatment.

Figure 2.

(a) BDD microsensor for the detection of bumetanide was inserted into the perilymph of the scala tympani (ST) of the guinea pig cochlea, and a double-barreled glass microcapillary encasing a reference (RE; Ag/AgCl) and counter electrode (CE; platinum) was placed in close proximity. The EP was simultaneously monitored using a glass microelectrode in the endolymph of the scala media (SM). SV, scala vestibule; StV; stria vascularis. (b) In vivo measurements in a guinea pig cochlear using the microsensor electrode. The BDD microelectrode subtraction current (SC; green with left axis) in the perilymph of the scala tympani and the EP (red with right axis) measured with a microelectrode in the endolymph of the scala media are shown. (Reproduced with permission from Ref. 16. Copyright 2017 Springer Nature.)

2.3 Electrochemical organic synthesis

It is known that the efficient generation of OH radicals occurs by applying a potential using BDD electrodes in aqueous solutions.^28,29 In fact, waste water treatment using BDD electrodes is one important direction for electrochemical applications.^30–32 From this point of view, it is expected that active species could be produced by electrolysis in an organic solvent using BDD electrodes. In fact, when electrochemical oxidation was performed in methanol at 1.0 V (vs. SEC) using BDD electrodes to investigate whether radical species were generated, efficient methoxy radical generation could be observed.²³ Then, as an example, isoeugenol, which is an inexpensive material found in plants, was electrochemically oxidized in methanol and licarin A was produced, a product having known anti-inflammatory activity. It is considered that this process was due to the methoxy radical, which is an electrolyzed active species generated on the BDD electrodes. This radical participates in the reaction and enables the organic synthesis of useful compounds. In addition, it has been found that the use of BDD electrodes is also effective for the production of superoxide anions by means of a reduction reaction.²⁵ The research direction of electrochemical organic synthesis via active species that can be specifically generated by BDD electrodes is expected to be important in the future.

3. Electrochemical Reduction of CO₂

3.1 Introduction

Electrochemical reduction is one of the methods for producing useful chemicals from CO₂, which is an abundant carbon resource. Compounds such as formic acid (HCOOH), carbon monoxide (CO), hydrocarbons, and alcohol can be obtained.³³ It is known that the Faradaic efficiency and selectivity of these products depends on the electrode materials and electrolyte. So far, many CO₂ reductions by metal electrodes have been reported. The metal electrodes used as cathodes are mainly categorized into three groups. The first group of electrodes (Sn, Hg, and Pb), on which the intermediate CO₂ anion radical (CO₂^•−) is difficult to adsorb, produce mainly HCOOH. The second group (Au, Ag), onto which the intermediate adsorbs, produce carbon monoxide (CO). The third group (Cu), which further adsorbs CO, produces hydrocarbons and alcohols.³⁴ It is important to note that some of these metals are toxic, some rare metals and some have low durability. In addition, when CO₂ reduction is performed in an aqueous solution, the applied potential has to be largely negative. It means that a hydrogen evolution reaction occurs as a competitive reaction. Therefore, it is necessary to select an electrode with a wide potential window.

BDD electrodes are metal-free next-generation electrode materials consisting only of carbon and boron.^1–10 Our expectation is of a wide potential window in an aqueous solution and that they would be highly durable due to the diamond structure, so we have applied BDD electrodes as CO₂ reduction cathodes.

So far, through the process of CO₂ reduction using BDD electrodes, the production of C1 compounds such as formic acid,^35–38 carbon monoxide,^39–44 and formaldehyde⁴⁵ have been reported. The production of C2 and C3 compounds such as methanol,⁴⁶ ethanol,^47,48 acetone,⁴⁷ and acetic acid⁴⁹ by using metal modified BDD electrodes have also been reported.⁵⁰ Here, our recent attempts on the electrochemical reduction of CO₂ on BDD electrodes, especially HCOOH formation and CO formation, are introduced.

