Reports: ND1056999-ND10: Photocatalytic Systems for CO2 Reduction

Jennifer Cha, PhD, University of Colorado, Boulder

Charles Musgrave, PhD, University of Colorado, Boulder

The focus of the proposed research is to design, engineer, and evaluate reaction cascades to produce alternative chemicals for petroleum technologies from carbon dioxide and water. In this study, we first show a simple method to construct CO2-reducing PECs that can generate formate and methanol at neutral pH. These cells are composed of CdS/NiOx photoanodes and NiO/CdTe or CuFeO2/CuO cathodes. To produce the CuFeO2/CuO cathodes, clean ITO electrodes were inserted into a solution of KCl, CuSO4 and FeCl3 and. Pt and Ag/AgCl was used as the counter and reference electrode respectively. Next, -0.31 V was applied for 2 h to form a 2.4 μm/1.8 μm thick layer of CuFeO2/CuO. The electrodeposited CuFeO2/CuO substrates were then annealed at 650°C for 3 h under air, which caused the electrode to change from an orange-pink color into dark gray-black. For the CdS/NiOx photoanode, a paste of CdS was first prepared, deposited onto ITO and calcined at 200°C for 24 h. Next a layer of NiOx (Solaronix) was deposited on the CdS layer followed by a second calcination at 200°C under air. The CdS/NiOx photoanodes were also tested for oxygen production and were then coupled with the CuFeO2/CuO photocathodes to form one of the two types of PECs studied and tested for CO2 reduction at neutral pH. UV-Vis absorbance measurements showed that the CdS/NiOx and CuFeO2/CuO electrodes strongly absorb in the visible. Next, the photoelectrochemical responses of the CdS/NiOx and CuFeO2/CuO electrodes were measured under CO2 atmosphere. The CdS/NiOx electrodes showed an anodic photocurrent with onset at -0.8 V vs. Ag/AgCl, with a maximum current at -0.35 V while the CuFeO2/CuO electrode showed an onset cathodic current at 0.1 V with maximum current at -0.4 V vs Ag/AgCl. PECs were next built from the CdS/NiOx and CuFeO2/CuO photoelectrodes and upon photoillumination with 465 ± 30 nm light emitting diode light for each electrode, an open circuit voltage of ca. 1 V was observed.

A second photocathode of NiO/CdTe was also fabricated and tested with the CdS/NiOx photoanode. For this, CdTe nanoparticles were first synthesized and from the absorption onset, the bandgap of the CdTe quantum dots was determined to be about 2 eV which also corresponds to the ~3.5 nm diameter as measured by TEM. To prepare the CdTe photocathodes, a NiO layer was first deposited on ITO to act as a hole collector as well as stabilize the electrode. Next, a paste was formed from the CdTe nanoparticles by mixing the nanocrystals with ethylene glycol which was then brushed on the NiOx film to form the p-type photoactive layer followed by calcination at 200°C for 24 h. The NiO/CdTe photocathodes were then coupled with the CdS/NiOx photoanodes to form the second type of autonomous PEC tested for CO2 reduction. Similar to the CuFeO2/CuO photoelectrodes, UV-Vis measurements of the NiO/CdTe cathode showed strong absorption in the visible region. The photoelectrochemical responses of the NiO/CdTe electrodes were also measured under Ar or CO2 atmosphere which showed a cathodic photocurrent onset at 0 V vs. Ag/AgCl, with a maximum current at -0.5 V. By assembling the NiO/CdTe cathode with the CdS/NiOx anode an OCV of ca. 0.7 V was obtained. The semiconductor based electrodes showed limited stability, which effected the total fuel conversion gained.

Using either the CuFeO2/CuO or NiO/CdTe photocathodes, PECs were next fabricated with the CdS/NiOx photoanodes. In order to test the PECs for CO2 reduction, 12C or 13C labeled CO2 was introduced under vacuum into a single or 3-electrode compartment PEC chamber. After 17 h photoirradiation with 465 nm light, a combination of electrospray ionization (ESI) and 1H NMR was utilized to analyze the products formed. In the first studies, in the presence of 12C CO2, after photoirradiation, we were able to determine from NMR analyses that 60 μM formate could be produced from the CuFeO2/CuO//CdS/NiOx PEC. By biasing the CuFeO2/CuO PEC by using Pt as counter electrodes and passing 17C through the cathode, a significantly higher amount of formate could be produced reaching 0.4 mM which equals to approximately 6.81% electron to formate efficiency. Replacing the CuFeO2/CuO cathode with the NiO/CdTe led to lower amounts of CO2 photoreduction, where in unbiased experiments, the NiO/CdTe photocathodes generated ~5-10 μM of formate. Biasing the NiO/CdTe electrode could only slightly increase the total amount of formate to ~27 μM which equals to 0.5% efficiency.

Next, in order to study the use of an organohydride with the PECs, we tested the utilization of CpRh(bpy)Cl2 as a co-catalyst. One of the primary reasons for choosing CpRh(bpy)Cl2 was that this molecule has been used previously for reducing NAD+ to NADH at neutral pH which is an optimal pH to run water oxidation at the photoanodes in the PECs, and as a hydride mediator for CO2 reduction in solution in the presence of an electron donor. Cyclic voltammogram analysis first showed that the addition of the catalyst in the presence of dissolved CO2 with Pt as the cathode could generate increased current and an onset shift of ca. 100mV. Interestingly, by using CpRh(bpy)Cl2 as a co-catalyst, much higher amounts of CO2 reduction were observed from the NiO/CdTe photocathodes as compared to the CuFeO2/CuO which is summarized in Table 1. As demonstrated, NMR analysis showed that the unbiased PECs in the presence of CpRh(bpy)Cl2 could yield 400 μM and 187μμM formate from the NiO/CdTe and CuFeO2/CuO respectively. Therefore, in the presence of CpRh(bpy)Cl2, the unbiased NiO/CdTe PEC showed a 40-fold increase in product yield as compared to the unbiased NiO/CdTe PEC with no organocatalyst. In direct contrast, the CuFeO2/CuO system showed little improvement with only a 3-fold increase in formate. Furthermore, with the biased NiO/CdTe/CpRh(bpy)Cl2 PECs we could also detect 10 μM methanol being generated in addition to the 2.3 mM formate.