研究导读
氢能作为实现碳中和的终极能源，其生产技术一直备受关注。传统碱性电解水制氢虽成本低，但受制于氢氧根离子交换膜（HEM）的传输速率，始终难以突破效率天花板。
天津大学
周凯歌
、
姜忠义
、
武美玲
携手
中山大学
周业成
、兰州大学
张浩力
课题组，
以二维BiOI基材料为平台，构建由短氢键（SHB）组成的离子传递网络，在二维限域通道中形成了具有Grotthuss模式的阴离子超快传递通道-“离子高速公路”。
离子电导率可提升至168 mS cm
-1
（90°C），并在膜式碱性电解槽中展现出高效耐用的电解制氢性能。这项工作揭示了限域空间对层间短氢键网络的调控作用，为阴离子快速传输通道设计提供了新范式，有望推动下一代氢能装备的研发进程。
Fig. 1. Interlayer confinement strategy to construct SHB networks.
(A) Types of hydrogen bonds depending on the donor-acceptor distance. (B) Construct SHB with an interlayer confinement strategy.
研究内容
首先，为了准备用于氢氧根离子传输通道的碘氧化铋(BiOI)纳米片，我们通过水热插层和微流体均质技术辅助法剥离BiOI晶体，实现了BiOI纳米片自上而下的成功剥离。制备的BiOI纳米片厚度与宽度分别为4 nm与1 μm，且其中的部分碘被羟基取代。利用该方法均一的尺寸分布，可通过真空抽滤辅助自组装方法，制备出具有良好柔韧性的半透明膜。BiOI纳米片在膜中形成了长程有序的二维通道，并可以通过控制氢氧根交换量调控纳米通道的尺寸和化学性质。可控氢氧根交换的二维BiOI膜为后续层间氢键网络和阴离子传导的研究提供具有可调控的限域空间与亲水基团的2D纳米通道。
Fig. 2.
Fabrication of BiOI nanosheets and hydroxide-substituted BiOI membranes.
(
A
) Schematic of the fabrication of BiOI nanosheets from BiOI crystal by the two-step procedure of hydrothermal intercalation and microfluidization for hydroxide conductor. (
B
) Atomic force microscopic (AFM) image of BiOI nanosheets. Height profiles are indicated by white lines with thicknesses of 3.89, 3.97 and 3.81 nm. (
C
) Lateral size and thickness distribution of the BiOI nanosheets synthesized at 160 °C and 24 h for the hydrothermal exfoliation procedure.
The data was sourced from 30 to 60 distinct measurements taken across various samples. (
D
) UV absorbance intensity at 226 nm of BiOI supernatants fabricated with different hydrolysis times and its according atomic ratio between Bi and I in BiOI nanosheets. (
E
) Cross-sectional SEM image of a BiOI membrane. Inset digital photo showing the flexibility of a BiOI membrane. (
F
) Exchange ratio of O−H received from EDS measurement of BiOI with varied immersion times in 1 M NaOH aqueous solution. Inset images indicate the schematic diagram of a single large BiOI exchanged by the different ratios of O−H.
Error bars were derived from the relative errors obtained via EDS measurements.
(
G
) XRD spectra of BiOI with varied O−H exchanging times. Peaks at 9.6° and 10.8° are referred to as Peak I and Peak II, respectively.
为了研究BiOI二维限域空间中的氢键网络及其与羟基取代的关系，该工作利用介电谱与红外光谱监控其强度与数量。通过介电谱追踪氢键网络的介电弛豫过程，可以发现，随着羟基取代量的增加，BiOI限域空间中的氢键网络强度增加。同时，红外光谱显示，处于3244 cm
−1
处的短氢键特征峰强度快速增加。而通过扣除BiOI基材料背景信号后，可以发现短氢键主要存在于膜内受限水中。基于以上实验结果，该工作进一步采用从头算分子动力学模拟（AIMD）研究了羟基取代率对BiOI限域通道内氢键网络的微观影响。其中，SHB（2.5 Å）的比例增加了12倍（从0.34%到4.2%），表明羟基取代明显促进了氢键网络的延伸。同时，氢键的平均距离减少了1.8%，表明形成了更多的短氢键，这与实验观测中短氢键含量增加相吻合。此外，BiOI膜通道宽度也随着羟基取代率增加而降低。因此，限域效应和界面相互作用的协同效应是BiOI限域通道中短氢键网络的形成原因。这一发现为理性设计短氢键超快传输通道，提升阴离子传输性能提供了重要参考。
Fig. 3. Characterizations of hydrogen bond networks in the hydroxide-substituted BiOI membrane.
(
A
) Dielectric spectra of hydroxide-substituted BiOI membrane. The spectra were fitted by the Havriliak–Negami relaxation model, and relaxation peaks were indicated by the color rectangles. (
B
) Relaxation time
τ
1
and
τ
2
under different O−H densities was obtained for Peak I and Peak II, respectively. FT-IR spectra of (
C
) hydrated BiOI
1-x
(OH)
x
membranes and (
D
) confined water in nanochannels after excluding the signal of unhydrated BiOI
1-x
(OH)
x
membrane. (
E
) Area ratios of SHBs and NHBs in hydrated/unhydrated BiOI
1-x
(OH)
x
membranes and confined water in the nanochannels of BiOI
1-x
(OH)
x
membrane. (
F
) Probability statistics of hydrogen-bond lengths in BiOI
0.67
(OH)
0.33
, BiOI
0.44
(OH)
0.56
and BiOI
0
(OH)
1
, respectively. (
G
) Hydrogen bond amounts and mean distance of BiOI
0.67
(OH)
0.33
, BiOI
0.44
(OH)
0.56
and BiOI
0
(OH)
1
, respectively. (
H
) Hydrogen bond distribution within the nanochannels of BiOI
1-x
(OH)
x
membranes.
The membranes BiOI
0.03
(OH)
0.97
, BiOI
0.17
(OH)
0.83
, BiOI
0.35
(OH)
0.65
, and BiOI
0.64
(OH)
0.36
, with respective thicknesses of 4.5, 5.0, 5.3, and 5.4 µm, were used in the experiments.
BiOI膜内因可调节的氢键网络特性，成为研究短氢键对阴离子传输影响的理想实验平台。随着通道中短氢键网络的扩展，羟基取代BiOI膜在90°C时的电导率提升至168 mS cm
-1
，高于大多数已报道的聚合物基HEM。为了深入探究短氢键网络对离子输运的影响规律，本研究通过分子动力学计算了氢氧根离子在通道内的均方差位移（MSD）。模拟结果表明，随着短氢键网络的扩展，氢氧根离子通过Grotthuss输运机制的比例显著增加，从而提高了羟基取代BiOI膜中的阴离子传输速率。因此，在较低离子交换容量条件下，羟基取代的BiOI基限域通道能显著提升氢氧根离子传输速率，从而避免高离子交换容量可能引起的对HEM其它性能的不利影响，如溶胀率高、气体隔绝性低等问题。
