Vapor Synthesis and Thermal Evolution of Supportless, Metal Nanotubes and Application as Electrocatalysts

One of the major limitations of proton exchange membrane fuel cells (PEMFCs) is the high cost and poor durability of the currently preferred catalyst design, small Pt nanoparticles supported on high surface area carbon (Pt/C). Unsupported, high-aspect ratio nanostructured catalysts, or extended surface catalysts, are a promising paradigm as electrocatalysts for a number of electrochemical reactions. These extended surface catalysts generally exhibit higher specific activities compared to their carbon-supported nanoparticle counterparts that have been ascribed to their unique electronic, surface and structural properties. Extended surface catalysts frequently maintain enhanced durability over supported catalysts during fuel cell operation because they are not susceptible to the same modes of degradation inherent to small supported nanoparticles. Considering the success of extended surfaces as catalysts, we have synthesized metallic, mixed-phase, and alloyed bimetallic nanotubes by a chemical vapor deposition (CVD) technique to catalyze a number of reactions relevant for fuel cells. In this CVD process, metalorganic precursors, namely metal-acetylacetonates, are decomposed by a mild thermal treatment and deposited as conformal nanoparticulate layers within a sacrificial anodic alumina template. Following vapor deposition, the nanotube samples may be annealed while still in the template to induce a series of changes with implications on electrocatalysis, including nano-porosity, alloying, and surface coordination. This synthesis technique and successive thermal modification is applicable for the deposition of a number of metals. The metallic nanotubes prepared by this method are highly active catalysts for a host of electrochemical reactions that are promising for fuel cell applications. The effects of composition, heat treatment temperature and gas environment on the activity and durability of these materials have been studied for oxygen reduction, methanol oxidation, formic acid oxidation, and hydrogen oxidation

Nanoparticles as Reactive Precursors: Synthesis of Alloys, Intermetallic Compounds, and Multi-Metal Oxides Through Low-Temperature Annealing and Conversion Chemistry

Alloys, intermetallic compounds and multi-metal oxides are generally made by traditional solid-state methods that often require melting or grinding/pressing powders followed by high temperature annealing (> 1000 degrees C) for days or weeks. The research presented here takes advantage of the fact that nanoparticles have a large fraction of their atoms on the surface making them highly reactive and their small size virtually eliminates the solid-solid diffusion process as the rate limiting step. Materials that normally require high temperatures and long annealing times become more accessible at relatively low-temperatures because of the increased interfacial contact between the nanoparticle reactants. Metal nanoparticles, formed via reduction of metal salts in an aqueous solution and stabilized by PVP (polyvinylpyrrolidone), were mixed into nanoparticle composites in stoichometric proportions. The composite mixtures were then annealed at relatively low temperatures to form alloy and intermetallic compounds at or below 600 degrees C. This method was further extended to synthesizing multi-metal oxide systems by annealing metal oxide nanoparticle composites hundreds of degrees lower than more traditional methods. Nanoparticles of Pt (supported or unsupported) were added to a metal salt solution of tetraethylene glycol and heated to obtain alloy and intermetallic nanoparticles. The supported intermetallic nanoparticles were tested as catalysts and PtPb/Vulcan XC-72 showed enhanced catalytic activity for formic acid oxidation while Pt3Sn/Vulcan XC-72 and Cu3Pt/y-Al2O3 catalyzed CO oxidiation at lower temperatures than supported Pt. Intermetallic nanoparticles of Pd were synthesized by conversion chemistry methods previously mentioned and were supported on carbon and alumina. These nanoparticles were tested for Suzuki cross-coupling reactions. However; the homocoupled product was generally favored. The catalytic activity of Pd3Pb/y-Al2O3 was tested for the Heck reaction and gave results comparable to Pd/y-Al2O3 with a slightly better selectivity. Conversion chemistry techniques were used to convert Pt nanocubes into Ptbased intermetallic nanocrystals in solution. It was discovered that aggregated clusters of Pt nanoparticles were capable of converting to FePt3; however, when Pt nanocubes were used the intermetallic phase did not form. Alternatively, it was possible to form PtSn nanocubes by a conversion reaction with SnCl2

