Development of sustainable carbon-based cathodes for electrocatalytic degradation of perfluorooctanoic acid (PFOA)

다국어 초록 (Multilingual Abstract)

Nowadays, substances containing PFAS (Per- and polyfluoroalkyl substances) are widely used in various industrial applications due to their exceptional stability. However, their persistence and bioaccumulation have led to global environmental contamination and adverse health effects. Previous efforts to decompose PFAS using anode systems have been constrained by the use of expensive noble metal-based materials or heavy metal-based materials such as tin and nickel, raising concerns regarding economic feasibility and secondary pollution.
In this study, we explore the electrocatalytic decomposition of PFOA using carbon and copper-based cathode systems, capitalizing on their abundance and low cost. Initially, the challenge of hydrogen peroxide production using carbonized melamine foam (CMF) was addressed by passivating N6 sites, resulting in an improved passivated carbonized melamine foam (PCMF). This modification enhanced hydrogen peroxide production by approximately 60 % (> 90 μM min-1).
Further modification with copper phosphide (CuxP) demonstrated that hydrogen peroxide could be effectively converted into other reactive oxygen species (ROS) within the system. Cyclic voltammetry (CV) confirmed that the modified catalyst enhanced catalytic activity in the oxygen reduction reaction (ORR). The highest decomposition efficiency was achieved at pH 13, where superoxide anion radicals (O2 ) were identified as the primary agents of decomposition. Under highly alkaline conditions, the system generated ROS, including O2•⁻, which significantly contributed to the degradation process. In reuse tests, the polytetrafluoroethylene (PTFE)-modified electrode (PCMF-Cu) exhibited superior stability and reusability while maintaining catalytic activity. This improvement is attributed to PTFE acting both as a binder and a passivator.
This remarkable electrocatalytic system has significant potential for future studies on the decomposition of unclassified PFAS , suggesting that it provides a scalable and effective solution for the removal of difficult to decompose organic pollutants such as PFAS . Future studies will be able to apply this system to other pollutants and further improve the catalyst to enhance its performance.

국문 초록 (Abstract)

오늘날 PFAS(Per- and polyfluoroalkyl substances)를 함유한 물질은 뛰어난 안정성으로 인해 다양한 산업 분야에서 널리 사용되었다. 그러나 이러한 물질의 지속성과 생물 축적으로 인해 지구적 환경 오염과 건강에 부정적인 영향을 가져왔다. 이전에 양극 시스템을 사용하여 PFAS 를 분해하려는 시도는 고가의 귀금속 기반 재료나 주석과 니켈과 같은 중금속 기반 재료의 사용에 의해 제약을 받아, 경제적 타당성과 2 차 오염에 대한 우려를 야기했습니다.
본 연구에서는 풍부하고 저렴한 탄소 및 구리 기반 음극 시스템을 활용하여 PFOA 의 전기촉매적 분해를 탐구하였다. 처음에는 Carbonized Melamine Foam(CMF)을 사용하여 과산화수소를 생산하는 단계에서 생성된 과산화수소가 분해되는 과제를 N6 사이트를 부동태화하는 개선된 PTFE 개질 Carbonized Melamine Foam(PCMF)으로 해결되었다. 이 개질을 통해
과산화수소 생산을 약 60 % (> 90 μM min -1) 향상시켰다.
Copper Phosphide(CuxP)을 이용한 추가 개질은 과산화수소를 생성하는 시스템 내에서 다른 반응성 산소종(ROS)으로 효과적으로 전환될 수 있음을
보여주었다. 순환 전압 전류법(CV)은 개질된 촉매가 산소 환원 반응(ORR)에서 촉매 활성을 향상시킨다는 것을 확인했다. 가장 높은 분해 효율은 pH 13 에서 달성되었으며, 여기서 Superoxide radical(O2•⁻)이 분해의 주요 작용제로 확인되었다. 고도로 알칼리성인 조건에서 시스템은 O2•⁻ 를 포함한 ROS를 생성했으며, 이는 분해 과정에 상당한 기여했다. 재사용 테스트에서 PTFE 개질 전극(PCMF-Cu)은 촉매 활성을 유지하면서도 뛰어난 안정성과 재사용성을 보였다. 이러한 개선은 PTFE 가 바인더와 부동태화 역할을 모두 하였기 때문이다.
이 놀라운 전기 촉매 시스템은 아직 분류도지 못한 PFAS 의 분해에 대한 미래 연구에 상당한 잠재력을 가지고 있으며, PFAS 와 같은 분해하기 어려운 유기 오염 물질을 제거하기 위한 확장 가능하고 효과적인 솔루션을 제공한다. 향후 연구에서는 이 시스템을 다른 오염 물질에 적용하고 촉매를 더욱 개선하여 성능을 향상시킬 수 있을 것이다.

