Coupling microbial electrolysis and photoelectrochemical cell toward highly efficient green hydrogen production

Abstract

Since the Industrial Revolution, the world has experienced an increase in population and economic expansion by both an increase of millions of people and a corresponding increase in social and economic activities around it. Continuous growth in the industrialization of industries, urban centres, and transportation networks requires a great supply of energy to help keep the modern mode of life and an expanding population. This demand has been greatly satisfied by the burning of fossil fuels (e.g., coal, oil, and natural gas), which together still comprise over 80% of total primary energy consumption as of 2024. Due to this, the increasing fossil fuel consumption has led to an unprecedented anthropogenic CO2 release, which contributes to global warming and causes widespread and potentially irreversible climate disruptions. Global energy-related CO2 emissions hit an all-time high of approximately 37.4 gigatonnes in 2024, according to the International Energy Agency (IEA), despite international efforts such as the 2015 Paris Agreement, which aimed to limit the global temperature rise to well below 2 °C above pre-industrial levels. These findings highlight the need for developing both environmentally friendlier and sustainable energy conversion techniques for climate change mitigation, and they don't have to be limited to limiting the consumption of fossil fuels, as they are capable of replacing conventional energy systems, if they are to have the capacity to address changes in climate change. As populations expanded, thus did the size of global wastewater, which has emerged as one of the most integral but energy-intensive aspects of modern urban infrastructure. Many wastewater treatment plants (WWTPs) use enormous amounts of energy for aeration, pumping, and sludge management. By the end of the twentieth century, the global electricity consumption was about 1.5% generated for wastewater treatment, and increased to around 3% by 2005, while the expansion of wastewater networks in developing countries will probably drive this volume to increase. Wastewater is rich in chemical energy and is abundant when stored as organic compounds like carbohydrates, proteins, and fatty acids, an amount that is around nine times more than that required for conventional treatment techniques. If such an inherent chemical energy can be recovered, wastewater could be used not only for producing clean water, but also for feeding renewable energy, and converting treatment plants from energy-consuming systems to energy-producing systems. In the wake of this idea, energy-positive wastewater treatment plants and resource recovery facilities were recently constructed, integrating energy generation with nutrient recovery and carbon management processes. The use of bioelectrochemical systems (BESs), including microbial fuel cells (MFCs) and microbial electrolysis cells (MECs) as a promising option for the simultaneous treatment of wastewater and renewable energy, has been a prominent area of focus among innovative energy-recovery technologies. In the MFC, exoelectrogenic micro-organisms (exoelectrogens) can also be used as biocatalysts by oxidizing organic waste cells and migrating electrons towards the anode via their metabolic pathways. These electrons are transferred through an external circuit to the cathode, where they start to generate electricity, bypassing chemical catalysts and high-temperature technologies. Unlike, MECs, MECs are an augmented version of MFCs formulated for the production of hydrogen wherein (1) added external biasing for the thermodynamic barrier is inserted to overcome the thermodynamic barrier for hydrogen evolution and (2) sustainable generation of H₂ from wastewater can also be achieved. The recent progress in physical progress in material science and nanostructured electrodes and the development of microbe communities have enabled MFC and MEC engineering systems to achieve improved and scalable performance. High-conductivity carbon materials, metal–organic frameworks and biofilm-compatible electrode interfaces have increased electron transfer efficiency and power density. Additionally, the combination of MFCs with membranes through bioreactors and even anaerobic digesters has provided the ability for hybrid, highly effective organic degradation and bioenergy recovery systems simultaneously. These developments are leading to potential sustainable and energy-positive wastewater treatment, placing MFCs and MECs at the core of the transition to a sustainable, circular bio-economy. In this thesis, I demonstrate a set of experiments for improved