NSRRC Activity Report 2023

034 NSRRC ACTIVITY REPORT 2023 structures in the solution of the hydrogen evolution reaction. Recent research suggests that the kinetics of hydrogen formation favor interactions between hydrogen atoms on different polymer chains rather than within the same polymer chain. Consequently, the observed superior efficiency in the hydrogen evolution reaction for A 1 –A 2 -type conjugated polymers compared to D–A-type conjugated polymers can be attributed to the more compact network structure formed in the reaction solution. To evaluate the stability of PBDTTTSOS in a photocatalytic hydrogen evolution reaction, the team conducted a continuous six-cycle experiment involving visible-light irradiation in an NMP/water/ascorbic acid (AA) mixture. The results indicated a gradual decrease in the quantity of generated hydrogen over the six cycles, with approximately 70% of the initial hydrogen evolution rate (HER) sustained after the sixth cycle. PBDTTTSOS maintained an HER of 48.6 mmol h −1 g −1 ( Fig. 2(e) ). To gain deeper insights into the efficiency decline, the team utilized USAXS and SAXS techniques to examine morphological and compactness changes post-photocatalytic reaction. Further analysis revealed that PBDTTTSOS underwent morphological alterations in the NMP/water/AA mixed solution, leading to a more loosely packed network following the hydrogen evolution reaction. Specifically, the USAXS analysis indicated a reduction in the α value from –3.14 (as prepared) to –2.88 post-photocatalytic activity ( Fig. 2(f) ). Moreover, the self-inhibition phenomenon of AA and its oxidation product dehydroascorbic acid (DHA) emerged as a contributing factor to the overall decrease in hydrogen evolution efficiency. In summary, these findings suggest that the observed reduction in photocatalytic efficiency can be attributed to morphological changes in PBDTTTSOS within the NMP/water/AA mixture, along with the self- inhibition effects of AA and its oxidation product DHA. To date, the majority of reported polymer photocatalysts have relied on freshwater as the primary source for hydrogen evolution, posing a significant challenge due to the limited availability of freshwater resources (< 1% of Earth’s water). This scarcity exacerbates an already critical global challenge. Therefore, a promising avenue involves the direct splitting of seawater without purification, capitalizing on its abundance (> 96.5% of Earth’s water) for sustainable solar-to-hydrogen energy conversion. PBDTTFSOS and PBDTTTSOS were chosen as representative compounds for investigating photocatalytic hydrogen evolution through seawater splitting under visible-light irradiation. In simulated seawater, PBDTTFSOS and PBDTTTSOS exhibited an impressive HER rate of 86.9 and 92.6 mmol g −1 h −1 , marking a 23.0% and 27.0% increase compared to that observed in deionized (DI) water ( Figs. 3(a) and 3(b) ). The concurrent USAXS–SAXS analysis revealed more compact network structures of the polymers in the aqueous solution with 0.6 M NaCl. Specifically, the USAXS data for PBDTTTSOS in simulated seawater exhibited a larger α value of –3.42, in contrast to –3.14 in DI water. This indicates a more compact chain packing density of polymer chains in simulated seawater, elucidating the enhanced photocatalytic efficiency observed in this medium ( Fig. Fig. 2 : (a) Time-dependent HER of the photocatalysts under visible-light illumination, (b) comparison of the HER between D–A-type conjugated polymers and A 1 –A 2 -type conjugated polymers, (c) combined USAXS (0.002–0.006 Å -1 ) and SAXS data of the D–A-type conjugated polymers, (d) parallel USAXS–SAXS data of the corresponding A 1 –A 2 -type conjugated polymers, (e) photocatalytic cycling stability test of PBDTTTSOS; and (f) combined USAXS (0.002–0.006 Å -1 ) and SAXS data of PBDTTTSOS before and after photocatalytic reaction. [Reproduced from Ref. 1]