3.2 Production of formic acid

In 2018, an example in which formic acid (HCOOH) was mainly produced by applying a current of −2.0 mA cm⁻² in an electrochemical CO₂ reduction using BDD electrodes was reported. A two-chamber flow cell separated by a Nafion membrane, which is a cation exchange membrane, was designed (Fig. 3).³⁵ A BDD electrode with a boron concentration of 0.1 % was used for the cathode, a Pt plate was used for the anode, and Ag/AgCl was used for the reference electrode. KCl aqueous solution (0.5 mol L⁻¹) saturated with CO₂ gas was used as the catholyte. The flow rate was controlled (20, 50, 100, 200, 500 mL min⁻¹), and the electrochemical reduction was performed for 1 hour at 2.0 mA cm⁻². The main product obtained was HCOOH, although a small amount of carbon monoxide (CO) and hydrogen (H₂) was also observed. It was suggested that HCOOH is likely to be produced on BDD electrodes because the surface of the electrodes are inert,^7,10 whereas CO₂^•−, which is a reaction intermediate, is not easily adsorbed.³⁴ When the flow rate was optimized (200 mL min⁻¹), a remarkable Faradaic efficiency of 94.7 % was observed.

Figure 3.

Schematic diagram of two-compartment flow cell. (Reproduced with permission from Ref. 35. Copyright 2018, Wiley-VCH Verlag GmbH & KGaA.)

To increase the amount of HCOOH production, the applied current density was then increased. Although the Faradaic efficiency of HCOOH production decreased, the amount of HCOOH production increased to 473 µmol m⁻² s⁻¹, which is more than that produced in the same experimental conditions using Sn and Pb electrodes (440 µmol m⁻² s⁻¹).⁵¹

The electrochemical properties of BDD electrodes are known to be boron concentration-dependent.⁵² Therefore, the effect of the boron concentration of the BDD electrodes on CO₂ reduction was studied. Electrochemical reduction was performed for 1 hour at an applied current density of −2.0 mA cm⁻² using BDD electrodes with boron concentrations of 0.01 %, 0.1 %, 0.5 %, 1 %, and 2 %, respectively (Fig. 4).³⁶ The maximum current efficiency of HCOOH production was observed when the boron concentration was 0.1 %. When the boron concentration increased, the Faradaic efficiency of CO production increased slightly. Since CO is easily generated on the electrode on which CO₂^•− is adsorbed,³⁴ it is suggested that a BDD electrode with a high boron concentration is more likely to adsorb CO₂^•−.

Figure 4.

Faradaic efficiencies for producing formic acid (red), hydrogen (green), and carbon monoxide (blue) by the electrochemical reduction of CO₂ on BDD. (Reproduced with permission from Ref. 36. Copyright 2018, Elsevier.)

Generally, it is known that the Faradaic efficiency and selectivity of reduction products are affected by the electrolyte.^33,53 Therefore, we have investigated the effect of the electrolyte on CO₂ reduction on BDD electrodes.³⁷ Firstly, the effects of cations in the electrolyte were studied.

When CO₂ reductions were performed in electrolytes with alkali metal halides (MX: M = Li, Na, K, Rb, Cs; X = Cl, Br, I), mainly HCOOH and a small amount of CO and hydrogen were produced. When the cation size was large (K⁺, Rb⁺, Cs⁺), the Faradaic efficiency of HCOOH production was high and that of hydrogen was low. On the other hand, when the cation size was small (Li⁺, Na⁺), the opposite tendency was observed (Fig. 5a). It is suggested that the highly efficient production of HCOOH with large alkali metal ions is due to the buffering effects of the hydrated alkali metal ions.⁵³

Figure 5.

Faradaic efficiencies for producing formic acid (black) and hydrogen (white) by the electrochemical reduction with a BDD electrode in catholytes of (a) 0.5 mol L⁻¹ MCl (M = Li, Na, K, Rb, and Cs), and (b) 0.5 mol L⁻¹ KNO₃, 0.25 mol L⁻¹ K₂SO₄, 0.5 mol L⁻¹ KX (X = Cl, Br, and I). (Reproduced with permission from Ref. 37. Copyright 2018, Wiley-VCH Verlag GmbH & KGaA.)

During the electrolysis, the pH near the electrode increases because the concentration of protons (H⁺) in the solution is decreased due to the CO₂ reduction reaction and the hydrogen evolution reaction. Here, hydrated cations are known to release protons in solution. The reaction is promoted as the cation size is larger and as the cation is less likely to be hydrated. Therefore, the larger the size of the cation, the more the pH increase near the electrode is suppressed. As a result, the concentration of hydrated CO₂, which is considered to be electrochemically reduced, is maintained high and HCOOH could be produced with high efficiency.⁵³

