ABSTRACT

Solid oxide fuel cell (SOFC) is a promising future technology for a highly efficient, fuel flexible, and clean source of energy. SOFC using oxide protonics materials, has obtained more interests as compared to traditional SOFC with oxide ion conductors due to its possible low temperature of operation between 400℃ to 700℃ which is a desirable temperature range for both chemical and energy conversion processes in view of combined system with steam reforming reactions. The search for a good solid electrolyte with high protonic conductivity accompanied with high mechanical and chemical stability is the foremost challenge for the realization of this intermediate temperature solid oxide fuels cells. One approach is either to continue searching for new materials or the other is to engineer the existing ones in order to enhance their specific properties. Considering the latter choice, one exciting and promising way which is still a virtually untapped area is the nano-scaled structuring of proton conducting solid electrolytes. Combining the superior characteristics of nanosized grain materials with the proton conducting properties of solid oxides is an interesting strategy that may show different properties in contrast to its large grain counterparts.

This PhD work mainly focuses on developing nanostructured proton conducting solid electrolytes for intermediate temperature solid oxide fuel cells. The key motivation is to answer whether nanocrystalline materials with its renowned superior characteristics will help in enhancing the properties of the existing oxide solid electrolytes and in the process gaining an understanding of the so-called 'nanoionics' phenomena. The objective seems easy. However, obtaining a nanosized grain samples with the desired stoichiometry and structure is a challenge especially with multinary metal oxide system and with the existing synthetic techniques typically employed (i.e. classical solid state reaction). Hence, this PhD research started from developing new synthetic processes in order to obtain nanosized grain complex metal oxides at very low processing temperatures.

Solid electrolytes for intermediate temperature solid oxide fuel cells (IT-SOFC) applications are usually complex multicomponent metal oxides. The typically studied are those of acceptor-doped ABO3 type perovskite structures. In principle, protonic defects can be incorporated in the ABO3 lattice structure in the form of hydroxyl anions as replacement for oxygen vacancies created by the acceptor dopant on the B-site sublattice, whereas a new and interesting strategy for incorporation of protonic defects is to incorporate these defects in-situ during crystallization by employing low temperature synthetic processes. Here, instead of creating oxygen vacancies for charge compensation, the normal lattice oxygen is replaced by protonic defects. This is one of the basic ideas of this PhD that drives us in finding materials that is stable under the co-existence of protonic defects at intermediate temperatures.

The first part of my Ph.D. research is the successful development of a new synthetic process to synthesize nanograined materials within just several nanometers at temperatures about 350℃ with an in-situ incorporation of protonic defects. This process is especially advantageous in synthesizing complex metal oxides with the goal of obtaining nanosized grain powders fully incorporated with protonic defects. Using high pressure pressing (4GPa) fully compacted (no porosities) bulk-nanograined samples of rare-earth doped BaZrO3 have been achieved even at room temperature.

Having a synthetic process as a key tool to obtain nanograined solid electrolytes, synthesis of different types of protonic conductors and even creation of new class of oxide protonics materials are examined. Important new findings not only about protonic conduction of nanograined rare-earth doped BaZrO3 and other material properties for nanocrystalline solid electrolytes have been obtained but interestingly, new types of protonic conductors stable only at intermediate temperatures but decompose at higher typical processing temperatures (>1000℃) has also been found. BaScO2(OH), an ABO3 perovskite type metal oxide where one third of the oxygen in the lattice is replaced by OH forming a new general ABO2(OH) type compound termed as hydroxyperovskite, has been successfully synthesized at low processing temperature (<400℃) using a newly developed synthetic approach. With the combined results, basic ideas have been formulated for nanograined oxide protonic conductors.

In this dissertation, in the case of complex metal oxide like rare-earth doped BaZrO3 synthesize via wet chemical process, the effect of nanosized grain (several nm) solid electrolyte is first dictated by the thermodynamic stability of the phases at low processing temperatures with water. In the case of Y-doped BaZrO3, real doping with the desired stoichiometry for nanosized grain is difficult to achieve. Even with Sc-doped BaZrO3, the different competing formation of the BaZrO3 and BaScO2(OH) creates a mixed phases of this solid electrolyte. Upon high pressure pressing, the high amount of surface adsorbed water species of the nanocrystals will increase its water activity due to the induced high pressure and will attack the surface cations forming a thin amorphous grain boundary core. Whether such amorphous phase at the grain boundary core is decomposed upon heat treatment at intermediate temperatures (300-600℃) depends on thermodynamics of phase formation and kinetics, hence, if not will create a blocking path for protonic motion. In the case of BaScO2(OH), the relatively easy reaction of the pressed induced hydroxides of Ba and Sc at the grain boundary core as detected by FTIR and TG/DTA can easily transform again to the original phase at intermediate temperature (<350℃) which explain the high protonic conductivity (10(-3) S/cm) of this new class of nanosized grain oxide protonics materials.

For Y and Sc-doped BaZrO3 case, upon annealing at higher temperatures, extraneous phases are desorbed and amorphous grain boundary phases are swept by grain growth and those different phases will merge forming the desired stoichiometric compound. Hence, a dramatic increase of the conductivity is noticed for both Y and Sc-doped BaZrO3 upon annealing whereby real desired doping is achieved. From the results, even with 50 nm grain size samples, a high total protonic conductivity has been observed.

