Research statement

August 16, 2015

(This article is based on what was posted on January 30, 2015, for Linkedin)

Toru Hara, PhD

Quest for Ultimate Energy Storage Device

1. Introduction and background

With supercapacitor-like rate capability and battery-like energy density, the energy storage device can be the ultimate "one."

For the better rate capability, it is possibly known that the ideal electrode architecture is three-dimensionally (3D) interpenetrated electron/ion network leading to efficient ion and electron transport [1, 2]. For the higher energy density, active materials for batteries are formed in the 3D architecture: Zhang et al. used electrodeposition to form NiOOH (cathode for nickel metal hydride battery) and MnO2 (cathode for lithiµm-ion battery) electrodes directly onto 3D nickel current collectors [3].

Battery that works in water-based (aqueous) electrolyte solutions, such as nickel metal hydride batteries, has some advantages: (1) it does not need an inert atmosphere for manufacturing unlike lithiµm-ion batteries thereby offering low cost; (2) it does not use flammable electrolyte solutions unlike lithiµm-ion batteries thereby unlike lithiµm-ion batteries it does not catch a fire. In contrast, the merit of lithiµm-ion batteries that work in organic electrolyte solutions is, they can deliver higher output voltage because of greater electrochemical windows thereby offering high energy density. So far the market seeking smaller and lighter batteries has been driving economy, i.e., consumer electronics and lithiµm-ion batteries. From now onward, it is said that environmental-protection-driven (= or energy-sustainability-driven) or security-of-materials-driven (in a broad meaning) may drive economy. This may shed light on aqueous batteries, particularly non-lithiµm-ion batteries: lithiµm is not abundant compared with sodium, proton, etc. From this point of view, aqueous batteries with 3D-architectured electrodes, particularly Na+ or H+ ion batteries can be promising.

2. Mono-atomic layer FeOOH / NiOOH battery

Among many candidates materials combinations, I have chosen (-) FeOOH / NiOOH (+). The novelty is forming mono-atomic layers of FeOOH and NiOOH. The main objective is to elude the beta-MOOH -> gamma-MOOH <-> alpha-M(OH)2 -> beta-M(OH)2 <-> beta-MOOH loop [M = Fe, Ni] that can result in crystal structure destruction during charge/discharge. The phase change is just a matter of the stacking pattern change [4]; the mono-atomic FeOOH and NiOOH layers have no MOOH stacking thereby they can offer a long cycle life.

2.1. FeOOH / NiOOH battery chemistry

At the anode (negative electrode) the following reaction takes place:

FeOOH + H+ + e- <-> Fe(OH)2, -0.56 V vs. SHE,

here, SHE denotes standard hydrogen electrode. Theoretical capacity is 301 mAh/g (470 mAh/g is possible when overdischarged). Note that hydrogen evolution reaction can happen at -0.83 V vs. SHE at pH = 14; thus, this side reaction can be suppressed even the anode is somewhat overdischarged.

At the cathode (positive electrode) the following reaction takes place:

NiOOH + H+ +e- <-> Ni(OH)2, +0.49 V vs. SHE

Theoretical capacity is 292 mAh/g (467 mAh/g is possible when overcharged but usually accompanying oxygen evolution side reaction). Note that oxygen evolution reaction can happen at 0.4 V vs. SHE at pH = 14 although the overpotential of the reaction may be expected to some extent.

The full cell reaction becomes

NiOOH + Fe(OH)2 <-> Ni(OH)2 + FeOOH, Ecell = 1.05 V.

This battery eliminates a slow reaction of the conventional NiFe battery reaction: that is 2NiOOH + Fe + 2H2O <-> 2Ni(OH)2 + Fe(OH)2, Ecell = 1.37 V. This battery only uses the second reaction of the conventional NiFe battery: NiOOH + Fe(OH)2 <-> Ni(OH)2 + FeOOH, Ecell = 1.05 V. Recently, many researchers are trying to use nanosized iron oxide as the anode instead of metal iron, e.g., Fe2O3 [5] or Fe3O4 [6]; however, the redox reactions via iron oxides tend to be irreversible or slow.

