图1 电解液中Fe对水合氧化镍电极OER过电位(a)和Tafel斜率(b)的影响[34]

Fig.1 Effect of Fe in electrolyte on OER overpotential (a) and Tafel slope (b) of hydrous nickel oxide electrode[34]

图2 (a) Ni(OH)2薄膜的Fe含量与在未纯化的1 mol/L KOH中老化天数的关系；(b) 在无Fe和未纯化的1 mol/L KOH中，Ni(OH)2薄膜在10 mA/cm2下的老化过电位和 Tafel 斜率[46]

Fig.2 (a) Fe content of Ni(OH)2 films versus days of aging in unpurified 1 mol/L KOH;(b) Overpotentials and Tafel slopes of Ni(OH)2 films in Fe-free and unpurified 1 mol/L KOH at 10 mA/cm2[46]

图3 在无Fe的1 mol/L KOH溶液中，Ni1-z M z O x H y 薄膜在400 mV过电位下，第5个循环和第50个循环的OER转化频率；报告值为三个样品的平均值，误差条为标准偏差；Ni0.9Fe0.1O x H y 的误差条较小，在此刻度上不明显；TOFtm的计算假设所有金属阳离子都是活性的（因此是下限）[53]

Fig.3 OER turnover frequency of Ni1-z M z O x H y films at 400 mV overpotential at cycle 5 and cycle 50 in Fe-free 1 mol/L KOH; Values reported are the average, and error bars are the standard deviation of three samples; The error bars of Ni0.9Fe0.1O x H y are small and not visible on this scale; TOFtm are calculated assuming all metal cations are active (and thus are lower limits)[53]

图4 线性扫描伏安图：(a) NF-NiOOH电极在添加过渡金属阳离子（100 μg/L M n+）的纯化1 mol/L KOH电解液中进行CP调节后的OER性能变化；(b) Fe3+浓度对NF-NiOOH电极电化学性能的影响[55]

Fig.4 Linear sweep voltammograms： (a) variations of OER performance of NF-NiOOH electrodes after CP conditioning in purified 1 mol/L KOH electrolyte spiked with transition metal cations (100 μg/L M n+)； (b) Influence of the Fe3+ concentration on the electrochemical performance of NF-NiOOH electrodes[55]

图5 在0.1 mol/L KOH中，混合Ni-Fe催化剂的OER活性随Fe含量变化的测量结果；对于0%Fe催化剂，测试在除Fe后的电解液中进行；上图：Fe含量对在γ-NiOOH中竞争形成高活性Fe位点和相分离的低活性γ-FeOOH 的影响[56]

Fig.5 Measured OER activity of mixed Ni-Fe catalysts as a function of Fe content in 0.1 mol/L KOH; For 0% Fe, measurements were performed in electrolyte which was carefully purified to remove any Fe contamination; Top: a schematic illustrating the influence of Fe content on the competing formation of highly active Fe sites in γ-NiOOH and of phase-separated low-activity γ-FeOOH[56]

图6 Fe-MO x H y 的活性-稳定性趋势以及同位素标记实验对动态Fe交换的观察：在1.7 V的电压下，Fe-MO x H y 在‘无Fe’KOH (a) 与含0.1 mg/L Fe的KOH溶液 (d) 中进行1 h计时电流实验的活动性-稳定性结果；Fe-NiO x H y 电极在1.7 V下的总Fe含量 [(b),(e)] 及OER活性 [(c),(f)] 随时间变化；(c)中的示意图描述了溶解过程；在含0.1 mg/L 57Fe电解质中进行的类似恒电位测试 [(e),(f)] 显示了OER催化过程中的Fe动态交换（溶解与再沉积）；(f)中的示意图描述了Fe溶解和再沉积过程[26]

Fig.6 Activity-stability trend of Fe-M hydr(oxy)oxides and observation of dynamic Fe exchange by isotopic labelling experiments: summary of the results of the activity-stability study of Fe-M hydr(oxy)oxides during chronoamperometry experiments at 1.7 V for 1 h in ‘Fe-free’ purified KOH (a) and in a KOH solution containing 0.1 mg/L Fe (d); the total amount of Fe in the Fe-NiO x H y electrode [(b),(e)] and OER activity [(c),(f)] during chronoamperometry measurements at 1.7 V; the schematic diagram in (c) depicts the dissolution process; similar chronoamperometry experiments performed in electrolyte containing 0.1 mg/L 57Fe[(e),(f)] reveal Fe dynamic exchange (dissolution and redeposition) at the interface during OER catalysis; the schematic diagram in (f) depicts both the Fe dissolution and redeposition processes[26]

