What is your goal?

Figure 1. Structure of a single cell of PEFC

The reactions taking place in fuel cells are actually identical to breathing. Breathing derives energy needed for organic activity by burning (oxidizing) hydrocarbons with dioxygen, but in a fuel cell, hydrogen is oxidized to extract electrical energy. At this time, a dioxygen molecule is converted to two water molecules by accepting four electrons and four protons (H⁺). When breathing occurs inside an organism, the oxygen reduction reaction (ORR) progresses efficiently without any energetic loss, using metalloenzyme as the electrocatalyst. For ORR in a polymer electrolyte fuel cell (PEFC), on the other hand, even when a state-of-the-art platinum-group alloy (PGM) catalyst is used, it is impossible to avoid the energy loss called overpotential. Why is the ORR overpotential much smaller for a metalloenzyme inside a living organism? The reaction center of a metalloenzyme is a multinuclear metal complex, and the dioxygen molecules (O₂) are linked with multiple metal ions configured on organic ligands with a cross-linked pattern, causing the stepwise transfer of four electrons and four protons, and breaking the O-O bonds during these steps to finally form water (H₂O). The metalloenzyme called cytochrome c oxidase (CcO) that drives ORR, for example, forms a reaction center by combining iron (Fe) and copper (Cu) ions, and another metalloenzime called laccase (Lac) forms a reaction center with three Cu ions. Although it is easy to think that the metalloenzymes can be directly applied as electrocatalysts resulting in a high performance PEFCs, it ends up being a PEFC stack so large to supply enough current to drive an automobile motor, since the diameter of the metalloenzyme described above is about 10 nm, far larger than the platinum atom (ca. 0.3 nm). The limited operating conditions for metalloenzymes, such as temperature and pH, etc. also make it impractical. Thus, our aim is to develop high performance electrocatalysts by extracting and accumulating the skeletal structure of the reaction center.

 

What kind of research are you carrying out?

Figure 2. View of an electrochemical XAFS measurement (top) and the EXAFS curve obtained by a Fourier transform of the experimental data and the molecular structure of bicopper complex.

At the present stage, the artificial multinuclear metal complex catalyst mimicking the ORR center does not achieve the performance of metalloenzymes and the PGM catalysts. The differences in the three-dimensional configuration of the metal ions and their surrounding environment are hypothesized as reasons for the reduced ORR activity. When a bicopper complex (Fig. 2) was prepared on carbon black supports, a relatively high ORR activity in neutral or alkaline electrolyte solutions was confirmed, but we still observed overpotential larger than that of Lac. Compared with the reaction center of Lac, its nuclear number is one short and it is exposed in a hydrophilic environment in the artificial system, although the reaction center of Lac is originally located in a hydrophobic nano-slit, resulting in the overpotential. However, determining the structure of a bicopper complex molecule that was actually constructed at the electrode in situ and combining results of electrochemical measurements to answer the questions “How does the reaction progress with this catalyst?” and “What kind of complex structure can lower the overpotential?” obtains the correlation between structure and performance. Feeding back the results to molecular structure design permits the development of superior electrocatalysts. We have carefully applied X-ray absorption fine structure (XAFS) spectroscopy using a synchrotron radiation source (KEK-PF in Tsukuba or SPring-8 in Harima) and the vibrational sum frequency generation (VSFG) spectroscopy using a femtosecond (10^-15 s) laser, allowing us to obtain a picture of the electrochemical interface in situ with an atomic or molecular view. We have also started research to measure ORR at metalloenzyme-modified electrodes. With the addition of Assistant Professor Masaru Kato, who has performed research on biomolecule-modified electrodes, to our laboratory in March 2014, we are looking forward to future progress.

 

What is your next goal?

When we can achieve the molecular design of multinuclear metal complex electrocatalysts by clarifying and referring to the correlation between the functions and structure of biomolecules, we will be also able to apply the strategy in other fields. Since our laboratory is part of the Graduate School of Environmental Science, we should be able to conduct research that directly contributes to purifying or restoring the environment. For example, to deal with nitrate ions that have polluted ground water as a result of the overuse of chemical fertilizers, we need to develop a catalyst that reduces it to nitrogen in a solution of neutral pH. If we can advance by overcoming such goals, the day may come when we can achieve an electrocatalyst group that can drive the nitrogen and carbon cycles. Recently, I am always wondering what the key is to resolving the mystery of how metalloenzymes inside living organisms are able to drive reactions, which cannot be driven artificially without using rare elements such as noble metals, more efficiently using combinations of common elements abundant in the earth's crust.