Heteroatom-doped highly porous carbon from human urine

Human urine, otherwise potentially polluting waste, is an universal unused resource in organic form disposed by the human body. We present for the first time “proof of concept” of a convenient, perhaps economically beneficial, and innovative template-free route to synthesize highly porous carbon containing heteroatoms such as N, S, Si, and P from human urine waste as a single precursor for carbon and multiple heteroatoms. High porosity is created through removal of inherently-present salt particles in as-prepared “Urine Carbon” (URC), and multiple heteroatoms are naturally doped into the carbon, making it unnecessary to employ troublesome expensive pore-generating templates as well as extra costly heteroatom-containing organic precursors. Additionally, isolation of rock salts is an extra bonus of present work. The technique is simple, but successful, offering naturally doped conductive hierarchical porous URC, which leads to superior electrocatalytic ORR activity comparable to state of the art Pt/C catalyst along with much improved durability and methanol tolerance, demonstrating that the URC can be a promising alternative to costly Pt-based electrocatalyst for ORR. The ORR activity can be addressed in terms of heteroatom doping, surface properties and electrical conductivity of the carbon framework.


Thermal stability of URC materials
The carbon content in these URC materials can be readily determined by TGA. As shown in the Figure S3, the weight losses near 100 o C are mainly attributed to the evaporation of adsorbed water. The combustion of carbon slowly begins around 460 o C for URC-700 and URC-800, and around 570 o C for URC-900, URC-1000, and URC-1100, which is comparable (565 o C) to that of graphite (Junsei Chemical Co., Ltd. Japan), indicating good thermal stability. The decomposition of URC samples is nearly complete after 700 o C. The final weight Figure S2 (A) A photograph showing rock salt composite and an USA one cent coin for comparison; The rock salt composite was obtained by drying the filtrate removed by 0.1 M HCl from the mixture of carbon and salt materials synthesized using human urine, (B) XRD spectra of (a) dried rock salt mixture (see (C) for the SEM image) obtained from filtrate for URC-1000-BW and (b) evaporated salt particles (see (D) for the SEM image) collected on the inner wall out of heating zone of quartz tube inserted into the tube furnace for URC-1000-BW. loss reaches to 95.5, 91.3, 89.6, 93.3, and 92.5 % for URC-700, URC-800, URC-900, URC-1000, and URC-1100, respectively. Secondly, thermal stability of the carbon materials is also important in determining their end use for numerous applications. These URC materials show a good thermal stability, and the peak temperature on DTG curve represents the temperature at which the maximum weight loss rate is reached, as shown in Figure   S3B. According to the DTG curves, it is found that the ranges of decomposing temperature move toward higher temperature and significantly increased by about 47 o C from URC-700 to URC-1100, indicating that the increase in carbonization temperature considerably enhances the thermal stability of the URC materials.

Raman spectra of the URC materials
The presence of non-metallic heteroatoms such as S, Si, and P except N in all our samples is found comparatively low by XPS analysis techniques (e.g., each <1 atom %). Raman spectroscopy was used to investigate the structural changes induced by the presence of the heteroatoms in the URC material as shown in Figure S4. Raman spectrum of each of URC samples displays two broad bands at 1340 and 1575 cm -1 , which are assigned to the D band and G band, respectively. The D band at 1340 cm -1 is related to the breaking of symmetry caused by structural disorders and defects, while the G band at 1575 cm -1 represents the in-plane tangential stretch vibration mode (E 2g ) of the graphite sheet (ref. S1, S2). Raman spectroscopy indicates that the peak intensities increase with the carbonization temperature. The positions of these bands are similar to each other for all five URC materials, suggesting that the structures of the all carbon are similar, showing turbostratic feature as observed in XRD spectra of Figure S1B. Interestingly, however, the differences in the I D /I G ratio are observed, and the increase in the intensity ratio suggests that the carbon structure becomes more disordered with increasing carbonization temperature. Moreover, it should be noted as well that at higher temperature i.e. for URC-1100, the second order Dʹ and Gʹ bands become more noticeable than for the other samples prepared at lower carbonization temperature. This can be attributed to the increase of the disorder in sp 2 hybridized graphitic carbon. The strong D-and Dʹ-band peaks for URC-1100 indicate that defects are increasing in the carbon obtained at higher carbonization temperature owing to the various bonding structures and defects in the URC-1100. The G-band peaks of all the samples down-shifted to around 1575 cm -1 , compared to the pristine graphene peak at 1589 cm -1 , signifying the increase of defects and disorder due to the presence of the heteroatoms within the carbon structure. Nevertheless, the defects and disorder are considered to be favorable for the improvement of electrochemical properties (ref. S3).

XPS spectra of C 1s for URC materials
The major C 1s peak at 284.6 eV can be assigned to sp 2 (C-C) followed by sp 3 at 285.3 eV, and the peak at 286.5 eV corresponds to the bonding configurations of carbon with oxygen or nitrogen. (ref. S4-S8) As shown in Table 1, the carbon content increases from URC-700 to URC-1100. Similarly, the main peaks of C-C bonding i.e. sp 2 -type (284.6 eV) and sp 3 -type (285.3 eV) are also increasing along with the total carbon content ( Figure   S5). However, comparatively the increase in sp 2 hybridization is slightly higher over the sp 3 hybridization. Table   S1 summarizes the full width at half maximum (FWHM) values of sp 2 (C-C) and sp 3 (C-C) constituent peaks.
The FWHM values of sp 2 and sp 3 constituent peaks decrease by only 0.1 eV from URC-700 to URC-1100. This could be due to the effect of carbonization temperature and the presence of various heteroatoms in the URC. (ref. S9) Secondly, for the deconvoluation of the C 1s signal, the energies were chosen at 284.6 and 285.3 eV for sp 2 and sp 3 , respectively.