In this PhD research work, an important step towards exploring the nanoscience behind nanograined oxide proton conducting solid electrolytes -which can also be generally applied to most nanograined advance ceramics- has been studied and discussed. The findings of this PhD research will surely open new pathways for synthesizing new nanosized grain functional materials thermodynamically stable to be formed at low processing temperatures with the used of the developed approach. Other classes of hydrated phases and oxides stable under the coexistence of protonic defects are still probably out there waiting to be explored. Understanding the nanosized oxide protonics materials may open a new frontier and opportunities in combining nanotechnology with energy related devices for future technology applications.

高い総合エネルギー変換効率を有する固体酸化物型燃料電池(SOFC)の実用化研究が進んでいるが,次世代型SOFCとして期待されているのが,プロトン伝導を示す酸化物プロトニクス材料を電解質に用いた中温度域作動型SOFC(IT-SOFC)である.このために必要な酸化物系プロトン伝導体の探索が行われているが,実用化に向けては経済的に有利な化学的方法による薄膜やナノ構造体・複合化という材料プロセスが鍵を握っている.本研究はソフト化学的方法による酸化物プロトン伝導体の低温合成とナノ粒子効果について検討したものであり,全6章から構成される.

第1章は序論であり、本研究の背景、及びIT-SOFCについて詳述するとともに、Bサイトをアクセプタードーパントである希土類元素で置換したBaZrO3をホストとする物質群を現時点で最も実用化可能性の高い系として選定した理由について述べるとともに,その合成における問題点,ナノ粒子化により期待される効果をまとめ,本研究の位置付け及び目的を明確化している.

第2章では,金属アルコキシドを出発原料とする多成分系ペロブスカイト型化合物ナノ粒子の合成方法について検討し,従来から多くの系に適用されてきたゾル-ゲル法の問題点を指摘した.すなわち通常のゾル-ゲル法で得られるX線的アモルファスゲル構造は残留アルキル基が立体障害となって形成されるものであり,その後の高温熱処理による熱分解による結晶化と炭酸塩形成が不均質化や粒成長をもたらす.これらの結果を踏まえて,(1)アルキル基交換による単一アルキル基を有する複合アルコキシド合成,溶液添加加水分解法によるゲル形成,およびアルコール沸点より高温での加水分解・アルコール蒸発による結晶質ナノ粒子合成法(改良ゾル-ゲル法)と,(2)前述(1)と同様に合成した複合アルコキシドのアルコール沸点~400℃における水蒸気添加による加水分解とアルコール蒸発による結晶質ナノ粒子合成法(ゲル結晶化法)の2つの方法を考案・検討した.得られた粉末に関する特性評価により,200℃以下の合成温度においてもゲル結晶化法により均質な数nm程度の結晶体ナノ粒子が得られることを見いだし,この方法により得られたナノ粒子を以後の研究に供することとした.

第3章では,Zrサイトに20mol%のYで置換したBaZrO3(20YBZ)および25及び50mol%ScをドープしたBaZrO3(それぞれ25ScBZ, 50ScBZ)を前章で開発した方法により合成し,ナノ粒子バルク体を合成して高温熱処理時の粒成長過程を検討した.本研究では,焼結による粒成長を抑制するために超高圧(4GPa)下で常温プレスすることにより緻密体を得る方法を開発した.これはナノ粒子化によって初めて得られるものであり,4~7nm程度のナノ粒子バルク体の合成に成功した.20YBZでは800~1500℃の熱処理により数百nmまで粒成長し,1500℃ではBa3Zr2YO(8.5)相と低濃度のYを固溶したBaZrO3相に分離した.しかし超高圧プレス過程において生成した水酸化物によると思われる移動度の小さいプロトンが存在し,熱処理温度の上昇と共にその比率が急激に減少する.プロトン伝導度は熱処理温度の上昇と共に急激に上昇し,粒界に形成された高抵抗の水酸化物層が温度と共に解消される浸透的伝導が起こっていると推定した.

第4章では,ScBZの研究から発見した新しいプロトン伝導体であるBaScO2OH相の低温合成を試み,これまでその安定性が明らかになっていなかったこのペロブスカイト相が1000℃以下では安定に存在し可逆的に水溶解・放出が起こること,完全に脱水するとBrownmillerite構造であるBa2Sc2O5相へ不可逆的に変化すること,OH基存在下で高いプロトン伝導性を示すことを明らかにした.

第5章では, MAS NMRによる測定によりH+ およびSc(3+)イオンの状態分析を行い,酸化物表面が終端OH基(化学吸着)および水分子(物理吸着)から主に構成され,超高圧プレス時に水活量が上昇するために物理吸着水が構成酸化物と反応して水酸化物が生じ,粒界に導入されたOH基がプロトン伝導に深く関与していると推定した.さらに ScおよびYをドーパントとした場合の相安定性を熱力学データより推定した相関係図を基に議論し,BaZrO3と平衡するBaLnO(2.5)相の存在が均質固溶体合成に不可欠な熱力学条件であることを示した.

第6章は本研究の総括であり,今後のナノ粒子イオン伝導性バルク体ならびに酸化物プロトニクス材料の研究展開の展望と課題をまとめている.

以上を要するに、本研究は複雑な相安定を有する多元系複合酸化物のナノ粒子の低温合成法に関して熱力学的視点からそのプロセスの可能性を議論すると共に,ゲル-結晶化法という新しい方法を開発し,これを利用して低温合成による結晶性ナノ構造酸化物プロトン伝導体の合成とその特性を明らかにしたものであり,イオン伝導性酸化物の材料化学・材料プロセスに対する貢献は大きい。よって本論文は博士(工学)の学位請求論文として合格と認められる。