太陽能模組銀元素回收提取及其電催化二氧化碳還原反應銅銀電極之選擇性提升應用

Abstract

近年碳中和成為國際主流趨勢，減碳技術與再生能源成為各國關注重點。為替代化石能源，以矽晶太陽能模組為首的再生能源電廠大量建置，然而其發電期間有限，汰舊之太陽能模組數量逐年增加，衍生資源閒置與廢棄物處理議題。太陽能模組製造業每年耗費全球8%的銀礦產，其為模組中最經濟價值的金屬元素。本研究著重於自模組中回收銀效率的提升，並開發銀提升電催化二氧化碳還原反應選擇性之應用。 模組中銀元素可從以硝酸、甲基磺酸與過氧化氫混合液提取。研究發現模組背板之鋁元素並不會影響初期甲基磺酸溶解銀的效率，僅在過氧化氫後，過量的鋁元素才會將銀置換析出。而在硝酸提取的含銀溶液中，以銅、鋅進行金屬置換無法提高還原率與純度。若以沈澱法將銀離子形成氯化銀沈澱、陰離子交換製備氧化銀，最後以工業界常見之高溫熱分解氧化銀，最終可得93%的整體金屬銀還原效率，而自甲基磺酸中以沈澱法純化銀元素，亦可獲得90％還原效率。 銀元素修飾奈米多孔銅(NPC)電極，以提升具經濟價值之二碳產物選擇性。共濺鍍銅鋁以微米顆粒沈積於氣體擴散電極表面，與去合金後平均23.8±4.8 nm奈米多孔支架共同展現為階乘式結構。以電子槍蒸鍍修飾銀的NPC-E gun電極中，銀均勻覆蓋奈米支架，僅留下微米結構。而間歇性電鍍銀的NPC-PED電極中，銀附著於奈米支架使其粗化至33.0±5.7 nm，保留NPC的多層次結構。XPS、XRD分析奈米多孔銅以一價銅為主，為去合金時電解液滲透梯度不同，使NPC頂部銅元素過度氧化導致。為維持奈米多孔銅電極催化之結構穩定性，噴塗高分子Nafion提升電極疏水性。 以gas flow cell提升催化反應物質傳遞，有效將二氧化碳還原反應電流密度提升至-400 mA/cm2，為工業化所需電流密度的兩倍。在高電流下，奈米多孔銅電極之產氫反應抑制在40%下，且將高價值二碳產物之發法拉第轉換效率提升至32%。而經過銀修飾之電極，氫氣更可抑制於30%以內，二碳產物之發法拉第轉換效率達到52%，相對無修飾之奈米多孔銅電極提升44%。塔佛斜率顯示NPC-E gun與NPC-PED相較NPC有更低的反應能量障礙。銀可增加關鍵中間產物*CO的覆蓋率，提升其偶合成二碳產物的機會。而NPC-PED的銅銀化學界面使得表面應力進一步降低中間產物耦合能障，故其二碳產物選擇性可與銀比例高的NPC-E gun相當。

Carbon neutrality has become a global target currently, so carbon reduction and renewable energy are prospering. To replace fossil fuels, crystalline silicon (Si) solar modules are widely used, but their limited lifespan leads to an emerging issue for recycling end-of-life solar modules. Silver (Ag) is the most valuable element, and solar module manufacturers consume 8% of world Ag production. In this work, an improvement in Ag recovery and the enhancement of the electrochemical CO2 reduction reaction (EC-CO2RR) were studied. Ag was extracted from a mixture of methanesulfonic acid (MSA) with hydrogen peroxide (H2O2), and nitric acid (HNO3). In MSA, the solubility of Ag+ would not be affected by the aluminum (Al) on the back sheet with a short immersion time. Al only replaced Ag+ when the hydrogen peroxide was consumed. In HNO3, the metal exchange by zinc and copper (Cu) could not perform well in terms of recovery yield and purity. In contrast, more than 93% of Ag+ in HNO3 could be recovered through AgCl precipitation, anion exchange to silver oxide (Ag2O), and thermal decomposition of Ag2O. The Ag+ in MSA could also achieve a 90% recovery yield with the precipitation method. Ag was deposited on the nanoporous Cu (NPC) to enhance the faradaic efficiency (FE) toward multi-carbon (C2+) products. Through co-sputtered CuAl on a gas diffusion layer (GDL) and dealloying, the spherical structure of the precursor remained in NPC and contributed to hierarchical morphology. E-gun decorated NPC (NPC-E gun) covered the top of the nanoporous ligament with Ag, so it only remained microporous structure. Pulse-electrodeposition decorated NPC thickened the nanoligament size from 23.8±4.8 nm to 33.0±5.7 nm, preserving the hierarchical structure. The major Cu species in NPC on GDL was Cu2O by XRD and XPS, revealing that the top of NPC was oxidized due to a different dealloying gradient. Thus, a protected polymer, Nafion, was coated to maintain the electrode's stability. EC-CO2RR was conducted in a gas flow cell to accelerate mass transfer. The current density was over -400 mA/cm2, which was twice the commercial standard. At -400 mA/cm2, the gas flow cell constrained the hydrogen evolution reaction below 40%, and FE towards the C2+ products achieved 32% in NPC. Moreover, the Ag decorated NPC: NPC-E gun and NPC-PED further suppressed HER below 30% so that the FE towards the C2+ product reached 52%, which was a relatively 44% enhancement compared to NPC. Tafel slope also proved a lower energy barrier in NPC-E gun and NPC-PED than in NPC. The enhancement might result from the vital intermediate *CO enrichment on the catalyst by decorated Ag to promote C-C coupling. Furthermore, the surface strain in NPC-PED reduced the activation energy for C-C coupling, so it could have similar performance to the NPC-E gun even though it had less Ag.