Next, the effects of anions in the electrolyte were studied. The cations were fixed to K⁺ and anions NO₃⁻, SO₄²⁻, Cl⁻, and ClO₄⁻ were examined (Fig. 5b). In the case of NO₃⁻, no CO₂ reduction product was obtained and almost no hydrogen evolution occurred. This is because the main reaction was the reduction of NO₃⁻.⁵⁴ HCOOH was produced with high efficiency when SO₄²⁻ and Cl⁻, which can be specifically adsorbed on the electrode, were used. Anions that are specifically adsorbed on the electrode are thought to keep CO₂ near the electrode and promote electron transfer from the electrode to CO₂.⁵⁵ On the other hand, when ClO₄⁻, which does not specifically adsorb on the electrode, HCOOH production was suppressed, but the Faradaic efficiency of CO production was higher than when other anions were used. Details will be discussed in the next section.

As described above, we have achieved the highly efficient and selective production of HCOOH in CO₂ reduction on BDD electrodes. It was found that the Faradaic efficiency was affected by the boron concentration of the BDD and the type of electrolyte.

3.3 Production of carbon monoxide

In 3.2, it was shown that when KClO₄ was used as the electrolyte, the production of CO was higher than that when other electrolytes were used. In order to improve the Faradaic efficiency and selectivity of CO production further, we investigated in detail the CO₂ reduction reaction when the electrolyte was KClO₄.³⁹

First, the applied potential dependence was investigated in KClO₄ solution using 0.1 % BDD for the cathode. Both CO and HCOOH occurred as products, and a maximum Faradaic efficiency for the CO production was seen at −2.1 V (vs. Ag/AgCl). Then, the CO₂ reduction performance using a KCl solution was compared with that using a KClO₄ solution at −2.1 V (vs. Ag/AgCl) (Fig. 6a). When KCl was used, the Faradaic efficiency of HCOOH production was very high, as shown in 3.2. This suggests that CO is likely to be produced when KClO₄ is used. Considering that CO is easily produced on the electrode on which intermediate CO₂^•− is adsorbed and that HCOOH is easily generated on the electrode on which CO₂^•− is difficult to adsorb,³⁴ the obtained results were consistent with expectations. That is, when the electrolyte is KClO₄, intermediate CO₂^•− can be adsorbed on the electrode, and when the electrolyte is KCl, CO₂^•− it is difficult to adsorb this on the electrodes.

Figure 6.

(a) Faradaic efficiencies for producing carbon monoxide (gray), formic acid (black), and hydrogen (white) by the electrochemical reduction of CO₂ using a 0.1 % BDD electrode at −2.1 V (vs. Ag/AgCl) in 0.1 mol L⁻¹ KClO₄ and KCl aqueous solution. (b) A schematic image of attenuated total reflection infrared (ATR-IR) measurements on BDD electrode deposited on Si ATR-IR prism. (c) Comparison of ATR-IR spectra taken (30 minutes) during electrochemical reduction of CO₂ in 0.1 mol L⁻¹ KClO₄ and KCl aqueous solutions at −2.1 V (vs. Ag/AgCl). (Reproduced with permission from Ref. 39. Copyright 2019, American Chemical Society.)

To confirm the generation of CO₂^•− during electrolysis on the BDD surface, attenuated total reflection infrared spectroscopy (ATR-IR), which enables in situ observation of the reaction, was performed.³⁹ For the ATR-IR measurements, a sub-micrometer-thick BDD film was deposited onto a Si ATR-IR prism using the microwave plasma-assisted chemical vapor deposition (MPCVD) system (Fig. 6b). When ATR-IR measurements during CO₂ reduction were performed in aqueous solutions of KClO₄ and KCl, a peak attributed to an OCO antisymmetric vibration of CO₂^•− was observed at around 1600 cm⁻¹. The peak was at around 1634 cm⁻¹ in KClO₄ solution, while at around 1616 cm⁻¹ in KCl solution (Fig. 6c). It is suggested that this peak shift is due to a difference in the nature of the adsorption of the CO₂^•− on the BDD electrodes in the two different solutions.

To check the state of the CO₂^•−, a similar ATR-IR measurement was performed using a BDD electrode synthesized from ¹³CH₄ gas as a carbon source (¹³C-BDD). The peak attributed to CO₂^•− showed a shift from 1634 cm⁻¹ to 1624 cm⁻¹ only in the KClO₄ solution. The results suggest that CO₂^•− is adsorbed on the electrode in the KClO₄ solution and exists freely in KCl solution. It is considered that the strength of the adsorption of the CO₂^•− intermediates on BDD surface can be controlled by the electrolyte, and then, the selectivity of the product can be controlled.

