The photovoltaic reactor for artificial photosynthesis was designed as Fig. 1.

Figure 1 The photovoltaic reactor. The photovoltaic reactor consists of a solar panel, an accumulator and an electrolytic cell. Which electrolytic cell consists of an anode chamber, an anode plate and cathode plate. And a 2.5 μm micro porous membrane of PTFE was used as the separation membrane. Full size image

The photovoltaic reactor consisted of a solar panel, an accumulator and an electrolytic cell. The solar panels were used to absorb solar energy, which was then converted into electric energy. The electrolytic cell was the sites of reactions, where the high energy hydrogen atoms and oxygen were produced by electrolysis of water. Then oxalate and polymer were subsequently synthesized.

The reactor was operated under the conditions studied for 480 hours, thereby a produced solution were obtained. Then the solution was analyzed by high performance liquid chromatography (HPLC) (Fig. 2), in which results showed the oxalate concentration was 17.32 g/L. For the 600 ml solution, total of 10.39 g of oxalate was obtained. The produced solution was treated in the following order: 1) strong acidic cation-exchange resin, 2) strong basic anion-exchange resin to remove the generated oxalate and other electrolytic compounds and finally a neutral solution was obtained. The neutral solution thus obtained was concentrated and dried, yielding 0.76 g neutral solid product, which was a transparent or translucent solid, shown in the supplementary figure 1 (S. Fig. 1).

Figure 2 The HPLC spectrums for the produced solution. It is the HPLC spectrum of the produced polymer, in which results show the oxalate absorption peak. Full size image

The neutral solid product was analyzed by a gel permeation chromatography (GPC). The GPC spectrum contains two distinct peaks at 11.92 and 19.04 min, which accounted for 86.6% and 13.4%, respectively of the total peak area (S. Fig. 2). Based on the molecular weight calibration from globular proteins, the weight-average molecular weight (Mw) of the product at 11.92 min was 2.37 × 105 g/mol (Mw). The average molecular weight for the product at 19.04 min was 191.56 g/mol (S. Fig. 3 and 4). Therefore, the neutral solid product was regarded as a polymer.

To evaluate the performance efficiency of the reactor, the current efficiency, the electronic energy consumption efficiency and the cathode plate area efficiency were investigated. The current efficiency (E i ) is the ratio of the actual mass of special product (M i ) to the theoretical mass (M th ) of that product liberated according to Faraday's law, %; expressed as equation (1). The electronic energy consumption efficiency (E EEC ) is the actual mass of special products (M i ) divided by the electronic energy consumption (EEC), g/kwh; expressed as equation (2). The cathode plate area efficiency of (E CPA ) is the actual mass of special products (M i ) divided by the area of cathode plate (CPA) and time (t), g/(m2. h); expressed as equation (3).

Under the conditions studied, the current efficiency of the reactor was about 18.4% for oxalate and 5.18% for the polymer. The electronic energy consumption efficiency was 123.69 g/kwh for oxalate and 9.05 g/kwh for the polymer. The cathode plate area efficiency was 17.32 g/(m2. h) for oxalate and 1.27 g/(m2. h) for the polymer.

The elemental composition of the polymer is (wt%): 40.90% as carbon, 54.53% as oxygen and 4.55% as hydrogen. So, the formula of polymer is expressed as C 8 H 10.7 O 8 . Based on the NMR results, the proposed structure of the polymer was given in Fig. 3. The polymer is made of oxalate, glycol and α-hydroxyl acetic acid.

Figure 3 The 1H NMR results of the polymer. It is the 1H NMR spectrum of the polymer. The proposed chemical structure of the polymer is attached in. Full size image

Shown in Fig. 3, except the peak of solvent (D 2 O, δ 4.70), there are three different hydrogen atoms in the 1H NMR spectrum. Those four peaks at δ 3.225, 3.635, 3.745 and 4.035, noted as H a, connected with C atoms at δ 62.29, may be assigned to CH 2 groups (S. Fig. 5 and 6)13,14,15,16. The peak at δ 8.544, noted as H b , not connected with any C atoms (S. Fig. 6), but in the range of active H, is assigned to OH group13,16,17. The peak at δ 3.892, noted as H c , connected with C atoms at δ 71.80, is assigned to CH group (S. Fig. 5 and 6)16. The peak at δ 2.007, not connected with any C atoms and out of the range of active H (S. Fig. 6), can be regarded as an interference peak.

Shown in Fig. 4, there are four different C atoms in the 13C NMR spectrum. Those at δ 171.13–171.16, noted as C a , not connected with any H atoms (S. Fig. 6), may be assigned to C 2 O 4 group15,16,18. The peak at δ 62.29, noted as C b , connected with H atoms δ 3.225–4.035, is assigned to CH 2 group (S. Fig. 5 and 6)16,19. The peak at δ 71.80, noted as C c , connected with H atoms of δ 3.892, is assigned to CH group (S. Fig. 5 and 6)16,19,20. The peak at δ 164.09, noted as C d , not connected with any H atoms (S. Fig. 6), may be assigned to COO group16,20,21.

Figure 4 The 13C NMR results of the polymer. It is the 13C NMR spectrum of the polymer. The proposed chemical structure of the polymer is attached in. Full size image

The proposed structure of the polymer was further confirmed by Fourier transform infrared spectrometer (FTIR) and mass spectrometry (MS) results. There are five characteristic peaks in the FTIR spectrum. They are assigned to O-H16,22,23,24, C = O22,23,25,27, C-O23,24 and H-C-H23,24,26 (Fig. 5). These functional groups are consistent with the proposed polymer structure. Shown in Fig. 6 are the MS results, there are eight fragments that were less than 438 m/z and all of them can be found in the proposed polymer structure.

Figure 5 The FTIR results of the polymer. It is the FTIR spectrum of the polymer. The functional groups of O-H, C = O, C-O and H-C-H are noted. Full size image