在2016年的各大車展中，來自豐田(Toyota)、本田(Honda)、現(xiàn)代(Hyundai)和梅賽德斯-奔馳(Mercedes-Benz)的多款全新燃料電池汽車和SUV，引發(fā)了廣泛的討論。但由于這種車的燃料電池組必須采用昂貴的鉑催化劑，以加速產(chǎn)生能量的化學(xué)反應(yīng)，因此第一批商用燃料電池車型相當(dāng)昂貴。
“過去20年中，燃料電池中鉑催化劑的用量已經(jīng)下降了到了原來的10%，”紐瓦克市特拉華大學(xué)(Universityof Delaware)化學(xué)工程系教授嚴玉山介紹道，“但我認為在未來很長一段時間內(nèi)，鉑的用量可能很難繼續(xù)下降。”
當(dāng)前的燃料電池技術(shù)，其成本到底能否降至經(jīng)濟上真正可負擔(dān)的水平，嚴教授對此持懷疑態(tài)度。大約10年前，嚴教授和他的同事放棄了現(xiàn)行的質(zhì)子交換膜型（下簡稱PEM）燃料電池，轉(zhuǎn)而將精力放在了一種完全不需要鉑催化劑的燃料電池研發(fā)中。
酸vs堿
1839年，當(dāng)WilliamGrove發(fā)明燃料電池原理時，他選擇了硫酸作為電池的電解質(zhì)。大約過了漫長的100年，人們才終于研制出另一種燃料電池，即堿性燃料電池。具體來說，堿性燃料電池會使用氫氧化鉀溶液作為電解質(zhì)。嚴教授指出，總體來說，酸性燃料電池和堿性燃料電池內(nèi)的化學(xué)反應(yīng)非常類似，都是電池正極氧氣減少，電源負極的氫氣被氧化。
“但當(dāng)你寫下標注電荷移動的化學(xué)方程式時，你就會發(fā)現(xiàn)用堿性電解質(zhì)中的OH-替代酸性電解質(zhì)中的H+還是會產(chǎn)生一些差別。”
上世紀90年代，當(dāng)汽車行業(yè)著力研發(fā)PEM氫燃料電池時，大家并未過多考慮酸性電池內(nèi)極具腐蝕性的酸性反應(yīng)環(huán)境。那時的研究重點是研制允許氫質(zhì)子(H+)通過的交換膜。后來，杜邦(DuPont)公司推出的Nafion半透性質(zhì)子交換膜改變了整個燃料電池領(lǐng)域的面貌，即使這種膜看起來與廚房常見的塑料紙沒什么差別。
嚴教授回憶說，雖然當(dāng)時這種氟化聚合物薄膜的價格非常昂貴，但那時的研究人員“好像就是認定了這種東西，他們似乎對其他技術(shù)完全沒有興趣了。”但嚴教授和他在特拉華大學(xué)的團隊并未止步于杜邦公司的Nafion膜，他們堅信，氫氧質(zhì)子交換膜燃料電池的概念一定可以提供與氫質(zhì)子交換膜同等級別的高性能，而且可以將成本降至前所未有的水平。
嚴教授解釋說，選擇堿性環(huán)境有一個優(yōu)勢：我們可以用鎳、銀等相對便宜的金屬替代非常昂貴的鉑催化劑。“堿性的反應(yīng)環(huán)境更好，”他表示說，“這是因為多數(shù)催化金屬在堿性環(huán)境下都穩(wěn)定的多，而酸性環(huán)境會溶解一切金屬，包括鉑。”
最近，嚴教授團隊發(fā)表的報告描述了團隊進行的氫氧質(zhì)子交換膜燃料電池研究，這種燃料電池原型選擇了一種以鎳為基礎(chǔ)的低成本復(fù)合催化劑，并在電池陽極附近的氫氧化反應(yīng)（見《自然·通訊》，1月14日內(nèi)容，點擊此處直達）。這種復(fù)合催化劑特別采用摻氮碳納米管上的鎳納米顆粒，可在堿性反應(yīng)環(huán)境中，展現(xiàn)與鉑族金屬類似的催化氧化效果。
目前，堿性燃料電池面對的最大問題在于，與酸性電池相比相對較慢的反應(yīng)速度。“這是一個問題，堿性環(huán)境下的反應(yīng)速度是酸性環(huán)境的1%。”嚴教授指出，“但我們已經(jīng)有思路，可以利用催化劑加速反應(yīng)的發(fā)生，不過這可能還需要幾年時間才能實現(xiàn)。”
堿性質(zhì)子膜挑戰(zhàn)Nafion膜
幾年前，特拉華大學(xué)的團隊為自己的堿性燃料電池研制了一種與“Nafion”很類似的膜。如同Nafion膜允許氫質(zhì)子通過一樣，特拉華大學(xué)的這層薄膜也允許氫氧質(zhì)子通過。嚴教授表示，“我們在氫氧質(zhì)子交換膜上做的相當(dāng)不錯。”
從技術(shù)層面而言，這種質(zhì)子膜的本質(zhì)是“銀-磷高分子離聚物的交匯界面”，效果非常好。使用這種季磷功能化的聚合物可以產(chǎn)出一種材料，這種材料在水中不易膨脹，可以用來打造性能絕佳的氫氧質(zhì)子交換膜燃料電池。嚴教授表示，這種材料是一種納米級拼接物，其疏水區(qū)段周圍遍布親水通道，氫氧離子正是通過這些微小通道才能穿過交換膜。
