(Invited) Electrooxidation of Urea and Creatinine on a Nickel Electrocatalyst
Authors: Stuve Eric M. , Carpenter Kelly 
Published: May 2020

Urea is one of the world’s most abundant waste products, commonly found in waste streams in varying concentrations. Urea is the compound in largest abundance in human urine (other than water), and creatinine is its second most abundant organic compound [1]. Typical concentrations of urea and creatinine in human urine are 0.22 M and 0.013 M, respectively. In treatment of human and animal urine streams, a method for reactive removal of organic compounds must include elimination of both urea and creatinine. A previous study by Schrank et al. [2] of the electrochemistry of urine compounds showed that, on a nickel cobaltite catalyst at urine-relevant concentrations, creatinine suppressed the reaction of urea at lower potentials, while creatinine oxidized preferentially over urea at higher potentials. In dialysate, however, urea and creatinine are present in lower concentration, typically 6 mM and 60 μM, respectively, and whether co-reaction of a urea/creatinine solution occurs is of strong interest in removal of uremic toxins.
Electrochemical reaction of urea has been studied for a variety of electrodes and is currently receiving increased interest due to its potential in treating wastewater to produce hydrogen and ammonia [3–5]. Urea electrooxdiation is also suitable for a direct urea fuel cell [6]. To date, one of the most active electrodes is Ni(OH)2, which converts to the catalytically active (Ni3+) form of nickel oxyhydroxide (NiOOH) above 0.45 V vs. a Hg/HgO electrode at pH 14 [3]. Urea electrooxidation is a 6-electron process; the anode reaction in alkaline media is
(NH2)2CO + 6 OH− → N2 + CO2 + 5 H2O + 6 e
−

and has a standard potential of −0.896 VHg/HgO (0.070 VRHE) at pH 14. Electrooxidation of creatinine is a 21-electron process
C4H7N3O + 21 OH− → 3/2 N2 + 4 CO2 + 14 H2O + 21 e
−

and has a standard potential of −1.021 VHg/HgO (−0.055 VRHE) at pH 14.

Materials and Methods

Cyclic voltammetry (CV) and chronoamperometry were performed in a three-electrode cell (Pine electrochemistry) utilizing a Pt coil counter electrode (CE) and a Hg/HgO reference electrode (RE) appropriate for alkaline solutions. The working electrode (WE) for each experiment was either a polished Ni disk (Basi), Ni hydroxy foam (NHF), or NHF with 1% Fe (NHF-Fe). All solutions contained 1 M KOH as a supporting electrolyte, chosen to represent the strongly alkaline environment of an anion exchange membrane that would be used in a larger reaction cell. Urea and creatinine were added in concentration ranges typical of healthy and uremic human serum. Data collection and visualization were performed with a Solartron 1287A potentiostat coupled with CorrWare/CorrView software. All cyclic voltammetry was performed at a scan rate of 10 mV/s.

Results

Figure 1 shows CVs of blank solution, urea solution, and a urea/creatinine solution on a NHF-Fe electrode. The Ni2+/Ni3+transition [Ni(OH)2 + OH− ↔ NiOOH + H2O + e
−] occurs over the range of 0.45–0.6 VHg/HgO for oxidation and 0.2–0.55 VHg/HgO for reduction. The oxygen evolution reaction (OER) occurs at potentials of 0.65 VHg/HgO and higher. Urea oxidation (in 10 mM urea) occurs over the range of 0.5–0.65 VHg/HgO , and its overlap with the Ni2+/Ni3+ transition demonstrates the catalytic nature of Ni3+. These results are in agreement with previous work. When 59 μM creatinine was added, the CV shows an increased oxidation peak in the urea oxidation region, attributed to co-oxidation of urea and creatinine. Chronoamperometry results (not shown) show that steady-state oxidation is established within one minute at 0.55 VHg/HgO. The steady-state oxidation current was 15 mA/cm2 for the urea/creatinine solution vs. 12 mA/cm2 for urea alone.

Conclusion

Electrocatalytic co-oxidation of urea and creatinine occur on a NHF-Fe electrode in alkaline (pH 14) media at potentials of 0.5–0.65 VHg/HgO. Demonstration of co-oxidation is of primary importance in reactive removal of organic compounds from dialysate and – by extension – from urine.

Acknowledgements

This work was supported by the University of Washington.

References

1. D. F. Putnam, “Composition and concentrative properties of human urine,” McDonnell-Douglas Astronautics Company/NASA Contractor's Report (1971).
2. A. Schranck, R. Marks, E. Yates, and K. Doudrick, Environ. Sci. & Tech. 52, 8638–8648 (2018).
3. B. K. Boggs, R. L. King, and G. G. Botte, Chem. Comm. 32, 4859–4861 (2009).
4. R. L. King and G. G. Botte, J. Power Sources 196, 9579–9584 (2011).
5. V. Vedharathinam and G. G. Botte, Electrochim. Acta 108, 660–665 (2012).

Figure 1