Figure S5
Deconvoluted XPS spectra of C 1s for URC materials prepared at different carbonization temperatures. Figure S6 shows the deconvoluted XPS spectra of N 1s for URC materials prepared at different carbonization temperatures. Figure S7 shows relative ratios of the deconvoluted peaks of N 1s as a function of carbonization temperature. It is clearly seen that N 1s signal is split into three major N species peaks centered at ~398.4 (pyridinic-N1), ~400 (pyrrolic-N2), and ~401.1 (quaternary-N3) (ref. S10). Interestingly, relative amount of N bonding configurations significantly changes with increasing carbonization temperature. It is worthy to note that all the N species decrease with the increase of temperature. Quaternary N species is found to be relatively more stable and predominant compared to pyridinic and pyrrolic N species although overall N content decreases with increasing temperature. In particular, pyrrolic N largely decreases probably owing to Table S1. FWHM values of deconvoluted peaks of C 1s in the URCs.

C1s
URC their lower stability at high temperature ( Figure S7). Interestingly, pyridinic N species also decreases but to the less amount. Thus, the relative amount of the quaternary N species increases with increasing temperature probably due to relatively better stability of quaternary N species compared to pyridinic and pyrrolic N species.
Moreover, all the URCs show presence of a minor terminal-N after 402 eV, which was excluded from the deconvoluted XPS spectra for clarity. In the case of XPS spectrum of N 1s, the FWHM values decrease distinctly for the deconvoluted constituent peaks for N1-pyridinic, N2-pyrrolic, and N3-quaternary configurations as shown in the Table S2. This may be attributed to the relatively higher nitrogen content (9.8 %) for URC-700 compared to 2.0 % in case of URC-1100 (ref. S7, S9). Furthermore, for the deconvoluation of the N 1s signal, the energies were chosen at 398.4 (pyridinic-N1), 400 (pyrrolic-N2), and 401.1 eV (quaternary-N3).  URC-1100 0.9 1.0 1.5

Cyclic voltammogram (CV) measurements for the URC materials
In general, the onset potential of ORR and cathodic reduction peak increase with increasing carbonization temperature as shown in Figure S8. Compared with all the carbon electrodes, the URC-1000 electrode shows better and more positive shift in both the onset potential (-0.04 V) and the peak potential (-0.19 V) with additional notable increase in the current density. Interestingly, the ORR onset and cathodic peak potentials at the URC-700 and URC-1100 electrodes shifted negatively compared to those of the rest three carbon electrodes, indicating a reduced ORR electrocatalytic activity. In order to determine the stability of the URC materials, ORR forward peak maximum currents were recorded for URC-1000 and commercial 20 wt% Pt/C catalysts during the repeated potential cycling up to 5000 ( Figure S9A). Methanol tolerance was compared for both URC-1000 and 20 wt% Pt/C catalysts in an electrolyte containing O 2 -saturated 3.0 M methanol in Figure S9B and S9C.

Resistivity measurement for the URC materials
To study the variation of electrical conductivity with pressure, a cell in four probe configuration was used for the measurement as shown in Figure S10. The cell was fabricated with some modification of an earlier report on measurement of electrical conductivity of powder samples (ref. S11). The cell consists of a hollow cylinder constructed with a non-conducting material (Teflon) in which two metallic pistons (Brass) form a pressure chamber. Current is applied to the sample through the metallic pistons and voltage is measured across the leads as shown in Figure S10. Keithley model 6220 and model 2182A were used as the DC current source and voltmeter, respectively. The current was varied from 0 to 10 mA, and the corresponding voltages were measured.
The electrical conductivity of the samples were estimated using the formula shown in following equation (3) l RA   where σ is the electrical conductivity, R is the resistance of the sample, A is the area of cross section of the sample (0.126 cm 2 ) and l is the distance between the voltage probes (0.2 cm). The pressure was varied by applying known weights on the metallic piston.

Figure S10
A diagram of the cell for measurement of electrical conductivity of powder samples.

Electrochemical impedance spectroscopy
To understand the electrocatalytic performances of the URC materials, electrochemical impedance spectroscopic (EIS) investigation was carried out and shown as Nyquist plots for URC-800 to URC-1100 materials in Figure S11A. The impedance diagrams are fit, considering the electrical equivalent circuits as shown in Figure S11B, and selected parameters are displayed in Table S3. This impedance data can be used to describe the interface properties. resistance, which is more favorable for promoting both the electrocatalytic activity and the ORR kinetics than other URC materials. This can be attributed in part to high mesopore surface area and volume of URC-1000, which facilitates the O 2 and electrolyte movement toward the ORR active surface sites as summarized in Table 1.
Thus, URC-1000 exhibits comparatively enhanced electrocatalytic activity as compared to the other URC materials, consequently proving to be the best electrode material for the ORR in terms of efficiency and kinetics.