應用於高值化產氫系統之幾丁聚醣衍生物多層離子傳導膜

Abstract

大多數的氫氣產生過程涉及在離子導電膜兩側進行水電解，從而同時產生氧氣和氫氣。然而，水分解是一個昂貴的過程，需高過電位，且副產品氧氣價值較低。在本研究中，氧氣析出反應（OER）被碘化物氧化反應（IOR）或乙二醇氧化反應（EGOR）所取代，以降低過電位，從而降低電力消耗。此外，IOR產生的I2和I3-或EGOR產生的HOCH2COOH和HCOOH等有價值的副產品將提高氫氣生產的經濟效益。無論操作系統如何，離子導電膜是影響系統性能和穩定性的關鍵組件，同時也是設備成本的重要因素。 在IOR系統中，水溶液中的KI溶液經氧化產生I3-，並同時產生多餘的鉀離子（K+），這些鉀離子需要迅速通過離子導電膜到達氫氣析出反應（HER）的一側，以確保快速的氫氣產生。因此，需要一種高效的鉀離子導電膜（PCM），該膜具有低碘離子（I-）滲透性。對於EGOR系統，HER側產生氫氣（H2）和氫氧根離子（OH-），這些OH-通過陰離子交換膜（AEM）到達EGOR側，為乙二醇（EG）的氧化提供OH-。因此，需要一種穩定的AEM，該膜具有高氫氧根導電性和低乙二醇滲透性。 在本研究中，我們旨在開發基於幾丁聚醣衍生物的低成本和環保的離子導電膜，以高效綠色氫氣生產。幾丁聚醣是一種可再生、環境友好且價格低廉的多醣，可以很方便地在幾丁聚醣上接枝各種功能基團，以賦予其所需的離子導電性並調整膜的機械性能。在碘化物氧化反應（IOR）系統中，高密度接枝的鉀磺酸根被引入到幾丁聚醣主鏈上，以提供鉀離子導電的磺酸化幾丁聚醣（SCS-K）。在乙二醇氧化反應（EGOR）系統中，四級銨鹽被接枝到幾丁聚醣主鏈上，以產生導電氫氧根的季銨化幾丁聚醣（QCS）。SCS/QCS高度水溶，通過環保的水基過程，使用戊二醛（GA）和醛基功能化氧化石墨烯（FGO）對SCS/QCS進行交聯，可以製備相應的複合膜。將多孔尼龍膜（PA）夾在兩層緊密排列的SCS/QCS複合膜之間，所形成的三層鉀離子導電膜（PCM）/陰離子交換膜（AEM）具有良好的離子導電性、高機械強度、良好的尺寸穩定性和低I-/EGOR滲透性。我們自製的多層PCM/AEM全電池展示出與商用Nafion/Sustanion膜相媲美的高效氫氣生產能力，證明了這些膜的實用性。

Most hydrogen evolution processes involve water electrolysis at both sides of an ion conducting membrane, and which simultaneous generation of oxygen and hydrogen. However, water splitting is an expensive process requiring high overpotential, and the byproduct oxygen is non-profitable. In this research, oxygen evolution reaction (OER) is replaced with either iodide oxidation reaction (IOR) or ethylene glycol oxidation reaction (EGOR) to reduce overpotential to lower the electricity consumption. Additionally, valuable by-products, such as I2 and I3- from IOR, or HOCH2COOH and HCOOH from EGOR will be generated to increase economic benefits of hydrogen production. Regardless the operation system, the ion conducting membrane is the key component affecting the performance and stability of the system, while also determinative to the infrastructure cost. In the IOR system, aqueous KI solution undergoes oxidation to generate I3- and excess potassium ions (K+) are concurrently produced, which need to pass through the ion conducting membrane to the hydrogen evolution reaction (HER) side promptly to ensure fast H2 production. Therefore, an efficient potassium ion conducting membrane (PCM) with low I- permeation is necessary. For the EGOR system, the HER side produces H2 and OH-, which passes through an anion exchange membrane (AEM) to the EGOR side to provide OH- for the oxidation of ethylene glycol (EG). Hereby, an stable AEM with high hydroxide conductivity and low EG crossover is required. In this study, we aim to develop low-cost and eco friendly ion conducting membranes based on chitosan derivatives for efficient green hydrogen production. Chitosan is a bio-renewable, environmental friendly and inexpensive polysaccharide, and a wide variety of functional groups can be feasibly grafted onto CS to incorporate desirable ion conductivity and tailor the mechanical properties of the membranes. In the IOR system, potassium sulfonates are introduced to chitosan backbone in high grafting density to afford K+ conductive sulfonated chitosan (SCS-K). For the EGOR system, quaternary ammonium salts are grafted onto chitosan backbone to produce hydroxide conducting quaternized chitosan (QCS). SCS/QCS is highly water-soluble and the corresponding composite membranes could be fabricated via environmental friendly water-based processes by crosslinking SCS/QCS with glutaraldehyde (GA) and aldehyde functionalized graphene oxide (FGO). By sandwiching mesoporous Nylon membrane (PA) between two densely packed SCS/QCS composite membranes, the three-layer PCM/AEM exhibit good ion conductivity, high mechanical strength, good dimensional stability, and low I-/EGOR crossover. The full cells empolying our custom-made multi-layer PCM/AEM demonstrate highly efficient hydrogen production comparable with commercial Nafion/Sustanion membranes, proving the practicality of these membranes.