這種新型膜技術(shù)的成本更低，主要原因是其采用的碳氫化合物材料比PEM膜采用的氟化聚合物膜更加便宜。
“我們最終的目標是將氫氧質(zhì)子膜燃料電池的應(yīng)用推廣至汽車領(lǐng)域，打造真正買的起的燃料電池汽車，那時豐田Mirai的價格大概可以下降至23000美元。”嚴教授推測，“一旦真正讓用戶買得起的燃料電池汽車成為現(xiàn)實，那氫能源經(jīng)濟所帶動的各種基礎(chǔ)設(shè)施建設(shè)與發(fā)展也將很快跟上。”
嚴教授以輕松的語氣講述了他與他的團隊是如何在2009年和2010年應(yīng)對美國能源署部長朱棣文(Steven Chu)，以及其他對燃料電池持懷疑態(tài)度人士的質(zhì)疑。雖然當(dāng)時進行了非常嚴格的評估，但最終朱棣文還是批準了嚴教授團隊的經(jīng)費申請。
如果打動朱棣文的不是嚴教授的研究成果，那可能是嚴教授努力研制新型燃料電池時的大膽無畏打動了這位諾貝爾物理學(xué)獎得主，因為朱棣文很少為氫燃料電池技術(shù)撥付經(jīng)費。
作者：Steven Ashley
來源：SAE《汽車工程雜志》
翻譯：SAE 上海辦公室
Rethinking the route to lower-cost fuel cells
New fuel cell-powered cars and SUVs from Toyota, Honda, Hyundai and Mercedes-Benz are capturing plenty of publicity on this year's auto show circuit. But the first commercial models are expensive, in part because their fuel cell stacks use costly platinum catalysts to speed the key power-producing chemical reactions.
“The level of platinum use in fuel cells has come down ten-fold in the last 20 years,” observed Yushan Yan, chemical engineering professor at the University of Delaware in Newark, “but I have a feeling that the platinum level will stay where it is for some time to come.”
Yan is skeptical that current fuel cell technology can be become truly affordable. About a decade ago he and his colleagues turned away from current proton exchange membrane (PEM) fuel cells in favor of a type that needs no platinum at all.
Acid versus base
When William Grove invented the principle of the fuel cell in 1839, he used sulfuric acid as the electrolyte. But it took another 100 years for the alternative—the alkaline, or basic, fuel cell—to be developed. The alkaline fuel cell used KOH, or potassium hydroxide, as its electrolyte. Yan noted that the reactions of both types are similar at a high level—oxygen reduces on the positive electrode, and the hydrogen oxidizes on the negative electrode.
"But when you write down the chemical reaction with the charge-carrying ions, it’s different because you use an OH- instead of an H+,” he said.