Nanoalloy Electrocatalysts for Electrochemical Devices

This Special Issue reprint covers the most recent advances in nanoalloy electrocatalysts, concerning not only the synthesis, characterization, and modeling, but especially reports of their activity, functionality, durability, and low cost

Ionomer-stabilised Pt and Pt-Ti bimetallic electrocatalysts for the proton exchange membrane fuel cell

This work aims to address the need for more durable electrocatalysts with lower precious metal content for proton exchange membrane fuel cells (PEMFCs), through the development of novel electrocatalyst materials and preparation routes. In this work, 'Nafion-Pt/C' electrocatalysts have been derived from ionomer-stabilised Pt nanoparticles synthesised via a novel, wet-chemical route that offers unprecedented control over the formation of the Pt-ionomer interface, with a view towards maximising the utilisation of the electrocatalyst. Nafion-Pt/C electrocatalysts have been characterised using ex-situ electrochemical techniques, and single-cell PEMFC testing to determine their activity and selectivity towards the oxygen reduction reaction (ORR), and to compare their utilisation and durability with commercially-available electrocatalysts. Nafion-Pt/C catalysts with agglomerated Pt particles exhibited a twofold improvement in durability vs. commercial catalysts, whilst offering similar ORR activities. Their enhanced durability was attributed to inhibition of Pt particle growth mechanisms by a passivating layer of Nafion introduced during the synthesis of Nafion-stabilised colloidal Pt. The second part of this work investigated methods for the synthesis of bimetallic nanoparticles consisting of an early transition-metal core (Ti) enclosed in a Pt shell, expected to offer higher intrinsic activity towards oxygen reduction than Pt alone, whilst being less prone to degradation than other alloys of Pt such as Pt-Ni, Pt-Co and Pt-Fe

