As Florian said, if your goal is simply to keep track of your participants and your recruiting platform already provides unique IDs, I strongly recommend you just use that instead.
If you still need to use an encryption method, most browsers now implement the SubtleCrypto API. There are some limitations though: some browsers like Edge do not support some algorithms, and most importantly, the API will only work on secure domains (the PCIbex Farm uses a secure domain).
If your goal is to store encrypted email addresses in your results file that only you can later decrypt, you will need an algorithm that takes a public encryption key and a private decryption key. There are a few of those, one of them is the ECDSA, illustrated here. You would also need to export your public key, as illustrated here.