microbial electrolysis cell (MEC) performance using different procedures. To improve hydrogen production, I have proposed a new hybrid MEC system combining a TiO2 nanotube (TNT) array photoanode in Chapter 3. As a result, TNT photoanode served as an additional electron source and reduced fuel depletion in the bio-anode while speeding proton reduction, leading to improvements in current density, power density and hydrogen production of 34.4%, 26.0% and 30.8%, respectively. In Chapter 4, I work on rational design of the photoanode to increase water splitting efficiency of photoelectrochemical (PEC) water splitting. I introduced a multi-functional TiO2/Mn-CdS/ZnS/CoPi photoanode that directly improves the light absorption, surface stability, and interfacial charge transfer together. TiO2 acts as a conductive UV-absorbing scaffold, whereas Mn-doped CdS enables high levels of absorption into the visible range. ZnS passivation reduces the photocorrosion of the CdS, and CoPi serves as a surface catalyst to speed oxygen evolution. This hierarchical structure encourages the effect on synergistic interactions between the components, which leads to improvement in charge separation, transport, and PEC performance. Chapter 5-The second half of the thesis studies heavy-metal–free ternary I–III–VI quantum dots (QDs) as efficient sensitizers for solar-driven photoelectrochemical (PEC) hydrogen production. First, I explore how CuInS2 QDs are formed, indicating that the strength of precursors for Lewis acid affects different pathways for nucleation: weaker precursors produce CuxS intermediates, while stronger precursors allow direct CuInS2 nucleation. Such pathways will fundamentally shape the electrical characteristics of QDs, where direct-formed QDs display lower electron trap densities and improved PEC performance. Upon this understanding, I methodically adjust the chalcogenide composition of CuIn(S1-ₓSex)2 QDs to modulate anion vacancy concentrations to achieve the most favorable hole density and carrier lifetimes. Notably, CuIn(S0.5Se0.5)2 QDs have the smallest anion vacancy density and the highest photocurrent density of 15.1 mA cm-2 (at 0.6 VRHE) when directly incorporated into TiO2 photoanodes. With regard to optimizing optoelectronic properties for high-performance PEC hydrogen production, this research underscores the key function of QD formation path, compositional engineering, and defect control on an integrated level.|지구 온난화와 기후 변화는 인류 생존을 위협하는 전 지구적 위기이며, 이를 완화하기 위해서는 온실가스의 주된 원인인 이산화탄소(CO2) 배출을 근본적으로 감축할 필요가 있다. 이러한 배경에서 수소(H2)는 연소 시 오직 물만을 생성하고 단위 질량당 122 kJ g-1의 높은 에너지 밀도를 가지는 청정 연료로 주목받고 있다. 그러나 현재 상용 수소의 대부분은 천연가스의 수증기 개질 반응(SMR: CH4 + 2H2O ⇌ CO2 + 4H2)을 통해 생산되고 있어, 여전히 화석연료 의존성과 CO2 배출이라는 구조적 한계를 지니고 있다. 따라서 탄소중립형 재생 수소 생산 기술의 확보가 시급하다. 본 연구에서는 생물전기화학적 및 광전기화학적 수소 생산을 융합한 통합형 시스템을 구축하기 위해 세 단계의 연구를 수행하였다. 제1장에서는 폐수 내 유기물의 화학적 에너지를 전기에너지로 전환하는 미생물 전기분해 셀(Microbial Electrolysis Cell, MEC)을 기반으로, 외부 광원을 이용하여 전자전달 효율을 높이는 하이브리드 MEC 시스템을 개발하였다. 특히, 티타늄 이산화물 나노튜브 배열(TiO2 nanotube array, TNT array)을 광보조형 양극(photoanode)으로 도입하여 미생물 전극(bioanode)의 전자 고갈을 억제하고, 광여기에 의한 추가 전자 공급을 통해 수소 발생 효율을 향상시켰다. 또한, 수소 발생 반응의 촉매 활성 및 안정성을 극대화하기 위해 생체 촉매인 hydrogenase를 기반으로 한 생물음극(biocathode)을 도입하였다. hydrogenase는 낮은 과전압에서 수소 생성 반응(H⁺ + e⁻ → ½H₂)을 효율적으로 촉진하며, 귀금속 촉매(Pt) 대비 높은 선택성과 생체적합성을 제공한다. 이러한 양극–음극의 이중적 개선을 통해, 하이브리드 MEC는 기존 시스템 대비 전류밀도(3714.29 ± 0.00 mA m-2), 전력밀도(1415.31 ± 23.94 mW m-2), 수소 발생속도 (1434.27 ± 114.17 mmol m-3 h-1)가 각각 약 30% 이상 향상되었다. 전극 전위 분석 결과, 광활성 TiO2 전극이 bioanode로의 전자 공급원을 보조함으로써 전자전달 저항을 감소시키고, hydrogenase 음극은 효율적인 수소 환원 반응을 유도하여 전체 MEC 구동 전압을 크게 낮추는 것으로 나타났다. 제2장에서는 MEC 시스템의 광보조 전극으로 응용 가능한 고효율 TiO₂ 기반 다층 광전극(photoanode)을 설계하였다. TiO₂는 저비용·무독성·우수한 화학적 안정성 등의 장점을 갖지만, 약 3.0–3.2 eV의 넓은 밴드갭으로 인해 가시광 흡수가 제한된다는 단점이 있다. 이를 극복하기 위해 본 연구에서는 좁은 밴드갭(≈2.4 eV)을 가지는 CdS 양자점(CdS QDs)을 감광층으로 적용하여 가시광 영역까지 흡수 범위를 확장하였다. 또한 Mn2+ 도핑을 통해 CdS 내의 전하 재결합을 억제하고, ZnS 피복층을 형성하여 광부식(photocorrosion)을 억제하였다. 더 나아가 CoPi(코발트 인산염) 산소발생촉매(OER catalyst)를 표면에 증착하여, 광양극에서의 산화 반응 속도와 전하 분리 효율을 동시에 향상시켰다. 이 다층 구조의 전극은 자외선–가시광 전반에 걸친 흡수 스펙트럼, 향상된 광전류(>2 mA cm⁻²), 장시간의 광안정성을 확보하였으며, 이는 MEC 시스템의 광보조 양극으로 직접 통합될 수 있는 높은 잠재력을 지닌다. 제3장에서는 환경적·독성적 한계를 극복하기 위해 무독성 I–III–VI족 계열 양자점(Quantum Dots, QDs)인 CuIn(S1-xSex)2 QDs를 합성하고, 음이온 공공(anion vacancy)을 제어하는 전략적 합성법을 제시하였다. 합성 경로에 따라 Cu 및 In의 화학적 환경과 결함 구조가 달라지며, 이는 전하 수송 및 재결합 특성에 직접적인 영향을 미친다. 본 연구에서는 전구체 조성에 따라 서로 다른 성장 경로(CuxS 중간체 형성 vs 직접 CIS 핵생성)를 조절함으로써, 전기적 특성이 개선된 CIS QDs를 제조하였다. 또한, S/Se 비율 조절을 통한 칼코게나이드 합금화(chalcogenide alloying)를 통해 음이온 결함 밀도를 제어하였다. 그중 CuIn(S0.5Se0.5)2 QDs 조성은 높은 음이온 조정 비율, 짧은 결합 거리, 향상된 전하 수명 특성을 보였으며, TiO2 전극에 감광층으로 적용 시 광전류 밀도(Jph ≈ 15.1 mA cm-2 @ 0.6 VRHE)의 우수한 PEC 성능을 나타냈다. 이 결과는 Cd 기반 QDs를 대체할 수 있는 친환경 감광소재로서의 실용적 가능성을 제시한다.