액체 기판 스퍼터링을 통한 담지 촉매 제작 및 전기화학 촉매로의 적용

학위논문 (박사)-- 서울대학교 대학원 : 화학생물공학부, 2014. 8. 성영은.탄소 담지체 위에 고르게 분산된 백금 나노 입자는 다양한 전기화학 반응의 촉매로 사용되고 있는 유용한 물질이다. 백금은 대부분의 전기화학 반응에서 높은 교환 전류밀도 가지고 있어 촉매의 활성이 뛰어나며, 나노 크기의 물질로 합성할 경우 표면적 대 부피비가 급격히 증가하여 높은 전기화학적 성능을 기대할 수 있게 된다. 뿐만 아니라 탄소 담지체는 금속 나노 입자의 안정성을 높여주고 탄소에서 백금으로의 전자 전달로 인한 촉매 활성 증가 역시 기대할 수 있다. 이러한 나노 담지 백금 입자는 주로 백금 전구체를 환원시켜 입자를 형성하는 케미컬 메소드를 통해 합성되어 왔다. 그러나, 이 환원 합성법은 전구체의 높은 가격 및 실험 과정에서 사용되는 환원제와 계면활성제로 인한 환경 문제 등 경제•환경적인 측면에서 단점을 가지고 있다. 또한 전구체와 환원제의 환원 전위에 강하게 영향을 받기 때문에 원하는 조성 및 형태의 입자를 합성하는 데에도 한계가 존재한다. 본 논문에서는 금속 전구체 및 환원제, 계면활성제의 사용을 최소화하여 나노 담지 백금 입자를 합성할 수 있는 새로운 합성법을 개발하고, 합성된 입자를 전기화학적 촉매로 적용할 수 있는 방법을 고안해 보고자 한다. 전기화학적 촉매 적용 여부는 산소 환원 반응을 이용하여 평가하였다. 탄소 담지체 위에 고르게 분산된 백금 나노입자를 합성하기 위한 방법으로 이온성 액체 기판 위에 스퍼터링 하는 방법을 선정하였다. 탄소 담지체가 분산된 이온성 액체 위에 직접 백금을 스퍼터링 한 후 후 열처리를 통해 백금 입자와 탄소 담지체 사이의 결합을 강화시켜 주는 방법을 통해 나노 답지 백금 입자를 손쉽게 합성할 수 있었다. 또한 담지 메커니즘은 금속 입자 표면에 흡착된 음이온이 탄소 담지체와 직접 반응하기 때문임을 밝혀내었다. 그러나 이렇게 합성된 입자는 음이온의 강한 흡착으로 인해 백금의 표면이 전해질에 노출 되지 않아 전기화학적 촉매로 사용하기에는 적합하지 않았다. 비휘발성 고분자 물질인 PEG를 대체 기판으로 적용하여 백금 담지 촉매를 합성하였다. 이 경우 이온성 액체와 달리 후 열처리 없이도 바로 담지 입자가 합성되었으며, 이렇게 합성된 촉매는 전기화학적으로도 높은 활성을 띄는 것으로 확인 되어 액체 기판 스퍼터링이 담지 나노 촉매를 합성하는 매우 효율적인 방법임을 증명하였다. 이러한 합성법으로 백금 단일 촉매를 합성하는 데에만 그치지 않고, 다양한 구조의 촉매를 합성하는 방법으로 응용하고자 합성법의 개선을 시도하였다. 두 개 이상의 타겟을 동시에 스퍼터링 하는 코스퍼터링을 통해 백금-코발트 및 백금-니켈 합금 나노 입자를 합성하였다. XRD 및 STEM EDS 분석을 통해 단순한 코스퍼터링을 통해 합금 구조가 성공적으로 형성되었음을 알 수 있었으며 합금 촉매는 전자 구조 및 원자간 거리의 변화로 인해 개선된 성능을 보임을 확인하였다. 또한 마이크로웨이브 장비의 도움을 받아 코어-쉘 구조를 갖는 나노입자를 합성하였다. 스퍼터링을 이용하여 코어가 되는 코발트 나노입자를 비휘발성 용액 내에 합성하였고, 백금 쉘 물질의 전구체를 주입한 후 마이크로 웨이브를 조사하여 백금 쉘을 증착시켰다. 