As a green alternative,the electrochemical nitrate reduction reaction (e-NO3RR) enables the conversion of nitrates from industrial wastewater and domestic sewage into ammonia using renewable energy.However,e-NO3RR catalysts often face challenges such as insufficient activity and stability.Herein,copper ruthenium nanoparticles supported on carbon black (CuRu/C) were synthesized by a simple Joule heating method.The morphology,composition and structure of the catalyst were investigated by various characterization methods.The electrochemical test show that CuRu/C exhibits the optimal NH3 yield rate and faradaic efficiency(FE) at -0.4 V versus reversible hydrogen electrode (vs.RHE).Furthermore,there is no obvious attenuation of the performance and FE is above 75% after 6 cycles of electrolysis experiment.Encouraged by its excellent ammonia yield,the CuRu/C electrode was employed as a cathode to assemble a Zn-nitrate battery,which achieved a power density of 5.407 mW/cm2,indicating its promising potential in energy conversion and storage.
如图7(a)所示,为避免偶然性误差,通过3组独立的恒电压电解实验得到CuRu/C对NH3的FE和产率误差棒图。随着电位正向增加,FE逐渐增加,产率逐渐降低在工作电位为-0.2 V和-0.4 V vs.RHE时,FE均接近80%,对NH3选择性较高。
为了探究硝酸盐还原产物的分布情况,对电解液中${\mathrm{NO}}_{2}^{-}$同步检测,结果如图7(b)所示。从柱状堆积图中可以看出,较正电压下(0、-0.2 V vs.RHE)还原产物含有较多${\mathrm{NO}}_{2}^{-}$,说明低过电势下CuRu/C无法将${\mathrm{NO}}_{3}^{-}$还原为${\mathrm{NO}}_{2}^{-}$,材料对${\mathrm{NO}}_{2}^{-}$选择性较高;在较负电压下(-0.6、-0.8 V vs.RHE)还原产物中${\mathrm{NO}}_{2}^{-}$极少,说明高过电势下CuRu/C能够克服${\mathrm{NO}}_{3}^{-}$还原为${\mathrm{NO}}_{2}^{-}$的能垒。同时,随着电位负向增加,${\mathrm{NO}}_{2}^{-}$和${\mathrm{NO}}_{3}^{-}$的总体FE逐渐降低,推测可能的原因为竞争性HER活性逐渐增强。综合来看,当工作电位为-0.4 V vs.RHE时CuRu/C对NH3的选择性和产率最优。
为探索CuRu/C催化剂用于实际能源转换与存储的可能性,将其应用于Zn-${\mathrm{NO}}_{3}^{-}$电池。其中,CuRu/C和锌片分别作为阴极和阳极材料,组装水系Zn-${\mathrm{NO}}_{3}^{-}$电池。与传统锌-空气电池相比,Zn-${\mathrm{NO}}_{3}^{-}$电池耦合NO3RR反应免受气液传质影响。如图9(a)所示,基于CuRu/C的Zn-${\mathrm{NO}}_{3}^{-}$电池表现出恒定的开路电压,持续3 600 s稳定在1.49 V vs.Zn/Zn2+,与万用表测得数值一致。从图9(b)可以看出Zn-${\mathrm{NO}}_{3}^{-}$电池的放电极化曲线显示放电电流密度随着电池电压的降低而增加,其间功率密度最高为5.407 mW/cm2。为了探究电池在不同电流密度下的放电稳定性,在1~5 mA cm-2范围内进行恒电流放电测试。如图9(c)所示,组装的Zn-${\mathrm{NO}}_{3}^{-}$电池在各个电流密度下电池电压均保持稳定30 min以上,显示出电池反应过程高效的电子和质量传递过程。此外,为了验证Zn-${\mathrm{NO}}_{3}^{-}$电池的循环耐久性,对电池进行循环充放电测试。如图9(d)所示,经过32个充放电循环后电池电压无明显衰减,表明该电池体系具备良好的稳定性。
4 结论
采用快速脉冲焦耳热的方法合成了负载于碳黑上的双金属催化剂CuRu/C。结果表明,纳米颗粒与碳黑载体结合暴露出丰富的催化活性位点,Ru优化了Cu的电子结构,利于提升材料的NO3RR活性。在-0.4 V vs.RHE下CuRu/C表现出最优的产氨活性,FE最高可达82.4%,NH3产率为397 μmol/(h·cm2)。同时,循环稳定性测试中经过12 h的恒电压电解后CuRu/C的FE和NH3产率未见明显衰减,稳定性良好。此外,CuRu/C催化剂成功应用于Zn-${\mathrm{NO}}_{3}^{-}$电池阴极材料,开路电压稳定在1.49 V vs.Zn/Zn2+,功率密度最高可达5.407 mW/cm2,且经过32个充放电循环电池电压无明显衰减。制备的双金属催化剂CuRu/C为环境中硝酸盐的去除和能源转换提供了一种潜在的解决方案。
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