In the 1990s when the auto industry focused development on PEM hydrogen fuel cells, there wasn't much concern about their extremely corrosive, acidic operating environment. The main issue was the other key component, the membrane that passes protons (H+) between the two electrodes.The ready availability of DuPont’s Nafion semi-permeable polymer film was the game-changer, despite it looking like ordinary plastic kitchen wrap.
Though the fluorinated polymer membrane was itself premium-priced, researchers “felt like that was it; they never wanted to work with other technologies,” Yan recalled. Rather than settle on Nafion, he and his group at Delaware bet that their hydroxide (OH-) exchange membrane fuel cell concept can offer high performance at an unprecedented low cost.
Opting for the high end of the pH range has an advantage: it enables replacement of platinum catalysts with cheaper metals like nickel or silver, Yan explained. “A basic operating environment is better," he said, "because many catalytic metals are much more stable, while everything dissolves in acid, including platinum.”
Yan’s team recently published an account of their work on a hydroxide exchange membrane fuel cell that uses a prototype low-cost nickel-based catalyst for the hydrogen oxidation reaction at the anode. (See www.nature.com, January 14.) The composite catalyst, which features nickel nanoparticles that are supported on nitrogen-doped carbon nanotubes, exhibits levels of hydrogen oxidation activity similar to those of platinum-group metals in an alkaline electrolyte.
The key remaining issue to address in the catalyst is the comparative slowness of the alkaline reaction compared to its acidic platinum counterpart. “It’s a problem; the reaction occurs 100 times slower in basic conditions," Yan noted, "but we have our ideas about how we can get the catalyst to do what we want. Still, it’s probably a couple of years away.”
Basic membrane challenges Nafion
Several years ago the Delaware group developed a “Nafion-equivalent” membrane for its alkaline fuel cell, a thin polymer membrane that does for hydroxide ions what Nafion does for protons. “We have a good handle on the hydroxide exchange membrane,” Yan asserted.
Technically, the prototype membrane is classified as “an efficient silver-phosphonium ionomer interface.” Using a quaternary phosphonium-functionalized polymer yields a material that is less susceptible to swelling with water while providing excellent hydroxide exchange membrane fuel cell performance. According to Yan, the material is a nanoscale patchwork of hydrophobic domains abutting hydrophilic water channels; it is via these tiny passages that hydroxide ions come streaming through.
The new membrane technology would also be cheaper because it would replace the PEM’s high-priced fluorinated polymer membrane with a cheaper hydrocarbon material, another boost to economic viability.
“Our real hope is that we can put hydroxide exchange membrane fuel cells into cars and make them truly affordable—maybe $23,000 for a Toyota Mirai,” Yan speculated. “Once the cars themselves are more affordable, that will drive development of the infrastructure to support the hydrogen economy.”
Yan recounted with amusement how he and his team’s contrary R&D path somehow passed measure with Steven Chu, the U.S. Department of EnergySecretary and notorious fuel-cell skeptic, in 2009 and 2010. Despite a hard-eyed evaluation, Chu green-lighted Yan's group for funding.
If it wasn't the result of sheer spite on the part of the Nobel Prize-winning physicist, perhaps it was the sheer audacity of building a new kind of fuel cell that impressed the Secretary, because it was one of only a few grants that Chu ever provided for hydrogen fuel cell technology.
Author: Steven Ashley
Source: SAE Automotive Engineering Magazine
電子掃描顯微鏡下的圖像可以展示嚴教授團隊研發(fā)的新型燃料電池催化劑，完全不需要昂貴的鉑金屬。
數(shù)年前，嚴教授與曾在特拉華大學(xué)攻讀博士后的研究員顧爽一起觀察氫氧質(zhì)子交換膜的實驗室樣本。
數(shù)年前，嚴教授在特拉華大學(xué)的研究團隊為自己的堿性燃料電池研發(fā)了一種有效的氫氧質(zhì)子交換膜。
近日，豐田Mirai燃料電池車已經(jīng)進入量產(chǎn)。2015年，一輛Mirai燃料電池車亮相特拉華大學(xué)的某次校園活動
長時間以來，嚴玉山教授在特拉華大學(xué)的研究團隊一直在研發(fā)一種成本更低的燃料電池，用來替代成本高昂的傳統(tǒng)PEM燃料電池，供燃料電池汽車使用。