Page 65 - ILLS Annual Report 2018

SCIENTIFIC HIGHLIGHTS
62-63
Figure 2
Schematic illustration of the hydrogen evolving half-reaction using conjugated microporous polymers (CMPs). F-CMP and S-CMP
are the two porous CMPs examined in this study, corresponding
to X = C(CH3)2 and X = SO2, respectively. Non-porous analogues are also illustrated at the bottom of the right-hand panel.
A photocatalyst is used to harness the sunlight and assist
the water-splitting reaction. To be scalable, photocatalysts should be based on earth-abundant and non-toxic elements. Conjugated polymers are promising materials as they have been shown to evolve hydrogen using visible light [3], offering great flexibility too in terms of chemical design. For instance, porosity has the potential to increase the number of catalytic sites if water can travel through the pores.
In this study [4], we focused on two conjugated microporous polymers (CMPs)—labelled F-CMP and S-CMP—showing larger hydrogen evolution than their linear, non-porous analogues (figure 2). In F-CMP,
X = C(CH3)2 can be replaced by a sulfone group (X = SO2), in S-CMP, as sulfone groups have been shown to increase hydrophilicity, leading to higher hydrogen evolution [5]. The aim of this study was primarily to understand water transport within the complex pore networks of F-CMP and S-CMP.
Water and CMPs form respectively the ‘guest’ and ‘host’ components of the catalytic system, calling for a targeted microscopic probe of its emerging structural dynamics behaviour and evolution. To achieve this, we examined differences between F-CMP and S-CMP in terms of their local dynamics by using the Quasi-Elastic Incoherent Neutron Scattering (QENS) technique. A unique feature offered by this technique is the ability to use D2O in addition to H2O for contrast variation purposes, as illustrated in figure 3.
Figure 3 shows the Q-averaged dynamical structure factor, S(E), at 300 K, highlighting on the one hand the impact of hydration on the dynamics of S-CMP, and on the other, the impact of pore confinement on the water dynamics. For both CMPs, the QENS signal broadens
as the amount of D2O increases up to a content of 35 wt. % for F-CMP and 55 wt. % for S-CMP. We assign this threshold to the maximum water uptake of each CMP. Although the dry signal of both CMPs is comparable,
S-CMP in D2O exhibits a broader QENS signal, thus pointing to faster dynamics in S-CMP than in F-CMP when in water, on the probed time scale. For both CMPs, the water signal for partially hydrated samples is narrower than the signal of bulk water, with a clear increase in the elastic contribution. This indicates that the dynamics of the water in the pores is frustrated within the accessible 5–50 ps instrumental time window. This effect is more pronounced
in the case of F-CMP. For all the water contents studied,
the QENS signal exhibits two contributions: one from the CMP itself and one from the water. Above the maximum water uptake, the QENS signal from the water shows two contributions: one from confined water inside the pores and one from free water. The water contributions to the QENS signal for both CMPs are summarised in figure 3; the rest of the signal is attributed to the CMP itself.
The lower water uptake of F-CMP in comparison with S-CMP, combined with more frustrated water dynamics inside the pores, points to reduced mass transport in the F-CMP network that probably contributes to its reduced photocatalytic activity.
Solar hydrogen could help to mitigate the impact of humans on climate change. If CMPs are shown to evolve hydrogen using visible light, understanding mass transport within their complex pore networks has the potential
to boost the design of such materials for water-splitting applications. We have shown that QENS is a robust method for probing mass transport, in addition to shedding light on the crucial water uptake process.
Figure 3
Top: Schematic illustration of the use of D2O (left) and H2O (right) to study hydration of the CMPs. Middle: Quasi-elastic neutron scattering (QENS) spectra of dry and hydrated S-CMP (left) and confined water in the pore of S-CMP in comparison with free water (right). Bottom: Summary of the various contributions from water to the QENS signals of F-CMP (left) and S-CMP (right) as a function of water content. The remaining contribution is assigned to the CMP itself. The maximum water uptake is shown by a red line.
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