2.2. 3D interpenetrated electron/ion network for mono-atomic layer FeOOH / NiOOH battery

In order to form 3D interpenetrated electron/ion network, 3D current collector can be used. There are various types of 3D current collectors; among them graphene paper is a promising candidate. Suppose 30 mg/cm2 (corresponding to 9 mAh/cm2) of FeOOH is directly deposited onto current collectors:

(1) metal foil, surface area =1.0 cm2 (real)/cm2 (nominal), FeOOH thickness = 211 µm;

(2) carbon fiber paper [e.g., TGP-H-60 (Toray Industry Inc., 190-μm thick)], surface area = 33.4 cm2 (real)/cm2 (nominal), FeOOH thickness = 6.3 µm;

(3) activated carbon fiber paper (e.g., 190-µm thick), surface area = 3 340 cm2 (real)/cm2

(nominal), FeOOH thickness = 63 nm;

(4) carbon-fiber-paper-supported carbon nanofoam (e.g., 190-µm thick); surface area = 64 000 cm2 (real)/cm2 (nominal), FeOOH thickness = 3.3 nm;

(5) graphene paper (e.g., 200-µm thick), surface area = 2630 m2/g x 16 mg/cm2 = 420 800 cm2

(real)/cm2 (nominal), FeOOH thickness = 0.50 nm;

(6) reduced graphene oxide & carbon nanotube paper [7], surface area = 651 m2/g x 16 mg/cm2

(supposing a thickness of around 200 µm) = 104 160 cm2 (real)/cm2 (nominal), FeOOH thickness = 2.0 nm.

There can be a less expensive 3D current collector: (i) starting from Ni foam, NiSn intermetallic layer is electrodeposited onto theNi foam; (ii) next, Sn is dissolved into alkaline solution, remaining porous Ni layer.

2.3. FeOOH- and NiOOH-forming onto 3D current collector

There are two options regarding the FeOOH- and NiOOH-forming process, chemical synthesis and electrodeposition. I have chosen electrodeposition.

Particularly when using graphene, chemical synthesis tends to result in a dot-on-graphene structure or a multi-atomic MOOH layer [8,9]. Electrodeposition is promising even though mono-atomic layer formation has not been reported so far [3, 10-13]. Particularly, pulse electrodeposition at a high current density is preferable.

2.4. Electron (or hole) conductivity of FeOOH and NiOOH

The reduced forms of active materials, M(OH)2 are highly insulating: Ni 3d-t2g orbitals are relatively well hybridized with O 2p orbitals; however, it is not the case with Fe 3d-e g

orbitals. Thus mono-atomic thick MOOH is preferable particularly for FeOOH.

2.5. Other anode materials choices

Besides NiFe battery, NiOOH cathode has been used with various types of anode materials such as Zn, Cd, metal hydride, etc.

As for Zn anode, the current problems are inefficient zinc utilization and internal short circuit failure resulting from zinc dendrite formation. In order to improve zinc utilization and to suppress zinc dendrite formation, a redesign of zinc electrode will be required, i.e., a porous, monolithic, 3D aperiodic architecture [14, 15]: according to the authors of refs. 14 and 15, the redesigned Zn sponge electrode provides (i) an inner core of Zn that is retained throughout battery charge/discharge to facilitate long-range electronic conductivity, (ii) amplification of electrified interfaces to distribute current uniformly throughout the electrode structure, and thus eliminate dendrite forming high current densities, and (iii) confined void volume elements within the interior of the porous anode that expedite saturation/dehydration of zincate to ZnO, thus

preventing shape change. However, the Zn sponge electrode may be difficult for scaling up electrode size. The same functions can be offered by directly electrodepositing Zn onto 3D current collector (I already initiated one R&D project on Zn/NiOOH battery).

Cadmium is not a choice because of its toxicity.

Metal hydride may survive: Kawasaki Heavy Indutry delivers 30-year-life nickel metal hydride battery, Gigacell.

Zn still has a strength regarding its energy density: it delivers the most negative potential and the highest theoretical capacity among anode materials that can be used in aqueous media. Note that NiZn battery is a hybrid ion system (Zn2+ and H+) like Leclanche cell or alkaline manganese cell.

There may be other choices.

Vanadium oxide may not be a choice since it is lethally toxic.

MoO3 is not cheap and the supply can be restricted. Molybdenum, country of origin: 1. China (104 000 ton/year), 2. USA (60 400), 3. Chile (35 090), 4. Peru (16 790), 5. Mexico (11 000) up, 6. Canada (9 005), 7. Iran (6 300), 8. Turkey (5 000), 9. Armenia (4 900), 10. Russia (3,900), 11. Mongol (1,903), 12. Uzbekistan (550). Molybdenum, amount of deposit: 1. China (4 300 000 ton), 2. USA (2 700 000), 3. Chile (1 100 000), 4. Peru (450 000), 5. Russia (250 000), 6. Armenia (200 000), 7. Canada (200 000), 8. Mongol (160 000), 9. Mexico (130 000), 10. Kazakhstan (130 000), 11. Kyrgystan (100 000), 12. Uzbekistan (60 000), 13. Iran (50 000). Uneven distribution of countries of origin has often generated international friction. In addition, molybdenum is a key material for various types of specialty steel. The omnipresence of raw materials is one of key requirements for supplying cheap batteries.