图7 可逆相偏析机制：在氧化还原电位下，Fe的溶解和Fe位点选择性再沉积导致了相偏析，而在还原态下，金属阳离子的再分布和均匀的Fe再沉积缓解了相偏析[59]

Fig.7 Mechanism for reversible phase segregation： at the OER potential, the dissolution of Fe and site-selective redeposition of Fe lead to phase segregation, whereas at the reduction state, the redistribution of metal cations and homogeneous Fe redeposition alleviate the phase segregation[59]

图8 OER过程中NiFe-LDH和NiFe合金在KOH中的Fe溶解/再沉积示意图[61]

Fig.8 Schematic diagram of the Fe dissolution/redeposition for NiFe-LDH and NiFe alloys in KOH during the OER process[61]

图9 （a）KOH溶液中Ni/Fe摩尔比与Ni-Fe合金电极OER起始电位关系；（b）电极衰减范围和恢复范围示意[62]

Fig.9 （a）Change of OER onset potential with Ni/Fe molar ratio in KOH solution; （b）Schematic of electrode degradation and recovery potential ranges[62]

图10 (a) NiOOH在1.55 V （vs. RHE）的CA测试，在除Fe后的1 mol/L KOH中开始测量，后加Fe(NO3)3到0.1 mg/L；(b) 在 (a) 中Fe掺杂达到峰值后，进行CV测试[58]

Fig.10 (a) CA measurements of NiOOH at 1.55 V (vs. RHE), after starting the measurement in purified Fe-free 1 mol/L KOH electrolyte, aqueous Fe(NO3)3 was added to a concentration of 0.1 mg/L; (b) CV test after peak Fe doping in (a)[58]

图11 在90℃、3 kA/m²、含10 mg/L Fe的33%（质量分数） NaOH条件下，阴极HER电位随时间变化；虚线表示可逆电位[63]

Fig.11 Variation of the cathode potential with time during the course of the HER from 33%(mass) NaOH at 90℃ and 3 kA/m2 in the presence of 10 mg/L Fe; The dashed line represents the reversible potential[63]

图12 在 37℃、30%（质量分数） KOH、250 mA/cm2条件下，Ni阴极电位随时间变化的情况（： 0.03 mg/L Fe；：3.0 mg/L Fe）[64]

Fig.12 Nickel cathode potential behavior with time under galvanostatic control of 250 mA/cm2 in 30%(mass) KOH at 37℃ ( for 0.03 mg/L dissolved iron; for 3.0 mg/L dissolved iron)[64]

图13 (a) 长期实验中的阴极极化电位；(b) 长期实验中的氢渗透电流；温度：70℃，充电电流：100 mA/cm²，裸Ni和Fe镀Ni的三个重复实验的100 s平均电流[67]

Fig.13 (a) Cathode polarization potentials during longer term experiments; (b) Hydrogen permeation currents during longer term experiments; T = 70℃, charging current = 100 mA/cm2, 100 s averaged currents from three replicate experiments for bare Ni and Ni with Fe coating[67]

图14 91℃下，电解液中Fe浓度分别为6、20、40和357 μmol/L时IR校正后的HER电位I-V曲线(a)和HER Tafel图(b)；Tafel图中采用的是电流密度（单位是A/cm²）的对数值，数据以平均值±标准差的形式呈现[68]

Fig.14 I-V curves for internal resistance corrected HER potential (a) and Tafel plots (b) for HER at 91℃ for Fe electrolyte concentrations of 6, 20, 40, and 357 μmol/L; Note that for the Tafel plots the logarithmic value of the current density was used in A/cm2; Data are represented as mean ± standard deviations[68]

图15 氢气泡内/下腐蚀产物的形成及其在脱离氢气泡后还原成金属的示意图[69]

Fig.15 Schematic diagram of the formation of corrosion products at/under hydrogen bubble and their reduction to metal upon detachment of the bubble[69]