Comment on "CO2 Footprint of Thermal Versus Photothermal CO2 Catalysis".

The previously published report by Wang et al. provides universal equations for evaluating the net CO2 emission via photothermal/thermal CO2 hydrogenation reactions in batch and flow reactors, respectively. However, it remains to be discussed whether the original feed amount or feed rate of H2 should be changed accordingly when the CO2 reduction rate is selected as the variable to investigate the net CO2 emission rate of the system. If not, the reaction would not be in accordance with the stoichiometric ratio, which is inconsistent with reality, bringing about the optimistic scenario in the assessment of CO2 footprint. This work has taken the potential relationship between the original feed amount or feed rate of H2 and CO2 conversion rate into account. The effects of CO2 conversion rate on the net CO2 emission rate in the photothermal catalytic system are re-examined, the obtained trends exhibit more challenging preconditions to achieve net-zero carbon emission than those in the work by Wang et al. The quantitative results indicate that green hydrogen source is indeed vital for carbon neutrality in photothermal CO2 catalysis. Here, the viewpoints will be worth considering and be seen as complementary to the proposed carbon emission assessment equations.

Synergizing Photo-Thermal H2 and Photovoltaics into a Concentrated Sunlight Use.

Solar hydrogen and electricity are promising high energy-density renewable sources. Although photochemistry or photovoltaics are attractive routes, special challenge arises in sunlight conversion efficiency. To improve efficiency, various semiconductor materials have been proposed with selective sunlight absorption. Here, we reported a hybrid system synergizing photo-thermochemical hydrogen and photovoltaics, harvesting full-spectrum sunlight in a cascade manner. A simple suspension of Au-TiO2 in water/methanol serves as a spectrum selector, absorbing ultraviolet-visible and infrared energy for rapid photo-thermochemical hydrogen production. The transmitted visible and near-infrared energy fits the photovoltaic bandgap and retains the high efficiency of a commercial photovoltaic cell under different solar concentration values. The experimental design achieved an overall efficiency of 4.2% under 12 suns solar concentration. Furthermore, the results demonstrated a reduced energy loss in full-spectrum energy conversion into hydrogen and electricity. Such simple integration of photo-thermochemical hydrogen and photovoltaics would create a pathway toward cascading use of sunlight energy.