두 단계로 합성된 나노 구조는 다양한 분석 방법을 통해 2-3 nm의 매우 작은 크기에도 불구하고 코어-쉘 구조를 띄고 있음을 확인하였다. 원자의 크기가 작은 코어에 원자 간 거리 감소 및 전자 구조 변화 역시 확인하였으며, 단일 물질 촉매에 비해 높은 전기화학적 활성을 보였다. 백금 기반의 입자뿐 아니라, 자성을 띄는 코발트 옥사이드 나노 입자 역시 합성이 가능했다. 탄소 담지체를 나노 입자 합성이 끝난 후 주입하는 방식을 통해 자성을 띄는 코발트임에도 뭉침 없이 고르게 분산된 담지 입자를 합성할 수 있었으며, 이렇게 합성된 촉매는 산소 환원 반응 및 산소 생성 반응 모두에 좋은 활성을 보여 양방향 기능성 촉매로 적용 가능함을 확인했다. 본 연구를 통해 단순한 스퍼터링을 통해 고른 크기를 갖는 나노 담지 촉매를 합성할 수 있음을 보였고, 촉매 합성 과정에서 발생하는 담지 메커니즘 역시 최초로 밝혔다. 또한 합성 방법을 개선하여 백금 기반 합금 및 코어-쉘 구조의 나노 입자와 자성 산화물 나노입자 합성에도 성공하였으며, 이들 촉매는 개선된 산소 환원 성능을 보임을 알 수 있었다.Chapter 1. Introduction 1.1. Fuel cells and nanocatalyst 1.2. Basis of sputtering technique 1.3. Nanoparticle synthesis via sputtering technique 1.4. Subject of research in the thesis 1.5. References Chapter 2. Experimental 2.1. Synthesis of carbon supported metal nanoparticle 2.1.1. Carbon supported Pt nanoparticles 2.1.2. Carbon supported Co3O4 nanoparticles 2.1.3. Carbon supported PtCo and PtNi alloy nanoparticles 2.1.4. Carbon supported Pt@Co nanoparticles 2.2. Electronic structure characterization 2.3. Physicochemical characterization 2.4. References Chapter 3. Results and Discussion 3.1. Establishment of the liquid sputtering method 3.1.1. Sputterin onto ionic liquids 3.1.1.1. Carbon supported metal nanoparticle synthesis 3.1.1.2. Supporting mechanism and electrocatalytic activities 3.1.2. Sputtering onto PEG 3.2. Application of the liquid sputtering method 3.2.1. Alloy nanoparticles via co-sputtering 3.2.2. Core-shell nanoparticles via microwave associated 2-step method 3.2.3. Co oxide/C electrocatalyst 3.3. References Chapter 4. ConclusionsDocto

