Electrochemical Reduction of Carbon Dioxide

社会严重依赖使用原始化石燃料作为化工和塑料制造的原料。随着脱碳压力的增加，这对所有行业来说都是不可能的，导致无法完全脱碳的高二氧化碳排放者面临越来越大的经济处罚威胁，如水泥，钢铁和玻璃生产商。电化学CO2还原提供了利用间歇性可再生能源（风能、太阳能）将CO2转化为有用产品的机会。这些产品包括从CO（合成气成分）到可持续航空燃料。电化学CO2还原是一项成熟的技术，但是，由于需要快速发展以在2050年之前实现净零排放，因此对工艺规模扩大的挑战和机遇的研究已经加速。在这项工作中，我展示了使用廉价的锰电催化剂在两个常见的电解槽，流动电池和零间隙电池，旨在实现更高的电流比一个典型的批量电化学实验。我表明，Mn操作与CO（jCO）的13.7 mA cm-2的部分电流密度增加比以前报道的近中性电解质中的流动电池。然而，器件稳定性差，5小时后jCO降至<5 mA cm-2。在碱性环境中，Mn催化剂表现出低活性，其中jCO <10 mA cm-2，尽管在酸性环境中，对于CO的选择性良好，其中jCO ~35 mA cm-2。随着我们迈向脱碳工艺的工业化，了解不纯气体进料对减少二氧化碳的重要性至关重要。通常，研究集中在超纯CO2流作为电解原料，然而，实际上烟道气和捕获的CO2都含有O2等杂质。因此，必须了解即使是很小的浓度对电解选择性和设备的影响。通常，如果探索O2的影响，则无法解释器械失效机制，从而导致知识缺口。以下从我的工作使用纯CO2，我探讨了Mn络合物和一个简单的Au催化剂上的小浓度的O2的影响。我表明，存在5%的O2阻碍CO2还原酸，我继续研究Au作为催化剂。I基准Au在纯CO2中的活性显示出对CO的高选择性，jCO ~90 mA cm-2，尽管当引入0.25%O2时，jCO降低至80 mA cm-2。值得注意的是，由于过氧化物的形成导致碳酸（氢）盐的形成增加，O2的引入降低了器械稳定性（5小时至3小时的操作）。我表明，提供一个酸性环境有一个小的影响，提高O2耐受性，但牺牲CO的选择性（45 mA cm-2至24 mA cm-2）和电池电压（2.6 V至3.4 V）。最后，我筛选聚合物添加剂来抑制O2-还原。我发现，通过选择限制O2-运输到催化剂（Nafion）的聚合物，我们可以抑制O2还原。然而，CO2运输也受到阻碍，导致CO形成的选择性差。

Society relies heavily on the use of virgin fossil fuels as feedstock for chemical and plastics manufacturing. As pressure to decarbonise builds, this is not possible for all industries, leading to an increasing threat of financial penalties for high CO2 emitters that are unable to completely decarbonise, like cement, steel and glass producers. Electrochemical CO2 reduction provides an opportunity to utilise intermittent renewable energy sources (wind, solar) to drive the conversion of CO2 into useful products. These products range from CO (component of syngas) to sustainable aviation fuel. Electrochemical CO2 reduction is a mature technology, however, due to the requirement for rapid development to achieve Net Zero by 2050, research into the challenges and opportunities for process scale-up has accelerated. In this work, I show the use of a cheap Mn electrocatalyst in two common electrolysers, a flow-cell and a zero-gap cell, designed to achieve higher currents than a typical batch electrochemical experiment. I show that the Mn operates with increased partial current densities for CO (jCO) of 13.7 mA cm-2 than has previously been reported in near-neutral electrolyte in a flow-cell. However, device stability is poor with jCO dropping to <5 mA cm-2 after 5 hours. In basic environments, the Mn catalysts demonstrates low activity with jCO <10 mA cm-2, although in an acidic environment selectivity for CO is good with jCO ~35 mA cm-2. As we move towards industrialisation for decarbonisation processes, it is critical to understand the importance of impure gas feeds on CO2 reduction. Typically, studies have focussed on an ultra-pure CO2 stream as feedstock for electrolysis, however, in reality both flue gas and captured CO2 contain impurities like O2. Therefore, it is imperative to understand the impact of even small concentrations on electrolysis selectivity and devices. Often, if the impact of O2 is explored, device failure mechanisms are not explained, leading to a knowledge gap. Following on from my work using pure CO2, I explore the impact of small concentrations of O2 on both the Mn complex and a simple Au catalyst. I show that the presence of 5% O2 impedes CO2 reduction in acid, and I move on to study Au as a catalyst. I benchmark Au activity in pure CO2 showing high selectivity for CO with jCO ~90 mA cm-2, although when 0.25% O2 is introduced jCO decreased to 80 mA cm-2. Significantly, the introduction of O2 decreases device stability (5 hours to 3 hours operation) due to peroxide formation leading to increased (bi)carbonate formation. I show that providing an acidic environment has a small impact on improving O2-tolerance but sacrifices CO selectivity (45 mA cm-2 to 24 mA cm-2) and cell voltage (2.6 V to 3.4 V). Finally, I screen polymer additives to suppress O2-reduction. I find that by selecting a polymer that restricts O2-transport to the catalyst (Nafion), we can suppress O2 reduction. However, CO2 transport is also impeding, leading to poor selectivity for CO formation.