Next, to improve the selectivity of CO production and the Faradaic efficiency, various conditions such as the boron concentration of BDD electrodes and the flow rate of the solutions were optimized. CO₂ reductions at various applied potentials were performed using 0.1 % BDD electrodes in both KClO₄ and KCl solution. At any potential, the Faradaic efficiency of CO production was higher in the KClO₄ solution, while that of HCOOH was higher in the KCl solution. Then, when 1 % BDD electrodes were used for the CO₂ reduction in KClO₄ solution, the hydrogen generation increased, but the Faradaic efficiency of CO production also increased. Also, the selectivity of CO production (ratio of CO to HCOOH production) also increased. It is suggested that BDD electrodes with a higher boron concentration (1 %) have more acceptor levels and holes than for 0.1 % BDD⁵⁶ and are more likely to adsorb CO₂^•−. More so, when the flow rate of the reactor was increased, hydrogen generation was significantly reduced and the production of CO and HCOOH was improved. This is because the amount of CO₂ supplied to the electrode surface increased. As described above, we have achieved the production of carbon monoxide with a current efficiency of up to 68 % by using KClO₄ as the electrolyte and BDD electrodes with a higher boron concentration.

3.4 Scale up of the electrolysis cell

As described in 3.2, a Faradaic efficiency of ca. 100 % for HCOOH production was achieved by optimizing the electrolysis conditions using BDD electrodes. The cell used at the time was a small one designed for the laboratory experiments with an electrode area of 9.6 cm² and a solution volume of 100 mL. In order to consider industrialization, a medium-sized reactor with an electrode area of 30 cm² and a solution volume of 1000 mL was designed. In the beginning, the Faradaic efficiency of HCOOH production was observed to be 10 % or less, and there were clearly problems in increasing the size of the electrolysis reactor.

3.5 Intermittent flow system

In the medium-sized cell described in 3.4, we tried to control flow conditions of the solution. The medium-sized cell in 3.4, in which the Faradaic efficiency of formic acid was 10 % or less, was based on continuous flow using a centrifugal pump, and the solution coming to the cell was continuous. Here, the flow conditions, i.e., the pressure of the solution to the cell, was controlled as a continuous liquid-fed intermittent flow system by using a synchronized dual-phase double-action cylindrical pump. The solution was supplied to the cell by intermittent flow in which the solution pressure goes to zero at every 1 second (Fig. 7a: right axis). At the same time, the current value follows this change in the solution pressure (Fig. 7a left axis).

Figure 7.

(a) Current (black) and pressure (red) for the electrochemical reduction of CO₂ in the intermittent flow cell. (b) Formic acid production and (c) Faradaic efficiency for the electrochemical reduction of CO₂ using a BDD electrode in the continuous (black) and intermittent (red) flow cells for 4 hours. (Reproduced with permission from Ref. 57. Copyright 2021, American Chemical Society.)

As a result, not only the amount of HCOOH production increased with time, but also the Faradaic efficiency reached to 96.1 %, demonstrating the importance of the intermittent flow system (Fig. 7c).⁵⁷ By the way, recently, we found that the flow conditions of the small cell designed for the laboratory experiments (shown in 3.2), which showed ca. 100 % Faradaic efficiency of HCOOH production, was also coincidentally intermittent flow, although we did not care with the flow conditions at that time.

Generally, it is known that the electrochemical reduction of CO₂ at BDD electrodes occurs via two steps, the generation of CO₂^•− and the further reduction of the CO₂^•− with protons (H⁺) to HCOOH. In this process, it is suggested that the efficiency of the chemical reaction between CO₂^•− and H⁺ is affected by the flow of the solution. Although more detailed studies based on the reaction mechanism and reaction rate theory are needed, we can conclude that design of the reactor and flow conditions of the solution are important for the optimized production of chemicals by electrochemical methods.

4. Conclusion

We have focused this report on the electrochemical CO₂ reduction by BDD electrodes as it is one of the most attractive applications these days. It was found that useful chemicals such as HCOOH and CO can be electrochemically synthesized with a high Faradaic efficiency by using BDD electrodes. We also found that the product could be controlled by electrochemical conditions such as the applied potential, applied current density, and electrolyte, etc. Also, the construction of appropriate reaction systems considering the flow conditions and the cell structure is also critical.