DEVELOPMENT OF NEW SUPPORTED CATALYSTS FOR DIRECT ETHANOL FUEL CELLS

Fuel cells are electrochemical energy conversion devices to be fed with a fuel and oxidant in order to generate electricity. Ethanol has, recently, shown a promising potential to replace hydrogen as a fuel directly fed into the cell due to its liquid state and sustainability. The challenge, however, is the significantly slow ethanol oxidation kinetics. Therefore, an affordable, highly active, and stable catalyst is necessary for activating such reaction. Platinum (and its alloys) is known as the most active metal for fuel cell catalysis, but its scarce presence has made the fuel cell commercialisation non-feasible. Also, it is very susceptible to poisoning by carbonaceous species which considerably decreases the catalyst lifetime. Palladium has shown a good potential to replace Pt. It is more abundant than Pt and has shown a comparable performance. Furthermore, it is more tolerant for poisoning species. In this thesis, Pd nanoparticles (NPs) are prepared by chemical reduction on five different carbon supports all of which are physically characterised and evaluated for ethanol oxidation. Because of a critical balance amongst the physiochemical characteristics (surface area, porosity, crystallinity extent), Vulcan carbon (XC72) has presented the highest support functionality for Pd ethanol oxidation as measured by the obtained current density. Numerous research efforts have co-concluded that adding a 2nd metal to Pd is beneficial economically and technically. Yet, a few groups have given attention to the trimetallic Pd-based electrocatalysts. Therefore, the main target of this thesis is to investigate various trimetallic combinations and synthesis methods to prepare C-supported trimetallic catalysts. Three different borohydride reduction synthetic protocols were deployed to prepare PdAuNi catalysts. The sodium borhydride-2-propanol (SBIPP) method gives the highest performing PdAuNi/C catalyst with 9-A/mgPd current density peak and - 0.36-V-vs-NHE onset potential while the single Pd counterpart gives only 2 A/mgPd and - 0.26 V, respectively. The physical characterisation shows enhanced physical alloy structure of this PdAuNi. Using the SBIPP protocol, 12 other catalyst combinations from Pd and two other metals were prepared with 12 wt.% metal loading. PdAu-based catalysts have shown significant enhancement of the Pd activity and stability towards ethanol oxidation. PdAuRh, in particular, produces a remarkable oxidation current peak of 10 A/mgPd and - 0.4-V onset potential. The PdAuRh and PdAuNi catalysts seem to present serious candidates to replace Pt and facilitate the transition into an affordable low-carbon technology to supply sustainable electricity.====================================================================================== ِArabic Abstract تعتبر خلايا الوقود أجهزة تحويل طاقة يتم تغذيتها بوقود ومؤكسد لكي تولد كهرباء. وحديثا لاقي وقود الايثانول اهتمام الباحثين كوقود لتغذية الخلية بدلا من الهيدروجين نظرا لطبيعته السائلة واستدامة وجوده. ولكن التحدي امامه هو البطء الشديد في تفاعل اكسدته داخل الخلية. لذا من الضروري إيجاد حفاز نشط جدا لتحفيز اكسدة الايثانول لوقت طويل نسبيا. يعرف معدن البلاتين بأنه انشط المعادن لتحفيز تفاعلات خلايا الوقود ولكن ندرته الشديدة تجعل تسويق خلايا الوقود تجاريا غير مجدي. أيضا فان معدن البلاتين لا يقام السموم الكربونية التي قد توجد على شكل شوائب او تنتج اثناء التفاعلات في خلايا الوقود مما يجعل عمر الحفاز الفعلي قصير جدا. على الجانب الآخر معدن البلاديوم يقدم بديل مناسب للبلاتين حيث انه اقل ندرة من البلاتين في القشرة الأرضية و يعمل بكفاءة تناظر البلاتين. إضافة الي ذلك فان عمر حفاز البلاديوم أطول من البلاتين نظرا لقدرة الأول على مقاومة التسمم بالعينات الكربونية. في هذه الرسالة جسيمات البلاديوم النانوية تم تحضيرها بالاختزال الكيمياء و تم تثبيتها على خمسة أنواع كربون مختلفة كل منهم تم تحليه فيزيائيا و كيميائيا و تم تطبيقها لأكسدة الايثانول. نظرا لتداخل تأثيرات الخواص الفيزوكيميائية مثل المساحة السطحية والمسامية ودرجة التبلور فان الكربون من نوع فولكان 72 قدم افضل أداء هندسي كحامل لحفاز البلاديوم لأكسدة الايثانول يعرف من خلال شدة التيار الكهربي الناتج من التفاعل. العديد من الدراسات السابقة توصلت الى ان إضافة عنصر آخر الي البلاديوم يحقق مصلحة مزدوجة برفع الكفاءة و تقليل الكمية المستهلكة من معدن البلاديوم النبيل. و لكن القليل من الدراسات ركز على دراسة إضافة عنصرين الي البلاديوم بدلا من عنصر واحد. لذا الهدف في هذا العمل هو فحص تركيبات محفزة مختلفة من ثلاث معادن (بلاديوم إضافة الى معدنين اخرين) وطرق تحضير مختلفة. ثلاث طرق اختزال باستخدام البوروهيدريد تم تطبيقاها لانتاج ثلاث حفازات ثلاثية من البلاديوم و الذهب و النيكل. الحفاز المنتج باستخدام تركبية بوروهيدريد الصوديوم و البوروبانول الثنائي هو اعلاهم أداء بكثاقة تيار 9 امبير/مجم (بلاديوم) و جهد بدأ أكسدة -0.36 فولت بينما الحفازي الأحادي من البلاديوم اعطي فقط 2 امبير/مجم (بلاديوم) و ابتدء اكسدة الايثانول مع فرق جهد -0.26 فولت. التشخيص الفيزيائي لهذا الحفاز بينية انشائية محسنة من سبيكة البلاديوم و الذهب و النيكل. و بنفس طريقة التحضير تم تحضير 12 حفاز ثلاثى اخري من البلاديوم و معدنين اخرين و معدل تحميل المعدن على الكربون يكافئ 12% بالوزن. بصفة عامة الحفازات التي تحتوي على بلاديوم و ذهب إضافة الى معدن اخر تقدم أداء محسن تجاه اكسدة الايثانول اكثر من الحفازات الأخرى. انشط هؤلاء هو الحفاز الذي يحتوي على بلاديوم و ذهب و روديوم و الذي انتج كثاقة تيار مرتفعة بقيمة 10 امبير/مجم (بلاديوم) و بدأ أكسدة الايثانول عند تطبيق جهد -0.4 فولت. ان هذا الحفاز الثلاثي و أيضا الحفاز المحتوي على بلاديوم و ذهب و نيكل يقدمون بدائل ناجحة و جدية لاستبدال البلاتين و يمكنهم تسهيل الانتقال الي تقنية منعدمة الكربون لإنتاج الكهرباء بشكل مستدام