Surface-modified carbon-based materials such as HNO3 treated carbon nanotubes and/or graphene oxide can be candidate alternatives. In order to re-obtain the electronic conductivity of above-mentioned carbonaceous materials, a mild reduction process (resulting in O content = 10-15%) such as a heat treatment at 200-250 degrees Celsius in an inert atmosphere is preferable than harsh chemical reduction processes since oxygen-based functional groups are required in order to obtain a high capacity in aqueous media. However, gravimetric capacities of the above-mentioned materials tend to be not high enough. It has been known that Russian companies, ELTON (ESMA), delivers asymmetric electrochemical capacitors, (-) activated carbon / NiOOH (+): carbon nanotube- or graphene-based materials can be good alternatives for activated carbon.

2.6. Other cathode materials choices

In Leclanche cells and alkaline manganese cells, MnO2 is used as the cathode material (I prefer the expression, MnOOH, since I consider that it should be clearly described as a proton intercalation/deintercalation material). The problems are a relatively low gravimetric capacity (usually 100-150 mAh/g is available) and dissolution into aqueous media because of the multi-

valency of manganese. It is noteworthy that MnO2 can be directly electrodeposited onto current collectors [3].

3. Practical issues

3.1. Oxygen evolution at NiOOH cathode

While charging, oxygen gas may evolve at the NiOOH cathode. The overpotential of oxygen evolution at pure NiOOH electrode can be +0.23 V in 1 M KOH solution at 10 mA/cm2

[16,17], for example. Since the electrical conductivity of Ni(OH)2/NiOOH is not high (101 - 106

Ω/cm) [18-21] (cf. FeOOH, 105 - 107 Ω/cm [22,23]), thick NiOOH films tend to suffer from overlap of its redox potential with oxygen evolution potential because of a high internal resistance (therefore, hole doping such as Co2+/3+ doping is often carried out); however, it is not the case with mono-atomic layer NiOOH on graphene sheet. Instead, the pH of electrolyte solution (by decreasing the concentration of KOH, oxygen evolution potential becomes more positive thereby oxygen cannot start evolving until reaching a more positive potential.), a relatively low oxygen evolution overpotential at a low current (meaning that oxygen starts evolving at a less positive potential when the battery is operated at a low C-rate) must be taken into consideration.

Although I am not going to dope Co2+/3+ into NiOOH for this mono-atomic layer FeOOH / NiOOH battery, it might be interesting to investigate the doping effect on the mono-atomic layer NiOOH or FeOOH when considering the electron/hole conduction modulation by internal hole/electron doping and by ad-atom or ad-ion surface doping [24-27].

3.2. Electrochemical impedance spectroscopy (EIS)

Since mono-atomic layer FeOOH / NiOOH battery has a great surface area particularly when deposited onto graphene paper, thereby has a great capacity (the response speed limit may still be a mere 1000 C corresponding to 0.36 s, though), and since a few layers of graphene and FeOOH or NiOOH can have a so-called quantum capacitance (In addition, graphene oxide, FeOOH, and NiOOH may exhibit current relaxation that is related to charge trapping/detrapping into/from defective states or their genuine 3d orbitals [28-30]: this relaxation is also a redox reaction.), I may not recommend EIS to explore the underlying rate-determining step(s) because of its high CR time constant etc. [C denotes capacitance that is the sum of electrical double layer capacitance and pseudo-capacitance, R denotes series equivalent resistance (SER)] (Note that SER does not mean DC resistance or a resistance at the lowest frequency; if one wants to analyze charge transfer process etc., one must use the impedance value at the intersection with the real axis.).

4. Future prospects

Considering the possibility of 9 mAh/cm2 with an electrode thickness of 200 µm (assuming a mass-loading of 30 mg/cm2), the mono-atomic layer FeOOH/NiOOH battery deposited onto 3D graphene papers can be competitive enough with lithium-ion batteries that usually offers around 2.5 mAh/cm2 with an electrode thickness of 100 µm although its output voltage of 1 V is lower than that of lithium-ion batteries, 3.6 V. Furthermore, eluding the β-MOOH -> γ-MOOH <-> α-M(OH)2 -> β-M(OH)2 <-> β-MOOH loop [M = Fe, Ni] [4] by forming mono-atomic layer can lead to a super long cycle life that has not been realized: this can be called a 0.5-step beyond the cutting edge.

However, a practical compromise or a 1-step-beyond-the-cutting-edge technology may be required since conventional batteries performance has been improved year by year.

4.1. Compromise for increasing volumetric capacity

In order to increase the volumetric capacity, a few to several atomic layer thick FeOOH and NiOOH electrodes may be preferred.

4.2. One step beyond the cutting edge

By taking full advantage of the 3D architecture and the combination of mono-atomic layer active materials, even all-solid-state energy storage device with a practically high enough capacity/pseudo-capacitance may be possible. In that case, redox reactions should be re-considered the current relaxation that is charge trapping/detrapping into/from defective states or genuine 3d orbitals [28-30] etc.

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