Furthermore, until now, our fundamental research has been mainly conducted with a 3-electrode system using Ag/AgCl reference electrodes. However, recently, we have conducted a similar study on a 2-electrode system by controlling the applied voltage, which is important for industrialization. The study has shown that formic acid could be produced with high efficiency as well.⁵⁸

Moreover, BDD electrodes retain their durability as a consequence of the diamond material structure and it has been confirmed that stable products could be obtained without corrosion even after long-term operation. This durability is attractive for electrodes for industrial CO₂ reduction.

On the other hand, although BDD electrodes, which are carbon electrodes, showed unique electrochemical properties compared with conventional electrode materials, further studies on fundamentals of BDD electrodes are still required. Also, the detailed mechanism of the CO₂ reduction reaction on the electrode has not yet been fully clarified. Therefore, further fundamental studies such as theoretical analysis and in situ spectroscopy will be important to elucidate the mechanisms at work on BDD electrodes.

Acknowledgments

The author would like to express thanks to all co-workers, especially Dr. K. Natsui, Dr. M. Tomisaki, Dr. J. Xu, Dr. P. K. Jiwanti, Dr. Irkham, Mr. S. Nagashima (Keio University), and Prof. K. Nakata (Tokyo University of Agriculture and Technology) for CO₂ reduction research. Also, I would like to express thanks to Dr. T. Yamamoto, Dr. G. Ogata, Dr. M. Murata and Prof. S. Nishiyama (Keio University), Dr. T. A. Ivandini (University of Indonesia), Dr. T. Watanabe (Aoyama Gakuin University), Dr. T. Saitoh (Tsukuba University), Prof. H. Saya (Medical school, Keio University), and Prof. H. Hibino (Medical school, Niigata University) for collaboration on BDD electrodes. This work was partially supported by JST-CREST (2011–2014) and JST-ACCEL (2014–2020).

CRediT Authorship Contribution Statement

Yasuaki Einaga: Conceptualization (Lead), Funding acquisition (Lead), Investigation (Lead), Project administration (Lead), Supervision (Lead), Writing – original draft (Lead), Writing – review & editing (Lead)

Conflict of Interest

The authors declare no conflict of interest in the manuscript.