목차 (Table of Contents)

• TABLE OF CONTENTS.................................................................................... i
• LIST OF FIGIRES ........................................................................................... iv
• LIST OF TABLES .......................................................................................... viii
• ABSTRACT...................................................................................................... ix
• Chapter 1 Introduction ......................................................................................1
• 1.1. Background ........................................................................................................1
• 1.1.1. Overview of PFAS ............................................................................................... 1
• 1.1.2. Comparative Analysis of PFAS Degradation technologies ..................................... 7
• 1.1.3. Materials selection and design for electrochemical decomposition........................10
• 1.2. Research Objectives ......................................................................................... 17
• 1.3. Dissertation Outline ......................................................................................... 18
• Chapter 2 Materials and Methods ...................................................................19
• 2.1. Materials Synthesis and Preparation............................................................... 19
• 2.1.1. Synthesis of Carbonized Melamine Foam (CMF) .....................................................19
• 2.1.2. Synthesis of Copper phosphide (CuxP) .....................................................................19
• 2.1.3. Modification of CMF with CuxP and PTFE. .............................................................20
• 2.2. Experiment Set-up ........................................................................................... 21
• 2.2.1. Electrochemical Experiment Configuration ..............................................................21
• 2.2.2. Electrochemical production of H2O2 ........................................................................22
• 2.2.3. Quantification of H2O2 ............................................................................................22
• 2.2.4. Calculation of Faradaic Efficiency ...........................................................................23
• 2.2.5. Detection ∙OH (hydroxyl radical) and O2∙- (superoxide radical).................................23
• 2.2.6. Scavenging ∙OH (hydroxyl radical) and O2∙- (superoxide radical)..............................24
• 2.3. Characterization of PCMF-Cu ........................................................................ 24
• 2.3.1. Morphology Characterization ..................................................................................24
• 2.4. Optimization of Electrodes with Varying Copper Phosphide (CuxP) Loading ... .......................................................................................................................... 25
• 2.5. Perfluorooctanoic acid (PFOA) Detection ....................................................... 25
• 2.5.1. PFOA Detection using with HPLC ...........................................................................25
• 2.5.2. Quantification of Fluoride (F-) .................................................................................26
• 2.5.3. Analysis of PFOA decomposition using with HPLC-MS/MS ....................................26
• Chapter 3 Result and Discussion .....................................................................28
• 3.1. Morphology analysis of CMF, PCMF, PCMF-Cu ........................................... 28
• 3.2. FT-IR Analysis of Electrode Materials ............................................................ 32
• 3.3. XPS Analysis of Electrode Materials ............................................................... 34
• 3.4. Electrochemical Performance .......................................................................... 39
• 3.4.1. Cyclic Voltammetry Analysis ...................................................................................39
• 3.4.2. H2O2 Production and Efficiency ...............................................................................41
• 3.5. PFOA Decomposition and Defluoridation ....................................................... 44
• 3.5.1. Decomposition degree according to use of each electrode.........................................44
• 3.5.2. Optimization by catalyst weight...............................................................................47
• 3.5.3. Decomposition depending on pH .............................................................................49
• 3.5.4. Reuse test ................................................................................................................50
• 3.5.5. Performance Comparison Among Systems Based on the Same ROS.........................53
• 3.5.6. Decomposition-Induced Formation of Short-Chain Perfluorocarboxylic Acids (PFCAs) ...........................................................................................................................56
• 3.6. Mechanistic Insights......................................................................................... 58
• Chapter 4 Conclusion .......................................................................................67
• REFERENCES .................................................................................................70
• ABSTRACT IN KOREAN...............................................................................77