Funding

Japan Science and Technology Corporation: JPMJCR1071

Japan Science and Technology Corporation: JPMJAC1402

Footnotes

Y. Einaga: ECSJ Active Member

References

• 1)  Diamond Electrochemistry (Eds. A. Fujishima, Y. Einaga, T. N. Rao, and D. A. Tryk), BKC and Elsevier (2005).
• 2)  A. Kraft, Int. J. Electrochem. Sci., 2, 355 (2007).
• 3)  Y. Einaga, J. S. Foord, and G. M. Swain, MRS Bull., 39, 525 (2014).
• 4)  J. V. Macpherson, Phys. Chem. Chem. Phys., 17, 2935 (2015).
• 5)  N. Yang, J. S. Foord, and X. Jiang, Carbon, 99, 90 (2016).
• 6)  N. Yang, S. Yu, J. V. Macpherson, Y. Einaga, H. Zhao, G. Zhao, G. M. Swain, and X. Jiang, Chem. Soc. Rev., 48, 157 (2019).
• 7)  Y. Einaga, Bull. Chem. Soc. Jpn., 91, 1752 (2018).
• 8)  S. J. Cobb, Z. J. Ayres, and J. V. Macpherson, Annu. Rev. Anal. Chem., 11, 463 (2018).
• 9)  B. C. Lourencao, R. R. Brocenschi, R. C. Rocha, B. C. Lourencao, R. F. Brocenschi, R. A. Medeiros, F. O. Fatibello, and R. C. Rocha-Filho, ChemElectroChem, 7, 1291 (2020).
• 10)  Diamond Electrodes (Ed. Y. Einaga), Springer Nature (2022).
• 11)  Y. Einaga, J. Appl. Electrochem., 40, 1807 (2010).
• 12)  A. Suzuki, T. A. Ivandini, K. Yoshimi, A. Fujishima, G. Oyama, T. Nakazato, N. Hattori, S. Kitazawa, and Y. Einaga, Anal. Chem., 79, 8608 (2007).
• 13)  S. Fierro, M. Yoshikawa, O. Nagano, K. Yoshimi, H. Saya, and Y. Einaga, Sci. Rep., 2, 901 (2012).
• 14)  Y. Ishii, T. A. Ivandini, K. Murata, and Y. Einaga, Anal. Chem., 85, 4284 (2013).
• 15)  T. Matsubara, M. Ujie, T. Yamamoto, M. Akahori, Y. Einaga, and T. Sato, Proc. Natl. Acad. Sci. U.S.A., 113, 8981 (2016).
• 16)  G. Ogata, Y. Ishii, K. Asai, Y. Sano, F. Nin, T. Yoshida, T. Higuchi, S. Sawamura, T. Ota, K. Hori, K. Maeda, S. Komune, K. Doi, M. Takai, I. Findlay, H. Kusuhara, Y. Einaga, and H. Hibino, Nat. Biomed. Eng., 1, 654 (2017).
• 17)  A. Hanawa, G. Ogata, S. Sawamura, K. Asai, S. Kanzaki, H. Hibino, and Y. Eianga, Anal. Chem., 92, 13742 (2020).
• 18)  Y. Triana, G. Ogata, M. Tomisaki, Irkham, and Y. Einaga, Anal. Chem., 94, 3948 (2022).
• 19)  Irkham, T. Watanabe, A. Fiorani, G. Valenti, F. Paolucci, and Y. Einaga, J. Am. Chem. Soc., 138, 15636 (2016).
• 20)  A. Fiorani, Irkham, G. Valenti, F. Paolucci, and Y. Einaga, Anal. Chem., 90, 12959 (2018).
• 21)  Irkham, A. Fiorani, G. Valenti, N. Kamoshida, F. Paolucci, and Y. Einaga, J. Am. Chem. Soc., 142, 1518 (2020).
• 22)  Irkham, R. R. Rais, T. A. Ivandini, A. Fiorani, and Y. Einaga, Anal. Chem., 93, 2336 (2021).
• 23)  T. Sumi, T. Saitoh, K. Natsui, T. Yamamoto, M. Atobe, Y. Einaga, and S. Nishiyama, Angew. Chem., Int. Ed., 51, 5443 (2012).
• 24)  T. Yamamoto, B. Riehl, K. Naba, K. Nakahara, A. Wiebe, T. Saitoh, S. R. Waldvogel, and Y. Einaga, Chem. Commun., 54, 2771 (2018).
• 25)  Y. Zhang, T. Sugai, T. Yamamoto, N. Yamamoto, N. Kutsumura, Y. Einaga, S. Nishiyama, T. Saitoh, and H. Nagase, ChemElectroChem, 6, 4194 (2019).
• 26)  M. Murata, T. A. Ivandini, M. Shibata, S. Nomura, A. Fujishima, and Y. Einaga, J. Electroanal. Chem., 612, 29 (2008).
• 27)  T. Watanabe, K. Akai, and Y. Einaga, Electrochem. Commun., 70, 18 (2016).
• 28)  B. Marselli, J. Garcia-Gomez, P. A. Michaud, M. A. Rodrigo, and C. Comninellis, J. Electrochem. Soc., 150, D79 (2003).
• 29)  A. Kapałka, G. Fóti, and C. Comninellis, Electrochem. Commun., 10, 607 (2008).
• 30)  C. Comninellis, A. Kapalka, S. Malato, S. A. Parsons, I. Poulios, and D. Mantzavinos, J. Chem. Technol. Biotechnol., 83, 769 (2008).
• 31)  J. Farrell, F. J. Martin, H. B. Martin, W. E. O’Grady, and P. Natishan, J. Electrochem. Soc., 152, E14 (2005).
• 32)  P.-A. Michaud, M. Panizza, L. Ouattara, T. Diaco, G. Foti, and Ch. Comninellis, J. Appl. Electrochem., 33, 151 (2003).
• 33)  Y. Hori, Electrochemical CO₂ Reduction on Metal Electrodes, Springer New York (2008).
• 34)  D. D. Zhu, J. L. Liu, and S. Z. Qiao, Adv. Mater., 28, 3423 (2016).
• 35)  K. Natsui, H. Iwakawa, N. Ikemiya, K. Nakata, and Y. Einaga, Angew. Chem., Int. Ed., 57, 2639 (2018).
• 36)  J. Xu, K. Natsui, S. Naoi, K. Nakata, and Y. Einaga, Diamond Relat. Mater., 86, 167 (2018).
• 37)  M. Tomisaki, K. Natsui, N. Ikemiya, K. Nakata, and Y. Einaga, ChemistrySelect, 3, 10209 (2018).
• 38)  N. Spataru, K. Tokuhiro, C. Terashima, T. N. Rao, and A. Fujishima, J. Appl. Electrochem., 33, 1205 (2003).
• 39)  M. Tomisaki, S. Kasahara, K. Natsui, N. Ikemiya, and Y. Einaga, J. Am. Chem. Soc., 141, 7414 (2019).
• 40)  P. K. Jiwanti and Y. Einaga, Phys. Chem. Chem. Phys., 21, 15297 (2019).
• 41)  S. A. Yao, R. E. Ruther, L. Zhang, R. A. Franking, R. J. Hamers, and J. F. Berry, J. Am. Chem. Soc., 134, 15632 (2012).
• 42)  N. Roy, Y. Shibano, C. Terashima, K. Katsumata, K. Nakata, T. Kondo, M. Yuasa, and A. Fujishima, ChemElectroChem, 3, 1044 (2016).
• 43)  N. Roy, N. Suzuki, Y. Nakabayashi, Y. Hirano, H. Ikari, K. Katsumata, K. Nakata, A. Fujishima, and C. Terashima, ChemElectroChem, 5, 2542 (2018).
• 44)  N. S. R. Cuellar, K. Wiesner-Fleischer, O. Hinrichsen, and M. Fleischer, ChemistrySelect, 3, 3591 (2018).
• 45)  K. Nakata, T. Ozaki, C. Terashima, A. Fujishima, and Y. Einaga, Angew. Chem., Int. Ed., 53, 871 (2014).
• 46)  P. K. Jiwanti, K. Natsui, K. Nakata, and Y. Einaga, RSC Adv., 6, 102214 (2016).
• 47)  P. K. Jiwanti, K. Natsui, K. Nakata, and Y. Einaga, Electrochim. Acta, 266, 414 (2018).
• 48)  Y. Liu, Y. Zhang, K. Cheng, X. Quan, X. Fan, Y. Su, S. Chen, H. Zhao, Y. Zhang, H. Yu, and M. R. Hoffmann, Angew. Chem., Int. Ed., 56, 15607 (2017).
• 49)  Y. Liu, S. Chen, X. Quan, and H. Yu, J. Am. Chem. Soc., 137, 11631 (2015).
• 50)  P. K. Jiwanti, K. Natsui, and Y. Einaga, Electrochemistry, 87, 109 (2019).
• 51)  M. Alvarez-Guerra, A. Del Castillo, and A. Irabien, Chem. Eng. Res. Des., 92, 692 (2014).
• 52)  T. Watanabe, Y. Honda, K. Kanda, and Y. Einaga, Phys. Status Solidi A, 211, 2709 (2014).
• 53)  M. R. Singh, Y. Kwon, Y. Lum, J. W. Ager, and A. T. Bell, J. Am. Chem. Soc., 138, 13006 (2016).
• 54)  C. Reuben, E. Galun, H. Cohen, R. Tenne, R. Kalish, Y. Muraki, K. Hashimoto, A. Fujishima, J. M. Butler, and C. Lévy-Clément, J. Electroanal. Chem., 396, 233 (1995).
• 55)  K. Ogura, J. R. Ferrell, III, A. V. Cugini, E. S. Smotkin, and M. D. Salazar-Villalpando, Electrochim. Acta, 56, 381 (2010).
• 56)  Z. Futera, T. Watanabe, Y. Einaga, and Y. Tateyama, J. Phys. Chem. C, 118, 22040 (2014).
• 57)  Irkham, S. Nagashima, M. Tomisaki, and Y. Einaga, ACS Sustainable Chem. Eng., 9, 5298 (2021).
• 58)  J. Du, A. Fiorani, and Y. Einaga, Sustainable Energy Fuels, 5, 2590 (2021).

Biographies

Yasuaki Einaga (Professor, Department of Chemistry, Keio University)

Yasuaki Einaga was born in 1971. He received his BS (1994), MS (1996), and PhD (1999) from the University of Tokyo. After 2 years as a research associate at the University of Tokyo, he started a faculty career as an assistant professor in Keio University in 2001, where he was promoted to full professor in 2011. He has been also a research director of JST-CREST (2011–2014), and JST-ACCEL (2014–2020). He was awarded “The Chemical Society of Japan Award for Creative Work” in 2016 for his pioneering work in diamond electrodes. His research interests include functional materials science, photochemistry, and electrochemistry.